New insights on microstructural parameters controlling mechanical properties of rolled molybdenum

The aim of this paper is to understand microstructural evolution and their key effects on mechanical properties of rolled molybdenum (Mo) from as-received sintered sheet. A batch of pure Mo sheets with various thickness reductions have been characterized in detail. The results reveal that the microstructure evolves by three stages including grain refinement, dynamic recrystallization (DRX) and grain elongation. For the rolled Mo, the fractions of low angle grain boundaries (LAGBs) are much higher than that of as-sintered Mo but do not increase monotonically with increasing thickness reductions. The occurrence of DRX results in the decrease of LAGBs at the intermediate stage while a larger rolling reduction results in grain boundaries bursting with high angle characters at the later stage. All rolled sheets are textured with dominant orientation locating at gamma-fiber and theta-fiber line. Room-temperature tensile tests demonstrate that all rolled Mo sheets are much stronger and more ductile (ultimate tensile strength, UTS = similar to 650-1000 MPa and total elongation, TE = similar to 10-35%) than as-sintered Mo (UTS = similar to 400 MPa and TE = 0), but a larger thickness reduction unexpectedly results in the deteriorated ductility which can be attributed to the increased number of HAGBs. The elongated microstructure with refined grains and the high fraction of LAGBs induced by rolling are considered to be the dominant parameters responsible to the improved mechanical properties of Mo. We suggest that rolling parameters should be optimized carefully to avoid the detrimental effect of crack initiation from the increase in high angle grain boundaries (HAGBs).