Short-Range Order in α-Brass

Abstract

Quenched equilibrium states of Cu–31.1 at.% Zn and Cu–22.4 at.% Zn single crystals (prepared with the Cu-65 isotope) were investigated by elastic diffuse neutron scattering. The diffuse intensity showed maxima which are attributed to the flat portions of the Fermi surface in the <110> directions. Short–range order parameters and linear displacement parameters were obtained from a fit to the measured data. Pair interaction energies were determined based on the inverse Monte–Carlo method. An ordered low-temperature phase Cu3Zn with the DO23 structure is suggested.

Understanding the Cu-Zn brass alloys using a short-range-order cluster model: significance of specific compositions of industrial alloys

“…In accordance with the relatively - large negative Warren-Cowley short-range-order parameters for the 1 st neighbor (e.g., α 1 = −0.1373 in 31.1 at.% Zn 12 ), a Zn solute atom is nearest-neighbored by twelve Cu solvent atoms occupying the (1,1,0) × 2/ a positions ( a being the FCC lattice constant), forming a Zn-centered cuboctahedral cluster, [Zn-Cu 12 ].…”

“…In accordance with the relatively - large positive Warren-Cowley short-range-order parameters for the 2 nd neighbor (e.g., α 200 = 0.1490 in 31.1 at.% Zn 12 ), the central solute Zn is 2 nd -neighbored with Zn situating at the (2,0,0) × 2/ a positions, expressed by the formula type This formula (1) describes the Cu-Zn alloys showing relatively strong short-range-order tendencies in both the 1 st and the 2 nd neighbors, covering a composition range of [Zn-Cu 12 ]Zn 1 and [Zn-Cu 12 ]Zn 6 , or 14.3 ≤ at.% Zn ≤ 36.8, 14.6 ≤ wt.% Zn ≤ 37.5.…”

“…Thereof, we propose the following structural model for stable solid solutions to describe the ideal solute distribution in FCC α-brasses: In accordance with the relatively - large negative Warren-Cowley short-range-order parameters for the 1 st neighbor (e.g., α 1 = −0.1373 in 31.1 at.% Zn 12 ), a Zn solute atom is nearest-neighbored by twelve Cu solvent atoms occupying the (1,1,0) × 2/ a positions ( a being the FCC lattice constant), forming a Zn-centered cuboctahedral cluster, [Zn-Cu 12 ]. In accordance with the relatively - large positive Warren-Cowley short-range-order parameters for the 2 nd neighbor (e.g., α 200 = 0.1490 in 31.1 at.% Zn 12 ), the central solute Zn is 2 nd -neighbored with Zn situating at the (2,0,0) × 2/ a positions, expressed by the formula type This formula (1) describes the Cu-Zn alloys showing relatively strong short-range-order tendencies in both the 1 st and the 2 nd neighbors, covering a composition range of [Zn-Cu 12 ]Zn 1 and [Zn-Cu 12 ]Zn 6 , or 14.3 ≤ at.% Zn ≤ 36.8, 14.6 ≤ wt.% Zn ≤ 37.5. In accordance with the weaker Warren-Cowley short-range-order parameters in the Zn-lean alloys 13 , the six 2 nd -neighbors at the (2,0,0) × 2/ a positions are occupied by a mixture of Cu and Zn, expressed by the formula type This formula (2) describes the Cu-Zn alloys showing relatively weak short-range-order tendencies in the 2 nd neighbors, covering a composition range of [Zn-Cu 12 ]Cu 6 and [Zn-Cu 12 ](Zn 1 Cu 5 ), or 5.3 ≤ at.% Zn ≤ 10.5, 5.4 ≤ wt.% Zn ≤ 10.8.…”

“…It has been long suspected that behind the many “anomalous” behaviors at specific Zn concentrations lies a short-range-order mechanism in α-brasses (see for instance 10 11 and the references quoted therein), involving internal friction, stress relaxation, yielding, work-hardening, activation energy of creep, activity coefficient, specific heat, cold-working, electrical resistance, etc. The first direct evidence of short-range ordering was provided by a neutron-diffuse-scattering experiment in combination with a Monte Carlo simulation on an α-brass single crystal containing 31.1 atomic percent (at.%) Zn 12 . The Warren-Cowley short-range-order parameter for the nearest-neighbor position (1,1,0), α 1 = −0.1373, is negative, signifying that the dissimilar Cu-Zn nearest order is favored.…”

Composition formulas of Cu-Ni industrial alloy specifications

“…Obviously, the choice of industrial specifications deter-mines the properties, and the correlation between composition and short-range-order of the solid solutions is the key to understanding industrial alloy selection. However, the short-range-order description generally using Cowley short-range-order parameter α [11,12] is problematic, because it only tells the composition deviations on each neighboring shells from the average ones, but provides no information on the possible local structural units that may guide the selection of alloy compositions. This problem has been addressed in the case of FCC solid solutions as exemplified by Cu-Zn alloys [1], using the cluster-plus-glue-atom model for stable solid solutions.…”