Automated <i>In Situ</i> Thermomechanical Analysis of Cobalt under EBSD

The present study aims to investigate the mechanical behaviour of pure cobalt in plasticity at varying temperatures, through in situ thermomechanical loadings under Electron Backscattered Diffraction (EBSD) in order to gain a deeper understanding of the various mechanisms that occur. EBSD analysis allows for the determination of microstructural parameters at a refined scale including grain size, grain misorientation, crystallographic and morphologic texture, and phase ratios, which can be employed to establish a correlation between microstructural changes and deformation mechanisms with temperature. Analyses were conducted in situ using either a furnace that can reach 1000 °C, or a 10 kN thermomechanical device, which enables simultaneous heating and mechanical loading. Both test types were automated in the Scanning Electron Microscope (SEM) by correlating the stage movement with the region of interest, enabling EBSD mappings to be acquired always at the same location. Mappings were post-processed via the spherical indexing process, which yields high-quality indexation (with a reduced number of points exhibiting a Confidence Index of less than 0.1), even at high strain levels. Such experiments conducted on cobalt demonstrated austenitic and martensitic transformations between hexagonal close packed and face centred cubic phases with temperature. Indeed, approximately 31.6 % of the initial face centred cubic phase has transformed into the hexagonal phase for 8 % strain during an in situ tensile test. This transformation is initiated in plasticity by dislocation motions in basal planes and subsequently accelerated by the concurrent activation of mechanical twinning. Additionally, an in situ thermal treatment in the SEM enabled the accurate determination of the phase transformation temperature: 460 °C during heating and 350 °C during cooling, corresponding to the points where the cubic phase fraction reaches a 50 % relative change.