The aim of this study is to enhance the plasticity and strength of M2 high-speed steel through electron beam melting (EBM) technology. Therefore, by controlling different melting times to change the energy input, a gradient distribution of grain size and uniform distribution of carbides can be achieved. During the deformation process, the gradient structure causes geometrically necessary dislocations (GND) to accumulate near the boundary of regions in the soft zone, resulting in compressive stress in the soft zone and tensile stress in the hard zone, ultimately achieving heterogeneity-induced (HDI) work hardening. This study is to EBM technology for the design and fabrication of three types of M2 high-speed steel samples, denoted as SS-1, SS-2, and SS-3, utilizing three distinct scanning strategies: single melting, double melting, and a combination thereof. The microstructure and phase evolution of these samples were comprehensively characterized using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) techniques. The findings demonstrate that when sample SS-3 was subjected to the combined scanning strategy involving both single melting and double melting approaches, its crystal structure exhibited a gradient distribution along with the presence of back stress during tensile testing. Sample SS-3 achieved an average tensile strength of 1479 MPa with an average elongation rate of 1.8%. This represents an enhancement in average tensile strength by approximately 8.6% compared to sample SS-1 (∼1362 MPa) and a significant increase in average elongation rate by around 20% compared to SS-1 (∼1.44%). The gradient grain structure materials show the microstructure transformation from fine grain to coarse-grain in the same material, thus reducing the stress concentration caused by the change of mechanical properties in each region. The SS-3 sample can maintain significant ductility while improving tensile strength. Consequently, this represents a pivotal direction for the advancement of high-performance structural materials. The conducted experimental studies have demonstrated that geometrically necessary dislocations (GNDs) and their associated strain hardening play a crucial role in enhancing the strength of grain structure materials. Additionally, these are essential to achieve an exceptional synergy between strength and toughness in these materials.

中文翻译：

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电子束熔炼异质组织改善M2高速钢力学性能

本研究的目的是通过电子束熔炼（EBM）技术提高M2高速钢的塑性和强度。因此，通过控制不同的熔化时间来改变能量输入，可以实现晶粒尺寸的梯度分布和碳化物的均匀分布。在变形过程中，梯度结构导致几何必要位错（GND）在软区区域边界附近积累，导致软区产生压应力，硬区产生拉应力，最终实现异质性诱导（HDI） ）加工硬化。本研究采用 EBM 技术设计和制造三种类型的 M2 高速钢样品，分别表示为 SS-1、SS-2 和 SS-3，采用三种不同的扫描策略：单熔化、双熔化和它们的组合。使用光学显微镜、扫描电子显微镜 (SEM)、透射电子显微镜 (TEM) 和电子背散射衍射 (EBSD) 技术对这些样品的微观结构和相演化进行了全面表征。研究结果表明，当样品 SS-3 采用单熔化和双熔化方法的组合扫描策略时，其晶体结构在拉伸测试过程中表现出梯度分布以及背应力的存在。SS-3 样品的平均拉伸强度为 1479 MPa，平均伸长率为 1.8%。这表明与样品 SS-1 (~1362 MPa) 相比，平均拉伸强度提高了约 8.6%，与 SS-1 (~1.44%) 相比，平均伸长率显着提高了约 20%。梯度晶粒结构材料表现出同一材料中由细晶粒到粗晶粒的微观结构转变，从而减少了各区域因力学性能变化而引起的应力集中。SS-3样品可以保持显着的延展性，同时提高拉伸强度。因此，这代表了高性能结构材料发展的关键方向。进行的实验研究表明，几何必要位错（GND）及其相关的应变硬化在提高晶粒结构材料的强度方面发挥着至关重要的作用。此外，这些对于在这些材料中实现强度和韧性之间的卓越协同作用至关重要。