梯度结构硬质合金的疲劳断裂

Fatigue fracture of functionally-graded cemented carbides

采用三点抗弯的方法研究WC-6Co梯度结构硬质合金和均质WC-6Co硬质合金的疲劳行为,探讨疲劳断口形貌与破坏机制的关系。结果表明:梯度结构硬质合金的疲劳裂纹在亚表面萌生;梯度结构硬质合金表层Co相发生明显塑性变形,WC相以沿晶断裂为主;中间层Co相变形也很明显,WC相解理断裂增加;内层Co相塑性变形很少,WC、η相以解理断裂为主;均质硬质合金Co相塑性变形明显,WC以沿晶、解理断裂为主,各部位断口形貌接近;梯度结构硬质合金的疲劳极限比均质硬质合金高约100 MPa;梯度结构硬质合金中疲劳裂纹沿垂直于试样下表面、平行于Co相梯度的方向形核,而均质硬质合金的疲劳裂纹沿平行于试样外表面方向形核。在应力集中效应、循环应力的作用下,Co相的马氏体相变是裂纹在亚表面萌生的主要原因;马氏体相变使Co相成为裂纹形核的快速通道,裂纹沿Co相梯度方向形核。

Three-point-bending fatigue tests were conducted on both WC-6（mass fraction,the same below）% Co functionally-graded cemented carbides（FGCCs） and conventional WC-6%Co cemented carbides.The relationship between fatigue fracture morphology and failure mechanism were investigated.The results show that the fatigue cracks are nucleated in the subsurface area of both FGCCs and conventional cemented carbides;obvious plastic deformation of Co-phase and intergranular fracture of WC-phase are observed at the surface layer of FGCCs;plastic deformation of Co-phase is still obvious,and cleavage fracture of WC-phase increases in the middle layer of FGCCs;there is little plastic deformation of Co-phase observed at inner layer,and the fracture of WC-phase and ？ phase are mainly cleavage fracture;obvious plastic deformation of Co-phase is observed at conventional cemented carbides,and the fracture of WC-phase are mainly intergranular fracture and cleavage fracture;the fatigue fracture morphology of each part is nearly the same in the conventional cemented carbides;the fatigue endurance limit of FGCC is higher than that of conventional cemented carbides by 100 MPa;the direction of the fatigue crack nucleation seems to perpendicular to the surface and along the Co gradient in FGCCs,while it seems to be parallel to the surface in conventional cemented carbides.The fatigue crack initiation at the subsurface area is mainly attributed to the effect of stress concentration and the martensitic phase transformation of cobalt;due to the Co-phase martensitic transformation,the fatigue cracks are nucleated along the Co gradient.