Determination of the Fe-rich portion of the Fe-Ni-S phase diagram

L Ma
D Williams
Joseph I. Goldstein, University of Massachusetts - Amherst

Abstract

The low-temperature, Fe-rich portion of the Fe-Ni-S phase diagram was determined from Fe-Ni-S alloys (2.5,5,10,20, and 30 wt.% Ni, 10 wt % S, balance Fe) heat treated at 100 °C intervals from 900 to 300 °C. The microstructure and microchemistry of the phases in the heat treated Fe-Ni-S alloys were studied using a high-resolution field-emission gun (FEG) scanning electron microscope (SEM), electron probe microanalyzer (EPMA), and analytical electron microscope (AEM). Tieline compositions were obtained by determining the average phase composition and by measuring compositional profiles across interphase interfaces with the EPMA and AEM. At 600 °C and below, at least one phase was <1 >Μm in size requiring the use of the AEM for analysis. The measured α + FeS, γ+ FeS, and α + γ + FeS boundaries in the Fe-rich corner of the Fe-Ni-S isotherms are consistent with previous studies. However, two new phases were observed for the first time coexisting with γ and FeS phases: FeNiγ′′ (∼52 wt.% Ni) at 600 and 500 °C and Ni 3 Fe, ordered Ll 2,γ′ (∼64 wt.% Ni) at 400 °C. New ternary isotherms are given at 600,500, and 400 °C that include the newly determined γ+γ′′ + FeS and the γ + γ′ + FeS three-phase fields. The effects of S on the phase boundaries of the α + γ phase field and the application of the Fe-Ni-S phase diagram to explain the microstructure and microchemistry of the metallic phases of stony meteorites are also discussed.