Types and properties of molybdenum alloys
Industrial molybdenum alloys can be divided into Mo-Ti-Zr, Mo-W and Mo-Re alloys, as well as Mo-Hf-C alloys strengthened by hafnium carbide particle precipitation. TZM alloy has excellent comprehensive properties and is the most widely used molybdenum alloy. TZC (Mo-1.25Ti-0.15Zr-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 alloy has some disadvantages such as low temperature brittleness, welding brittleness and high temperature oxidation, so its development is limited. It is difficult to improve the oxidation resistance of molybdenum alloy at high temperature by alloying. The main problems in molybdenum alloy research are to improve the high temperature strength and recrystallization temperature and to improve the low temperature plasticity of materials. The main problem in the study of pure molybdenum material is to improve the low temperature plasticity, that is, to reduce its plastic-brittle transition temperature.
The main strengthening ways of molybdenum alloy are solution strengthening, precipitation strengthening and work hardening. Titanium, zirconium and hafnium are the main alloying elements of molybdenum. The influence of alloying elements on the hardness of molybdenum rolling bar is shown in the figure on the next page. Titanium, zirconium and hafnium can not only strengthen and maintain the low temperature plasticity of materials by solid solution, but also form stable and dispersed carbide phase, and improve the strength and recrystallization temperature of materials.
Interstitial carbon and nitrogen, especially oxygen, have a serious effect on the plastic-brittle transition temperature. Their solubility in molybdenum is very low (less than 1ppm at room temperature), and the excess intergranular elements are distributed on grain boundaries in the form of molybdenum compounds, which reduce the strength of grain boundaries and lead to intergranular brittle fracture. Adding trace boron to molybdenum alloy can refine grain, purify grain boundary and change grain boundary morphology, thus improving the plasticity of molybdenum: adding trace iron and yttrium can also improve the plasticity at low temperature. It is found that rhenium can obviously improve the plasticity of molybdenum and tungsten, and reduce the plastic-brittle transition temperature of molybdenum to -200℃.
