## 1 Introduction  
Diffusion bonding is a solid-state process in which two nominally flat surfaces are joined at an elevated temperature using an applied interfacial pressure for a period ranging from a few minutes to a few hours [1]. The process is usually carried out in a vacuum or inert gas atmosphere at temperatures in the range of 0.5-0.8 \(T_{m}\), where \(T_{m}\) is the melting temperature in Kelvin [2, 3]. Diffusion bonding has been exploited in a variety of applications ranging from electronics and sensors to the aerospace and nuclear industries [4, 5, 6, 7]. It is a batch process, which, together with the need for optimisation, can limit component throughput [3]. Nevertheless, it does offer a means of joining metals that may not be bonded using more common techniques such as welding titanium alloys and stainless steels. Diffusion bonding has been found to offer an attractive alternative to fusion welding [8], which has been found to cause the formation of brittle intermetallic compounds at the interface and consequent degradation in the joint strength. Distortion of the component can also occur owing to wide disparities in coefficients of thermal expansion and the presence of residual stresses [9]. During the bonding process, pores formed by the surface roughness of the two materials are filled by the interdiffusion of atoms across the interface, resulting in the formation of diffusion layers. Depending on the mutual solubility of the parent materials, these layers may be solid solutions or intermetallic compounds formed of separate crystallographic phases. The process variables that affect the strength and integrity of the bond are: (i) bonding temperature; (ii) bonding pressure; and (iii) holding time at pressure [3, 10]. The bonding temperature will influence the extent of interdiffusion across the interface, as well as affect the formation of any reaction layers and the thickness of such layers. Under pressure at high temperature, plastic deformation of the asperities of the two surfaces takes place [5],closing the pores between them and fracturing the oxide layers on the two parent material surfaces, which can impede diffusion and hinder the bonding process, for example in stainless steels [6]. The time at pressure, like the bonding temperature, also influences the formation and extent of any reaction layers. However, excessive bonding temperatures and/or holding times can be detrimental to the bond strength if it causes grain growth [11] or the formation of undesirable intermetallic compounds at the interface [12; 13]. The latter was demonstrated by [PERSON] et al. [13] who investigated the effect of holding times of between 15 and 90 min for magnesium (Mg) bonded to aluminium (Al) at 390 \({}^{\circ}\)C and a pressure of 5 MPa. They found that the shear strength of the bond was at its highest for a holding time of 30 min. Longer times resulted in reduced bond strengths, which was attributed to the formation of brittle Mg-Al intermetallic compounds at the interface. Similar findings were reported by [PERSON] et al. [14] who bonded Mg to Al at temperatures of between 475 and 500 \({}^{\circ}\)C and times of between 80 and 100 min (the pressure was 1 MPa). They found that increasing both temperature and holding time increased the thickness of the Al\({}_{3}\)Mg\({}_{2}\) and Al\({}_{12}\)Mg\({}_{17}\) at the interface: the growth of these compounds also resulted in a reduced bond strength, as demonstrated by fractography of the bonded joints following shear strength tests, which revealed fracture to occur at the bond interface in an almost wholly brittle manner. One way by which the formation of undesirable intermetallic compounds at the interface may be suppressed is to use interlayers to inhibit elemental diffusion between the two parent materials [12]. Such interlayers have been used in many forms, including foils, coats and nanoparticles [8].