Karen Perkins

A Critical Analysis of the Conventionally Employed Creep Lifing Methods

The deformation of structural alloys presents problems for power plants and aerospace applications due to the demand for elevated temperatures for higher efficiencies and reductions in greenhouse gas emissions. The materials used in such applications experience harsh environments which may lead to deformation and failure of critical components. To avoid such catastrophic failures and also increase efficiency, future designs must utilise novel/improved alloy systems with enhanced temperature capability. In recognising this issue, a detailed understanding of creep is essential for the success of these designs by ensuring components do not experience excessive deformation which may ultimately lead to failure. To achieve this, a variety of parametric methods have been developed to quantify creep and creep fracture in high temperature applications. This study reviews a number of well-known traditionally employed creep lifing methods with some more recent approaches also included. The first section of this paper focuses on predicting the long-term creep rupture properties which is an area of interest for the power generation sector. The second section looks at pre-defined strains and the re-production of full creep curves based on available data which is pertinent to the aerospace industry where components are replaced before failure.

Influence of shot peening on high-temperature corrosion and corrosion-fatigue of nickel based superalloy 720Li

High-temperature corrosion fatigue, a combination of corrosion with a fatigue cycle, is an emerging generic issue affecting power generation and aero gas turbine engines and has the potential to limit component life. Historically, surface treatments, such as shot peening have been used to improve component life and have been optimised for fatigue response. Research into optimisation of shot peening techniques for hot corrosion and high-temperature corrosion fatigue has shown 6–8A 230H 200% coverage to provide overall optimum performance for nickel-based superalloy 720Li based on the limited data within this study. Utilisation of electron backscatter diffraction techniques, in combination with detailed assessment of corrosion products have been undertaken as part of this work. The resultant cold-work visualisation technique provides a novel method of determining the variation in material properties due to the shot peening process and the interaction with hot corrosion. Through this work it has been shown that all three shot peening outputs must be considered to minimise the effect of corrosion fatigue, the cold work, residual stress and surface roughness. Further opportunity for optimisation has also been identified based on this work.

Insights into microstructural interfaces in aerospace alloys characterised by atom probe tomography

Atom probe tomography (APT) is becoming increasingly applied to understand the relationship between the structure and composition of new alloys at the micro- and nanoscale and their physical properties. Here, we use APT datasets from two modern aerospace alloys to highlight the detailed information available from APT analysis, along with potential pitfalls that can affect data interpretation. The interface between two phases in a Ti–6Al–4V alloy is used to illustrate the importance of parameter choice when using proximity histograms or concentration profiles to characterise interfacial chemistry. The higher number density of precipitates and large number of constituent elements in a maraging steel (F1E) present additional challenges such as peak overlaps that vary across the dataset, along with inhomogeneous interface chemistries.

The Influence of Lath, Block and Prior Austenite Grain (PAG) Size on the Tensile, Creep and Fatigue Properties of Novel Maraging Steel

The influence of martensitic microstructure and prior austenite grain (PAG) size on the mechanical properties of novel maraging steel was studied. This was achieved by looking at two different martensitic structures with PAG sizes of approximately 40 µm and 80 µm, produced by hot rolling to different reductions. Two ageing heat-treatments were considered: both heat-treatments consisted of austenisation at 960 °C, then aging at 560 °C for 5 h, but while one was rapidly cooled the other was slow cooled and then extended aged at 480 °C for 64 h. It is shown that for the shorter ageing treatment the smaller PAG size resulted in significant improvements in strength (increase of more than 150 MPa), ductility (four times increase), creep life (almost four times increase in creep life) and fatigue life (almost doubled). Whereas, the extended aged sample showed similar changes in the fatigue life, elongation and hardness it displayed yet showed no difference in tensile strength and creep. These results display the complexity of microstructural contributions to mechanical properties in maraging steels.

Influence of environmental factors on the corrosion-fatigue response of a nickel-based superalloy

Nickel disc corrosion is an important area within the gas turbine engine, with the potential to affect the lives of critical rotating parts. To understand the influence of corrosion on such components, it is essential to generate material data in a representative environment; hot corrosion in conjunction with cyclic loading. A comprehensive assessment of the behaviour of alloys U720Li and RR1000 in a laboratory-simulated environment has been undertaken to explore and replicate corrosion-fatigue features that have been identified in service. Key parameters such as salt loading (flux) and variations in mechanical stress cycle have been examined and have shown to modify the corrosion morphology and resulting fatigue life of the material. This paper is part of a thematic issue on the 9th International Charles Parsons Turbine and Generator Conference. All papers have been revised and extended before publication in Materials Science and Technology.

Research data supporting "A SANS and APT study of precipitate evolution and strengthening in a maraging steel"

Small angle neutron scattering (SANS)j and imaging atom probe field ion microscope data on a marageing steel destined for aerospace applications. The steel is strengthened with NiAl and Laves phase precipitates and is designed to be strong at both ambient and elevated temperatures, while maintaining a panoply of other properties such as machinability, fatigue resistance, toughness, creep strength and coatability.

On the Effect of Environmental Exposure on Dwell Fatigue Performance of a Fine-Grained Nickel-Based Superalloy

The influence of sulfur contamination on the corrosion-fatigue behavior of a polycrystalline superalloy used in aero-engines is considered. Samples tested under a variety of environmental conditions (including exposures to air, SOx gas, and salt) are characterized through a suite of high-resolution characterization methods, including transmission electron microscopy (TEM), secondary ion mass spectroscopy (nanoSIMS), and atom probe tomography (APT). The primary effect of sulfur contamination is to accelerate the crack growth rate by altering the failure mechanism. The SIMS and TEM analyses indicate Cr-Ti sulfide particle formation at grain boundaries ahead of and around oxidized cracks. The APT analysis suggests that these particles then oxidize as the crack propagates and are enveloped in chromia. The chromia is surrounded by a continuous layer of alumina within the cracks. All of the sulfur detected was confined within the particles, with no elemental segregation found at grain boundaries.

Optimisation of a salt deposition technique for the corrosion-fatigue testing of nickel based superalloys

ABSTRACT This paper summarises the work undertaken to develop a salt deposition technique that can be utilised for hot corrosion and high temperature corrosion-fatigue testing of high performance alloys for gas turbine application. The optimisation process has yielded a new method of application of a sodium sulphate based solution to various test piece geometries. The technique employs the use of a micro-pipette to deposit a pre-calculated solution, consisting of methanol, water, Na2SO4–NaCl onto preheated specimens, in preparation for corrosion testing. This optimised salting technique has reduced the variability in the salting of test specimens, when compared with existing salt spray methods and has therefore enabled test repeatability along with an associated significant reduction in scatter within high temperature corrosion-fatigue results.

The Effect of a Two-Stage Heat-Treatment on the Microstructural and Mechanical Properties of a Maraging Steel

Maraging steels gain many of their beneficial properties from heat treatments which induce the precipitation of intermetallic compounds. We consider here a two-stage heat-treatment, first involving austenitisation, followed by quenching to produce martensite and then an ageing treatment at a lower temperature to precipitation harden the martensite of a maraging steel. It is shown that with a suitable choice of the initial austenitisation temperature, the steel can be heat treated to produce enhanced toughness, strength and creep resistance. A combination of small angle neutron scattering, scanning electron microscopy, electron back-scattered diffraction, and atom probe tomography were used to relate the microstructural changes to mechanical properties. It is shown that such a combination of characterisation methods is necessary to quantify this complex alloy, and relate these microstructural changes to mechanical properties. It is concluded that a higher austenitisation temperature leads to a greater volume fraction of smaller Laves phase precipitates formed during ageing, which increase the strength and creep resistance but reduces toughness.

Laser ablation: Optimising material removal rate with limited oxidation of TI6AL4V

Laser ablation is a method of removing relatively small volumes of material from a substrate and this technique is finding a wide range of exploitation avenues in industry; such as large scale paint remover to composite resin removal. It has particular relevance to the aerospace industry due to the precision and speed of the material removal. In order for this technique to be effective, the laser parameters need to be optimised. In this instance, the volume of material removed has been studied using an Nd:YAG Q-Switched pulsed laser. The laser parameters: speed of the laser, intensity, pulse duration, frequency etc were varied to determine the optimum parameters for the removal of approximately 0.1cm3 of material from Ti6Al4V. It was determined that the rate of material removal is dependent on the laser parameter interaction rather than the overall energy of the system. In parallel, the effect of parameters on the surface of the sample were studied and optimum parameters determined for maximum material removal rate with little or no oxidative effects. In addition, the samples have been mechanically tested using High Cycle Fatigue, and found to be favourable compared to more conventional material removal techniques.Laser ablation is a method of removing relatively small volumes of material from a substrate and this technique is finding a wide range of exploitation avenues in industry; such as large scale paint remover to composite resin removal. It has particular relevance to the aerospace industry due to the precision and speed of the material removal. In order for this technique to be effective, the laser parameters need to be optimised. In this instance, the volume of material removed has been studied using an Nd:YAG Q-Switched pulsed laser. The laser parameters: speed of the laser, intensity, pulse duration, frequency etc were varied to determine the optimum parameters for the removal of approximately 0.1cm3 of material from Ti6Al4V. It was determined that the rate of material removal is dependent on the laser parameter interaction rather than the overall energy of the system. In parallel, the effect of parameters on the surface of the sample were studied and optimum parameters determined for maximum material re...