Titanium-Boron (Ti-B) Phase Diagram
Figures 1 and 2 show the equilibrium titanium-boron phase diagrams (expressed in °C and °F, respectively) calculated with Thermo-Calc, coupled with SSOL4 thermodynamic database.
Figure 1. Ti-B phase diagram shows which phases are to be expected at equilibrium for different combinations of boron content (expressed in weight %) and temperature (in °C). The Ti-B phase diagram was calculated with Thermo-Calc, coupled with SSOL4 thermodynamic database.
Figure 2. Ti-B phase diagram shows which phases are to be expected at equilibrium for different combinations of boron content (expressed in weight %) and temperature (in °F). The Ti-B phase diagram was calculated with Thermo-Calc, coupled with SSOL4 thermodynamic database.
The equilibrium phases of the titanium-boron system are:
• Terminal solid solutions — high-temperature body-centered cubic (beta) titanium, low-temperature hexagonal close-packed (alpha) titanium, and rhombohedral (beta) boron.
• Intermetallic compounds — TiB (monoboride), Ti3B4, and TiB2 (diboride).
According to SSOL4 thermodynamic database, the technically important titanium-rich section of the Ti-B binary system has a eutectic reaction (liquid —> beta titanium + TiB) at 1541 °C (2806 °F) and 1.64 wt. % B (6.88 at. % B). TiB has a tendency to rapidly segregate to the grain boundaries, and boron itself has negligible solid solubility in the high-temperature BCC (beta) and low-temperature HCP (alpha) titanium.
The strength and stiffness of titanium-base alloys may be increased by the precipitation of TiB intermetallic phase. However, the ductility and fracture toughness are retained only if the amount of boron is less than the eutectic composition (hypo-eutectic Ti-B alloys). It is important to emphasize that the eutectic point can shift significantly when other alloying elements are added.
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