Three kinds of Mo-Si-B ternary alloys and a 1 at.% Al added Mo-Si-B alloy with the compositions near Mo-8.7 at.% Si-17.4 at.% B that is in the Mo 5 SiB 2 and Mo ss two-phase compositional region were produced by Ar arc-melting followed by the heat treatment at 1800°C for 24 h. These alloys have the characteristic fine microstructure composed of small Mo ss solid solution (Mo 55 ) particles in the Mo 5 SiB 2 (T 2 ) matrix with the primary phase (Mo 55 or T 2 depending on composition). The volume fraction of the Mo ss particles ranges from 25.5 to 30.5 % and its average size from 3.0 to 6.4 μm in the fine microstructure of the alloys. Micro cracks were introduced by Vickers hardness tests into the microstructures, and their propagation is disturbed by the small Mo ss particles. Thus, each hardness value seems to relate to the cracking behavior around each indent. On the other hand, Vickers hardness values do not show correlation with the volume fraction of the Mo ss particles, but clearly decrease with increasing the average particle size of Therefore, it should be concluded that the increase in the particle size of Mo ss could enhance the toughness of the Mo 5 SiB 2 /Mo-based alloys effectively by ductile phase toughening.
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