Types of molybdenum alloys
The molybdenum alloys produced by industry can be divided into Mo-Ti-Zr, Mo-W and Mo-Re alloys, as well as Mo-Hf-C alloys strengthened by precipitation of carbonized crucible. 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.
Mo alloy has the disadvantages of low temperature brittleness, welding brittleness and high temperature oxidation, so the development is limited. It is difficult to improve the high temperature oxidation resistance of molybdenum alloy by alloying method, which is only improved by protective coating at present. The main problem in the research of molybdenum alloy 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 is to improve the low temperature plasticity, that is, to reduce its plastic-brittle transition temperature.
The main strengthening pathways of molybdenum alloys are solid solution strengthening, precipitation strengthening and work hardening (see metal strengthening). titanium, zirconium, and thallium are the main alloying elements of molybdenum. the effect of alloying elements on the hardness of the rolled bar of molybdenum is shown in the figure below. titanium, zirconium and thallium can not only strengthen and maintain the low temperature plasticity of the material by solid solution, but also form a stable and dispersed carbide phase, which can increase the strength and recrystallization temperature of the material.
Gap impurity carbon, nitrogen and especially oxygen have a serious effect on the plastic-brittle transition temperature. their solubility in molybdenum is extremely low (not more than 1 ppm at room temperature), while the excess interstitial elements are distributed on grain boundaries as compounds of molybdenum, reducing grain boundary strength and leading to brittle fracture between crystals. the addition of trace boron to the molybdenum alloy can refine the grains, purify the grain boundaries and change the grain boundary morphology, thus improving the plasticity of molybdenum: the addition of elements such as trace iron and yttrium can also improve the plasticity at low temperature (see interface). the gich (G.Geach) and hughes (J.Hughes) found in 1955 that \9651;\9651;\9651;\9651;\9651;\9651;\9651;\9651; can significantly improve the plasticity of molybdenum and tungsten plasticity of molybdenum and tungsten, which can reduce the plastic-brittle transition temperature of molybdenum to ℃\9651;200.
