Title: Summary Part Structure

Choose the correct statement.

- Many materials used by engineers are crystalline in nature meaning their atomic-scale structure can be described as regular and repeating, despite the presence of defects. (correct)
- Many materials used by engineers are crystalline in nature. These materials have no atomic-scale structure.
- Many materials used by engineers are crystalline in nature meaning their atomic-scale structure can be described as regular and repeating with no defects present.

- Typical metals are polycrystalline. Polycrystalline metals consist of many crystals of the same crystal structure, called grains, in random orientations. (correct)
- Typical engineering metals are polycrystalline. Polycrystalline metals combine many crystals which are of different crystal structure.

- Crystalline materials most often contain various defects and imperfections, such as dislocations, vacancies, impurities, inclusions, and grain boundaries. (correct)
- Crystalline materials typically feature a nearly perfect crystal lattice.

- Chemically more complex compounds such as ceramics exhibit a wide variety of crystalline structures that are typically more complex than the crystal structure of metals. (correct)
- Similar to metals, most ceramics feature simple crystallographic structures: fcc, bcc and hcp lattices are most commonly observed.

- Glasses are amorphous materials: they feature short-range order, but long-range order – that is, crystallinity – is lost in the glass. (correct)
- Glasses are amorphous materials: the arrangments of atoms in a glass is completely random.

- Polymers are typically made up of long chains which as a group can be amorphous, semi-crystalline, or fully crystalline. (correct)
- Polymers cannot be crystalline as their long chains inhibit them from forming regular repeating patterns.

- Thermoplastics are polymers which become deformable or "melt" at elevated temperatures while thermosets do not. (correct)
- Thermosets are polymers which become deformable or "melt" at elevated temperatures while thermoplastics do not.

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