Low Temperature Recrystallization and Formation of an Ultra Fine (γ σ) Microduplex Structure in a SUS316L Stainless Steel

Abstract

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. 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 paper, the HS-PM process was applied to an SUS316L stainless steel, and the mechanical properties of the material at room temperature asmore » well as the microstructural changes during the process are discussed.« less