Questions
3–5 questions per paper
Difficulty
Medium
Importance
High yield; direct scoring potential.
Overview
Material Science covers the relationship between internal structure, processing, and properties of materials, which is vital for understanding mechanical design constraints. In PSU exams, this topic is highly factual, focusing on crystallographic constants, phase transformations, and thermal properties of engineering metals.
Crystal Structures
This section examines the arrangement of atoms in metallic solids, primarily focusing on unit cell calculations. Understanding coordination numbers and atomic packing factors is essential for solving numerical problems frequently asked in BHEL and NTPC papers.
- SC: APF=0.52, CN=6
- BCC: APF=0.68, CN=8
- FCC: APF=0.74, CN=12
- HCP: APF=0.74, CN=12
- Miller indices (hkl) for planes and directions
Iron-Carbon Phase Diagram
The Iron-Carbon diagram is the backbone of metallurgy, detailing the phase transformations that dictate the properties of steel and cast iron. Memorizing invariant reactions and temperature ranges is a must for identifying microstructural changes.
- Peritectic reaction: L + delta = Austenite at 1493°C
- Eutectic reaction: L = Austenite + Cementite at 1147°C
- Eutectoid reaction: Austenite = Ferrite + Cementite at 727°C
- Formation of Pearlite, Ledeburite, and Martensite
- Carbon solubility limit in Ferrite is 0.022% at 727°C
Heat Treatment
Heat treatment processes modify the mechanical properties of steel by altering its microstructure. Aspirants should prioritize the cooling rates and the resulting hardness levels produced by various processes.
- Annealing: Softening and ductility improvement
- Normalizing: Uniform grain structure refinement
- Quenching: Maximum hardness (Martensite formation)
- Tempering: Relief of internal stresses and brittleness
- Jominy End-Quench test for hardenability
Fatigue & Creep
Fatigue and creep explain material failure under cyclic loading and high-temperature stress over time, respectively. These concepts are frequently tested through conceptual questions regarding the S-N curve and endurance limit.
- Endurance limit is 0.5 * Ultimate Tensile Strength for steel
- S-N curve depicts Stress vs Number of cycles to failure
- Creep stages: Primary (transient), Secondary (steady-state), Tertiary
- Fatigue failure occurs at stresses lower than yield strength
Formula Sheet
APF = (Volume of atoms in unit cell) / (Total volume of unit cell)
n = (N_corner/8) + (N_face/2) + (N_body/1)
Endurance Limit Ratio: S_e / S_ut = 0.5
Exam Tip
Focus heavily on the Iron-Carbon phase diagram invariant points and their respective temperatures, as these appear in almost every mechanical engineering PSU paper.
Common Mistakes
- Confusing the specific temperatures of the Eutectic and Eutectoid points in the Fe-C diagram.
- Neglecting the difference between hardenability and hardness during heat treatment questions.
- Miscalculating the Atomic Packing Factor by confusing the number of atoms per unit cell for BCC and FCC structures.
More Revision Notes
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