The massive transformation in titanium aluminides: Initial stages of nucleation and growth
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ge in crystal structure without a change in composition. It is well accepted that massive transformations occur by a nucleation and growth process, where nucleation is usually the rate-controlling step.[1] Once a critical nucleus forms, growth takes place in most alloy systems at velocities estimated at 1 ⫻ 10⫺4 m/s to 1 ⫻ 10⫺2 m/s by diffusional jumps across the massive-product/matrix interface.[2,3] Detailed studies have been made on the effect of crystallography on both nucleation and growth in the Cu-Zn system.[4,5,6] The nuclei of the massive-product ␣ phase at : grain boundaries have a rational orientation relationship with at least one of the parent  grains, although growth often occurred into the  grain with the irrational orientation.[5,6] Analysis of the interface between the massive ␣ and retained  interface showed no low-index orientation relationship and no evidence of regular interfacial dislocation arrays.[4] These data suggest that the massive transformation grows without regard to the crystallography of the ␣: interface. Investigations of the massive transformation in Ag-26 at. pct Al of the bcc  phase to the hcp m phase have determined that a Burgers orientation relationship was obtained, with at least one of the  grains forming the grain boundary.[7] In an J.E. WITTIG, Associate Professor of Materials Science, is with the Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN 37335. Contact e-mail:[email protected] This article is based on a presentation made at the symposium entitled “The Mechanisms of the Massive Transformation,” a part of the Fall 2000 TMS Meeting held October 16–19, 2000, in St. Louis, Missouri, under the auspices of the ASM Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
alternative view of the growth of the massive product,[3,7–9] Aaronson and co-workers present experimental evidence with theoretical backing that, for systems such as Ag-Al, the massive transformation proceeds similar to precipitation reactions, where crystallographic considerations at the product:parent interface influence growth. The only fundamental difference is that while the composition changes during precipitation, massive transformations occur without a change in composition.[7] Numerous studies[10–19] in near-equiatomic Ti-Al systems have shown that a massive solid-state transformation of the hexagonal ␣ phase into the tetragonal L10 ␥m phase can occur when the cooling rate is sufficiently high. However, there have been some variations in these investigations concerning the mechanism of nucleation and the crystallographic orientation relationships of the parent and product phases. Wang et al.[10] first reported that water- or ice-brinequenched samples of Ti-48 at. pct Al exhibit this massive ␣ → ␥ transformation. Alloys with an aluminum composition between 46.5 and 50 at. pct, which were equilibrated in the ␣ -phase field at 1400 ⬚C, could be massively transformed with water quench rates.[11] However, these studies[10
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