JNTU B.Tech II PHYSICAL METALLURGY Previous Question Paper (March - 2022 - R18)

Alloying is the process of adding one or more elements to a base metal to modify its properties to suit specific engineering applications. The primary necessities include increasing mechanical strength, improving corrosion resistance, enhancing electrical or thermal conductivity, modifying magnetic properties, and improving castability or formability.

The Hume-Rothery rules define the conditions for extensive solid solubility of a solute in a solvent metal:

1. Atomic Size Factor: The atomic radii of the solute and solvent atoms must differ by no more than 15% to minimize lattice strain.
2. Crystal Structure: For complete solid solubility, both the solute and solvent must possess the same crystal structure (e.g., both FCC).
3. Electronegativity: The elements should have similar electronegativities. If the difference is large, the elements tend to form intermetallic compounds rather than solid solutions.
4. Valency: A metal with a lower valency is more likely to dissolve a metal with a higher valency than vice versa, known as the relative valency effect.

Solid solution strengthening is a method of increasing the yield strength of a metal by adding solute atoms to form a solid solution. This mechanism relies on the interaction between solute atoms and dislocations within the crystal lattice.

When solute atoms are introduced, they create local lattice distortions due to differences in atomic size or modulus. These distortions act as obstacles to dislocation motion:

• Elastic Interaction: Oversized or undersized solute atoms create stress fields that interact with the strain fields of dislocations, effectively pinning them in place.
• Modulus Interaction: Differences in the shear modulus between the solute and solvent atoms result in a change in the energy required to move a dislocation through the lattice.
• Electrical Interaction: In some alloys, differences in valency create electrical fields that interact with the charge density of dislocation cores.

By impeding the movement of dislocations, the material requires a higher applied stress to cause plastic deformation, thereby increasing the overall strength and hardness of the alloy.