Martensitic and Massive Transformations in Ni–Cu–Sn Alloy

Abstract

The martensitic and massive transformations in a Ni₇₁Cu₄Sn₂₅ alloy were studied by means of electron microscopy. In this alloy, the high temperature DO₃ (cubic) phase underwent a martensitic transformation to the 2H structure (orthorhombic) during quenching and a massive transformation to the DO₁₉ structure (hexagonal) during slow cooling. The morphology of the martensite and the massive phases was studied. The 2H martensite had the internal (121) twins. Misfit dislocations were observed at curved boundaries of the DO₁₉ grains. The 2H structure obtained by quenching changed into the DO₁₉ structure during annealing at 873 K. In a specimen annealed at 873 K for 120 s, a crystal with the DO₁₉ structure contained thin plates of the 2H structure as growth faults; the orientation relationship was such that, (100)_2H⁄⁄(10\bar10)_DO19, [001]_2H⁄⁄[0001]_DO19. The mechanism of the successive DO₃\ ightleftarrows2H\ ightleftarrowsDO₁₉ transformations was discussed briefly.