CHEE 317 - QUIZ 1 SOLUTIONS

ASSERTION: Grain boundaries are the sites of atomic mismatch. REASON: Grain boundaries have higher atomic density.

Assertion is correct statement and reason is wrong statement.

What is not related with Nitinol?

Free of surface defects

Which point defect in ionic crystals does not alter the density of the relevant solid?

Frenkel

Critical size of the nucleus during solidification of metals is expected to affect what properties?

Grain size, surface reactivity, number of grain boundaries

What is the coordination number?

Number of anions that surrounds the central cation

What is the Schottky defect in ceramic materials?

Pair of nearby cation and anion vacancies

If temperature increases, how do hardness and ductility of a material change?

Decreases, increases

How is the Schottky defect observed in crystals?

Equal number of cation and anion is missing from the lattice

Distinguish between homogeneous and heterogeneous nucleation for the solidification of a pure metal.

Homogeneous: First and simplest case. Metal itself will provide atoms to form nuclei. Metal, when significantly undercooled, has several slow moving atoms which bond with each other to form nuclei. Cluster of atoms below critical size is called embryo. If the cluster of atoms reach critical size, they grow into crystals If not, they are dissolved. Cluster of atoms that are greater than critical size are called Nucleus. Heterogeneous: Nucleation occurs in a liquid on the surfaces of structural materials. These structures, called nucleating agents, lower the free energy required to form a stable nucleus. Nucleating agents also lower the critical size. Smaller amount of undercooling is required to solidify. Used excessively in industries.

Why is hydrogen bonding only possible with hydrogen?

Hydrogen's nucleus is electron deficient when it bonds with an electronegative atom

What is solid solution strengthening? Describe two types. What are two main factors that affect solid solution strengthening?

In solid solutions, addition of one or more impurities to the host metal can increase the strength of metals. Substitutional solid strengthening is when solute atoms substitute for a parent solvent atom in a crystal lattice. Interstitial solid solutions are when solute atoms fit in between the voids of solvent atoms. Two factors that affect SSS are the size of the solute atoms and the amount of solute atoms.

At very low undercooling, would you expect the critical radius of the nuclei to be larger or smaller than the same nuclei at larger undercooling?

Smaller

What defect takes place due to the imperfect packing of atoms during crystallization?

Surface defect