Disorder trapping and recrystallization-induced grain refinement in undercooled Ni3Ga melts

The mechanism of grain refinement in undercooled Ni₃Ga melts was investigated. An in situ diagnosis of the dendrite growth velocity using a high-speed camera indicates direct crystallization of the Ni₃Ga compound and disorder trapping at a critical undercooling of 68 K and 120 K, respectively. An EBSD analysis shows a grain-refined microstructure for samples with undercoolings of 120 K and 148 K in contrast to a coarse-grained microstructure for the sample with an undercooling of 89 K. HRTEM investigation shows a mixture of a disordered solid solution and the ordered Ni₃Ga compound in the atomic-scale microstructure of each sample in agreement with X-ray and neutron diffraction analyses. Microhardness tests show that disorder trapping reduces the mechanical strength of the samples in spite of grain refinement. It is concluded that disorder trapping promotes plastic deformation of the Ni₃Ga compound and that grain refinement is induced by recrystallization after rapid solidification processing.