Neutron diffraction, X-ray diffraction and calorimetry were used to id
entify the structural states of Ni3Mn-Ni3Al and Ni3Mn-Ni3Si alloys. Th
e Kurnakov's temperatures, parameters of long-range order, and lattice
parameters were measured. The phase diagrams were constructed for the
ordered alloys. The concentrational transitions were found to be isom
orphic in Ni3Mn-Ni3Al and Ni3Mn-Ni3Si. The substitution sites were ide
ntified for a third alloying element.