Ultrasonic detection of ductile-to-brittle transitions in free-cutting aluminum alloys

Resonant ultrasound spectroscopy (RUS) is applied to detect the ductile-to-brittle and brittle-to-ductile transitions in an AA6262 free-cutting aluminum alloy during thermal cycling. It is shown that the RUS method is sensitive enough to detect reliably the melting and solidification temperatures of micro-sized Mg3Bi2-Bi eutectic particles responsible for these transitions, although the volume fraction of the particles is very small (~1%) and is even decreasing with the cycling. The proposed RUS approach is compared with differential scanning calorimetry (DSC); the latter method is shown to be unable to detect the transition temperatures especially when transition intervals are broad. The results reveal that the phase transitions of the eutectic particles exhibit a significant hysteresis and pronounced asymmetry between melting and solidification.