Mech 285 Lectures
Professor Rodney Herring
At the end of this lecture you should be able to:
• Know the properties of Al and its alloys.
• Know the 4 basic types of Al alloys and why they are categorized this way.
• Know the properties of Cu, Ni, Co, Ti, the refractory metals and the
• Write a discussion of the important properties of the common nonferrous
alloys for particular applications.
Structural metals and alloys are often divided into two major categories,
ferrous and nonferrous materials.
• Ferrous alloys are those based on iron as the principal metal and include
steels, stainless steels, cast iron, etc, as discussed in our last lecture.
• Approximately 90% of the world’s production of metals/alloys are ferrous
because of their good strength, toughness, ductility and relatively low cost.
Nonferrous alloys are those based on other metals with particular emphasis on
Al, Cu, Ti, Zn, Zr, and Mg.
Aluminum is an extremely useful engineering material as:
• they are light weight and strong; Al has a density of 2700 kg/m3, which is
about 1/3 that of steel.
• their strength to weight ratio is excellent.
• Al is non-toxic.
• Al is one of the best “metal” electrical conductors (what is the best?)
• Al has good corrosion resistance due to its natural oxide layer, which is thin
and passive once formed.
Aluminum is an extremely useful engineering material as:
• Al stays ductile at low temperatures (why?)*
• Al has a relatively low price
• It is the third most plentiful element on earth (next to oxygen
• Al is easily alloyed and many of its alloys are stronger than
• Al alloys are non-magnetic.
• Al alloys have a stable or predictable microstructure.
• Al alloys have an excellent machining, forming, and forging
• Al alloys have a relatively high thermal expansion.
• Al alloys have a relatively high thermal conduction.
* - Al’s FCC crystal structure retains its strength, ductility and toughness at
cryogenic temperatures. This is why we see many cryogenic tanks made
Disadvantages of Aluminum include:
• Al has a low melting temperature so can’t be used at high
temperatures (above ~ 400 C)
• Al has low hardness so it is not good for wear resistance.
• Because Al’s elastic modulus is 1/3 that of steel, it’s deflection
as a structural component may be too great for the application.
• Al’s FCC structure work hardens so it may become brittle
after plastic deformation and fracture easily.
• Al’s high thermal expansion sometimes causes problems with
its use as an interconnect for electronic devices. (Good
electronics use Cu alloys and the best use Au.) Why isn’t Ag or
a superconductor material used instead?
Production of Al in an
electrolytic cell in the
Alcan Aluminum, a
Canadian company has
it’s head-quarters based
in Kingston, Ontario
and leads the World in
the production of
Aluminum. The starting
material is a cryolite or
bauxite ore, which
Canada has plenty.
Because Al production requires electricity, very often there is a
electricity-generating power station close by.
Aluminum alloys can be subdivided into two major groups based
on their method of fabrication:
• Wrought alloys
• Cast alloys
Wrought alloys are shaped by plastic deformation and have
microstructures and compositions different from the casting
alloys because of the differences in manufacturing
Within each of these two major groups, they are further
subdivided into two subgroups:
• Heat treatable alloys
• Nonheat treatable alloys So, you need to know 4 groups of Al alloys.
Heat treatable alloys are strengthened by “age hardening” (what
does this mean?), whereas nonheat treatable alloys are
strengthened by strain hardening.
There are three steps to age harden materials such as Aluminum.
1. Solution Treatment – the alloy is heated above the solvus temperature
into a single phase region of the phase diagram to dissolve any secondary
phases such as precipitates. The material is held at this temperature until
a homogeneous solid solution is produced. Al is usually solution-treated
between 500 oC and 548 oC.
2. Quench – the alloy is rapidly cooled so the atoms do not have enough
time to diffuse to potential nucleation sites. The alloy remains as a single
phase material that is supersaturated with alloying elements. If the
material is work hardened, the increase in dislocations density can be
used as nucleation sites during aging. (we’ll see an example)
3. Aged – The alloy is heated to a temperature below the solvus so the
atoms can diffuse to numerous nucleation sites to produce precipitates.
Nucleation of precipitates is enhanced by the presence of dislocations.
Ideally, uniform highly dispersed, ultrafine precipitates give the best
effect in age hardening or precipitate strengthening.
GPI and GPII are
nuclei of the q
is eventually forms
in Al-Cu Alloys.
The yield strength hardens by the formation of precipitates, which
after longer times rippen (get large) and the strength falls off.
The strength does not fall off at low aging temperatures.
Age Hardening of Weldalite
• The hardness as a function of aging time of the high-strength aluminum
alloy, Weldalite developed by Lockheed Martin.
R. A. Herring, F. W. Gayle and J. R. Pickens, "High-resolution electron microscopy study of two high-strength
aluminum alloys", J. of Material Science 28 (1993) 69 - 75
Age Hardening of Weldalite
• The strength of Weldalite initially softens due to annealing of dislocations
and then hardens by the formation of precipitates. When the precipitates
rippen, the strength falls off slightly.
Requirements for Age Hardening
Not all alloys are age hardenable. Four conditions must be satisfied for an
alloy to have an age-hardened response during heat treatment.
1) The alloy system must display a decreasing solid solubility with
decreasing temperature. In other words, the alloy must form a single
phase on heating above the solvus line, then enter a two-phase region on
2) The matrix should be relatively soft and ductile, and the precipitate
should be hard and brittle. In most age hardenable alloys, the
precipitate is a hard, brittle “intermetallic” compound.
3) The alloy must be quenchable. Some alloys cannot be cooled rapidly
enough to suppress the formation of the precipitate. Quenching may,
however, introduce residual stresses that cause distortion of the part. To
minimize residual stresses aluminum alloys are quenched in hot water, at
about 80 oC, i.e., a hot quench.
4) A coherent precipitate must form. (see next slide)
Many important alloys are age-hardenable including stainless steels and alloys
based on aluminum, magnesium, titanium, nickel, chromium, iron, and
Noncoherent & Coherent Precipitates
a) A noncoherent precipitate has no relationship with the
crystal structure of the surrounding matrix.
b) A coherent precipitate forms so that there is a definite
relationship between the precipitate and the matrix’s crystal
Age Hardening of Al During Welding
Initially, there’s a fine
a) dispersion of precipitates
in the grains.
Microstructure now has
b) rippened precipitations in
grains and secondary
phase, q, at grain
boundaries next to fusion
Microstructural changes that occur in age-hardened alloys during fusion
welding a) microstructure in the weld at the peak temperature and b)
microstructure in the weld after slowly cooling to room temperature.
Welding is now routine but care is required and there are limitations on the
types of combinations, which can be joined. Usually a specified weld-filler is
required for a particular alloy.
Note the significant increase in strength of age hardened alloys.
What is the perfect application for pure Al?
Aluminum alloys are designated by Aluminum Association into a
numbering system shown in the following Tables of Al Alloys.
The four digit numerical designation indicates the following
• The first digit indicates the principal material(s) to be added to
• The second digit indicates modifications of the original alloys
or impurity limits.
• The last two digits identify the Al alloy or Al purity.
Why are Al-Si-
Mg alloys age
The temper designations for wrought Al alloys follow the alloy
designations and are separated by a hyphen.
Subdivisions of a basic temper are indicated by one or more digits
and follow the letter of the basic designations.
Basic temper designations
• F – as fabricated.
• O – annealed and recrystallized
• H – strain hardened
• T – heat treated to produce stable tempers other than F or O.
• H1 – strain hardened only
• H2 – strain hardened and partially annealed
• H3 – strain hardened and stabilized.
Heat treated subdivisions
• T1 – naturally aged
• T3 – solution heat treated, cold worked, and naturally aged to
a substantial stable condition.
• T4 – solution heat treated and naturally aged to a substantially
• T5 cooled from an elevated temperature shaping process and
then artificially aged
• T6 – Solution heat treated and then artificially aged
• T7 - Solution heat treated and stabilized
• T8 – solution heat treated, cold worked and then artificially
What does naturally aged and artificially aged mean?
The second large class of nonferrous alloys is the Cu-based alloys.
Cu has a variety of properties, which make it useful in many
• High thermal conductivity
• High electrical conductivity (Silver (Ag) is the highest metal!)
• Good corrosion resistance
• Cu and most of its alloys are FCC -> therefore toughness and
ductility are retained at cryogenic temperatures.
• Ease of fabrication
• Medium tensile strength
• Controllable microstructure – easily alloyed for higher strength
• Easily soldered and joined, although it’s not easy to weld (why?)
• Relatively inexpensive
Cu is normally produced from copper sulfide ores as tough-pitch
copper (98 – 99% Cu) or as electrolytically refined Cu (99.95%
Cu alloys are classified according to a designation of the Copper
Development Association, which is a different designation that
that of the Aluminum Association. The tempering designations
are also different.
Some example of Cu alloys are shown in the follow slide.
There are several ways to increase the strength of Cu alloys
• They can be cold-worked or solid-solution strengthened
• Cold-working has a substantial work hardening effect, much
like it did for Al Alloys.
• The process of age hardening is an important method of
strengthening alloys of materials and it illustrates the power of
knowledge of the phase diagrams and microstructures of
The solid solution strengthening of Cu has produced many
common engineering materials such as:
Cu – Zn alloys are called “Brasses”
• 70% Cu – 30% Zn is called alpha brass because it is a single
phase solid solution.
• 60% Cu – 40% Zn has two phases, alpha and beta brass.
Cu – Sn alloys are called “Bronzes”
• 1-10% Sn forms solid solution strengthened alloys
Cu – Be alloys typically contain 0.6-2% Be and 0.2-2.5% Co.
• Precipitation hardened and cold working is possible – extremely
high strength alloys are possible (~215,000 psi), which is the
highest of any commercial copper alloys
• Non-sparking, excellent fatigue resistant
• Used for springs, gears, diaphrams, valves, welding nozzles –
Nickel and Cobalt Alloys
Nickel and Cobalt alloys are used for corrosion protection and for
high-temperature resistance, taking advantage of their high
melting points and high strengths.
Nickel is FCC and has good formability.
Cobalt is FCC above 417 C and HCP below this temperature.
Cobalt is used because of its exceptional wear resistance, and
because of its resistance to body fluids, as a biomedical material
for prosthetic devices such as hip and knee socket replacement.
Typical alloys and their uses are listed in the following table.
Turbine blade design for active cooling by a gas shown in a) and
the increase in the high-temperature capability of Ni superalloys
as a result of improved manufacturing methods from producing a
polycrystalline material to a single crystal material.
While there are numerous other nonferrous alloys such as Mg, Zn,
and Zr, which can be considered for special applications, the
alloys that has received considerable attention is Titanium, Ti.
• Ti is relatively light weight (4505 kg/m3)
• Ti has excellent corrosion resistance, especially in sea water
• Ti has a high strength to weight ratio
• Some Ti alloys have a strength of over 150,000 psi (1000 MPa)
• It’s surface oxide (TiO2) breaks down at ~500 C, which limits
the range of its use, but surface coatings help to increase it’s
range of use to much higher temperatures (~1000 C).
• Ti has a HCP structure at room temperature and changes to
BCC at 882 C – alloying with niobium stabilizes the BCC phase
to room temperature, which enhances fabricability. Why?
• Typically, other alloying additions (Al, Sn, V) increase Ti’s
strength and machinability (see table next slide).
Refractory metals include tungsten (W), molybdenum (Mo),
tantalum (Ta) and niobium (Nb).
Their metallic bonds are weak resulting in low yield strengths.
They have exceptionally high melting temperature (>1925 C) and so
have high-temperature service.
They have a BCC structure so they display a ductile to brittle
transition temperature. Hot forming these materials aids their
fabrication into useable products.
Applications include filaments for light bulbs (tungsten),
penetrators, rocket nozzles, nuclear power generators (niobium),
electronic capacitors (tantalum and niobium) and chemical
These metals have a high density. The US army is developing W to
be a penetrator (high impact bullet) to replace Urania. Urania is
currently being used but it contaminates the combat zone and
These include gold, silver, palladium and rhodium.
They are precious and expensive.
These materials resist corrosion and make very good conductors of
As a result alloys of these materials are often used as electrodes for
devices and to measure temperature as the thermocouples.
Because of their corrosion resistance, they are used as nano-sized
particles as catalysts in automobile exhaust systems and in
petroleum refining. In exhaust gases, they facilitate the
oxidation of CO to CO2 and NOx to N2 and O2.
Recently, Au nanoparticles have been used to fight cancer by being injected into
and taken up by the cancer, which is illuminated by a laser. The laser creates
a surface plasmon (collection of charges) on the Au particle surface, which
decay to phonons (atomic lattice vibrations), which heat the cancer cells and