The effect of heat treatment and cyclic loading on nanoindentation behaviour of FeSiB amorphous alloy

Highlights: • Single-step and multi-step nanoindentation studies in FeSiB amorphous alloy • A peculiar strain hardening behaviour during multi-step nanoindentation • The disappearance of step-like pile-up during multi-step nanoindentation in crystallized sample • Observation of strain-localization around the indent in multistep nanoindentation • The role of initial free-volume on the work-hardening response In this study we investigate the mechanical properties of Fe_80.75Si₈B_11.25 amorphous alloys produced by melt-spinning by means of single-step and multi-step nanoindentation methods. Single-step nanoindentation showed that annealing treatment increases the hardness (by ≈ 13%) and Young's modulus (by ≈ 46%); however, unnecessary prolonged annealing time (500 °C for 3 h) decreases the hardness (by ≈ 20%) and Young's modulus (by ≈ 16%) considerably. Positron annihilation spectroscopy (PALS) showed two lifetime components corresponding to the interstitial defects and free volume, the lifetime of both decreased after stress-relaxation treatment at temperature far below the glass transition point. The strain hardening behaviour was observed during multi-step nanoindentation in amorphous state leading to a slight increase of hardness (≈ 5%); however, multi-step nanoindentation in the stress-relaxed sample (400 °C for 0.5 h) resulted in an overall strain-softening phenomenon and any increase in the number of cycles (from 3 to 10) did not alter the global softening behaviour. Atomic force microscopy (AFM) of the indented surface in the amorphous sample revealed the strain localization around the residual indent after multi-step nanoindentation (leading to more material pile-up as compared to single-step indent) that could be due to the densification induced by stress and/or interaction and entanglement of the shear bands during unloading/reloading process.