Intermediate Materials & Metallurgical interview questions

150 questions with answers (MME-151 to MME-300). Try each one aloud before reading the answer.

Crystallography & Defects

MME-151 · What are dislocations and why do they matter?

Dislocations are line defects in the crystal lattice. Their glide under shear stress is the main mechanism of plastic deformation, which is why metals yield at stresses far below the theoretical shear strength.

MME-152 · How do you derive the atomic packing factor of BCC?

Atoms touch along the body diagonal, so a = 4r/√3. With 2 atoms per cell, APF = 2·(4/3)πr³ / a³ = √3π/8 ≈ 0.68.

MME-153 · How do you calculate theoretical density? Give the example of copper.

ρ = nA/(Vc·NA). For FCC Cu, n = 4, A = 63.55 g/mol and a = 0.3615 nm: ρ = (4 × 63.55)/(6.022×10²³ × (3.615×10⁻⁸ cm)³) ≈ 8.94 g/cm³.

MME-154 · What is the Burgers vector in FCC metals?

The shortest lattice translation along the close-packed direction, b = (a/2)⟨110⟩, with magnitude a/√2. For Cu, |b| ≈ 0.256 nm.

MME-155 · How does vacancy concentration depend on temperature?

Nv = N·exp(−Qv/kT), where N is the number of atomic sites, Qv the vacancy formation energy and k Boltzmann's constant. Near the melting point the vacancy fraction reaches roughly 10⁻⁴ to 10⁻³.

MME-156 · Why are FCC metals generally more ductile than BCC and HCP metals?

FCC has 12 slip systems on close-packed {111} planes with low Peierls stress, available at all temperatures. HCP usually has only 3 easy basal systems, and BCC slip needs thermal activation, so it becomes difficult at low temperature.

MME-157 · Why does γ-Fe (FCC) dissolve far more carbon than α-Fe (BCC)?

Although FCC packs more densely, its octahedral interstice (radius ≈ 0.414r) is much larger than BCC's (octahedral ≈ 0.155r, tetrahedral ≈ 0.291r). Maximum solubility is 2.14% C in austenite versus 0.022% C in ferrite.

MME-158 · What is Schmid's law?

The resolved shear stress on a slip system is τR = σ·cosφ·cosλ, where φ is the angle between the load and the slip-plane normal and λ the angle between the load and the slip direction. Slip begins when τR reaches the critical resolved shear stress.

MME-159 · How do you calculate interplanar spacing in a cubic crystal?

d(hkl) = a/√(h² + k² + l²). For example, (110) in BCC Fe with a = 0.2866 nm gives d = 0.2866/√2 ≈ 0.2027 nm.

MME-160 · What is stacking fault energy and why does it matter?

The energy per unit area of a fault in the stacking sequence of close-packed planes. Low SFE (brass, austenitic stainless steel) gives wide partial dislocations, little cross-slip, high work hardening and planar slip. High SFE (Al) allows easy cross-slip and recovery.

MME-161 · What is the difference between low-angle and high-angle grain boundaries?

Low-angle boundaries (misorientation below about 10–15°) are arrays of dislocations such as tilt or twist walls. High-angle boundaries have disordered structures, higher energy, and act as stronger barriers and faster diffusion paths.

MME-162 · What is the difference between dislocation glide and climb?

Glide is conservative motion on the slip plane driven by shear stress. Climb moves an edge dislocation out of its slip plane by absorbing or emitting vacancies, so it needs diffusion and becomes important at high temperature, as in creep.

MME-163 · What is the planar density of the FCC (111) plane?

The (111) plane contains the equivalent of 2 atoms in an area of (√3/2)a². Planar density is 4/(√3·a²), the highest of any FCC plane, which is why it is the slip plane.

Phase Diagrams & Phase Transformations

MME-164 · What are the key phases and points in the Fe–Fe₃C diagram?

Phases are ferrite (α, BCC), austenite (γ, FCC), δ-ferrite, cementite (Fe₃C, 6.67% C) and liquid. Microconstituents are pearlite and ledeburite. The eutectoid is at 0.76% C and 727 °C, and the eutectic at 4.3% C and 1147 °C.

MME-165 · What is a TTT diagram?

A time–temperature–transformation (isothermal) diagram showing the start and finish times of austenite transformation to pearlite, bainite or martensite at constant temperature. Its C-shaped curves have a "nose" at the fastest transformation.

MME-166 · What are the phase fractions in a 0.40% C steel just below 727 °C?

Proeutectoid ferrite = (0.76 − 0.40)/(0.76 − 0.022) ≈ 0.49, so about 49% ferrite and 51% pearlite by mass.

MME-167 · How does a CCT diagram differ from a TTT diagram?

A CCT (continuous cooling transformation) diagram shows transformations during continuous cooling at various rates, which matches real heat treatment. Its curves are shifted to longer times and lower temperatures compared with the TTT diagram.

MME-168 · What is the critical nucleus radius in homogeneous nucleation?

r* = −2γ/ΔGv, where γ is the interfacial energy and ΔGv the volume free-energy change (negative). Larger undercooling makes ΔGv more negative, shrinking r* and the nucleation barrier ΔG* = 16πγ³/(3ΔGv²).

MME-169 · What is the difference between diffusional and diffusionless transformations?

Diffusional transformations (pearlite, proeutectoid ferrite) need long-range atom movement and depend on time and temperature. Diffusionless (martensitic) transformations happen by coordinated shear of atoms at near sound speed, with no change in composition.

MME-170 · What is the difference between upper and lower bainite?

Upper bainite forms at about 400–550 °C and has cementite between ferrite laths, so it is less tough. Lower bainite forms at about 250–400 °C and has fine carbides inside the ferrite plates, giving better strength and toughness.

MME-171 · What are Ms and Mf, and how does carbon affect them?

Ms is the temperature where martensite starts to form on cooling and Mf where it is essentially complete. Increasing carbon and most alloying elements lower both. Above about 0.5–0.6% C, Mf falls below room temperature, leaving retained austenite.

MME-172 · What is retained austenite and why does it matter?

Austenite that does not transform to martensite after quenching because Mf is below the quench temperature. It lowers hardness and can transform later in service, causing dimensional instability and cracking.

MME-173 · What are Fick's laws, and how does diffusivity vary with temperature?

First law: J = −D(dC/dx). Second law: ∂C/∂t = D(∂²C/∂x²). Diffusivity follows an Arrhenius relation, D = D₀·exp(−Q/RT).

MME-174 · What is coring in non-equilibrium solidification?

When an alloy cools too fast for diffusion in the solid, each dendrite has a composition gradient from center to edge. Coring lowers the effective solidus and can cause incipient melting. Homogenization annealing removes it.

MME-175 · How does spinodal decomposition differ from nucleation and growth?

Spinodal decomposition occurs inside the spinodal region, where ∂²G/∂c² < 0. Small composition fluctuations grow continuously by uphill diffusion with no nucleation barrier, giving a fine interconnected structure. Nucleation and growth need a critical nucleus.

MME-176 · What are the characteristics of the Pb–Sn eutectic?

The eutectic is at 61.9 wt% Sn and 183 °C. It freezes at a single temperature into a lamellar mixture of Pb-rich α and Sn-rich β, which is why 63/37 solder has no pasty range.

Heat Treatment

MME-177 · Why is steel quenched and then tempered?

Quenching forms martensite, which is very hard but brittle and full of residual stress. Tempering precipitates carbides and relieves stresses, restoring toughness and ductility at some cost in hardness.

MME-178 · What is the Jominy end-quench test?

A standard 25 mm diameter bar is austenitized and water-sprayed on one end only. Hardness is then measured along its length. The hardness–distance curve characterizes hardenability.

MME-179 · How do alloying elements improve hardenability?

Elements such as Mn, Cr, Mo, Ni and a small amount of boron (≈ 0.001–0.003%) slow ferrite and pearlite formation and shift the TTT/CCT curves to longer times. This lets martensite form at slower cooling rates and in thicker sections.

MME-180 · What is the difference between martempering and austempering?

Martempering quenches into a bath just above Ms, equalizes the temperature, then air-cools to form martensite with less distortion. It still needs tempering. Austempering holds above Ms until bainite forms fully, giving toughness without a separate temper.

MME-181 · What are temper embrittlement and tempered martensite embrittlement?

Temper embrittlement comes from slow cooling through or holding at about 375–575 °C, where P, Sb, Sn and As segregate to prior austenite grain boundaries. Mo reduces it. Tempered martensite embrittlement occurs on tempering at about 250–400 °C from carbide films on boundaries.

MME-182 · What causes quench cracking and how is it prevented?

Thermal and transformation stresses, made worse by high carbon, sharp corners, abrupt section changes and severe quenchants. Prevent it with generous radii, uniform sections, milder quenchants, martempering and prompt tempering.

MME-183 · How does carburizing case depth vary with time?

Case depth is roughly proportional to the square root of time, x ≈ k√t, where k depends strongly on temperature through diffusivity. Doubling the case depth needs about four times the time.

MME-184 · What is decarburization and how is it avoided?

Loss of surface carbon when steel is heated in an oxidizing atmosphere, leaving a soft surface with poor fatigue strength. It is avoided with controlled-carbon-potential atmospheres, vacuum or salt baths, protective coatings, or by machining off the affected layer.

MME-185 · What is Grossmann's ideal critical diameter (DI)?

The diameter of a bar that would harden to 50% martensite at its center under an ideal quench of infinite severity. It is calculated from carbon content and grain size, multiplied by factors for each alloying element.

MME-186 · Why are hypereutectoid steels hardened from just above A₁ rather than above Acm?

Hardening from above A₁ keeps some undissolved cementite for wear resistance and limits the carbon in austenite. That reduces retained austenite, grain growth and cracking risk.

MME-187 · What is the difference between carbonitriding and ferritic nitrocarburizing?

Carbonitriding adds C and N in the austenite range (about 800–870 °C) and then quenches, giving a shallow hard case. Ferritic nitrocarburizing adds N and C at about 570 °C, below A₁, forming a thin compound layer with little distortion.

MME-188 · What are the advantages of vacuum heat treatment?

No oxidation or decarburization, bright clean surfaces, precise control, and high-pressure gas quenching that reduces distortion. It is widely used for tool steels and aerospace parts.

MME-189 · What is sub-zero (cryogenic) treatment?

Cooling hardened steel to about −80 °C or lower right after quenching, to transform retained austenite to martensite. It improves dimensional stability and hardness, and must be followed by tempering.

Mechanical Behavior & Testing

MME-190 · How are true stress and true strain related to engineering values?

Before necking, σT = σ(1 + ε) and εT = ln(1 + ε). True stress uses the instantaneous area, so it keeps rising after the UTS.

MME-191 · What is the strain-hardening exponent n?

In the Hollomon relation σT = K·εTⁿ, n measures work-hardening capacity. Necking starts when true strain equals n. Typical n is about 0.2–0.25 for low-carbon steel and about 0.4–0.5 for austenitic stainless steel.

MME-192 · What causes the yield point phenomenon in mild steel?

Carbon and nitrogen atoms form Cottrell atmospheres that pin dislocations. A higher upper yield stress is needed to break free, followed by a lower yield plateau as Lüders bands spread along the gauge length.

MME-193 · What is the stress intensity factor K?

K = Y·σ·√(πa), where σ is the remote stress, a the crack length and Y a geometry factor. Fracture occurs when K reaches the material's fracture toughness, KIC, in MPa√m.

MME-194 · How do ductile and brittle fracture surfaces differ?

Ductile fracture is fibrous with dimples from microvoid coalescence, often a cup-and-cone shape, and visible necking. Brittle fracture is flat and shiny, with cleavage facets or river patterns (transgranular) or grain facets (intergranular), and little deformation.

MME-195 · What is an S–N curve and an endurance limit?

An S–N curve plots stress amplitude against cycles to failure. Steels and titanium show an endurance limit, a stress below which life is effectively infinite, roughly 0.5 × UTS for steels up to about 1400 MPa UTS.

MME-196 · What are the three stages of creep?

Primary creep has a decreasing strain rate as work hardening dominates. Secondary (steady-state) creep has a constant minimum rate where hardening balances recovery. Tertiary creep accelerates because of necking, voids and microstructural damage, leading to rupture.

MME-197 · What is the Bauschinger effect?

After plastic deformation in one direction, a metal yields at a lower stress when loaded in the reverse direction. It is caused by back stresses from dislocation pile-ups and matters in cyclic loading and pipe forming.

MME-198 · How is tensile strength estimated from Brinell hardness for steel?

UTS (MPa) ≈ 3.45 × HB for many carbon and low-alloy steels. It is a useful approximation but not a substitute for a tensile test.

MME-199 · What is the Griffith criterion?

For a brittle solid, the fracture stress is σf = √(2Eγs/(πa)), where E is Young's modulus, γs the surface energy and a the half crack length. Strength falls with the square root of flaw size.

MME-200 · What is the Larson–Miller parameter?

LMP = T(C + log tr), where T is in kelvin, tr is rupture time in hours and C ≈ 20. It lets short, high-temperature creep tests be extrapolated to long service times at lower temperature.

MME-201 · What is the stress concentration factor for an elliptical hole?

Kt = σmax/σnom = 1 + 2(a/b) = 1 + 2√(a/ρ), where a is the half-length normal to the load, b the other semi-axis and ρ the tip radius. A circular hole gives Kt = 3.

MME-202 · How do Rockwell C and Rockwell B scales differ?

HRC uses a diamond cone with a 150 kgf total load, for hardened steel. HRB uses a 1/16 in steel ball with a 100 kgf load, for softer metals such as annealed steel, brass and aluminium.

Iron & Steelmaking

MME-203 · What are the main reduction reactions in a blast furnace?

In the upper stack, CO reduces iron oxides stepwise: 3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂, then Fe₃O₄ → FeO → Fe. Lower down, the Boudouard reaction C + CO₂ → 2CO regenerates CO, and direct reduction FeO + C → Fe + CO occurs at high temperature.

MME-204 · What conditions favour desulfurization of steel or hot metal?

A basic slag (high CaO/SiO₂), a strongly reducing (low-oxygen) environment, high temperature and good slag–metal mixing. The reaction is [S] + (CaO) → (CaS) + [O].

MME-205 · What conditions favour dephosphorization?

An oxidizing, basic slag rich in FeO and CaO, at relatively low temperature, since the reaction is exothermic. The reaction is 2[P] + 5[O] + 3(CaO) → (3CaO·P₂O₅).

MME-206 · Why does the BOF use pure oxygen instead of air?

Air would bring in nitrogen, which dissolves in steel and causes embrittlement and strain ageing. It would also carry away heat as hot N₂, lowering the scrap melting capacity.

MME-207 · What does vacuum degassing achieve?

Low pressure (RH or VD/VOD processes) removes dissolved hydrogen to prevent flaking, lowers nitrogen, and promotes C + O → CO for ultra-low carbon and deoxidation without forming inclusions.

MME-208 · Why is calcium treatment used in steelmaking?

Calcium modifies solid alumina inclusions into liquid calcium aluminates, which prevents nozzle clogging during casting. It also converts elongated MnS into harder globular CaS–MnS, improving transverse toughness.

MME-209 · What are common continuous casting defects and their causes?

Centerline segregation and porosity (solidification shrinkage), surface longitudinal cracks (uneven mould heat transfer), transverse cracks (straightening in the low-ductility range), and inclusions from slag entrainment. Electromagnetic stirring and soft reduction reduce segregation.

MME-210 · What is pulverized coal injection (PCI)?

Injecting fine coal through the blast furnace tuyeres to replace part of the expensive coke. Coke still supports the burden and keeps it permeable, so it cannot be replaced entirely.

MME-211 · Why is hot metal pretreated before the BOF?

To remove sulfur (and sometimes Si and P) before the oxidizing BOF, where desulfurization is poor. Common reagents are lime, CaC₂ or magnesium injected into the transfer ladle.

MME-212 · What is the difference between sinter and pellets?

Sinter is made by partially fusing fine ore, fluxes and coke breeze on a moving grate, usually at the steel plant. Pellets are balled from very fine concentrate with a binder and fired, giving uniform size and strength that suit long transport and DRI.

MME-213 · What does mould powder do in continuous casting?

It melts into a slag layer that lubricates the shell against the mould, controls heat transfer, insulates the meniscus from heat loss and oxidation, and absorbs rising inclusions.

MME-214 · How does AOD remove carbon from stainless steel without large chromium loss?

Argon–oxygen decarburization dilutes the injected O₂ with Ar, lowering the partial pressure of CO. This favours C oxidation over Cr oxidation at practical temperatures, and any oxidized Cr is later recovered with Si.

MME-215 · What are the main DRI processes?

Gas-based shaft furnaces using reformed natural gas (Midrex, HYL/Energiron), and coal-based rotary kilns (common in India). Fluidized-bed processes exist for fines.

Non-Ferrous Metallurgy

MME-216 · What are heat-treatable and non-heat-treatable aluminium alloys?

Heat-treatable alloys (2xxx, 6xxx, 7xxx) gain strength by precipitation hardening. Non-heat-treatable alloys (1xxx, 3xxx, 5xxx) are strengthened by solid solution and cold work.

MME-217 · What do aluminium temper designations O, H, T4, T6 and T73 mean?

O is annealed. H is strain-hardened. T4 is solution-treated and naturally aged. T6 is solution-treated and artificially aged to peak strength. T73 is overaged for stress corrosion resistance.

MME-218 · What are α, β and α–β titanium alloys?

α alloys (with Al, O, Sn) are weldable and creep-resistant. β alloys (with V, Mo, Nb) are heat-treatable and formable. α–β alloys such as Ti-6Al-4V balance strength, toughness and processability and account for most Ti use.

MME-219 · Why are titanium alloys difficult to machine and weld?

Low thermal conductivity concentrates heat at the tool, Ti reacts chemically with tools, and its low modulus causes chatter. In welding, it absorbs O, N and H above about 500 °C and becomes embrittled, so it needs full inert-gas shielding.

MME-220 · Why is beryllium copper used for springs and non-sparking tools?

Precipitation hardening of Cu with about 2% Be gives tensile strengths above 1000 MPa, the highest among copper alloys. It keeps good conductivity and does not spark.

MME-221 · How are nickel-based superalloys strengthened?

Mainly by coherent, ordered γ′ precipitates of Ni₃(Al,Ti) in the FCC γ matrix, plus solid-solution strengthening (Mo, W, Re, Co) and grain-boundary carbides. γ′ shows anomalous yield strength that rises with temperature up to about 700–800 °C.

MME-222 · What is dezincification of brass?

Selective leaching of zinc from brass in some waters, leaving porous, weak copper. Alloys with less than about 15% Zn, or additions of As, Sb or P to α-brass, resist it.

MME-223 · What is season cracking?

Stress corrosion cracking of cold-worked brass in environments containing ammonia, typically intergranular. It was first seen in cartridge cases. Stress-relief annealing prevents it.

MME-224 · What does the magnesium alloy designation AZ91 mean?

A Mg alloy with about 9% aluminium (A) and 1% zinc (Z). It is a common die-casting alloy.

MME-225 · How do shape memory alloys work?

In NiTi (Nitinol), deformation of low-temperature martensite by twin rearrangement is recovered when it is heated and reverts to austenite. Above the austenite finish temperature, stress-induced martensite gives superelasticity.

MME-226 · What is %IACS?

Electrical conductivity expressed as a percentage of the International Annealed Copper Standard, where annealed copper at 20 °C is 100% (58 MS/m). Pure aluminium is about 61% IACS.

MME-227 · Why are Al–Si casting alloys modified with Sr or Na?

They change the eutectic silicon from coarse, brittle plates to a fine fibrous form, improving ductility and toughness substantially.

MME-228 · How is the grain size of cast aluminium refined?

By adding Al–Ti–B master alloys. TiB₂ and Al₃Ti particles act as heterogeneous nuclei, and Ti in solution restricts growth, giving fine equiaxed grains with less hot tearing.

Extractive Metallurgy & Thermodynamics

MME-229 · How is an Ellingham diagram used?

A metal can reduce the oxide of any metal whose line lies above its own at that temperature. Most lines slope upward because ΔS is negative, while the 2C + O₂ → 2CO line slopes downward, so carbon becomes a stronger reductant at higher temperatures.

MME-230 · How are ΔG, ΔH, ΔS and the equilibrium constant related?

ΔG = ΔH − TΔS, and at standard state ΔG° = −RT·ln K. A negative ΔG means the reaction is thermodynamically spontaneous, and a more negative ΔG° means a larger K.

MME-231 · Why is aluminium not produced by carbon reduction?

Al₂O₃ is so stable that carbon could reduce it only above about 2000 °C, where aluminium carbide forms and Al vaporizes. Electrolysis in molten cryolite is far more practical.

MME-232 · How is copper produced from sulfide concentrate pyrometallurgically?

Smelting produces matte (Cu₂S·FeS) and slag. Converting with air oxidizes FeS to slag and then Cu₂S to blister copper (about 98–99% Cu). Fire refining and anode casting follow, then electrorefining.

MME-233 · How does copper electrorefining work?

Impure copper anodes dissolve in an acidified CuSO₄ electrolyte and pure copper (about 99.99%) plates on cathodes. Nobler impurities such as Au, Ag and Se fall as anode slime, and less noble ones such as Ni and Fe stay in solution.

MME-234 · How much copper does 1000 A deposit in one hour at 100% efficiency?

Faraday's law: m = ItM/(nF) = 1000 × 3600 × 63.55/(2 × 96485) ≈ 1186 g, so about 1.19 kg.

MME-235 · Why is cryolite used in the Hall–Héroult cell?

Alumina melts at about 2050 °C. Dissolving it in molten cryolite (Na₃AlF₆) lets electrolysis run at about 950–970 °C, and the bath conducts well and is less dense than molten Al, which sinks.

MME-236 · What is the Kroll process?

Titanium production: TiO₂ ore is chlorinated with carbon to TiCl₄, which is purified and reduced with molten magnesium (TiCl₄ + 2Mg → Ti + 2MgCl₂) under argon, forming titanium sponge.

MME-237 · How is zinc produced by the roast–leach–electrowin route?

ZnS concentrate is roasted to ZnO (calcine) and SO₂, leached in sulfuric acid, purified (iron precipitated as jarosite or goethite, Cu and Cd cemented with Zn dust), then electrowon onto aluminium cathodes.

MME-238 · What happens to the SO₂ produced when roasting sulfide ores?

In modern plants it is captured and converted into sulfuric acid in a contact acid plant, which controls emissions and earns by-product revenue.

MME-239 · What is SX–EW?

Solvent extraction–electrowinning. A metal-bearing leach solution is contacted with an organic extractant that selectively loads, for example, Cu. The metal is stripped into a clean, concentrated electrolyte and electrowon with inert anodes.

MME-240 · What are activity, Raoult's law and Henry's law?

Activity is the effective concentration, a = γX. Raoult's law (a = X) applies to the solvent in dilute or ideal solutions. Henry's law (a = γ°X, with γ° constant) applies to a dilute solute. Steelmaking often uses the 1 wt% Henrian standard state.

Corrosion & Protection

MME-241 · What is galvanic corrosion?

Accelerated corrosion when two dissimilar metals are electrically connected in an electrolyte. The more active (anodic) metal corrodes faster and the nobler (cathodic) one is protected.

MME-242 · What is a Pourbaix diagram?

A potential versus pH map, at a specified temperature and ion activity, showing regions of immunity, corrosion and passivity for a metal in water. It shows thermodynamics only, not corrosion rates.

MME-243 · What is the Nernst equation?

E = E° − (RT/nF)·ln Q. At 25 °C this is E = E° − (0.0592/n)·log Q. It gives the electrode potential at non-standard concentrations.

MME-244 · Why does anode-to-cathode area ratio matter in galvanic corrosion?

The total anodic current must equal the cathodic current. A small anode next to a large cathode concentrates the current, so the anode corrodes very fast. Steel rivets in copper plate fail quickly, while copper rivets in steel plate are tolerable.

MME-245 · What is the pitting resistance equivalent number (PREN)?

PREN = %Cr + 3.3 × %Mo + 16 × %N. It ranks stainless steels for chloride pitting resistance: about 18 for 304, about 24 for 316 and over 40 for super duplex.

MME-246 · How do sacrificial anode and impressed current cathodic protection compare?

Sacrificial anodes (Zn, Al, Mg) need no power and suit small or well-coated structures and low-resistivity soils or seawater. Impressed current uses a rectifier and inert anodes, so it delivers more current over larger areas and in high-resistivity soils, but it needs monitoring.

MME-247 · What is a differential aeration cell?

A corrosion cell formed by differences in oxygen concentration. The oxygen-poor area, such as under a drop, deposit or below the waterline, becomes anodic and corrodes, while the oxygen-rich area acts as the cathode.

MME-248 · What is hydrogen embrittlement?

Loss of ductility and delayed cracking caused by atomic hydrogen diffusing into a metal from corrosion, cathodic protection, pickling, plating or welding. High-strength steels above about 1000 MPa are most susceptible.

MME-249 · How is corrosion rate calculated from weight loss?

CR = K·W/(ρ·A·t). For CR in mm/y, K = 8.76 × 10⁴, W is mass loss in g, ρ is density in g/cm³, A is area in cm² and t is time in hours.

MME-250 · What is the Pilling–Bedworth ratio?

The ratio of oxide volume to the volume of metal consumed. Below 1 (e.g. Mg) the oxide is porous and non-protective. Between 1 and 2 (e.g. Al, Cr) it tends to be protective. Well above 2 it builds stress and tends to spall.

MME-251 · What is microbiologically influenced corrosion (MIC)?

Corrosion accelerated by microorganisms, notably sulfate-reducing bacteria, that create local acidic, sulfide-rich or differential-aeration conditions under biofilms. It often causes pitting in pipelines, tanks and cooling systems.

MME-252 · What are erosion corrosion and cavitation damage?

Erosion corrosion is accelerated attack where high-velocity or turbulent flow strips protective films, as at pipe bends and pump impellers. Cavitation is damage from collapsing vapour bubbles in low-pressure regions of liquids, which pits the surface mechanically.

Powder Metallurgy

MME-253 · How do water-atomized and gas-atomized powders differ?

Water atomization quenches droplets quickly, producing irregular, oxidized particles with good green strength, commonly used for press-and-sinter iron. Gas atomization (N₂, Ar) gives spherical, clean powder with good flowability for MIM, HIP and additive manufacturing.

MME-254 · What are the stages and mechanisms of solid-state sintering?

Stages are neck formation, densification with interconnected pores, and pore isolation and rounding. Surface diffusion and evaporation–condensation grow necks without shrinkage. Grain boundary and volume diffusion move matter from boundaries and cause densification.

MME-255 · What is liquid-phase sintering?

Sintering where a minor constituent melts and wets the solid particles, giving rapid densification by particle rearrangement and solution–reprecipitation. Examples are WC–Co cemented carbides and W–Ni–Fe heavy alloys.

MME-256 · Why do uniaxially pressed compacts have density gradients?

Friction between the powder and die walls makes pressure fall with distance from the moving punch. Double-action pressing, die-wall lubrication, lower height-to-diameter ratios and cold isostatic pressing reduce the gradients.

MME-257 · What sintering atmospheres are used and why?

Endothermic gas, H₂–N₂ mixtures, dissociated ammonia, pure H₂ or vacuum. They reduce surface oxides, prevent oxidation, control carbon content and remove lubricant residues.

MME-258 · What is infiltration in powder metallurgy?

Filling the pores of a sintered skeleton with a lower-melting metal by capillary action, such as copper into iron or steel. It raises density, strength and pressure tightness.

MME-259 · How does porosity affect powder metallurgy part properties?

Strength, ductility, fatigue and impact resistance all fall sharply with porosity because pores act as stress concentrators. Ductility and toughness are especially sensitive. Elastic modulus and conductivity fall too.

MME-260 · How is powder particle size measured?

By sieve analysis (mesh number), laser diffraction, sedimentation and microscopy or image analysis. Results are reported as a distribution, for example D10, D50 and D90.

MME-261 · What is powder compressibility?

The green density reached at a specified compaction pressure. It is higher for soft, pure, large, irregular particles with low oxide, and lower for prealloyed or hardened powders.

MME-262 · What is sinter hardening?

Using alloyed powders (with Mo, Ni, Cu) and accelerated cooling in the sintering furnace to form martensite directly, so a separate quench is not needed.

MME-263 · What is mechanical alloying?

High-energy ball milling that repeatedly cold-welds and fractures powder particles to produce homogeneous alloys or fine dispersions, such as oxide dispersion strengthened (ODS) alloys, that cannot be made by melting.

MME-264 · What powder properties are needed for powder bed fusion additive manufacturing?

Spherical particles with a controlled size range (typically about 15–45 µm for laser PBF), good flowability and packing density, low oxygen and moisture, and minimal satellites or internal gas porosity.

Welding & Joining Metallurgy

MME-265 · What are the sub-zones of a steel weld HAZ?

From the fusion line outward: the coarse-grained HAZ (heated well above A₃, lowest toughness), the fine-grained HAZ (just above A₃), the intercritical HAZ (between A₁ and A₃) and the subcritical HAZ (below A₁, tempered or spheroidized).

MME-266 · What is the IIW carbon equivalent formula?

CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15, in wt%. Steels with CE below about 0.40 are generally easy to weld, and higher values need preheat and low-hydrogen practice.

MME-267 · What causes hydrogen-induced (cold) cracking?

Three factors together: diffusible hydrogen, a susceptible hard microstructure (martensite in the HAZ), and tensile stress, often with a temperature near ambient. Removing any one of them prevents it.

MME-268 · What is solidification (hot) cracking?

Cracking along the weld centerline or grain boundaries during the final stage of solidification, when low-melting films such as FeS, phosphides or Al eutectics cannot carry shrinkage strains. A wide freezing range, high S and P, and poor bead shape make it worse.

MME-269 · How is arc welding heat input calculated?

HI (kJ/mm) = η·V·I·60/(1000·S), where S is travel speed in mm/min and η the arc efficiency (about 0.8 for SMAW and GMAW, 0.6 for GTAW, 1.0 for SAW). For example, 24 V, 180 A, 250 mm/min and η = 0.8 gives about 0.83 kJ/mm.

MME-270 · What is the Schaeffler diagram used for?

It predicts the weld metal microstructure (austenite, ferrite, martensite and mixtures) of stainless and dissimilar welds from Cr-equivalent and Ni-equivalent values, including filler dilution. The WRC-1992 diagram gives more accurate ferrite numbers.

MME-271 · How are shielding gases chosen?

Ar gives a stable arc and is used for Al, Ti and TIG generally. He increases heat input and penetration. CO₂ or Ar–CO₂ mixtures suit GMAW of steel. Small O₂ or CO₂ additions stabilize spray transfer on stainless steel.

MME-272 · What does post-weld heat treatment (PWHT) do?

Heating a weldment, typically to about 550–700 °C for C–Mn and Cr–Mo steels, relieves residual stresses, tempers hard HAZ martensite, lets hydrogen escape and improves dimensional stability and resistance to stress corrosion cracking.

MME-273 · What is lamellar tearing?

Step-like cracking in rolled plate beneath a weld, caused by stress through the thickness (Z-direction) acting on elongated MnS inclusions. It is prevented with low-S, Z-grade plate, buttering layers and better joint design.

MME-274 · What does the coating on an SMAW electrode do?

It generates shielding gas, forms protective slag, stabilizes the arc, deoxidizes the weld pool, adds alloying elements and controls bead shape. Low-hydrogen basic coatings limit diffusible hydrogen.

MME-275 · What is weld dilution?

The fraction of weld metal that comes from melted base metal: dilution = base metal melted/(base metal + filler melted). It controls weld composition, especially in dissimilar joints and overlays.

MME-276 · What are diffusion bonding and explosive welding?

Diffusion bonding joins clean surfaces under pressure at high temperature through atomic diffusion, with no melting. Explosive welding drives plates together at high velocity to form a wavy metallurgical bond, used for clad plates such as Ti or stainless on steel.

Materials Characterization

MME-277 · What do secondary and backscattered electrons show in an SEM?

Secondary electrons are low-energy electrons from the top few nanometres, so they give topographic contrast. Backscattered electrons are high-energy and increase with atomic number Z, so they give compositional (Z) and orientation contrast.

MME-278 · At what 2θ does the Al (111) peak appear with Cu Kα radiation?

With a = 0.405 nm, d(111) = 0.405/√3 = 0.2338 nm. sinθ = 0.15406/(2 × 0.2338) = 0.3295, so θ ≈ 19.2° and 2θ ≈ 38.5°.

MME-279 · What is the Scherrer equation?

t = Kλ/(β·cosθ), where t is the crystallite size, K ≈ 0.9, λ the wavelength, β the peak broadening in radians (after removing instrumental broadening) and θ the Bragg angle. It applies to crystallites below about 100 nm.

MME-280 · What are the XRD reflection rules for cubic lattices?

Simple cubic shows all (hkl). BCC shows only planes where h + k + l is even: (110), (200), (211). FCC shows only planes where h, k and l are all odd or all even: (111), (200), (220).

MME-281 · What is EBSD?

Electron backscatter diffraction in the SEM. Kikuchi patterns from a tilted sample are indexed to map crystal orientation, phase, grain size, texture, grain boundary types and local misorientation.

MME-282 · What are bright-field and dark-field TEM imaging?

Bright-field forms the image with the transmitted beam, so diffracting regions appear dark. Dark-field uses a selected diffracted beam, so only regions diffracting into it appear bright. This helps identify precipitates and defects.

MME-283 · What do DSC and DTA measure?

Differential scanning calorimetry measures the heat flow difference between sample and reference. Differential thermal analysis measures their temperature difference. Both detect melting, crystallization, glass transitions and precipitation reactions.

MME-284 · What is dilatometry used for?

It measures length change with temperature. That gives the thermal expansion coefficient and transformation temperatures (Ac₁, Ac₃, Ms) and is used to build CCT diagrams.

MME-285 · How is Vickers hardness calculated?

HV = 1.8544·F/d², where F is the load in kgf and d the mean indent diagonal in mm. The 136° diamond pyramid gives geometrically similar indents, so HV is largely independent of load.

MME-286 · How is thickness measured with pulse-echo ultrasonics?

Thickness = v·t/2, where v is the longitudinal wave velocity (about 5900 m/s in steel) and t the round-trip time. A 10 µs round trip in steel corresponds to about 29.5 mm.

MME-287 · What is eddy current testing?

An AC coil induces eddy currents in a conductive part. Cracks, conductivity changes or thickness changes alter the coil impedance. It suits surface and near-surface flaws, tube inspection, coating thickness and alloy sorting.

MME-288 · How are grain size and phase fraction measured quantitatively?

Grain size uses the linear intercept method (mean intercept length) or planimetric counting. Phase volume fraction equals the area fraction and the point fraction (Vv = AA = PP), measured by point counting or image analysis.

Composites, Ceramics & Polymers

MME-289 · What is the critical fibre length?

lc = σf·d/(2τc), where σf is fibre strength, d fibre diameter and τc the fibre–matrix interfacial shear strength. Fibres much longer than lc (about 15lc) behave like continuous fibres.

MME-290 · What are the isostrain and isostress composite modulus models?

Isostrain (load parallel to fibres): Ec = VfEf + VmEm, an upper bound. Isostress (load transverse): 1/Ec = Vf/Ef + Vm/Em, a lower bound. Transverse stiffness is dominated by the matrix.

MME-291 · What is the Weibull modulus?

The parameter m in Ps = exp[−(σ/σ₀)^m], which describes the scatter in brittle fracture strength. Higher m means more consistent strength: about 5–15 for ceramics versus about 100 for metals.

MME-292 · How is zirconia transformation-toughened?

Metastable tetragonal ZrO₂ grains, retained with Y₂O₃, MgO or CaO, transform to monoclinic near a crack tip. The roughly 4% volume expansion puts the crack in compression and raises fracture toughness.

MME-293 · How are technical ceramics processed?

Powder preparation, then shaping (dry pressing, isostatic pressing, slip casting, extrusion, injection moulding, tape casting), then drying, binder burnout and sintering, sometimes hot pressing or HIP, followed by diamond grinding.

MME-294 · How does crystallinity affect polymer properties?

Higher crystallinity raises density, stiffness, strength, chemical resistance and melting temperature, but lowers transparency and toughness. HDPE is more crystalline and stiffer than LDPE.

MME-295 · What is viscoelasticity?

Behaviour combining elastic solid and viscous fluid response, so deformation depends on time and rate. It shows up as creep under constant stress, stress relaxation under constant strain, and hysteresis, and is modelled with Maxwell and Kelvin–Voigt elements.

MME-296 · How is carbon fibre made from PAN?

Polyacrylonitrile fibre is stabilized in air at about 200–300 °C under tension, carbonized in inert gas at about 1000–1500 °C, optionally graphitized at higher temperature for high modulus, then surface-treated and sized.

MME-297 · What are prepregs and autoclave curing?

Prepregs are fibres pre-impregnated with partially cured resin. Laid-up plies are vacuum-bagged and cured in an autoclave under heat and pressure, which consolidates laminates with low voids (below about 1%) and high fibre content for aerospace.

MME-298 · What determines thermal shock resistance?

High fracture strength and thermal conductivity, and low elastic modulus and thermal expansion. Fused silica and Si₃N₄ resist thermal shock well, while dense alumina is more vulnerable.

MME-299 · What is vulcanization?

Crosslinking rubber with sulfur (or peroxides) under heat. It turns sticky, plastic raw rubber into an elastic, durable elastomer with higher strength and less creep.

MME-300 · What are Mn, Mw and polydispersity?

Mn is the number-average molecular weight, Σ(NiMi)/ΣNi. Mw is the weight-average, Σ(NiMi²)/Σ(NiMi). The polydispersity index Mw/Mn is at least 1 and measures the breadth of the distribution.

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