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Fatigue damage development in pure polycrystalline magnesium under cyclic tension-compression loading

机译:循环拉伸压缩载荷作用下纯多晶镁的疲劳损伤发展

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摘要

Cyclic deformation and fatigue damage development of pure polycrystalline Mg were investigated by conducting fully reversed strain-controlled tension-compression experiments along the extrusion direction at two strain amplitudes of 1 % and 0.12%, respectively, in ambient air. At a strain amplitude of 1%, the tensile peak stress exhibited significant cyclic softening while the compressive peak stress showed moderate cyclic hardening. At a strain amplitude of 0.12%, marginal cyclic hardening was observed symmetrically at the tensile and compressive peak stresses. To study the fatigue damage development, a series of experiments using companion specimens were interrupted at the specified loading cycles. The morphology of fatigue damage on the specimen surface was examined by high resolution scanning electron microscope (SEM). With a strain amplitude of 1%, grain boundary cracking and twin tip damage were detected right after the second loading cycle. During the initial~80% fatigue life, microcracks were incessantly initiated with limited propagation that was observed to inhabit on both the grain boundary and the twin boundary. Final fracture surface was characterized by densely distributed cleavage facets along twin lamella boundaries together with tearing/shearing ridges. At a strain amplitude of 0.12%, initiation of micro-crack by grain boundary cracking occurred at the loading cycle between ~50% and ~70% fatigue life. Both intergranular and transgranular propagation modes were observed. The early-stage transgranular propagation was dominated by slip-induced cleavage cracking on both basal and non-basal slip planes. Crystal sugar-like structure and cleavage facets were found on the initiation site on the final fracture surface. Early-stage initiation and transgranular propagation modes were discussed on their dependences of cyclic loading magnitudes.
机译:通过在环境空气中分别沿两个应变幅度分别为1%和0.12%的挤压方向进行完全反向的应变控制拉伸压缩实验,研究了纯多晶Mg的循环变形和疲劳损伤发展。在应变幅度为1%时,拉伸峰值应力表现出明显的循环软化,而压缩峰值应力表现出中等的循环硬化。在应变振幅为0.12%时,在拉伸和压缩峰值应力下对称地观察到边缘循环硬化。为了研究疲劳损伤的发展,在指定的加载周期下中断了使用伴生试样的一系列实验。用高分辨率扫描电子显微镜(SEM)检查试样表面的疲劳损伤形态。当应变幅度为1%时,在第二次加载循环后立即检测到晶界裂纹和双尖端损坏。在最初的〜80%疲劳寿命中,微裂纹以有限的传播不断地引发,观察到在晶界和孪晶界都存在。最终的裂缝表面的特征是沿着双薄片边界的致密切割面以及撕裂/剪切脊。在应变幅度为0.12%时,在大约50%至70%的疲劳寿命之间的加载周期内,会发生晶界裂纹引发的微裂纹。观察到了晶间和晶间传播模式。在基层和非基层滑动面上,滑移引起的卵裂开裂控制了早期的跨颗粒传播。在最终断裂表面的起始位置发现了晶体状糖结构和卵裂面。讨论了早期萌生和跨颗粒传播模式对循环载荷大小的依赖性。

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