Impurities in Solids. Crystal Electro- Element R% Structure negativity Valence

Transcription

1 4-4 Impurities in Solids 4.4 In this problem we are asked to cite which of the elements listed form with Ni the three possible solid solution types. For complete substitutional solubility the following criteria must be met: 1) the difference in atomic radii between Ni and the other element ( R%) must be less than ±15%, 2) the crystal structures must be the same, 3) the electronegativities must be similar, and 4) the valences should be the same, or nearly the same. Below are tabulated, for the various elements, these criteria. Crystal Electro- Element R% Structure negativity Valence Ni FCC 2+ C 43 H 63 O 52 Ag +16 FCC Al +15 FCC Co +0.6 HCP 0 2+ Cr +0.2 BCC Fe -0.4 BCC 0 2+ Pt +11 FCC Zn +7 HCP (a) Pt is the only element that meets all of the criteria and thus forms a substitutional solid solution having complete solubility. At elevated temperatures Co and Fe experience allotropic transformations to the FCC crystal structure, and thus display complete solid solubility at these temperatures. (b) Ag, Al, Co, Cr, Fe, and Zn form substitutional solid solutions of incomplete solubility. All these metals have either BCC or HCP crystal structures, and/or the difference between their atomic radii and that for Ni are greater than ±15%, and/or have a valence different than 2+. (c) C, H, and O form interstitial solid solutions. These elements have atomic radii that are significantly smaller than the atomic radius of Ni.

2 This problem asks that we determine, for a hypothetical alloy that is composed of 25 wt% of metal A and 75 wt% of metal B, whether the crystal structure is simple cubic, face-centered cubic, or body-centered cubic. We are given the densities of the these metals (ρ A 6.17 g/cm 3 and ρ B 8.00 g/cm 3 for B), their atomic weights (A A g/mol and A B g/mol), and that the unit cell edge length is nm (i.e., 3.32 x 10-8 cm). In order to solve this problem it is necessary to employ Equation 3.5; in this expression density and atomic weight will be averages for the alloy that is ρ ave na ave V C N A Inasmuch as for each of the possible crystal structures, the unit cell is cubic, then V C a 3, or ρ ave na ave a 3 N A And, in order to determine the crystal structure it is necessary to solve for n, the number of atoms per unit cell. For n 1, the crystal structure is simple cubic, whereas for n values of 2 and 4, the crystal structure will be either BCC or FCC, respectively. When we solve the above expression for n the result is as follows: n ρ ave a3 N A A ave Expressions for A ave and ρ ave are found in Equations 4.11a and 4.10a, respectively, which, when incorporated into the above expression yields n 100 C A + C a B 3 N A ρ A ρ B 100 C A + C B A A A B Substitution of the concentration values (i.e., C A 25 wt% and C B 75 wt%) as well as values for the other parameters given in the problem statement, into the above equation gives

3 4-26 n 100 ( nm) 3 ( atoms/mol) 25 wt% 6.17 g/cm wt% 8.00 g/cm wt% g/mol + 75 wt% g/mol 1.00 atom/unit cell Therefore, on the basis of this value, the crystal structure is simple cubic.

4 This problems asks that we compute the unit cell edge length for a 90 wt% Fe-10 wt% V alloy. First of all, the atomic radii for Fe and V (using the table inside the front cover) are and nm, respectively. Also, using Equation 3.5 it is possible to compute the unit cell volume, and inasmuch as the unit cell is cubic, the unit cell edge length is just the cube root of the volume. However, it is first necessary to calculate the density and average atomic weight of this alloy using Equations 4.10a and 4.11a. Inasmuch as the densities of iron and vanadium are 7.87g/cm 3 and 6.10 g/cm 3, respectively, (as taken from inside the front cover), the average density is just ρ ave C V ρ V C Fe ρ Fe wt% 6.10 g /cm wt% 7.87 g /cm g/cm 3 And for the average atomic weight A ave 100 C V + C Fe A V A Fe wt% g /mole + 90 wt% g /mol g/mol Now, V C is determined from Equation 3.5 as V C na ave ρ ave N A (2 atoms /unit cell)(55.32 g /mol) (7.65 g /cm 3 )( atoms /mol)

5 x cm 3 /unit cell And, finally a (V C ) 1/3 ( cm 3 /unit cell) 1/ x 10-8 cm nm

6 The surface energy for a crystallographic plane will depend on its packing density [i.e., the planar density (Section 3.11)] that is, the higher the packing density, the greater the number of nearest-neighbor atoms, and the more atomic bonds in that plane that are satisfied, and, consequently, the lower the surface energy. From 3 the solution to Problem 3.54, the planar densities for BCC (100) and (110) are 16R 2 and 3 8R 2 2, respectively that is and. Thus, since the planar density for (110) is greater, it will have the lower surface energy. R2 R2

7 This problem calls for the mass of hydrogen, per hour, that diffuses through a Pd sheet. It first becomes necessary to employ both Equations 5.1a and 5.3. Combining these expressions and solving for the mass yields M JAt DAt C x ( m 2 /s)(0.25 m kg /m3 )(3600 s/h) m 4.1 x 10-3 kg/h

8 For this problem we are given D 0 (1.1 x 10-4 ) and Q d (272,000 J/mol) for the diffusion of Cr in Ni, and asked to compute the temperature at which D 1.2 x m 2 /s. Solving for T from Equation 5.9a yields T Q d R(ln D 0 ln D) 272,000 J/mol [ ] (8.31 J/mol - K) ln ( m 2 /s) - ln ( m 2 /s) 1427 K 1154 C Note: this problem may also be solved using the Diffusion module in the VMSE software. Open the Diffusion module, click on the D vs 1/T Plot submodule, and then do the following: 1. In the left-hand window that appears, click on the Custom1 box. 2. In the column on the right-hand side of this window enter the data for this problem. In the window under D0 enter preexponential value viz. 1.1e-4. Next just below the Qd window enter the activation energy value viz It is next necessary to specify a temperature range over which the data is to be plotted. The temperature at which D has the stipulated value is probably between 1000ºC and 1500ºC, so enter 1000 in the T Min box that is beside C ; and similarly for the maximum temperature enter 1500 in the box below T Max. 3. Next, at the bottom of this window, click the Add Curve button. 4. A log D versus 1/T plot then appears, with a line for the temperature dependence of the diffusion coefficient for Cr in Ni. At the top of this curve is a diamond-shaped cursor. Click-and-drag this cursor down the line to the point at which the entry under the Diff Coeff (D): label reads 1.2 x m 2 /s. The temperature at which the diffusion coefficient has this value is given under the label Temperature (T):. For this problem, the value is 1430 K.

9 It is possible for two screw dislocations of opposite sign to annihilate one another if their dislocation lines are parallel. This is demonstrated in the figure below.

10 This problem asks that we compute the magnitudes of the Burgers vectors for copper and iron. For Cu, which has an FCC crystal structure, R nm (Table 3.1) and a 2 R nm (Equation 3.1); also, from Equation 7.1a, the Burgers vector for FCC metals is b a Therefore, the values for u, v, and w in Equation 7.10 are 1, 1, and 0, respectively. Hence, the magnitude of the Burgers vector for Cu is b a 2 u 2 + v 2 + w nm 2 (1 ) 2 + (1 ) 2 + (0 ) nm For Fe which has a BCC crystal structure, R nm (Table 3.1) and a 4 R nm (Equation 3.3); also, from Equation 7.1b, the Burgers vector for BCC metals is b a Therefore, the values for u, v, and w in Equation 7.10 are 1, 1, and 1, respectively. Hence, the magnitude of the Burgers vector for Fe is b nm 2 (1) 2 + (1) 2 + (1) nm

11 We are asked to compute the critical resolved shear stress for Zn. As stipulated in the problem, φ 65, while possible values for λ are 30, 48, and 78. (a) Slip will occur along that direction for which (cos φ cos λ) is a maximum, or, in this case, for the largest cos λ. Cosines for the possible λ values are given below. cos(30 ) 0.87 cos(48 ) 0.67 cos(78 ) 0.21 Thus, the slip direction is at an angle of 30 with the tensile axis. (b) From Equation 7.4, the critical resolved shear stress is just τ crss σ y (cos φ cos λ) max (2.5 MPa) [ cos(65 ) cos(30 ) ] 0.90 MPa (130 psi)