## 6 Comparison of Nanoindentation Data with Mechanical Testing of Diffusion-Bonded Systems  
The studies discussed above have demonstrated the ability of nanoindentation to identify and measure the mechanical properties of discrete regions adjacent to diffusion-bond interfaces. Often these regions will contain deleterious phases which can degrade the bond strength, making their presence undesirable. When used in conjunction with techniques such as EPMA and XRD to understand the elemental and crystallographic nature of the interface, nanoindentation can therefore provide an indication of when a bond is likely to be unsuitable. In many systems, the presence of phases of high hardness will equate to a lower ductility and reduced ability to resist deformation. This has been noted in a number of studies [13; 53; 67]. In contrast, when the formation of intermetallics at the interface is suppressed in favour of a solid solution (for example, by the use of interlayers) a higher-strength bond is achieved [12]. Indeed, for TiAl diffusion bonded to AISI Type 310 stainless  
Figure 11: Nanoindentation results of (**a**) microhardness profile across the TC4/Al\({}_{5}\)(HfNbTiZr)\({}_{95}\) joint bonded at 950 \({}^{\circ}\)C/60 min/5 MPa, (**b**) corresponding experimental load–displacement curves at different positions [11]. steel, the use of a Ni/Al interlayer resulted in the joint having a lower hardness than those without an interlayer but also an approximately fourfold increase in the shear strength [20]. [PERSON] et al. [13] studied the influence of interfacial intermetallic phases on the bond strength in Mg/Al joints. They performed microhardness measurements across the interface of Mg bonded to Al; an interlayer of Ag was used to prevent the formation of hard and brittle Mg-Al intermetallic compounds that can form at the interface when these two metals are bonded directly. For this reason, a 2.5 \(\upmu\)m coating of Ag was deposited by magnetron sputtering onto both Mg and Al prior to bonding. Diffusion bonding was carried out at a range of temperatures from 380 \({}^{\circ}\)C to 440 \({}^{\circ}\)C for 30 min at a pressure of 5 MPa; the effect of holding time was also investigated by bonding specimens at 400 \({}^{\circ}\)C for durations of between 15 and 90 min. Microhardness measurements across the interfaces were carried out using a load of 10 g. Graphs showing the variation in microhardness across the interface, together with the effect of bonding temperature on shear strength of the joint, can be seen in Figure 12. The mean hardness values of the two parent metals (Mg and Al) were 44 HV (0.43 GPa) and 75 HV (0.74 GPa), respectively. The bond with the highest shear strength was that formed at 390 \({}^{\circ}\)C (at this temperature, the Mg-Al compounds were absent); the diffusion zone formed at this temperature exhibited the lowest hardness--125 HV (1.23 GPa)--of all the bonding temperatures; the bond was found to consist of Mg/Mg\({}_{3}\)Ag/MgAg/Al. The shear strength declined at higher bonding temperatures, a trend accompanied by an increase in the hardness of the diffusion zone, the highest value--368 HV (3.61 GPa)--being recorded on the specimens bonded at 440 \({}^{\circ}\)C. XRD of fracture surfaces showed that at temperatures of 410 \({}^{\circ}\)C or above MgAl intermetallic compounds (e.g., AgMg\({}_{6}\)Al\({}_{4}\), Mg\({}_{17}\)Al\({}_{12}\), Mg\({}_{2}\)Al\({}_{3}\)) had formed, which was accompanied by a decreasing shear strength and a concomitant increase in hardness of the diffusion zone. The fracture surfaces also exhibited a brittle appearance. In another study of Mg/Al, [PERSON] et al. [67] reported that, for Mg bonded to Al with a Ag interlayer, the mean shear strength of the bond was 14.5 MPa, while for Mg/Al the corresponding figure was 3.4 MPa; in contrast, the maximum microhardness values measured at the interface for the two systems were 1.23 GPa (Mg/Ag/Al) and 2.21 GPa (Mg/Al).