Neutron Measurement of Clustering in the Alloy CuNi

Abstract

Measurements have been made of the cluster diffuse neutron scattering from a polycrystalline sample of ${\mathrm{Cu}}_{0.525}$$\mathrm{Ni}^{62}_{0.475}$. This isotope and composition are ideal for such measurements since the effective scattering length for the Bragg reflections and for the temperature-diffuse and Huang scattering from static displacements is zero, while that for the cluster scattering is 0.811\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}12}$ cm. The short-range order parameters ${\ensuremath{\alpha}}_{1}$ through ${\ensuremath{\alpha}}_{9}$, representative of the system at approximately 550\ifmmode^\circ\else\textdegree\fi{}C, were determined by a least-squares fit to the scattering data. ${\ensuremath{\alpha}}_{1}=0.121$, while all other $\ensuremath{\alpha}$'s are less than $\frac{1}{9}{\ensuremath{\alpha}}_{1}$. Magnetic scattering effects in this sample are too small to have been detected within the accuracy of our measurements. Using an approximate theory of Clapp and Moss relating the short-range order parameters and pairwise interaction energies, first- and second-neighbor interaction energies of -0.0130 to -0.0122 eV and +0.0073 to +0.0078 eV are determined. From these, a critical temperature for phase separation of 233 to 263\ifmmode^\circ\else\textdegree\fi{}C is predicted, which is in reasonable agreement with that extrapolated from thermodynamic data of Rapp and Maak and of Oriani and Murphy.