A Multi-scale Approach to the Development of Microstructure-aware Constitutive Models for Magnesium

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 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.

4. Model development

Results from the aforementioned activities will be used to complement the development of new constitutive and failure models for magnesium. These models will be underpinned by crystal plasticity based finite element method simulations that are themselves put through upscaling methodologies to arrive at numerically informed predictions of the bulk constitutive models. Deriving these models in this manner opens the potential to utilise statistical methods and machine learning approaches to tailor the design of microstructure in magnesium alloys for the specific needs of a component.

Once these models are developed, there exists the possibility to collaborate with researchers at Brunel University who have developed the MCAST technique for producing new textures of magnesium with uniform microstructure; thus, providing a direct application for this project.