STRUCTURAL CHANGES IN INVAR IRON-PALLADIUM ALLOYS BEFORE THE f.c.c.- b.c.t. MARTENSITIC TRANSFORMATION

Abstract
In several iron-palladium alloys (25-32 at % Pd) metastable f.c.c. long range disordered austenite transforms into b.c.c. or b.c.t. martensite according to the M_s temperature is above or below the Curie temperature. In this last case, the austenite becomes tetragonal with a distribution of the c/a parameter as the temperature decreases. This phenomenon may be connected with the strong Invar effect present in these alloys and is characterized by special features in electron microscopy. As discussed in Fe₃Pt alloys, such structural changes may be explained by shears of same magnitude on two {110} planes lying at 60° from each other and may be considered as precursor of the transformation of f.c.c. austenite into b.c.t. martensite.