Optimization of shear lag model and influencing mechanisms of primary and eutectic TiB on the microstructure and properties of Ti-4Al-6Cr-5Mo-8Nb

To quantitatively evaluate the strengthening mechanism of in situ TiB in titanium matrix composites, the shear-lag model was optimized, and Ti-4Al-6Cr-5Mo-8Nb-xB alloys (x = 0, 0.4, 0.8, 1.2, 1.6 wt%) were fabricated via ultrasonic-assisted casting. Microstructural evolution with varying boron content was systematically investigated to elucidate the corresponding strengthening mechanisms. The optimized model enhances applicability across different microstructures and quantifies their impact on mechanical properties. The microstructure underwent a transition from eutectic TiB networks to mixed structures with large primary TiB. The β grain size decreased from 1.07 mm to 25 μm. The phase transformation kinetics of eutectic TiB are constrained by β grain boundaries, resulting in aspect ratios of 2.11–5.47 for eutectic TiB and 8.77–23.26 for primary TiB. A modified shear-lag model incorporating TiB aspect ratio and volume fraction corrections reduced the average absolute error to 18.46 % and achieved a correlation coefficient of 0.89. Consistent with model predictions, tensile strength increased from 979.92 MPa to a maximum of 1125.5 MPa when both TiB cluster networks and primary TiB phases were present. This enhancement is mainly due to increased grain boundary density and the load-bearing effect of TiB. Additionally, the TiB cluster network and β grain boundaries effectively impede the motion of geometrically necessary dislocations within individual grains, thereby enhancing the strength of the polycrystalline alloy.