## 1 Introduction  
Currently technological advances, in particular, the development of rocket and space technology, nuclear energy, heat power engineering, microelectronics, aviation, mechanical engineering and medicine highlight the necessity for new materials and technologies or modification of known ones. The surface of parts in modern machines, airplanes or engines is under the heaviest load, since it is affected by the working environment and contacting machine parts at an early operation stage. Therefore, improvement of the product reliability and its surface parameters is a very acute issue. Applying special coatings on the surface of different products is one of the common processing methods, while the composition, structure and properties of the coatings may differ from the base metal.  
Various coating methods are used in industry currently. Scientific and technical experience of dealing with concentrated flows has shown that the electron beam is the most effective source of energy for external material processing [1, 2]. Moreover, it has the highest energy and absorption coefficient. Energy power and concentration in the beam vary from 1 MW and higher, which is not the case with laser and plasma. Consequently, the material is heated at a very high speed before evaporation. High speed evaporation of most refractory elements and chemical compounds is available, and the condensate can be applied on any material, (even incompatible) with the best adhesion and controlledstructure and properties. The coating thickness is easily controlled from the nanoscale and above (e.g., to millimeter range) [3,4].  
In the modern world, the development of nuclear energy leads to the increase in radioactive waste. It is well-known that tantalum (Ta) has a high radiation resistance to neutrons and the gamma radiation. At the same time, it has a high melting point. The most common substrate for applying a tantalum coating is aluminum, as it is a lightweight metal with a low melting point. Such materials can be widely used in nuclear reactors, containers for storing and transporting radioactive waste and in space rocket technology.  
Various factors influence physical and mechanical characteristics of tantalum coatings, the main of which are residual stresses significantly changing the surface morphology and adhesion to an aluminum substrate. The chief reasons for occurrence of the residual stresses are: the uneven temperature distribution in the substrate-coating system; a significant difference in thermal expansion coefficients and in the parameters of crystal lattices; phase and structural transformations in the substrate, coating, and the transition zone between them and the presence of various inclusions in the coating [5].  
The paper [6] presents the deposition method of Ni, Nb and Ta coatings with very good adhesion to an aluminum substrate at special temperature conditions. In order to form different temperature conditions for vapor phase condensation, a temperature gradient in the range of 100-600 \({}^{\circ}\)C has been created on the substrate. Research data were obtained after testing samples from different temperature zones. According to the results, the optimum range of tantalum condensation on aluminum substrate is 200-320 \({}^{\circ}\)C. The nature of the processes occurring at the substrate-Ta-coating interface is revealed at various stages of phase formation and is an integral part of the corresponding studies. Thus, the goal of the paper is to study the phase and elemental compositions, microhardness, and morphology of tantalum coatings on an aluminum gradient substrate obtained by electron beam technology at 300-450 \({}^{\circ}\)C condensation temperature.