High temperature properties of molybdenum-lanthanum and TZM alloys
Molybdenum, a refractory metal, is widely used in electronics, aerospace and energy industries because of its excellent high temperature strength, creep resistance, low thermal expansion coefficient, good thermal conductivity and corrosion resistance. Under normal conditions, the use of pure molybdenum is limited to its recrystallization temperature (about 1000 ℃), because once molybdenum recrystallization occurs, harmful impurity elements are easy to concentrate on the equiaxed grain boundaries, so that molybdenum not only loses its excellent high temperature performance, but also greatly reduces its normal temperature performance. Since molybdenum was recognized by people, improving its high temperature use range has always been a goal of scientists. As with other metals, alloying can significantly improve its high temperature performance. Solid solution strengthening and second phase strengthening are the most typical methods. For solid solution strengthening, the experimental results show that the possibility of molybdenum alloys is extremely limited, except for tungsten and rhenium, the total amount of elements added is usually less than 1%(mass fraction), and zirconium, titanium and hafnium are the most effective and commonly used addition elements. The representatives of this alloy are Mo-0.5Ti alloy, TZM and TZC alloy. As solid solution strengthening reaches close to the limit, in order to further improve its heat resistance strength, considering a diffuse distribution of second phase group, but because of the high melting point of molybdenum, all made of molybdenum metal melting point is lower than molybdenum compounds, and use of molybdenum metal compounds is impossible to improve the high temperature strength, usually choose molybdenum oxide and carbide to strengthen metal, In the past 20 years, dispersion rare earth oxides have been used to improve their high temperature strength. Typical alloys include molybdenum copper and molybdenum billion alloys.
After annealing at 1100 ℃ for 1 h, a small part of grains recrystallized in both molybdenum lanthanum and TZM alloys. With the increase of annealing temperature, the proportion of recrystallized grains increases until the annealing treatment at 1550℃ for 1h.The recrystallization temperature is more than 300°C higher than that of pure molybdenum, and the recrystallized grains are elongated, which is obviously different from the equiaxed grains in pure molybdenum recrystallized state.
When the temperature is less than 1400 ℃, molybdenum lanthanum alloy has a higher comprehensive properties of strength and plasticity. When the temperature is higher than 1400 ℃, the tensile strength and plasticity of molybdenum lanthanum alloy decrease obviously. With the increase of test temperature, the tensile strength of TZM alloy decreases, but its plasticity increases, which is contrary to that of molybdenum lanthanum alloy. At the same time, both strength and plasticity of TZM alloy have obvious advantages over the molybdenum lanthanum alloy at the same temperature.
Lanthanum molybdenum alloy at the time of test temperature is greater than or equal to 1400 ℃ show different from TZM alloy hot brittleness, the occurrence of this phenomenon is mainly because when the annealing temperature is greater than or equal to 1400 ℃, lanthanum molybdenum alloys can be obviously observed streamline distribution of second phase group, and with the increase of annealing temperature, streamline organizations tend to fracture, Its shape tends to be circular; However, the size, shape and distribution of the second phase of TZM alloy have no obvious change in either the processing state or the recrystallization state.
