Development and application of 3D X-ray diffraction for the study of phase transformations in metallic materials

Ball, James A.D. ORCID: 0000-0001-5830-8268 (2024). Development and application of 3D X-ray diffraction for the study of phase transformations in metallic materials. University of Birmingham. Ph.D.

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Abstract

Many steel alloy types, both currently in use and under development, exploit a deformation-induced phase transformation to achieve a combined high strength and ductility. As deformation is applied to these alloys, a metastable retained austenite phase transforms to martensite. This process acts as a significant carrier of plasticity, increasing the work-hardening rate and therefore the ductility. The "stability", or resistance against martensitic transformation, of the austenite phase is the main parameter that governs the martensitic transformation rate and therefore the work-hardening behaviour of the steel. In the last few decades, the stability of an individual austenite grain has been shown to depend on a number of microstructural properties, such as the size of the grain, its orientation relative to the loading axis, the alloy chemistry, and the configuration of the grain's immediate crystallographic neighbourhood. A good understanding of how exactly these properties modify austenite grain stability is crucial to the development of accurate models of deformation-induced phenomena, which themselves directly contribute to the design of new and improved alloys that better exploit said phenomena.

In the past, austenite grain stability has usually been evaluated for a steel sample either through phase-averaged behaviour, where the stability of the phase overall is characterised, or on an individual grain level, where typically only a few grains are considered. This is primarily due to the difficulties involved with measuring the martensitic transformation in situ at a per-grain level for a large number of grains simultaneously. The recent development of far-field Three-Dimensional X-Ray Diffraction (3DXRD) has enabled such measurements on a range of polycrystalline materials, capturing the grain-level position, orientation and strain tensor for many thousands of grains in situ. However, the 3DXRD technique poses a number of significant challenges related to data analysis and post-processing, both crucial steps that must be carefully implemented to enable detailed measurements of complicated polycrystal samples.

In this study, 3DXRD was implemented at the I12 Joint Engineering, Environmental, and Processing (JEEP) Beamline at the Diamond Light Source X-ray synchrotron. Then, the capabilities of the technique were explored by examining how a microstructurally "simple" single phase ferritic steel responds to in-situ tensile deformation on a per-grain level. A number of micromechanical phenomena were investigated, including a small (but statistically significant) grain neighbourhood effect, where the stress state of a central grain was found to depend on the orientation of its immediate neighbourhood grains, a finding never before seen for large numbers of grains in a cubic polycrystal. During this 3DXRD implementation, a sophisticated automated data analysis and post-processing pipeline was developed, that enabled rapid exploration of such micromechanical effects.

With 3DXRD implemented and a data analysis pipeline developed, a novel metastable stainless steel alloy system was devised that enabled the exploration of the martensitic transformation at very low applied strains, as 3DXRD is typically limited to ~2% maximum strain. This alloy system was extensively characterised non-destructively in three dimensions with laboratory electron-based and X-ray based techniques, and was used to evaluate both the performance of multi-phase laboratory-based Diffraction Contrast Tomography (DCT), as well as a novel registration algorithm that accurately located two-dimensional planes measured with Electron Back-Scatter Diffraction (EBSD) within the three-dimensional DCT dataset.

Finally, the deformation response of the alloy was measured in-situ with 3DXRD at the ID11 beamline of the European Synchrotron Radiation Facility, coupled with in-situ EBSD scans using an in-chamber tensile stage. Substantial martensite transformations were found even within the ~2% maximum strain window, proving the alloy design was successful and enabling extensive in-situ analyses of austenite grain stability in the bulk material with 3DXRD. Austenite grain stability was found to be influenced by grain size, orientation, and local neighbourhood. Larger grains, grains oriented with {100} close to the loading axis, and grains with more ferrite/martensite-dense neighbourhoods were found to have reduced stability against deformation. The minimum strain work criterion model was also evaluated against the experimental data — it was found to correctly predict the orientation of martensite that formed in the majority of grains, given the parent orientation and macroscopic applied load. Grains where the model failed tended to have reduced levels of stress just before forming martensite, which was attributed to the use of the global stress state by the model as opposed to more granular measurements of the immediate stress field around a grain.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):

Supervisor(s)	Email	ORCID
Collins, David	UNSPECIFIED	orcid.org/0000-0003-2458-9713
Connolley, Thomas	UNSPECIFIED	orcid.org/0000-0002-1851-3467
Michalik, Stefan	UNSPECIFIED	orcid.org/0000-0001-9459-322X
Jenkins, Mike	UNSPECIFIED	UNSPECIFIED