Phase Field Simulations of Autocatalytic Formation of Alpha Lamellar Colonies in Ti-6Al-4V
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THE microstructures of two-phase Ti alloys are composed of the b phase that has a body-centered cubic structure and the a phase that has a hexagonal close-packed structure. b is stable at high temperature and a precipitates in the form of plates from b on cooling. The existence of a Burgers orientation (BO) relationship between b and a given by ½1 ð101Þb jjð0001Þa ; 111 b jj 1120 a results in 12 possible variants of a based on the crystal symmetries of b and a.[1] Two distinctly different microstructures are produced depending upon the undercooling of the b phase field. Typically, a basket weave structure is produced at a large undercooling where a high thermodynamic driving force favors the
BALA RADHAKRISHNAN and SARMA GORTI, Senior Research Staff, are with the Oak Ridge National Laboratory, 1 Bethel Valley Rd, Oak Ridge, TN 37831. Contact email: [email protected] SURESH SUDHARSANAM BABU, Professor, is with the Department of Mechanical, Aerospace and Biomedical Engineering, University of Tennessee, Knoxville, TN, 37996 This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). Manuscript submitted September 11, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
intragranular nucleation and growth of multiple variants of a. When the undercooling is lower, a lamellar a in the form of colony morphology is obtained. Formation of the colony structure under these conditions has been mainly attributed to the formation of Widmansta¨tten side-plates (WS) of the same variant originating from a grain boundary allotriomorph (GBA) of a.[2] The nucleation of WS occurs in general by one of the following mechanisms: (1) the instability of the GBA-b interface that grows with the same variant into a b grain that has BO relationship with GBA[3] or (2) sympathetic nucleation[4] of WS ahead of the GBA-b interface with a misorientation to account for the deviation of the b grain orientation from BO with respect to GBA. Recent phase field calculations[5] have shown that the variant selection in the latter mechanism may be driven by the minimization of the elastic strain energy due to transformation strains and may be more appropriately considered as autocatalytic nucleation. However, the mechanisms of WS based entirely on grain boundary nucleation may not be adequate to explain the formation of intragranular colony structure in coarse-grained materials. An interesting example is the formation of a colonies during laser additi
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