## 3 Results and Discussion  
### Stress-Strain Response  
Figure 1a-c show the stress-strain curves of the NiAl alloys with 5 at%, 6 at% and 7 at% Cu, respectively. It is seen that for all specimens, the flow stress varies with both the strain rate and the Cu content. For a given Cu content, the flow stress increases with increasing strain rate. Moreover, for a given strain rate, the maximum stress increases with increasing Cu content. Overall, it is seen that the strain rate has a greater effect on the flow stress than the Cu content. In addition, it is noted that specimen fracture occurs more readily in the specimens with a lower Cu content. For the specimens containing 5% Cu, the fracture strain decreases with increasing true strain (Figure 1a). However, as the Cu content is increased to 6%, the fracture strain increases (Figure 1b). For the maximum considered Cu content of 7%, fracture occurs only at the highest strain rate of 5 \(\times\) 10\({}^{3}\) s\({}^{-1}\). In other words, both the strengthening properties and the ductility of the NiAl specimens increase with an increasing Cu addition. Figure 1d compares the unalloyed NiAl alloy with different Cu contents under strain rate of 10\({}^{-1}\) s\({}^{-1}\) and 25 \({}^{\circ}\)C. It can be seen that the stress of unalloyed NiAl (binary) is the lowest.  
The stress-strain curves presented in Figure 1 show that the specimens deformed at different strain rates have a different work hardening response. In general, the stress-strain response of engineering metals and alloys can be described by the power law:  
\[\sigma\,=\,A\,+\,B\epsilon^{n} \tag{1}\]  
where \(A\) is the yield strength, \(B\) is the material constant and \(n\) is the work hardening coefficient.  
Table 1 presents the results obtained for \(A\), \(B\) and \(n\) for the present NiAlCu specimens by curve-fitting the experimental data in Figure 1. It is seen that for a constant Cu content, the yield strength, material constant and work hardening coefficient all increase with increasing strain rate. Moreover, it is noted that the material constant (\(B\)) has a higher value under quasi-static loading than under dynamic loading. Thus, it is inferred that the dislocation density and multiplication rate increase at higher strain rates and give rise to an enhanced flow resistance. For a given strain rate, the yield strength, material constant and work hardening coefficient all increase with increasing Cu addition. In other words, a higher Cu content not only increases the flow stress, but also the fracture strain. Table 2 shows the hardness values of the different NiAlCu specimens. It is seen that the hardness increases with both an increasing strain rate and an increasing Cu content.  
Figure 1: Stress–strain curves of NiAl intermetallic alloys deformed at strain rates of \(10^{-3}\)–\(5\times 10^{3}\) s\({}^{-1}\): (a) Ni\({}_{63}\)Al\({}_{32}\)Cu\({}_{5}\); (b) Ni\({}_{63}\)Al\({}_{32}\)Cu\({}_{6}\); (c) Ni\({}_{63}\)Al\({}_{32}\)Cu\({}_{7}\) and (d) comparison of four NiAl alloy.  
In general, the precipitation of secondary phase \(\gamma^{\prime}\) from the \(\beta\)-phase results in alloys with a greater hardness and a reduced susceptibility to brittle fracture [27]. Furthermore, lattice defects in the intermetallic structure may increase or decrease the intermetallic hardness depending on lattice distortion [28]. In the present study, a notable strengthening of the binary NiAl intermetallic alloy system occurs as the Cu content is increased. Hence, it is inferred that Cu addition results in major lattice distortion due to substructure defects, and leads to a greater microhardness as a result.