15 For each of the following classes of materials, give two specific examples that are

1 © 2014 Cengage Learning. All Rights Reserved. May not be scanned, copied, or duplicated, or posted to a publicly available website, in whole or in part.Chapter 1 Introduction to Materials Science and Engineering

1–5 For each of the following classes of materials, give two specific examples that are

a regular part of your life:

(a) metals, (b) ceramics, (c) polymers, and (d) Semiconductors.Specify the object that each material is found in and explain why the material is used in each specific application.

Solution: (a) Aluminum is a metal used in the base of some pots and pans for even heat distribution. It is also a lightweight metal that makes it useful in kitchen cookware. Stainless steel is used for flatware. It is easily formed and has good corrosion resistance, strength, and hardness.(b) Ceramics are compounds that contain metallic and nonmetallic elements. Two specific examples are tungsten carbide and magnesia.Tungsten carbide is often bonded with cobalt and/or nickel. Tungsten carbide is used mainly in tips for metal cutting tools (knives can be made with this) because of its good wear resistant characteristics.Magnesia is a heat-resistant ceramic that is used in liners for ovens.Magnesia can resist high temperatures but is susceptible to thermal stress cracking.(c) Two specific examples of polymers are polystyrene and polytetrafluoroethylene also known as Teflon. Styrofoam is polystyrene rigid foam insulation that is used for cups that keep hot drinks warm.Teflon is used as a coating on the inside of kitchen cookware such as frying pans because it prevents food from sticking to the pan while cooking.(d) Two examples of semiconductors are silicon doped with phosphorus (n- type) and silicon doped with boron (p-type). Both types of impurities convert silicon from a poor into a useful conductor. Both n and p-type semiconductors are contained in the semiconductor device called a diode, so that at the junction between both types, current is able to flow.A diode blocks current in one direction while allowing current flow in the other direction. A device that uses batteries, e.g. a remote control or a calculator, often contains a diode that protects the device if the batteries are inserted backward. The diode blocks the current from leaving the battery if it is reversed, protecting the sensitive electronics in the device. Another semiconductor is the compound semiconductor Al xGa1-xAs, which is used in lasers.

(Essentials of Materials Science and Engineering 3e Donald Askeland, Wendelin Wright ) (Solution Manual all Chapter) 1 / 4

2 © 2014 Cengage Learning. All Rights Reserved. May not be scanned, copied, or duplicated, or posted to a publicly available website, in whole or in part.1–6 Describe the enabling materials property of each of the following and why it is so: (a) steel for I-beams in skyscrapers, (b) a cobalt chrome molybdenum alloy for hip implants, (c) polycarbonate for eyeglass lenses, and (d) bronze for artistic castings.

Solution: (a) Steel for I-beams in skyscrapers must be strong in order to bear large mechanical loads.(b) A cobalt chrome molybdenum alloy for hip implants must be biocompatible, meaning that it must not degrade when inserted into the body nor be toxic or otherwise dangerous.(c) Polycarbonate for eyeglass lenses must be transparent and impact- resistant.(d) Bronze can be melted and poured into molds to be shaped. It is also fairly corrosion resistant (which is important for art displayed outdoors).Over long periods of time when subjected to an outdoor environment, bronze will develop an oxide known as a patina. The patina protects the bronze from further corrosion.

1–7 Describe the enabling materials property of each of the following and why it is so: (a) aluminum for airplane bodies, (b) polyurethane for teeth aligners (invisible braces), (c) steel for the ball bearings in a bicycle’s wheel hub, (d) polyethylene terephthalate for water bottles, and (e) glass for wine bottles.

Solution: (a) Aluminum has a high strength to weight ratio. Thus it has the strength needed to withstand the forces imposed on an airframe, but keeps the weight of the airplane low compared to other metals. The lighter the airplane body, the less force it takes to lift the plane into the air. This results in less fuel being used and a reduction in operating costs.Aluminum also has good corrosion resistance.(b) Polyurethane for teeth aligners must be highly formable and transparent.Since teeth aligners are worn during the day, a transparent material is desirable to make them less conspicuous. A unique set of teeth aligners must be produced for each individual, and so the polyurethane must be easily molded. Computer software is used to produce computer models of a person’s teeth during various stages of the correction process. A rapid prototyping tool is used to create physical models of the person’s teeth at each stage. A sheet of polyurethane is then heated and formed onto the models to produce the teeth aligners.

• / 4

3 © 2014 Cengage Learning. All Rights Reserved. May not be scanned, copied, or duplicated, or posted to a publicly available website, in whole or in part.(c) Ball bearings in a bicycle’s wheel hub reduce friction between metal surfaces. Therefore, steel is used because it has high strength and hardness.(d) Polyethylene terephthalate is easily formed by a blow-molding process and is recyclable, critical properties for mass-produced water bottles.(e) Glass has high chemical resistance; thus, glass bottles are used to preserve the taste of the wine contained in them.

1–9 Steel is often coated with a thin layer of zinc if it is to be used outside. What characteristics do you think the zinc provides to this coated, or galvanized, steel?What precautions should be considered in producing this product? How will the recyclability of the product be affected?

Solution: The zinc provides corrosion resistance to the iron in two ways. If the iron is completely coated with zinc, the zinc provides a barrier between the iron and the surrounding environment, therefore protecting the underlying iron. If the zinc coating is scratched to expose the iron, the zinc continues to protect the iron because the zinc corrodes preferentially to the iron (see Chapter 18). To be effective, the zinc should bond well to the iron so that it does not permit reactions to occur at the interface with the iron and so that the zinc remains intact during any forming of the galvanized material. When the material is recycled, the zinc will be lost by oxidation and vaporization, often producing a “zinc dust” that may pose an environmental hazard. Special equipment may be required to collect and either recycle or dispose of the zinc dust.

1–10 We would like to produce a transparent canopy for an aircraft. If we were to use a traditional window glass canopy, rock or birds might cause it to shatter. Design a material that would minimize damage or at least keep the canopy from breaking into pieces.

Solution: We might sandwich a thin sheet of a transparent polymer between two

layers of the glass. This approach, used for windshields of automobiles, will prevent the “safety” glass from completely disintegrating when it fails, with the polymer holding the broken pieces of glass together until the canopy can be replaced.Another approach might be to use a transparent, “glassy” polymer material such as polycarbonate. Some polymers have reasonably good impact properties and may resist failure. The polymers can also be toughened to resist impact by introducing tiny globules of a rubber, or elastomer, into the polymer; these globules improve the energy- absorbing ability of the composite polymer, while being too small to interfere with the optical properties of the material.

• / 4

4 © 2014 Cengage Learning. All Rights Reserved. May not be scanned, copied, or duplicated, or posted to a publicly available website, in whole or in part.1–11 Coiled springs should to be very strong and stiff. Silicon nitride (Si3N4) is a strong, stiff material. Would you select this material for a spring? Explain.

Solution: Springs are intended to resist high elastic forces, where only the

atomic bonds are stretched when the force is applied. The silicon nitride would satisfy this requirement; however, we would like to also have good resistance to impact and at least some ductility (in case the spring is overloaded) to ensure that the spring will not fail catastrophically. Ceramic materials such as silicon nitride have virtually no ductility, poor impact properties, and often are difficult to manufacture without introducing at least some small flaws that cause failure even for relatively low forces. The silicon nitride is NOT recommended.

1–12 Temperature indicators are sometimes produced from a coiled metal strip that uncoils a specific amount when the temperature increases. How does this work; from what kind of material would the indicator be made; and what are the important properties that the material in the indicator must possess?

Solution: Bimetallic materials are produced by bonding two materials having

different coefficients of thermal expansion to one another, forming a laminar composite. When the temperature changes, one of the materials will expand or contract more than the other material. This difference in expansion or contraction causes the bimetallic material to change shape; if the original shape is that of a coil, then the device will coil or uncoil, depending on the direction of the temperature change. In order for the material to perform well, the two materials must have very different coefficients of thermal expansion and should have high enough moduli of elasticity so that no permanent deformation of the material occurs.

1–13 You would like to design an aircraft that can be flown by human power nonstop for a distance of 30 km. What types of materials properties would you recommend? What materials might be appropriate?

Solution: Such an aircraft must possess enough strength and stiffness to resist

its own weight, the weight of the human “power source,” and any aerodynamic forces imposed on it. On the other hand, it must be as light as possible to ensure that the human can generate enough work to operate the aircraft. Composite materials, particularly those based on a polymer matrix, might comprise the bulk of the aircraft. The polymers have light weight (with densities of less than half that of aluminum) and can be strengthened by introducing strong, stiff fibers made of glass, carbon, or other polymers. Composites, having the strength and stiffness of steel, but with only a fraction of the weight, can be produced in this manner.

• / 4