**1. Introduction**

New strategies in the development of metallic materials for advanced structural applications involve the synthesis of bulk compounds that contain metallurgical bond. The bi- or multi-layered composites with built-in specific functionalities are an example of such materials. They o ffer an optimum balance between manufacturing and service costs, and the durability to perform under various conditions of usage. For materials used in the chemical industry, the proper combination of strength and high anticorrosive resistance, usually at high temperatures are especially important. The tantalum (Ta) and stainless steel composite is one of the industrially relevant bi-layered metallic material used in this field [1–4]; tantalum provides excellent corrosion resistance, while the stainless steel substrate is typically used as a load-bearing component. In most of the corrosion situations, it is capable to protect all installations exposed to highly oxidizing or caustic environments, where glass-lined equipment is subject to mechanical damage or thermal shock failures. Since the high cost of tantalum has traditionally been a major impediment in wide-scale industrial applications, such as large pressure vessels, therefore, it is advisable to use them rather as a coating on carbon, stainless or 'duplex'-type steels. Ta cladding is often used as an alternative to Ta coatings for fabricating coating parts out of solid Ta. Compound materials of this type (e.g., large vessels/tanks) are both structurally and cost-e ffective.

The Ta/stainless steel composites in the form of sheets/plates are di fficult (or impossible) to produce via conventional methods of joining due to metallurgical incompatibility between joined components (high di fference in the melting points of these metals, Ta at 3290 K and Fe at 1811 K). Therefore, explosive welding (EXW) is, at present, the only e fficient way of surface joining of Ta and stainless steel sheets (Figure 1). However, further processing of bi-layered Ta/stainless or carbon steel composites is strongly restricted. This results from serious di fficulties of a butt joint formation during 'conventional' welding due to limited heat transfer from the heat-a ffected zone. Moreover, if one attempts to use fusion welding to joint Ta to steel, the molten pool tends toward the eutectic composition (they are formed even well below the melting point of Ta) and then form brittle intermetallics upon solidification [1]. To solve problems associated with conventional welding of the Ta/stainless steel clads an intermediate layer, made of soft material and high thermal conductivity, such as copper (Cu), is used. Besides rapid heat dissipation, Cu has one more advantage, i.e., it does not react metallurgically with Ta.

**Figure 1.** (**a**) Schematic representation of explosive welding in two steps: (**a**) stage I—formation of a Cu/stainless steel (SS) clad, (**b**) stage II—formation of Ta/Cu/SS clad, and (**c**) cutting of Cu/SS base plate before the second stage of joining.

In earlier works on di fferent metal compositions, a lot of attention was put into an explanation of the correlation between the clad strength and the interface waviness and the quantity of solidified melt zones, e.g., [5,6]. However, it becomes increasingly apparent that not the details of the interface waviness but the complex microstructure of interfacial layers determines the mechanical and some physical properties of the clad [7–13]. In the light of such evidence, various microstructural transformations can be distinguished. On the one hand, the shear stresses, which occur due to oblique collision of the sheets are responsible for strain hardening and turbulent flow of the interfacial layers. This leads to the formation of wavy interfaces between the joined sheets. Since the interfacial layers are subjected to severe plastic deformation [14–18] they can easily undergo recovery and recrystallization. On the other hand, the processes of fast heating followed by fast cooling during clad preparation result in the formation of solidified melt zones of di fferent structures, phase composition and mechanical properties [7–11,19,20].

Explosive welding of Ta and other metals have received much attention so far, referring to, e.g., Ta/Cu/steel [5,21], Cu/steel [22–24] and Cu/Ta [25–27]. The Cu–Ta system is characterized by nearly zero mutual solubility of the components in the solid-state [28,29] and high structural and mechanical stability at elevated temperatures. Greenberg et al. [30], Maliutina et al. [31] as well as Bataev et al. [11] have shown that explosively welded Ta and Cu sheets exhibit a heterophase mixture in the reaction region with the size of the dispersed Ta and Cu particles similar to those of colloids. As also documented by Bataev et al. [11] and Parchuri et al. [32], the TaxCu1-x based intermetallics or decagonal quasicrystals were found to coexist along with pure Ta and Cu particles in the solidified melt zones at the Ta/Cu interface. The formation of metastable phases can also be expected due to rapid cooling during the solidification of melted volumes. In earlier works, the metastable phases in Ta–Cu system were investigated by Cullis et al. [33] who observed the metastable substitutional solid solution in the form of thin films. Furthermore, amorphous phases were observed by Natasi et al. [34] and Gong et al. [35], whereas the nano-crystalline phases by Purja Pun et al. [36] and Rajagopalan et al. [37]. On the other hand, a Cu/stainless steel interface contains solidified melt zones that are exclusively composed of intermetallic phases of different chemical compositions and various morphology of grains, e.g., [16,19]. Moreover, independently on metals composition, radical temperature changes [10,11] can lead to remarkable microstructural transformation in near-the-interface layers of the bonded sheets. In earlier works, a lot of attention was paid to the role of solidified melt zones with respect to strength properties, whilst significantly less effort has been directed towards characterizing their internal microstructure. To the best of the authors' knowledge, there is an absence of detailed studies of the Ta/Cu/stainless steel metals combination that specifically discussed the strain hardening, recovery, and recrystallization of interfacial layers.

Therefore, this work is intended to show interconnected phenomena that must be considered in the interfacial layers of joined sheets in the Ta/Cu (M1E)/(304L stainless steel) composite at the Ta/Cu (M1E) and Cu/304 L stainless steel interfaces. The microstructures and chemical composition changes are analyzed using scanning (SEM) and transmission (TEM) electron microscopes equipped with energy dispersive spectrometry (EDS) detectors. Since the interfacial layers are subjected to severe plastic deformation, which can undergo partial recrystallization, the high-resolution electron backscattered diffraction (EBSD) facility was used as a suitable tool to study the microstructural changes occurring in the parent sheets (areas of not mixed original sheets excluding melted zones). In order to support microstructural findings, the mechanical properties were evaluated using microhardness measurements, whereas the integrity of the joints via lateral bending test.
