過共晶組織を有するMo-ZrC in-situ複合材料の高温強度と室温靭性

抄録

For high temperature structural applications such as turbine blades, high temperature strength and fracture toughness at room temperature of Mo-ZrC in-situ composites with hyper-eutectic structure were investigated. Samples of Mo-30, 40, 50 mol%ZrC were prepared by arc-melting in an argon gas atmosphere, followed by annealing at 1673 K for 252 ks. Microstructures of all the annealed samples are found to consist of primary ZrC and fine Mo/ZrC eutectic region. In Mo/ZrC eutectic region, very fine ZrC precipitates are in Mo_ss matrix. The results of compression tests indicate relatively weak temperature dependence of yield stress. Particularly, the yield stresses of Mo-40 mol%ZrC and Mo-50 mol%ZrC at 1773 K are higher than that of monolithic ZrC reported previously. It is considered that ZrC phase in Mo-ZrC in-situ composite is remarkably solid-solution-strengthened by Mo.
From 3-point-bending tests examined at room temperature, fracture toughness of Mo-30 mol%ZrC and Mo-40 mol%ZrC was evaluated to be 14.7 MPa·m^1⁄2 and 13.7 MPa·m^1⁄2, respectively. In Mo-30 mol%ZrC, it is revealed that cleavage fracture occurs in primary ZrC particles while two types of intergranular fracture at Mo_ss grain boundaries and Mo/ZrC phase boundaries are observed in Mo/ZrC eutectic region. These intergranular fractures in eutectic region are associated with improvement in the fracture toughness of this composite by crack deflection and ligament formation.