Slip Band Characterisation in Ti‐6Al‐4V with Varying Degrees of Macrozones

Abstract

Electron Backscatter Diffraction (EBSD) orientation maps and Digital Image Correlation (DIC) was combined to analyse slip trace formation of Ti-6Al-4V with varying degrees of microstructural inhomogeneity. The focuses of the study are three products forms of Ti-6Al-4V that are strong-macrozone, intermediate-macrozone and no-macrozone material that were loaded to study the impact of the primary macrozone orientation on strain localisation. Strain localisation was characterised at the nanoscale by a applying a fine speckle pattern to the sample surface using the Gold remodelling technique. All materials exhibited a proportion of grains that exhibited high intensity planar slip traces, neighbouring regions within the same grain showed little deformation. In the no-macrozone material the majority of grains deformed but the level of transgranular strain remained relatively constant across the material. For the intermediate-macrozone condition, macrozones with their caxis parallel to the loading direction resulted in grains that did not deform and the neighbouring grains that had favourable orientations for easy slip showed highly planar strain slip traces. The strain map of the strong-macrozone condition loaded at 45 to the extrusion direction (ED) showed that a pronounced high strain band developed in the macrozone region. The primary crystallographic orientation of the macrozone had the c-axis perpendicular to the loading direction and the likely activated slip system was prismatic slip. There were higher overall transgranular strains across the macrozone region compared to the neighbouring region, which displays greater misorientation between grains. The 45 loading direction of the strong-macrozone condition produced finer slip trace spacing and more intense strain heterogeneity and increased localised deformation than the no-macrozone condition

Effect of nanoscale α2 precipitation on strain localisation in a two-phase Ti-alloy

“…When comparing maximum shear strain to average macroscopic strain the ratio reached in the α 2 -containing sample is about 20, while in the α 2 free sample this ratio is only 11-12. Previous studies using HR-DIC with a similar spatial resolution in austenitic stainless steel [27] and titanium [32] have shown peak-to-applied strain ratios of ≈10, which is in agreement with the observations for the α 2 -free sample. However, the higher peak-to-applied strain ratio observed in the sample with α 2 precipitates is likely to be a result of increased slip planarity and shearing associated with the precipitates [5]- [7].…”

“…To perform a correct crosscorrelation each subset must show unique features, therefore a distinctive pattern is applied to the surface of the specimen. In case of high spatial resolution strain mapping at a scale that enables the analysis of slip traces, vapourassisted remodelling of deposited gold has proven to be a highly effective pattern-forming methodology [27], [30]- [32]. The two main advantages of this method are the simplicity of creating non-repetitive but homogenous nanoscale patterns over large areas compared to other techniques (i.e.…”

“…lithography or ion-beam assisted deposition) and the high spatial resolution of the 2D displacement and strain maps that can be achieved from the nano-sized speckles [27]. It has been demonstrated that this method provides detailed strain information, as single slip traces can be resolved and can be compared quantitatively to assess the level of strain heterogeneity between samples, slip systems and single grains [27], [32], [33].…”

Microstructure and properties of electrically assisted stationary shoulder friction stir welded Ti6Al4V

“…Kernel average misorientation (KAM) diagram in Figure 9 calculates the average misorientation between every pixel and its surrounding pixels, and assigns the mean value to that pixel, for displaying small orientation changes on the map, highlighting regions of higher deformation with pixels having higher value [36], and qualitatively reflecting the degree of homogenisation of plastic deformation, which is not homogeneously distributed in the all the welded joints as indicated by scattering orientation colours with peak value [37]. The higher density of local misorientation by accumulating pixel colours was observed in the macrozone within the HAZ at both the AS and the RS of all the joints with more intense strain heterogeneity (Figure 10) [38], this meant that the material stress distribution during deformation and dislocation densities are both higher in the macrozones compared to the non-macrozones [39, 40]. It is also seen that the overall density of local misorientations decreased slightly with the increasing current from the BM to the FSW joint from BM to 0A and 300A then decreased to 400A-ESSFSW joint indicating increasingly homogenous deformation.…”

Investigation of the influence of macrozones in titanium alloys on the propagation and scattering of ultrasound

“…Cases 1 and 2 both have the same elongated macrozones but due to them having a different C-axis orientation, the attenuation and backscatter responses vary significantly. It is crucial to note that the macrozones are normally generated through deformation processes such as rolling, and the preferred C-axis orientations usually follow the deformation and grain elongation direction [55]. As such, cases 1 and 3 are more common than case 2.…”