An Analysis on Microstructure and Grain Size of Molybdenum Powder Material Processed by Equal Channel Angular Pressing

Pure molybdenum powder with a body center cubic lattice was processed by equal channel angular pressing (ECAP) for multi-pass at a processing temperature of 673 K, and subsequently, tests of relative density and Vickers microhardness were followed after processing. Additionally, the x-ray diffraction (XRD) was employed to analyze the crystallite size and dislocation density for both as-received powder and ECAP-processed samples. Electron backscatter diffraction (EBSD) was performed to characterize the grain structure and texture for the ECAP-processed samples. The results show that through processing by ECAP, bulk molybdenum sample with ultrafine-grained microstructure was achieved with the relative density of 0.93, the Vickers microhardness of 355 Hv, and the mean grain size of 0.24 μm. XRD profiles based on integral breadth method indicate that the crystallite size and dislocation density in the initial powder are 63.0 nm and 8.45E13 m⁻², respectively. After ECAP processing, the crystallite size decreases gradually from 52.7 to 39.1 nm and the dislocation density increases from 3.82E14 to 4.00E14 m⁻² after 1 and 2 passes. EBSD measurements show the significant grain refinement after 2 passes of ECAP. Therefore, grain refinement strengthening and dislocation tangling are likely to contribute to the increase of microhardness. The narrow range of grain size distribution after 2 passes of ECAP and the decrease of non-uniformity coefficient of microhardness both indicate the enhanced homogeneity compared with samples after 1 pass. The XRD profiles as well as inverse pole figures reveal that a texture of {110} <111> was developed through processing by ECAP.