Effects of Undercooling and Cooling Rate on Peritectic Phase Crystallization Within Ni-Zr Alloy Melt
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Effects of Undercooling and Cooling Rate on Peritectic Phase Crystallization Within Ni-Zr Alloy Melt P. LU¨ and H.P. WANG The liquid Ni-16.75 at. pct Zr peritectic alloy was substantially undercooled and containerlessly solidified by an electromagnetic levitator and a drop tube. The dependence of the peritectic solidification mode on undercooling was established based on the results of the solidified microstructures, crystal growth velocity, as well as X-ray diffraction patterns. Below a critical undercooling of 124 K, the primary Ni7Zr2 phase preferentially nucleates and grows from the undercooled liquid, which is followed by a peritectic reaction of Ni7Zr2+L fi Ni5Zr. The corresponding microstructure is composed of the Ni7Zr2 dendrites, peritectic Ni5Zr phase, and inter-dendritic eutectic. Nevertheless, once the liquid undercooling exceeds the critical undercooling, the peritectic Ni5Zr phase directly precipitates from this undercooled liquid. However, a negligible amount of residual Ni7Zr2 phase still appears in the microstructure, indicating that nucleation and growth of the Ni7Zr2 phase are not completely suppressed. The micromechanical property of the peritectic Ni5Zr phase in terms of the Vickers microhardness is enhanced, which is ascribed to the transition of the peritectic solidification mode. To suppress the formation of the primary phase completely, this alloy was also containerlessly solidified in free fall experiments. Typical peritectic solidified microstructure forms in large droplets, while only the peritectic Ni5Zr phase appears in smaller droplets, which gives an indication that the peritectic Ni5Zr phase directly precipitates from the undercooled liquid by completely suppressing the growth of the primary Ni7Zr2 phase and the peritectic reaction due to the combined effects of the large undercooling and high cooling rate. https://doi.org/10.1007/s11663-018-1189-8 The Minerals, Metals & Materials Society and ASM International 2018
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INTRODUCTION
MANY alloys of great commercial significance exhibit a phase diagram with a peritectic reaction, such as steels (Fe-C, Fe-Ni),[1–3] rare permanent magnetic alloys (Nd-Fe-B),[4] high-temperature structural materials (Ti-Al, Ni-Al),[5,6] as well as glass forming alloys (Cu-Zr, Ni-Zr).[7,8] According to such phase diagrams, peritectic alloys usually undergo crystallization of the primary phase and a subsequent peritectic reaction where the primary phase reacts with the liquid phase at a triple junction upon cooling to produce the peritectic phase. The peritectic reaction terminates once the primary phase is enwrapped by the peritectic phase.[9] The primary phase then transforms into the peritectic phase via peritectic transformation. However, peritectic P. LU¨ and H.P. WANG are with the MOE Key Laboratory of Space Applied Physics and Chemistry, Department of Applied Physics, Northwestern Polytechnical University, Xi’an 710072, P.R. China. Contact email: [email protected] Manuscript submitted August 31, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
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