On the determination of a local order parameter in a nickel-titanium alloy

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Abstract
A new method of determining the local degree of order by field-evaporation rate measurements (the plane stability ratio) is described. Its relationship with the field-evaporation behaviour of nickel and titanium atoms in partially ordered γ-Ni3Ti is fully discussed. Plane stability ratios measured at {200} poles were found to be the most sensitive to variation of order and yield information not only about the average local order in a small region but also about the progression of order through the interface between the disordered matrix and an ordered region. The field-ion microscope (f.i.m.) contrast of y’ precipitates in a nickeltitanium matrix and a method of distinguishing the nickel and titanium atomic species in the f.i.m. are also established. Thus the development of order as measured by plane stability ratio measurements can be complemented by direct studies of atomic arrangements.