19. (2 points) Activation energies are lower for interstitial diffusion than for

Question

19. (2 points) Activation energies are lower for interstitial diffusion than for vacancy diffusion. True Fal 20. (2 points) One of the best ways to decrease the time required for diffusion is to use lower temperatures or to make the diffusion distances as large as possible a))lrue (b) False 21. (2 points) In class you were taught that there are several classifications for defects in crystals based on their size/dimension. Vacancies, interstitials, and substitutional solutes are which one of the following? (a) Line defects b) Volume defects (c Point defects d) Planar defects (e) None of the above 22. (2 points) Tempering of a steel will: (a) Decrease the hardness by decreasing the amount of martensite b) Decrease the hardness by increasing the amount of martensite c)Increase the hardness by decreasing the amount of martensite Increase the hardness by increasing the amount of martensite (e) None of the above 23. (2 points) In steel, both pearlite and bainite are composed of which 2 phases: (a) Bainite and Cementite (b) Ferrite and Cementite Ferrite and Martensite! (d) Martensite and Cementite (e) Martensite and Tempered Martensite (f) Pearlite and Ferrite are known as: (a) Vitamins (b) Nucleants Growth inciters (d) Inoculants (e) None of the above

Explanation / Answer

19) The answer is True: Activation energy of interstitial diffusion is lower because interstices are small atoms which less energy or weak bond. So they can easily jump or diffuse than vacancies. Option A

20) The answer is False: Time for diffusion is directly proportional to the square of diffusion distance and inversely proportional to diffusion coefficient. If the temperature decreases, diffusive coefficient decreases so the time required for diffusion increases and distance increases the time for diffusion as they are directly proportional. Option B

21) Point Defects Option C

22) Tempering is a process of quenching which is done to steel in order to increase the mechanical properties. Therefore, tempering increases the hardness by increasing the amount of martensite. Option D

23) Cementite and Ferrite Option B

24) Chemicals added to molten metal to promote nucleation are known as inoculants or grain refiners, Option D

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