Deformation-thermal method for atomic ordering and mechanical improvement of Cu 3 Pd alloys

. The paper presents studies of the mechano-structural characteristics of the atomically ordered Cu 3 Pd alloy subjected to severe plastic deformations (SPDs) at room and cryogenic temperatures combined with subsequent annealings. Transmission and scanning electron microscopy (TEM and SEM), X-Ray diffraction (XRD) analysis and microhardness tests were used as research methods. The mechanical characteristics of the synthesized Cu 3 Pd alloy were compared after preliminary SPDs at room and cryogenic temperatures as well as during subsequent annealings. The significant acceleration of the atomic ordering kinetics and, consequently, effective diffusion during recrystallization annealing after cryodeformation was found. The developed deformation-thermal method can be used to synthesize high-strength nanostructured resistive and electrocontact materials.


Introduction
The most important characteristic of atomically ordered alloys is the possibility to obtain a low electrical resistance in them. However, an atomic ordering is a diffusion-controlled process and, in some cases, it takes a significant amount of time. It is inefficient in modern production environment.
The paper demonstrates that kinetics and atomic ordering degree of the Cu3Pd alloy can be influenced by both preliminary severe plastic deformations (SPDs) and subsequent annealings [1,2]. Moreover, it was found that it is very important to accurately select the temperature and time of annealing to create the most favorable thermokinetic conditions for atomic ordering [3][4][5].
The research goal is to compare the microstructure and mechanical characteristics of the Cu3Pd alloy after preliminary SPDs at room and cryogenic temperatures as well as subsequent annealings, in particular, to show the impact of the deformation degree and the temperature of subsequent annealings on the kinetics of atomic ordering. It is assumed that a decrease in the deformation temperature should suppress recrystallization processes, maintain a high dislocation density, and possibly activate mechanical twinning as a mechanism for additional plastic deformation in Cu3Pd alloys with low and medium stacking fault energies. This will increase the efficiency of cryogenic SPDs compared to those at room temperature and, therefore, will favorably impact the microstructure, mechanical characteristics and kinetics of atomic ordering.

Materials and methods
The ingot of the Cu3Pd alloy was synthesized with high-purity Cu (99.99 %) and Pd (99.98 %) by vacuum melting. The above ingot was cut into several Cu3Pd samples. The grinding and polishing of the samples were performed on a Buehler MetaServ 250 machine (Buehler, Germany).
The SPDs were performed by torsion under high hydrostatic pressure (6 GPa) on Bridgman anvils at room and cryogenic temperatures. The temperature before the deformation process was 78-80 K. Some of the samples after SPDs were annealed in evacuated Pyrex ampoules in the PM-1.0-7 electric furnace.
The annealing regimes (350, 400, 450, and 500 °C) were chosen taking into account the previous work [6] in order to obtain atomic ordering and simultaneously preserve the submicro-and nanocrystalline structure of the samples. The annealing exposures of the samples were 1 and 24 hours.
The XRD analysis was performed by DRON-4 diffractometer. The electron microscopic studies were carried out using SEM Quanta 200 Pegasus (FEI Company, Netherlands) and TEM JEM-200 CX (JEOL Ltd., Japan) microscopes [7]. The PMT-3 microhardness tester was used for microhardness research.

Results and discussion
The structural changes in the Cu3Pd samples after SPDs at room and cryogenic temperatures are qualitatively similar (Figure 1). The dislocation clusters and networks with appeared the cellular structure were observed while maintaining the local short-range atomic order at the low cryodeformation degree (Figure 1, a). The grain refinement occurred with the increase of the deformation degree up to the nanostructured state with the grain size of about 50 nm. In this case, the electron diffraction patterns are characterized by an annular shape which also indicates a severe grain refinement. The maximum degree of the deformation achieved per 10 revolutions was 7.3. According to Figure 1, the nanofragmentation degree under the same conditions at cryogenic or room temperatures is qualitatively similar.
Cryogenic SPDs proceed according to the same mechanism as at room temperature with the exception of differences in the degree of accumulated hardening [1]. This is evidenced by the data on microhardness obtained in the study. Figures 2 and 3 present the analysis of the microstructure and diffraction patterns of the synthesized Cu3Pd samples. The formation of the identical ultrafine-grained atomically ordered structure during annealing occurs faster in the sample subjected to SPD at cryogenic temperature. The atomically ordered structure of the Cu3Pd sample shown in Fig.  2 was formed after annealing at 450 °C for 1 h. Such the ultrafine-grained structure for the Cu3Pd sample in Fig. 3, b appeared only after annealing for 24 h at the same temperature.  The microhardness of the Cu3Pd samples was measured from the center in three directions along the radius. Figure 4 shows the microhardness dependences of the Cu3Pd samples after SPDs at cryogenic (curve 1) and room (curve 2) temperatures depending on the number of revolutions and annealing temperature. It was found that microhardness of the Cu3Pd samples after SPD at cryogenic temperature is approximately 1000-1500 MPa higher than at room temperature. At the same time, the values of the microhardness of the Cu3Pd samples subjected to SPDs at room and cryogenic temperatures and after subsequent annealing take similar values under increasing of the annealing temperature. It is important that the microhardness values of the Cu3Pd samples remain at a sufficiently high level.
Abnormal grain growth in Cu3Pd samples after SPD begins at the certain temperatures ( Figure 5). The recrystallization of the samples after cryogenic SPD and subsequent isothermal annealing begins at significantly lower temperatures (above 300 °C) than at room temperature (from 450 °C).

Conclusion
The paper demonstrates research of the mechano-structural characteristics of the atomically ordered Cu3Pd alloy subjected to SPDs at room and cryogenic temperatures combined with subsequent annealings. The following main results were obtained: 1.
The SPDs by torsion per 5-10 revolutions, both at room and cryogenic temperatures, ensures the refinement of the grain structure of the Cu3Pd samples up to the nanostructured state.

2.
The significant acceleration of the atomic ordering kinetics and, consequently, effective diffusion during recrystallization annealing after cryodeformation was found. The obtained Cu3Pd samples are characterized by the ultrafine-grained structure. 3.
The developed deformation-thermal method can be used to synthesize high-strength nanostructured resistive and electrocontact materials.