Mössbauer and structural studies of Fe_0.75Ni_0.25-xCr_x and Fe_0.65Ni_0.35-xCr_x alloy systems

X-ray diffraction patterns for Fe_0.75Ni_0.25-xCr_x show that the samples with x = 0 and 0.25 form a single bcc-type phase with a lattice parameter of 2.870 Å, while the samples with x = 0.05 and 0.10 form the bcc-type phase with an additional γ-fcc phase, as a minor phase with a lattice parameter of 3.585 Å. On the other hand all the Fe_0.65Ni_0.35-xCr_x samples show a single γ-fcc phase. The lattice parameter for the Fe_0.65Ni_0.35-xCr_x is found to increase linearly with increasing chromium concentration. Mössbauer spectra for Fe_0.75Ni_0.25-xCr_x show that all the samples are magnetically ordered and some of them have a central paramagnetic line with a very low intensity (between 2 and 15%) for the samples with x = 0, 0.05, and 0.10. However, Mössbauer spectra for Fe_0.65Ni_0.35-xCr_x show that the sample with x = 0 is magnetically ordered and the hyperfine field splitting starts to collapse at x = 0.05 and disappears completely at x = 0.10 and above. The average isomer shift of this system is negative and it decreases with increasing Cr concentration.