《材料科学与工程基础》英文影印版习题及思考(4)

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Which of these elements would you expect to form the following with copper:

(a) A substitutional solid solution having complete solubility?

(b) A substitutional solid solution of incomplete solubility?

(c) An interstitial solid solution?

5.9 For both FCC and BCC crystal structures, there are two different types of

interstitial sites. In each case, one site is larger than the other, which site is

normally occupied by impurity atoms. For FCC, this larger one is located at the center of each edge of the unit cell; it is termed an octahedral interstitial site. On the other hand, with BCC the larger site type is found at 0, __, __ positions—that is, lying on _100_ faces, and situated midway between two unit cell edges on this face and one-quarter of the distance between the other two unit cell edges; it is termed a tetrahedral interstitial site. For both FCC and BCC crystal

structures, compute the radius r of an impurity atom that will just fit into one of these sites in terms of the atomic radius R of the host atom.

5.10 (a) Suppose that Li2O is added as an impurity to CaO. If the Li_ substitutes for

Ca2_, what kind of vacancies would you expect to form? How many of these vacancies are created for every Li_ added?

(b) Suppose that CaCl2 is added as an impurity to CaO. If the Cl_ substitutes for O2_, what kind of vacancies would you expect to form? How many of the

vacancies are created for every Cl_ added?

5.28 Copper and platinum both have the FCC crystal structure and Cu forms a

substitutional solid solution for concentrations up to approximately 6 wt% Cu at room temperature. Compute the unit cell edge length for a 95 wt% Pt-5 wt% Cu alloy.

5.29 Cite the relative Burgers vector–dislocation line orientations for edge, screw, and

mixed dislocations.

6.1 Briefly explain the difference between selfdiffusion and interdiffusion.

6.3 (a) Compare interstitial and vacancy atomic mechanisms for diffusion.

(b) Cite two reasons why interstitial diffusion is normally more rapid than

vacancy diffusion.

6.4 Briefly explain the concept of steady state as it applies to diffusion.

6.5 (a) Briefly explain the concept of a driving force.