WC-12%Co超硬合金の引張・圧縮疲労特性

抄録

Fatigue behavior of WC-12%Co hardmetal has been investigated under tension-compression cyclic loading at various stress ratios by using a newly developed apparatus. Tests were made under pulsating tension, pulsating compression and stress ratios R of -1, -4 and -9. WC-12%Co hardmetal showed two types of fatigue failure depending on the stress ratio. Under pulsating tension (R=0) and alternating stress (R=-1), fatigue cracks were initiated from the inside of the specimens. In these cases, cleavage fracture of Co was usually observed in the area of crack initiation. This type of fracture of Co is considered to occur in hcp Co transformed, due to the cyclic plastic straining, from the fcc Co which is the as-sintered state. In the fatigue crack initiation area, a Co-rich region or Co-rich regions were always found by EDX, in the specimens fatigue fractured under R=0 and -1.
Fatigued specimens under pulsating compression showed a sharp decrease in tensile strength after a certain number of stress cycles. This decrease in tensile strength is caused by the grooves, which act as the fracture origin, formed on the specimen surface by scaling-off of surface layer. Fatigue fracture under tension-compression fatigue at R=-4 and -9 also occurred from the grooves formed on the specimen surfaces, just as the tensile fracture of fatigued specimens under pulsating compression did. In tension-compression (R=-4 and -9) fatigue, however, crack growth preceding unstable fracture occurred by fatigue due to the tensile stress component in cyclic stress, in addition to the groove formation by scaling-off of the surface layer.