Structure and Phase Transformation of Atom Cluster of Alloy

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In the present study, order-disorder and martensitic transformations in nano-scale particles of Cu-Au and Fe-Ni alloys, respectively, have been investigated by means of high resolution electron microscopy, electron diffraction and computer simulation, in order to make clear the origins of size effects on these phase transformations.Although atom clusters of Cu_3Au and CuAu alloys prepared by vacuum deposition have the same crystal structures and defect structures as those in bulk, it is found that there seem certain minimum sizes for the L1_2 and L1_0 atomic orderings of them, respectively, and that their critical temperatures, T_c, are remarkably depressed in the nano-scale particles. In order to make clear the origins of these size effects, we have made a computer simulation of the L1_2 ordering in the nano particles of Cu_3Au. The ground state energy of the nano particles is evaluated by utilizing the modified embedded atom method (MEAM), and then a computer simulation is made of th … More e order-disorder transition by Monte Carlo method with the many-body potential obtained by MEAM.The calculations have been made for three kinds of model crystals of the nano particles with 2 and 3 nm in diameter, containing 321 and 1061 atoms, respectively, and of a bulk with a size of 5x5x5 unit cells, on which the periodic boundary codition is imposed. As a result, it is found that the difference in ground state energy between perfectly ordered and disordered states becomes less in the nano particles than in the bulk, and that T_c decreases markedly with decreasing particle size. These results appear to account for the previous experimental ones consistently.Atom clusters of Fe-22.1-28.6at.%Ni alloys in as-deposited state are island-like, and consist of fine bcc martensite crystals and possibly oxide crystals of NiFe_2O_4. Upon heating at 773 K,they become an fcc single phase as a whole, consisting of isolated austenitic particles with average sizes of several nm in diameter. The austenitic particles thus formed are found to be entirely stable even when cooled to 105 K.This is true even in the case of an Fe-22.1at.% Ni alloy. But, when kept at 77 K for 10 d, the austenitic particles are found to be transformed into the martensitic ones. This fact indicates that contrary to previous reports the martensitic transformation does occur even in the nano-scale particles, although isothermally. Less