An extraordinary-performance gradient nanostructured Hadfield manganese steel containing multi-phase nanocrystalline-amorphous core-shell surface layer by laser surface processing

摘要

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 similar to 600 mu m-thick heterogeneous gradient nanostructured layer on an austenitic Hadfield manganese steel, in which the average grain size is gradually decreased from similar to 200 mu m in the matrix to only similar to 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 similar to 1055 MPa and ductility of similar to 10.5% in the laser-processed specimen. Particularly, the core-shell structural surface maintains ultra-strong (tensile strength of similar to 1.6 GPa, micro-pillar compressive strength of-4 GPa at a strain of similar to 8%, and nanoindentation hardness of similar to 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.

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