High-strength Cu–Zr binary alloy with an ultrafine eutectic microstructure

Abstract

In this study, we synthesized Cu–Zr binary alloys reinforced with an ultrafine eutectic microstructure. The alloys consisted of alternating layers of a hard superlattice phase and a ductile Cu phase with a very fine interlamellar spacing of ∼60 nm. The superlattice phase enhanced the strength of the alloys while the laminated composite structure helped improve their plasticity, making their mechanical properties comparable to those of the earlier reported high strength alloys. This paper discusses the fundamental microstructural aspects that influence the mechanical properties of these alloys.

Appendix

The structure of the superlattice phase was identified by HRTEM, HR simulation, and molecular statistics. Considering that the superlattice phase is based on the structure of Cu₅Zr intermetallic compound (fcc, F^¯₄3m), structural characterization was performed using crystallographic and thermodynamic data for Cu₅Zr. The HRTEM observation was interpreted using JEMS simulation (Electron Microscopy Software, JAVA version, JEMS, EMS). In addition, the enthalpy of formation (ΔH_f) was calculated by a modified embedded-atom method (MEAM) interatomic potential for pure Cu28 and Zr.29 The most stable atomic configuration of the superlattice phase was identified by calculating Zr atomic positions using the various enthalpies of formation by substituting each Cu site in Cu₅Zr with a Zr atom.

Figure A1 shows HRTEM micrographs of the superlattice observed at three zone axes corresponding to (a) [010], (b) [130], and (c) [031], respectively, and (d) a Kikuchi map of the Cu₅Zr phase. The map makes possible evaluation of the orientation relationship of the interposed plane in the superlattice. From the HRTEM image recorded at the zone axis of [010] in Fig. A1(a), a typical superlattice fringe can be observed with a modulated contrast perpendicular to [001]. Also, the HRTEM images were recorded at the zone axes corresponding to [130] and [031]. Figures A1(b) and A1(c) were acquired at the zone axes of [130] and [031], respectively. In Figs. A1(b) and A1(c), the modulated contrast was still observable after tilting around the [001] axis, whereas it disappeared when tilting around the [100] axis. This suggests that the superlattice plane was interposed only in the [001] direction, even though the cubic system has three equivalent directions for 〈100〉. From the structural characterization of the HRTEM images and diffraction patterns, the specific growth direction of the superlattice corresponds to the [001] direction.

FIG. A1

HRTEM images and corresponding SADPs of the superlattice recorded at three different zone axes of(a) [010],(b) [130], and (c) [031];(d) Kikuchi map of Cu₅Zr.

Figure A2(a) shows HRTEM micrographs of the superlattice phase observed at the zone axis of [001], showing a typical superlattice fringe with the modulated contrast. Hereafter, the bright and dark layers in Fig. A2(a) are referred to as layer I and layer II, respectively. To investigate the chemical composition of the modulated layer in the superlattice phase, a line profile across layers I and II was examined at the zone axis of [010] using electron energy loss spectroscopy (EELS), as shown in the Fig. A2(b). The EELS spectra shows that layer I has a lower Cu content than layer II. Considering that the basic structure of the superlattice is based on the Cu₅Zr intermetallic compound, a lower Cu content in layer I is thought to be due to the substitution of Cu with Zr.

FIG. A2

(a) HRTEM image and electron diffraction pattern recorded from the superlattice at the zone axis of [010].(b) Variations in the Cu content across layers I and II, obtained by EELS and EDS.

Figure A3(a) shows a magnified HRTEM micrograph near the interface of layers I and II, as indicated by the broken rectangle in Fig. A2(a), while Fig. A3(b) is the HRTEM image simulated under acceleration voltage of 200 keV, spherical aberration of C_s = 1.2 mm, energy spread of 0.6 eV, and defocus of −64 nm. The numbers marked on the calculated HRTEM image indicate the atomic sites of Cu that can be substituted by Zr in order to construct the superlattice phase. Enthalpies of formation were calculated using molecular statistics for each position of Zr in Fig. A3(b). The calculation result shows that the enthalpy of formation for fictitious atomic site 1 was −12.890 kJ/mol of atoms, while other positions had positive values. This fact reflects that the substitution of Cu atom in site 1 by a Zr atom induces the structural stability of the superlattice phase due to the minimum enthalpy of formation. Therefore, it is believed that the superlattice phase is a cubic system with the Cu atomic site 1 replaced by a Zr atom. Although this superlattice phase is in the same space group with Cu₅Zr and its atomic position is identical to Cu₅Zr phase, they—i.e., the Cu₅Zr-based super lattice and the Cu₅Zr intermetallic compound—are composed of totally different crystal structures. The specific atomic sites of Cu atomic position [(0.25, 0.75, 0.75), (0.75, 0.75, 0.25)] in Cu₅Zr, are periodically replaced by Zr atoms, and then the atomic sequence is finally changed. Considering the above experimental results, HR simulation was performed. As shown in Fig. A4, the simulated HR image and the real image show the same superlattice fringe with the modulated contrast. To the best of our knowledge, the superlattice phase is a new phase, which has not yet been reported in the Cu–Zr equilibrium phase diagram.23

FIG. A3

(a) Magnified view of the broken rectangle indicated in Fig. A2(a).(b) Simulated HR image of the same region, showing the atomic positions of Cu and Zr.

FIG. A4

(a) Simulated HR image and(b) real image of the superlattice at the zone axis of [010].