Quantitative evaluation of the effect of process-induced defects on the fatigue limit of additively manufactured Ni-based superalloy in a hydrogen environment

在这一年里，我使用具有各种小缺陷和微观结构的样品，从小的疲劳裂纹扩展门槛的角度研究了添加制造的镍基高温合金718的疲劳极限。在宏观和微观分析的基础上，对疲劳极限的确定机理、小缺陷的影响以及影响疲劳抗力的因素进行了深入的探讨。主要结论如下：1)疲劳试验表明，在循环扭转载荷和推拉载荷作用下，小裂纹行为均受最大剪应力控制。这意味着所有样品的疲劳极限都是剪切型小疲劳裂纹扩展的门槛条件，类似于锻造材料的相关研究结果。2)在每种情况下，都估计了剪切型小疲劳裂纹扩展门槛值，以量化小缺陷的影响。得到了疲劳门槛值与缺陷尺寸之间的关系，不仅得到了与锻造材料结果一致的裂纹尺寸依赖关系，而且添加剂制造材料的疲劳抗力也相当低。3)通过综合考察潜在决定因素对小疲劳裂纹扩展抗力的影响，得出结论：添加制造材料的劣化疲劳门槛值最大的影响因素是相邻晶内两个有利滑移面之间的扭转角。

During this year, I have examined the fatigue limit of an additively-manufactured, Ni-based superalloy 718 in terms of the small, fatigue-crack growth-threshold, using samples with various small defects and microstructures. Based on the macro- and microscopic analyses, the determining mechanism of the fatigue limit, the effects of small defects and the factors impacting fatigue resistance were all thoroughly explored. The principal conclusions are summarized as follows:1) Fatigue tests revealed that small-crack behavior was governed by maximum shear stress under both cyclic torsional loading and push-pull loading. This implied that the fatigue limit of all the samples was the threshold condition of a shear-mode, small fatigue-crack propagation, similar to the findings related to wrought materials.2) In each case, the shear-mode, small fatigue-crack growth-threshold was estimated to quantify the effects of small defects. As a result, the relationship between the fatigue threshold and defect size was acquired, showing not only a crack-size dependence consistent with the results of wrought materials, but also a substantially low fatigue resistance of additively-manufactured materials.3) Through the comprehensive examination of the effects of potential determining factors on small fatigue crack-growth resistance, it was concluded that the twist angle between two favorable slip planes in neighboring grains was the most influential factor in the inferior fatigue threshold of the additively-manufactured material.