Modele possible de germination de la martensite c.c. ou q.c. dans les alliages c.f.c. application aux alliages Fe 3Pt

Abstract

Although many models on the b.c.c. or b.c.t. martensite nucleation exist, none of them are able to explain the premartensitic effects observed by X-ray diffraction and transmission electron microscopy at low temperature in ordered Fe 3Pt alloys. These effects show a tetragonal distortion of the austenitic lattice with distribution of the tetragonality parameter around an average value that decreases with temperature. They are interpreted by the same amount of double-shear mechanism γ〈11̄0〉 γ on planes at 60°. For a sufficient amount of shear, the extension of this mechanism gives a possible model of b.c.c. or b.c.t. martensite nucleation in f.c.c. alloys. It forecasts the Nishiyama-Wassermann orientation relationships. It involves no specific change of volume. The strain that it imposes on the austenitic lattice, according to the mathematical theory of the martensitic transformation, is equivalent to the strain due to the product of the Bain distorsion by a rigid body rotation. The establishment of the habit plane needs an additional lattice invariant deformation which is a martensite twinning on α′, planes in Fe 3Pt alloys.