## 1 Introduction  
Reactive joining utilises the heat produced by reactive nano-multilayers for soldering or brazing. The reactive nano-multilayers can be employed either as free-standing reactive foils (RFs) [1, 2] or as nano-multilayer coatings which are directly deposited onto the joining components [3]. With the localised, in situ produced heat provided by the exothermal reaction of the nano-multilayers, reactive joining represents a very flexible joining technique, e.g., enabling to solder heat-sensitive materials and components at room temperature without use of a furnace. Envisioned and partly already realised fields of applications are, for instance, the assembly of microsystems (e.g., mounting of strain gauges and infra-red emitters [4]), hermetic sealings [5, 6], and metal-ceramic bonds for manufacturing of sputter targets [7]. However, successful joining with RF requires careful adjustment of the produced heat (e.g., via foil composition and/or foil thickness): On the one hand, an overproduction of the heat can negatively affect the integrity of joining components, e.g., by local grain coarsening [8]or re-melting [9] of the component material in the vicinity of the RF, or by mechanical damage of the material via thermal shock [10]. On the other hand, if an insufficient amount of heat is provided by the exothermal reaction, the soldering or brazing material will not (or only partly) be melted, thus hindering the creation of a firm bond between RF, solder and components. Notably, the time-temperature profile evolving in the joining zone depends not only on the properties of the RF, i.e., the total amount of heat and the rate of heat production, but also on the thermal properties of the components to be joined, i.e., on their thermal conductivity and heat capacity [11]: For a given amount of heat produced by the reactive foil, a high thermal conductivity of the components can lead to rapid heat dissipation through the joining components (in extreme cases, even the self-heating reaction within the reactive nano-multilayer can be quenched), while a low thermal conductivity of the joining parts can lead to a pronounced heat pile-up within the joining zone [10]. This problem is similar to other joining processes which utilise a localised, dynamic heat source, e.g., electron-beam welding or laser welding [12, 13]. Accordingly, comparable approaches are utilised to model the dynamic joining process, although the description of the localised heat source naturally greatly differs (cf. e.g., [12, 13] and [11, 14, 15]). The relation between the exothermal reaction of the nano-multilayers, the thermal properties of the joining components and the ensuing time-temperature-profile has been modelled in detail and experimentally investigated for reactive joining with Ni-Al RF and AuSn solder in combination with a stainless steel and an aluminium alloy, i.e., materials with relatively low to moderately high thermal conductivities [1, 11]. In comparison to conventional furnace soldering with AuSn, superior mechanical properties were observed for the reactively soldered joints, attributed to the refinement of the solder microstructure due to the rapid cooling upon reactive joining [1]. In addition, a pronounced effect of the thermal conductivity of the different alloys on the lamellar microstructure of the AuSn solder was found [11]. Hence, in order to fully exploit the possibilities of reactive joining, understanding of reactive joining down to the microstructural level is required. Therefore, in the present work, the influence of thermal conductivity on the reactive joining process is systematically investigated with focus on the evolution of the microstructure within the bonding zone and its influence on the mechanical performance of the created joint.