METALS
• Metallic Solids, also simply called “metals” consist entirely
of metal atoms.
IMAGE SOURCE:
Brown, 2011
IMAGE SOURCE:
Brown, 2012
IMAGE SOURCE:
Brown, 2011
• An alloy is a material that contains more than one element
and has the characteristic properties of a metal.
IMAGE SOURCE:
Brown, 2012
CATEGORIES OF ALLOY
1. Substitutional alloy – when atoms of the solute in a solid
solution occupy positions normally occupied by a solvent
atom
They are formed when the two metallic components have
similar atomic radii and chemical-bonding characteristics.
When two metals differ in radii by more than about 15%,
solubility is generally more limited.
IMAGE SOURCE:
Brown, 2012
CATEGORIES OF ALLOY
2. Interstitial alloy - When the solute atoms
occupy interstitial positions in the “holes”
between solvent atoms.
For an interstitial alloy to form, the solute
atoms must have a much smaller bonding
atomic radius than the solvent atoms.
Typically, the interstitial element is a
nonmetal that makes covalent bonds to the
neighboring metal atoms. The presence of
the extra bonds provided by the interstitial
component causes the metal lattice to
become harder, stronger, and less ductile.
IMAGE SOURCE:
Brown, 2012
CATEGORIES OF ALLOY
3. Heterogeneous alloy – when the components in the alloy
are not dispersed uniformly.
In general, the properties of heterogeneous alloys depend on
both the composition and the manner in which the solid is
formed from the molten mixture.
IMAGE SOURCE:
Brown, 2012
CATEGORIES OF ALLOY
4. Intermetallic compounds- are compounds rather than
mixtures; hence, they have definite properHes and their composiHon
cannot be varied.
Unlike the atoms in substitutional and interstitial alloys, the
different types of atoms in an intermetallic compound are
ordered rather than randomly distributed.
The ordering of atoms in an intermetallic compound generally
leads to better structural stability and higher melting points than
what is observed in the constituent metals.
On the negative side, intermetallic compounds are often more
brittle than substitutional alloys.
CATEGORIES OF ALLOY
IMAGE SOURCE:
Brown, 2012
IMAGE SOURCE:
Brown, 2012
IMAGE SOURCE:
Brown, 2012
IMAGE SOURCE:
Callister, 2007
IMAGE SOURCE:
Callister, 2007
For a 40 wt% Sn–60 wt% Pb alloy at 150 C (300 F), (a) What
phase(s) is (are) present? (b) What is (are) the composition(s)
of the phase(s), in terms of mass fraction?
IMAGE SOURCE:
Callister, 2007
REFERENCES
Brown, T.L., Lemay Jr., H.E., Bursten, B.E., Murphy, C.J., and
Woodward, P.M. (2012) Chemistry: The Central Science, 12th
Ed., USA: Pearson Education, Inc.
Brown, L.S. and Holme, T.A. (2011) Chemistry for Engineering
Students, 2nd Edition, USA, Brooks/Cole, Cengage Learning
Callister Jr., W., (2007) Material Science and Engineering: An
Introduction, USA, John Wiley & Sons