Institute of Structural Materials

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, undergoes significant temperature variation in service and is therefore often subject to thermo-mechanical fatigue (TMF) damage. Therefore more recent research projects have sought to investigate the TMF behaviour in the alloy, specifically investigating lifing methodologies, the effect of phase angle and the effect of peak cycle temperature as part of the Innovate UK funded project Materials And Lifing Improvements in Turbines (MALIT) exhibits highly anisotropic mechanical properties, both in the elastic and plastic loading regimes.

Historical programmes at Swansea have also focused on the off-axis effects and multi-axial fatigue behaviour of CMSX-4 along with assessing the impact of internal casting porosity on fatigue life at numerous temperatures.

Smith, R., Lancaster, R., Jones, J., & Mason-Flucke, J. (2019). Lifing the Effects of Crystallographic Orientation on the Thermo-Mechanical Fatigue Behaviour of a Single-Crystal Superalloy. Materials, 12(6), 998.

Lancaster, R. & Jeffs, S. (2015). Creep lifing methodologies applied to a single crystal superalloy by use of small scale test techniques. Materials Science and Engineering: A, 636, 529-535.

Lancaster, R. & Jeffs, S. (2015). Elevated Temperature Creep Deformation of a Single Crystal Superalloy through the Small Punch Creep Method. Materials Science & Engineering A, 626, 330-337.

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-crystalline, nickel based superalloy, disc materials cannot be ignored and also require consideration. 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 can justifiably claim to be an internationally leading research facility in the field of TMF. Research programmes have produced extensive experiments supporting lifing methodologies in both single crystals and polycrystals, resulting in significant cost avoidance to Rolls Royce (link to REF Impact Case study)

TMF research topics

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 the ISM 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 and keynote presentations at international conferences resulted which put forward the “Evans-Bache” model. The application of electron back scattered diffraction for measuring facet inclination on fracture surfaces was pioneered at the ISM in the early and 1990s 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.

Further reading:

• Bache M.R., Li, J., Davies H.M. (2024) A re-assessment of Ti-685 as a dwell sensitive titanium alloy and a definition for engineering relevant dwell behaviour. International Journal of Fatigue Volume 178.
• J. Li, H.M. Davies , K. Fox , M. Mulyadi , M.G. Glavicic , M.R. Bache. (2023). An appraisal of dwell sensitive fatigue in Ti-6Al-4V and the governing role of inhomogeneous micro-texture. International Journal of Fatigue Volume 171.
• Bache, M., Davies, H., Davey, W., Thomas, M. and Berment-Parr, I. (2019). Microstructural Control of Fatigue Behaviour in a Novel α + β Titanium Alloy. Metals 2019, 9(11), 200.
• Davey, W., Bache, M., Davies, H. and Thomas, M. (2018). Fatigue Performance of the Novel Titanium Alloy Timetal 407. MATEC Web Conf. Volume 165.
• Whittaker, M.T., Harrison, W., Lancaster, R.J., Williams, S. (2013). An analysis of modern creep lifing methodologies in the titanium alloy Ti6-4. Materials Science & Engineering A 577 (2013) 114-119. 
• Harrison, W.J., Whittaker, M.T., Lancaster, R.J. (2013). A Model for Time Dependent Strain Accumulation and Damage at Low Temperatures in Ti-6Al-4V. Materials Science & Engineering A, 574, 130-136.
• Whittaker, M.T., Evans, W.J., Lancaster, R.J., Harrison, W., Webster, P.S. (2009). The effect of microstructure and texture on mechanical properties of Ti6-4. International Journal of Fatigue 31, Issues 11-12, 2022-2030.
• Bache, M.R. (2003). A review of dwell sensitive fatigue in titanium alloys: the role of microstructure, texture and operating conditions. Int. J. Fatigue, 25, pp.1079-1087.
• Evans W.J. and Bache, M.R. (1994). Dwell-sensitive fatigue under biaxial loads in the near alpha titanium alloy IMI685. Int. J. Fatigue, 16, pp.443-45
  
  

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.

Further reading:

• G. Norton, M.R. Bache, R. Hurst “Small Punch Creep Testing Applied to a Brittle Intermetallic Material”. Proceedings of the 2^nd International Conference on Small Scale Test Techniques, October 2-4^th, 2012, Ostrava, Czech Republic.
• R.C. Hurst, R.J. Lancaster, G. Norton, R. Banik, M.R. Bache “A Renaissance in Small Punch Testing at Swansea University”. Proceedings of the IX Baltica Conference on Fast tools for condition and life assessment of power plants, June 11-13^th 2013, Helsinki, Finland.

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. The ISM research has assessed the mechanical performance of continuous fibre reinforced silicon carbide and alumina based materials at test temperatures up to 1400°C.

Ongoing research is concentrated on novel test development to monitor damage accumulation in CMCs under static and cyclic loading, utilising techniques such as digital imaging, acoustic emission, electrical resistance and X-ray computed tomography. In addition, condition assessment methods are being investigated through methods including vibration and microwave spectroscopy. All of which form part of a significant global strategy for the development and implementation of CMC components.

Further reading:

• Quiney, Z., Jeffs, S.P., Gale, L., Pattison, S. and Bache, M.R. (2023). Matrix cracking onset stress and strain as a function of temperature, and characterisation of damage modes in SiCf/SiC ceramic matrix composites via acoustic emission. J Eur Ceram Soc, vol. 43, no. 7, pp. 2958–2967.
• Newton, C.D., Jeffs, S.P., Gale, L., Pattison,S. and Bache, M.R. (2023). Determining the interlaminar tensile strength of a SiCf/SiC ceramic matrix composite through diametrical compression testing. J Eur Ceram Soc, vol. 43, no. 7, pp. 2951–2957.
• Quney, Z., Weston, E., Nicholson, P.I., Pattison, S. and Bache, M.R. (2020). Volumetric assessment of fatigue damage in a SiCf/SiC ceramic matrix composite via in situ X-ray computed tomography. J Eur Ceram Soc, vol. 40, no. 11, pp. 3788-3794.
• Bache, M.R., Newton, C.D., Jones, J.P, Pattison, S., Gale, L., Nicholson, P.I. and Weston, E. (2019). Advances in Damage Monitoring Techniques for the Detection of Failure in SiCf/SiC Ceramic Matrix Composites. Ceramics, 2, pp. 347-371.
• Di Salvo, D.T., Sackett, E.E., Johnston, R.E., Thompson, D., Andrews, P., Bache, M.R. (2015). Mechanical characterisation of a fibre reinforced oxide/oxide ceramic matrix composite. J Eur Ceram Soc, vol. 35, no. 16, pp. 4513-4520.
• Wilshire, B. and Bache, M.R. (2008). Creep of monolithic and fibre-reinforced silicon carbide. J Eur Ceram Soc, vol. 28, no. 7, pp. 1535-1542.
• Wilshire, B. and Bache, M.R. (2007). Creep damage resistance of ceramic-matrix-composites. J. Eur. Ceram Soc, vol. 27, no.16, pp. 4603-4611.

The Small Punch (SP) and Shear Punch (ShP) tests are well established mechanical test approaches that have found application in several industrial sectors for material ranking and mechanical property estimation, particularly where more conventional approaches are inhibited. Such methods are receiving increased attention from engineers who recognise the significant cost benefits that these test methodologies have to offer in terms of far less material wastage for specimen manufacture. In addition, test specimens for SP and ShP experiments can be extracted from larger components to determine a localised distribution of properties or for remnant life assessment of service parts, benefits that may be restricted when looking to employ larger scale test pieces.

The team in the ISM were some of the key contributors to the standardisation of the SP test, which was standardised by ISO and ASTM in 2020. The test method can be used to obtain creep rupture lives and tensile fracture data for a wide variety of different material systems. More recently fatigue live estimations have been approximated from the test, enabling a new collection of researchers to adopt such test methodologies. Indeed, the number of publications relating to small scale testing research is increasing every year, with numerous worldwide laboratories now utilising the SP test for material characterisation for all manner of purposes.

Key Publications:

Evaluating the efficacy of alternative small scale test methodologies in deriving the mechanical properties of additive manufactured materials. Lancaster, R.J., Barnard, N.C., Haigh, B., Sackett, E., May, P.E., Douglas, R., Britton, D., Jeffs, S.P. Journal of Materials Research and Technology, 2023, 26, pp. 9328–9345

Investigation of the strain rate sensitivity of CoCrFeMnNiTix (x = 0, 0.3) high-entropy alloys using the shear punch test. González, S., Sfikas,. A.K., Kamnis, S., John, S.E., Barnard, N.C., Gammer, C., Eckert J., Garay-Reyes C.G., Martínez-Sánchez R.,Naung S.W., Rahmati M., Keil T., Durst, K., Lancaster, R.J. Materials and Design, 2023, 233, 112294

Computational and experimental investigation of the strain rate sensitivity of small punch testing of the high-entropy alloy CoCrFeMnNi. González, S., Sfikas, A.K., Kamnis, S., John S.E., Nye Z.W., Spink M., Allen C., Martínez-Sánchez R., Naung S.W., Rahmati M., Keil T., Durst K., Lancaster R.J. Journal of Alloys and Compounds, 2023, 936, 168219

Derivation of material properties using small punch and shear punch test methods. Lancaster, R.J., Jeffs, S.P., Haigh, B.J., Barnard, N.C. Materials and Design, 2022,215

Characterising the fatigue performance of additive materials using the small punch test. Lewis, D.T.S., Lancaster, R.J., Jeffs, S.P., Illsley, H.W., Davies, S.J., Baxter, G.J. Materials Science & Engineering A, 2019, 754, 719-727

Development of a novel methodology to study fatigue properties using the small punch test. Lancaster, R.J., Jeffs, S.P., Illsley, H.W., Agryakis, C., Hurst, R.C., Baxter, G.J. Materials Science & Engineering A, 2019, 748, 21-29

Creep strength and minimum strain rate estimation from Small Punch Creep tests. Holmström S., Li Y., Dymacek P., Vacchieri E., Jeffs S.P., Lancaster R.J., Omacht D., Kubon Z., Anelli E., Rantala J., Tonti A., Komazaki S. Materials Science & Engineering A, 2018, 731, 161-172

The contribution of small punch testing towards the development of materials for aero-engine applications. Hurst, R.C., Lancaster, R.J., Jeffs, S.P., Bache, M.R. Theoretical and Applied Fracture Mechanics, 2016, 86, 69-77

An analysis of small punch creep behaviour in the γ titanium aluminide Ti-45Al-2Mn-2Nb. Lancaster, R.J., Harrison, W.J., Norton, G. Materials Science & Engineering A, 2015, 626, 263-274

Creep lifing methodologies applied to a single crystal superalloy by use of small scale test techniques. Jeffs, S.P., Lancaster, R.J., Garcia, T.E. Materials Science & Engineering A, 2015, 636, 529-535