Design/Materials Selection for an Electrical Switch
Design the contacts for a switch or relay that opens and closes a high-current electrical circuit.
Example SOLUTION
When the switch or relay opens and closes, contact between the conductive surfaces can cause
wear and result in poor contact and arcing.
Therefore, our design must provide for both good electrical conductivity and good wear
resistance. A relatively pure copper alloy dispersion strengthened with a hard phase that does
not disturb the copper lattice would, perhaps, be ideal. In a Cu-Al2O3 alloy, the hard ceramicoxide particles provide wear resistance but do not interfere with the electrical conductivity of
the copper matrix.
Design
of
a
Heat
Treatment
for
a
Cu-Al
Alloy
Gear
Design the heat treatment required to produce a high-strength aluminum-bronze gear
containing 10% Al.
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Figure 13.6 Binary phase diagrams for the (c) copper-aluminum
Example 13.6 SOLUTION
1. Heat the alloy to 950oC and hold to produce 100% β.
2. Quench the alloy to room temperature to cause β to transform to martensite, β´, which
is supersaturated in copper.
3. Temper below 565oC; a temperature of 400oC might be suitable. During tempering, the
martensite transforms to α and γ2. The amount of the γ2 that forms at 400oC is:
4. Cool rapidly to room temperature so that the equilibrium γ does not form.
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.
 Superalloys - A group of nickel, iron-nickel, and cobalt-based alloys that have
exceptional heat resistance, creep resistance, and corrosion resistance
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Figure The effect of temperature on the tensile strength of several nickel-based alloys.
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Figure 13.8 (a) Microstructure of a superalloy, with carbides at the grain boundaries and γ΄
precipitates in the matrix ( 15,000). (b) Microstructure of a superalloy aged at two
temperatures, producing both large and small cubical γ΄ precipitates ( 10,000). (ASM
Handbook, Vol. 9, Metallography and Microstructure (1985), ASM International, Materials
Park, OH 44073.)
Example Design/Materials Selection for a High-Performance Jet Engine Turbine Blade
Design a nickel-based superalloy for producing turbine blades for a gas turbine aircraft engine
that will have a particularly long creep-rupture time at temperatures approaching 1100oC.
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Figure 13.9 (a) A turbine blade designed for active cooling by a gas. (b) The high-temperature
capability of superalloys has increased with improvements in manufacturing methods (for
Example 13.7).
Example 13.7 SOLUTION
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First, we need a very stable microstructure. Addition of aluminum or titanium permits the
precipitation of up to 60 vol% of the γ´ phase during heat treatment and may permit the alloy
to operate at temperatures approaching 0.85 times the absolute melting temperature.
Second, we might produce a directionally solidified or even single-crystal turbine blade
(Chapter 8). In directional solidification, only columnar grains.
We would then heat treat the casting to assure that the carbides and γ´ precipitate with the
correct size and distribution.
Finally, the blade might contain small cooling channels along its length. Air for
combustion in the engine can pass through these channels, providing active cooling to the
blade, before reacting with fuel in the combustion chamber.
Titanium Alloys
 Titanium’s excellent corrosion resistance provides applications in chemical processing
equipment, marine components, and biomedical implants such as hip prostheses.
 Titanium is an important aerospace material, finding applications as airframe and jet
engine components.
 Titanium alloys are considered biocompatible (i.e., they are not rejected by the body).
By developing porous coatings of bone-like ceramic compositions known as
hydroxyapatite, it may be possible to make titanium implants bioactive (i.e., the natural
bone can grow into the hydroxyapatite coating).
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Figure 13.10 Portions of the phase diagrams for (a) titanium-tin, (b) titanium-aluminum, (c)
titanium-molybdenum, and (d) titanium-manganese.
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Figure 13.11 The effect of temperature on the yield strength of selected titanium alloys.
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Figure 13.12 (a) Annealing and (b) microstructure of rapidly cooled alpha titanium ( 100).
Both the grain boundary precipitate and the Widmanstätten plates are alpha. (From ASM
Handbook, Vol. 7, (1972), ASM International, Materials Park, OH 44073.)
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Figure 13.13 Annealing of an alpha-beta titanium alloy. (a) Annealing is done just below the α–β
transformation temperature, (b) slow cooling gives equiaxed α grains ( 250), and (c) rapid
cooling yields acicular α grains ( 2500). (From Metals Handbook, Vol. 7, (1972), ASM
International, Materials Park, OH 44073.)
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Figure 13.14 (a) Heat treatment and (b) microstructure of the alpha-beta titanium alloys. The
structure contains primary α (large white grains) and a dark β matrix with needles of α formed
during aging (250). (From ASM Handbook, Vol. 7, (1972), ASM International, Materials Park, OH
44073.)
Example 13.8 Design of a Heat Exchanger
Design a 5-ft-diameter, 30-ft-long heat exchanger for the petrochemical industry (Figure 13.15).
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Figure 13.15 Sketch of a heat exchanger using titanium tubes (for Example 13.8).
Example 13.8 SOLUTION
Provided that the maximum operating temperature is below 535 oC so that the oxide film is
stable, titanium might be a good choice to provide corrosion resistance at elevated
temperatures. A commercially pure titanium provides the best corrosion resistance.
Pure titanium also provides superior forming and welding characteristics and would,
therefore, be our most logical selection. If pure titanium does not provide sufficient strength,
an alternative is an alpha titanium alloy, still providing good corrosion resistance, forming
characteristics, and weldability but also somewhat improved strength.
Example 13.9
Design of a Connecting Rod
Design a high-performance connecting rod for the engine of a racing automobile (Figure 13.16).
Figure 13.16 Sketch of connecting rod (for Example 13.9).
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Example 13.9 SOLUTION
To achieve high strengths, we might consider an alpha-beta titanium alloy. Because of its
availability, the Ti-6% Al-4% V alloy is a good choice. The alloy is heated to about 1065 oC, which
is in the all-β portion of the phase diagram.
When the heat treatment is performed in the all-β region, the tempered martensite has
an acicular structure, which reduces the rate of growth of any fatigue cracks that might
develop.
Example 13.10
Materials for Hip Prosthesis
What type of a material would you choose for an implant to be used for a total hip replacement
implant?
Example 13.10 SOLUTION
We need to consider the following factors: biocompatibility, corrosion resistance, high-fracture
toughness, excellent fatigue life, and wear resistance.
These requirements suggest 316 stainless steel or Ti-6% Al-4% V. Titanium is biocompatible and would be a better choice. Perhaps a composite material in which the stem is
made from a Ti-6% Al-4% V alloy and a head that is made from a wear-resistant, corrosion
resistant, and fractured tough ceramic, such as alumina, may be an answer.
Another option is to coat the implant with a material like porous hydroxyapatite to
encourage bone growth.
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