Single Crystal Nickel-Based Superalloys: Rationalisation of the Breakdown of the Anomalous Yielding Effect

(a) Brief Description of the context of the research including potential impact

Nickel-based superalloys are exemplar structural materials, particularly for applications in aeronautics and space. They are unique in their capacity to maintain substantial strength to elevated temperatures, in part due to Kear-Wilsdorf locking which leads to a thermal activation (and hence increasing density) of dislocation pinning sites. This is known as the anomalous yielding effect.

Nevertheless, the anomalous yielding effect fails at about 750 deg C, the exact temperature depending upon the alloy and then also with a significant strain-rate dependence. The physical origins of this failure are not well understood; this represents a failure of our basic knowledge of the way these materials behave. This project aims to address this.

(b) Aims and Objectives

We aim to elucidate the physical effects arising in these materials as a function of temperature and strain rate, in particular forming a physical basis for the operative mechanisms of deformation.

We aim in particular to rationalise (in the form of physically-faithful models) the deformation characteristics of microtwinning and stacking fault shear and their inter-relationship, and their relationship to more established modes of climb-assisted creep (McLean, Dyson) at high temperature and APB-shearing in the athermal regime at ambient temperatures.

We aim to carry out original experimentation in order to test and validate our ideas, probably making use of model systems such as single crystal superalloys. (c) Novelty of research methodology We will bring particular novelty to the research project by

First, using multiscale modelling methods involving density functional theory, molecular dynamics, crystal plasticity modelling and finite elements. We will find ways to bridge length scales to provide more accurate and physically-faithful modelling.

Where appropriate, we will also use analytical models to aid in the lengthscale bridging and too facilirate communication of our findings

Our DFT calculations for the planar fault energies in these systems will include eventually temperature and composition effects which will lead them to the state-of-the-art.

To validate our models, we will use state-of-the-art materials provided by our industrial collaborators. We will make use of our unique mechanical testing apparatus which allows for multi-angle videography. (d) Alignment to EPSRC's strategies and research areas

EPSRC funds research in chemistry, engineering, information and communications technologies, mathematical sciences, physics and materials. In the field of materials, physical metallurgy is a major theme. Metals/alloys and the production and recycling of metallic products account for about 10% of the UK's GDP. Major international companies such as Rolls-Royce rely upon the nickel-based superalloys for their commercial products.

(e) Any companies or collaborators involved

Professor Roger Reed has a number of industrial collaborators: Siemens, Rolls-Royce, Proterial, IHI, Severn Thermal Solutions, HTRC, the Max Planck institute (Dusseldorf). Over time, we will seek advice from these companies/organisations and probably they will be willing to provide sponsorship of our activities.