## 5 Nanoindentation of Diffusion-Bonded Interfaces
### [PERSON] et al. [62] used nanoindentation to study the diffusion bond of uranium (U) to copper using vacuum hot pressing. Bonding was carried out at temperatures of between 350 and 650 \({}^{\circ}\)C for between 200 and 240 min. Examination of the bonded specimens revealed the presence of a mixed U-Cu interfacial region approximately 10 \(\upmu\)m in thickness, which was identified as UCu\({}_{5}\) from XRD. The hardness of this region was 5.2 GPa, compared with 4.0 GPa for U and 1.6 GPa for Cu. The elastic modulus of UCu\({}_{5}\) was 121 GPa, which was between that of U (160 GPa) and Cu (88 GPa).  
[PERSON] et al. [63] bonded NiTi to Ti\({}_{3}\)SiC\({}_{2}\) at temperatures of between 800 and 1000 \({}^{\circ}\)C in an Ar atmosphere for durations of between 1 and 10 h. Examination of the bonded materials in transverse section indicated an interfacial structure consisting of Ti\({}_{2}\)Ni, Ti\({}_{5}\)Si\({}_{3}\) and NiTiSi. Using nanoindentation to map this region with a grid of 10 \(\times\) 10 indents at a spacing of 3 \(\upmu\)m, it was found to exhibit both a higher hardness and elastic modulus than either the NiTi or Ti\({}_{3}\)SiC\({}_{2}\).  
[PERSON] et al. [11] used nanoindentation to evaluate a refractory high-entropy alloy (RHEA)-Al\({}_{5}\)(HfNbTiZr)\({}_{95}\)--bonded to a Ti-6 Al-4V alloy. The materials were bonded at a pressure of 5 MPa for 1 h at temperatures of between 800 and 1000 \({}^{\circ}\)C. Nanoindentation and shear testing were used to assess the mechanical properties of the bonded specimens. The highest shear strengths were recorded on specimens bonded at 950 \({}^{\circ}\)C; examination of the fracture surface revealed a ductile appearance, in contrast to the specimens bonded at lower temperatures which were found to behave in a more brittle manner. The microstructure of the interface was found to consist of two distinct zones, one of which was a transformed Ti structure resulting from interdiffusion between the two parent alloys; the other zone was a  
Figure 10: Nanoindentation measurements showing the distribution in hardness values across the (**a**) W/V, (**b**) V/Ni and (**c**) Ni/steel interfaces of a W/V/Ni/steel diffusion bond [60].  
solid solution with a similar structure to the RHEA. Nanoindentation of the interface (see Figure 11) indicated the hardness and elastic modulus to be between those of Ti-6 Al-4V and Al\({}_{5}\)(HfNbTiZr)\({}_{95}\). XRD of the specimen bonded at 950 \({}^{\circ}\)C revealed the presence of Ti on both fracture surfaces, which suggested that failure had occurred within the Ti alloy rather than at the interface and demonstrates the strength of the bond formed.  
[PERSON] et al. [9] bonded stainless steel Type 321 to a Ti-5 Al-2.5 Sn alloy by diffusion bonding at temperatures ranging from 750 \({}^{\circ}\)C to 950 \({}^{\circ}\)C for between 10 and 30 min. The properties of the bond were evaluated by both shear strength testing and nanoindentation. Microstructural examination of the bonded materials revealed four distinct layers at the interface, the thickness of which increased both with bonding temperature and time. The highest shear strength, as well as greatest ductility, was recorded on specimens bonded at 850 \({}^{\circ}\)C. At temperatures lower than 850 \({}^{\circ}\)C the bond was found to be incomplete (residual porosity), while the lower strengths of specimens bonded at higher than 850 \({}^{\circ}\)C were attributed to the increased presence of intermetallic phases (e.g., FeTi and Fe\({}_{2}\)Ti) which also led to more brittle behaviour with failure occurring through these regions. Nanoindentation measurements, which were carried out by making a 6 \(\times\) 6 grid of indents across the interface, also showed the hardness of the interfacial region to be significantly higher on the specimens bonded at 950 \({}^{\circ}\)C than those bonded at 850 \({}^{\circ}\)C.