Reducing grain size (i.e. increasing the fraction of grain boundaries) could effectively strengthen nanograined metals but inevitably sacrifices the ductility and possibly causes a strengthening-softening transition below a critical grain size. In this work, a facile laser surface remelting-based technique was employed and optimized to fabricate a ∼600 μm-thick heterogeneous gradient nanostructured layer on an austenitic Hadfield manganese steel, in which the average grain size is gradually decreased from ∼200 μm in the matrix to only ∼8 nm in the nanocrystalline-amorphous core-shell topmost surface. Atomic-scale microstructural characterizations dissected the gradient refinement processes along the gradient direction, i.e. transiting from the dislocations activities and twinning in sub-region to three kinds of martensitic transformations, and finally a multi-phase nanocrystalline-amorphous core-shell structural surface. Mechanical tests (e.g. nanoindentation, bulk-specimen tensile, and micro-pillar compression) were conducted along the gradient direction. It confirms a tensile strength of ∼1055 MPa and ductility of ∼10.5% in the laser-processed specimen. Particularly, the core-shell structural surface maintains ultra-strong (tensile strength of ∼1.6 GPa, micro-pillar compressive strength of ∼4 GPa at a strain of ∼8%, and nanoindentation hardness of ∼7.7 GPa) to overcome the potential strengthening-softening transition. Such significant strengthening effects are ascribed to the strength-ductility synergetic effects-induced extra work hardening ability in gradient nanostructure and the well-maintained dislocation activities inside extremely refined nanograins in the multi-phase nanocrystalline-amorphous core-shell structural surface, which are evidenced by atomic-scale observations and theoretical analysis. This study provides a unique hetero-nanostructure through a facile laser-related technique for extraordinary mechanical performance.

减小晶粒尺寸（即增加晶界分数）可以有效地强化纳米晶粒金属，但不可避免地会牺牲延展性，并可能导致低于临界晶粒尺寸的强化-软化转变。在这项工作中，采用简单的基于激光表面重熔的技术并对其进行了优化，以在奥氏体 Hadfield 锰钢上制备约 600 μm 厚的异质梯度纳米结构层，其中平均晶粒尺寸从约 200 μm 逐渐减小。在纳米晶 - 非晶核 - 壳最顶层表面中的基质仅为~8 nm。原子尺度微观结构表征剖析了沿梯度方向的梯度细化过程，即从次区域的位错活动和孪晶过渡到三种马氏体转变，最后是多相纳米晶-非晶核-壳结构表面。沿梯度方向进行机械测试（例如纳米压痕、大块试样拉伸和微柱压缩）。它证实了激光加工试样的抗拉强度约为 1055 MPa，延展性约为 10.5%。特别是，核壳结构表面保持超强（抗拉强度为~1.6 GPa，微柱抗压强度为~4 GPa，应变为~8%，纳米压痕硬度为~7.7 GPa）以克服潜在的强化- 软化过渡。这种显着的强化效果归因于强度-延展性协同效应引起的梯度纳米结构中的额外加工硬化能力和多相纳米晶-非晶核-壳结构表面中极其精细的纳米晶粒内保持良好的位错活动，这得到了证明通过原子尺度的观察和理论分析。这项研究通过一种简便的激光相关技术提供了一种独特的异质纳米结构，以实现非凡的机械性能。