Discriminating beta, alpha and alpha '' phases in metastable beta titanium alloys via segmentation: A combined electron backscattering diffraction and energy-dispersive X-ray spectroscopy approach

摘要

Annealed metastable beta titanium (Ti) alloys comprise body-centred-cubic beta and hexagonal-close-packed alpha phases and possibly, orthorhombic alpha '' martensite that forms on quenching or deformation. Electron backscattering diffraction is amongst the most popular methods for characterising such multi-phase microstructures. However, the crystallographic similarity between alpha and alpha '' martensite renders unambiguous discrimination of these phases via electron backscattering patterns (EBSPs) virtually impossible; thereby limiting the use of EBSD in characterising beta-Ti alloys. In this study, we demonstrate that alpha and alpha '' martensite are primarily misindexed due to an indiscernible difference between these phases along their [(1) over bar 10](alpha) and [010](alpha '') zone axes. Furthermore, the slight compositional difference between alpha and alpha '' is insufficient to discriminate these phases using on-the-fly energy-dispersive X-ray spectroscopy (EDS) spectrum matching. Consequently, a segmentation method was developed that relies on a combination of reindexed EBSPs and grain-median EDS elemental data to unambiguously discriminate beta, alpha and alpha '' martensite in metastable beta Ti alloys. All steps are implemented in an open-source and freely available computer program called phaseSegmenter that makes use of the MTEX toolbox in MATLAB. The program is readily applicable to Ti alloys containing alpha', alpha '' or massively transformed alpha as well as other phase transforming alloy systems with similar phase discrimination issues.