拉伸变形时梯度结构的塑性协调

Plastic accommodation during tensile deformation of gradient structure

梯度结构是一种典型跨尺度微观结构,其内部不同尺度晶粒具有迥异的内禀塑性稳定性,塑性协调是其获得优异强韧性的关键微观机制.本研究采用同步辐射原位拉伸测试方法,对梯度结构在拉伸变形过程中,由表及里不同深度结构层的点阵应变演化进行了研究.结果表明,梯度结构拉伸时具有两类塑性协调响应:一是由逐层微观屈服导致的瞬态弹-塑性变形,使纳米结构层的弹-塑性应变范围延后和扩展;二是纳米结构表层的塑性局域化和软化.梯度结构塑性协调引起了层间和层内应力状态的转变,导致层间微观力学行为响应的显著差异,利用异质变形诱导应变硬化和林位错硬化,促使梯度结构中纳米结构表层获得与芯部粗晶层相当的拉伸均匀应变.

Gradient structure(GS)possesses a typical trans-scale grain hierarchy with varying internal plastic stability,and the mutual plastic accommodation plays a crucial role in its superior strength-ductility combination.Using the in-situ synchrotron X-ray diffraction(XRD)during tensile loading,we measured lattice strains sequentially from the nanostructured(NS)surface layer to the central coarsegrained(CG)layer to elucidate when and how plastic accommodation occurs and evolves within the GS,along with their roles in plastic deformation and strain hardening.Throughout the tensile deformation,two types of plastic incompatibility occur in the GS.One is an extended elastoplastic transition due to layer-by-layer yielding.The other is strain localization and softening in the NS layer,in contrast with the stable plastic deformation in the CG layer.Plastic accommodation thus occurs concurrently and manifests as both an inter-layer and intra-layer change of stress state throughout tensile deformation.This produces different micromechanical responses between layers.Specifically,the NS layer initially experiences strain hardening followed by an elastoplastic deformation.The hetero-deformation induced hardening,along with forest hardening,facilitates a sustainable tensile strain in the NS layer,comparable to that in the CG layer.