Reducing Emissions by Exploiting Field-Induced Martensitic Transformations

The aim of the the fellowship will be to develop the analysis tools to design and use materials that exploit stress- and electromagnetic field-affected phase transformations. This area extends from bainite and martensite in steels to the variant selection problem during the beta->alpha transformation in titanium and zirconium alloys, from omega superelasticity in the beta-Ti alloy GUM metal to NiTi shape memory alloys (SMAs) and ferromagnetic SMAs. In the component context, conventional SMAs rely on a temperature change to provide actuation, which is achieved either passively in response to the environment or by heating / cooling using bleed air, resistance heating or heating filaments. Ferromagnetic SMAs use an electromagnetic field, which allows much faster switching, for example in a pump or to improve flow control. While the crystallography of these transformations is well understood, models are not generally available for the micromechanics that can be incorporated into Finite Element (FE) descriptions of component behaviour used by designers. In addition, whilst these systems are clearly tractable to atomistic approaches, atomistic modeling is still too immature to reliably design new alloys without experimental support; however approaches such as density functional theory (DFT) can enable insight into alloy design approaches to be developed. A subsidiary aim will be to start to bridge the gap to the DFT community. In conventional alloys the problem is often complicated by a diffusional component to the transformation, or nucleation may be the limiting step. However, we have recently shown clearly that applied stress can bias variant selection, leading to the production of mono-variant transformed beta grains in Ti-6246, with consequent effects on properties. The ability to model variant selection in diffusionless transformations, such as in martensite in steels, omega in Ti and Zr, and in (f)SMAs will be a prerequisite to modeling the more complicated problem in Ti-64 and Ti-6246. Industrially, the major goal of the fellowship will be to build a capability to model such transformations and to design alloys exploiting them for use in aerospace, automotive and power applications, with QinetiQ, Rolls-Royce, Timet, Corus and DSTL.