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

Highlights: • Abnormal grain growth caused by cyclic heat treatment through phase transformation temperature was found. • A large grain over 30 mm in length was obtained by repeating the cyclic heat treatment. • Boundary energy of subgrains introduced by precipitation of γ second phase contributed to the abnormal grain growth. • The γ-fcc precipitates exhibited the K-S OR with the α-bcc matrix. Abnormal grain growth (AGG) and related microstructural features were investigated for Fe-Mn-Al-Ni shape memory alloy sheets subjected to thermal cycling through α (bcc) → α + γ (fcc) → α phase transformations by means of microstructural observations including the electron backscatter diffraction (EBSD) technique. In the initial cooling from the α single-phase to the α + γ two-phase region, a high density of subgrain boundaries with small angle deviation of about 1° through 2° is formed around the γ precipitates in the α matrix. The subgrain structure remains in the α single phase even after dissolution of the γ 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 α + γ two-phase region is performed at temperatures below 1000 °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.