《材料科学与工程基础》英文影印版习题及思考(5)

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(b) What is the driving force for steadystate diffusion?

6.6 Compute the number of kilograms of hydrogen that pass per hour through a

5-mm thick sheet of palladium having an area of 0.20 m2 at 500℃. Assume a diffusion coefficient of 1.0×10- 8 m2/s, that the concentrations at the high- and low-pressure sides of the plate are 2.4 and 0.6 kg of hydrogen per cubic meter of palladium, and that steady-state conditions have been attained.

6.7 A sheet of steel 1.5 mm thick has nitrogen atmospheres on both sides at 1200℃

and is permitted to achieve a steady-state diffusion condition. The diffusion coefficient for nitrogen in steel at this temperature is 6×10-11 m2/s, and the diffusion flux is found to be 1.2×10- 7 kg/m2-s. Also, it is known that the concentration of nitrogen in the steel at the high-pressure surface is 4 kg/m3. How far into the sheet from this high-pressure side will the concentration be 2.0 kg/m3? Assume a linear concentration profile.

6.24. Carbon is allowed to diffuse through a steel plate 15 mm thick. The

concentrations of carbon at the two faces are 0.65 and 0.30 kg C/m3 Fe, which are maintained constant. If the preexponential and activation energy are 6.2 _

10_7 m2/s and 80,000 J/mol, respectively, compute the temperature at which the diffusion flux is 1.43 _ 10_9 kg/m2-s.

6.25 The steady-state diffusion flux through a metal plate is 5.4_10_10 kg/m2-s at a

temperature of 727_C (1000 K) and when the concentration gradient is _350

kg/m4. Calculate the diffusion flux at 1027_C (1300 K) for the same

concentration gradient and assuming an activation energy for diffusion of

125,000 J/mol.

10.2 What thermodynamic condition must be met for a state of equilibrium to exist? 10.4 What is the difference between the states of phase equilibrium and metastability? 10.5 Cite the phases that are present and the phase compositions for the following

alloys:

(a) 90 wt% Zn–10 wt% Cu at 400℃

(b) 75 wt% Sn–25wt%Pb at 175℃

(c) 55 wt% Ag–45 wt% Cu at 900℃

(d) 30 wt% Pb–70 wt% Mg at 425℃

(e) 2.12 kg Zn and 1.88 kg Cu at 500℃

(f ) 37 lbm Pb and 6.5 lbm Mg at 400℃

(g) 8.2 mol Ni and 4.3 mol Cu at 1250℃.

(h) 4.5 mol Sn and 0.45 mol Pb at 200℃

10.6 For an alloy of composition 74 wt% Zn–26 wt% Cu, cite the phases present

and their compositions at the following temperatures: 850℃, 750℃, 680℃, 600℃, and 500℃.

10.7 Determine the relative amounts (in terms of mass fractions) of the phases for

the alloys and temperatures given in Problem 10.5.

10.9 Determine the relative amounts (in

terms of volume fractions) of the phases for

the alloys and temperatures given in

Problem 10.5a, b, and c. Below are given the

approximate densities of the various metals

at the alloy temperatures:

10.18 Is it possible to have a copper–silver

alloy that, at equilibrium, consists of a _