Research on microstructure and properties of Cu-0.41%Cr-0.20%Zr alloy after solution-aging treatment

3.1. SEM

Fig. 1(a) shows the microstructure morphology of Cu-0.41%Cr-0.20%Zr alloy observed by scanning electron microscopy. The phase size in the alloy microstructure is micrometer scale, and there are mainly three phases: A phase (black spheroid), B phase (white strip) and C phase (black and white mixed phase). The C phase is further enlarged, as shown in Fig. 1(b).

Fig. 1Microstructure of Cu-0.41%Cr-0.20%Zr alloy (SEM)

3.2. EDXS

X-ray and EDXS qualitative and quantitative analysis methods were used to analyze the composition of phases in the microstructure of Cu-0.41%Cr-0.20%Zr alloy. Typical EDXS qualitative analysis spectra of these phases are shown in Fig. 2. It can be determined that there are mainly three phases in the microstructure of the alloy copper matrix phase, chrome-rich phase and zirconium-rich phase. Fig. 2(a) shows that the phase is Cr rich, and there are copper and zirconium peaks in addition to chromium peaks. The corresponding quantitative analysis results are shown in Table 1. The results show that the chromium content in the particles of A-phase is as high as 97.14. It can be determined that the A-phase is pure chromium phase. Since the solid solution treatment temperature of the alloy is 980°C, according to the Cu-Cr solid solubility curve, the solid solubility of Cr in pure Cu is low at this temperature. It can be judged that the phase is formed during the solidification process of the alloy and is not dissolved in the solution treatment.

The qualitative results in Fig. 2(b) show that there are only copper peaks and zirconium peaks. It can be judged that this phase is Zr-rich phase. It is possible that the rapid diffusion of Zr in Cu may lead to the reaction of the two elements to form Cu₂Zr. On the other hand, it may spread out to form Cu₅Zr. Combined with Fig. 1 and Fig. 2, it is assumed that the white precipitates and pits are two aspects of the same characteristics. If the pits of pure chromium are detected and surrounded by Zr-rich phase, their compositions most correspond to Cu₅Zr. Because it is assumed that chromium precipitates around the Cu₅Zr core in a pure copper matrix, most of the Zr-rich phase and Cr phase are distributed separately in the SEM observation experiment. However, these two phases can exist simultaneously, as shown in Fig. 1(b). The Zr-rich phase and Cr phase exist simultaneously, and the Cr-rich phase is embedded in the Cu₅Zr matrix (particle C). There is a clear interface between the two stages, and there is no intermetallic phase. According to the foregoing analysis, Cr phase should be formed in the solidification process of alloy. It can be assumed that the Cr-rich phase is solidified by the melt, and then the Zr-rich phase solidifies at the interface between the chrome-rich phase and the melt, with the Cr-rich phase acting as the core of the Zr phase solidification.

Chromium and zirconium can be enriched at the same time. The intermetallic phases of chromium and zirconium are not easily combined. Table 1 shows that the zirconium content of particles is as high as 14.45 %, and the Cu/Zr atomic ratio is 5.7. Considering the existence of matrix and the detection of actual data, it can be sure that the B phase of Cu-0.41%Cr-0.20%Zr alloy is Cu₅Zr. This result is consistent with the experimental results of Cu-Cr-Zr ternary system [11].

Fig. 2EDXS qualitative analysis of microstructure of Cu-0.41%Cr-0.20%Zr alloy

3.3. CTEM

Fig. 3 is the TEM microscopic image of the alloy, which is an analysis of the microstructure at the nanoscale. Fig. 3(a) is the bright field image; Fig. 3(b) is leaf contrast; Fig. 3(c) is the SAED of particle B in Fig. 3(a). Usually, SAED and XRD analysis can be used to analyze the crystal system of the material and the optimal growth orientation.

Fig. 3Microstructure of Cu-0.41%Cr-0.20%Zr alloy (TEM)

Fig. 3(a) is atypical TEM image of the solution aging sample. Particle A is foliated with coarse B and C particles. The aged precipitated phase is dispersed within the particles in the copper matrix, with the sizes ranging from 5 nm to 20 nm, as shown in Fig. 3(b). Since no additional reflection points were detected by selected electron diffraction and morphology and size except for the matrix, it was presumed that the particle is a coherent chromium particle. The results are similar to those of Cr particles in copper matrix. Fig. 3(c) shows the selected electron diffraction pattern point index of particle B shown in Fig. 3(a), with vertex B being Cu₅Zr.

3.4. HRTEM

Fig. 4(a) is the HRTEM micrograph of the alloy, and Fig. 4(b) is a typical EDXS qualitative analysis. In the CTEM observation, it is found that the three are decomposition products in different evolutionary states. Fig. 4(a) depicts HRTEM micrographs of a lattice fringe associated with (220) Cu. The fringe spacing measured under the microscope is about 0.13 nm, which is in good agreement with d220 (0.1278 nm) of Cu [12]. As the HRTEM micrograph shown in Fig. 4, the ordered bcc phases are similar to that in Fig. 3(c). A typical EDXS qualitative analysis spectrum for the chrome-rich phases is shown in Fig. 4(b) with copper peaks and chromium peaks. It can be assumed that the leaf contrast is pure chromium.

Fig. 4Microstructure of Cu-0.41%Cr-0.20%Zr alloy (HRTEM)

The HRTEM results show that the precipitated phase is pure Cr, and the particle size ranges from 1 nm to 20 nm. The purpose of observing and analyzing the microstructure of Cu-0.41%Cr-0.20%Zr alloy is to understand the strength mechanism and composition of precipitated phase, so as to provide some theories for strengthening the properties. The aging precipitated phase is the strengthened phase formed by the decomposition of supersaturated solid solution during the process of solid solution aging. According to the data, because the precipitated reinforced phase size is between 1-100 nm, coarse particles have little influence on the performance. The improvement of the mechanical properties of the alloy depends on the successful redistribution of the alloy content of the aging precipitated phase during solidification. The zirconium content of the alloy is reduced to 0.1 % or less.

3.5. Discussion

Table 2 shows the mechanical properties and electrical conductivity of Cu-0.41Cr-0.20Zr alloy after 980C×2h solution, water quenching, 450C×20 h aging and air cooling. Fig. 3(a) shows the precipitation enhanced form 1nm and 500 nm. According to the type of precipitated phase and its orientation relationship with the matrix, the strengthened phase of Cu-0.41 cr-0.20 Zr is Cu₅Zr and elemental Cr, and the elemental Cr of body-centered cubic and Cu conform to the N-W orientation relationship [13].

These aging precipitates together with the copper matrix produce peak hardness [14]. Hardness increases with an empirical O’Rowan relationship as shown in Eq. (1):

where $\tau$ is the increase of shear stress; $k$ is a constant; $f$ is the volume fraction of precipitated strengthened phase, and $R$ represents the diameter of the precipitated strengthened phase. According to the TEM analysis in Fig. 3, the volume fraction of precipitated phase $f$ is about 30 %. The constant $k$ is 2.7 N/m. The more the volume fraction of precipitated phase, the smaller the aging precipitated phase, the more the increase of $\tau$, and the higher the alloy hardness.

The empirical formula for the electrical conductivity of aging precipitated alloys is shown in Eq. (2) [15]:

where $\sigma$ is the electrical conductivity of alloy; ${\sigma }_0$ is the electrical conductivity of solid solution matrix; ${\sigma }_i$ is the conductivity of precipitated phase, and $V_i$ is the volume fraction of precipitated phase. The electrical conductivity of the alloy is proportional to the volume fraction of the precipitated phase.

It can be seen that the heat treatment process with a 2-hour aging of 20 hours in a 980 °C solution can improve the strength and conductivity of Cu-0.41%Cr-0.20%Zr alloy. The volume fraction of the precipitated phase can be further improved by improving the process conditions, which further increases the hardness and conductivity.