Questions
8–10 questions in major PSU papers
Difficulty
Medium-Hard
Importance
Core — never skip
Overview
Physical Metallurgy explores the relationship between the structure of metals and their macroscopic properties through thermodynamic phases and microstructural evolution. It is a cornerstone topic for mechanical engineering aspirants as it forms the basis for material selection and failure analysis in industrial power plants and manufacturing sectors. Mastery requires a deep understanding of phase transitions and the kinetics of heat treatment processes.
Crystal Structures
Crystal structures define the atomic arrangement within a metallic lattice, directly influencing ductility, hardness, and mechanical deformation behavior. PSU exams frequently test the coordination number, atomic packing factor (APF), and number of atoms per unit cell for these systems.
- BCC: CN=8, APF=0.68, n=2
- FCC: CN=12, APF=0.74, n=4
- HCP: CN=12, APF=0.74, n=6
- Atomic radius relation: BCC r = a(sqrt(3)/4)
- Atomic radius relation: FCC r = a(sqrt(2)/4)
Iron-Carbon Phase Diagram
The Fe-Fe3C equilibrium diagram is the most critical conceptual tool for understanding steel and cast iron properties. Aspirants must know the specific invariant reactions and temperature-composition boundaries of various phases.
- Peritectic reaction: L + delta = gamma at 1493C
- Eutectic reaction: L = gamma + Fe3C at 1147C
- Eutectoid reaction: gamma = alpha + Fe3C at 727C
- Maximum carbon solubility in austenite is 2.14%
- Maximum carbon solubility in ferrite is 0.022%
Heat Treatment, TTT & CCT Diagrams
These diagrams represent the non-equilibrium transformation of austenite into various microstructures based on time and cooling rate. Understanding the nose of the TTT diagram is essential for predicting the final phases like martensite, bainite, or pearlite.
- Annealing: Slow cooling in furnace for maximum softness
- Normalizing: Air cooling for uniform grain structure
- Quenching: Rapid cooling to form Martensite
- Tempering: Reheating below eutectoid to increase toughness
- Martensite is a BCT (Body Centered Tetragonal) structure
Formula Sheet
APF = (n * Volume of atom) / (Volume of unit cell)
n_BCC = 2
n_FCC = 4
r_BCC = a*sqrt(3)/4
r_FCC = a*sqrt(2)/4
Exam Tip
Memorize the invariant reactions (Peritectic, Eutectic, Eutectoid) by their temperature and phase components; these are the highest yield points for quick marks.
Common Mistakes
- Confusing the temperature and carbon composition values of the three major invariant reactions.
- Assuming martensite forms at a specific temperature regardless of cooling rate, ignoring the Ms and Mf temperatures.
- Miscalculating APF by using incorrect atom counts per unit cell.
More Revision Notes
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