Experimental study of grain refinement mechanism in undercooled Ni-15at.%Cu alloy

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ying glass fluxing and cyclic superheating, rapid solidification of undercooled Ni–15at.%Cu alloy was performed by rapidly quenching the sample after recalescence. The evolution of microstructure and microtexture has been analyzed. At both low and high undercoolings, well-developed dendrites, within and around which are distributed by the fine equiaxed grains, are observed. At low undercooling, the completely grain-refined microstructure shows a highly oriented texture without annealing twins, whereas at high undercooling a fully random texture as well as a number of annealing twins is observed. On this basis, all the possible mechanisms for grain refinement, as well as their effects on the microstructure formation, were discussed. The grain refinement at both low and high undercoolings is concluded to originate from dendrite fragmentation. Particularly, at high undercooling, recrystallization, as a consequence of dendrite deformation (by fluid flow) and dendrite fragmentation (which provides grain boundary sites for recrystallization nucleation and for the “appearing” recrystallized grains), occurs and plays a role in the grain refinement and the formation of fully random texture.

I. INTRODUCTION

Rapid solidification of bulk undercooled melts has been studied extensively because it is a good method for preparation of metals in metastable states, such as grain-refined materials.1 As compared with the fine equiaxed grains produced by conventional solidification (e.g., magnetic and mechanical stirring, copious nucleation induced by grain refiner, etc.), the grain refinement upon rapid solidification of undercooled melts occurs spontaneously. To date, a number of mechanisms have been proposed to account for the origin of grain refinement: copious nucleation due to cavity collapse,2 development of growth instabilities,3 dendrite remelting,4–6 recrystallization upon or after solidification,7–11 dynamic nucleation,12–14 etc. Copious nucleation can be excluded for a system with fast growth kinetics, e.g., the pure metal or the completely miscible alloy, since recalescence could prohibit further nucleation and a single nucleation event is sufficient to initiate and complete crystallization.15,16 Such a viewpoint can be verified by the as-solidified microstructure (e.g., Refs. 7, 8, and 14), where the directional dendrites, starting from the nucleation site and then spreading throughout the entire specimen, are observed. Mullis and Cochrane3 proposed that grain refinement should originate from unstable dendrite growth, a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0257 J. Mater. Res., Vol. 25, No. 10, Oct 2010

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which is, however, in apparent contradiction with the rigorous stability analysis of dendrite growth (e.g., Ref. 17) as discussed by Karma,6 and hard to be testified experimentally. On the basis of Jackson et al.,4 a model of dendrite fragmentation was developed by Karma.5,6 So far, as the most promising mechani