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The Science and Engineering of Materials, 4th edDonald R. Askeland – Pradeep P. Phulé

Chapter 13 – Nonferrous Alloys

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Objectives of Chapter 13

Explore the properties and applications of Cu, Al, and

Ti alloys in load-bearing applications.

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Chapter 13 Outline

13.1 Aluminum Alloys

13.2 Magnesium and Beryllium Alloys

13.3 Copper Alloys

13.4 Nickel and Cobalt Alloys

13.5 Titanium Alloys

13.6 Refractory and Precious Metals

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Hall-Heroult process - An electrolytic process by which aluminum is

extracted from its ore.

Temper designation - A shorthand notation using letters and numbers to

describe the processing of an alloy. H tempers refer to cold-worked

alloys; T tempers refer to age-hardening treatments.

Section 13.1

Aluminum Alloys

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Production of aluminum in an electrolytic cell.

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(a) FeAl3 inclusions in annealed 1100 aluminum ( 350).

(b) Mg2Si precipitates in annealed 5457 aluminum alloy ( 75).

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Portion of the aluminum-magnesium phase diagram.

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(a) Sand-cast 443 aluminum alloy containing coarse silicon and inclusions.

(b) Permanent-mold 443 alloy containing fine dendrite cells and fine silicon due to faster cooling.

(c) Die-cast 443 alloy with a still finer microstructure ( 350).

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A steel cable 0.5 in. in diameter has a yield strength of 70,000 psi. The density

of steel is about 7.87 g/cm3. Based on the data in Table 13-5, determine (a) the

maximum load that the steel cable can support, (b) the diameter of a cold-

worked aluminum-manganese alloy (3004-H 18) required to support the same

load as the steel, and (c) the weight per foot of the steel cable versus the

aluminum alloy cable.

Example 13.1

Strength-to-Weight Ratio in Design

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Example 13.1 SOLUTION

a. Load = F = σy A = 70.000 (π/4) (0.5 in.)2 = 13,744 lb

b. The yield strength of the aluminum alloy is 36,000 psi. Thus:

A = (π/4)d2 = F/σy = 13,744/36,000 = 0.38 in.2 d = 0.697 in.

Density of steel = ρ = 7.87 g/cm3 = 0.284 lb/in.3 Density of aluminum =

ρ = 2.70 g/cm3 = 0.097 lb/in3

c. Weight of steel = Alρ = (π/4)(0.5in)2(12)(0284) = 0.669 lb/ft

Weight of aluminum = Alρ = (π/4)(0.697)2 (2) (12) (0.097) = 0.444 lb/ft

Although the yield strength of the aluminum is lower than that of the steel

and the cable must be larger in diameter, the aluminum cable weighs only

about half as much as the steel cable. When comparing materials, a proper

factor-of-safety should also be included during design.

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Design a method for recycling aluminum alloys used for beverage cans.

Example 13.2 SOLUTION

One approach to recycling the cans is to separate the two alloys from the cans. The cans

are shredded, then heated to remove the lacquer that helps protect the cans during use.

We could then further shred the material at a temperature where the 5182 alloy begins to

melt. The small pieces of 5182 can therefore be separated by passing the material through

a screen. The two separated alloys can then be melted, cast, and rolled into new can

stock.

An alternative method would be to simply remelt the cans. Once the cans have

been remelted, we could bubble chlorine gas through the liquid alloy. The chlorine reacts

selectively with the magnesium, removing it as a chloride. The remaining liquid can then

be adjusted to the proper composition and be recycled as 3004 alloy.

Example 13.2

Design of an Aluminum Recycling Process

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Design the material to be used to contain liquid hydrogen fuel for the space

shuttle.

Example 13.3 SOLUTION

Liquid hydrogen is stored below 253oC; therefore, our tank must have good

cryogenic properties.

Lightweight aluminum would appear to be a good choice.

Aluminum does not show a ductile to brittle transition. Because of its good

ductility, we expect aluminum to also have good fracture toughness,

particularly when the alloy is in the annealed condition.

One of the most common cryogenic aluminum alloys is 5083-O.

Aluminum-lithium alloys are also being considered for low-temperature

applications to take advantage of their even lower density.

Example 13.3

Design/Materials Selection for a Cryogenic Tank

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Design a casting process to produce automotive wheels having reduced weight

and consistent and uniform properties.

Example 13.4 SOLUTION

Thixocasting process in which the material is stirred during

solidification, producing a partly liquid, partly solid structure that behaves as a

solid when no external force is applied, yet flows as a liquid under pressure. We

would select an alloy with a wide-freezing range so that a significant portion of

the solidification process occurs by the growth of dendrites. A hypoeutectic

aluminum-silicon alloy might be appropriate. In the thixocasting process, the

dendrites are broken up by stirring during solidification. The billet is later

reheated to cause melting of just the eutectic portion of the alloy, and it is then

forced into the mold in its semi-solid condition at a temperature below the

liquidus temperature.

Example 13.4

Design of a Casting Process for Wheels

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Magnesium alloys are used in aerospace applications, high-speed

machinery, and transportation and materials handling equipment.

Instrument grade beryllium is used in inertial guidance systems where

the elastic deformation must be minimal; structural grades are used in

aerospace applications; and nuclear applications take advantage of the

transparency of beryllium to electromagnetic radiation. Beryllium is

expensive, brittle, reactive, and toxic.

Section 13.2

Magnesium and Beryllium Alloys

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The magnesium-aluminum phase diagram.

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Blister copper - An impure form of copper obtained during the copper

refining process.

Applications for copper-based alloys include electrical components

(such as wire), pumps, valves, and plumbing parts, where these

properties are used to advantage.

Brass - A group of copper-based alloys, normally containing zinc as the

major alloying element.

Bronze - Generally, copper alloys containing tin, can contain other

elements.

Section 13.3

Copper Alloys

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Binary phase diagrams for the

(a) copper-zinc,

(b) copper-tin,

(c) copper-aluminum, and

(d) copper-beryllium systems.

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Design the contacts for a switch or relay that opens and closes a high-current

electrical circuit.

Example 13.5 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 ceramic-oxide particles provide

wear resistance but do not interfere with the electrical conductivity of the copper

matrix.

Example 13.5

Design/Materials Selection for an Electrical Switch

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Design the heat treatment required to produce a high-strength aluminum-bronze

gear containing 10% Al.

Example 13.6

Design of a Heat Treatment for a Cu-Al Alloy Gear

Binary phase diagrams

for the copper-aluminum.

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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.

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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.

Section 13.4

Nickel and Cobalt Alloys

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The effect of temperature on the tensile strength of several nickel-based alloys.

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(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).

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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.

Example 13.7

Design/Materials Selection for a High-Performance

Jet Engine Turbine Blade

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Example 13.7 SOLUTION

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. 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.

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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).

Section 13.5

Titanium Alloys

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Portions of the phase diagrams

for

(a) titanium-tin,

(b) titanium-aluminum,

(c) titanium-molybdenum, and

(d) titanium-manganese.

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The effect of temperature on the yield strength of selected titanium alloys.

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(a) Annealing and (b) microstructure of rapidly cooled alpha titanium ( 100). Both

the grain boundary precipitate and the Widmanstätten plates are alpha.

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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),

(c) rapid cooling yields acicular α grains ( 2500).

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(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).

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Design a 5-ft-diameter, 30-ft-long heat exchanger for the petrochemical

industry.

Example 13.8

Design of a Heat Exchanger

Sketch of a heat exchanger using titanium tubes.

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Example 13.8 SOLUTION

Provided that the maximum operating temperature is below 535oC 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.

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Design a high-performance connecting rod for the engine of a racing

automobile.

Example 13.9

Design of a Connecting Rod

Sketch of connecting rod.

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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 1065oC, 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.

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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 bio-compatible 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.

Example 13.10

Materials for Hip Prosthesis

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Refractory metals – These include tungsten, molybdenum, tantalum,

and niobium (or columbium), have exceptionally high-melting

temperatures (above 1925oC) and, consequently, have the potential for

high-temperature service.

Applications of Refractory metals include filaments for light bulbs,

rocket nozzles, nuclear power generators, tantalum- and niobium-based

electronic capacitors, and chemical processing equipment.

Precious Metals - These include gold, silver, palladium, platinum, and

rhodium. From an engineering viewpoint, these materials resist

corrosion and make very good conductors of electricity.

Section 13.6

Refractory and Precious Metals

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