Impurity segregation at grain boundaries

1 A crystal of ferrous oxide, FeyO, is found to have a lattice parameter a = 0.43 nm, and a density of 5.72 g/cm. What is the value of y State all assumptions. 

2 For Feo.gsO, the lattice parameter a = 0.4352 nm and the density is 5.7 g/cm. Calculate the site fraction of iron vacancies and the number of iron vacancies per cubic centimeter. 

3 Indicate the following types of defects by Kroger-Vink notation  

4 Write two possible defect reactions that would lead to the formation of a metal-deficient compound. Cite an example of an oxide that you think would likely form each of the defect reactions you have chosen. 

5 Write down two possible defect reactions for the dissolution of CaO in Zr02. 

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Fig. 3 Impedence spectra at 340°C of two 3 mol% Y203-Zr02 (3YSZ) compositions with different S1O2 impurities (O 20 ppm Si02, D 800 ppm Si02) showing the effect of impurity segregation at grain boundaries on the grain boundary impedance (arc on the right hand side) (Badwal et al. 1997).

Explain the variation of solute segregation at grain boundaries with increasing temperature. Draw schematically and explain the variation in grain boundary velocity with temperature for pure, slightly impure and highly impure materials. 

Step cooling will not simulate embrittlement of 1 WCM Mb, though it occurs (e.g., a 100°F [38°C] increase in transition temperature was reported after 8 y at 930° F [500° C]). This is because the embrittlement in 11/iCr-1 Mo is caused by precipitation of carbides in the ferrite phase rather than segregation of impurities to the grain boundaries. Temper embrittlement can be reversed by heating at 1,150°F (620°C) for 2 h per inch of thickness. 

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