A high-resolution time-resolved study of incoherent interface motion during the massive transformation in TiAl alloy
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I. INTRODUCTION
IN contrast to the case of coherent and partly coherent interfaces,[1,2] the atomic mechanisms by which solid-solid incoherent interfaces move are not well understood.[3,4,5] This is true for both homophase interfaces, such as high-angle grain boundaries,[2,6–8] and heterophase interfaces, such as massive transformation interfaces.[4,5,9] Previous in-situ heating high-resolution transmission electron microscope (HRTEM) studies of transformation interfaces during growth and dissolution of precipitate particles have shown that fluctuations involving concurrent changes in the structure and composition occur at ledges and precipitate edges, and that the fluctuations involve the cooperative motion of many atoms at the interfaces.[1,10,11] During growth, these fluctuations have been shown to be due to local nucleation barriers at the interfaces, e.g., due to the formation of critical-size double-kinks,[11,12] which leads to oscillatory behavior in ledges and plate edges.[10] Larger nucleation barriers associated with cusp-oriented coherent and partly coherent interfaces lead to the commonly observed start-stop behavior associated with interface motion in diffusional phase transformations.[1,11,13] During dissolution or at static interfaces when nucleation is not an issue, local fluctuations can occur at interfaces due to thermal effects.[12,14] In-situ HRTEM studies on grain boundaries have revealed such reversible fluctuations.[7] Recent in-situ heating HRTEM experiments performed on massive transformation interfaces in TiAl alloy have shown evidence of both continuous and stepwise motion NASTARAN RAFFLER, Visiting Student, is with the University of Applied Sciences, Fachhochschule Bonn-Rhein-Sieg, 53359 Rheinbach, Germany. JAMES M. HOWE, Professor, is with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903-4745. Contact e-mail: [email protected] This article is based on a presentation made in the “Hume-Rothery Symposium on Structure and Diffusional Growth Mechanisms of Irrational Interphase Boundaries,” which occurred during the TMS Winter meeting, March 15–17, 2004, in Charlotte, NC, under the auspices of the TMS Alloy Phases Committee and the co-sponsorship of the TMS-ASM Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
during growth at the atomic level, depending on the orientation relationship and interface plane.[5] These two types of growth behavior were previously observed at much lower levels of resolution during cinematographic studies of the massive transformation in Cu-Ga alloys[15] and during in-situ heating TEM experiments of massive interfaces in Ag-Al[16,17,18] and Cu-Zn[19] alloys. The work on Ag-Al also showed that whether an interface grows continuously or by steps depends on the driving force for transformation. One problem associated with all of the previous, lower-resolution in-situ studies is that the actual structures of the massive transformation interfaces were not known, but assumed to be incoherent
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