MAGNETIC ASPECTS OF MARTENSITIC TRANSFORMATIONS IN FENI ALLOYS

Fe100-xNix alloys for x<34 at% i.e., up to the structurally stable Inv ar concentration, undergo a martensitic transformation. In the parent fcc-phase these alloys show a volume enhancement which is known as the anti-Invar effect. This effect disappears at a concentration at which the martensitic transformation also no longer takes place. In a narro w concentration range of about 30 less than or equal to x<34 at% Invar , anti-Invar and the martensitic transformation can be observed simult aneously. The Invar and the anti-Invar effects are understood to be du e to the instability of the magnetic moment with respect to a volume c hange, i.e. the moment-volume instability. Using thermal expansion mea surements on Fe100-xNix, with 0 less than or equal to x less than or e qual to 37 at%, we determine critical volumes of the gamma-phase at th e martensitic transformations. Furthermore we show the existence of In var behavior in the rest Austenite in Fe70Ni30.