微/中尺度塑性变形中钛板的韧脆断裂尺寸效应

Lei Sun, Zhutian Xu, Linfa Peng, X. Lai
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摘要

随着微型化钛薄壁产品需求的显著增加,利用薄板进行微成形是一种具有高生产率的有前途的方法。然而,一旦薄片厚度缩小到微观尺度,尺寸效应及其对断裂行为的影响就存在许多未知因素。本研究采用单轴拉伸试验结合数字图像相关测量系统,研究了晶粒尺寸对0.1 mm商用纯钛板断裂行为的影响。随着晶粒尺寸从33.07 μm增大到107.70 μm,断裂行为呈现出韧脆转变。宏观上,CP-Ti试样的延伸率和临界断裂应力随晶粒尺寸的增大而减小。扫描电镜观察发现,随着晶粒尺寸的增大,韧窝数量逐渐减少,粗晶断口逐渐以解理面和河纹为主。为了探究断裂机制,进一步用透射电镜观察了不同晶粒尺寸的位错演化。在不同晶粒尺寸下,裂纹尖端出现位错发射。晶粒尺寸为33.07 μm的试样在晶界处出现了明显的位错堆积现象。这些强烈的位错降低了裂纹尖端的有效应力,从而提高了裂纹扩展的阻力。裂纹尖端位错密度随晶粒尺寸的增大而减小,导致裂纹尖端有效应力增大,裂纹塑性降低。因此,粗晶CP-Ti片材以解理断裂为主。
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Ductile-to-Brittle Fracture Size Effect of Titanium Sheets in Micro/Meso-Scale Plastic Deformation
With a significantly increasing demand for miniaturized titanium thin-walled products, micro forming using sheet metals is a promising approach with high productivity. However, once the sheet thickness is scaled down to a micro-scale, there are many unknowns in terms of size effect and its affected fracture behavior. In this research, the influence of grain size on the fracture behavior of commercially pure titanium sheets with a thickness of 0.1 mm was investigated by the uniaxial tensile tests combined with a digital image correlation measurement system. The ductile-to-brittle transformation of fracture behavior with the grain size increasing from 33.07 to 107.70 μm was revealed. Macroscopically, the elongation and critical fracture stress of CP-Ti samples decrease with the increase of grain size. According to the scanning electron microscopic observations, the number of dimples decreases with grain size increasing, while the cleavage planes and river patterns gradually dominate in the coarse grain fracture surface. To explore the fracture mechanism, the dislocation evolution of various grain sizes is further observed by a transmission electron microscope. The dislocation emission from crack-tips was revealed at different grain sizes. Significant dislocation pile-up at grain boundaries was observed in the specimen with a grain size of 33.07 μm. Those intense dislocations reduce the effective stress at the crack tip resulting in higher crack propagation resistance. Nevertheless, the dislocation density at crack-tip decreases strongly with the increase of grain size leading to high crack-tip effective stress and less crack plasticity. Hence cleavage fracture was dominated in coarse grain CP-Ti sheets.
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