JNTU B.Tech II Year METALLURGY AND MATERIAL SCIENCE Previous Question Paper (September - 2021 - R16)

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 provides a higher density of grain boundaries, which effectively impedes dislocation movement, thereby increasing both the yield strength and the hardness of the material.

Furthermore, fine-grained metals exhibit superior toughness and ductility, as smaller grains allow for more uniform plastic deformation and reduce the likelihood of stress concentrations that lead to premature fracture. At elevated temperatures, however, coarse-grained materials are often preferred because they exhibit lower creep rates; this is due to the reduction in grain boundary sliding and diffusion-assisted deformation mechanisms that occur more readily in fine-grained structures.

The formation of substitutional solid solutions is governed by the Hume-Rothery rules, which dictate the conditions under which a solute atom can replace a solvent atom in a crystal lattice without forming a new phase:

• Atomic Size Factor: The difference in atomic radii between the solute and solvent atoms must be less than 15%. If the difference exceeds this limit, the lattice strain becomes too high, limiting 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 electronegativity values. A large difference promotes the formation of intermetallic compounds rather than a solid solution.
• Valency: A metal is more likely to dissolve a metal of higher valency than one of lower valency. This rule reflects the tendency of the solvent to maintain electronic stability during the substitution process.