Flow Stress and Microstructural Evolution of TZM and Mo-La Alloys at Temperatures from 1400 to 1700 °C

Flow stress and microstructural evolution at high temperatures are studied in a commercially pure Mo and two Mo alloys. The alloys are powder metallurgy TZM (Ti-Zr-Mo) and Mo-0.6La (Mo with 0.6 wt% added La). TZM contains dispersoids in the form of oxides and carbides containing Ti and Zr, and Mo-La contains dispersoids in the form of La₂O₃. Experiments performed include static annealing tests at temperatures from 1400 to 1700 °C and uniaxial tensile tests at 1500 and 1700 °C under constant true-strain rates of 10^-3, 10^-4 and 3×10^-5 s^-1. The effects of dispersoids in TZM are greatly diminished at the highest temperatures, leading to grain sizes similar to the unalloyed Mo. The ultimate tensile strength of TZM is only slightly higher than that of the unalloyed Mo at these temperatures, offering as little as 21% higher strength at 1700 °C. On the other hand, the dispersoids in Mo-0.6La provide effective pinning across the elevated temperatures, resulting in a high recrystallization temperature and small average grain size. Mo-0.6La has up to twice the strength of TZM at 1700 °C and contains finer subgrains. The relationship between high-temperature strength and subgrain sizes during deformation is explored.