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

为了阐明粗大细长晶组织的氧化物弥散强化高温合金的蠕变疲劳失效机理，采用表面光滑的圆棒试件和带中心缺口的平板试件，在1173K和1273K的应力控制蠕变疲劳条件下观察了裂纹的萌生和扩展行为。虽然沿晶横纹小裂纹从寿命早期就开始在光滑表面萌生，但在萌生后立即被相邻的纵向晶界所阻止。穿晶细小裂纹从寿命中期开始萌生并继续扩展，而纵向晶界阻碍了扩展。宏观裂纹是由穿晶小裂纹的扩展和聚合形成的。由缺口试件观察到的宏观裂纹扩展受一系列纵向晶界的局部影响。而细长粗晶组织的波动在整个裂纹扩展过程中被平滑，整个试件的厚度约为15个晶粒度。扩展速率与蠕变J积分这一非线性断裂力学参数有很好的相关性。

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.