Creep Properties of a Zinc-Aluminum Die-casting Alloy as a Function of Grain Size

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Title: Creep Properties of a Zinc-Aluminum Die-casting Alloy as a Function of Grain Size
Author: Gobien, Jeremy Michael
Advisors: Lewis Reynolds, Committee Member
Korukonda Murty, Committee Member
Ronald Scattergood, Committee Member
Carl Koch, Committee Chair
Abstract: Zinc and its alloys are a large component of the metal die-casting industry. Recent research on zinc alloys have focused on both creep resistant formulations for high temperature applications as well as high fluidity formulations for ultra-thin die-casting technology. The described research describes the creep behavior of an ultra-fine grained Zn-Al die-casting alloy which was optimized for high fluidity. The creep studies were carried out over a range of grain sizes. It was found that the limit of refinement of the grain size during cryogenic ball-milling was the low end of the ultra-fine grained regime. The chosen milling time for use in the research that follows was found to have a volume average grain size of 260nm with a relatively large standard deviation of 85nm. Targeted heat treatments were performed at varying temperature and time. Pre and post-heat treatment microscopic examination revealed a well behaved Hall-Petch relationship within the ultra-fine grained regime. Using this information an additional microstructure of 510 nm volume average grain size was chosen for an additional microstructure of interest for study using the impression creep technique. A highly annealed microstructure was also produced using near melting temperatures and long terms in order to remove most of the grain boundaries from the specimen to show the effect they have on the creep behavior. It was found that for both the as-milled condition and the first annealed condition, having volume average grain sizes of 260 and 510nm respectively, that stress exponents near 1 were seen at low stresses transitioning to values in the range of 4-7 at elevated stresses. A stress exponent of 1 along with activation energy values near that of grain boundary diffusion for both cases suggest Coble creep as a potential creep mechanism for the ultra-fine grained samples. Evidence of a threshold stress was seen in the as-milled condition suggesting an interference mechanism with the grain boundary diffusion process. The possibility of oxide dispersions could explain the threshold stress that provides the high grain growth exponents (relative to a nominally pure material) encountered; however, microscopy of the specimens was inconclusive showing relatively few nanocrystalline dispersions to explain such a result. This suggestion also does not explicitly explain the threshold stress disappearing for the ultra-fine grained annealed microstructure. An additional explanation is that of the ball-milling process leading to non-uniform segregation of Al to the grain boundaries as has been suggested in the literature on other alloys processed by ball milling. If true this process could explain the disappearance of the effect after annealing since it is energetically favorable for Al to form a stable second phase under equilibrium conditions. This precipitation could explain the removal of the non-uniform segregation and subsequent absence of a threshold stress for annealed specimens. Highly annealed ball-milling specimens as well as sand-castings of the alloy of interest were tested for comparison to coarse grained tensile creep results. The data shows stress exponent values very close to those of tensile creep tests on coarse grained sand castings.
Date: 2010-04-30
Degree: PhD
Discipline: Materials Science and Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/6196


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