Consider a diffusion couple setup between pure tungsten and a tungsten alloy containing 1 at% thorium. After several minutes of exposure at 2000 C, a transition zone of 0.01 cm thickness is established. What is the flux of thorium atoms at this time if diffusion is due to (a) volume diffusion, (b) grain boundary diffusion, and (c) surface diffusion? TABLE 5-2 I The effect of the type of diffusion for thorium in tungsten and for self-diffusion in silver* Diffusion Coefficient (D) Diffusion Type Thorium in Tungsten Silver in Silver Do cm?/s Q cal/mole Do cm2/s Q cal/mole 66,400 90,000 120,000 0.47 0.068 8,900 22,750 45,700 Surface Grain boundary 0.74 0.24 Volume 1.00 0.99

Problem 4 The preexponential and activation energy for the diffusion of iron in cobalt are 1.1 x 105 m²/s and 253,300 J/mol, respectively. At what temperature will the diffusion coefficient have a value of 2.1 × 10-¹4 m²/s?

A diffusion couple of two metals, A and B, was fashioned as shown: Diffusion of A atoms A-B alloy B Diffusion of B atoms After a 690 hour heat treatment at 1120°C the concentration of A is 1.8 wt% at the 3.2 mm position within Metal B. At what temperature (in Kelvin) must the diffusion couple need to be heated to produce this same concentration at a 2.3 mm position after 690 h? The preexponential and activation energy values for the diffusion of A in B are 1.9 x 104 m²/s and 239 kJ/mol, respectively.

[Decarburization] An FCC iron-carbon alloy initially containing carbon with the concentration, C0, is exposed to anoxygen-rich and virtually carbon-free atmosphere. Under the given conditions, the carbon diffuses from the alloy andreacts at the surface with the oxygen in the atmosphere, meaning that the carbon concentration at the surface position is maintained essentially at 0 wt% C. The subproblems are independent.(1) Determine the diffusion coefficient at the temperature T given below.  Use T = 1250 °C(2) When the value of D is given below, what position will the carbon concentration be Cx after a treatment for thetime, t? Use D = 1.93 × 10−10m2 /s; C0 = 0.35 wt%; Cx = 0.11 wt%; t = 12 hr