# Introduction to Materials Exam 2

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• Topic: Dislocation, Materials science, Solid
• Pages : 3 (564 words )
• Published : April 14, 2012

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MIE Introduction to Materials, Fall ‘08
Exam #2

(Please keep your writing neat, use full sentences, and make the writing large enough so that it can be read.)

Problem #1

a) Describe the difference between the substitutional and the interstitial mechanism for mass transport - diffusion in metals.

b) Do these two mechanisms control mass transport – diffusion in 1) ceramics and 2) polymers? Explain your answers.

c) Given the table of data below, which metal (iron - Fe, tungsten - W, or copper - Cu) has the highest self diffusion coefficient, D, at 950oC. Explain your reasoning.

MetalMelting Point(oC)Crystal Structureσy (MPa) KIC (MPam1/2)

Fe 1538fcc 18050
W 3410bcc 52280
Cu 1085fcc 6935
Problem #2

a) Draw a typical true stress - strain curve, σT vs εT, for a metal. Indicate the yield point, and the point where necking takes place.

b) Using this curve describe the process of strain hardening including the effect of dislocations. Describe the strain hardening relationship, σT = K(εT)n , where K and n are constants. Finally, give a practical process that will strain harden an alloy.

c) Thermal processing of a metal allows for the recovery of the initial mechanical properties of an alloy. Describe briefly the recrystallization process indicating the driving force for the process, the change in the defect structure of the alloy, and the change in microstructure, if any.

d) Describe a second process which can strengthen a metal. Also, explain the importance of dislocations to the process of strengthening in this example. Problem #3

a) Polymer and ceramic deformation are not controlled by dislocations. Explain this statement.

b) Semi-crystalline polymers have stress – strain curves similar to those of metals (Problem #2, part a). Explain how elastic and plastic deformation takes place...