## 1 Introduction
For alloys with bcc structure TiVZrNb (a = 3.3047 A), TiVZrNbHf (a = 3.3664 A) and TiV\({}_{0.5}\)ZrNbHf (a = 3.3911 A), the enthalpy of hydrogen absorption decreases in absolute value with increasing the cell parameter from 67.6 to 61.8-59.1 kJ/mol H\({}_{2}\)[8]. This trend is the opposite of that observed for other types of metal hydrides (AB, AB\({}_{2}\) and AB\({}_{5}\)) where the hydride stability (that is the formation enthalpy) increases with an increase in the interstitial void size [33; 34; 35]. The reported data on the enthalpy of hydride formation are of particular interest considering some well-known metal-hydrogen systems with similar equilibrium hydrogen absorption pressure, but a higher enthalpy of hydrogenation (for example, \(-\)75 kJ/mol H\({}_{2}\) for MgH\({}_{2}\), [36]). In addition to the conventional analysis of pressure-composition isotherms (PCI) by means of the Van 't Hoff equation, calorimetric studies can provide some useful items of information on the hydrogenation behavior of alloys. Calorimetric measurements directly give the values of the enthalpy of the process at a certain fixed temperature at all its stages, and not only in the region of the phase transition. This is of special importance in the case when there are no clear boundaries of the phase regions on the isotherms. The effectiveness of such a thermochemical analysis of metal-hydrogen systems, the so-called calorimetric titration, has been demonstrated in [37; 38], including for disordered alloys. The need for high-temperature activation seriously complicates the use of this experimental research technique. We have proposed the deposition of a thin palladium layer at the alloy surface in order to eliminate the high-temperature pre-treatment stage and ensure the hydrogen interaction with the HEA at room temperature. Palladium coating is a known approach to improving different characteristics of alloys in the reaction with hydrogen. It was found for the AB\({}_{5}\) alloy (A = La\({}_{0.34}\)Ce\({}_{0.50}\)(Pr,Nd)\({}_{0.16}\); B = Ni\({}_{0.63}\)Co\({}_{0.13}\)Mn\({}_{0.12}\)Al\({}_{0.12}\)) that surface modification with palladium produced by electrodeposition from the solution greatly improves the activation characteristics and the resistance to poisoning at the interaction with hydrogen. In particular, the hydrogen absorption rate after prolonged exposure to air increases about 100 times for surface-modified materials compared to the unmodified alloy [22]. Also, an increase in the rate of hydrogen uptake and easier activation after the palladium coating has been observed for TiFe-based alloys [39] and TiMn\({}_{1.52}\)[40]. In this work, the hydrogen absorption properties of EBM-PDME (electron beam melting with pendent drop melt extraction) TiZrVNbTa fibers with a palladium surface coating were studied. This crucible-less processing mode is suitable for materials with a high melting point [41; 42]. Due to the high (up to 10\({}^{6}\) K/s) cooling rates, microcrystalline and amorphous materials can be obtained. The absorption isotherm for the first cycle has been measured at room temperature without any preliminary activation; the maximum sorption capacity was 1.7 wt.%. The enthalpy of absorption in a wide range of hydrogen concentrations has been evaluated using Tian-Calvet calorimetry. The data obtained allows us to suggest the prospects of the proposed approach to a deeper study of the processes of hydride formation in HEAs.