Structure and phase composition of Ti-Nb alloy produced by the method of selective laser melting
Features of the structure and phase composition of Ti-40 mas % Nb obtained in the process of selective laser melting were investigated in this paper. The alloy of such a composition has the lowest Young’s modulus among Ti-Nb alloys system and is applied as a material for medical implants. The structure of the specimens consisting of one and eight layers obtained at the same conditions of selective laser melting are studied by the methods of X-ray diffraction, scanning electron microscopy and energy-dispersive microanalysis. It is shown that the alloy consists of two phases. They are equilibrium β-Ti-Nb and non-equilibrium α''-phase. Presence of α''-phase is caused by nonequilibrium conditions of cooling and crystallization of material. The structure of multi-layered specimen repeats structure of monolayer. Monolayer in turn has gradient structure. Throughout the height of the monolayer the grain size and porosity change. Grain size of monolayer is in the range of 1-35 μm. Grain size slightly increases from low to up boundary of alloying zone. Circular form pinholes with the size 1-10 μm are formed in the low alloying zone. Pores are situated on the grains boundaries. There is less number of pores in the up alloying zone. The gradient structure of monolayer is defined by conditions of heatsink. With the growth of the distance from the surface of the contact with the powder the grain size becomes larger and the porosity becomes smaller. The multilayer structure is translated monolayer structure. It is recommended in the process of selective laser melting to create such conditions of cooling and crystallization of melted material when the bulk of the layer has the structure similar to the up zone of monolayer with sufficiently fine grain and low porosity.