Study on The Order-Disorder Phase Transition And Long-Range Order Parameter of Ni-Pt Alloy

Abstract

Study on The Order-Disorder Phase Transition And Long-Range Order Parameter of Ni-Pt Alloy Order-disorder phase transition of Ni05Pt05 equiatomic alloy and doped with Fe and Co with composition Ni045Fe005Pt05 and Ni0 Ni0.45 Fe0.05Pt0.5 were studied X-ray diffraction (XRD) method. The specimens are prepared, using arc furnace, with high purity constituent elements. Ni-Pt system undergoes disorder-order transformation at high temperature from fcc (face centered cubic) structure to fct (face centered tetragonal) in the Ni-Pt alloy. Lattice parameters of the three disordered and three-ordered Ni-Pt and doped alloys were determined. A disordered alloy of all these three compositions are fcc with almost no change in lattice parameter Ni-Pt in the disordered state has lattice parameter of a=3.7468Å, while ordered one is fct having a=3.8187Å and c=3.6028Å with c/a ratio of 0.943. A slight increase of c/a ratio has been observed for Fe and Co doped alloys. The lattice parameter of the ordered fct phase of Ni0.45 Fe0.05Pt0.5 and Ni0.45Co0.05Pt0.5are a=3 .8141Å, c=3.6069Å and a=3.8174Å c= 3.6154Å with c/a ratio of 0.945 and 0.947 respectively. The average value of long-range order parameter "s" has been found to be 0.951 for the equiatomic NiPt alloy using in-situ high temperature attachment of XRD, indicating the alloy to be almost perfectly ordered. It has been calculated that out of 1 000Pt sites in the crystal, 975Pt atoms occupied their places and the rest 25Pt sites were occupied by Ni atoms. It has been also observed the difference in the lattice parameter due to order-disorder transition is explained as arising from the associated change of local environment. Fe and Co doped Ni-Pt alloys have been found to be multiphase in contrast to the undoped counterpart. The phases have been identified to be FePt, FeNi, NiPt for Fe doped and CoPt, NiCo, NiPt for Co doped alloys.