Room-temperature fracture toughness of MoSiBTiC alloys

Room-temperature fracture toughnesses of TiC-added Mo-Si-B alloys were evaluated for samples of three different compositions prepared using a conventional Ar arc-melting technique. The first alloy (TiCp) had a primary phase during solidification of NaCl-type TiC including an amount of Mo, with a Mo solid solution (Mo_ss) volume fraction of approximately 49% and a TiC volume fraction of approximately 19%, while the volume fraction of Mo₅SiB₂ (T₂) was approximately 31% and the remaining 1% was Mo₂C including an amount of Ti. The second alloy (T2p) had a primary phase of T₂, with volume fractions of Mo_ss, TiC, Mo₅SiB₂ (T₂), and Mo₂C of approximately 38%, 4%, 45%, and 13%, respectively. The third alloy (Mop) had a primary phase of Mo_ss, with volume fractions of Mo_ss, TiC, Mo₅SiB₂ (T₂), and Mo₂C of approximately 55%, 8%, 32%, and 6%, respectively. Room-temperature fracture toughness was evaluated by three different bending tests using Chevron-notched specimens. Fracture toughness values obtained by the three methods were relatively close with good reproducibility. Consequently, the fracture toughness values of TiCp, T2p, and Mop were evaluated to be ∼15.2 MPa(m)^1/2, ∼10.5 MPa(m)^1/2, and ∼13.6 MPa(m)^1/2, respectively. Fracture surface observations indicated that the Mo_ss phase is subject to severe plastic deformation during the fracture process. The TiC phase was also noted to leave river patterns behind through crack propagation. These fractographic results suggest that not only the ductile-phase toughening by the Mo_ss phase but also an extra-toughening mechanism by the TiC phase are responsible for the goodness of the room-temperature fracture toughness of the MoSiBTiC alloys.