A Study of Curie Temperatures and the Effect of Carbon in Face‐Centered Cubic Iron–Nickel Alloys

The direct determination of Curie temperatures (θc) in metastable f.c.c. iron‐nickel solid solutions can be thwarted by a martensitic transformation. However, at the lower nickel contents, interstitially dissolved carbon which inhibits this phase transformation, fortuitously increases the propensity for ferromagnetism. An experimental scheme has been constructed for the determination of virtual Curie temperatures (θNi) down to 0 °K in these alloys. θc was determined (using a simple torque magnetometer) at various dissolved carbon levels for a given substitutional percent nickel; linear extrapolation to zero carbon was possible in each case. Derived results include: a linear variation, θ vs. nickel content, intercepting for 0 °K at ≈ 25 at% Ni; an approximately constant local effect of dissolved carbon, calculated for constant volume, from the observed dθc/dC, and known variations in these Invar‐type alloys; and the construct of ferromagnetic f.c.c. alloys at nickel contents as low as 22.6%.