介观尺度金属孪晶变形的强烈晶体尺寸效应

Strong Crystal Size Effect on Deformation Twinning at Meso-scale

主导许多材料力学行为的孪晶变形是一种局部晶体高度协调一致的非弹性剪切变形过程,其发生原因与时空特性仍然保持着某种神秘色彩。本项研究利用纳米压入仪下微柱体压缩与相应的透射电镜原位定量变形表征技术,发现随所用钛铝合金单晶外观尺度逐步减小到1个μm时,孪晶切变所需应力随之显著提高,表现出很强的尺度依赖性。当晶体的外部几何尺度进一步减小到亚μm量级时,材料的塑性变形方式发生了根本性的转变,孪晶变形完全由通常的位错滑移变形取代,而材料所能承受的最大流变应力亦呈现出一种接近于所用材料理想强度水平的"应力饱和"平台现象。本研究提出了以螺位错为媒介的孪晶变形"受激滑移"模型,完美地解释了孪晶变形具有强烈晶体尺寸效应的内在原因。

Deformation twinning in crystals is a highly coherent inelastic shearing process that controls the mechanical behaviour of many materials,but its origin and spatio-temporal features are shrouded inmystery.Using micro-compression and in situ nano-compression experiments,here we find that the stress required for deformation twinning increases drastically with decreasing sample size of a titanium alloy single crystal until the sample size is reduced to one micrometre,below which the deformation twinning is entirely replaced by less correlated,ordinary dislocation plasticity.Accompanying the transition in deformation mechanism,the maximum flow stress of the submicrometre-sized pillars was observed to saturate at a value close to titanium’s ideal strength.We develop a ＇stimulated slip’ model to explain the strong size dependence of deformation twinning.