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Lecture 13 - Nonferrous Alloys

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Lecture 13 - Nonferrous Alloys Powered By Docstoc
					 Mech 285 Lectures

Professor Rodney Herring
                              Objectives
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
   precision metals.
• Write a discussion of the important properties of the common nonferrous
   alloys for particular applications.
                          Nonferrous Alloys
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.
(cont’d)                   Aluminum
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
   and silicon).
• Al is easily alloyed and many of its alloys are stronger than
   pure Al.
• Al alloys are non-magnetic.
• Al alloys have a stable or predictable microstructure.
• Al alloys have an excellent machining, forming, and forging
   characteristics.
• 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
    from Al.
                          Aluminum
(cont’d)
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
                                        Hall-Heroult process.
                                        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
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
  requirements.
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.
                           Age 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.
Age Hardening
                      Age Hardening


GPI and GPII are
nuclei of the q
precipitate, which
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
                       (Al-6.2Cu-1.25Li-0.4Mn-0.4Ag-0.2Zr)




• 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
                  (Al-6.2Cu-1.25Li-0.4Mn-0.4Ag-0.2Zr)




• 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
     cooling.
     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
    copper.
  Noncoherent & Coherent Precipitates




            a)                              b)

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
structure.
        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
                                                        zone.
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
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
  information.
• The first digit indicates the principal material(s) to be added to
  the Al.
• 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
hardenable or
not?
                     Aluminum Alloys
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.
For example.
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.
Strain-hardened subdivisions
• H1 – strain hardened only
• H2 – strain hardened and partially annealed
• H3 – strain hardened and stabilized.
                     Aluminum Alloys
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
  stable condition
• 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
  aged.

             What does naturally aged and artificially aged mean?
                         Copper Alloys
The second large class of nonferrous alloys is the Cu-based alloys.
Cu has a variety of properties, which make it useful in many
  applications.
• 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
                       Copper Alloys
Cu is normally produced from copper sulfide ores as tough-pitch
  copper (98 – 99% Cu) or as electrolytically refined Cu (99.95%
  Cu).
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.
                        Copper Alloys
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
  materials.
                   Copper Alloys
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 –
  relatively inexpensive.
                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.
                      Titanium Alloys
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 Alloys
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
  processing equipment.
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
  surrounding area.
                          Precious Metals
These include gold, silver, palladium and rhodium.
They are precious and expensive.
These materials resist corrosion and make very good conductors of
  electricity.
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
  kill them.

				
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