X-ray analysis of zinc-copper alloys

Abstract

Alloys containing up to 38 per cent. zinc constituting the α brasses are found to possess a face centred cubic lattice, the parameter increasing from 3.608 A for pure copper to 3.696 A for an alloy containing 38 per cent. zinc. Solution takes place by the substitution of zinc atoms for copper atoms. (a) The X-ray analysis of a brass containing 48.5 per cent. zinc by weight showed that the β brasses possess a centred cubic structure, the side of the unit cube being 2.946 A. (b) Specimens quenched from 550° C. and annealed below 470° C. gave identical spectra and showed no sign of the presence of the α phase. The evolution of heat observed at 470°C. is not accompanied by a change of crystal structure, and is similar to the α-β transformation in iron. It is not accompanied by a breaking up of the β constituent into a mixture of α and γ. (c) Specimens containing 43.7 per cent. zinc by weight were examined. The first annealed at 750° C. and quenched showed the reflections due to β brass only. The second annealed at 500° C. and quenched, and the third annealed at 400° C. consisted of a mixture of the α and β phases. The results of X-ray analysis are in agreement with the constitutional diagram of Shepherd. The γ phase is found to crystallise on a rhombohedral hexagonal structure of side 4.136 A and axial ratio 0.6495. The epsilon phase gave a spectrum characteristic of a close-packed hexagonal lattice of side 2.718 A and axial ratio 1.585. (a) The zinc-copper system crystallises on a rhombohedral hexagonal lattice from copper to γ brass, and on a close-packed hexagonal system from (or possibly δ) brass to zinc. The observed parameter of the lattice of brass suggests that the copper atom causes the zinc atom to rotate about an axis perpendicular to a 1120 plane, until the zinc atoms in successive 0001 planes are separated by a distance equal to the side of the lattice of pure zinc. Fusion would then take place at about the same temperature in both cases. (b) The very low value of the density previously ascribed to an alloy containing 10 per cent. copper is not confirmed. The value found for the density of this alloy is 7.35 grms./c.c. The observed density of the alloys examined is in good agreement with the value computed from X-ray data. (c) The hardness of the brasses attains a maximum in the region of γ brass, where the atomic volume curve shows the greatest departure from the straight line joining the atomic volumes of copper and zinc. The hardness of the α phase is ascribed to local distortion and that of the β phase to the difference in type of lattice. The relatively very great hardness of γ brass is due to small atomic volume and loss of symmetry. (d) The fact that the atomic volume curve consists of two straight lines suggests the existence of an allotrope of zinc with an atomic volume of 13.92 A3. Consideration of the value of the parameter of the lattice of the β phase suggests that this phase is due to an allotropic modification of copper, with an atomic volume of 12.79 A3.