Analysis of second-phase particle fracture in hypereutectic aluminum-silicon alloys

Vickers micro-indentation tests were performed on second-phase particles of Al-8.5wt.% Si and results were compared with those of monolithic silicon. Damage microstructures beneath indented second-phase silicon particles were studied to determine the crack morphologies causing particle fracture, and also to identify the role of indentation-induced phase transformations on subsurface damage. Plastic deformation at low loads and volume expansion due to subsurface crack formation at high loads (>650 mN) were responsible for pile-up formations around the indentations. The probability of lateral cracks reaching the surface and causing particle fracture was shown to obey Weibull statistics with a low modulus. Diamond cubic Si-I transformed into bee Si-III and rhombohedral Si-XII, under an estimated indentation pressure of 19.3 GPa. Cross-sectional FIB and TEM revealed a semi-circular plastic core and subsurface lateral crack pattern below the residual indents, in addition to a localized, amorphous silicon zone below the plastic core at the median crack edges.