Industrially produced molybdenum alloys can be divided into Mo-Ti-Zr series, Mo-W series and Mo-Re series alloys, as well as Mo-Hf-C series alloys that are precipitation strengthened with hafnium carbide particles. TZM alloy has excellent comprehensive properties and is the most widely used molybdenum alloy. TZC (Mo-1.25 Ti-0.15 Zr-0.15C) alloy has higher high temperature strength and recrystallization temperature than TZM, but it is difficult to process and its application is limited.
Molybdenum alloys have shortcomings such as low-temperature brittleness, welding brittleness, and high-temperature oxidation, so their development is limited. It is difficult to improve the high temperature oxidation resistance of molybdenum alloys by alloying. At present, only protective coatings are used to improve this performance. The main problem in the research of molybdenum alloys is to improve the high-temperature strength and recrystallization temperature, and to improve the low-temperature plasticity of the material. The main problem in the study of pure molybdenum materials is to improve low-temperature plasticity, that is, to reduce its plasticity-brittle transition temperature.
The main strengthening methods of molybdenum alloys are solid solution strengthening, precipitation strengthening and work hardening (see strengthening of metals). Titanium, zirconium and hafnium are the main alloying elements of molybdenum. The influence of alloying elements on the hardness of molybdenum rolled bars is shown in the figure on the next page. Titanium, zirconium and hafnium can not only solid-solution strengthen and maintain the low-temperature plasticity of the material, but also form a stable and dispersed carbide phase, which improves the strength and recrystallization temperature of the material.
Interstitial impurities carbon, nitrogen, especially oxygen, have a serious impact on the plasticity-brittle transition temperature. Their solubility in molybdenum is extremely low (not more than 1ppm at room temperature), and the excess interstitial elements are distributed on the grain boundaries in the form of molybdenum compounds, reducing the strength of the grain boundaries and causing brittle fracture between the grains. The addition of trace boron to the molybdenum alloy can refine the grains, purify the grain boundaries and change the morphology of the grain boundaries, thereby improving the plasticity of molybdenum: adding trace elements such as iron and yttrium can also improve the low-temperature plasticity (see interface). In 1955, G. Geach and J. Hughes discovered that rhenium can significantly improve the plasticity of molybdenum and tungsten, and can reduce the plasticity-brittle transition temperature of molybdenum to -200℃.