**3.2 Microstructure of brazed joints of molybdenum with stainless steel at the application of brazing filler metal of Cu-Mn-Ni-0.2Si**

Lowering of silicon concentration in brazing filler metal from 1 to 0.2 wt. % does not eliminate the formation of reaction layers on the interface of brazing filler metal molybdenum (**Figure 6(a)** and **(b)**). At brazing temperature of 1100°C (τ = 5 min), reaction layers also form along the brazed seam at the interface with molybdenum.

The results of micro X-ray spectral analysis showed that silicon concentration in the reaction layer based on molybdenum (63.41%) does not exceed 0.92% (**Figure 6**), which is significantly lower than in the previous sample (in brazing with Cu-Mn-Ni-Fe-1Si brazing filler metal). The quantity of the other component elements, namely, iron, chromium, nickel, and manganese, is on the same level as in the previous sample (**Table 3**—Spectrum 1). The obtained investigation results show that microcracks form in some regions of the reactive layer (**Figure 6 (a)**). They are located normal to the reaction layer and base metal plates. Cracks are absent in the central zone of the brazed seam, consisting of a solid solution based on copper and dispersed inclusions.

According to binary phase diagrams, consisting of metallic systems, the molybdenum-iron system has considerable regions of solubility at high temperatures.

**Figure 6.**

*Microstructure (a) and studied regions, in which chemical composition (b) was determined in the brazed joint of molybdenum-stainless steel (Cu-Mn-Ni-0.2Si brazing filler metal).*

However, with temperature lowering, these regions are quickly reduced, and at room temperature, the mutual solubility is practically absent. Some intermetallic phases form between the considered areas [13].
