8.15 From the data in Table 8.2, calculate the diffusion coefficient of carbon in ferrite ( ) and austenite ( ) at 900°C. Explain the difference in the values on the basis of the two crystal structures.

 

8.14 Make a plot of the activation energy Q for diffusion of different species as a function of the melting point of the species. Comment on your result.

 

8.16 In a diffusion anneal, to what level should the initial temperature of 900°C be increased to double the depth of penetration? D0 = 0.4 m2 s–1 and Q = 100 kJ mol–l.

 

8.17 An amount Q of a dopant is deposited on the surface of a silicon substrate. During a subsequent anneal without the dopant in the atmosphere, the concentration c of the dopant as a function of depth x and time t is given by c = (Q/ 􀀀DT ) exp [–x2/(4Dt)] Show that this is a solution of Fick’s Second Law, when D is independent of concentration.

 

8.18 A steel containing 0.002% N is to be nitrided to yield a nitrogen content of 0.12% at depth of 4 mm from the surface. The nitriding atmosphere is equivalent to a surface concentration of 0.35% N. How long is to be the nitriding process? The steel is BCC ( ) at the nitriding temperature of 700°C.

 

From the Fe–Fe3C phase diagram, for a 0.2% C steel, name the phases and their fractions at equilibrium at the following temperatures: (i) just above 1493°C, (ii) just below 1493°C, (iii) just above 725°C, and (iv) just below 725°C.

 

What is the fraction of proeutectoid cementite in (i) 1.4% C, (ii) 1.0% C, and (iii) 0.7% C steels?

 

One solid phase on heating through an invariant temperature becomes two solid phases. Name the invariant reaction. Sketch the phase boundaries near the invariant line.

 

In the Pb–Sn system, calculate the alloy composition at which the fraction of total 􀀀 is 2½ times the fraction of the phase at 182°C.

 

For soldering with Pb–Sn alloys, at least 85% of the eutectic mixture is preferred in the microstructure. Determine the composition limits of tin that will satisfy this condition.

 

Calculate the fraction of proeutectoid ferrite, eutectoid ferrite and total ferrite in a 0.2% C steel.