Experimental Investigation of Allotropic Transformation of Cobalt: Influence of Temperature Cycle, Mechanical Loading and Starting Microstructure

Abstract

The allotropic phase transformation in polycrystalline high-purity cobalt is incompletely reversible and exhibits a temperature hysteresis. This leads to the presence of a FCC metastable phase at room temperature, which alters the mechanical properties. Moreover, this phase transformation seems to be induced by the plastic deformation. The influence of thermal cycling and initial microstructure on the phase transformation has been analyzed with different experimental approaches, namely in situ x-ray diffraction, differential scanning calorimetry and high-temperature digital image correlation analysis. A multiscale analysis, under an in situ tensile test, has been adopted to follow the phase transformation induced by the plastic deformation. The main result shows that the transformation is initiated by basal slip mechanisms, in competition with twinning mechanisms during the second work-hardening stage.