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Single crystal blade materials have found extensive use in high temperature service applications such as rotating aerofoils and static vanes in the gas turbine due to a favourable combination of mechanical properties including impressive high temperature fatigue, creep and environmental behaviour. Since internal grain boundaries are absent in single crystals, CMSX-4, the material of choice for many HP turbine applications, exhibits highly anisotropic mechanical properties, both in the elastic and plastic loading regimes. As such, recent research focused on the off-axis effects and multi-axial fatigue behaviour of the second generation Ni-based single crystal superalloy CMSX-4 as part of the TSB funded SAMULET programme (Strategic Affordable Manufacturing of UK Leading Environmental Technologies), with future studies scheduled under the SILOET II (Strategic Investment in Low-carbon Engine Technology) project.
Previous research undertaken at Swansea has detailed the effect of plain and notched fatigue on Ni-based single crystal alloys orientated in <001> direction. This work focussed on CMSX-4 and two experimental alloys, exploring their LCF characteristics whilst assessing the impact of internal casting porosity on fatigue life at numerous temperatures.
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Within the gas turbine engine, the high transient thermal stresses developed due to variations in power requirements during a typical flight cycle 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 reduced disc rim thickness and further increases in turbine entry temperature has led to a situation where TMF in poly-crystaline, nickel based superalloy, disc materials cannot be ignored. TMF is now regarded as one of the most significant challenges facing the gas turbine sector as designers strive to meet the demanding emissions and fuel burn ACARE 'Flightpath 2050' targets. Swansea UTC has devoted significant effort over the past decade in developing test facilities and techniques for the characterisation of TMF behaviour in a laboratory environment and research is currently taking place in the following fields:
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Two main topics of titanium research have developed International renown at Swansea UTC; fundamental understanding of “cold dwell sensitivity” in near-alpha titanium alloys and advanced lifing correlations supporting component design and operation.
Cold dwell behaviour was first acknowledged from the in-service failure of RB211 fan discs in the 1970s. Work based at Swansea UTC demonstrated that the combination of anisotropic crystal plasticity and stress redistribution was a pre-requisite for “quasi-cleavage facet formation” and dwell induced failures. A series of seminal academic papers, keynote presentations at International conferences resulted describing the “Evans-Bache” model. The application of electron back scattered diffraction for measuring facet inclination on fracture surfaces was pioneered at Swansea UTC and recent codes have endorsed automated quantitative tilt fractography. Ultimately, this led to the award of the IoM3 Harvey Flower Titanium Prize for “contribution to an improved understanding of titanium metallurgy or alloy development” to Prof WJ Evans in 2007 and Prof MR Bache in 2013.
Unrivalled testing facilities for the characterisation of constitutive behaviour have underpinned the development of advanced lifing algorithms applied to “Class A” safety critical titanium components. Research conducted under EPSRC grants along with Rolls-Royce private venture funding has married fundamental materials knowledge to non-linear, strain based lifing procedures applied to Ti6/4 fan disc materials employed across the fleet of Rolls-Royce Trent engines. Similar studies were completed on Ti 6246 compressor disc alloy. Such techniques encompassed intimate knowledge of creep-fatigue-environmental interactions gathered from precise, empirical experimentation in the Swansea laboratories. Fatigue crack initiation and crack propagation were predicted based only on deformation characteristics in the alloy.
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Vision 20 Materials and Advanced Manufactured Components
The requirements for advanced gas turbine jet engines continue to evolve in order to improve operational efficiency through weight reduction and increased operating temperatures. This has led to the development of new high temperature materials for these applications. However, for any material that is to be considered, it is necessary to acquire a fundamental understanding of the associated creep properties. To perform a traditional creep assessment of any novel material can be expensive due to the relatively large volume of material usually required, in addition to the restricted material availability which is a typical hindrance in alloy development. As such, this has led to the utilisation of several miniature characterisation techniques for creep assessment, and in particular, the small punch (SP) creep test.
The SP test is a miniaturised mechanical testing method that can be used to obtain both creep and fracture data from relatively small quantities of experimental material. Since the advent of the Code of Practice for Small Punch Testing in 2006, increased attention has been paid to show that laboratory test data produced through SP testing is comparable to that acquired from conventional uniaxial creep assessment. Current research concentrates on the potential for SP testing as an effective tool for ranking both creep and monotonic properties of novel alloy variants and complex geometries produced through advanced manufacturing methods such as electron beam melting (EBM), blown powder deposition and metal injection moulding (MIM) where anisotropic mechanical responses may be expected due to interactions between processing and microstructure evolution.
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Ceramic systems have long been considered as potential replacements for metallics for selected gas turbine applications due to their superior high temperature capabilities and significant reductions in density. The fundamental brittle nature of ceramics, however, does not support damage tolerant design and component lifing which is a major consideration for safety critical parts. The preferred approach adopted to improve damage resistance, crack growth and toughness of ceramics has been to process them in the form of ceramic matrix composites, i.e. embedding a second ceramic reinforcing phase within a ceramic matrix. Previous Swansea UTC research has assessed the mechanical performance of continuous fibre reinforced silicon carbide and alumina based materials at test temperatures up to 1400oC.
Ongoing research is now concentrating on novel test development to monitor damage accumulation in CMCs under static and cyclic loading, utilising digital imaging and automated potential drop techniques, forming part of a significant global strategy for the development of CMC components.
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We have equipment for standard mechanical property assessment whilst at the same time addressing bespoke forms of specimen or sub-element testing where novel test development is required.
Our laboratories offer an extensive range of static and dynamic test rigs.
Find out more about the facilities at the Institute of Structural Materials.
