material science homework

ENGR 1210: FA 2016; HW 6

Homework 6 on chapter 6; (Total points: 100)

Reading assignments: Chapter 6 from textbook, notes and slides Student Name: MTSU ID:

Honor pledge: I acknowledge that I have neither received

nor given any unauthorized help/aid during this assignment.

Part 1 Key Engineering Terms: fill in the blanks – 30 points

1. Hot working: permanent deformation of metals and alloys above the recrystallization temperature.

2. Cold working: permanent deformation of metals and alloys above he recrystallization temperature.

3. Hardness and ductility exhibit inverse relationship.

4. Ductility: The measure of degree of plastic permanent

deformation that has been sustained at fracture.

5. Elastic deformation: if a metal is deformed by a force

Return to its original dimensions after force is removed.

6. Engineering stress: average uniaxial force divided by original cross-sectional area.

7. Engineering strain: change in length of the sample divided by unit

Original length.

8. Shear stress: ratio of shear force divided by area on which it acts.

9. Modulus of elasticity: stress divided by strain in the

Elastic region of an engineering stress-strain diagram.

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10. Yield strength: the stress at which material just begins to plastically deform.

11. Offset yield strength: value of stress at 0.2% strain.

12. Resilience: it is the capacity of material to absorb energy when deformed

Elastically .

13. Toughness: Property that is indication of material’s resistance to

Brittle fracture when a crack is present.

14. Ultimate tensile strength: the maximum

stress into engineering stress strain curve before material fractures.

15. True stress: load divided by instantaneous cross-section area.

16. Hardness: Measure of material’s resistance to localized

Plastic deformation.

17. Dislocation: a line defect where an array of atoms are missing resulting in lattice distortion.

18. Slip: the process by which plastic deformation is produced by

Dislocation motion.

19. Critical resolved shear stress: the minimum amount of shear stress required to initiate

slip.

20. Twin boundary: a special grain boundary across which the grains show

Mirror lattice symmetry.

21. Recovery: the first stage in the annealing process that results in removal of

Residual stresses and formation of

Low energy dislocation configurations.

22. Grain boundary acts as a barrier to dislocation movement.

23. Recrystallization: the second stage of annealing process in which new grains start to grow and dislocation density _ decreases significantly.

24. Solid solution strengthening: alloying materials with

Solute atoms that go into Substitutional or interstitial solid solution.

25. Strain hardening: it is a phenomenon by which ductile material becomes harder and stronger as it is plastically/permanently deformed.

26. Superplasticity: the ability of some metals to deform plastically by

1000% to 2000% at

high temperatures and low loading rates.

Part 2 Learning concepts/reflections/problems – 50 points

1. (a) Explain the three stages of annealing treatment in 2-3 sentences each.

(b) Explain any five properties that we can measure using a stress-strain curve under uniaxial tensile loading. 10 points

2. What are the 4 strengthening mechanisms of metals? Explain each mechanism in 4-5 sentences. 10 points

3. Consider a cylindrical specimen of some hypothetical metal alloy that has a diameter of

10.0 mm. A tensile force of 1500 N produces an elastic reduction in diameter of 6.7 × 10– 4 mm. Compute the elastic modulus of this alloy, given that Poisson’s ratio is 0.35.

10 points

4. A 0.505-in.-diameter aluminum alloy test bar is subjected to a load of 25,000 lb. If the diameter of the bar is 0.490 in. at this load, determine (a) the engineering stress and strain and (b) the true stress and strain. 10 points

5. (a) A 10-mm-diameter Brinell hardness indenter produced an indentation 2.50 mm in diameter in a steel alloy when a load of 1000 kg was used. Compute the HB of this material.

(b) What will be the diameter of an indentation to yield a hardness of 300 HB when a 500-kg load is used? 10 points

Part 3 Materials design – 20 points

1. Why are nanocrystalline materials stronger? Explain answer based on dislocation activity. 5 points

2. What are the five important factors that affect the recrystallization process in metals?

5 points

3. Consider casting a cube and a sphere on the same volume from same metal. Which one would solidify faster? Why? 5 points

4. Why is it difficult to improve both strength and ductility simultaneously? 5 points

Bonus questions: -10 points

1. Why are cast metal sheet ingots hot rolled first instead of being cold rolled?

2. Why does slip in metals usually take place on closed packed planes and in closed packed directions?