Mark T Whittaker

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Verified

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• MPhys, PhD, FIMMM,
• Research Director at Swansea University

Nickel-based superalloys operate primarily in high-temperature environments where their unique microstructure provides operational capability far in excess of many other engineering materials. As such these materials often operate in applications where thermal cycles occur alongside mechanical fatigue, giving rise to the phenomenon of thermo-mechan...

A bespoke TMF crack growth test set-up has been developed and validated for use throughout this study and the effects of phasing between mechanical loading and temperature have been investigated. The study shows that TMF cycles may show increased crack growth rate behaviour when compared to isothermal fatigue. The phase angle of the applied TMF cyc...

The development of high strength alloys with complex chemistries has led to modern nickel disc alloys being produced through a powder metallurgy process which often produces melt anomalies within the material microstructure. The current research investigates the effect of these melt anomalies on overall lifing approaches for the alloy RR1000, utili...

As turbine entry temperatures (TET) in modern gas turbine engines continue to increase, Type II hot corrosion has the potential to be a significant damage mechanism at turbine disk rims as temperatures approach 700°C. Therefore, assessment of the effect of hot corrosion damage is essential. In the current research, low cycle corrosion-fatigue behav...

This paper describes work to study hot corrosion damage in the nickel-based gas turbine alloy RR1000 coated with various salts at 600℃. Interest in type-II hot corrosion on the fatigue resistance of gas turbine alloys has increased to minimise power/thermal inefficiencies. Stress-free corrosion demonstrated a greater impact of sulphate (versus chlo...

The nickel-based superalloy Haynes 282 is a promising candidate material among the existing batch of aerospace alloys for manufacture via laser powder bed fusion (LPBF). LPBF Haynes 282 has a strong preference for epitaxial grain growth in the (001) orientation, promoting inhomogeneous grain morphologies and anisotropic mechanical behaviour. In thi...

This is a summary of the section “High Temperature Extreme Alloys and Composites” for the Elsevier Encyclopedia of Materials: Metals and Alloys. The summary illustrates the structure of the section and gives a brief description of how each article fits in to the overall section.

The solidification process of Ti45Al2Nb2Ta0.8B (B4522Ta) and Ti45Al2Nb2Hf0.8B (B4522Hf) Bridgman samples were studied through directional solidification. Boron addition at 0.8 at. % was found to have changed the orientation relationship of peritectic α with β dendrites. The peritectic alpha is randomly oriented in both B4522Ta and B4522Hf, it was l...

Boron has been used to refine the microstructures in TiAl castings, such as low-pressure turbine (LPT) blades, to improve mechanical properties. However, boride precipitates with undesirable morphologies could reduce ductility and even entirely remove the benefits of grain refinement. Boride size and morphology in variant alloys based on Ti45Al2Mn2...

The current paper presents work on identification and evaluation of a range of factors influencing accuracy and comparability of data generated by three laboratories carrying out stress-controlled thermo-mechanical fatigue crack growth tests. It addresses crack length measurements, heating methods and temperature measurement techniques. It also pro...

A number of thermal exposures were undertaken to determine the effect oxidation has on the fatigue lives of the nickel-based superalloy RR1000. The thermal exposures initially caused a considerable reduction in the fatigue lives with increasing oxidation. However, the longest exposure time resulted in an S-N curve that lay between the shortest and...

Surface treatments such as shot peening to inhibit fatigue crack initiation are essential processes when designing gas turbine components for aerospace applications. It is therefore crucial to understand the effects of shot peening in representative service environments. Here, the influence of surface treatment on the high temperature corrosion fat...

The polycrystalline nickel-base superalloy RR1000 is used as turbine rotor material in Rolls-Royce aero engines and has to withstand a wide variety of load and temperature changes during operation. In order to maximize the potential of the material and to improve component design, it is of great interest to understand, and subsequently be able to a...

An understanding of rate dependency over a wide range of time scales is vitally important in approximating the transient response of critical components operating in extreme environments. Many examples of viscoplastic model formulations can be found in the literature, wherein all rate dependency is assumed to occur after yielding. Such models negle...

The crack driving mechanisms in a coarse grained nickel-base superalloy RR1000 when subjected to in- and out of phase thermo mechanical fatigue are investigated. It is found that the difference in fatigue crack growth rate between these two load conditions is accounted for by the different mechanical conditions at the crack tip region, rather than...

The current paper describes TMF crack growth behaviour in an advanced nickel-based superalloy. Changes in behaviour are examined which occur as a function of the phase angle between applied stress and temperature. The fractography of the failed specimens reveals changes from transgranular to intergranular growth between high and low phase angle tes...

The thermo-mechanical fatigue (TMF) behaviour of a Ti-6Al-4V matrix composite reinforced with SCS-6 silicon carbide fibres (140 μm longitudinal fibres, laid up hexagonally) has been investigated. In-phase and out-of-phase TMF cycles were utilized, cycling between 80–300 °C, with varying maximum stress. The microstructure and fracture surfaces were...

In the current study, dislocation activity and storage during creep deformation in a nickel based superalloy (Waspaloy) were investigated, focussing on the storage of geometrically necessary (GND) and statistically stored (SSD) dislocations. Two methods of GND density calculation were used, namely, EBSD Hough Transformation and HR-EBSD Cross Correl...

Thermo-mechanical fatigue data is critical for the generation of appropriate lifing methodologies for a range of in-service applications where non-isothermal conditions are prevalent. Recently the development of more standardised testing approaches through appropriate code of practice documents and international standards has proved crucial. In the...

Strain-controlled cyclic deformation of a nickel-based single crystal superalloy has been modelled using three-dimensional (3D) discrete dislocation dynamics (DDD) for both [0 0 1] and [1 1 1] orientations. The work focused on the interaction between dislocations and precipitates during cyclic plastic deformation at elevated temperature, which has...

Lifing of components which are likely to be subject to high temperature creep deformation is a critical area to a range of industries, particularly power generation and aerospace. In particular, extrapolation of short term data to predict long-term allowable creep stresses is an area of significant importance, since no appropriate method of acceler...

There are many applications where the combination of stress and elevated temperature require creep to be considered during the design process. For some applications, an evaluation of rupture life for given conditions is sufficient, however, for components such as those in gas turbine aeroengines, the accumulation of creep strain over time and the e...

Oxidation-promoted crack growth, one of the major concerns for nickel-based superalloys, is closely linked to the diffusion of oxygen into the crack tip. The phenomenon is still not well understood yet, especially the full interaction between oxygen diffusion and severe near-tip mechanical deformation. This work aimed at the development of a robust...

Low cycle fatigue (LCF) of a low-carbon (LC) directionally-solidified (DS) nickel-base superalloy, CM247 LC DS, was investigated using both experimental and computational methods. Strain-controlled LCF tests were conducted at 850°C, with a loading direction either parallel or perpendicular to the solidification direction. Trapezoidal loading-wavefo...

This paper explores the concept of creep activation energy, comparing the currently used Arrhenius equation to an outcome of the using Gibbs free energy. The consequence of these differing approaches is illustrated using large datasets. By examining the fundamental approach to creep activation energy, this article highlights potential advances in t...

In developing more accurate lifing models for critical aerospace components, knowledge of the development of evolving stress conditions is required to make appropriate predictions. Traditional approaches have been based around data conducted under isostress and isothermal conditions, enabling the development of constitutive relationships. However,...

In developing more accurate lifing models for critical aerospace components, knowledge of the development of evolving stress conditions is required to make appropriate predictions. Traditional approaches have been based around data conducted under isostress and isothermal conditions, enabling the development of constitutive relationships. However,...

Creep tests of the polycrystalline nickel alloy Waspaloy have been conducted at Swansea University, for varying stress conditions at 700 °C. Investigation through use of Transmission Electron Microscopy at Cambridge University has examined the dislocation networks formed under these conditions, with particular attention paid to comparing tests perf...

Non-isothermal conditions during flight cycles have long led to the requirement for thermo-mechanical fatigue (TMF) evaluation of aerospace materials. However, the increased temperatures within the gas turbine engine have meant that the requirements for TMF testing now extend to disc alloys along with blade materials. As such, fatigue crack growth...

A three dimensional structure of varying wall thickness has been manufactured from an alloy similar to 718 and subjected to metallographic characterisation. The technique is evaluated as a process capable of generating complex geometries. This can be used to add features or as a free form fabrication method. However, in order to allow for compariso...

An Experimental Study of Low-Cycle-Fatigue behaviour of a Directionally Solidified Nickel-Based Superalloy at high temperature in air. Fractographic analysis using SEM showed a brittle fracture dominated failure. Crystal plasticity modelling was carried out and numerical simulations were in good agreement with experimental findings

Infrared thermography (IRT), a non-invasive temperature measurement technique has been investigated and developed for use with cyclic high-temperature loading. The technique utilizes an infrared thermography camera (IRTC) and Rolls-Royce HE23 black thermal paint (TP). The TP is applied to a test-piece surface to provide a stable emissivity value an...

Although originally discovered in the 18th century [1], the titanium industry did not experience any significant advancement until the middle of the 20th century through the development of the gas turbine engine [2]. Since then, the aerospace sector has dominated worldwide titanium use with applications in both engines and airframe structures [3]....

This paper details the development and assessment of a new empirical creep model that belongs to the limited ranks of models reproducing full creep curves. The important features of the model are that it is fully standardised and is universally applicable. By standardising, the user no longer chooses functions but rather fits one set of constants o...

It has been noted that creep models may appear as simple equations but when it comes to actually using them it isn’t so straight forward. This document has been written to show how to predict failure or rupture time using well known creep models. NOTE: This document does not evaluate the models, nor makes recommendations in choosing a model. It is...

This letter explores issues surrounding region splitting, a current technique used to improve creep life predictions. The history of the technique is discussed briefly and its current implementation by major research groups described in more detail. An example of novel or non-traditional region splitting is also explored with its first known applic...

The creep behaviour of the nickel superalloy RR1000 is studied through a number of constantload creep tests. It is often assumed that creep data generated by constant-load testing are unsuitable for building a generalised creep model due to the non-constant stresses incurred. Analysis of existing models shows that significant errors may occur in ma...

This conference presentation applies the Wilshire Equations to Grade 91 Steels demonstrating the method step-by-step, and results.

A non-invasive temperature measurement, control and profiling technique has been investigated for use with thermomechanical fatigue loading. The technique utilises an infrared thermography camera and Rolls–Royce developed thermal paint to control and monitor cyclic temperature. Thermal paint is used to maintain a stable surface emissivity upon the...

In the past decade, a new approach to predictive creep lifing has been developed, known as the Wilshire equations. Having been applied to a range of power generation and aerospace materials, the understanding of material behaviour associated with the equations has developed significantly. With the equations based around the dominance of diffusion c...

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...

Gamma titanium aluminides (γ-TiAl) display significantly improved high temperature mechanical properties over conventional titanium alloys. Due to their low densities, these alloys are increasingly becoming strong candidates to replace nickel-base superalloys in future gas turbine aeroengine components. To determine the safe operating life of such...

Thermo-mechanical fatigue (TMF) is an important factor for consideration when designing aero engine components due to recent gas turbine development, thus understanding failure mechanisms through crack growth testing is imperative. In the current work, a TMF crack growth testing method has been developed utilising induction heating and direct curre...

The fatigue properties of the high strength stainless steel CSS42L have been evaluated under strain and stress controlled conditions. The results have been used to derive a predictive approach based on the Walker strain equation. Accurate predictions are obtained for VCN and DEN specimens although the lower stress concentration RCN specimen is show...

Microstructural damage and subsequent failures resulting from thermo-mechanical fatigue (TMF) loading within the temperature range 300-700 °C are investigated for the polycrystalline nickel superalloy, RR1000. Strain controlled TMF experiments were conducted over various mechanical strain ranges, encompassing assorted phase angles, using hollow cyl...

Creep tests of the polycrystalline nickel alloy Waspaloy have been conducted at Swansea University, for varying stress conditions at 700 °C. Investigation through use of Transmission Electron Microscopy at Cambridge University has examined the dislocation networks formed under these conditions, notably those with stresses above and below the yield...

Recently developed TIMETAL639 is a high strength, forgeable alpha-beta alloy which is stronger, harder and critically no denser than Ti-6Al-4V. The alloy has been developed as a material for rotating components in future generations of gas turbine engines. The aim of the current work is to characterise TIMETAL639 both microstructurally and mechanic...

The ability to predict the stress-rupture behaviour of high temperature components is of paramount importance in both power generation and aerospace applications. Historically, the power law-based techniques such as the Larson-Miller, Manson-Haferd and Goldhoff-Sherby have typically been utilised to describe the creep behaviour of metals and alloys...

Assessment of thermo-mechanical fatigue behaviour of the polycrystalline nickel alloy RR1000 reveals a significant effect of phase angle on fatigue life. The current paper explores two scenarios: the first where the mechanical strain range is held constant and comparisons of the fatigue life are made for different phase angle tests; and secondly, t...

Current creep models are derived using data from constant stress (or load) creep tests and are capable of accurately predicting creep behaviour when applied conditions are constant or near constant. However, analyses of creep curve shapes for the nickel based superalloy Waspaloy, when applied stress and/or temperature vary greatly during testing, h...

Dwell fatigue effects at low temperatures in a range of titanium alloys have been well documented since the 1970s. However, attempts to model dwell fatigue phenomena are limited. The time dependent effects have been shown to be due to the inherent strain accumulation behaviour at ambient or low temperatures, loosely termed ‘cold creep’ in alloys su...

The fact that Gamma titanium aluminides (γ-TiAl) offer strong potential for replacing conventional titanium and nickel-base alloys in future gas turbine engine designs has resulted in the requirement to accurately describe the creep behaviour of these complex alloys. Consequently, the system Ti–45Al–2Mn–2Nb has been extensively studied under high t...

Within the gas turbine engine, the high transient thermal stresses resulting from throttle movement from idle to high settings give rise to the phenomenon of thermo-mechanical fatigue (TMF). These effects have been widely explored for turbine blade materials, typically single crystal nickel alloys. More recently however, a combination of thinner di...

Traditional creep lifing techniques based on power law equations have shown themselves to be extremely limited, particularly in the prediction of long term data based only on short term experiments. More recently, alternative approaches such as the Wilshire equations and hyperbolic tangent methods have been proposed which offer a new insight into t...

Recent work in the creep field has indicated that the traditional methodologies involving power law equations are not sufficient to describe wide ranging creep behaviour. More recent approaches such as the Wilshire equations however, have shown promise in a wide range of materials, particularly in extrapolation of short term results to long term pr...

Impressive data sets have been produced for 316H stainless steel (18Cr–12Ni–Mo with up to 0.08C) by the National Institute for Materials Science (NIMS), Japan, to reveal the dependencies on stress and temperature of the high-temperature creep and creep fracture behavior of nine batches of tube, six of bar and two of plate. Using these long-term pro...

Compared to conventional titanium alloys, gamma titanium aluminides (gamma-TiAl) offer highly desirable properties to the design engineer, with relatively high specific stiffness and high temperature capability. However, a significant drawback of this class of material is the low ductility at room temperature under monotonic tensile loading, which...

A critical review of recent creep studies concluded that traditional approaches such as steady-state behavior, power law equations, and the view that diffusional creep mechanisms are dominant at low stresses should be seriously reconsidered. Specifically, creep strain rate against time curves show that a decaying primary rate leads into an accelera...

The fatiigue properties of different microstructural variants of the titanium alloy Ti 6A1 4V have been shown to vary significantly, with attempts to correlate the data based purely upon optical measurements to describe the microstructure showing no discernable success. An alternative approach, where the effective structural unit size of the materi...

Compared to conventional titanium alloys, gamma titanium aluminides (gamma-TiAl) offer highly desirable properties to the design engineer, with relatively high specific stiffness and high temperature capability. However, a significant drawback of this class of material is the low ductility at room temperature under monotonic tensile loading, which...

Intermetallic materials such as gamma titanium alumindes (gamma-TiAl) offer an extremely attractive balance of properties. In support of potential engineering applications it is essential that the material properties are well understood across a wide temperature range. For applications where static loading will predominate at relatively high temper...

Variability in the mechanical properties of steel strip has a significant effect on the structural performance of manufactured components. This study provides a statistical analysis of the variability in yield strength, Ultimate Tensile Strength (UTS) and elongation to failure by examining the chemistry of the cast and rolling conditions of the ste...

The effect of crystallographic texture has been considered in attempting to derive a total life prediction capability for the titanium alloy Ti6-4. Orientation effects due to texture are shown to be a result of stress relaxation in strain control specimens, and consequently occur in some cases of notched specimens. It is shown that these effects ca...

Processing trials have produced a three-dimensional, thin-walled structure of representative aerospace component geometry, fabricated directly by laser melting of Ti 6Al4V powder. This additive-built form has been subjected to metallographic characterization. The fabrication technique is evaluated as an economic, commercial process that can add fea...

Processing trials have produced a three dimensional, thin-walled structure of representative aerospace component geometry, fabricated directly by laser melting of Ti 6Al4V powder. This additive-built form has been subjected to metallographic characterisation. The fabrication technique is evaluated as an economic, commercial process capable of addin...

Using new data analysis procedures, 100 000 h creep strengths are estimated by extrapolation of stress rupture values with creep lives <5000 h for Grade 22 tube as well as for annealed/ tempered and quenched/tempered plates. In addition to allowing accurate prediction of long term strengths, the resulting property sets can be discussed sensibly in...

The cut-edge characteristic properties of automotive structures formed during the mechanical blanking and laser-cutting processes significantly influence fatigue life performance. This factor is becoming increasingly important as S355MC and DP600 high strength steels (HSS) grades under investigation exhibit an increased sensitivity to fatigue crack...

Recent trials have produced tungsten-inert-gas (TIG)-welded structures of a suitable scale to allow an evaluation of the technique as an economic and commercial process for the manufacture of complex aeroengine components. The employment of TIG welding is shown to have specific advantages over alternative techniques based on metal inert gas (MIG) s...

The low and high temperature creep properties of the α+β titanium alloy Ti834 have been studied for various levels of applied plastic prestrain. Evidence is shown that the prestrain at room temperature causes the formation of quasi-cleavage facets in a manner described by the Evans–Bache model. These facets are also produced when the material is cr...

Understanding the role of grain boundaries has been impeded by continued adoption of power law equations for identification of the processes governing creep and creep fracture of metals and alloys. In contrast, the dominant mechanisms can be clarified through new relationships which involve normalisation of the applied stress by the yield and ultim...

The creep and creep rupture properties of Grade 23 (2.25Cr–1.6W) steels have been analysed for material produced in pipe (P23) and tube (T23) forms. The similarities and differences in the behaviour patterns observed for the pipe and tube products are then discussed in terms of the dislocation processes governing creep strain accumulation and the v...

Increased efficiency within the aero engine can be achieved through higher operating temperatures. In order to meet this requirement designers seek either to implement new alloys or show that existing alloys are capable of operating under more extreme conditions. At higher temperatures fatigue is no longer the sole damage mechanism and contribution...

The widely used titanium alloy, Ti6-4, is available in several different microstructural conditions, with variations being due to a number of factors including processing route and subsequent heat treatments. These differing microstructures can produce significant variations in the fatigue life of the material and as such it is essential that predi...

The paper focuses on CMSX-4 and two experimental alloys, LDSX-5 and LDSX-6, developed to provide alternative performance attributes. The specific objective in this work was an exploration of the low cycle fatigue (LCF) characteristics of these three alloy variants and the assessment of methods for predicting the observed lives. A comparison of the...

The effects of cold dwell on titanium alloys have been widely studied in recent years, with many alloys showing detrimental mechanical properties when dwell periods are introduced at peak load. Failures in such situations are often characterised by the formation of quasi-cleavage facets, and models have been suggested to explain the mechanism by wh...

To explain the seemingly complex results obtained when creep property measurements are described using power-law equations, it is commonly assumed that a transition from dislocation to diffusional creep processes occurs with decreasing applied stress. However, this assumption is negated by observations recorded for aluminium, copper and various par...

Practical experiences of service failures of components manufactured from HK40 (25Cr-20Ni-0.4C) are discussed in relation to the stress rupture behaviour identified using traditional and new approaches to creep property analysis. In particular, recently devised relationships allow reasonable prediction of 100,000h rupture strengths for centrifugall...

Improved understanding of the interactions of fatigue, creep and environmental damage mechanisms has enabled design engineers to extend the operating temperatures of existing alloys. The current paper investigates these effects in the titanium alloy Ti6246 in terms of the growth of fatigue cracks from notched specimen geometries and plain samples....

This paper focuses on the application of numerical models for predicting LCF initiation lives in notched Ti 6246 specimens tested in air at ambient and elevated temperatures. Model predictions are based on the results of un-notched strain control specimens and rely on the assumption that, for an equivalent stress–strain state, both notched and plan...

The paper explores the impact of texture in the titanium alloy Ti-6-4 on the fatigue response of plain and notched test-pieces. In particular it examines the behaviour of specimens orientated parallel (0 degrees) and perpendicular (90 degrees) to the rolling direction in uni-directionally rolled plate. The measurements clearly demonstrate the impor...

Manufacturing trials in support of shaped metal deposition (SMD) as a commercial process for the near net shape processing of aero engine components are reported. Initially, relatively simple multi-pass linear weld deposition beads employing the nickel-based polycrystalline superalloy Alloy 718 were characterized, to define the microstructural cond...

A methodology has been developed which is capable of predicting creep/fatigue crack growth rates at ambient and elevated temperatures in Ti 6246. Predictions are based on finite element analysis and strain-control testing of plain specimens. The prediction of fatigue crack growth rates for a given crack configuration and cyclic plastic zone size is...

Life prediction methods are essential in the selection of materials for high performance applications. These design criteria allow safe lives to be predicted for areas of geometrical discontinuity where localized increases in stress lead to the early initiation and propagation of fatigue cracks under cyclic loading. This paper explains two methods...

The paper derives from a major research programme on texture evolution and characterisation in the titanium alloys Ti-6Al-4V and Ti550. The present publication focuses on the mechanical characterisation of the texture in rolled plate. It focuses specifically on monotonic strengths and strain control fatigue under tension and torsion loading. Depend...