Introduction
Titanium does not occur free in Nature. However, when combined
with other elements, it is quite abundant, occurring in small
amounts in most of the volcanic, sedimentary and metamorphic rocks.
Its more important minerals are ilmenite, rutile, arizonite (iron titanate),
brookite, anatase, leucochene (titanium dioxide), perovskite
(calcium titanate), and others. The first two have commercial importance,
and can be found in deposits spread all over the world. There are important rutile
and ilmenite deposits in Australia, Argentina, USA, Central Africa, Brazil, Canada,
Egypt, India and Norway. The largest well-known deposits of rutile are located in
Australia.
Titanium and its alloys are relatively new engineering metals since they have been
in use only since about 1952. They are extremely attractive materials for engineers
because they have a high strength to weight ratio, high elevated temperature properties
to about 550oC, and excellent corrosion resistance particularly in oxidising
acids and chloride media. This metal is being increasingly used for marine applications.
Its resistance to seawater attack combined with its mechanical properties make it a prime
choice for equipment operating within the sea or transferring seawater.
Titanium is not an 'exotic' metal, it is the fourth most abundant structural metal in the
earth's crust, and the ninth industrial metal. This metal has become the prime selection
for a wide range of critical and demanding applications
Pure Titanium
Titanium is a light metal having a density of about 4540 kg/m3. This compares
to steel at 7900 kg/m3 and Aluminium at 2710kg/m3. Titanium has a melting point
of about 1668oC which is higher than iron at1560oC. Titanium has a Modulus of Elasticity
of 110 x 109 Pa. compared to steel at 210 x 109 Pa. Therefore Titanium
has a significantly high deflection under the same load than steel. Pure Titanium can be
cold rolled to 90% reduction in thickness at room temperatures with cracking.
There are a number of grades of commercially pure
(unalloyed) titanium as identified in ASTM B265 (Gr.1,2,3,4,7,11,and 12) (see table below for Gr 1 to 4)).
Each grade has a different amount of impurity content
(Grade 1 have lowest impurities). Tensile strengths vary from 172 x 106Pa
for Grade 1 to 620 x 106Pa for Grade 12. The yield strength can be up to 480 x 106Pa (Gr 12)
Titanium Alloys
Titanium Alloys are generally divided into three groups (Alpha, Alpha-Beta and Beta).
The Alpha group contain most importantly aluminum and tin. They can also contain
molybdenum, zirconium, nitrogen, vanadium, columbium, tantalum, and silicon.
Alpha alloys are not suitable for heat treatment. Alpha alloys are used for aircraft parts
and cryogenic equipment.
The Alpha-Beta group can be strengthened by heat treatment.
The alloys are used in aircraft and aircraft turbine parts, chemical
processing equipment, marine hardware.
The Beta Alloys have good hardenability. Beta alloys are
slightly more dense than other titanium alloys, having densities ranging from
4800 to 5050 kg/m3. They are the least creep resistant alloys, they are
weldable, and can have yield strengths up to 1345 x 106 Pa.(Solution treated and age hardened)
Beta alloys are the smallest group. They are used for heavier duty purposes on aircraft.