Application of directional reflectance microscopy in in-situ observation of deformation behaviour of pure aluminium

Abstract

Surface strain distribution plays an important role in the study of deformation mechanisms of materials. As digital image correlation (DIC) and electron backscattered diffraction (EBSD) has their own strengths and limitations, both often need to be incorporated together in order to have the simultaneous study of the deformation of materials macroscopically and microscopically. Directional reflectance microscopy (DRM), being a novel optical microscopy characterisation technique, provides a facile method to examine the deformation of materials on both macroscopic and microscopic view simultaneously. As surface roughness is a function of strain while DRM was proven to be able to characterise surface features of materials, this makes DRM has the potential of characterising strain through roughness measure by DRM. Also, the DRM data set is dependent on the crystallographic plane orientation of the sample, where different plane orientations have different deformation behaviours. There are two key factors that result in the deviation of the DRM data set, which are the waviness of the overall surface caused by the deformation of the material and the localised deformation behaviour of the grain. In order to investigate the localised deformation behaviour of the grain, the effect of the waviness of the overall surface towards the DRM data set has to be eliminated. Hence, the objectives of this study are to find the relationship between the surface roughness and DRM reflectance intensities and also investigate the effect of tilting of sample towards the analysis of DRM, which lead to the ultimate goal of characterising the strain distribution by DRM. In order to achieve the first objective mentioned above, the data set obtained through DRM was compared with the roughness values obtained through confocal laser microscopy. The second objective was achieved by comparing the DRM data sets under flat and tilted condition for the same sample. The results from this project have shown the possibility for DRM being used in the field of experimental mechanics, specifically in the deformation behaviour of metals.