Multiscale Modelling of Micro/Nano-Scale Structure and Damage Evolution in Superplastically Deforming Materials

Superplastic forming (SPF) is being used increasingly by a range of UK and international industries, e.g. aerospace (Ti-alloy gas turbine components), high-performance automotive (Al- and Ti-alloys), architecture and defence. SPF is carried out by several UK wealth-generating companies (e.g. Doncasters, Aeromet, Superform) for a range of customers (including Airbus, Aston Martin, Boeing, Bombardier, BAE Systems, Ford Motor, GKN Lockheed, Goodrich, Raytheon, Siemens). Increasing use of SPF correlates roughly with availability of suitable materials capable of superplasticity, and with improvements in cost, speed and energy consumption of the SPF process. It is evident from literature of the last decade that a new generation of superplastic materials are emerging (e.g. high strain rate ceramics, metal matrix composites). New materials will lead to new applications, such as superplastic forming of complex-shaped ceramic armour plating. Furthermore, availability of new low cost structural alloys (e.g. Al-alloys) suitable for superplastic forming will lead to increased use of SPF in making complex shapes and replacing multi-part assemblies with single contiguous parts for increased structural integrity. Formability at lower temperatures reduces cost and energy consumption considerably, making SPF viable in industries with small profit margins and environmental restrictions (e.g. consumer automotive). Although modelling superplasticity is the end goal of this project, it is first necessary to model phenomena (e.g. grain growth, grain shape change and recrystallisation) common to a host of other material deformation modes relevant to many other industrial materials processing methods. UK metals forming industries (e.g. Doncasters, Corus) are increasingly interested in developing accurate models of their forming processes. Thus, the proposed work is apt and timely for advancement of a large and diverse sector of UK industry. Although superplasticity has been studied experimentally for over 80 years, there is not yet a comprehensive understanding of the physical processes of this important material phenomenon. Past modelling efforts have been hindered by there being no unique superplastic flow process. Rather, many small-scale (from atomic to micro) mechanisms combine with relative strengths that depend principally on grain size, temperature and strain rate to produce superplastic flow. It is essential for the further development of superplastic forming as a viable manufacturing method that a new modelling framework be developed in order to better understand the relation between the microstructure and mechanisms of superplasticity. This would enable materials to be thermomechanically processed with efficacious microstructures for lowering the process temperature and increasing the strain rate (and thereby reducing cost and increasing throughput) of superplastic forming applications in industries that exploit this phenomenon for forming metals and ceramics in contiguous complex shapes (e.g. aerospace, automotive, armour). This would be possible if low temperature mechanisms could be accessed via suitable changes to the microstructure, e.g. grain refinement, grain boundary/dislocation pinning or formation of vacancy/impurity complexes. Although the focus of the proposed work is modelling superplastic deformation, the impact will be far greater; a product of this effort will be a new multiscale modelling finite element package made of commercial software and custom subroutines. This will be the foundation of further studies into how deformation mechanisms at the micro and nano scales dictate overall material behaviour (e.g. fatigue, strain hardening and processes such as hot forming, forging, rolling, extrusion, drawing and machining).