Magnetic Aspects of Martensitic Transformations in FeNi Alloys

Feloo-xNi, alloys for x<34 at% i.e., up to the structurally stable Invar 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 narrow concentration range of about 301x<34 at% Invar, anti-Invar and the martensitic transformation can be observed simultaneously. The Invar and the anti-Invar effects are understood to be due to the instability of the magnetic moment with respect to a volume change, i.e. the moment-volume instability. Using thermal expansion measurements on Feloo-xNi,, with OSxS37 at%, we determine critical volumes of the yphase at the martensitic transformations. Furthermore we show the existence of Invar behavior in the rest Austenite in FeT0Ni3,,.