How an individual constituent zone behaves during the deformation of a heterostructured metallic material is a fundamental issue for understanding heterostructure deformation, but it remains a challenge to experimentally observe it. Here we report a study on the stress-strain behavior of the nanostructured gradient layer (NGL) in an integrated gradient specimen that consists of a coarse-grained (CG) central layer sandwiched between two NGLs. Constraint from the CG central layer led to the formation of dense and dispersed stable strain bands (SBs) in the NGL, which regained dislocation hardening after initial recovery and grain coarsening. Consequently, the NGL exhibited a transient plateau of flow stress after yielding, and then regained extra strain hardening to achieve excellent uniform elongation. These unique behaviors are dramatically different from those of a freestanding NGL, indicating a fundamentally different deformation principle that is intrinsic to heterostructures, i.e., inter-zone constraint modifies the constitutive behavior of constituent zones.

異構金屬材料變形過程中, 成分單元域的應力-應變行為是理解異構材料變形和強韌化機理的關鍵科學問題. 本研究以由納米結構梯度表層和粗晶芯部層構成的梯度結構為例, 實驗觀測了其拉伸變形過程中納米結構梯度層的應力-應變行為. 結果發現, 納米結構梯度層表現出區別於其獨立變形的獨特回應: 低應變階段, 加工硬化率先急劇降低, 出現流應力平臺; 然後, 加工硬化率異常回復, 流應力升高; 高應變階段, 加工硬化率緩慢降低, 展現出優異的均勻延伸率. 該獨特回應的力學機制為, 粗晶層約束納米結構梯度層的應變局域化, 誘導形成彌散分佈且穩定的高密度應變帶; 晶體學機制為, 應變帶內先發生變形驅動納米結構回復和晶粒長大, 然後因位錯增殖、累積而硬化.