Improved fracture toughness of a Mo-12Si-8.5B-3Zr alloy by grain coarsening and its multiple toughening mechanisms

Abstract

Mo-12Si-8.5B-3Zr alloy was manufactured via a mechanical alloying (MA) followed by hot pressing (HP) sintering technology. The correlation between microstructure and mechanical properties of the alloy annealed at different temperatures (i.e., 1600 °C, 1700 °C and 1800 °C) for 3 h was investigated. The microstructure of the as-HPed alloy exhibited that the Mo3Si/Mo5SiB2 (T2) particles were distributed dispersedly in α-Mo matrix. Additionally, spherical or ellipsoidal ZrO2/Mo2Zr particles with nano-scale and submicro-scale were located at the grain boundaries (GBs) and partially in grain interiors (GIs). Increasing annealing temperature resulted in the growth of grains including α-Mo, Mo3Si and T2. The α-Mo average grain size of 1800 °C-annealed alloy reached the maximum value (∼1.2 μm) which was more than double that of as-HPed alloy. Consequently, compared with the as-HPed alloy, the 1800 °C-annealed alloy containing the coarsest α-Mo phase demonstrated excellent fracture toughness values of 13.9 MPa·m1/2 and 15.3 MPa·m1/2, which were determined from single edge notch bend tests and indentation tests, respectively. Based on theoretical analysis, α-Mo phase coarsening provided remarkable improvement in fracture toughness primarily by promoting crack trapping mechanism. Besides, coarsened Mo3Si/T2 particles elevated cracks deflection and interfacial debonding effects, which provided further resistance to cracks growth. Although the compression strength of alloy decreased with increasing annealing temperature, it maintained a high value of 2.86 GPa at 1800 °C.