Study on initiation and growth of microstructurally small cracks in creep-fatigue of oxide-dispersion-strengthened superalloy

为了阐明氧化物弥散强化（ODS）高温合金的蠕变疲劳失效机理，采用表面光滑的圆棒试样和中心缺口的平板试样，在1173K和1273K应力控制蠕变疲劳条件下，观察了裂纹的萌生和扩展行为。虽然晶间横向小裂纹从寿命的早期阶段在光滑表面上发起，但它们在发起后立即被相邻的纵向晶界阻止。穿晶小裂纹从寿命中期开始萌生并继续扩展，同时纵向晶界对裂纹的扩展起了阻碍作用。宏观裂纹是由穿晶小裂纹扩展和贯通形成的。宏观裂纹扩展，这是由缺口试样检查，是局部的纵向晶界阵列的影响。然而，由于细长的粗晶组织的波动被平滑在整个裂纹扩展的整个厚度的试样，其中包括约15个晶粒。扩展速率与非线性断裂力学参数蠕变J积分相关性良好。

In order to elucidate the creep-fatigue failure mechanism of an oxide-dispersion -strengthened (ODS) superalloy with the coarse elongated grain structure, the behavior of crack initiation and propagation was observed under stress- controlled creep-fatigue conditions at 1173K and 1273K using a round bar specimen with smooth surface and a plate specimen with a center notch. Although intergranular traverse small cracks initiated on the smooth surface from the early stage of life, they were arrested immediately after the initiation by adjacent longitudinal grain boundaries. Transgranular small cracks initiated from the middle stage of life and continued to propagate while longitudinal grain boundaries interfered the growth. The macrocrack was subsequently formed by the propagation and the coalescence of the transgranular small cracks. The macrocrack propagation, which was examined by the notched specimen, was locally influenced by an array of longitudinal grain boundaries. However, the fluctuation due to the elongated coarse grain structure was smoothed out in the whole crack propagation throughout the thickness of specimen which included about 15 grains. The propagation rate was correlated well with a nonlinear fracture mechanics parameter, creep J-integral.