Measurement of Transformation Stress in Metastable High Entropy Alloys by Spherical Nanoindentation

Pop-in events due to stress-induced phase transformation in metastable HEAs have been observed in nanoindentation tests with Berkovich indenter. This phase transformation assists with deformation plasticity that strengthens the material without ductility reduction, similar to TWIP/TRIP effects in advanced steels. To determine the deformation mechanism, if twinning or dislocation gliding attributes to the phase transformation, the stress during indentation at crystal-level need to be quantified. In the present work, the contact area function of the indent impressed by a spherical tipped indenter was calibrated and applied for strain and stress calculations. Since twinning and gliding activation energies are different, as are their strain magnitudes, the transformation mechanism in metastable HEAs was able to be determined in the initial phase transformation stage.