BIOCOMPATIBLE METAL-MATRIX COMPOSITES BASED ON TWO-PHASE TI-18NB-8ZR ALLOY REINFORCED WITH BORIDES

This investigation intends to study composites based on Ti18Nb8Zr alloy, which have a two-phase structure consisting of equiaxed β grains with acicular intra-granular precipitates of α-martensite. The use of this alloy as matrix alloy and the implementation of martensitic transformation together with the introduction of TiB particles will allow to achieve a possible synergistic effect from the implementation of a whole set of strengthening mechanisms: solid-solution, deformation, grain boundary strengthening, dispersion hardening, as well as strengthening from boride fibers. Ti18Nb8Zr/(Ti,Nb)B metal matrix composites were vacuum-arc remelted in pure argon; the weight percentage of TiB2 in the initial charge before melting was 0.7 and 2.0 wt.%, respectively. For comparison, the unreinforced Ti18Nb8Zr alloy was obtained by the same method. The initial microstructure of the unreinforced Ti-18Nb-8Zr alloy was represented by finely dispersed acicular α martensite, consisting of thin laths of submicron thickness, located in the β matrix. The microstructure of the cast composites consisted of a two-phase α+β Ti-13Nb-13Zr matrix and (Ti,Nb)B boride fibers randomly located in the matrix, the proportion of which increased with the amount of TiB2 in the initial charge. The volume fraction of (Ti,Nb)B fibers in the structure of two states of the composites was ~ 2.2% and ~ 10.6% for alloys with 0.7 and 2.0 wt.% TiB2 in the initial charge, respectively. The initial non-reinforced Ti18Nb8Zr alloy showed yield strength of 560 MPa and relative elongation of 15%. The composite with 0.7 wt.% TiB2 in the initial charge demonstrated an increase in yield strength to 605 MPa and a decrease in relative elongation to 5%. Increasing the content of the reinforcing component TiB2 to 2 wt. % led to the most significant increase in the strength properties of the composite, the yield strength was 745 MPa, and a decrease in ductility - relative elongation of 0.5%. Complete biocompatibility of the obtained composites was shown upon direct contact with cells in vitro.