Strain Hardening in Extruded Pure Magnesium

Abstract

There has been a surge in interest in wrought magnesium alloys for structural applications in automotive industry. However, the application of wrought alloys is limited by its poor plasticity, due to its inherent strong texture and hcp structure having limited active slip systems. Due to limited slip, deformation twinning acts as an additional mode of deformation in textured Mg polycrystals, whose mechanical behaviour is strongly orientation dependant. In this context, the broad aim of this thesis was to address the knowledge gaps in understanding the mechanical response in extruded pure magnesium, as a function of strain, strain-rate, and orientation. Towards, this end, RT mechanical tests in two orientations (Mg || ED) and (Mg ꓕ ED), were studied to understand the strain hardening behaviour. These mechanical tests were complemented with microstructural characterization techniques involving SEM-EBSD, TEM, XRD etc to understand the evolution of microstructure affecting the strain hardening response. The first part of investigation concentrates on low strain rate deformation of extruded Pure Mg. The strain hardening response due to the activation of extension twinning had an unusual sigmoidal shape, with three distinct stages A, B and C. The goal of this study was to fill the existing knowledge gap in understanding the mechanisms affecting the evolution of sigmoidal 𝛩 in Mg, at low strain rates. Microstructural investigation based on SEM-EBSD, TEM was used to understand the mechanisms at distinct stages. Thereafter, a new three-parameter model based on dislocation density evolution was developed to predict the evolution of Θ in Stage A, B and C. The model also incorporates the effect of strain-rate and showed excellent agreement with the Θ evolution from experiments. The second part of the investigation concentrates on strain hardening response of Mg at high strain-rates, as a function of strain, strain-rate and orientation (Mg || ED and Mg ꓕ ED). A marked asymmetry in strain hardening was observed as a function of test orientation, at high strain rates. Thus, the aim was to understand the factors affecting strain hardening in Mg as a function of test orientation, at high strain-rates. Towards this end, high strain rate jump tests were performed. During a HSR jump test, the mobile dislocation density 𝜌𝑚is expected to be constant. Thus, an increase in strain-rate Δ𝛾̇ would lead increase in dislocation velocities. For these high dislocation velocities, dislocation drag effects dominate resulting in enhanced flow stress; the increment is flow stress (Δ𝜎) after the jump being linearly proportional to Δ𝑣. Thus, by systematic high strain rate jump tests in two orientations, it was concluded that 𝜌𝑚 decreased with increase in strain, in Mg || ED case, while 𝜌𝑚 increased with strain in Mg ꓕ ED orientation. It is proposed that this contrasting behaviour of 𝜌𝑚 between the two test directions is the primary cause of asymmetry in strain hardening at high strain rates. This observation was also corroborated with evidence from TEM studies.