Page 115 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Chapter 3 Physical Properties of Materials
for casting and metalworking operations (/feat checking). Thermal shock is the term
generally used to describe development of cracks after a single thermal cycle.
To alleviate some of the problems caused by thermal expansion, a family of
iron-nickel alloys with very low thermal-expansion coefficients has been developed,
called low-expansion alloys. The low thermal expansion characteristic of these alloys
is often referred to as the Invar effect, after the metal Ini/ar. The thermal coefficient
of expansion is typically in the range of from 2 >< 1O`6 to 9 >< 10`6 per °C. Typical
compositions are 64% Fe-36% Ni for Invar and 54% Fe-28% Ni-18% Co for
Koz/ar. Low-expansion alloys also have good thermal-fatigue resistance and good
ductility; as a result, they can easily be formed into various shapes. Applications in-
clude (a) bimetallic strips consisting of a lovv-expansion alloy metallurgically bonded
to a high-expansion alloy (the strip bends when subjected to temperature changes)
and (b) high-quality glass-to-metal seals in which the thermal expansions are
matched.

3.7 Electrical, Magnetic, and Optical Properties

Electrical conductivity and the dielectric properties of materials are important not
only in electrical equipment and machinery, but also in such manufacturing processes
as the magnetic-pulse forming of sheet metals (Section 16.11), resistance Welding
(Section 31.5), and the electrical-discharge machining and electrochemical grinding
of hard and brittle materials (Chapter 27). The units of electrical conductivity are
mho/m where ni/vo is the reverse of 0/vm, the unit of electrical resistance. The influ-
,
ence of the type of atomic bonding on the electrical conductivity of materials is the
same as that for thermal conductivity. Alloying elements have a major effect on the
electrical conductivity of metals: The higher the conductivity of the alloying element,
the higher is the electrical conductivity of the alloy.

Dielectric Strength. An electrically insulating material’s dielectric strength is the
largest electric field it can encounter without degrading or losing its insulating prop-
erties. This property is defined as the voltage required per unit distance for electrical
breakdown and has the units of V/m.
Conductors. Materials with high electrical conductivity, such as metals, are gener-
ally referred to as conductors. Electrical resistivity is the inverse of electrical con-
ductivity. Materials With high electrical resistivity are referred to as dielectrics or
insulators.

Superconductors. Superconductii/ity is the phenomenon of near-zero electrical
resistivity that occurs in some metals and alloys below a critical temperature. The
temperatures involved often are near absolute zero (0 K, or -273°C). The highest tem-
perature at which superconductivity has been exhibited to date is -123°C, but ad-
vances in high temperature superconductivity continue to be made.
The main application of superconductors is as high-povver magnets. Also, su-
perconductors are the enabling technology for magnetic resonance imaging (MRI),
which is Widely used for medical imaging. Other applications proposed for super-
conductors include magnetic levitation (maglev) trains, efficient power transmission
lines, and extremely fast computer components.

Semiconductors. The electrical properties of semiconductors, such as single-crystal
silicon, germanium, and gallium arsenide, are extremely sensitive to temperature and
to the presence and type of minute impurities. Thus, by controlling the concentration