Cut pieces of cast Mg_2Ni ingot were heated in a vacuum of about 10~(-2) Pa for 1.8 approx 14.4 ks at 773, 823 and 873 K, arid then examined using X-ray diffraction, optical microscopy and electron probe X-ray microanalyzer with EDX. The vacuum heating resulted in the demagnesization of Mg_2Ni phase by sublimation of magnesium from the surface, forming porous layer of MgNi_2. Thickness of the layer increased with increasing temperature and/or heating time. Approximate estimation, using these kinetic data, showed that apparent activation energy of the growth of the layer was 210 kJ/mol. Based on the estimation, the whole Mg_2Ni specimen with 0.7 X 10~(-3) m thickness was found to change into the porous MgNi_2 after vacuum-heating for 14.4 ks at 923 K.
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