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.

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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.

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Surface Oxidation and Segregation of TZM Alloy

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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.

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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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