Physical Properties of Titanium and Its Alloys

Titanium is lightweight, strong, corrosion resistant and abundant in nature. Titanium and its alloys possess tensile strengths from 30,000 psi to 200,000 psi (210-1380 MPa), which are equivalent to those strengths found in most of alloy steels. The density of titanium is only 56 percent that of steel, and its corrosion resistance compares well with that of platinum. Of all the elements in the earth’s crust, titanium is the ninth most plentiful.

Physical Properties

If all the elements are assembled in order of atomic number, it can he noticed that there is a relationship in properties corresponding to the atomic number.

Titanium is found in column four along with chemically similar zirconium, hafnium, and thorium. Therefore, it was not unexpected that titanium would possess some properties similar to those found in these metals.

Titanium has two electrons in the third shell and two electrons in the fourth shell. When this arrangement of electrons, where outer shells are filled before the inner shells are completely occupied, occurs in a metal, it is known as a transition metal. This arrangement of electrons is responsible for the unique physical properties of titanium. To mention a few, chromium, manganese, iron, cobalt, and nickel are found in the transition series.

The atomic weight of titanium is 47.88, while aluminum has an atomic weight of 26.97, and iron 55.84.

A crystal structure may he thought of as a physically homogeneous solid in which the atoms are arranged in a repeating pattern. This arrangement is instrumental in the physical behavior of a metal. Most metals have either a body-centered cubic, face-centered cubic, or a hexagonal-close-packed structure.

Titanium has a high melting point of 3135°F (1725°C). This melting point is approximately 400°F above the melting point of steel and approximately 2000°F above that of aluminum.

Thermal Conductivity. The ability of a metal to conduct or transfer heat is called its thermal conductivity. Thus, a material, to be a good insulator, would have a low thermal conductivity, whereas a radiator would have a high rate of conductivity to dissipate the heat. The physicist would define this phenomenon as the time rate of transfer by conduction, through unit thickness, across unit area for unit temperature gradient.

Linear Coefficient of Expansion. Heating a metal to temperatures below its melting point causes it to expand or increase in length. If a bar or rod is uniformly heated along its length, every unit of length of the bar increases. This increase per unit length per degree rise in temperature is called the coefficient of linear expansion. Where a metal will be alternately subjected to beating and cooling cycles and must maintain a certain tolerance of dimensions, a low coefficient of thermal expansion is desirable. When in contact with a metal of a different coefficient, this consideration assumes greater importance.

Titanium has a low coefficient of linear expansion which is equal to 5.0x10^-6 inch per inch/°F, whereas that of stainless steel is 7.8x10^-6, copper 16.5x10^-6, and aluminum 12.9x10^-6.

Electrical Conductivity and Resistivity. The flow of electrons through a metal due to a drop in potential is known as electrical conductivity. The atomic structure of a metal strongly influences its electrical behavior.

Titanium is not a good conductor of electricity. If the conductivity of copper is considered to be 100%, titanium would have a conductivity of 3.1%. From this it follows that titanium would not be used where good conductivity is a prime factor. For comparison, stainless steel has a conductivity of 3.5% and aluminum has a conductivity of 30%.

Electrical resistance is the opposition a material presents to the flow of electrons. Since titanium is a poor conductor, it follows that it is a fair resistor.

Magnetic Properties. If a metal is placed in a magnetic field, a force is exerted on it. The intensity of the magnetization, called M, can be measured in terms of the force exerted and its relation to the magnetic field strength, H, depending upon the susceptibility, K, which is a property of the metal.

Metals have a wide variance in susceptibility and can be classified in three groups:

• The diamagnetic substances in which K is small and negative, and thus are feebly repelled by a magnetic field; examples are copper, silver, gold and bismuth.
• The paramagnetic substances in which K is small and positive, and thus are slightly attracted by a magnetic field; the alkali, alkaline and the nonferromagnetic transition metals fall in this group (it can be seen that titanium is slightly paramagnetic).
• The ferromagnetic substances, which have a large K value and are positive; iron, cobalt, nickel, and gallium fall under this heading.

An important feature of Group 3, besides the strong attraction in a magnetic field, is the fact that these metals retain their magnetization after being removed from the magnetic field.

Most of the more important physical properties of titanium have now been indicated.