Mechanisms of plastic deformation in magnesium alloys

Lead Research Organisation: University of Oxford
Department Name: Engineering Science

Abstract

Alignment to EPSRC's strategies and research areas:
This project falls within the EPSRC Materials engineering - metals and alloys research area and aligns with the strategic focus of 'reducing material demand through resource efficiency and reducing lead times to product development through greater understanding of the microstructure/processing/performance triangle.' Advances in modelling and experimentation are expected outcomes of this project, promoting interdisciplinary collaboration, and further aligning it with the EPSRC's vision.

Description of Project:
Magnesium alloys enjoy one of the highest strength-to-weight ratios compared to other structural metals, making them potential weight savings alternatives in high-performance environments. Applications include components in aircraft, where lighter weight alternatives can represent significant fuel and cost savings. Despite these advantages, widescale use of magnesium has been limited by its complex deformation behaviour; its limited slip systems and high anisotropy lead to complex and often competitive deformation modes characterised by dislocation slip, twinning, and recrystallisation. This means magnesium suffers from poor workability, leading to early failure during conventional forming processes.
In recent years, novel pre- and post-processing techniques such as melt shearing and severe plastic deformation have shown promise in improving both the workability and bulk mechanical properties of magnesium. These techniques alter the microstructure and texture of the material to promote more uniform deformation and ultimately delay failure. The effect of these techniques on high strain-rate properties however remains largely unknown. This project seeks to develop an improved understanding of the mechanisms of deformation in magnesium alloys, and their sensitivity to mechanical loading (strain-rate, stress state), thermal environment (elevated, cooled temperatures), and microstructure (grain size, textures).
The project will comprise several key activities:
1. Bulk constitutive characterisation
Characterise the constitutive behaviour of magnesium alloys using the suite of mechanical loading equipment (quasi-static load frames, Split- Hopkinson pressure bars, single-stage gas guns) and leading diagnostic techniques (high-speed imaging, DIC, velocimetry) within the University of Oxford's Impact Engineering Laboratory.
2. In-situ texture evolution
Utilise dynamic X-ray diffraction capabilities at ESRF to monitor changes in texture (grain rotations and twinning) during mechanical loading.
3. Dynamic failure
Identify the primary mechanisms of plastic deformation (e.g. slip vs twinning) as a function of temperature, rate, stress-state, etc. and understand how microstructure can encourage/discourage failure. Deduce the conditions which trigger adiabatic shear. Adiabatic shear is a primary mode of failure in low symmetry metals (magnesium, titanium), and is encountered more frequently with increasing strain-rate. A Hopkinson pressure bar system will be used to drive high-rate compression into specially-shaped magnesium specimens which contain a small shear ligament.
4. Model development
Results from the aforementioned will be used to develop new constitutive and failure models for magnesium. This model will initially be based off the existing Oxford model for titanium (similar crystal structure and sensitivities), but will be extended to capture the effects of texture/microstructure. There is also potential to utilise statistical methods and machine learning approaches to guide experiments and model optimisation. The project will be conducted in collaboration with researchers at Brunel University. Brunel have developed the MCAST technique for producing new textures of magnesium with uniform microstructure, and this project will seek to develop an understanding of their influence on overall mechanical performance.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509711/1 01/10/2016 30/09/2021
2283233 Studentship EP/N509711/1 01/10/2019 31/03/2023 David Wason