Effect of boron on the microstructure and properties of Ti-6Al-4V

Ti-6Al-4V is an important alpha+beta titanium alloy with an interesting and challenging correlation between the mechanical properties and its complex microstructure. The microstructure presents a three dimension complexity with features at different scale levels (prior-beta grains and alpha/beta colonies in millimeter scale, and alpha laths at micrometer scale). Titanium alloys are also very sensible to small amounts of impurities (such as O, H and B) and recently it has been found that boron addition in small quantities refine the microstructure and improve some of the mechanical properties. However, the effect of boron in the microstructure and its correlation to mechanical properties is not well understood. In this work a quantitative characterization of microstructure parameters in Ti-6Al-4V and Ti-6Al-4V-0.11B alloys is done. Furthermore, we characterize the microstructure at different ingot depths to observe the change of microstructure parameters through the solidification stages. The results show that the addition of boron (1) increases the thickness of grain boundary alpha in 54% and the thickness of alpha laths in 87%. (2) Reduces the length of alpha laths in 15%, which in turn reduces the alpha/beta colony size in 80%, and the aspect ratio of alpha laths in 55%. Throughout the ingots various microstructural features changed (1) on both alloys, the thickness of alpha laths decreases with the depth in the ingot. (2) In Ti-64- 0.11B alloy the thickness of grain boundary alpha increases at the center of the ingot and (3) The alpha/beta colony size is increased with the depth of the ingot for Ti-6Al-4V alloy, but decreases slightly in the Ti-6Al-4V-0.11B alloy. The other microstructural features remain approximately constant within the ingot.Validerat; 20101217 (root