Engineering Materials Multiple Choice Questions (MCQs)

✅ Correct: Finer grains strengthen the material due to grain boundary strengthening (Hall–Petch effect). However, more grain boundaries also make it more prone to corrosion attack.

✅ Correct: ASTM grain size number is related to grain count by \( N = 2^{n-1} \). This means each increase in grain size number doubles the number of grains per unit area.

✅ Correct: Grain boundaries are regions of atomic disorder and higher energy. This makes them more chemically reactive and prone to corrosion or etching.

✅ Correct: Recrystallization annealing removes strain hardening and forms new fine grains. It restores ductility and improves toughness in worked metals.

✅ Correct: Twinning is a shear deformation where part of the crystal reorients symmetrically. It differs from slip, as it involves a coordinated movement of atoms rather than dislocation glide.

✅ Correct: In screw dislocations, the Burgers vector is parallel to the dislocation line. This distinguishes it from edge dislocations, where the Burgers vector is perpendicular.

✅ Correct: For screw dislocations, the Burgers vector lies parallel to the dislocation line. This parallelism defines the screw nature of the distortion in the lattice.

✅ Correct: A Schottky defect is a vacancy defect where equal numbers of cations and anions are missing. It maintains electrical neutrality but reduces the density of the crystal.

✅ Correct: A twin boundary separates two regions that are mirror images of each other. Twinning often occurs during deformation or annealing and influences mechanical properties.

✅ Correct: Anisotropic materials have properties that vary with direction, such as crystals or composites. In contrast, isotropic materials have uniform properties in all directions.

✅ Correct: At the eutectic point, three phases coexist in a binary system. By Gibbs phase rule, F = C − P + 1 = 2 − 3 + 1 = 0, so there are no degrees of freedom.

✅ Correct: According to Hume-Rothery rules, atomic size difference is the most critical factor. For complete solubility, the size difference should be less than about 15%.

✅ Correct: In a peritectic reaction, a liquid and a solid phase combine on cooling to form a new solid. This is different from eutectic or eutectoid reactions where two phases form simultaneously.

✅ Correct: The Fe–C diagram is based on equilibrium cooling, while TTT curves are obtained under non-equilibrium conditions. TTT diagrams capture time-dependent transformations like martensite formation.

✅ Correct: In a monotectic reaction, one liquid transforms into a solid and another liquid of different composition. This occurs at a specific temperature and composition in certain binary systems.

✅ Correct: Spheroidising heat treatment produces globular (spherical) cementite in steel. This structure improves machinability and ductility by reducing hardness compared to lamellar pearlite.

✅ Correct: Pearlite is a eutectoid mixture of ferrite and cementite formed at 0.8% carbon and 723°C. It results from the eutectoid transformation: austenite → ferrite + cementite.

✅ Correct: Induction hardening is used for crankshafts. It rapidly heats the surface by electromagnetic induction and then quenches it, producing a hard wear‑resistant surface while keeping the core tough.

✅ Correct: In carburizing, carbon is added to the surface in the form of free carbon (from solid, liquid, or gaseous carbonaceous materials). This diffuses into the steel surface, creating a high‑carbon case for wear resistance.

✅ Correct: The correct matching ensures that each heat treatment process is paired with its intended property improvement. For example, annealing improves ductility, normalizing refines grain, hardening increases hardness, and tempering improves toughness. (Option 1 represents the correct mapping).

✅ Correct: Quenched martensite is very hard but brittle. Tempering relieves internal stresses and improves ductility and toughness while slightly reducing hardness. This makes the steel more useful in engineering applications.

✅ Correct: Annealed and hot‑rolled mild steel shows a distinct yield point on its stress–strain curve. Cast iron and brass do not show a clear yield point, while cold‑rolled steel often shows strain hardening instead.

✅ Correct: If the martensite finish temperature (Mf) is below room temperature, some austenite does not transform and remains as retained austenite. This is common in high‑alloy steels where Mf is depressed.

✅ Correct: Molybdenum increases resistance to creep and fatigue, thereby improving endurance strength. Nickel improves toughness, vanadium refines grain, and tungsten improves hot hardness.

✅ Correct: Spheroidal graphite (SG) cast iron, also called ductile iron, combines castability with high strength and toughness. This makes it ideal for pipe fittings like elbows and tees, which must withstand pressure and impact.

✅ Correct: Hydrogen in steel leads to hydrogen embrittlement, making the steel brittle and prone to cracking. This is a serious issue in welding and high‑strength steels.

✅ Correct: High speed steels contain alloying elements in different proportions. The order of increasing proportion is tungsten → titanium → vanadium. Tungsten provides hot hardness, titanium improves wear resistance, and vanadium refines grain structure.

✅ Correct: Adding small amounts of sulphur and phosphorus improves machinability. They form inclusions that act as chip breakers, reducing tool wear and making machining easier. However, excess amounts reduce toughness and ductility.

✅ Correct: Vanadium forms stable carbides and oxides, which improve resistance to oxidation at elevated temperatures. It also refines grain size, but its key role is enhancing high‑temperature strength and oxidation resistance.

✅ Correct: Aluminium is used to improve steel’s strength at elevated temperatures. It forms stable oxides and nitrides, which refine grain size and enhance creep resistance. Tungsten improves hot hardness, but aluminium specifically boosts elevated‑temperature strength.

✅ Correct: Austempering is a heat treatment where steel is quenched to a temperature above martensite start (Ms) but below pearlite range, and held until transformation is complete. This produces a 100% bainitic structure, which has high toughness and strength with less distortion compared to martensite.

✅ Correct: Recrystallization occurs when cold‑worked metals are heated to a suitable temperature. New strain‑free grains form, refining the structure and restoring ductility lost during cold working.

✅ Correct: Widmanstätten structure is a coarse, plate‑like microstructure formed during annealing or slow cooling of certain steels and alloys. It appears as needle‑like ferrite or cementite plates within austenite, often undesirable due to brittleness.

✅ Correct: Nickel is an austenite stabilizer. It lowers the critical temperature and expands the austenite phase field, improving toughness and corrosion resistance in steels. Chromium and molybdenum, by contrast, are ferrite stabilizers.

✅ Correct: Chromium is a ferrite stabilizer. It raises the transformation temperature and promotes ferrite formation, improving corrosion resistance and hardness. Nickel and manganese, on the other hand, are austenite stabilizers.

✅ Correct: Cobalt increases the red hardness of high speed steels. This means the tool retains its hardness and cutting ability even at elevated temperatures, making it ideal for high‑speed machining.

✅ Correct: Brittle materials fail easily in tension because microscopic cracks and flaws open up under tensile stress. In compression, these cracks tend to close, so compressive strength is much higher than tensile strength.

✅ Correct: A typical creep curve has three stages: - Transient (primary): decreasing creep rate due to strain hardening. - Steady state (secondary): constant creep rate, longest stage. - Accelerated (tertiary): rapidly increasing creep rate leading to failure. Hence, the order is transient → steady state → accelerated.

✅ Correct: Resilience is the ability of a material to absorb energy within the elastic limit and release it upon unloading. It is especially important under shock loading, where sudden energy absorption is required without permanent deformation.

✅ Correct: Adding a small amount of magnesium to molten gray cast iron modifies the graphite flakes into spheroidal (nodular) graphite. This improves ductility, toughness, and fatigue resistance compared to ordinary gray cast iron.

✅ Correct: Adding a small amount of sulphur to copper improves its machinability. Sulphur forms sulfide inclusions that act as chip breakers, making machining easier.

✅ Correct: Phosphor bronze is an alloy of copper, tin, and about 1% phosphorus. Phosphorus increases wear resistance, stiffness, and fatigue strength, making it useful for springs and bearings.