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This study and a second part dedicated to the mechanical characterization provide a better knowledge of tungsten (W)-vanadium (V) alloys reinforced with yttrium oxide (Y2O3) particles, which have been scarcely investigated. Two W alloys (W-2 or 4 wt.% V-0.5 wt.% Y2O3) and a pure W material processed by powder metallurgy and consolidated by hot isostatic pressing were analysed. Along this part, the microstructure of the materials at room temperature is mainly analysed with a field emission scanning electron microscope. The densification in the compacts shows an increase with the V and Y2O3 additions. Porosity is reduced because of the formation of a W-V solid solution and V pools that fill the pores between the grains, although such effect is mainly observed in the W2V0.5Y alloy. The microstructure of pure W is composed of coarse polyhedral grains whereas a few coarse W grains, V pools and a nanostructured area, composed of fine W grains with dispersed Y, form the alloys. In contrast to previously studied W-4 wt.% V alloys, the V pools exhibit a reduction in the oxygen content, which prevents the formation of acicular oxide structures. Finally, the refinement of the microstructure induced by the addition of V and Y2O3 was analysed by electron backscattered diffraction measurements. Pure W presents high amount of grains over 1 mu m (around 60% of the volume fraction) and only 2% below 100 nm. In the new alloys meanwhile, the population of micron size grains is highly reduced to less than 10% and grains smaller than 100 nm represent the 20%. (C) 2015 Elsevier Ltd. All rights reserved.