Microstructure control and ultra-high strengthening of stainless steel by the room temperature recrystallization

高应变粉末冶金（HS-PM）工艺是一种新型的粉末冶金工艺，其结合了机械球磨（MM）或机械合金化（MA）、热处理和烧结工艺，并且使人们能够产生超细晶粒结构。它有三个显著的特点：[1]非平衡相，[2]纳米晶粒显微结构，[3]良好的可加工性。在室温下由研磨过程给出的极高的应变能产生大量的缺陷，这增强了原子在低温下的扩散，并有助于形成非平衡相，例如过饱和固溶体或非晶相。通过控制热处理条件，非平衡相获得纳米晶粒结构，影响相变、回复、再结晶、晶粒长大等（2）。晶粒细化对提高材料的力学性能具有重要意义，细晶材料易发生超塑性变形。因此，HS-PM工艺是最有效和有用的非平衡粉末冶金工艺，因为它允许同时改善机械性能和加工性能的微观结构的控制。在这份报告中，HS-PM工艺应用于SUS 316L不锈钢，并在室温下的材料的力学性能以及在该过程中的微观结构的变化进行了讨论。经HS-PM处理的粉末的HIP压坯具有均匀分散的σ相的超细组织。HIP压坯在室温和高温下也表现出出色的机械性能。

The high strain powder metallurgy (HS-PM) process is a novel powder metallurgy process combining mechanical milling (MM) or mechanical alloying (MA), heat treatment and sintering processes, and enables one to produce an ultra-fine grain structure. It has three remarkable features : [1] a non equilibrium phase, [2] a nano grain microstructure, and [3] good workability. The extremely high strain energy given by the milling process at room temperature produces a large number of defects and this enhances the diffusion of atoms at low temperature and contributes to the formation of a non equilibrium phase such as a supersaturated solid solution or an amorphous phase. A nano grain structure is obtained from the non-equilibrium phases by controlling the heat treatment conditions, which influences the phase transformation, recovery, recrystallization, grain growth, etc (2). Grain refinement is very important to improve the mechanical properties of materials, and fine grained materials easily deform by superplasticity. Therefore, the HS-PM process is the most efficient and useful non-equilibrium powder metallurgy process because it permits simultaneous improvement of mechanical properties and workability by the control of microstructure.In this report, the HS-PM process was applied to an SUS316L stainless steel, and the mechanical properties of the material at room temperature as well as the microstructural changes during the process are discussed. The HIP compact of the HS-PM processed powder showed ultra-fine microstructure with a homogeneously dispersed σphase. The HIP compact also showed outstanding mechanical properties at room temperature as well as at the elevated temperatures.