Microstructure and texture evolution during thermomechanical processing of β-quenched Zr

The microstructure and texture evolution of an α-Zr alloy during thermomechanical processing was investigated, starting from the β-quenched microstructure. The material was rolled at 550 °C to reductions of 10, 20, 40 and 60%, and held at 550 °C for 24 h. EBSD was used to measure the texture at the different reductions and characterize the microstructural evolution, and crystal plasticity finite element modelling was used as a theoretical framework to help understand the changes in texture observed. Our results show that slip, twinning and recrystallization all play a role in the microstructure development during hot rolling. Their contribution to texture development, lamellae break-up and the ultimate development of a bimodal microstructure are discussed

Inspection of surface strain in materials using dense displacement fields

We have developed high density image processing techniques for finding the surface strain of an unprepared sample of material from a sequence of images taken during the application of force from a test rig. Not all motion detection algorithms have suitable functional characteristics for this task, as image sequences are characterised by both short- and long-range displacements, non-rigid deformations, as well as a low signal-to-noise ratio and methodological artefacts. We show how a probability-based motion detection algorithm can be used as a high confidence estimator of the strain tensor characterising the deformation of the material. An important issue discussed is how to minimise the number of image brightness differences that need to be calculated. We give results from three studies: mild steel under axial tension, the formation of kink bands in compressed carbon-fibre composite, and non-homogeneous strain fields in a welded aluminium alloy. Because the algorithm offers increased accuracy near motion contrast boundaries, its application has resulted in new mesomechanical observations

The effect of loading direction on strain localisation in wire arc additively manufactured Ti–6Al–4V

Ti–6Al–4V microstructures produced by high deposition rate Wire Arc Additive Manufacturing (WAAM) can be both heterogeneous and anisotropic. Key features of the as-built microstructures include; large columnar ß grains, an α transformation texture inherited from the β solidification texture, grain boundary (GB) α colonies, and Heat Affected Zone (HAZ) banding. The effect of this heterogeneity on the local strain distribution has been investigated using Digital Image Correlation (DIC) in samples loaded in tension; parallel (WD), perpendicular (ND) and at 45° (45ND) to the deposited layers. Full-field surface strain maps were correlated to the underlying local texture. It is shown that loading perpendicular to the columnar β grains leads to a diffuse heterogeneous deformation distribution, due to the presence of regions containing hard, and soft, α microtextures within different parent β grains. The ‘soft’ regions correlated to multi-variant α colonies that did not contain a hard α variant unfavourably orientated for basal or prismatic slip. Far more severe strain localisation was seen in 45° ND loading at ‘soft’ β grain boundaries, where single variant α GB colonies favourably orientated for slip had developed during transformation. In comparison, when loaded parallel to the columnar ß grains, the strain distribution was relatively homogeneous and the HAZ bands did not show any obvious influence on strain localisation at the deposit layer-scale. However, when using high-resolution DIC, as well as more intense shear bands being resolved at the β grain boundaries during 45° ND loading, microscale strain localisation was observed in HAZ bands below the yield point within the thin white-etching α colony layer

Multi-dimensional study of the effect of early slip activity on fatigue crack initiation in a near-α titanium alloy

During service of gas turbine engines, high cycle fatigue of titanium is a leading cause of component failure highlighting the need for better understanding of the crack initiation mechanism to predict initiation sites. In this study, the relationship between plastic slip activity and fatigue crack initiation was investigated in a near-α titanium alloy using cyclic four-point bending at up to 90% of the proof stress. The finding from surface characterization show that plasticity at such low stress levels was dominated by the basal slip and two types of cracking were seen parallel to basal slip traces. Detailed 3D analysis of both crack types highlighted out-of-plane Burgers vector activity for the observed basal slip associated with crack initiation, consistent with the classic surface roughening mechanism. The transgranular crack initiation was accompanied by the formation of crystallographic facet which was identified to be 6° away from the basal plane due to additional prismatic slip activation during multi-step crack formation. The intergranular crack facet along the boundary between primary α grain pairs, which have their c-axes aligned nearly parallel to each other but with mis-aligned prismatic planes, was formed by an easy cleavage in one step along the basal plane. Statistical evaluation demonstrated that grains combining a moderately high Schmid factor for basal slip, high resolved tensile stress along the c-axis and the Burgers vector being oriented strongly out-of-surface plane favoured transgranular crack initiation. Based on those observations a new parameter involving these three geometrical factors was developed to predict transgranular surface crack initiation sites

JQFonseca/hrdic_az31: AZ31 HRDIC data visualization notebook

Latest version, updating the reference to the published article