M.T. Whittaker

Recent Advances in Creep Modelling of the Nickel Base Superalloy, Alloy 720Li

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 predictions. In the aerospace industry however, long term creep behaviour is not critical and more focus is required on the prediction of times to specific creep strains. The current paper illustrates the capability of the Wilshire equations to recreate full creep curves in a modern nickel superalloy. Furthermore, a finite-element model based on this method has been shown to accurately predict stress relaxation behaviour allowing more accurate component lifing.

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.

Long term creep life prediction for Grade 22 (2·25Cr—1Mo) steels

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 terms of the deformation and damage processes controlling creep and creep fracture.

Microstructural Characterization of a Prototype Titanium Alloy Structure Processed via Direct Laser Deposition (DLD)

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 features such as bosses or flanges as a hybrid-manufacturing route for existing forms of gas turbine components. The samples were extracted from six locations with different wall thickness, varying forms, and intersecting ligament geometries. A fine-scale Widmanstätten colony microstructure was consistent throughout the structure within grains elongated parallel to the axis of epitaxy. Evidence of limited grain boundary α was detected; however, this was never continuous around individual grains. A moderate Burgers texture was measured employing electron backscatter diffraction (EBSD), which is consistent with the melt/cast titanium alloy forms cooling through the β transus.

A Model for Creep and Creep Damage in the γ-Titanium Aluminide Ti-45Al-2Mn-2Nb

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 components, a good understanding of their creep properties is essential. Of particular importance to gas turbine component design is the ability to accurately predict the rate of accumulation of creep strain to ensure that excessive deformation does not occur during the component’s service life and to quantify the effects of creep on fatigue life. The theta (θ) projection technique is an illustrative example of a creep curve method which has, in this paper, been utilised to accurately represent the creep behaviour of the γ-TiAl alloy Ti -45Al-2Mn-2Nb. Furthermore, a continuum damage approach based on the θ-projection method has also been used to represent tertiary creep damage and accurately predict creep rupture.

Creep behaviour of Waspaloy under non-constant stress and temperature

Abstract 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, have shown that predictive methods based purely on strain, time or life fraction are insufficient and cannot predict the observed creep rates. This is important when considering stress concentration features where stress relaxation due to creep can significantly alter the distribution of stress and thus affect fatigue life. When both stress and temperature are changed during a creep test, dislocation movement must proceed through a dislocation network formed under different conditions, resulting in greater than expected creep rates. It is proposed that this is due to a reduction in effective internal stress due to changes in dislocation structure.

An Empirical Approach to Correlating Thermo-Mechanical Fatigue Behaviour of a Polycrystalline Ni-Base Superalloy

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, the difference between the behaviour of In-phase (IP) and −180° Out-Of-Phase (OOP) tests over a variety of applied strain ranges. It is shown that different lifing approaches are currently required for the two scenarios, with a mean stress based approach being more applicable in the first case, whereas a Basquin-type model proves more applicable in the second. However, it is also demonstrated that the crack propagation phase should also be considered in these types of tests for high strain ranges and projects that future modelling approaches should attempt to unify mean stress, stress range and a crack propagation phase.

Titanium in the Gas Turbine Engine

The development of the gas turbine engine over the past 60 years has been mirrored by the success of the titanium industry, with a clear symbiant relationship existing between the two industries. Immediately apparent in the early days of the evolution of the gas turbine was the need for a material which could provide the strength required for component operation, whilst at the same time providing a low enough density to allow for successful flight applications. Whilst aluminium based alloys offer an excellent strength to weight ratio, their operation is limited to temperatures below approximately 1300C, reducing possible applications within the gas turbine to a minimum. 300 series stainless steels offer a similar strength to most conventional titanium alloys, but come with a significant density penalty of over 50% and, whilst offering reductions in cost, do not provide significant benefits in terms of operating temperatures. Titanium however, has long been viewed as having a desirable balance of properties for applications towards the front end of the gas turbine engine (i.e. fan discs/blades, compressor discs/blades, along with other smaller components). Titanium has a density of 4.5g/cm3 (which, apart from a limited number of alloys such as Ti811, does not vary significantly in alloys considered for aerospace applications) which is higher than aluminium, but lower than nickel and steel alloys. Titanium is allotropic with a HCP lattice ( phase) stable to 8820C, transforming to a BCC (β phase) lattice above this temperature. Alloying elements act to stabilize either of these phases (Al, Sn for example stabilize the alpha phase, whereas Mo, V, Cr stabilize the beta phase) meaning that the transformation temperature can be altered, and subsequently the proportions of each phase existing at room temperature can be varied. The morphology of these phases may however vary, dependent on the process history, with alpha phase material being classed as primary alpha (persisting during heat treatment in the  phase field) or secondary alpha (structures arising from the → phase transformation). This allows for the development of a range of bimodal microstructures which provide titanium alloys with inherent strength and also allows for further refinement of properties through various heat treatment and processing regimes. For example designers requiring creep strength and good elevated temperature properties may choose to opt for alloys with more alpha stabilizers (alpha or near alpha alloys), whereas metastable beta alloys, which are heavily beta stabilized offer improved forgeability. Alphabeta alloys contain a more balanced mix of stabilizers and are widely used due their balance of properties. Ti6-4 (Ti-6Al-4V) for example has been a stalwart of the titanium industry since the 1950s due to its good weldability, relatively high strength and good fatigue properties.

Creep Deformation by Dislocation Movement in Waspaloy

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 performed above and below the yield stress. This paper highlights how the dislocation structures vary throughout creep and proposes a dislocation mechanism theory for creep in Waspaloy. Activation energies are calculated through approaches developed in the use of the recently formulated Wilshire Equations, and are found to differ above and below the yield stress. Low activation energies are found to be related to dislocation interaction with γ′ precipitates below the yield stress. However, significantly increased dislocation densities at stresses above yield cause an increase in the activation energy values as forest hardening becomes the primary mechanism controlling dislocation movement. It is proposed that the activation energy change is related to the stress increment provided by work hardening, as can be observed from Ti, Ni and steel results.

Non-invasive temperature measurement and control techniques under thermomechanical fatigue loading

Abstract 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 test piece. The accuracy of the technique is compared against type N thermocouples and a pyrometer for both temperature control and monitoring purposes. Diverse test specimen geometries and alloy compositions are used over a 100–700°C temperature range. Effects on temperature measurement accuracy such as thermocouple shadowing are highlighted and quantified. The non-invasive technique has proved accurate to within ±2°C of the reference thermocouples when in combination with the thermal paint coating.