The dissociation of an alloy of copper, iron and nickel Further X -ray work

Abstract
In a previous paper it was shown that X-ray evidence led to the conclusion that a series of intermediate structures is formed during the dissociation of the alloy Cu 4 FeNi 3 . Attempts to determine these structures failed because they led to X-ray intensities that did not agree with those observed, although their positions were adequately accounted for. In the present paper it is shown that the intensities are modified in a systematic way by extinction, and that, after introducing a correction for extinction, the theory advanced can account for all the data. In the type of structure postulated there is a periodic variation of the lattice parameter. Concentration differences are set up in the alloy at regular intervals, while the coherence of the single-phase lattice remains. The wave-length of the periodicity is of the order of 10 -6 cm. On prolonged annealing at constant temperature the diffraction pattern changes gradually. It is possible to follow the wave-length and the amplitude of the periodic variation as a function of time and temperature over a considerable interval. It has been found that the amplitude is independent of time at a given temperature and that the wave-length increases linearly with the logarithm of time. The values of wave-length and amplitude as a function of time and temperature are reproducible within the limit of error, so that the measurements have a quantitative value. After longer times of annealing the diffraction pattern of t-he periodic state gradually changes into another pattern, which can be explained either by a periodic structure with a range of longer periods or by tetragonal lamellae. The bearing of the present observations on the -kinetics of phase change is discussed.

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