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

在这一年中，我使用具有各种小缺陷和微结构的样品，从小疲劳裂纹扩展阈值的角度研究了一种增材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，深入探讨了疲劳极限的决定机理、小缺陷的影响以及影响疲劳抗力的因素。主要结论如下：1)疲劳试验表明，在扭转载荷和推拉载荷下，小裂纹行为受最大剪切应力支配。这意味着所有样品的疲劳极限都是剪切模式的阈值条件，疲劳裂纹扩展很小，与变形材料的研究结果相似。2)在每种情况下，估计剪切模式，小疲劳裂纹扩展阈值，以量化小缺陷的影响。结果表明，增材制造材料的疲劳阈值与缺陷尺寸之间的关系不仅与变形材料的结果一致，而且具有较低的抗疲劳性。3)通过综合考察潜在决定因素对抗疲劳小裂纹扩展性能的影响，得出相邻晶粒两个有利滑移面之间的扭转角是增材制造材料下疲劳阈值的最大影响因素。

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.