Deformation Behavior and Processing Parameters of Cu-3Ag-0.6Zr Alloy during Compression at Elevated Temperatures

Hot compression tests of the Cu–3Ag–0.6Zr (wt.%) alloy were carried out on a Gleeble-3800 testing machine at various strain rates and at different deformation temperatures. The stress–strain behavior of the alloy during the deformation process are analyzed. The results show that the hot deformation behavior of the Cu–3Ag–0.6Zr alloy is markedly affected by the interaction between the work hardening and the dynamic recrystallization at different strain rates and temperatures. The thermal deformation activation energy is calculated as 248.8384 kJ mol 1 and the hot compression constitutive equation has been established. The processing map of the alloy has also been established. According to the obtained processing map, the preferable domains for hot working are identified at a strain rate of 10 s and at 900–940 K or temperatures higher than 1023 K.


Introduction
Recently, copper alloys have been considered for aerospace application such as the thrust chamber of liquid rocket engine because of their high thermal conductivity [1]. Although pure copper also has excellent thermal conductivity and is compatible with propellants, the low strength restricts its application in the thrust chamber. Consequently, a small amount of alloying elements (like Ag and Zr) are added to the pure copper to improve its mechanical properties [2]. NARloy-Z alloy is one such alloy designed to manufacture the combustion chamber liner of liquid rocket engine [3,4].
The plastic deformation performance of metals and alloys under high temperature conditions, which has a significant inÀuence on the hot workability, can be expressed by the relationship between Àow stress, strain rate and deformation temperature [5,6]. However, the hot deformation behavior and processing map of Cu-Ag-Zr alloys has not yet been extensively studied, which is a prerequisite for its usage in hot working processes.
In this paper, compression tests on one particular Cu-Ag-Zr alloy, Cu-3Ag-0.6Zr, were carried out at various strain rates and at different deformation temperatures to reveal its hot deformation behavior. The stress-strain curves of this alloy were investigated, and the constitutive equation and processing map were established using the experimental data.

Material and Experimental Procedure
The Cu-3Ag-0.6Zr (wt.%) alloy investigated was prepared by induction melting. Then the ingot was forged at the temperature range of 1073-923 K. A block with dimensions of 100 mm × 100 mm × 40 mm was cut from the forged piece, and then homogenized at 1193 K for 40 min and water quenched. The cylindrical specimens with 8 mm diameter and 12 mm height were machined from the heat-treated block. Hot deformation experiments were carried out on a Gleeble-3800 testing machine at imposed strain rates of 10 -3 s í1 , 10 -2 s í1 , 10 -1 s í1 , 1 s í1 and at testing temperature of 873 K, 923 K, 973 K, 1023 K, respectively, up to a total deformation of 50% (i.e., the true strain is about 0.693). The sample was resistance heated to deformation temperature by thermocoupled-feedback-controlled AC current at a heating rate of 10 K/s and held for 180 s before compression.

Results and Discussion
Stress-strain Curves of Cu-3Ag-0.6Zr Alloy. Figure 1 shows the typical true stress-strain curves of the Cu-3Ag-0.6Zr alloy compressed at various strain rates and at different deformation temperatures. At the lowest strain rate (10 -3 s -1 ) and the two lower temperatures, the flow stress increases to a maximum value at ¿rst and then decreases to a steady state ( Fig. 1(a)). Such Àow behavior is attributed to the interaction between the work hardening and the dynamic recrystallization (DRX) [6]. In the early stage of hot compression, the work hardening dominates the deformation process and the flow stress increases. Then the work hardening and the softening caused by DRX balance at the point of peak flow stress. As the deformation proceeds, the effect of DRX softening is gradually rising, so the flow stress decreases. DRX softening occurs much earlier as temperature increases to 973 K and 1023 K. When the strain rate rises to 10 -2 s -1 ( Fig. 1(b)), the flow stress slowly increases with increasing strain at a relatively low temperature (873 K). As the strain rate further increases to 10 -1 s -1 and 1 s -1 , the flow stress of this alloy rises markedly with increasing strain at a temperature lower than 1023 K, as shown in Fig. 1(c) and (d). This suggests that the work hardening plays the dominant role during the entire deformation process. However, the Àow stress remains on a steady value as the deformation temperature increases, indicating the balance between the work hardening and dynamic recovery. The flow stress of Cu-3Ag-0.6Zr alloy increases with an increase in strain rate when deformed at the same temperature. However, the thermal softening effect will be more pronounced as the deformation temperature rises.

Constitutive Equation.
In the hot deformation process of metals, the relationship between strain rate ( H ), temperature (T) and flow stress (V ) can be expressed as [7]: where, A is the structural factor, D is the stress multiplier, n and R are constants, Q is the activation energy. The coef¿cient Q can be used to characterize the hot workability of materials, which is an important thermodynamic parameter. The power law, Eq. 1, can be expressed as follow if 0.8 D V where, E is also a constant, and In order to calculateD , l n H is plotted versus ln V as shown in Fig. 2(a), and ln H is plotted versus V as shown in Fig. 2(b). From the above, 1 n and E are 7.90543 and 0.06372, respectively, and the value of D is calculated as 0.00806 MPa -1 from these values. According to the above natural logarithmic equations, the relationship between the activation energy (Q) and the deformation temperature (T) can be expressed by the following equation: l n s i n h l n 1 / l n s i n h The quantity ln[sinh(DV )] is plotted versus 1000/T in Fig 3(a) and ln[sinh(DV )] is plotted versus l n H in Fig. 3(b). From the slope of these curves, the value of Q is calculated as 248.8384 kJ mol í1 .

D V H
The relationship between strain rate, deformation temperature and Àow stress can also be analyzed using the Zener-Hollomon parameter (Z), which is proposed by Sellars In order to calculate n and lnA , ln Z is plotted versus ln[sinh(DV )] as shown in Fig. 4. The slope n is calculated as 5.87049 and the intercept lnA as 26.83336, so A is 4.50381×10 11 s -1 .
Based on above analysis, the constitutive equation of the Cu-3Ag-0.6Zr alloy is determined as: Processing Map. The processing map for the Cu-3Ag-0.6Zr alloy obtained at a true strain of 0.55 is shown in Fig. 5, which is constructed by overlapping a power dissipation map and an instability map. In fact, processing maps made at different strains are essentially similar [9]. The processing map is classi¿ed into two regions: stability domain (unshaded) and instability domain (shaded). The contour lines represent the ef¿ciency of power dissipation, which corresponds to the microstructural changes during hot deformation [6]. Figure 5 shows that the alloy is not suitable for processing under high strain rates. The ef¿ciency of power dissipation increases with decreasing strain rate. However, as the deformation temperature increases, the stability domain gradually expands, implying the reduction in the possibility of unstable deformation of the Cu-3Ag-0.6Zr alloy. The suitable hot processing temperature is about 900-940 K or a temperature higher than 1023 K according to this processing map. In both domains the ef¿ciency of power dissipation amounts to 31%.