Rapid solidification of highly undercooled Ni–Cu alloys

Bulk samples of Ni–25%Cu, Ni–50%Cu, Ni–67%Cu and Ni–75%Cu alloys were undercooled by up to 421K(0.25TL), 349K(0.22TL), 380K(0.25TL) and 381K(0.26TL) respectively with glass fluxing method. Their recalescence behaviors and final microstructures were investigated. A linear relationship between the duration time of the thermal plateau and undercooling was found and used to determine the hypercooling limits, which were 528.3, 448.4, 461.1 and 457.7K for these four Ni–Cu alloys. Using these values, the mean specific heats of these undercooled alloys were predicted to be 25.734, 29.902, 27.837 and 26.403J mol−1 K−1. The solidified microstructures were mainly characterized by a morphology transition from coarse dendrites to refined equiaxed grains. The critical undercoolings for this microstructural transition of these four Ni–Cu alloys were determined as 200, 170, 150 and 180K. The rapid crystal nucleation, dendrite growth and the marked solute trapping effect under high undercooling conditions are responsible for this morphology transition.