Ordering Reaction of the Cu₄Pd Alloy

Abstract

The alloy Cu₄Pd has a disordered face-centered-cubic structure when quenched from temperatures between 478°C and the melting point (about 1100°C). Below 478°C an ordered phase is stable. The results of a Debye-Scherrer X-ray analysis indicate that the ordered phase has a tetragonal unit cell described by the space group C²_4h − P _42/m with 2 Cu in 2a, 2 Cu in 2f, 4 Cu in 4j (x = 0.2, y = 0.6), 4Pd in 4j (x = 0.4, y = 0.2), and 8 Cu in 8k (x = 0.1, y = 0.3). The orientation relationship between the face-centered-cubic phase and the ordered tetragonal phase is given by: $\begin{array}{c}{[100]}_{\text{ord.}}//{[130]}_{\text{dis.}}\\ {[001]}_{\text{ord.}}//{[001]}_{\text{dis.}}\end{array}$ The behavior of Cu₄Pd is typical of ordering alloys except that the transformation is very sluggish. The increase in hardness and the microstructural and X-ray diffraction effects are interpreted in terms of coherency strains caused by the ordering.