Jawaharlal Nehru Technological University Hyderabad B.Tech II PHYSICAL METALLURGY Previous Question Paper (March - 2021 - R18)

A phase diagram is a graphical representation of the phases present in a material system as a function of temperature, pressure, and composition under equilibrium conditions. It maps the regions of stability for different phases within a multi-component system.

The following information can be obtained from a phase diagram:

• Phases present: Identification of the specific phases (e.g., solid solution, intermetallic compound) existing at a given temperature and composition.
• Phase composition: Determination of the chemical composition of each phase present using tie-lines.
• Phase fractions: Calculation of the relative amounts of each phase using the lever rule.
• Solubility limits: Determination of the maximum amount of solute that can dissolve in a solvent phase before a new phase forms.
• Transformation temperatures: Identification of critical temperatures such as liquidus, solidus, solvus, and eutectic or peritectic points.

Dispersion strengthening is a mechanism used to increase the yield strength of a metal by introducing a fine, uniform distribution of hard, non-coherent, and thermally stable second-phase particles (dispersoids) into the matrix. These particles act as obstacles to dislocation motion.

When a dislocation moves through the crystal lattice under an applied stress, it encounters these hard particles. Because the particles are non-coherent and do not shear easily, the dislocation must either bow around them (Orowan looping) or climb over them, both of which require higher stress. This effectively pins the dislocations and hinders plastic deformation.

A classic example is Thoriated Tungsten (W-ThO2). In this system, fine particles of thorium oxide (ThO2) are dispersed within the tungsten matrix. These particles remain stable at high temperatures, preventing grain growth and dislocation movement, which significantly enhances the creep resistance and mechanical strength of the tungsten, making it ideal for high-temperature applications like welding electrodes or filaments.