Gradient metallic materials, which can be made by surface mechanical attrition, non-equilibrium growth, or other thermomechanical means, have been deemed as an effective way to overcome the strength-ductility tradeoff as commonly seen in traditional metallic alloys. Although a large number of research works have been devoted to the unique strengthening mechanisms as resulting from the enhanced plastic-strain gradients and back stresses in these novel materials, the disproportionally fewer studies on ductility are rather restricted to the delay of the Considère necking limit due to the elevated work hardening. This view is incomplete, because the onset of necking (i.e., the uniform ductility) depends critically on both the amplitude and wavelength of initial perturbations. Even with a mere simplification of gradient materials by a film-on-substrate or a sandwich structure, this work shows that with random surface perturbations, the upper bound of uniform ductility is dictated by an extremely short-wavelength perturbation at the surface of the outer hard layer (thus denoted as Rayleigh mode). On the other hand, a pre-patterned surface with perturbations of varying periods is practicably unable to evolve into the Rayleigh asymptote and thus can achieve even higher uniform ductility. A revisit of literature experiments is suggested to be undertaken with a particular focus on the spectral growth kinetics of surface morphology and its connection to uniform ductility in these gradient materials.

梯度金属材料可以通过表面机械摩擦、非平衡生长或其他热机械方式制成，被认为是克服传统金属合金中常见的强度-延展性权衡的有效方法。尽管大量研究工作致力于研究这些新型材料中增强的塑性应变梯度和背应力所导致的独特强化机制，但对延展性的研究却少得不成比例，而是仅限于 Considere 颈缩极限的延迟。到高架加工硬化。这种观点是不完整的，因为颈缩的开始（即均匀延展性）在很大程度上取决于初始扰动的幅度和波长。即使仅通过基板上的薄膜或夹层结构对梯度材料进行简化，这项工作也表明，在随机表面扰动下，均匀延展性的上限由外表面的极短波长扰动决定。硬层（因此称为瑞利模式）。另一方面，具有不同周期扰动的预图案化表面实际上无法演变为瑞利渐近线，因此可以获得更高的均匀延展性。建议重新审视文献实验，特别关注表面形态的光谱生长动力学及其与这些梯度材料中均匀延展性的联系。均匀延展性的上限由外硬层表面的极短波长扰动决定（因此表示为瑞利模式）。另一方面，具有不同周期扰动的预图案化表面实际上无法演变为瑞利渐近线，因此可以获得更高的均匀延展性。建议重新审视文献实验，特别关注表面形态的光谱生长动力学及其与这些梯度材料中均匀延展性的联系。均匀延展性的上限由外硬层表面的极短波长扰动决定（因此表示为瑞利模式）。另一方面，具有不同周期扰动的预图案化表面实际上无法演变为瑞利渐近线，因此可以获得更高的均匀延展性。建议重新审视文献实验，特别关注表面形态的光谱生长动力学及其与这些梯度材料中均匀延展性的联系。具有不同周期扰动的预图案表面实际上无法演变为瑞利渐近线，因此可以实现更高的均匀延展性。建议重新审视文献实验，特别关注表面形态的光谱生长动力学及其与这些梯度材料中均匀延展性的联系。具有不同周期扰动的预图案表面实际上无法演变为瑞利渐近线，因此可以实现更高的均匀延展性。建议重新审视文献实验，特别关注表面形态的光谱生长动力学及其与这些梯度材料中均匀延展性的联系。