Effect of harmonic structure design with bimodal grain size distribution on near-threshold fatigue crack propagation in Ti-6Al-4V alloy
Effect of harmonic structure design with bimodal grain size distribution on near-threshold fatigue crack propagation in Ti-6Al-4V alloy
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DOI：
10.1016/j.ijfatigue.2016.02.038
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发表时间：
2016-11-01
影响因子：
6
通讯作者：
Ameyama, Kei
中科院分区：
文献类型：
--
作者：
Kikuchi, Shoichi;Imai, Takafumi;Ameyama, Kei
采用机械磨粉烧结的方法制备了具有双峰谐波结构的Ti-6Al-4V钛合金（Ti-6Al-4V），该合金由粗晶组织包裹在细小等轴晶粒的网状结构中，具有较高的强度和塑性。为了研究谐波组织Ti-6Al-4V合金的近阈值疲劳裂纹扩展，在应力比作用下对碟形致密试样（ASTM标准）进行了降k试验。在实验室气氛中，R从0.1到0.8，具有恒定的R加载制度。利用扫描电镜（SEM）观察断口形貌，利用电子背散射衍射（EBSD）分析裂纹轮廓，探讨疲劳裂纹扩展的机理。在相同的应力强度范围δ K下，谐波结构材料的裂纹扩展速率da/dN不断高于粗针状组织材料，而疲劳阈值δ K-th较低。这是由于细晶粒的存在降低了谐波组织Ti-6Al-4V合金的粗糙度致裂纹闭合程度和有效应力强度范围K-eff，K-th。此外，在某些区域，疲劳裂纹并不在具有较高抗疲劳裂纹扩展能力的粗晶组织中扩展，而是优先在谐波组织中的细晶网状组织中扩展。(C) 2016 Elsevier Ltd.版权所有。
Titanium alloy (Ti-6Al-4V) with a bimodal harmonic structure, which consists of a coarse-grained structure surrounded by a network structure of fine equiaxed grains, has been fabricated by sintering mechanically-milled powders to achieve high strength and good plasticity. To investigate the near threshold fatigue crack propagation in the harmonic structured Ti-6Al-4V alloy, K-decreasing tests are conducted on disk-shaped compact specimens (ASTM standard) under stress ratios.R from 0.1 to 0.8 with a constant-R loading regime in a laboratory atmosphere. The fracture surfaces are observed using scanning electron microscopy (SEM), and crack profiles are analysed using electron backscatter diffraction (EBSD) to discuss the mechanism of the fatigue crack propagation. The crack growth rates da/dN in the harmonic structured material are constantly higher than those in a material with coarse acicular microstructure under comparable stress intensity range Delta K, while the fatigue thresholds Delta K-th are lower. This is attributed to a decrease in the magnitude of roughness-induced crack closure and the effective stress intensity range Delta K-eff,K-th in the harmonic structured Ti-6Al-4V alloy due to the presence of fine grains. Furthermore, in some areas, fatigue cracks do not propagate in the coarse-grained structure with higher fatigue crack growth resistance, but they preferentially propagate across the network structure of fine grains in the harmonic structure. (C) 2016 Elsevier Ltd. All rights reserved.