Microstructural stress concentration: An important role in grain refinement of rheocasting structure
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I. INTRODUCTION
IT is well known that the morphology of the primary phase of semisolid slugs has a great influence on the semisolid metal processes (SSPs). During agitated solidification, the fine rosettelike primary phase will diminish the hot-tear phenomenon.[1] During die filling, rounder and finer primary grains generally result in smoother die filling.[2] Fine primary grains also are critical to fabricate thin-walled parts and to diminish the segregation of the liquid phase.[3,4,5] Another advantage of fine rheocasting structures, which is illuminated by the Hall–Petch law,[6] is the superior mechanical properties. However, because it is difficult to observe dendrites in semisolid slurry directly, the mechanisms of grain refinement in agitated solidification are still far from being completely understood.[7–14] It is generally accepted that grain refinement in agitated semisolid slurries is caused by dendrite fragmentation. Disagreement arises over the question of how dendrite branches detach. There are three major hypotheses:[15,16] (1) dendrite arms detach at the roots due to shear forces; (2) dendrite arms melt off at their roots as a consequence of accelerated diffusion in liquid, or thermal fluctuations, or stress-aided melting; and (3) repeated plastic bending creates numerous dislocations at the roots of dendrite arms. Recrystallization of the dislocations followed by rapid liquid penetration along the new grain boundaries detaches the side arms. Among the various hypotheses, the mechanisms of detaching through recrystallization followed by liquid penetration and the mechanism of detaching through melting off by thermosolutal advection are thought to be the most reasonable hypotheses. But there are some uncertainties about both Z. YANG, Postdoctor, and C.G. KANG, Professor, are with the National Research Laboratory of Thixo-Rheo Forming, School of Mechanical Engineering, Pusan National University, Busan 609-735, South Korea. Contact email: [email protected] Z.Q. HU, Professor, is with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China. Manuscript submitted December 24, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
hypotheses. Since primary dendrites in stirred solidifications are usually equiaxed and since dendrite branches are short and coarse, it is not certain that metallic flow can create adequate force to bend the dendrite arms plastically. It also should be noted that there is little interspace among dendrite branches, and so the boundary stagnant layer that covers the dendrite branches will naturally block the penetration of thermosolutal advections into the dendrite roots. Pilling and Hellawell[17] have calculated the stress that dendrite arms bear in interdendritic fluid flow. In their model, the dendrite is slim and the calculation is very conservative. However, through their calculation, readers can easily find that it is very difficult for metallic flow, even with a relatively hig
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