A gradient nanocrystalline layer with a thickness in a range of millimeter magnitude was successfully produced on the surface of Hadfield steel by a novel severe plastic deformation technology, high speed pounding. The surface hardness was measured, and the microstructure evolution during nanocrystallization process was characterized by X-ray diffraction and transmission electron microscopy. Results showed that the hardness increment and nanocrystallization in Hadfield steel were obtained at different stages under high speed pounding. The first stage was strain hardening, where surface hardness of Hadfield steel increased gradually during high speed pounding until a steady-state value was obtained. The hardening degree and rate of Hadfield steel were determined by deformation stress and strain rate, respectively. The second stage was microstructure nanocrystallization, at which twin boundaries interacted with dislocations to form general high angle grain boundaries. In this stage, the surface hardness of Hadfield steel remained basically the same. Moreover, a physical model was established to explain the strain hardening and surface nanocrystallization behaviors in accordance with the microstructure evolution at different stages in Hadfield steel.

通过一种新颖的严格塑性变形技术-高速冲击，在哈德菲尔德钢的表面成功地制造了厚度在毫米量级范围内的梯度纳米晶体层。测量了表面硬度，并通过X射线衍射和透射电子显微镜表征了纳米结晶过程中的微观结构演变。结果表明，在高速冲击下，哈德菲尔德钢在不同阶段获得了硬度增加和纳米晶化。第一步是应变硬化，在高速冲击过程中，哈德菲尔德钢的表面硬度逐渐增加，直到获得稳态值。哈德菲尔德钢的硬化程度和速率分别由变形应力和应变速率确定。第二阶段是微结构纳米晶化，在该过程中，孪晶边界与位错相互作用，形成了一般的高角度晶界。在此阶段，哈德菲尔德钢的表面硬度基本保持不变。此外，建立了一个物理模型来解释哈德菲尔德钢在不同阶段的显微组织演变过程中的应变硬化和表面纳米晶化行为。