Structural and composition changes in single crystals caused by heating the maraging alloys to within the α-γ- limits were investigated by X-ray. The heating is necessary either for producing the intermetallic phases on aging  or for initiating the reverse α-γ- transition. These processes in iron-nickel alloys are accompanied by the redistribution of the alloying element between the γ- and α-phases.

The test materials were Fe-29wt.%Ni-2wt.%Ti and Fe-28wt.%Ni-2wt.%Ti-2wt.%Al alloys, which are used for the fabrication of stamps and press molds. The specimens contained (70-85)% martensite at room temperature. The reverse transition occurred on heating in salt bath at (400-500)⁰C. The transition kinetics and the amount of martensite were controlled by magnetometry. Selected area X-ray diffraction patterns were taken on a KAMEBAX spectrometer. Nickel content of the γ-solid solution was estimated from the previously built concentration dependences of fcc-lattice parameter of binary iron-nickel alloys.

The tempering of freshly quenched alloys reduced the parameter c_α and the tetragonality c/a_α of the martensite lattice even at room temperature. At temperatures between 100 and 300^o C, the parameter c_α  reduced further, and the reverse α-γ-transition occurred during tempering at 400^o C. The reduce in c_α is associated with the loss of matching of the martensite and retained austenite lattices, as well as with the formation of a Ni₃Ti-type phase and the subsequent depletion of the α-solid solution in the alloying elements due to the formation of intermetallic compounds and the nickel redistribution between the γ- and α-phases.

Annealing of the two-phase specimens at 520^oC for several minutes shifted the martensite reflections having high third indices toward higher Bragg angles.

The retained austenite reflections were tailed at the side of low angles, and weak first-order satellite reflections appeared at the tailing at 520^oC (for 10 h) or higher annealing temperature (600^oC) attenuated and eliminated the latter reflections, and the retained austenite reflections started blurring. Holding at 600^oC increased the Bragg angles and intensities of the satellite reflections. The Bragg angles of the retained austenite reflections also increased.

   Thus, heating of two-phase iron-nickel alloys during the reverse α-γ- transition causes the surface lamination of the γ-solid solution. The surface of the quenched alloy is nickel because of the nickel redistribution between the γ- and α-phases during the α-γ-transition, selective iron oxidation, and increase  in the concentration of the unoxidized component in the superficial layer.

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