Metal

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Dr.rer.nat Heru Susanto

 Metals achieve engineering importance because of their abundance, variety, and
unique properties as conferred by metallic bonding.
 The two most abundant metallic elements, iron (5.0%) and aluminum (8.1%), are
also the most commonly used structural metals. Iron is the most-used metal, in
part because it can be extracted from its frequently occurring, enriched ores with
considerably less energy penalty than aluminum, but also because of the very
wide range of mechanical properties its alloys can provide (as will be seen
below).
 The cost of metals is strongly affected by strategic abundance as well as
secondary factors such as extraction/processing cost and perceived value. Plain
carbon steels and cast irons, iron alloys with carbon, are usually most costeffective for ordinary mechanical applications. These alloys increase in cost with
alloying additions.
 This bonding is different from other types of solids in that the electrons are free
to acquire energy, and the metallic ions are relatively mobile, and quite
interchangeable with regard to their positions in the crystal lattice, the threedimensional repeating arrangement of atoms in a solid.
 Metals are good conductors of heat and electricity because thermal and
electrical energy can be transferred by the free electrons.

 As a broad generalization, metallic elements with an odd
number of valence electrons tend to be better conductors
than those with an even number.
 Metals are opaque to and reflective of light and most of the
electromagnetic spectrum, because electromagnetic energy
is transferred to the free electrons and immediately
retransmitted.
 Metals are almost always crystalline solids with a regular
repeating pattern of ions. A number of atomic-level defects
occur in this periodic array. A number of atomic-level defects
occur in this periodic array.

 A large number of atomic sites
are “vacancies” (point defects)
not occupied by atoms
 The number and mobility of
vacant sites increase rapidly with
temperature.
 At a slightly larger level, linear
atomic packing defects known as
dislocations, give rise to the
ability of metallic materials to
deform substantially under load.
 Such a dislocation can break and
remake bonds relatively easily in
a metal and thereby shift an
atomic distance
 Many dislocations moving in this
fashion can give rise to significant
shape change in the material at
moderate stresses.

The interaction of deformation,
alloying elements, temperature,
and time can cause a wide
variety of microstructures in a
solid metal down to near atomic
levels with mechanical (and
other) properties which can vary
over a very wide range. It is
possible to manipulate the
properties of a single metal
composition over a very wide
range in the solid state — a
behavior which can be used to
mechanically form a particular
metal and then use it in a
demanding load-bearing
application.

 One of the important technological advantages of metals is their ability
to incorporate a wide variety of secondary elements in a particular metal
and thereby create alloys of the metal. Alloying can increase the
strength of a metal by several orders of magnitude and permit the
strength and ductility to be varied over a wide range by thermal and/or
mechanical treatment, resulting in ease of mechanical forming or
resistance to deformation.
 Casting methods include expendable mold casting
(investment/precision, plaster mold, dry sand, and wet sand casting),
permanent mold casting (ingot, permanent mold, centrifugal, and die
casting), and continuous casting (direct chill and “splat” casting).
 As cooling rate increases, the grain (crystal) size tends to be smaller
and the strength increases while compositional segregation decreases,
providing more uniform properties.

 Alloying additions can have profound
consequences on the strength of metals. Major alloying additions can lead
to multiphase materials which are stronger than single-phase materials.
 Small alloying additions may also substantially increase strength by so-lute
strengthening as solid solution substitutional or interstitial atoms and or by
particle strengthening as dispersion or
precipitation hardening alloys.
 As the amount of an alloying element
in solution increases, the strength increases as disloctions are held in place
by the “foreign” atoms. The greater the
ionic misfit (difference in size — Sn is a
much larger ion than Ni), the greater
the strengthening effect.
 Interstitial solid solution carbon contributes to the strength of iron and is one
contributor to strength in steels and
cast irons.

 Steels, perhaps the most
important of all engineering
metals, are alloys of iron and
carbon usually containing
about 0.02 to 1.0 w/o carbon.
 Steel strengthening treatments
require heating into the
austenite region (above the
Ac3) and then quenching.
 Steel forming and heat
treatment center on the
transformation from austenite,
γ phase, at elevated
temperature to ferrite (α phase)
plus cementite (iron carbide,
Fe3C) below 727°C (1340°F),
the Ac1 temperature, a
eutectoid transformation.

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