Significance of Strain Rate in Severe Plastic Deformation on Steady-State Microstructure and Strength

TL;DR

This study investigates how strain rate influences the steady-state microstructure and strength in materials subjected to severe plastic deformation, finding that certain properties are largely rate-independent within a specific strain rate range.

Contribution

It demonstrates that the steady-state microstructure and mechanical properties are largely unaffected by strain rate variations in high-speed high-pressure torsion processing.

Findings

Microstructure and properties are rate-independent between 0.004 and 20 s-1.

Grain refinement and recrystallization rates are proportional to strain rate.

Steady state is governed by a rate-independent balance of processes.

Abstract

The microstructure and mechanical properties of materials saturate to steady states after severe plastic deformation (SPD). Despite the well-known effect of temperature on the steady-state microstructure, there is no general agreement on the significance of strain rate and the applicability of the Zener-Hollomon parameter in this regard. In this study, several pure metals (aluminum, copper, titanium, and iron) and a Cu-30Zn (wt%) brass alloy have been processed by a high-speed high-pressure torsion (HPT) equipment with controllable rotation speeds in the range of 0.06 to 60 rpm. It is found that crystallite/grain size, dislocation density, microhardness and shear stress at the steady state are reasonably rate-independent for the von Mises strain rates in the range of 0.004 to 20 s-1. Because both rates of grain refinement and of dynamic recrystallization are proportional to the strain rate, it is suggested that their balance, which determines the steady state, is rate-independent.