Dislocation-Microstructure Interaction at a Crack Tip - In Search of a Driving Force for Short Crack Growth

Nickel-based superalloys are particularly used in applications involving high temperatures and stresses, such as the critical gas-turbine blades and discs in aerospace and power-generation industries. The behaviour of short cracks in nickel superalloys is of particular importance for component design and life prediction, as a large proportion of service life is spent in the growth of small cracks before final failure. Due to the strong influence of local microstructure and heterogeneous stress/strain fields, short cracks are known to grow anomalously under fatigue and tend to exhibit high, irregular and scattered growth rates. The physical driving force for short crack growth is still not well understood yet despite intensive research effort, mainly due to the limited understanding of crack-tip behaviour.

This proposal aims to investigate the fundamental deformation mechanism at the tip of a short crack for nickel-based superalloys under fatigue at a range of temperatures. The research will focus on the influence of evolving local plasticity, induced by dislocation dynamics at the crack tip, on short crack growth. The interaction between dislocation and material microstructure is the major source for heterogeneous plasticity and internal stress concentration, leading to initiation and growth of short cracks. Short crack growth testing in a controlled environment will be carried out to study the anomalous behaviour of short crack growth in these alloys under fatigue, which is the expertise of UoS. Temperature will be varied in order to observe the critical effect of temperature change on the slip behaviour near the crack tip. Following crack growth tests, post-mortem transmission-electron-microscopy analyses of crack-tip zone will be performed to reveal the detailed mechanisms for nucleation and multiplication of dislocations, pile-up and penetration of dislocations at phase/grain boundaries and the influence of grain misorientations on dislocation behaviour. In particular, match-stick samples will be extracted from the crack-tip fracture process zone of fatigue-tested specimens to allow in-situ measurements of crack tip deformation under fatigue, which are the established techniques at UoM. In this case, high resolution digital image correlation, with the assistance of grain orientation mapping and scanning-electron-microscopy imaging of gold remodelled surfaces, will be used to quantify shear strain in slip traces formed near the crack tip during fatigue loading. In addition, high energy synchrotron X-ray diffraction studies will be carried out to measure the elastic strain response and load transfer between different phases around the crack tip, which will provide insight regarding the penetration of dislocations into the gamma-prime precipitates.

To physically simulate the material plasticity behaviour, a three-dimensional discrete-dislocation-dynamics (DDD) approach will be developed to model the interaction between dislocations and material microstructures, which is the strength of LU, based on experimental results. The DDD model will be interfaced with viscoplasticity and crystal plasticity models, and further applied to investigate the role of dislocation dynamics in depicting short crack growth. A multi-scale finite element method will be established for the crack-tip deformation analyses, which aims to identify a micromechanics-based driving force for short crack growth. Computational simulations will be thoroughly validated against local strain measurements (at both mesoscale and microscale), in-situ and post-mortem measurements as well as X-ray tomography of extracted match-stick samples. The ultimate goal is to deliver an efficient finite element procedure to predict short crack growth, with full validation against the experimental data, for end users.

The research will have a direct impact on power generation and aero engine industries, as it addresses the fatigue behaviour of high temperature materials, a critical issue in the performance of gas turbine systems. Nickel-base superalloys, the materials studied in this proposal, are an important class of high temperature materials, and currently irreplaceable for application as gas turbine discs and blades. In response to fast growing energy demand and climate change concerns, gas turbine industries are striving to implement even higher operating temperature and longer maintenance intervals to produce power and energy systems with high efficiency. This can only be achieved through improved understanding of materials behaviour and development of accurate models for fatigue design and life prediction of modern gas turbine systems, which will be delivered by this research.

Through close collaboration with our project partners in energy and aerospace sectors (Alstom, Rolls-Royce and Dstl), exploitation of the research outcomes will be carried out in terms of optimisation of service conditions and material microstructures to achieve a maximal service life. This will contribute to fatigue design and life management of critical gas turbine components, with ensured structural integrity and safety, under service conditions. Furthermore, the developments of materials and fatigue life models will allow industries to use "numerical experiments" in product development wherever possible, and save costs by reducing the number of expensive, risky and time-consuming experimental tests. In addition, all three institutions have extensive collaborative research networks and experienced knowledge transfer services, which will be fully utilised by our research team to further promote the impact.

This research will deliver new scientific findings, particularly the physical measurements of micro-deformation near a crack tip and the predictive modelling tools for short crack growth under fatigue conditions. This will strengthen the international competitiveness of UK research in advanced metals and alloys, and also create impact in research communities worldwide who are working on high temperature materials. The underlying generic outcomes, delivered by this research, also provide benefit to researchers in mathematics, physics, manufacturing, petrochemical, biomedical and nuclear engineering. To maximise the impact across disciplines, publications and presentations will be sought for a range of journals, conferences, seminars and workshops, both nationally and internationally. In addition, the Southampton Heterogeneous Data Centre will be used to archive all research data and findings generated from this research for easy access by wider researchers and audiences.

Through this research programme, the three post-doctoral research associates (PDRAs), as well as the externally funded PhDs, will equip themselves with advanced knowledge, skills and experience in fatigue, characterisation and numerical modelling. This will provide a remedy to the critical shortage of engineering skills in the UK. The multi-institutional research project will also provide an important platform for the investigators to further establish their expertise in respective research fields, and develop and expand their leadership roles.

The research has a direct relevance to power generation and air travel, and can easily engage wider public audiences. This activity will be mainly led by UoS who will design specific, further engagement programmes for this research, with all researchers being involved. Significant findings and developments will be summarised and published in a timely manner on our project website which will be set up from the beginning and continuously updated throughout the project period. In addition, public engagement with this research programme will be promoted using the school visit and open day opportunities at each institution.