Effective Structural Unit Size in Polycrystals: Formation, Quantification and Micromechanical Behaviour

Lead Research Organisation: Imperial College London
Department Name: Materials


The concept of grain size playing an important role in the engineering application of polycrystalline metals is well established. During casting and subsequent wrought processing, tried and tested methods are used to refine grain size in order to enhance ductility and increase tensile, yield and fatigue strengths. The advent of electron microscopy based experimental techniques such as electron back scatter diffraction (EBSD) and focussed ion beam (FIB) plus nano-indentation have provided novel, intriguing insights into the deeper aspects of both structural evolution and structure / property relationships. This has included preliminary identification of the critical role of effective structural unit size (rather than grain size) in determining mechanical behaviour. However, understanding of the the relationship between processing and effective structural unit size remains in its infancy for most systems. Consequently, significant progress can now be made in understanding the evolution of structures including recrystallisation processes and variant selection during phase transformation. This offers the potential of refining the structure of a wide range of engineering materials for which phase transformation plays an important role during processing such as steel, titanium, zirconium etc. The fatigue process is very complex but can be simplified conceptually into initiation and crack growth. For high cycle fatigue (HCF) regimes where the number of applied stress cycles can easily exceed 10,000,000 material evaluation relies on specimen or component testing. The majority of the HCF life is spent initiating a defect that then grows rapidly to failure. For materials subject to such HCF regimes, the design principle is to stay below an empirically defined endurance stress so that initiation is prevented. For low cycle fatigue (LCF) the situation is different in that initiation life and growth life can both be used to predict a safe component life. Typically, initiation is again determined empirically by mechanical testing. The current inability to predict fatigue initiation from basic principles stems from the fact that crack initiation is dominated by interactions from grain to grain which are inherently difficult to quantify and to model. Thus, for significant end user applications, the engineer has minimal knowledge defining what aspects of a material, or its processing, influence its performance other than by mechanical testing, which is very time consuming and expensive.Considerable scientific exploration of fatigue has until recently largely failed to assist the material producer and end user in other important ways. In the specific case of the titanium-based alloys, the definition of grain boundaries and subsequent measurement of grain size are notoriously difficult through optical inspection alone. The existence of large colonies of similarly orientated crystallographic units can encourage extensive planar slip structures to develop. In turn, through a process of stress redistribution between relatively weak and strong units , this can have a potentially disastrous effect on component performance. Key issues which determine mechanical properties of interest to the end user include:a) How boundaries behave and what constitutes a boundary for a given load regime.b) Factors in processing and heat treatment that dictate effective structural unit size.c) Modelling capability to provide quantitative predictions of mechanical behaviour including HCF initiation and short crack growth rates.All of these issues form the basis of the current proposal for research.


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Description This grant provided key information on how Ti microstructures deform, which has been of wide benefit for the polycrystalline modelling of these materials.

It also provided insight into how titanium alloy microstructures with a greater variety of crystal orientations might be produced, which should enable materials with better fatigue resistance to be developed.
Exploitation Route To understand how jet engine titanium alloys deform, particularly around crack tips, and therefore to improved understanding of service limits, and an ability to engineer closer to these and to build more efficient jet engines. In addition, to engineer better materials with enhanced fatigue performance.
Sectors Aerospace, Defence and Marine

Description The work has been of great use in understanding how texture evolves during the thermomechanical processing of titanium fan blade materials, and how microstrains develop in both commercially pure Ti and Ti-6Al-4V. This has been used by gas turbine manufacturers (Rolls-Royce) and titanium manufacturers (Timet) to improve the performance of the materials they put into service. In particular an alluring result was obtained relating to the variant selection of secondary alpha, which may enable the development of improved alloys.
First Year Of Impact 2011
Sector Aerospace, Defence and Marine
Impact Types Economic

Description EPSRC Leadership
Amount £1,151,930 (GBP)
Funding ID EP/H004882/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 04/2010 
End 03/2015
Description EPSRC Programme Grant (Hexmat)
Amount £4,979,741 (GBP)
Funding ID EP/K034332/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 05/2013 
End 05/2018
Description EPSRC Programme grant (DARE)
Amount £3,226,486 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 09/2014 
End 09/2019
Description Rolls-Royce plc 
Organisation Rolls Royce Group Plc
Country United Kingdom 
Sector Private 
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