Effective Structural Unit Size in Polycrystals: Formation, Quantification and Micromechanical Behaviour
Lead Research Organisation:
University of Manchester
Department Name: Materials
Abstract
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.
People |
ORCID iD |
Michael Preuss (Principal Investigator) |
Publications
Babout L
(2014)
3D characterization of trans- and inter-lamellar fatigue crack in (a + ß) Ti alloy
in Materials Characterization
Birosca S
(2009)
Three-dimensional characterization of fatigue cracks in Ti-6246 using X-ray tomography and electron backscatter diffraction
in Acta Materialia
Birosca S
(2011)
3-D observations of short fatigue crack interaction with la2mellar and duplex microstructures in a two-phase titanium alloy
in Acta Materialia
Britton T
(2010)
Electron backscatter diffraction study of dislocation content of a macrozone in hot-rolled Ti-6Al-4V alloy
in Scripta Materialia
Leo Prakash D
(2013)
The effect of ß phase on microstructure and texture evolution during thermomechanical processing of a + ß Ti alloy
in Acta Materialia
Obasi G
(2013)
The effect of ß grain coarsening on variant selection and texture evolution in a near-ß Ti alloy
in Materials Science and Engineering: A
Obasi G
(2012)
The influence of rolling temperature on texture evolution and variant selection during a ? ß ? a phase transformation in Ti-6Al-4V
in Acta Materialia
Obasi G
(2012)
Effect of ß grain growth on variant selection and texture memory effect during a?ß?a phase transformation in Ti-6 Al-4 V
in Acta Materialia
Obasi G
(2012)
In situ neutron diffraction study of texture evolution and variant selection during the a ? ß ? a phase transformation in Ti-6Al-4V
in Acta Materialia
Description | The Manchester contribution to the project covered two aspects. Firstly, x-ray tomography was used to study the effect of microstructure on crack propagation in two-phase Ti-alloys. It was found that the crack shape is greatly affected by the microstructure of the material as well as the crack rate. Key microstructure parameters were identified that affect crack bifurcation. The second contribution in the project was related to variant selection during phase transformation in Ti alloys. Here, the project focused on the effect of beta grain growth on the formation of large alpha colonies when the material is cooled to room temperature. It was found that beta grain coarsening has a very large effect on the formation of a very homogeneous microstructure caused by variant selection at the beta grain boundary. It was also found that this mechanism becomes more dominant for certain rolling temperatures. |
Exploitation Route | Understanding the formation of heterogeneous microstructures in Ti components is very important as it will result in performance variation. the aerospace industry is very interested in reduced performance scatter and the findings of the present work clearly help them to identify key processing parameters to achieve this. We have been in discussion with TIMET UK to further fund a post doc position in Manchester to explore the effect of rolling temperature on variant selection and the formation of a heterogeneous microstructure. |
Sectors | Energy Transport |
Description | The results have been used continuously by researchers, industrial partners and the supply chain to improve Ti based products and prediction of performance. |
Sector | Aerospace, Defence and Marine,Energy,Transport |
Impact Types | Societal Economic |
Description | Rolls-Royce Plc |
Amount | £45,000 (GBP) |
Funding ID | co-funding of DTA studentship |
Organisation | Rolls Royce Group Plc |
Sector | Private |
Country | United Kingdom |
Start |
Description | Rolls-Royce Plc |
Amount | £30,000 (GBP) |
Funding ID | top up funding for CDT advanced metallic systems studentship |
Organisation | Rolls Royce Group Plc |
Sector | Private |
Country | United Kingdom |
Start |
Description | Timet UK Ltd |
Amount | £21,000 (GBP) |
Funding ID | top up for LATEST-2 PhD studentship |
Organisation | Timet UK Ltd |
Sector | Private |
Country | United Kingdom |
Start |
Description | Rolls-Royce plc |
Organisation | Rolls Royce Group Plc |
Country | United Kingdom |
Sector | Private |
Start Year | 2007 |
Description | TIMET |
Organisation | Timet UK Ltd |
Country | United Kingdom |
Sector | Private |
PI Contribution | Contribution to R&D |
Collaborator Contribution | fund PhD students and provide material |
Impact | better understanding of processing of Ti alloys |
Start Year | 2010 |