Physical Basis of Molybdenum and TZM Alloy Plastic Working

Before molybdenum and TZM alloy plastic should a certain understanding of its brittleness, toughness, fracture behavior and other physical properties, to understand the physical basis of the material, in order to better carry out plastic working. The plasticity of material refers to the deformation degree before breaking. The strength refers to a kind of ability to resist deformation and fracture. And toughness is the ability to absorb energy from plastic deformation to fracture of the whole process. Molybdenum and TZM alloy has high strength, but the plastic deformation is poor, namely poor toughness, having obvious brittleness.

The brittleness and toughness of material will changes as the temperature changes, namely, plastic-brittle transition temperature (DBTT). In the above DBTT temperature range, under high stress the plastic deformation is more smoothly, and the resulting products showed good toughness. Conversely, at temperatures below the DBTT to process deformation is prone to produce brittle fracture with different forms. Different metal materials, the plastic-brittle transition temperature is different, tungsten plastic-brittle transition temperature is generally about 400 ℃ and molybdenum DBTT is near the room temperature. High plastic-brittle transition temperature, the brittle of material is high which is not conducive to material processing. In order to reduce the plastic-brittle transition temperature of the material the measure is to overcome the brittleness and increase the toughness. The effect factors of material’s plastic-brittle transition temperature include purity, grain size, deformation degree, stress and alloying element material.

TZM alloy image

Molybdenum and its alloys low (or room) recrystallization brittleness is unlike copper, aluminum. After recrystallization annealed, copper and aluminum will form equiaxial recrystallization grain structure has excellent room temperature plastic processing, which can easily be processed at room temperature. However, molybdenum and TZM alloy after recrystallization has high brittleness at room temperature, so during the processing and using is prone to brittle fracture.

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High-energy High-speed Consolidation for TZM Alloy Manufacturing

Using hot isostatic for TZM mixed powder pressing due to time temperature deviation large, the mixed powder can be not fully densifying. And it will affect alloy’s mechanical properties. Using high-energy high-speed consolidation technology to replace the hot isostatic for mixed powder pressing, it is possible to produce high-density material and improve alloy’s strength. In this production process, the powder is pressed between the electrodes by means of high voltage which produced by unipolar generator to press the powder. When the voltage is about 25 to 5 volts, the unipolar generator can provide 100-150A/mm2 current density. Using high-energy high-speed consolidation method for TZM alloy powder pressing and the production process is as following:

1. TZM alloy powder was mixed evenly. And in TZM mixed powder the Ti content is 0.47%, Zr 0.109%, carbon 0.018%, oxygen 0.0016%, and average particle size is 74um.

2. The powder was placed in between the electrodes, and the current is about 10 megajoules to make the mixed solidified. During discharge, the copper electrode should to cover with a 250 microns molybdenum foil layer, or directly using tungsten electrodes. Besides, the weight of the powder is between 59 ~ 150g and molding pressure is between 270 to 690 MPa in the regulation.

Using high-energy high-speed consolidation for powder pressing, the powder has high density, moderate strength, low toughness and appropriate structural changes. After sintered, TZM alloy structure is unevenly, mainly because of copper electrode conduction cooling, so the copper electrode will affect the temperature of the powder.

With different pressures and unit input power, the midplane density of alloys is different. When the consolidation energy is higher than 3000 J/g, the densification is much larger than 98%. When consolidation energy is about 270-890 MPa can little affect the results.

TZM image

When the local density is greater than 96%, the alloy shows good mechanical properties. In the sintering process, the alloy exhibits low plasticity, mainly because of the presence of excess oxygen. However, the oxygen content is too small Mo also suffers severe embrittlement.

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Using Powder Metallurgy Method for TZM Alloy Plate Manufacturing

TZM alloy is the most used widely and the best performance molybdenum alloy. TZM alloy has good high temperature strength, creep resistance and higher recrystallization temperature, so it is often made of nozzle, piercing point, mold, heat shields and high-power ceramic tube gates, widely used in industry, military and aerospace fields. TZM alloy production methods are melting and powder metallurgy. Currently most of manufacturers choose to use powder metallurgy method, mainly because it doesn’t need have consumable arc furnace, large presses and high-temperature furnace and other large equipment, and the process is simpler. Besides, using powder metallurgy for TZM alloy plate manufacturing the production cycle is short and energy consumption is low, having high productivity. In addition, the alloy which produced by powder metallurgy has similar properties with smelting method produced alloy. To obtain good system performance alloy plate should improve alloy’s recrystallization temperature and ductility, and reduce plastic-brittle transition temperature, so in the process of rolling should roll in changed directions and process intermediate heat treatment, making the alloy performance has improved. TZM alloy plate produce by powder metallurgy method and the production processes are as following:

TZM image

1. Mixing 0.45% Ti, 0.08% Zr, 0.01% C and graphite powder with Mo for 6 hours is good for full mixing.

2. Using cold isostatic to press the mixed powder at 150MPa pressure obtain pressed blank.

3. Under the protection of hydrogen, the pressed blank is placed in sintering furnace at 2100 ℃ for 4 hours heat preservation and then to sinter to get TZM alloy blank.

4. Roll the 30mm blank at 1350℃, making the slab thickness to become 4.5mm, and the deformation is 83%. Then at 700 ~ 750 ℃, the slab is rolled to 1.2mm, and the total deformation is 95%.

5. Anneal at 900 ℃to eliminate stress, then at 600 ~ 700 ℃ longitudinal rolling to obtain 0.7mm.

6. After annealing at 850 ℃ to eliminate stress, then longitudinal cold rolling at 200 ~ 300 ℃ obtain 0.5mm alloy plate, and the total deformation is 98%.

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Powder Metallurgy Produced TZM Alloy Plate Properties Analysis

Modern TZM alloy industrial production usually uses powder metallurgy method. In order to improve the properties of the alloy, should process rolling in changed direction and intermediate heat treatment, which not only can eliminate the stress of the alloy, but also can improve its mechanical properties.

Analyze the mechanical properties and process performance tests on TZM alloy, which produces by powder metallurgy. TZM alloy plate blank cold rolling at 45°orientation has a certain degree tensile property. After annealing at 850 ℃, the plate in all directions at room temperature has tensile strength.

With incomplete pole figure method estimates the orientation distribution function (ODP) of 0.5mm powder metallurgy produced TZM alloy plate in the processing state and eliminate stress state (850 ℃/h) showing that the states of the alloy plate blank will affect alloy plate’ texture And the states and orientation of plate blank’s affected the mechanical properties at room temperature of alloy plate is connected with texture type of plate blank.

TZM ALLOY PICTURE

Alloy state and orientation will great impact curved plastic-brittle transition temperature. When alloy at cold state, the bending performance has better direction. After 850 ℃ annealed, the plastic - brittle transition temperature is decreased, and the lateral amplitude transformation temperature is lowered significantly. After eliminated stress, in all directions of alloy plate transition temperature is similar and below 0 ℃, so the alloy plate is preferably used after stress relief.

After complete recrystallization, there is not elongation on alloy plate. And the curved plastic-brittle transition temperature is increased to above room temperature. So we should avoid use alloy plate above the recrystallization temperature. Alloy plate starting recrystallization temperature is 1200 ℃ and the ending recrystallization temperature is 1600 ℃.

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Using Brazing Titanium Graphite and TZM Alloy to Produce Joint

Graphite is a low atomic weight materials, having low loudness density, high sublimation point (3850 ℃), high thermal conductivity, heat capacity and excellent impact resistance and other advantages. To produce a new type alloy material and this alloy is maked up by graphite W and Mo. As we know, graphite has good thermal performance and low-density advantages and other advantages. So doped with graphite into Mo or W to produce new type alloy is good for heat dissipation, but at the same time will reduce products’ quality, especially in some of the high-speed rotating components. Doped Ti, Zr and other trace elements in Mo, after powder metallurgy process and alloying process can produce Mo alloy, which is TZM alloying. And TZM alloy has good mechanical property, thermal physical property, thermal and electrical conductivity which is higher than other insoluble metal. Using TZM alloy and graphite to produce composite joints obtained by braze welding has widely applications in the aerospace field. Vacuum brazing is the preferred process for preparing high temperature composite joints. China domestic current has little research in this area. Smid, who studied the Mo-graphite composite materials for NET / ITER nuclear reactor to obtain joint which can use in 800 ~ 1200 ℃. Chan, who use 72Ag-28Cu and 95Ag-5Al brazing filer metal for Ti-6Al-4V and TZM alloy brazing, and the brazing temperature is 850 ℃ and 950 ℃. Besides, the weld of joint is good.

TZM ALLOY PICTURE

TZM alloy composition is (mass fraction, %) 99.4Mo-0.47Zr-0.1Ti, the rest is other trace elements. And the diameter is 100mm. After powder metallurgy sintering and 1400 ℃ forging, upsetting annealing can obtain TZM alloy. Graphite has high strength, high density, high purity, and its outer diameter is greater than 100mm. TZM alloy and Ti foil washed by ultrasonic with acetone, is placed in vacuum hot pressing brazing furnace sequentially. Chose titanium foil as brazing filler metal, and titanium foil thickness is 0.05mm and diameter is 100mm.

After brazing, under optical microscope and stereo microscope specimens found the joint is densification and has uniform width. The joint observed by SEM and microanalysis component analysis (EDX) showed that the brazing layer width is about 120um and complete penetration rate is above 95%, which has clearly two-layered structure. Graphite matrix is ​​loose, the brazing layer closing to graphite substrate side has deeper color. This is interface reaction layer produced by graphite substrate and brazing filler metal chemical reaction. This layer connects with graphite, so it is relatively smooth. Besides, there are a small number of cracks and voids. And the thickness of reaction layer occupies 1/3 of the entire brazing layer. Analyzed by EDS, the average component of mixed layer is (mole fraction, %) 46.22C-53.78Ti. And the mix layer is mixed by high melting point TiC and rest of brazing filler metal which is produced by interfacial reaction. The brazing layer closing to TZM matrix the color is lighter. This brazing layer is Ti-Mo solid solution, which accounted for 2/3 of the entire solder layer. Combined with TZM substrate, the structure is smooth, and there are knife-like carbide to grow up, improved joint performance. The EDS average component (mole fraction, %) is 51.48Ti-48.16Mo.

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Brazing Time and Temperature Affected Joints of Brazing Titanium Graphite and TZM Alloy

When the brazing filer metal is selected, process parameters of brazing process is the main reason to impact joint’s organization and performance. Use titanium alloy as brazing filer metal. The temperature of heat preservation, time of heat preservation and quantity of brazing filer metal will affect structure and properties of joint, and this joint is made up by graphite and TZM alloy braze welding. To analysis the mechanism and optimize the process parameters, making joint using temperature increases to 1400 ℃, and joint re-melting temperature is higher than 1600 ℃. Besides, the joint strength is about 141 ~ 150MPa.

Brazing process significantly affects the property and performance of joint. Brazing temperature and time determine the thickness and composition of the brazing layer. Combined with Fick law of diffusion and the research of ceramic and metal activity brazing found in certain reaction system the reaction layer thickness depends on the connection time and temperature. With the increase of the brazing time, temperature increasing, the thickness of the reaction layer is increase, till saturation. If the reaction temperature is low, or the time is short, and the reaction layer is not continuous. Besides, the wetting effect of brazing filler metal on graphite base material is insufficiency. What’s more, the interface bonding force is too small. Analyzed interface reaction layer’s thickness and morphology by SEM found within a certain range, the higher the temperature, the longer the time, the TiC reaction layer is more wider. However, the interface reaction does not increase with time and temperature without limit, when the reaction layer thickness reaches a certain extent will produce dense carbides to separate Ti and graphite. Ti difficult to penetrate or diffuse through the carbide layer to react with graphite, so long temperature preservation can not be significantly improved reaction layer thickness of the joint.

TZM ALLOY PICTURE

In addition, the brazing temperature and time also determines the thickness of the Ti-Mo solid solution layer. The weld of brazing and TZM substrate through diffusion reaction can change the composition of brazing filler metal to form a solid solution, and after isothermal solidification can obtain solid solution. Analyze the connection of TZM by TLP (transient liquid phase) diffusion mechanism. After brazing filler metal was melted, Mo of TZM element through the interface of brazing filler metal dissolves in liquid the brazing filler metal, and uniformly, resulting in solid solution melting point increase, solidification occurs in the brazing temperature. When the brazing filler metal thickness is 0.1mm, at different temperatures between 1700 ~ 1750 ℃, the thickness of the Ti-Mo layer has some differences. Showing from non-steady-state diffusion equation, the brazing temperature and time determines the diffusion amount of Mo to brazing filler metal. After solidification can form Ti-Mo solid solution layer with different thickness and composition. Brazing temperature and time optimized can produce interface reaction layer with appropriate thickness and the solid solution layer. Interface reaction layer of 30 ~ 40um is of TiC, and the joint is made up by 70 ~ 80um Ti-Mo solid solution layer has better performance. Further improve the brazing temperature and time to produce thicker interfacial reaction layer will weaken performance of joint and result in the TZM matrix grains to grow, reducing the mechanical properties.


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TZM Alloy Oxidation Resistance Property

TZM alloy has good high temperature mechanical property, but at a high temperature oxidation resistance  property is poor. Under high temperature, it unable to form a protective oxide layer, so it must be used at high temperatures in non-oxidizing atmosphere. When the temperature is below 400 ℃, TZM alloy oxidation rate is very slow, because alloy surface formed less volatile MoO2. When the temperature is between 400 ~ 750 ℃, the oxidation rate becomes faster, the alloy surface to generate volatile MoO3. When the temperature is higher than 750 ℃, due to MoO3 volatile oxide weight decrease sharply.

Improving alloy’s oxidation resistance usually has two methods: alloying and coating method. Alloying method can not provide sufficient high-temperature oxidation resistance. Therefore the more commonly used method is coating method.

TZM alloy picture

There are many coating production process. The pack cementation method has many advantages including low cost, easy control, strong combination, so it has been widely used. TZM alloy coating by pack cementation method, generally choose Al powder as coating materials, NH4Cl as catalyst, Al2O3 as filler. The optimum parameters for the coating growth is tm (Al2O3): m (Al): m (NH4Cl) = 7: 2: 1,1000 ℃, 12h.

Observing XDR found that coating consist of Al5Mo, Al2Mo and Al2 (Mo4) 3 phase. Near the substrate the coating thickness is about 30um and average Al content is lower. Outer coating thickness is 20um and average Al content is higher. TZM alloy under coating protective within 10h operation, the oxidation rate is very fast. But within 10 ~ 50h, the oxidation rate becomes slowly, because the Al2O3-layer has a better protective effect and can prevent oxidation further development. It is shows coating method can well improve the oxidation resistance of the TZM alloy.

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TZM Alloy Thermodynamic Analysis

Using thermodynamic calculation found, TZM alloy during the sintering process, Ti, Zr and O reaction’s Gibbs energy is much lower than the binding energy of C and O, so Ti and Zr priority react with O to form TiO2 and ZrO2, reducing C’s deoxidize effect greatly. Further, since the strengthening elements Ti and Zr reacts with oxygen or nitrogen adsorbed in the compacted material, which also reduces the deoxygenation of C, so that the C content in TZM alloy is difficult to control.

In TZM alloy, strengthening elements titanium (Ti), zirconium (Zr) and carbon (C) will produce second phase strengthening and solid solution strengthening in the alloy. TZM alloy during sintering, the oxygen (O) in molybdenum powder will react with C to generate CO, and in this process C acts as a deoxidizer. In addition, C works together H2 protective gas to reduce oxygen content of the alloy.

TZM alloy picture

After thermodynamic analysis, Ti, Zr, Mo reacts with C to generate TiC, ZrC, Mo2C decomposed at elevated temperatures, dissolved into the molybdenum substrate. If the alloy O content is higher, there may occur the following reaction: Ti (Zr) C + [O] → Ti (Zr) nO2n-1 + CO2 or Mo2C + [O] → Mo + CO2. And this is the reason TZM alloy after high temperature treatment the second phases are all Ti and Zr oxides.

If TZM alloy oxygen content is too high, the oxygen will react with Ti, Zr to form the corresponding oxide, so a large number of second phase produce at grain boundaries and make mechanical properties of the alloy deterioration. Thereby controlling the oxygen content in TZM alloy has an important influence on the mechanical properties of the material guarantee. During the alloy production, to reduce the oxygen content of the TZM alloy, the most effective measure is to reduce the oxygen content of the raw material and production process oxygen content. So the manufacturers can use hypoxia molybdenum powder and vacuum mixing powder measure.

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TZM Alloy High Temperature Properties

TZM alloy has good high temperature property, so it can be made of high temperature components used in various fields. TZM alloy High temperature properties include high-temperature tensile property, high temperature oxidation resistance property, high-temperature fatigue property, and high-temperature bending and creep property.

TZM alloy picture

High temperature tensile property: High-temperature tensile property influence by many factors, such as different forms TZM alloy, TiC and ZrC doping method, production method and temperature. TZM alloy after deformation, as the temperature rises, the tensile strength decreased significantly. Zr alloy element added in pure Zr has best results, the amount of 0.1%, having highest performance. And Ti element added in TiH2 has best results, the amount of 0.8%, the alloy having better performance. Using powder metallurgic method and melting method to produce TZM plate the performance is equivalent, but the strength of TZM bars prepared by melting method was significantly higher than prepared by powder metallurgy method.

High temperature fatigue resistance: TZM alloy fatigue life at 500 ℃ and 350 ℃ has little difference, but at 500 ℃ fatigue life is slightly lower than 350 ℃. At 350 ~ 500 ℃, the thermal fatigue life of the alloy is significantly lower than the fatigue life at 350 ~ 500 ℃. This may be caused by the synthetic effect of temperature and cyclic loading, such that the fatigue life is lower than the thermal fatigue life. With the increase of the maximum cyclic stress, fatigue life shortening, elongation increased.

High temperature oxidation resistance: TZM alloy in high temperature oxidation rate is very fast, it can not generate antioxidant protection layer to protect itself. There are two methods for improving high-temperature oxidation resistance property which are alloying methods and coating method, and the coating method has better effect.

High temperature bending and creep property: TZM alloy creep is toward compressive strain direction, and when fatigue life reaches at 2% to 10%, creep direction changes to stretching strain. Creep property is closely related with stress intensity and temperature. Cyclic stress range increases, the more serious cyclic creep. With increasing temperature, cyclic creep has become more apparently.

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Vacuum Arc Melting of TZM Alloy

There are two main methods for TZM alloy production: one is the powder metallurgy method and the other one is vacuum arc melting method. TZM alloy vacuum arc smelting process including: electrode production, water cooling effects, stable arc stirring and melting power. Good smelting process has a certain impact on the quality of TZM alloy. In order to produce good performance TZM alloy should be strict requirements on its smelting process.

TZM alloy picture

Electrode requirements: the ingredients of electrode should be uniform; electrode no bending, should meet the requirement of straightness; electrode should be dry, bright, no oxidation; electrode should tie up tightly.

Water cooling effect: in vacuum arc melting furnace, there are two mainly function of crystallizer water cooling effect: one is to take away the heat released during melting, making sure that crystallization will not be burned; the other is affecting the inside organization of ingot, which is heating ingot’s bottom and all around making ingot produced directional columnar grain structure. When melting TZM alloy, cooling water pressure controlling in 2.0 to 3.0 kg / cm2, the water layer about 10mm is best.

Arc stable stirring: during melting TZM alloy, to plus a coil parallel circling with crystallizer and, after powder on forming magnetic field. The main role of the magnetic field is constraining the arc and making the molten pool to solidify under stirring condition. The arc constraining effect is called "arc stable." In addition, to take appropriate arc stability magnetic field intensity can reduce crystallizer breakdown.

Melting power: the power of the melting means melting current and voltage, and it is important process parameters. Inappropriate parameters can cause failure of TZM alloy smelting. Selecting the appropriate melting powder is close related with electrical machine and crystallizer size. The "L" is refer to the distance between the electrode and the crystallizer wall, then the lower value L is, the greater coverage area of ​​the arc on weld pool, and at the same powder the pool heating state is better, the more active the pool. On the contrary, the operation will be difficult and crystallizer is easy to damage.

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TZM Alloy Properties

TZM alloy picture

TZM alloy has many good properties, such as physical property, chemical property and mechanical property, including high melting point, high strength, high elastic modulus, small linear expansion coefficient, low vapor pressure, good electrical and thermal conductivity, high corrosion resistance and good high temperature mechanical properties , so it is widely used in various fields.

High temperature oxidation resistance property: TZM alloy has high melting point, so that it can be widely used in many field as a high temperature material. However, at high temperature, the alloy has poor oxidation resistance property can not form anti-oxidation layer to protect itself, so the oxidation rate is faster and service life is shorter at high temperatures. In order to improve high temperature oxidation resistance property of the alloy there are two methods: alloying and coating technology.

Room temperature and high temperature tensile mechanical property: Compared with molybdenum, TZM alloy room temperature tensile property is better, but not as good as the extension of molybdenum. Experiments show that at 1200 ℃, the tensile strength of molybdenum has been a significant decline, but the tensile strength of the TZM alloy remains at a high level. This is mainly because the second phase strengthening impacts TZM alloy’s property.

High temperature bending and creep properties: TZM alloy at high temperatures exhibited excellent bending property, but with temperature increase, the anti-bending property of TZM alloy is poor. TZM alloy creep property is related with temperature and stress intensity. With cyclic stress increasing, cyclic creep becomes more serious. In the same cyclic stress range, the temperature has large impact on creep property, as the temperature rises, the more obvious cyclic creep showing.

High temperature fatigue resistance: The study found that with the increase of the maximum cyclic stress, high temperature fatigue property continues to shorten, the elongation gradually increasing.

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TZM Alloy Properties Improvement

TZM alloy picture

TZM alloy has high temperature property, but at the high temperature oxidation resistance is poor. At the high temperature TZM alloy can’t produce anti-oxidation layer to protect itself, leading to life service and application range greatly shortened. Improving alloy’s oxidation resistance there are two main methods: alloyage and coating method. Alloyage refers to adding trace elements to improve alloy’s oxidation resistance. And coating method refers to coating a protection layer on alloy surface to improve high temperature oxidation resistance of the alloy. Al powder is usually used in coating material as raw material. And after experiment found the optimum parameters for coating are as following: m (Al2O3): m (Al): m (NH4Cl) = 7: 2: 1,1000 ℃, 12h.

Improving the ductile-brittle transition temperature and mechanical property of the alloy usually use neutron irradiation method. It was found that the alloy after neutron radiation, tensile property significantly improved, although the ductility is decrease, but the hardness is increase. At present, the study of neutron radiation is not mature and the general consensus is obtained through research, which is the radiation will have a lot of small gap in the alloy at a lower temperature, but high temperature radiation the gap will increases and DBTT will decrease.

Alloy after extrusion process will produce a lot of stress. The stress will largely affect the mechanical properties of the alloy. To change the annealing temperature can reduce the stress to improve its machinability. It was found that annealing temperature changes will affect alloy internal organization to cause alloy properties change. Overall, as the annealing temperature increase, the elongation of the alloy is increase and work hardening decrease. On the other hand, improving the production process and quality of raw material is good for TZM alloy properties improvement.

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New Type Rare Earth Doped TZM Alloy

The experiment found that doping rare earth into molybdenum can refined molybdenum grains. Besides, rare earth molybdenum alloy not only has lower plastic-brittle transition temperature, but it can improve some molybdenum properties, such as increasing molybdenum recrystallization temperature and high temperature strength, improving toughness plasticity property and high temperature creep property. As we know, TZM alloy has high melting point, high strength, high elastic modulus, low expansion coefficient, low vapor pressure, good electrical and thermal conductivity, good corrosion resistance and other good properties. It is the one of most widely used alloy in molybdenum alloy. To make TZM alloy can be applied to more areas, some scholars to prepare a new type rare earth doped TZM alloy, thereby improving ductility and toughness properties of TZM alloy.

Common doped rare earth elements including La, Y, Ce, production method usually uses powder metallurgy method and vacuum arc melting method. Using powder metallurgy method to produce rare earth doped TZM alloy the production processes are as following: mixed powder, ball milling, cold isostatic pressing, sintered, cogging, hot-rolled, cold-rolled and alkali wash and other subsequent processing steps then to obtain the finished product. Taking La2O3 doped for example, observing the La2O3-TZM alloy found TZM alloy’s crystallization temperature is increased. Undoped TZM alloy recrystallization temperature is about 1100 ℃ and La2O3-TZM alloy recrystallization temperature is about 1200 ℃. Further, at the same temperature, La2O3-TZM alloy has smaller grains than TZM alloy. Besides, after complete crystallization, the grain size of La2O3-TZM alloy is also smaller than TZM alloy.

TZM alloy image

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TZM alloy Deoxidation Mechanism Analysis during Vacuum Sintering

The experiment found that TZM alloy during powder metallurgy sintering process mainly has two kinds of deoxidation mechanisms: the first one, metal oxide of carbon reduction system to form metal carbides and CO; the second one, MoO2 happening disproportionation reaction at vacuum high temperature to form metal Mo and MoO3 gas, where the MoO3 gas will be discharged by vacuum system.

Firstly, analysis the influence of carbon content on TZM alloy deoxidation during vacuum sintering. Experiment arrangement as follows: adding 0.5% TiHx and 0.09% ZrHx into molybdenum powder, and then dividing the sample into three parts, in the sample were added 0.04%, 0.07%, 0.10% different proportions of carbon, and finally made TZM alloy rods. Tested oxygen content and the carbon content of TZM alloy bar found C element content and TZM alloy deoxidation effect is proportional to, but excess carbon can cause alloy component failure.

In addition, the oxygenium of molybdenum mixed powder mainly decisions by oxygen content of molybdenum powder. Molybdenum powder after the reduction reaction, the mainly existing way of oxygen is molybdenum oxide, which the most stable is MoO3 and MoO2, and MoO3 in a vacuum and high temperature will volatilize into a gas, so at high temperature molybdenum oxide mainly exists by MoO2 form. The experiment found that in the case of high temperatures and the absence of air, MoO2 happen disproportionation reaction to produce metal Mo and gaseous MoO3, and gaseous MoO3 will be discharged by vacuum system.

TZM alloy deoxygenation process mainly achieved by carbon reaction and disproportionation reaction of MoO2 which occurs in high-temperature vacuum sintering process. The reaction temperature and air pressure of product in vacuum furnace will have great impact on the quality of these two reactions. Reducing the partial pressure of product and improving vacuum degree can reduce the deoxygenation reaction temperature of the beginning which is good for reducing oxygen content of TZM alloy.

TZM alloy image

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Surface Oxidation and Segregation of TZM Alloy

TZM alloy image

Segregation phenomenon refers to the constituent elements of the alloy uneven distribution in the crystallization. The experiment found that TZM alloy added element segregation occurs at ultrahigh vacuum and at different temperature ranges. Titanium segregation starts from 840 ℃ and zirconium segregation is beginning from 1100 ℃. Titanium’s maximum temperature segregation is 1150 ℃ and zirconium is 1350 ℃. Higher than the above temperature, the surface concentration of titanium and zirconium are decreasing, and higher than 1400 ℃titanium will disappear the same surface. Segregation temperature is similar to the required temperature of these two elements diffuse to the molybdenum substrate. The temperature of segregation disappearance depends on the balance of the additional diffusion kinetics.

In TZM alloys, titanium and zirconium element has a great affinity for oxygen, so oxidation reaction with oxygen is easily. Stability of TiO2 and ZrO2 promotes the surface material to absorbent the oxygen, free standard enthalpy of formation of these two oxides is lower than MoO2, and both vapor pressure is smaller than MoO2. Thus, the surface oxygen can diffuse to the alloy and preferential oxidation with the additive of alloy producing oxides precipitate. TZM alloy samples after 1390 ℃ treatment, the alloy surface is covered with a large amount of oxide precipitation. After milling alloy surface, the inside oxygen precipitates content only one-tenth of alloy surface.

We can confirm TZM alloy surface oxidation by microstructure oxide precipitation tests of the alloy. The shape and form of oxide precipitates has big difference with carbide precipitate. In an oxidizing atmosphere, at 1200 ℃ alloy starts decarburization reaction, oxygen is gradually dissolved and the original oxide oxidation occurs.

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TZM Alloy Second Phase Structure Analysis

The second phase of TZM alloy is mainly solid solution, carbides and oxides of Ti and Zr. These second phases have an important effect on the alloy’s mechanical strength, toughness, and creep strength. The second phase will occur alloy’s strengthening effect, such as solid solution strengthening and second phase strengthening. To understand and control the formation, particle size and distribution of second phase for improving the thermal deformation resistance and properties of the alloy has great significance.

Observing the SEM picture found, there are a lot of second phase at grain boundaries of the alloy. In TZM alloy carbon content and oxygen content both are very high, so it is considered that the second phase showing at grain boundaries is Mo2C, Ti and Zr’s oxides and carbides. The reason that a lot of second phases gathering at the grain boundaries is during sintered cooling process Ti, Zr and C reduce the solubility, and thus precipitated to the grain boundaries.

TZM alloy image

A large number of second phases at grain boundaries during metals deformation, due to differences in the elastic modulus can not be synchronized deformation, the grains will result from the grain boundaries, so that the second phase crushing, leading to reduce the tensile strength and elongation. It causes the material to break, reduce processing performance and mechanical properties of the material in the subsequent deformation.

The fine distribution second phases in intragranular and grain boundary can improve the recrystallization temperature and the tensile strength of the alloy, having a beneficial effect on the properties of the alloy. While the second phase distribution in the alloy depends on the heat treatment process. If the material can be rapidly cooled during sintering, hardening, so that Ti, Zr, C can be solid solution in the crystal lattice of molybdenum, then after tempering treatment Ti, Zr, C precipitation dispersed in the alloy to improve the properties of the alloy materials.

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Recrystallization TZM Alloy Hot Deformation Characteristics

Recrystallization state TZM alloy at room temperature is brittle, and therefore difficult to deform, so alloy cold rolling and warm rolling to avoid full recrystallization annealing. Processing at high-temperature, since the material becomes soft, lower intensity, is conducive to machining distortion. Studying the recrystallization state TZM alloy high temperature deformation has great significant for understanding the effect of recrystallization annealing on TZM alloy microstructure and hardness.

The alloys were annealing at 1100,1200,1300,1400, 1500 and 1600 ℃, and heat preservation for 1h, observing at different annealing temperature the organizational structure of the alloy. After 1100 ℃ annealing, alloy grains become processing state fiber organization, but after 1600 ℃ annealing, the grain becomes coarse strip and grains growth, indicating recrystallization completed. Hardness of the alloy with annealing temperature decreased, at 1600 ℃, the material has been significantly softened.

At 1600 ℃ to operate hot compression deformation on alloy. After compression deformation alloy has significant deformation, shorten its length, radius increases, and the intermediate raised to drum. When the strain capacity is less than 5%, the stress rapidly increases with strain increasing and hardening obvious. When the strain capacity is higher than 5%, the increase rate of stress is become slowed, work hardening rate becomes low as well. After compression, alloy microstructure is strip shape and both ends deformation is small. The middle grains shows oval-shaped, intermediate has large deformation. It is indicated that along the axial deformation of the alloy is uneven. Observation alloy radial direction microstructure found, there is not much difference between the center and the edge of the grain size and shape, indicating relatively uniform along the radial deformation. After heat compression there are many microcracks is formed in line with the compression direction on alloy’s grain boundaries. The alloy after recrystallization grains becomes coarse. And when the orientation is not conducive to plastic deformation there will a lot of stress concentration occurs at grain boundary. When the stress concentration reaches a certain degree, it will crack. Therefore, in the processing of proceeding, it should choose appropriate annealing temperature to prevent cracks.

TZM alloy image

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TZM Alloy High Temperature Properties

TZM alloy has good high temperature property, so it can be made of high temperature components used in various fields. TZM alloy High temperature properties include high-temperature tensile property, high temperature oxidation resistance property, high-temperature fatigue property, and high-temperature bending and creep property.

High temperature tensile property: High-temperature tensile property influence by many factors, such as different forms TZM alloy, TiC and ZrC doping method, production method and temperature. TZM alloy after deformation, as the temperature rises, the tensile strength decreased significantly. Zr alloy element added in pure Zr has best results, the amount of 0.1%, having highest performance. And Ti element added in TiH2 has best results, the amount of 0.8%, the alloy having better performance. Using powder metallurgic method and melting method to produce TZM plate the performance is equivalent, but the strength of TZM bars prepared by melting method was significantly higher than prepared by powder metallurgy method.

TZM alloy image

High temperature fatigue resistance: TZM alloy fatigue life at 500 ℃ and 350 ℃ has little difference, but at 500 ℃ fatigue life is slightly lower than 350 ℃. At 350 ~ 500 ℃, the thermal fatigue life of the alloy is significantly lower than the fatigue life at 350 ~ 500 ℃. This may be caused by the synthetic effect of temperature and cyclic loading, such that the fatigue life is lower than the thermal fatigue life. With the increase of the maximum cyclic stress, fatigue life shortening, elongation increased.

High temperature oxidation resistance: TZM alloy in high temperature oxidation rate is very fast, it can not generate antioxidant protection layer to protect itself. There are two methods for improving high-temperature oxidation resistance property which are alloying methods and coating method, and the coating method has better effect.

High temperature bending and creep property: TZM alloy creep is toward compressive strain direction, and when fatigue life reaches at 2% to 10%, creep direction changes to stretching strain. Creep property is closely related with stress intensity and temperature. Cyclic stress range increases, the more serious cyclic creep. With increasing temperature, cyclic creep has become more apparently.

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TZM Alloy High Temperature Properties

TZM alloy has good high temperature property, so it can be made of high temperature components used in various fields. TZM alloy High temperature properties include high-temperature tensile property, high temperature oxidation resistance property, high-temperature fatigue property, and high-temperature bending and creep property.

High temperature tensile property: High-temperature tensile property influence by many factors, such as different forms TZM alloy, TiC and ZrC doping method, production method and temperature. TZM alloy after deformation, as the temperature rises, the tensile strength decreased significantly. Zr alloy element added in pure Zr has best results, the amount of 0.1%, having highest performance. And Ti element added in TiH2 has best results, the amount of 0.8%, the alloy having better performance. Using powder metallurgic method and melting method to produce TZM plate the performance is equivalent, but the strength of TZM bars prepared by melting method was significantly higher than prepared by powder metallurgy method.

High temperature fatigue resistance: TZM alloy fatigue life at 500 ℃ and 350 ℃ has little difference, but at 500 ℃ fatigue life is slightly lower than 350 ℃. At 350 ~ 500 ℃, the thermal fatigue life of the alloy is significantly lower than the fatigue life at 350 ~ 500 ℃. This may be caused by the synthetic effect of temperature and cyclic loading, such that the fatigue life is lower than the thermal fatigue life. With the increase of the maximum cyclic stress, fatigue life shortening, elongation increased.

High temperature oxidation resistance: TZM alloy in high temperature oxidation rate is very fast, it can not generate antioxidant protection layer to protect itself. There are two methods for improving high-temperature oxidation resistance property which are alloying methods and coating method, and the coating method has better effect.

High temperature bending and creep property: TZM alloy creep is toward compressive strain direction, and when fatigue life reaches at 2% to 10%, creep direction changes to stretching strain. Creep property is closely related with stress intensity and temperature. Cyclic stress range increases, the more serious cyclic creep. With increasing temperature, cyclic creep has become more apparently.

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TZM Alloy and Lanthanum Molybdenum Alloy

Molybdenum (Mo) has excellent high temperature strength, creep resistance, low thermal expansion coefficient, good thermal conductivity and corrosion resistance property, so it is widely used in the electronics industry, aerospace and military fields. However, Mo has a low recrystallization temperature, and therefore the application of pure molybdenum is often limited. Molybdenum recrystallization temperature is about 1000 ℃. Once molybdenum recrystallization, harmful impurity element is easy enrichment at the grain boundaries, thereby reducing the room temperature and high temperature performance of molybdenum. Alloying can help to improve the molybdenum’s high temperature properties. After alloying molybdenum alloy will produce solid solution strengthening and second phase strengthening, making the properties of the alloys has been improved. Analyzing lanthanum molybdenum alloy and TZM alloy’s high temperature performance plays an important role on molybdenum alloy further development.

Lanthanum molybdenum alloy and TZM alloy annealing at 1100 ℃, there are a small number of grains recrystallization, and with increasing annealing temperature, the recrystallized grains gradually increased. Recrystallized structure of molybdenum alloy exhibits elongated tissue which is different with pure molybdenum. Pure molybdenum is an isometric grain.

When the annealing temperature is less than 1400 ℃, lanthanum molybdenum alloy has a strong overall performance of strength and plasticity. When the temperature is higher than 1400 ℃, the tensile strength and ductility is decrease. TZM alloy’s tensile strength decreases with increasing temperature, but the plasticity increase, which is different with lanthanum molybdenum alloy. In addition, after comprehensive comparison, at the same temperature TZM alloy high temperature performance is better than lanthanum molybdenum alloy.

When the annealing temperature is higher than 1400 ℃, lanthanum molybdenum alloy exhibits hot brittleness which is different from the TZM alloy. This is mainly because, as the annealing temperature increase, the flow-line organization of lanthanum molybdenum alloys has a tendency to fracture. However, TZM alloy, whether in process state or recrystallization state, the size, shape and distribution of second phase are not significant changes.

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Vacuum Arc Melting of TZM Alloy

There are two main methods for TZM alloy production: one is the powder metallurgy method and the other one is vacuum arc melting method. TZM alloy vacuum arc smelting process including: electrode production, water cooling effects, stable arc stirring and melting power. Good smelting process has a certain impact on the quality of TZM alloy. In order to produce good performance TZM alloy should be strict requirements on its smelting process.

Electrode requirements: the ingredients of electrode should be uniform; electrode no bending, should meet the requirement of straightness; electrode should be dry, bright, no oxidation; electrode should tie up tightly.

Water cooling effect: in vacuum arc melting furnace, there are two mainly function of crystallizer water cooling effect: one is to take away the heat released during melting, making sure that crystallization will not be burned; the other is affecting the inside organization of ingot, which is heating ingot’s bottom and all around making ingot produced directional columnar grain structure. When melting TZM alloy, cooling water pressure controlling in 2.0 to 3.0 kg / cm2, the water layer about 10mm is best.

Arc stable stirring: during melting TZM alloy, to plus a coil parallel circling with crystallizer and, after powder on forming magnetic field. The main role of the magnetic field is constraining the arc and making the molten pool to solidify under stirring condition. The arc constraining effect is called "arc stable." In addition, to take appropriate arc stability magnetic field intensity can reduce crystallizer breakdown.

Melting power: the power of the melting means melting current and voltage, and it is important process parameters. Inappropriate parameters can cause failure of TZM alloy smelting. Selecting the appropriate melting powder is close related with electrical machine and crystallizer size. The "L" is refer to the distance between the electrode and the crystallizer wall, then the lower value L is, the greater coverage area of ​​the arc on weld pool, and at the same powder the pool heating state is better, the more active the pool. On the contrary, the operation will be difficult and crystallizer is easy to damage.

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Using TZM Alloy to Produce Titanium Isothermal Forging Mold Material

Titanium alloy has some special requirements on isothermal forging, for example, there are some special requirements on mold temperature, pressure and dwell time. During isothermal forging the mold temperature should be at 850~950 ℃ and pressure should be control at 100~120 MPa. Besides, the dwell time should be at 5~15 minutes, and based on the above requirements, commonly used nickel-base superalloy, insoluble metals and their alloys (TZM alloy), ceramic material, such as silicon nitride, silicon carbide and so on as titanium isothermal forging material. The experiment found that compared to the performance of other material TZM alloy has more favorable advantages. TZM alloy not only has good temperature resistance and high strength property, but the die service life also has a strong advantage.

Using TZM alloy as sophisticated forgings materials during isothermal forging, there may be has some crack happening in forging die to cause damage, and forging die damage may be caused by the following reasons:

1. The forging die happen some local plastic deformation and dimensional change;

2. Effected by the lubricant and protective gas, the embedding of material cause wearing;

3. The cracks concentrated on the place where the local stress is highly.

In order to prevent forging die early damage, understanding the carrying capacity before the crack spread of TZM alloy forging die, understanding of the conditions of forging die appeared, have important significance for judging the operation reliability and evaluation the forging die service life.

The cracks in the forging die are often unavoidable, so to slow the pace of cracks expansion has a certain influence on the service life of the mold. It was found that if the dwell time is too long, there will have a certain impact on crack formation and expansion. So isothermal forging pressing time should be as short.On the other hand, forging die at preheating and cooling will produce high thermal stress, causing cracks. In order to avoid large thermal stress, people should pay special attention to preheat uniformity.

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TZM Alloy Hot-dip Aluminum Coating Microstructure

TZM alloy has good high temperature performance and mechanical properties, often as heat-resistant and high-temperature structural materials widely used in nuclear reactors, aerospace, power generation and chemical industry, but it is easily oxidized in air at high temperature, thus greatly limiting its applications. Use hot-dip aluminized process to produce aluminum layer on TZM alloy surface, which can improve the high temperature oxidation resistance. After 800 ℃, 100h cyclic oxidation experiment found that TZM alloy surface coated on aluminum layer its high-temperature oxidation resistance is greatly improved. The coating consists of surface pure aluminum alloy layer and inner alloy layer and the mainly metallurgical of alloy layer is Al4Mo and Al5Mo phase.

TZM alloy after hot dip aluminum coating, its surface shows shiny metallic luster and surface is roughness, seamless plating. Observed alloy hot dip aluminum coating microstructure found, after hot dip aluminum coating the metallographic structure of alloy includes brown inlay, pure aluminum layer (I), alloy layer (II) and matrix (III). Besides, the coating is uniform and compact, divided into outer and inner layers, distinct phase interface. In addition, the study found that, at the same temperature of hot-dip, longer dipping time, the thickness of the alloy layer also increases.

Measured aluminum coated layer cross section microhardness found that the outer surface has lower hardness. When the hardness of load is 25g, the hardness range is HV30 ~ 130, which is mainly dependent on α-Al. With distance from the surface increasing, the hardness also increases. In 120um, the hardness value reaches HV760. In 120 ~ 150um, hardness changes at the range of HV760 ~ 758 and the hardness load is 500g, mainly because alloy layer Al4Mo and Al5Mo is cemented carbide phase having greater hardness. Then, as the distance further increase, the hardness begins to decline sharply, remained steady at HV260, where the hardness load is 100g and TZM alloy matrix hardness is HV260.

TZM

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Titanium Zirconium Molybdenum Ternary Alloy - Oral Planting Material

Titanium zirconium molybdenum ternary alloy (Y30) is an excellent performance oral planting material. The main ingredient is titanium (Ti), in order to improve overall performance on its basis doped with 5% zirconium (Zr) and 2% molybdenum (Mo). Zirconium is a neutral element, and its solid solution strengthening effect can improve the strength of Ti, molybdenum doped mainly to improve the tensile strength and corrosion resistance of titanium. In addition, doing polarity toxicity test, accumulation toxicity test, rat bone marrow cell chromosomal aberration test, pyrogen test, hemolysis test in vitro Chinese hamster ovary cell proliferation and other tests on titanium, zirconium, molybdenum found that the elements is security, non-toxic elements and has good biocompatibility as well. At the same time, doing the test of the physical properties, mechanical properties, and corrosion resistance found that titanium zirconium molybdenum ternary alloy is a safe and reliable oral planting material which has broad application prospects.

In order to evaluate the toxicity and safety of Y30 were done six biological safeties testing, and test reports are as follows:

1. Y30 leach liquor extract polar toxicity test: Using Y30 leach liquor injected the abdominal cavity of mice was observed after 14 days. The mice's diet, sleep, activity and growth are normal. The leach liquor is non-toxic to mice.

2. Y30 leach liquor accumulation (subacute) toxicity test, 25 days in a row using Y30 leach liquor injected mice, the cumulative amount reached 700cc / kg, the mice showed no death. It is shows that there is not cumulate effect on mice’s body.

3. Y30 leach liquor on rat bone marrow chromosome aberration test found the bone marrow cells of mice chromosome no effect chromosomal aberrations occur.

4. Y30 pyrogen test: injected rabbits with Y30 leach liquor after body temperature test found rabbit body temperature raise were below 0.6 ℃, in line with pyrogen test requirements.

5. Y30 hemolysis test: observe containing Y30 leach liquor of red blood cells found no fracture phenomena which described Y30 leach liquor afford hemolytic reaction.

6. Different concentrations of Y30 leach liquor in vitro hamster ovary (CHO) cell proliferation text: Y30 leach liquor does not affect the growth of CHO cells and it is kind of safe metal planting material.

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Surface Oxidation and Segregation of TZM Alloy

Segregation phenomenon refers to the constituent elements of the alloy uneven distribution in the crystallization. The experiment found that TZM alloy added element segregation occurs at ultrahigh vacuum and at different temperature ranges. Titanium segregation starts from 840 ℃ and zirconium segregation is beginning from 1100 ℃. Titanium’s maximum temperature segregation is 1150 ℃ and zirconium is 1350 ℃. Higher than the above temperature, the surface concentration of titanium and zirconium are decreasing, and higher than 1400 ℃titanium will disappear the same surface. Segregation temperature is similar to the required temperature of these two elements diffuse to the molybdenum substrate. The temperature of segregation disappearance depends on the balance of the additional diffusion kinetics.

In TZM alloys, titanium and zirconium element has a great affinity for oxygen, so oxidation reaction with oxygen is easily. Stability of TiO2 and ZrO2 promotes the surface material to absorbent the oxygen, free standard enthalpy of formation of these two oxides is lower than MoO2, and both vapor pressure is smaller than MoO2. Thus, the surface oxygen can diffuse to the alloy and preferential oxidation with the additive of alloy producing oxides precipitate. TZM alloy samples after 1390 ℃ treatment, the alloy surface is covered with a large amount of oxide precipitation. After milling alloy surface, the inside oxygen precipitates content only one-tenth of alloy surface.

We can confirm TZM alloy surface oxidation by microstructure oxide precipitation tests of the alloy. The shape and form of oxide precipitates has big difference with carbide precipitate. In an oxidizing atmosphere, at 1200 ℃ alloy starts decarburization reaction, oxygen is gradually dissolved and the original oxide oxidation occurs.

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TZM Alloy Ti and Zr Determination by ICP-AES Method

In TZM alloy Ti and Zr content have great impact on its performance and through ICP-AES (Inductively Coupled Plasma - Atomic Emission Spectrometry) method can accurate measurement Ti and Zr content. ICP-AES has high sensitivity, high precision and wide linear working range and other properties, so it has widely application range.

Experimental Method: Weighed 0.2000g sample in 100ml beaker, added 2ml of sulfuric acid and 1g ammonium sulfate, and then placed on electric hot plate for heating. After the sample dissolved and cooled, poured into 100ml volumetric flask, diluted with water to the mark, for analysis.

Dissolving acid selection: The molybdenum alloy can dissolve by sulfuric acid - ammonium sulfate or nitrate - hydrofluoric acid mixture, but using nitric acid - hydrofluoric acid should ensure that sufficient amount of acid can make the sample completely dissolved. The viscosity of sulfuric acid is large, which is difficult to improve during lifting injections, but the amount is small, the results less affected. Overall consideration, the experiments will 2ml of sulfuric acid and 1g ammonium sulfate to dissolved molybdenum alloy.

Analytical line selection: To scan the spectral line by molybdenum, titanium, zirconium solution provided with the spectrum library, according to the shape of the peak, and condition of the peak value and matrix line to compare the interference of the matrix to the peak to chose small interference spectrum line, so choose 336.1nm as titanium determination spectral line, 343.8nm as zirconium determination spectral line.

Used ICP-AES method for Ti and Zr measurement and the values shows on the table. The recovery rate 98.33% -110.00%, RSD (n = 5) were less than 2.0%. This method avoids the conventional chemical analysis cumbersome process and greatly reduces the analysis time, which can simultaneously to measure Ti and Zr elements, so this method not only has fast, flexible and convenient features, but also can save a lot of manpower, reducing reagent consumption, improving the detection efficiency.

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