Abnormal grain growth induced by cyclic heat treatment in Fe-Mn-Al-Ni superelastic alloy

Abnormal grain growth (AGG) and related microstructural features were investigated for Fe-Mn-Al-Ni shape memory alloy sheets subjected to thermal cycling through alpha (bcc) -> alpha + gamma (fcc) -> alpha phase transformations by means of microstructural observations including the electron backscatter diffraction (EBSD) technique. In the initial cooling from the alpha single-phase to the alpha + gamma two-phase region, a high density of subgrain boundaries with small angle deviation of about 1 degrees through 2 degrees is formed around the. precipitates in the a matrix. The subgrain structure remains in the a single phase even after dissolution of the. precipitates by heating, and some limited grains grow faster by encroaching on neighboring grains, including the subgrains, resulting in AGG. The maximum grain size after the AGG becomes extremely large when annealing in the alpha + gamma two-phase region is performed at temperatures below 1000 degrees C, and a single crystal over 30 mm in length can be obtained by repeating the cyclic heat treatment. It is found that the boundary energy of the subgrains dominantly contributes to the AGG as the driving force. (C) 2016 Elsevier Ltd. All rights reserved.