Deformation Twinning in Polycrystalline Mg Microstructures at High Strain Rates at the Atomic Scales

Garvit Agarwal,Avinash M Dongare

doi:10.1038/s41598-019-39958-w

Abstract

Large scale molecular dynamics (MD) simulations are carried out to investigate the twinning behavior as well as the atomic scale micromechanisms of growth of tension and compression twins in polycrystalline Mg microstructures at high strain rates. A new defect characterization algorithm (extended-common neighbor analysis (E-CNA)) is developed that allows for an efficient identification of various types of twins in HCP microstructures. Unlike other local orientation analysis methods, the E-CNA method allows for atomic scale characterization of the structure of different types of twin boundaries in HCP microstructures. The MD simulations suggest that the local orientation of individual grains with the loading axis plays a critical role in determining the ability of grains to nucleate either compression twins or tension twins. The twinning behavior is observed through nucleation of a pair of planar faults and lateral growth of the twins occurs through nucleation of steps along the planar faults. The kinetics of migration of steps that determine the rate of growth of twins are investigated at the atomic scales. The twin tip velocity computed at high strain rates compares well with the experimentally reported values in the literature.

Highlights

Large scale molecular dynamics (MD) simulations are carried out to investigate the twinning behavior as well as the atomic scale micromechanisms of growth of tension and compression twins in polycrystalline Mg microstructures at high strain rates
The role of loading orientation with the c-axis of the grains on the nucleation of compression twin (CT) and tension twins (TTs) agrees well with previously reported MD simulations for single crystal Mg nanopillars that suggest the nucleation of {1121} TTs as the primary deformation mode when the c-axis is oriented at an angle of 15° < θ < 45° with the loading axis, while {1011} CTs are favored when c-axis is oriented at an angle of 60° < θ < 90° with the loading axis[20]
The capabilities of “common neighbor analysis” method is extended to identify the atomic scale environments that represent compression twins and tension twins in HCP microstructures. The capability of this “extended common neighbor analysis” method to characterize the evolution of deformation twins is demonstrated using polycrystalline Mg microstructures subjected to uniaxial tensile stress deformation using MD simulations

Summary

Introduction

Large scale molecular dynamics (MD) simulations are carried out to investigate the twinning behavior as well as the atomic scale micromechanisms of growth of tension and compression twins in polycrystalline Mg microstructures at high strain rates. Several experimental studies have been carried out to investigate the twinning mechanisms in HCP metals These studies suggest two commonly observed twinning modes in Mg, Ti and Co microstructures using transmission electron microscopy[1,2,3,4,5,6] comprising of tension twins (TTs) on {1012} and {1121}. The role of deformation twinning in the dynamic deformation response of single crystal Mg along the axis has been investigated at high strain rates using the plate impact experiments[9], wherein the shock recovered samples suggest the presence of horizontal as well as angled {1012} TTs. The average propagation velocities of the tip of the {1012} TTs in this study is reported to be in the range of ~430 m/s to ~1000 m/s for strain rates ranging from 103 to 105 s−1. While these studies provide significant insights in the presence of deformation twins and their structures, a clear

Methods

Results

Conclusion