Callister - An introduction - 8th edition

412 • Chapter 11 / Applications and Processing of Metal Alloys
(4.5 g/cm 3 ), a high melting point [1668C (3035F)], and an elastic modulus of 107
GPa (15.5 10 6 psi). Titanium alloys are extremely strong; room-temperature tensile
strengths as high as 1400 MPa (200,000 psi) are attainable, yielding remarkable
specific strengths. Furthermore, the alloys are highly ductile and easily forged and
machined.
Unalloyed (i.e., commercially pure) titanium has a hexagonal close-packed crystal
structure, sometimes denoted as the phase at room temperature. At 883C
(1621F) the HCP material transforms to a body-centered cubic (or ) phase. This
transformation temperature is strongly influenced by the presence of alloying elements.
For example, vanadium, niobium, and molybdenum decrease the -to-
transformation temperature and promote the formation of the phase (i.e., are
-phase stabilizers), which may exist at room temperature. In addition, for some
compositions both and phases will coexist. On the basis of which phase(s) is
(are) present after processing, titanium alloys fall into four classifications: alpha,
beta, alpha beta, and near alpha.
Alpha titanium alloys, often alloyed with aluminum and tin, are preferred for
high-temperature applications because of their superior creep characteristics. Furthermore,
strengthening by heat treatment is not possible inasmuch as is the
stable phase; consequently, these materials are normally utilized in annealed or
recrystallized states. Strength and toughness are satisfactory, whereas forgeability is
inferior to the other Ti alloy types.
The titanium alloys contain sufficient concentrations of beta-stabilizing elements
(V and Mo) such that, upon cooling at sufficiently rapid rates, the
(metastable) phase is retained at room temperature. These materials are highly
forgeable and exhibit high fracture toughnesses.
Alpha beta materials are alloyed with stabilizing elements for both constituent
phases. The strength of these alloys may be improved and controlled by
heat treatment. A variety of microstructures is possible that consist of an phase
as well as a retained or transformed phase. In general, these materials are quite
formable.
Near-alpha alloys are also composed of both alpha and beta phases, with only
a small proportion of —that is, they contain low concentrations of beta stabilizers.
Their properties and fabrication characteristics are similar to the alpha materials,
except that a greater diversity of microstructures and properties are possible for
near-alpha alloys.
The major limitation of titanium is its chemical reactivity with other materials
at elevated temperatures. This property has necessitated the development of nonconventional
refining, melting, and casting techniques; consequently, titanium alloys
are quite expensive. In spite of this high temperature reactivity, the corrosion
resistance of titanium alloys at normal temperatures is unusually high; they are
virtually immune to air, marine, and a variety of industrial environments. Table 11.9
presents several titanium alloys along with their typical properties and applications.
They are commonly utilized in airplane structures, space vehicles, and surgical implants,
and in the petroleum and chemical industries.
The Refractory Metals
Metals that have extremely high melting temperatures are classified as the refractory
metals. Included in this group are niobium (Nb), molybdenum (Mo), tungsten
(W), and tantalum (Ta). Melting temperatures range between 2468C (4474F) for
niobium and 3410C (6170F), the highest melting temperature of any metal, for
tungsten. Interatomic bonding in these metals is extremely strong, which accounts