Developing a uniform distribution of bimodal α-Mo grains in the Mo-Si-B alloy for toughness and strength improvement

Mo-Si-B alloys consisting of alpha-Mo, Mo3Si, and Mo5SiB2 phases have high melting point above 2000 degrees C and have potential as ultra-high temperature structural materials. However, the room-temperature fracture toughness falls short of meeting the practical application, and enhancing the toughness through grain coarsening inevitably leads to a trade-off with the strength of the Mo-Si-B alloy. Bimodal structure design is expected to improve both the toughness and strength simultaneously. In the present work, Mo-12Si-8.5B alloy with a unique bimodal microstructural design was successfully prepared via multistep mechanical alloying, followed by hot pressing. The bimodal structure comprised fine (similar to 0.65 mu m) alpha-Mo grains and partial coarse (similar to 1.25 mu m) alpha-Mo grains, which exhibited a uniform distribution and formed a continuous matrix embedded with fine Mo3Si and Mo5SiB2 intermetallics. Such microstructure enhanced the fracture toughness and compressive strength simultaneously, exhibiting 14.3 MPa m(1/2) and 3130 MPa, respectively. The coarse alpha-Mo grains involved increasing dislocation motion and storage capacity, playing an important toughening role as the crack trapper. The uniform distribution of the bimodal alpha-Mo grains combined with the continuous matrix promoted uniformity of the strain distribution and alleviated the plastic constraint on alpha-Mo imposed by intermetallics, which then maximized the extent of crack trap toughening of alpha-Mo. The existence of fine alpha-Mo grains strengthened the alloy.