## 6 Comparison of Nanoindentation Data with Mechanical Testing of Diffusion-Bonded Systems
These differences were attributed to the presence of different phases at the interface, with Mg\({}_{3}\)Ag and MgAg at the Mg/Ag/Al interface, while Mg\({}_{2}\)Al\({}_{3}\) and Mg\({}_{17}\)Al\({}_{12}\) were at the Mg/Al interface. The properties and behaviour of intermetallic phases were studied by [PERSON] et al. [53] who conducted microtensile tests and hardness tests on a range of bulk Al-Fe intermetallic specimens. The highest strengths and lowest hardness values were recorded on the AlFe\({}_{3}\) specimens, which were also significantly more ductile than the AlFe specimens, the mean  
Figure 12: Graphs showing (**a**) the variation of shear strength as a function of bonding temperature; and (**b**) the variation in hardness across the interface of diffusion bonded Al/Mg [13]. elongation at failure being 3.5% and 1.4%, respectively. The authors were unable to manufacture microdensile specimens from the harder Al\({}_{2}\)Fe, Al\({}_{5}\)Fe\({}_{2}\) and Al\({}_{3}\)Fe phases owing to their more brittle nature. [PERSON] et al. [68] diffusion bonded Ti-6 Al-4V to microduplex stainless steel at temperatures of between 850 and 1000 \({}^{\circ}\)C. Tensile and shear tests carried out to evaluate the mechanical properties of the bond revealed that the highest strengths were recorded on specimens bonded at 900 \({}^{\circ}\)C (see Figure 13). At lower temperatures, voids were seen at the interface due to incomplete bonding arising from insufficient thermal activation and elemental diffusion. The decline in both ductility and strength of the joint at bonding temperatures above 900 \({}^{\circ}\)C was attributed to the greater volume fractions of the intermetallic phases (\(\sigma\) and \(\lambda\) + FeTi) at the interface (at 850\({}^{\circ}\) C and below, the diffusion interface was free of intermetallic layers). This was apparent in microhardness measurements (also shown in Figure 13) made across the interface, which showed the maximum hardness to be 1260 HV\({}_{0.02}\) (12.36 GPa) on the specimen bonded at 1000 \({}^{\circ}\)C. In contrast, the maximum hardness of the interface recorded on the specimen bonded at 900 \({}^{\circ}\)C was 850 HV\({}_{0.02}\) (8.34 GPa). The properties are summarised in Table 2.  
In another study of Al bonded to steel, [PERSON] et al. [19] reported that although all specimens underwent brittle fracture, those heat treated at 500 \({}^{\circ}\)C for 180 min exhibited higher bond strengths than those treated at 550 \({}^{\circ}\)C for 10 min. This observation was attributed to the presence of a thicker intermetallic layer at the interface of the latter specimens, with thicker intermetallic layers leading to lower bond strengths. Examination of the fracture surface indicated that failure had occurred predominantly in this layer. This has also been reported by other researchers, notably [PERSON] et al. [66]. The differences between these two heat treatment conditions are shown in Table 3. [PERSON] et al. [19] also \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Bonding Temperature** & **Maximum Tensile** & **Maximum Shear** & **Maximum** \\ (°C) & **Strength (MPa)** & **Strength (MPa)** & **Interfacial Region** \\ \hline
900 & 520 & 405 & 8.34 \\ \hline
1000 & 293 & 212 & 12.36 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of mechanical property measurements made on Ti-6 Al-4V/microduplex stainless steel diffusion bonded at 900 and 1000 \({}^{\circ}\)C [68]. Figure 13: Mechanical properties of diffusion bonded Ti-6 Al-4V/microduplex stainless steel as a function of bonding temperature (**left**); and microhardness measurements made across the interfaces of specimens bonded at 900 and 1000 \({}^{\circ}\)C (**right**) [68]. reported that Al/Fe specimens treated at 550 \({}^{\circ}\)C for 60 min, which had thicker intermetallic layers at the interface, were found to fail during machining. Although nanoindentation cannot directly measure the strength of the bond, it nevertheless offers the ability to infer the likely characteristics of that bond.