materials assignement 2 attempt 1

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CT Quiz Homepage http://webct.mcmaster.ca/SCRIPT/MATLS_1M03_T2_06_newR

02/02/2006

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Assignment 2 (Ch 4 & 5)

Name: BRYCE JONES Attempt: 1 / 3 Out of: 16

Started: January 29, 2006 2:33pm Finished: February 2, 2006 10:55pm Time spent: 104 hr, 21 min., 48 sec.

Question 1 (1 point)

_________________ point defects occur when impurity atoms fill the voids or interstices between host atoms in the crystal structure.

Student response:

No. Student response Grade Correct answer (Value)1. Interstitial 100% Answer evaluated by the formula. (100%)

General feedback: Interstitial point defects involve impurity atoms filling the voids between atoms in the regular crystal lattice. See page 69 in the text.

Score: 1 / 1

Question 2 (2 points)

How many vacancies are present in a cubic meter of a material with the following properties at 556°C?

The energy for vacancy formation is 1.41 eV/atom.

The density of the material is 13.1 g/cm3.The atomic weight of the material is 174 g/mol.

Use scientific notation, significant figures in answer: 3

Student response: 1.23e20

Correct answer: 1.22E20 (1.22 * 1020

)

General feedback: See page 67-68 of the text book for a similar example.

Score: 2 / 2


Find the composition in weight percent (wt%) of the copper in an aluminum-copper alloy that consists of 74.52 at% aluminum.

Use decimal notation, digits after decimal: 2

Student response: 44.61 wt%

Correct answer: 44.61 wt%

General feedback: For help on composition conversions, please see page 71 of the textbook. An example similar to this question is given on page 73.

Score: 1 / 1


Consider a copper-aluminum solid solution containing 89.3 at% Al. How many atoms per cubic centimeter (atoms/cm^3) of copper are there in thissolution?

Take the density of copper to be 8.94 g/cm3 and the density of aluminum to be 2.71 g/cm3.


Student response: 9.07e21 atoms/cm^3

Correct answer: 6.68E21 (6.68 * 1021) atoms/cm^3

General feedback: If you're having trouble deriving the equation for this problem, it is introduced in exercise 4.17 of the text. Note that in this version of the equation C1 is supposed to be specified in wt%, but the composition of the solution in this question is given in at%.

Score: 0.2 / 2


True or False?

The surface energy of a single crystal depends on the crystallographic orientation with respect to the surface.

Student response: CorrectResponse

StudentResponse

Answer Choices

a. True

b. False

General feedback: True. The surface energy of a single crystal depends on crystallographic orientation because the atomic packing is different for thevarious crystallographic planes. The number of unsatisfied bonds will vary from plane to plane.

Score: 1 / 1



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A copper-aluminum alloy has an aluminum concentration of 357 kg/m3.

If the densities of aluminum and copper are 2.71 and 8.94 g/cm3, respectively, what is the concentration of aluminum in wt%?


Student response: 92.41 wt%

Correct answer: 4.40 wt%

General feedback: For more information on the mass per unit volume composition scheme, see page 71 of the text.

Score: 0.1 / 1


The process of atoms moving around within a pure metal is called __________________.

Student response: PercentValue

CorrectResponse

StudentResponse

Answer Choices

0.0% a. interdiffusion

0.0% b. impurity diffusion

0.0% c. pure metal diffusion

100.0% d. self-diffusion

General feedback: Diffusion occuring in a pure metal, where all atoms exchanging positions are of the same type, is called self-diffusion.

Score: 1 / 1


Consider a metallic species which diffuses in a host metal.

The values of D0 and Qd are 6.86 x 10-5 m2 /s and 232 kJ/mol, respectively.

If the temperature is 540 °C, calculate the value of the diffusion coefficient (in m^2/s).


Student response: 8.42e-20 m^2/s

Correct answer: 8.37E-20 (8.37 * 10-20) m^2/s

General feedback: This problem can be solved using the temperature-dependent formula for diffusion. See the section on factors that influence diffusionin the text for more information.

Score: 2 / 2


Carbon is allowed to diffuse through a steel plate 12.8 mm thick.

The concentrations of carbon at the two faces are 0.730 and 0.234 kg/m3, and are maintained constant. The preexponential (D0) and the activation

energy are 8.17 x 10-7m2 /s and 138 kJ/mol, respectively.

Calculate the temperature (in K) at which the diffusion flux is 2.38 x 10-7 kg/(m2-s).


Student response: 3e3 K

Correct answer: 3394 K

General feedback: The diffusion flux through the plate is given by Fick's law and is proportional to the concentration gradient between the ends of theplate:

Before you can use this equation, however, you need to determine the diffusion coefficient, D. That can be accomplished using therelationship between diffusion and temperature:

Remember to watch your signs. More information can be found in the sections of your text which discusses steady-state diffusion andfactors that influence diffusion.

Score: 0.2 / 2


A sheet of BCC iron is exposed to a carburizing atmosphere on one side and a decarburizing atmosphere on the other.

Diffusion is carried out at 725°C. After it reaches steady state, the iron is quickly cooled to room temperature. The carbon concentrations at the two

surfaces are determined to be 0.0179 and 0.0062 wt%. The diffusion flux is found to be 1.8 x 10-8 kg/m2-s and the diffusion coefficient is 8.3 x 10-11


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m2 /s.

Knowing that the densities of carbon and iron are 2.25 and 7.87 g/cm3, respectively, find the thickness of the sheet of iron (in mm).


Student response: 5.4e-2 mm

Correct answer: 4.2 mm

General feedback: To solve this problem, first convert the concentrations from weight percent to kilograms of carbon per cubic meter of iron using thefollowing equation:

The diffusion flux (in mass diffusing per unit area per unit time) is proportional to the concentration gradient as follows:

Here the high and low concentrations are denoted by subscripts 1 and 2, respectively.

Substitute the C 1 and C 2 concentration expressions from the first equation into the second equation, and solve for x2 (let x1=0mm).

For more information, see the section in the text on steady-state diffusion.

Score: 0.3 / 3

Total score: 8.8 / 16 = 55.0%

materials assignement 2 attempt 1

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