The following references should contain the information you need:
L. Zhang, Y. Du, H. Xu, S. Liu, Y. Liu, F. Zheng, N. Dupin, H. Zhou, and C. Tang, Phase Equilibria and Thermal Analysis in the Fe-Mn-Ni System, Int. J. Mater. Res., 2009, 100(2), p 160-175
V. Raghavan, Fe-Mn-Ni (Iron-Manganese-Nickel), Journal of Phase Equilibria and Diffusion, 2010, 31(2), p. 177-179