This page contains a selection of project decriptions which have been posted on the site over the years, and is intended to give an idea of the type of projects we work on. Broadly speaking, our research divides into nickel-base superalloys for turbine blade and turbine disc applications. Projects can be largely experimental, largely computational, or somewhere between the two.
Effects of Ru additions on high temperature phase stability and creep behaviours of Ni-base SX superalloys
Refractory elements, such as Re and W, are added to Ni-base superalloys to enhance the high temperature creep resistance, these elements also tend to promote the formation of intermetallic Topologically-Close-Packed (TCP) phases that eventually degrade the mechanical properties. Additions of the platinum group metal, ruthenium, to experimental alloys have hindered the precipitation of these deleterious intermetallic phases. My research is to establish the possible underlying mechanisms for Ru additions to improve the microstructural stability and its direct and indirect influences on the high temperature creep resistance.
Development of Next Generation Ni-base Single Crystal Superalloys
Turbine blades operate at high temperature within aggressive environments. In addition they undergo thermal cycles as aircrafts take off and land. To meet the needs of higher efficiency of modern engines, advanced Ni-based single crystal superalloys with excellent creep properties, microstructural stability and excellent corrosion resistance are required. Initial results have shown that Ru plays a key role in improving stability and creep strength when combined with other alloying additions. The aim of my project is to develop a next generation Ru-containing Ni-base single crystal superalloy. My research is focused on investigating the characteristic deformation mechanisms of alloys with and without Ru, the thermodynamics of contributing to microstructural stability, and the environmental effects associated with Ru additions.
Some aspects of deformation mechanisms of CMSX-4
CMSX-4 is an advanced single crystal nickel-base superalloy used by Rolls-Royce and others for turbine blades. The microstructure consists of cuboidal gamma prime phase precipitates that are coherent with the surrounding gamma phase matrix. These precipitates are very effective strengtheners, as they are very difficult for dislocations to penetrate. As turbine components operate at high temperatures, creep processes are critical in determining blade life. There has been significant progress in understanding anisotropic creep response in CMSX-4 and developing models that can be used to predict for creep life. However, near features such as notches and cooling holes, local stresses are likely to be high enough for plasticity to occur. Currently, there is relatively little information on intermediate/ high temperature cyclic loading where creep and plasticity interact. The aim of my project is to understand how notched samples behave under fatigue in order to interpret how a complex blade will behave under real loading conditions. This will contribute to the accurate prediction of blade life. Deformation is being studied in plain and notched bars tested under Load Controlled Low Cycle Fatigue (LCLCF). The emphasis is on detailed examination of the microstructure and dislocation structures by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
Transmission Electron Micrograph of main slip plane of CMSX-4 plain bar after above yield LCF at 750°C
Lifetime Prediction and Damage Modelling for Gas Turbine Components
A reasonable lifetime prediction is essential for gas turbine components subjected to complex thermal and mechanical loading at elevated temperature. To achieve this aim, both a good constitutive modelling for inelastic analysis and a good damage modelling are required. Previous work has shown a strong interaction between the effects of creep and fatigue and also an important role for oxidation at temperatures over 850°C. The mechanisms of creep and fatigue damage (including oxidation) are strongly dependant on temperature, stress and orientation, as are the interactions between the micro-structural damage from each process. It is thus important to understand the type of damage occurring as a function of condition in order to model this complex behavior and to understand the limits over which behavior can be extrapolated. Based on the current QinetiQ code, this project aims to produce a semi-validated anisotropic model representing microstructural damage to predict the life of components subject to a combination of creep, fatigue and oxidation over a range of temperatures and to validate the model using Thermo-mechanical Fatigue (TMF) testing.
Development of an Advanced Power Processed Materials for Rotor Disc Application
Turbine discs are responsible for holding the turbine blades in place and are subjected to a centrifugal force associated with rotation. There also exists a temperature gradient from the bore to the rim of the disc. High strength, fatigue and creep resistance, are all required in varying degrees, dependent on location within the component. In the bore of the disc there exist high stresses, of the order of 1000MPa and relatively low temperatures. Here it is desirable to have a fine grain size, to achieve good tensile strength. The rim experiences lower rotational forces, but much higher temperatures so good creep properties are required, which requires a coarse grained microstructure.
Schematic cross-section illustration of a turbine disc
For my PhD I am undertaking extensive thermodynamic and mechanical investigations into three new powder processed nickel-base superalloys developed by Rolls-Royce plc. These alloys are seen as long-term successors to currently used turbine disc alloys, due to their improved mechanical properties and temperature capabilities.
During the course of my PhD I have studied the effect of primary and secondary heat treatments on both the micro and nano-structures of these alloys and the resulting mechanical properties. This has produced a detailed understanding of the intricacies between alloy chemistry, heat treatment and cooling rate.
One key area of investigation in any new alloy system is the thermodynamic stability with regard to perceived operating times and temperatures and this as been extensively studied in this work. Nickel-base superalloys are prone to precipitate certain phases known to have a negative impact on mechanical properties. Part of this project has been to assess the effect of long term exposure to temperature and to quantify the microstructural changes that occur.
Some typical microstructures are shown below.
As heat treated – Fine grain size, precipitates present on grain boundary provide high temperature strength
After exposure at 750°C for 5000 hours – Precipitation of deleterious sigma phase (white blocky particles) on grain boundaries
Nano-size gamma prime precipitates which constitute approximately 55% of the volume of the alloys studied are responsible for high temperature strength. Their response to heat treatment and cooling rate is critically important for the overall strength of these alloys.
The nano-sized precipitates can change shape with time ad temperature. Here one has begun to split up into eight smaller precipitates. The size at which this happens is strongly dependent upon the cooling rate and lattice mismatch of the alloy. Analysis using FEGSEM (second image) enables secondary gamma prime to be viewed in isometric projection
High temperature deformation of disk alloy
High temperature deformation of Ni-base superalloys is very important since the blades and discs of aeroengine turbine need to work at elevated temperatures for a expected long period. For my PhD I am mainly concerned with investigating the microstructural evolution and the tensile properties of nickel-base superalloys at high temperature. Currently, the nickel-base alloy Inconel 718 has being investigated because it is one of the most widely used superalloys. Recently, an experimental Electrical Thermo-Mechanical Testing (ETMT) apparatus was used for tensile testing. With the help of ETMT apparatus, a large number of tension and compression tests under different high temperatures and various stresses would generate tensile property data for a wide range of Ni-based superalloys. Microstructure investigation of various Ni-based superalloy before and after high temperature deformation would be undertaken to analysis the tensile properties, including quantitative information of composition, size, morphology and distribution of strengthening precipitates. Tensile property data from ETMT testing would be compared with the data from conventional tensile tests.