Processing, microstructure, texture, and tensile properties of the Ti-6Al-4V-1.55B eutectic alloy

Boron (B) addition to conventional titanium (Ti) alloys instigates precipitation of intermetallic TiB whiskers that provide significant increases in strength and stiffness. The eutectic composition is the maximum boron concentration that provides these benefits via melt processing while maintaining reasonable ductility and damage tolerance. The eutectic point for the most widely used Ti alloy, Ti-6Al-4V (wt pct), modified with B, was determined to occur at 1.55 +/- 0.05B and 1545 +/- 5 degrees C. The microstructure, texture, and tensile properties of the eutectic alloy are sensitive to ingot solidification conditions and melt pool morphology. Several microstructural forms in the as-cast condition as well as their transformation during thermomechanical processing (TMP) were identified. Two types of eutectic structures, aligned and random, were obtained. Cast microstructures with random eutectic colony orientation had no marked crystallographic texture, while cast aligned microstructure had a strong [020] texture for the TiB phase and a < 110 > circular texture for the alpha-Ti phase. As-cast microstructure could be significantly modified via TMP. Fragmentation of TiB occurred during three-dimensional (3-D) forging and aligned microstructures were produced after rolling or extrusion. An attractive balance of strength and ductility was obtained after solution treatment in the full beta condition followed by aging. The microstructure of the Ti-6Al-4V-1.55B eutectic alloy can be controlled to tailor the property combinations.