Invar Behavior and Martensitic Transitions in FeNiPt Alloys

Abstract
We determined the structural and the magnetic phase diagram for the disordered, ternary system FeNiPt in the Fe-rich corner in the vicinity of the γ(fcc)-α(bbc) transition region along the quasi binary cross section Fe_1-x(Pt₅₀Ni₅₀)_x with x=0.28, 0.3, 0.31, 0.32 and 0.35. All alloys are characterized by X-ray investigations, magnetization measurements M(T,H) (4.2≤T≤850K), as well as resistivity R(T) and thermal expansion measurements α(T) below 300K. We observe for x≤0.31 a non reversible, γ-α martensitic phase transition. In the alloys with x> 0.31 the γ(fcc) phase is stable down to 4.2K, and pronounced magneto-volume effects (Invar-like behavior) is observed. For the alloys close to the γ-α transition, the magnetic behavior M(T,H) is also discussed.