纳米孪晶纯铜的极值强度及纳米孪晶提高金属材料综合强韧性

多晶材料的强度随晶粒尺寸减小而提高。但理论分析及分子动力学模拟结果显示,当材料晶粒尺寸小于某个临界值后,进一步减小晶粒尺寸可能导致材料软化。多晶体材料极值强度的出现是由于塑性变形的控制机制由晶格位错运动逐步转化为晶界行为。本研究组研究了具有不同孪晶片层厚度的纳米孪晶纯铜的极值强度,结果发现强度随孪晶片层厚度下降而提高。当孪晶片层厚度为15nm时,材料强度达到最大值。进一步减小孪晶片层,强度反而减小、出现软化现象。随孪晶片层减小,样品的塑性和加工硬化能力单调增加。分析表明纳米孪晶铜中极值强度的出现是由于随孪晶片层尺寸减小塑性变形机制从位错孪晶界相互作用主导转变为由孪晶片层结构中预存位错运动主导所致。

Revealing the Maximum Strength in Nanotwinned Copper

The strength of polycrystalline materials increases with decreasing grain size. Below a critical size, smaller grains might lead to softening, as suggested by atomistic simulations. The strongest size should arise at a transition in deformation mechanism from lattice dislocation activities to grain boundary-related processes. We investigated the maximum strength of nanotwinned copper samples with different twin thicknesses. We found that the strength increases with decreasing twin thickness, reaching a maximum at 15 nanometers, followed by a softening at smaller values that is accompanied by enhanced strain hardening and tensile ductility. The strongest twin thickness originates from a transition in the yielding mechanism from the slip transfer across twin boundaries to the activity of preexisting easy dislocation sources.