Microstructure and mechanical properties of medium-entropy TiNbZr alloy-based composites, reinforced with boride particles

The TiZrNb metal-matrix composites reinforced by in-situ formed borides were produced by vacuum arc melting using different amount of TiB₂ (0.2, 2.0 and 4.4 wt%). Structure, mechanical properties and biocompatibility of the composites were studied. The initial microstructures of the composites were composed of the bcc TiZrNb matrix and needle-like (Ti, Nb) B particles the volume fraction of which was ∼ 1.0%, 6.8% and 13.2%, respectively. Besides a small amount of the ω phase particles measuring ∼ 10 nm were found in the matrix of all the composites. Compression test at room temperature showed that the minimum content of the borides did not have noticeable strengthening effect on mechanical properties of the composite. An increase in the amount of (Ti, Nb) B to 6.8 vol% resulted in an increase in the yield stress by 22% to ∼ 900 MPa without a visible decrease in ductility. The composite with 13.2 vol% of the borides showed the yield strength of 1010 MPa and compression ductility (deformation till fracture) of 9% that can be associated with a hypereutectic composition and appearance of coarse particles of primary (Ti, Nb) B. Evaluation of main strengthening mechanisms suggested that the load transfer strengthening contributed mainly to the overall strength of the composite. Analysis of biocompatibility suggests some decrease in the proliferation rate of mesenchymal stem cells with an increase in the borides amount.