Measurements of atomic order with the field-ion microscope

Abstract

Field-ion studies of several near stoichiometric alloys exhibiting long-range order have been made with the intent of extracting quantitative data on atomic ordering parameters. The alloys studied include PtCo, Ni 4Mo, FePt and FePt 3. Methods of calculating the long-range order parameter directly from a field-ion micrograph (direct counting and optical transformation) have been developed and detailed techniques are described. Optical transforms of computer simulated masks indicate that order parameters can be deduced within 4 % accuracy. Similar measurements from experimental images are considerably less accurate, but there is promise that this may be greatly improved in the near future. The direct counting method is employed to determine the long-range order parameter of the alloy FePt as a function of temperature. These values are compared with those calculated by the theories of Bragg and Williams and Cowley and with experimental X-ray data. All values were in good agreement with one another in the low-temperature range; however, they diverged considerably as the temperature approached T c. The primary reason for this divergence is due to an observed region in which two phases (ordered and disordered) coexist in equilibrium. The experimental evidence supports an ordering mechanism based upon nucleation and growth near the critical temperature, accompanied by a homogeneous transformation within each domain.