85 Alloying Alloys And Phase Diagrams 851 Alloying Behavior

Titanium exists in two crystalline states: a low-temperature alpha (a) phase, which has a close-packed hexagonal crystal structure, and a high-temperature beta (P) phase, which has a body-centered cubic structure (Fig. 8.1) [8]. This allotropic transformation occurs at 880°C (1620°F) in nominally pure titanium. Titanium has certain features that make it very different from other light metals such as aluminum and magnesium [9]. The allotropic transformation allows the opportunity for formation of alloys composed of a, P, or a/P microstructures, in addition

FIGURE 8.1 Two allotropic forms of titanium. Transition from low-temperature a-phase to the high-temperature P-phase occurs at 882°C (1620°F) [8].

to compound formation in certain alloys. Because of its electronic structure as a transition element, titanium can form solid solutions with most substitutional elements having a size factor within 20%, giving the opportunity for many alloying possibilities. Titanium also reacts strongly with interstitial elements such as nitrogen, oxygen, and hydrogen at temperatures below its melting point. When reacting with other elements. titanium may form solid solutions and compounds with metallic, covalent, or ionic bonding.

The choice of alloying elements is determined by the ability of the element to stabilize either the a or P phases (Fig. 8.2) [10]. Aluminum, oxygen, nitrogen, gallium, and carbon are the most common a-stabilizing elements. Zirconium, tin, and silicon are viewed as neutral in their ability to stabilize either phase. Elements that stabilize the P phase can either form binary systems of the P-isomorphous-type or the P-eutectoid type (see next section). Elements forming the isomorphous-type binary system include Mo, V, and Ta, while Cu, Mn, Cr, Fe, Ni, Co, and H are eutectoid formers in which compounds may form. The P-isomorphous alloying elements, which do not form intermetallic compounds, have traditionally been preferred to the eutectoid-type elements as additional to a-P or P alloys to improve hardenability and increase response to heat treatment.

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