2018 Volume 59 Issue 5 Pages 717-723
The effect of Ag additions to the four-phase, η + ε → α + τ′, was studied experimental and thermodynamically using Zn–22 mass%Al–2 mass%Cu base alloys with four Ag contents from 0 to about 4 mass% Ag. SEM and XRD results indicated the presence of the τ′ phase decreased as the Ag content increased for the aged alloys which can be attributed to the stabilization of the ε phase because of its increase in Ag content which is in agreement with Thermo-Calc results. The Ag addition also promoted a slowest decrease in hardness in the aging curves at 200°C which can be attributed to the slowest diffusion process for the quaternary alloy.
Zn-base alloys are used as bearing materials and they have the following advantages: good castability, good resistance to wearing and low cost.1–4) The principal alloying element for this type of alloys is aluminum. Furthermore, its mechanical properties are based on either hyper-eutectoid (Zn–27 mass%Al), eutectoid (Zn–22 mass%Al) and monotectoid (Zn–40 mass%Al) alloy compositions.4–6) The mechanical strength of Zn–Al alloys can be improved by the addition of copper because of the formation of the intermetallic compound, CuZn4, known as the ε phase which has a cph crystalline structure.6–11) Nevertheless, the aging process of these alloys may promote the formation of the (Al4Cu3Zn) τ′ phase with a rhombohedral crystalline structure following a four-phase reaction, η + ε → α + τ′, where the α and η phases correspond to the fcc Al-rich and cph Zn-rich phases, respectively.6–8,12–14) However, this four-phase reaction is responsible for the dimensional instability, increase in volume of about 4%.7,8) Thus, the four-phase reaction has been considered an important issue to be studied. There are several works which have analyzed the effect of addition of silver to the Zn–Al and Zn–Al–Cu alloys.14–16) For instance, Casolco et al.15,16) reported that the Ag addition between 4 and 6.15 mass% to the Zn–22 mass%Al alloy caused the formation of an intermetallic compound AgZn3 with a hcp crystalline structure. Additionally, the addition of about 1 Ag mass% to the Zn–22 mass%Al alloy has been observed14) to produce a strong grain refinement of the α and η phases. This fact has permitted to increase the elongation of the order of 650% in tension specimens tested at 230°C. Flores et al.17) reported that the Ag addition of 3 mass% inhibited the four-phase reaction at 200°C after 350 h because of the absence of the cph ε phase (CuZn4). They observed that Ag was combined with the Cu and Zn to form a Ag-containing cph intermetallic phase, which was named as the φ phase by them.
Therefore, it is important to analyze the effect of silver addition on the four-phase reaction to know more precisely the growth kinetics of this reaction. Besides, the presence of the Ag-containing cph intermetallic phase in this alloy seems to be a good alternative to inhibit the four-phase reaction and thus to overcome the problem of dimensional instability.
Therefore, the goal of this work is to analyze the effect of Ag addition to the four-phase η + ε → α + τ′ reaction on the growth kinetics and on the alloy hardness for the Zn–22 mass%Al–2 mass%Cu base alloys.
Table 1 indicates the chemical analysis, atomic absorption spectroscopy, for four Zn–Al–Cu and Zn–Al–Cu–Ag alloys, as well as its designation for this work. The preparation of alloys was carried out by melting of 99.9 mass% Ag, 99.99 mass% Cu, 99.9 mass% Zn and 99.7 mass% Al under an argon gas atmosphere in an electric furnace. Melts were continuous cast to obtain square ingots. Ingots were hot extruded at 350°C using an area reduction of 90% and extrusion rate of 7.1 mm s−1 to obtain a square bar of 10 mm side and 2000 mm long. Specimens of 20 × 20 × 5 mm were cut with a low-speed precision cut-off machine and subsequently homogenized at 350°C for 168 h (7 days) and then furnace cooled. Homogenized specimens were aged at 200°C for times of 0.5, 1, 3.5, 5, 10, 25, 50, 100, 250 and 500 h. Homogenized and aged specimens were analyzed by X-ray Diffractometer (XRD) using monochromated Kα copper radiation. Specimens were also metallographically prepared to be observed with Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) with backscattered electron image (BEI) at 20 kV. EDX analysis of phases was also conducted. Vickers microhardness was determined using a load of 200 gf and testing time of 12 s.
Figures 1–4 show the XRD patterns of ZACA, ZACA1, ZACA2 and ZACA4 alloys, respectively, for a 2θ angle of 20–37° and 41–46°. The XRD pattern of ZACA shows the presence of XRD peaks corresponding to the ε, η and α phases for the homogenized alloy (0 h), Fig. 1. As the aging time progresses, the ε phase diffraction peaks disappears and the diffraction peaks of τ′ phase are present after aging for 3.5 h. This fact suggest that the following four-phase reaction took place:15)
| \begin{equation} \eta + \varepsilon \rightarrow \alpha + \tau' \end{equation} | (1) |
X-ray diffraction patterns of the ZACA alloy aged at different times.
X-ray diffraction patterns of the ZACA1 alloy aged at different times.
Evolution of the X-ray diffraction patterns of the ZACA2 alloy aged at different times.
Evolution of the X-ray diffraction patterns of the ZACA4 alloy aged at different times.
Comparison of the X-ray diffraction patterns of the ZAC, ZACA1, ZACA2 and ZACA4 after (a) annealed treatment at 350°C and (b) aged at 200°C during 500 h.
The SEM micrographs for the temporal evolution of microstructure are shown in Figs. 6(a–c) during aging at 200°C of ZACA alloy. In the case of the homogenized alloy, an interlamellar microconstituent is clearly observed which was formed by the eutectoid reaction β → α + η, where the Al-rich α phase is imaged in black color, the Zn-rich η phase in white color. Additionally, the (CuZn4) ε phase corresponds to the isolated white regions. As the four-phase η + ε → α + τ′ reaction takes place, the τ′ phase precipitates appears gray colored small and irregular regions within the ε phase. The volume fraction increases with aging time, Figs. 6(b and c). On the other hand, the microstructure evolution of ZACA4 alloy is shown in Figs. 6(k–l) during aging at 200°C for different times. The lamellar constituent, composed of the α and η phases, is also observed. The ε phase, large white areas, can be also detected together with isolated small dark areas corresponding to the α phase. That is, the reactants for the four-phase reaction are present; however, the τ′ phase is not formed even for aging as long as 500, like in Figs. 6(b and c) with gray color. This result is consistent with the XRD results. The microstructure evolution of ZACA 1 and ZACA1, Figs. 6(d–i), shows that the presence of τ′ phase is very limited and it decreases with the increase in Ag content.
SEM micrographs of (a–c) ZACA, (d–f) ZACA1, (e–g) ZACA2 and (h–i) aged at 200°C for 5, 100 and 500 h, respectively.
The SEM-EDS lineal chemical composition analysis of ZACA1 and ZACA4 alloys after annealing and aging at 200°C for 500 h is shown in Figs. 7 and 8(a–b), respectively. In the case of the annealed ZACA1 alloy, the ε phase is in white color and the lineal chemical analysis indicates a content of about 80 at.% Zn, 10 at.% Cu, 4 at.%Ag and almost no Al content, Fig. 7(a). In contrast, a content of approximately 80 at.% Zn, 5 at.% Cu, 9 at.%Ag and almost no Al content is observed for the same phase of the annealed ZACA4 alloy, Fig. 7(b). That is, the ε phase shows a silver enrichment. After aging at 200°C for 500 h, the τ′ phase formation is evident for ZACA1 alloy with 40 at.%Cu, 30 at.% Zn and 30 at.% Al, Fig. 8(a), which is consistent with the equilibrium composition reported in the literature.14) In the case of the remaining ε phase for the aged ZACA1 alloy, the Ag content increases from 4 to 9 at.%, Fig. 8(b), and it is similar to that of the Ag-rich ε phase for ZACA4 alloy. These results suggest that the ε phase becomes stabilized when the Ag content reaches a value of about 9 at.%. That is, the Ag-containing εAg phase is not metastable to take part in the four-phase reaction. In general, the four alloys exhibit the coarsening of all phases with the increase in aging time.
EDX Al, Cu, Zn and Ag line analysis of ZACA1 (a) annealed and (b) aged at 200°C for 500 h.
EDX Al, Cu, Zn and Ag line analysis of ZACA4 (a) annealed and (b) aged at 200°C for 500 h.
The stabilization of the ε phase with the increase in Ag content was analyzed using the Thermo-Calc, TC, software.19) It is important to mention that this section of computational calculation is based on the SSOL5 data base, which only includes the assessed thermodynamic data for Ag-containing binary, Al–Zn and Al–Cu–Zn alloy systems. This means that the TC software calculates the effect of Ag addition on the phase stability for the Al–Zn–Cu alloy system using the previously mentioned thermodynamic data bases. That is, this calculation is only considering a thermodynamic prediction for the effect Ag on the equilibrium phases of the ternary alloy. Figures 9(a–d) present the TC calculated ternary and pseudo ternary Al–Cu–Zn phase diagrams at 200°C for ZACA, ZACA1, ZACA2 and ZACA4 alloys, respectively. TC calculated single points at 400°C indicate only the presence of the fcc β phase for all the alloy compositions. The equilibrium diagrams for ZACA and ZACA1 alloys indicate that the stable phases are the τ′, α and η phases for any composition of this three-phase field. However, the equilibrium diagrams for ZACA3 and ZACA4 alloys show that the ε phase begins to become stable for ZACA3 alloy and it is completely stable alloy for ZACA4 alloy. These results suggest that the increase in Ag content promotes the stabilization of the cph εAg phase. Furthermore, Table 2 shows the TC calculated equilibrium phases at 200°C for the four alloy compositions. There is a tendency to decrease the amount of η and τ′ phases with the increase in Ag content. On the other hand, the percentage of α phase trends to increase with Ag content. The ε phase appears only in ZACA4 alloy. Another interesting point is that the τ′ phase is also present in ZACA4 alloy according to TC calculations; however, this phase was no detected for the ZACA4 alloy after aging at 200°C for 500 h which can be attributable to its low volume fraction. Additionally, the TC calculated composition of formed phases is shown in Table 3. These results indicate that the τ′ phase shows no Ag presence for all alloys which shows a good agreement with results of Fig. 8(c) where no silver content is observed for the formed τ′ phase. On the other hand, the Ag solution is more evident in the η phase than in the α phase for ZACA1, ZACA2 and ZACA4 alloys. The highest Ag content of about 12 at.% corresponds to the ε phase for ZAC4 alloy. This content is consistent with the value of approximately 10 at.% observed for the same phase in Fig. 8(b). Thus, the previous results suggests that the four-phase reaction, η + ε → α + τ′, is almost completely suppressed for the aged ZAC4 alloy because the Ag-containing ε phase becomes stabilized and it cannot take part into the reaction. That is, the four-phase reaction is not inhibited by the absence of the ε phase, but because of the stabilization of the ε with the Ag addition to Zn–Al–Cu alloys.
Ternary and pseudoternary at 200°C for (a) ZACA, and (b) ZACA1, (c) ZACA2 and (d) ZACA4, respectively.
The aging curves for ZACA, ZACA1, ZACA2, ZACA3 and ZACA4 alloys, aged at 200°C for different times, is shown in Fig. 10. The lowest and highest initial hardness corresponds to ZACA and ZACA4 alloys, respectively, because they show the lowest and highest Ag content, respectively. As aging progresses, the hardness decreases with aging time for the four alloys which seems to be associated with the coarsening process of the different phases. The largest and fastest decrease in hardness is observed for aging of ZACA alloy which can be associated to the presence of the τ′ phase and the fastest coarsening process of phase because of the fast diffusivity of the ternary alloy in comparison to the quaternary alloys. Furthermore, the smallest and slowest decrease in hardness corresponds to the Ag containing ZACA1, ZACA2 and ZACA4 alloys.
Aging curves for ZACA, ZACA1, ZACA2 and ZACA4.
A study of the effect of Ag additions to Zn22 mass%Al–2 mass%Cu alloys on the four-phase reaction, η + ε → α + τ′, was carried out and the conclusion are the following:
The authors wish to acknowledge the financial support from Conacyt and SIP-IPN.
