JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II MECHANICAL METALLURGY Previous Question Paper (November/December - 2020 - R16)

Crystal defects are irregularities or imperfections in the periodic arrangement of atoms within a crystalline solid. They are classified based on their dimensionality:

• Point defects (0D): Localized disruptions involving one or a few atoms, such as vacancies, self-interstitials, impurity atoms (substitutional or interstitial), Frenkel defects, and Schottky defects.
• Line defects (1D): Linear disruptions in the atomic lattice, primarily dislocations, including edge dislocations, screw dislocations, and mixed dislocations.
• Plane defects (2D): Interfaces or boundaries that separate regions of different crystallographic orientation or stacking sequences.
• Volume defects (3D): Large-scale imperfections such as pores, cracks, foreign inclusions, or precipitates.

Plane defects, also known as interfacial defects, act as boundaries between regions of the same crystal structure but different orientation or stacking. Key types include:

• External Surfaces: The surface of a crystal where the atomic bonds are incomplete, resulting in higher surface energy and increased chemical reactivity compared to the bulk.
• Grain Boundaries: Interfaces separating two small crystals (grains) having different crystallographic orientations. These are classified as low-angle or high-angle boundaries. They impede dislocation motion, thereby strengthening the material according to the Hall-Petch relationship.
• Twin Boundaries: A special type of grain boundary across which there is a specific mirror-image symmetry in the atomic arrangement. Twinning occurs due to mechanical deformation or annealing.
• Stacking Faults: Disruptions in the normal stacking sequence of close-packed planes (e.g., changing from ABCABC... to ABCABABC...). These are common in face-centered cubic (FCC) metals and significantly influence plastic deformation behavior.
• Phase Boundaries: Interfaces between two different phases or materials with distinct crystal structures, which are critical in determining the properties of multi-phase alloys.