Disorder–Order Phase Transformation in Ni50Pt50 and Ni20Pt45Al35 (at %) Alloys

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X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were used to investigate the disorder–order phase transformation of Ni50Pt50 and Ni20Pt45Al35 (at %) alloys. DSC analyses showed that the disorder–order transformation from the face centered cubic (fcc) structure to the face centered tetragonal (fct) L10 α-NiPt(Al) structure occurred in Ni50Pt50 at 660°C and in Ni20Pt45Al35 at 1250°C, respectively. Transmission electron microscopy and XRD analysis revealed L10 NiPt and L10 α-NiPt(Al) phases with superlattice reflections (110) and (100) appeared in diffraction patterns of the Ni50Pt50 and Ni20Pt45Al35 alloys annealed at 1100°C for 72 h and 500°C for 120 h. At room temperature, the annealed Ni50Pt50 alloy contained both disordered fcc NiPt and ordered L10 NiPt phases with (001)[010 ]_L1₀NiPt ] || (001)[010]fcc NiPt relationship, only ordered L10 α-NiPt(Al) phase was present in Ni20Pt45Al35 alloy indicating that Al is an α-NiPt(Al) stabilizer.