JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II Year METALLURGY AND MATERIALS SCIENCE Previous Question Paper (September - 2021 - R13)

Grain size significantly influences the mechanical and physical properties of metals. According to the Hall-Petch relationship, yield strength increases as grain size decreases because grain boundaries act as barriers to dislocation motion. A finer grain structure results in a higher density of grain boundaries, which effectively pins dislocations and requires higher stress for plastic deformation.

Key effects include:

• Strength and Hardness: Finer grains improve both yield strength and hardness.
• Ductility and Toughness: Fine-grained materials often exhibit better ductility and impact toughness compared to coarse-grained counterparts, as they distribute plastic deformation more uniformly.
• Creep Resistance: At high temperatures, coarse grains are generally preferred because grain boundary sliding, which contributes to creep, is minimized.
• Corrosion Resistance: Large grains are often more resistant to intergranular corrosion, as there is less total grain boundary area available for localized attack.

The formation of substitutional solid solutions is governed by the Hume-Rothery rules, which define the conditions under which a solute atom can replace a solvent atom in a crystal lattice without significantly disrupting the structure.

• Atomic Size Factor: The difference in atomic radii between the solute and solvent atoms must be less than 15%. If the difference exceeds this, the lattice strain becomes too high for extensive solid solubility.
• Crystal Structure: For complete solid solubility, both the solute and solvent must possess the same crystal structure (e.g., both FCC or both BCC).
• Electronegativity: The solute and solvent should have similar electronegativities. If the difference is large, the atoms tend to form intermetallic compounds rather than solid solutions.
• Valency Factor: A metal is more likely to dissolve a metal of higher valency than one of lower valency. This is known as the relative valency effect.

These rules provide a framework for predicting the extent of solubility in alloy systems, where adherence to these criteria promotes the formation of a stable, single-phase solid solution.