Disorder trapping during the solidification of βNi3Ge from its deeply undercooled melt

Abstract

A melt encasement (fluxing) technique has been used to solidify the congruently melting intermetallic βNi3Ge from its deeply undercooled parent melt. High speed photography and a photo-diode technique have been used to measure the resulting growth velocity. The maximum undercooling achieved was 362 K, wherein a growth velocity of 3.55 m s−1 was recorded. At an undercooling of 168 K, an abrupt increase in the gradient of the velocity-undercooling curve is observed and this is attributed to a transition from growth of the ordered L12 compound at low undercooling, to growth of the fully disordered compound at high undercooling. A change in the microstructure, from a distribution of very coarse (700 μm), randomly oriented grains to a fine-grained structure (15–20 μm) with a large number of low-angle grain boundaries is observed coincident with this transition in the velocity-undercooling curve, a grain refinement pattern that is different to that observed either in deeply undercooled solid solutions or other intermetallics. This transition is ascribed to a recovery and recrystallisation process in the disordered phase due to the low post-recalescence cooling rate.