Enhanced fracture toughness and toughening mechanisms of fine-grained Mo–12Si–8.5B alloy with a bi-modally structured α-Mo grain

Abstract

Fine-grained Mo–Si–B alloys possess superior strength and oxidation resistance at high temperature, but they usually exhibit poor toughness at room temperature. Improvement of toughness is critical to the practical application of alloys. Here, a series of fine-grained Mo–12Si-8.5B-0.57 wt% La2O3 alloys with bi-modally structured α-Mo grains were fabricated by hot pressing sintering of a mixture of ball milled Mo–Si–B–La2O3 powders and unmilled Mo–La2O3 powders in select proportion. When the mass fraction of unmilled Mo–La2O3 powders was 15%, the sintered alloy showed an optimal microstructure consisting of a continuous bimodal fine-grained (~0.62 μm)/coarse-grained (~3.06 μm) α-Mo matrix with embedded Mo3Si/Mo5SiB2 (~0.52–0.54 μm) particles. This bimodal-structural alloy displayed high yield strength (2460 MPa) and compression strength (2561 MPa) together with significant enhanced fracture toughness (12.5 MPa m1/2). The high strength resulted from the contributions of fine α-Mo grains and nanometer La2O3 particles, meanwhile the improved toughness might be attributed to (1) coarse-grained α-Mo regions promoting crack trapping, (2) the formation of intragranular microcracks within the coarse-grained α-Mo regions induced by La2O3, (3) occurrence of the α-Mo/intermetallic interface debonding.