ABSTRACT
Similar to iron and nickel, titanium is also an allotropic element existing in more than one crystallographic form. Titanium has a hexagonal close packed (hcp) crystal structure at room temperature and transforms to a body-centered cubic (bcc) structure above the transition temperature (βt) of 888°C. Hexagonal titanium is referred to as alpha (α) phase and body-centered titanium is referred to as beta (β) phase. Addition of specic alloying elements either increases or decreases the beta transus temperature, where the beta transus is a very important factor that aects processing and heat treatment operations. Based on the relative amount of these phases present in a particular alloy, titanium alloys are classied as α, near α, α + β, and β alloys. Some of the α stabilizers in titanium are aluminum, oxygen, and nitrogen. Some of the β stabilizers are vanadium, molybdenum, niobium, silicon, and chromium. A complete list of α and β stabilizers are given in Table 4.7. It is worthwhile to note that there are two classes of beta phase stabilizing elements: one is isomorphous, containing elements that are miscible in beta phase, while the other solute group forms eutectoid phase systems with titanium. Increasing the quantity of α stabilizers promotes the volume fraction of alpha phase, and similarly, increasing the quantity of β stabilizers promotes the beta phase. Figure 4.5 shows a pseudo phase diagram illustrating the variation of phases as the beta stabilizer is added to the titanium system (Lütjering and Williams 2007).
