Deformation mechanisms and plasticity of ultrafine-grained Al under complex stress state revealed by digital image correlation technique

Yonghao Zhao,Yanglin Gu

doi:10.1515/ntrev-2021-0007

Abstract

Abstract Conventional engineering stress–strain curve could not accurately describe the local deformability of the tensile necking part because the strain is calculated by assuming that the tensile specimen was deformed uniformly. In this study, we used 3D optical measuring digital image correlation to systematically measure the full strain field and actual flow stress in the necking region of ultrafine-grained (UFG) Al. The post-necking elongation and strain hardening exponent of the UFG Al were measured as 80% and 0.10, slightly smaller than those of the coarse-grained Al (117% and 0.28), suggesting the high plastic deformability of the UFG Al under complex stress state. Microstructural studies revealed the shear and ductile fracture, numerous micro-shear bands, and elongated UFG grains in the UFG Al, which are controlled by cooperative grain boundary sliding and multiple dislocation slips.

Highlights

Bulk ultrafine-grained (UFG) metals with grain sizes smaller than 1 μm made via severe plastic deformation typically have high strength but very low tensile ductility at ambient ∂σ ≤ σ, (1)∂ε εwhere σ and ε are true stress and true strain, respectively, and εis the strain rate
It is apparent that the plastic deformation induced by equal-channel angular pressing (ECAP) in UFG Al was not uniform, resulting in a mixture of micron and sub-micronsized grains
It suggests that the restoration occurs to some degree during the ECAP process, which can be attributed to accumulated strain and adiabatic heating because of large plastic deformation

Summary

Introduction

Bulk ultrafine-grained (UFG) metals with grain sizes smaller than 1 μm made via severe plastic deformation typically have high strength but very low tensile ductility at ambient ∂σ ≤ σ, (1)∂ε εwhere σ and ε are true stress and true strain, respectively, and εis the strain rate. For the UFG materials, their strength at the right-hand side of equation (1) is high and strain hardening rate at the left-hand side of equation (1) is low, making it easy for premature necking even at small tensile strain. Up to now only a few investigations have focused on the effect of the UFG microstructure on sheet formability, in which the UFG material is deformed in the multi-axial mode along a complex strain path These studies suggest that even though the UFG sheet materials show very limited deformability in the tensile test, their formability in stretch forming was relatively good [15,16]. There is no systematic study on the plasticity and deformation mechanisms of the UFG

Objectives

Results

Discussion

Conclusion