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13 Cards in this Set
- Front
- Back
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Do noncrystalline materials display the phenomenon of allotropy (polymorphism)? Why or why not? |
Noncrystalline materials do not display the phenomenon of allotropy; since a noncrystalline material does not have a defined crystal structure, it cannot have more than one crystal struture, which is the definition of allotropy. |
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Explain why lead has very limited solubility in solid tin (hardly any lead will dissolve in tin at room temperature) atomic radii are: Pb = 0.175NM and Sn = 0.151nm; both have electronegativity of 1.8. Pb: FCC; Sn: tetragonal The valencies are Pb: 2; Sn: 4 |
For Pb dissolving in Sn, the size factor is ((0.175-0.151)/0.151)(100) = 15.9%; which is greater than the limit of 15% for solubility. This criterion precludes solubility, despite the identical electronegativities. Also a metal will tend to dissolve a metal of higher valency, but Pb has a lower valency than Sn. |
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Explain why tin is moderately soluble in solid lead (approximately 3.5% of the atom in lead can be replaced by tin at room temperature). |
For Sn dissolving in Pb, the size factor is ((0.175-0.151)/0.175)(100) = 13.7%, which is below the limit of 15% for solubility. The identical electronegativities are conductive to solubility. Also, a metal will tend to dissolve a metal of higher valency, and this is indeed the case for Sn dissolving in Pb. |
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True or False: The Burgers vector of an edge dislocation is normal to the dislocation line and normal to the slip plane. |
False. The Burgers vector of an edge dislocation lies in (is parallel to) the slip plane. |
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True or False: The Burgers vector of a screw dislocation is normal to the dislocation line and parallel to the slip plane. |
False. The Burgers vector of a screw dislocation is parallel to the dislocation line. |
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True or False: When a screw dislocation enable slip in a single crystal, the dislocation line remains in the slip plane. |
The dislocation line of a screw dislocation lies in the slip plane. |
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True or False: BCC crystals do not contain dislocation. |
False. All crystals contain dislocations. |
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True or False: Fine- grained samples of gamma- iron fewer dislocations than coarse- rained samples of gamma iron. |
The grain size affects the mobility of dislocations, not the number of dislocations. |
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True or False: Given values of the diffusion coefficient for carbon in iron at 850C and 950C we can calculate the activation energy for diffusion of carbon in iron. |
False. Iron is a BCC at 850, and FCC at 950. So the given values of diffusion coefficient would pertain to diffusion of carbon through different structures. |
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True or False: Given values of (i) activation energy fro self- diffusion in copper and (ii) the activation energy for vacancy migration in copper, we can calculate the activation energy for vacancy formation in copper. |
True. The total activation energy for self diffusion is the sum of the activation energies for vacancy formation and vacancy migration. |
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True or False: Self- interstitials do not form in pure metals. |
False. Thermal energy leads to a finite concentration of self- interstitials in all metals regardless of purity. |
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True or False: Diffusion in crystalline solids can only occur if there are vacancies in the crystals |
False: (though the absence of vacancies is hypothetical). Vacancies are not needed for interstitial diffusion. |
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True or False: At room temperature, the stress needed to cause significant creep will be lower for pure copper than for pure aluminum. |
False. Pure Al has a lower melting temperature than pure Cu, and so the vacancy concentration (and capacity for self diffusion to occur) at room temperature will be higher in pure Al. Thus, the stress needed to cause significant creep at room temperature will be lower for pure Al. |
