Simultaneous and different nucleation modes in undercooled Pd–Cu–Ni–P melts

Both Pd40Cu30Ni10P20 alloys with a slightly off-eutectic composition and Pd42.5Cu30Ni7.5P20 alloys with a eutectic composition exhibit high glass-forming ability and a low critical cooling rate for glass formation of 0.100 and 0.067 K/s, respectively. Due to the difference in the critical cooling rate, an incubation time for crystallization at 683 K for the eutectic alloy is 2.5 times longer than that for the off-eutectic alloy. In order to clarify the reason for the difference, nucleation and crystal grain growth behaviors of the undercooled off-eutectic and eutectic melts were investigated. Nucleation frequency was evaluated as 3.00×1010 nuclei/m3 s for the off-eutectic alloy and 4.77×109 nuclei/m3 s for the eutectic alloy. In addition, it is found that the nucleation behavior changes from internal homogeneously dispersed mode to surface one. On the contrary, the crystal grain growth rates have almost the same value of about 3.20×10−7 m/s for the crystallized phases in both alloys. Based on these results, the compositional effect on the stability of undercooled melt is discussed in the present study.