The growth of structural modulations during the early stages of precipitation in gold-platinum alloys has been investigated using the Debye-Scherrer X-ray diffraction method. Three alloys having the compositions 40% Pt-60% Au, 60% Pt-40% Au, and 80% Pt-20% Au were examined over the aging temperature range 500°-600°C. The modulation wavelength was found to be nearly constant for short aging times and then to increase uniformly to a maximum value upon further aging. The maximum wavelength attained by the alloy of critical composition is smaller than the maximum observed for the 40-60 or 80-20 alloys: this was attributed to a maximum in elastic strain energy, due to coherency strains, at the critical composition. The change in wavelength growth rate with aging time is in qualitative agreement with the theory of spinodal decomposition.
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