Fig. 4.1. Crystal Structures of Pure Titanium shown in Fig. 4.2. Those elements that increase the beta transus temperature through stabilizing the alpha phase are called alpha stabilizers and include aluminum, oxygen, nitrogen, and carbon. Those elements that decrease the beta transus temperature are called beta stabilizers. The beta stabilizers are further subdivided into beta isomorphous elements, which have a high solubility in titanium, and beta eutectoid elements, which have only limited solubility and tend to form intermetallic compounds. The beta isomorphous elements are molybdenum, vanadium, niobium, and tantalum, while beta eutectoid elements include manganese, chromium, silicon, iron, cobalt, nickel, and copper. Finally, tin and zirconium are considered neutral because they neither raise nor lower the beta transus temperature, but since both contribute to solid solution strengthening, they are frequently used as alloy additions.

Titanium alloys are classified according to the amount of alpha and beta retained in their structures at room temperature. Classifications include alpha, near-alpha, alpha-beta, and metastable beta. As their name implies, alpha alloys do not contain beta phase at room temperature, while near-alpha alloys contain mostly alpha with only a small amount of beta. Alpha-beta alloys are two-phase alloys containing both alpha and beta phases. Metastable beta alloys contain mostly beta with small amounts of alpha present. The beta alloys are called metastable because thermodynamically, if given the time, the beta would transform to equilibrium phases of alpha and intermetallic compounds; however,

a Stabilizers P Isomorphous Stabilizers

Fig. 4.2. Phase Diagrams for Binary Titanium Alloys

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