Deformation heterogeneities and their role in life-limiting fatigue failures in a two-phase titanium alloy
Deformation heterogeneities and their role in life-limiting fatigue failures in a two-phase titanium alloy
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DOI：
10.1016/j.actamat.2014.08.034
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发表时间：
2015-01-01
期刊：
影响因子：
9.4
通讯作者：
Larsen, James M.
中科院分区：
文献类型：
--
作者：
Jha, Sushant K.;Szczepanski, Christopher J.;Larsen, James M.
Ti-6 Al-2 Sn-4 Zr-6 Mo（Ti-6-2-4-6）是一种用于涡轮机发动机的α + β钛合金，其疲劳裂纹萌生部位的特征在于区分产生寿命限制失效和促进平均寿命行为的微观结构邻域和机制。的表征方法包括定量倾斜断口，聚焦离子束铣削裂纹萌生方面，和电子背散射衍射分析。区分寿命限制和平均值占主导地位的裂纹萌生微观结构的邻居的动机源于以前开发的理解，即平均值和寿命限制行为对应力水平（和许多其他变量）的反应不同，导致两个亚群之间的分离增加，应力水平降低，从而增加寿命的可变性。发现这两种行为的不同响应速率的出现，因为寿命限制机制是由裂纹扩展寿命占主导地位的，与微观结构尺度的裂纹萌生发生在前几个疲劳周期内，而平均行为越来越多地占主导地位的裂纹萌生寿命的应力水平降低。裂纹萌生邻域的2-D表征的代表性样本选自在窄范围的施加应力幅下对Ti-6-2-4-6的双重α + β相微观结构进行疲劳测试所产生的寿命限制和平均值主导群体。汇编的裂纹萌生刻面和多个试样的刻面晶粒附近的数据指向至少四个类别的临界微观结构配置，每个代表一组必要的（但也许不是足够的）条件，在这种合金裂纹萌生。基于这一特性，提出了一个假设的寿命限制疲劳行为。该假设调用了疲劳变形不均匀性的层次结构的概念，建议在前几个疲劳周期内发展。变形的不均匀性被认为是与潜在的随机性和层次的微观结构安排。这一假设似乎解释了发生裂纹增长寿命为主，寿命限制故障的制度，高周疲劳，如本研究所示，并建议发生这种故障的概率，即使在非常高的循环疲劳制度，虽然随着应力水平降低的概率减少。(C)2014 Acta Materialia Inc.由爱思唯尔有限公司出版。保留所有权利。
Fatigue crack-initiation sites in Ti-6Al-2Sn-4Zr-6Mo (Ti-6-2-4-6), an alpha + beta titanium alloy used in turbine engine applications, were characterized with emphasis on distinguishing the microstructural neighborhoods and mechanisms that produce the life-limiting failures vs. those that promote the mean-lifetime behavior. The characterization methods included quantitative tilt fractography, focused ion beam milling across crack-initiation facets, and electron backscattered diffraction analysis. The motivation for discerning between the life-limiting and the mean-dominating crack-initiation microstructural neighborhoods stemmed from the previously developed understanding that the mean and the life-limiting behaviors respond differently to stress level (and many other variables), leading to an increasing separation between the two subpopulations as the stress level is decreased, thereby increasing the variability in lifetime. The different rates of response of the two behaviors was found to arise because the life-limiting mechanism was dominated by the crack-growth lifetime, with microstructural-scale crack-initiation occurring within the first few fatigue cycles, whereas the mean behavior was increasingly dominated by the crack-initiation lifetime as the stress level was decreased. Representative specimens for 2-D characterization of crack-initiation neighborhoods were selected from life-limiting and mean-dominating populations generated by fatigue tests on a duplex alpha + beta phase microstructure of Ti-6-2-4-6 under a narrow range of applied stress amplitudes. A compilation of data on the crack-initiation facet and the neighborhood of the faceted grain from multiple specimens pointed to at least four categories of critical microstructural configurations, each representing a set of necessary (but perhaps not sufficient) conditions for crack-initiation in this alloy. Based on this characterization, a hypothesis for the life-limiting fatigue behavior is presented. The hypothesis invokes the concept of hierarchy of fatigue deformation heterogeneities, which is suggested to develop within the first few fatigue cycles. The deformation heterogeneity is suggested to be linked to the underlying randomness and hierarchy in the microstructural arrangements. This hypothesis appears to explain the occurrence of crack-growth-lifetime-dominated, life-limiting failures in the regime of high-cycle fatigue, as shown in this study, and suggests a probability of occurrence of such failures even in the very-high-cycle fatigue regime, although with diminishing probability as the stress level is decreased. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.