ANTI-INVAR IN FE-NI

Anti-Invar behavior in a material can be characterized by an anomalously large and a weak temperature-dependent thermal-expansion coefficient, when compared to the respective Gruneisen lattice expansion. It is just the opposite of the Invar effect, which is characterized by an anomalously small thermal-expansion coefficient. Common to Invar and anti-Invar is the fact that both posess moment-volume instabilities. Anti-Invar is observed in the paramagnetic state. It occurs in gamma-Fe and in a number of 3d fcc binary and ternary alloys. In FexNi100-x alloys it is observed in the concentration range 70 less-than-or-equal-to x less-than-or-equal-to 100 at. % within the fcc stability range. The effect vanishes as the Invar concentration, x=65 at. %, is approached. To examine the valence electron concentration dependence of the anti-Invar effect the thermal expansion has been measured in the fcc state of FexNi100-x for 63 less-than-or-equal-to x less-than-or-equal-to 100 at. %. Using a model based on moment-volume instabilities in conjunction with a thermal activation process the size of the anti-Invar effect in these alloys has been determined. The volume enhancement is found to decrease with increasing Ni concentration from 2.8% in gamma-Fe to 0% at x = 65 at. %.