Origin of Tetragonality of BCT Martensite in Substitutional Fe–Pd(–Ni) Disordered Alloys

In order to study the origin of tetragonality of bct martensite of substitutional type disordered Fe-Pd alloys, the change in the axial ratios and internal structures of the martensites with thesystematic substitution of Pd for some of Ni in a typical bcc martensite of Fe-Ni alloys has been examined using optical and electron microscopy. It is found that the bct martensite is transformed from the fcc austenite not only via the fct martensite but also directly. The bct martensite is always accompanied by {112} internal twins in which the mottled contrast is observed