Evolution of Microstructure during Hot Deformation of the PM Molybdenum Alloy TZM

The molybdenum alloy TZM (Mo-0.5wt%Ti-0.08wt%Zr) is a commonly used structural material for high temperature applications. For these purposes a high strength at elevated temperatures and also a sufficient ductility at room temperature are being aimed. Preceding investigations revealed the existence of subgrains in hot deformed TZM. It was observed that with proceeding primary recrystallization and therefore with disappearance of subgrains the yield strength drops almost to a level of pure molybdenum. It is being assumed that the existence of a dislocation substructure has a pronounced effect on the yield strength of TZM. The aim of the present study was to evaluate the subgrain and texture formation and also to estimate the dislocation arrangement within subgrains during hot deformation. Hence, TZM rods were rolled to different degrees of deformation at a temperature above 0.5 Tm. The microstructure of the initial material was fully recrystallized. Texture formation, misorientation distributions and subgrain sizes were analyzed by electron backscattering diffraction (EBSD). Mechanical properties were characterized by tensile tests at room temperature and up to 1200°C. It was revealed, that with increasing degree of deformation a distinct substructure forms and therefore yield strength rises. Consequently, the misorientation between adjacent subgrains increases, their size decreases and a <110> fibre texture develops. To estimate the influence of texture on strength of TZM the Taylor factors are calculated from EBSD data.