Early Stages of Plastic Deformation in Low and High SFE Pure Metals

Marcello Cabibbo,Eleonora Santecchia

doi:10.3390/met10060751

Abstract

Severe plastic deformation (SPD) techniques are known to promote exceptional mechanical properties due to their ability to induce significant grain and cell size refinement. Cell and grain refinement are driven by continuous newly introduced dislocations and their evolution can be followed at the earliest stages of plastic deformation. Pure metals are the most appropriate to study the early deformation processes as they can only strengthen by dislocation rearrangement and cell-to-grain evolution. However, pure metals harden also depend on texture evolution and on the metal stacking fault energy (SFE). Low SFE metals (i.e., copper) strengthen by plastic deformation not only by dislocation rearrangements but also by twinning formation within the grains. While, high SFE metals, (i.e., aluminium) strengthen predominantly by dislocation accumulation and rearrangement with plastic strain. Thence, in the present study, the early stages of plastic deformation were characterized by transmission electron microscopy on pure low SFE Oxygen-Free High Conductivity (OFHC) 99.99% pure Cu and on a high SFE 6N-Al. To induce an almost continuous rise from very-low to low plastic deformation, the two pure metals were subjected to high-pressure torsion (HPT). The resulting strengthening mechanisms were modelled by microstructure quantitative analyses carried out on TEM and then validated through nanoindentation measurements.

Highlights

The improved mechanical tensile, fatigue, and ductile properties yield by ultrafine-grained (UFG)metallic materials and alloys compared to the conventional grained counterparts are well-known within the scientific community [1]
The microstructure evolution induced by high-pressure torsion (HPT) in Oxygen-Free High Conductivity (OFHC) 99.99% pure Cu and 6N-Al is shown in Figure 2, which reports strain levels of εeq = 0.40, 0.91, and 1.21 for Cu, and in Figure 3, reporting the microstructure after εeq = 0.02, 0.05, and 0.10 for Al
A representative Kikuchi pattern used to measuring the misorientation across the boundaries is reported in (d), in the case of OFHC 99.99% pure Cu at εeq = 1.21

Summary

Introduction

The improved mechanical tensile, fatigue, and ductile properties yield by ultrafine-grained (UFG). Metallic materials and alloys compared to the conventional grained counterparts are well-known within the scientific community [1]. Different technological methods were developed in the last three decades to obtain UFG metals and alloys. The first of them is a top-down approach by which a bulk metallic material is plastically forced to deform and refine the grain structure . Among the most effective and well-developed such top-down techniques the severe plastic deformation (SPD) methods are the ones showing the most promising and technologically reliable ones [2,3,4,5,6,7,8,9,10].

Results

Discussion

Conclusion