Basic Materials & Metallurgical interview questions

150 questions with answers (MME-001 to MME-150). Try each one aloud before reading the answer.

Crystallography & Defects

MME-001 · What are examples of BCC, FCC and HCP metals?

BCC: α-Fe, Cr, W, Mo, V. FCC: Al, Cu, Ni, Au, γ-Fe. HCP: Mg, Zn, Cd and α-Ti.

MME-002 · What is a unit cell?

The smallest repeating group of atoms that, when translated in three dimensions, builds the entire crystal lattice. It is defined by edge lengths a, b, c and inter-axial angles α, β, γ.

MME-003 · What are the atomic packing factors of BCC, FCC and HCP?

BCC is 0.68. FCC and HCP are both 0.74, the maximum possible for equal spheres, because both are close-packed structures.

MME-004 · What is the coordination number in BCC, FCC and HCP?

BCC has 8 nearest neighbours. FCC and HCP each have 12.

MME-005 · How many atoms belong to SC, BCC, FCC and HCP unit cells?

Simple cubic has 1, BCC has 2, FCC has 4, and the conventional hexagonal HCP cell has 6.

MME-006 · What are Miller indices?

A notation (hkl) for crystal planes, found by taking reciprocals of the plane's axial intercepts in lattice units and clearing fractions. Directions are written [uvw], and families as {hkl} and ⟨uvw⟩.

MME-007 · What is allotropy (polymorphism)?

The ability of an element or compound to exist in more than one crystal structure. Iron is BCC (α) below 912 °C, FCC (γ) from 912 to 1394 °C, and BCC (δ) up to its melting point at 1538 °C.

MME-008 · What are the main types of point defects?

Vacancies, self-interstitials, substitutional impurities and interstitial impurities. In ionic crystals, a Schottky defect is a cation–anion vacancy pair and a Frenkel defect is a vacancy–interstitial pair.

MME-009 · What is the difference between an edge and a screw dislocation?

In an edge dislocation the Burgers vector is perpendicular to the dislocation line, like an extra half-plane of atoms. In a screw dislocation it is parallel to the line, giving a helical distortion. Most real dislocations are mixed.

MME-010 · What is a slip system?

A combination of a slip plane and a slip direction, usually the most densely packed plane and direction. FCC slips on {111}⟨110⟩ with 12 systems.

MME-011 · What is a grain boundary?

The interface between two crystals (grains) of different orientation in a polycrystal. It is a planar defect that impedes dislocation motion and is a fast path for diffusion.

MME-012 · What is twinning?

A planar defect where the lattice on one side of a boundary is a mirror image of the other. Twins form by deformation (common in HCP and BCC at low temperature) or by annealing (common in FCC such as Cu and brass).

MME-013 · What is the difference between crystalline and amorphous solids?

Crystalline solids have long-range periodic atomic order and a sharp melting point. Amorphous solids, such as glass, have only short-range order and soften over a temperature range around the glass transition.

Phase Diagrams & Phase Transformations

MME-014 · What is the difference between steel and cast iron?

Steel contains less than about 2.1% carbon. Cast iron contains more than about 2.1% carbon, typically 2–4%, which makes it castable but less ductile.

MME-015 · What is an alloy?

A metallic substance made by combining a metal with one or more other elements, metallic or non-metallic, to improve properties such as strength, hardness or corrosion resistance. Examples are steel (Fe–C) and brass (Cu–Zn).

MME-016 · What is a phase?

A homogeneous portion of a system with uniform physical and chemical characteristics, separated from other phases by distinct boundaries. Ice and water in a glass are two phases.

MME-017 · What is the Gibbs phase rule?

F = C − P + 2, where F is degrees of freedom, C the number of components and P the number of phases. At constant pressure, as on most metallurgical diagrams, F = C − P + 1.

MME-018 · What is a eutectic reaction?

On cooling, a single liquid transforms at constant temperature into two solid phases: L → α + β. In Fe–C the eutectic is at 4.3% C and 1147 °C, forming ledeburite.

MME-019 · What is a eutectoid reaction?

On cooling, one solid phase transforms into two different solid phases: γ → α + β. In steel, austenite at about 0.76% C and 727 °C transforms to pearlite (ferrite + cementite).

MME-020 · What is a peritectic reaction?

On cooling, a liquid and a solid react to form a new solid: L + α → β. In Fe–C, L + δ → γ at about 1495 °C and 0.17% C.

MME-021 · What is the lever rule?

A mass balance along a tie line in a two-phase field: fraction of α = (Cβ − C₀)/(Cβ − Cα), where C₀ is the alloy composition and Cα and Cβ are the phase compositions.

MME-022 · What are the liquidus and solidus lines?

The liquidus is the temperature above which the alloy is fully liquid. The solidus is the temperature below which it is fully solid. Between them, liquid and solid coexist.

MME-023 · What is an isomorphous system?

A binary system in which the two components are completely soluble in each other in both liquid and solid states, giving a single solid solution. Cu–Ni is the classic example.

MME-024 · What are the Hume-Rothery rules?

Conditions for extensive substitutional solid solubility: atomic radii within about 15%, the same crystal structure, similar electronegativity, and similar valence.

MME-025 · What is pearlite?

A lamellar microconstituent of alternating ferrite and cementite (Fe₃C) plates formed from austenite at the eutectoid. Slow cooling gives coarse pearlite and faster cooling gives fine pearlite.

MME-026 · What are hypoeutectoid and hypereutectoid steels?

Hypoeutectoid steels have less than ~0.76% C and contain proeutectoid ferrite plus pearlite. Hypereutectoid steels have more than ~0.76% C and contain proeutectoid cementite plus pearlite.

Heat Treatment

MME-027 · What is the difference between annealing and normalizing?

Full annealing heats steel into the austenite range and furnace-cools it, giving maximum softness and coarse pearlite. Normalizing air-cools from a slightly higher temperature, giving finer grains and somewhat higher strength and hardness.

MME-028 · What is hardening of steel?

Heating steel into the austenite range (austenitizing) and quenching it fast enough to form martensite, which greatly increases hardness and strength.

MME-029 · What is tempering?

Reheating quenched steel to a temperature below A₁ (typically 150–650 °C) and holding it. It relieves stresses and decomposes martensite into tempered martensite, trading some hardness for toughness.

MME-030 · What is hardenability?

The ability of a steel to form martensite to a given depth when quenched. It depends mainly on alloy content and austenite grain size, not on carbon-controlled maximum hardness.

MME-031 · What is carburizing?

A case-hardening process that diffuses carbon into the surface of low-carbon steel at about 900–950 °C, followed by quenching and tempering. The result is a hard, wear-resistant case and a tough core.

MME-032 · What is nitriding?

A case-hardening process that diffuses nitrogen into steel at about 500–575 °C, below the transformation range, forming hard nitrides with Al, Cr, Mo and V. No quench is needed, so distortion is low.

MME-033 · What are common quenchants, from most to least severe?

Agitated brine, water, polymer solutions, oil, molten salt and still air. More severe quenchants give higher hardness depth but more distortion and cracking risk.

MME-034 · What is stress-relief annealing?

Heating below the lower critical temperature (often 550–650 °C for steel) and cooling slowly to reduce residual stresses from welding, machining or cold work without changing the microstructure significantly.

MME-035 · What is spheroidizing?

Prolonged heating just below or cycling around A₁ so that cementite forms spheroids in a ferrite matrix. It gives the softest, most machinable and formable condition for medium- and high-carbon steels.

MME-036 · What is austenitizing?

Heating steel above its upper critical temperature (A₃ or A₁ for hypereutectoid steels) and holding until it transforms to austenite, the starting point for hardening and normalizing.

MME-037 · What do A₁, A₃ and Acm mean?

A₁ is the eutectoid temperature (727 °C). A₃ is the boundary between γ and α + γ for hypoeutectoid steel. Acm is the boundary between γ and γ + Fe₃C for hypereutectoid steel.

MME-038 · What is recrystallization?

The formation of new strain-free grains in a cold-worked metal on heating, which restores softness and ductility. Recrystallization temperature is roughly 0.3–0.5 of the absolute melting temperature.

MME-039 · What is induction hardening?

Rapid surface heating of steel by high-frequency electromagnetic induction followed by immediate quenching. It hardens only a surface layer, commonly used for shafts, gears and bearing journals.

Mechanical Behavior & Testing

MME-040 · What is the difference between hardness and toughness?

Hardness is resistance to localized plastic deformation such as indentation or scratching. Toughness is the energy a material absorbs before fracture, combining strength and ductility.

MME-041 · What are engineering stress and strain?

Engineering stress σ = F/A₀ (load over original area, in Pa). Engineering strain ε = ΔL/L₀ (change in length over original length, dimensionless).

MME-042 · What is yield strength and the 0.2% offset method?

Yield strength is the stress at which plastic deformation begins. For metals without a sharp yield point, it is taken where a line parallel to the elastic slope, offset by 0.2% strain, intersects the stress–strain curve.

MME-043 · What is ultimate tensile strength?

The maximum engineering stress in a tensile test. Beyond it, necking begins and the engineering stress falls until fracture.

MME-044 · How is ductility measured?

By percent elongation, (Lf − L₀)/L₀ × 100, and percent reduction in area, (A₀ − Af)/A₀ × 100, measured from a tensile test.

MME-045 · What is Young's modulus, and typical values?

The slope of the elastic region, E = σ/ε. Steel is about 200 GPa, aluminium about 70 GPa, copper about 120 GPa and titanium about 110 GPa.

MME-046 · What are the common hardness tests?

Brinell (hardened steel or carbide ball), Rockwell (cone or ball, depth-based), Vickers (square diamond pyramid) and Knoop (elongated diamond, for thin layers). Brinell and Vickers use load divided by indentation area.

MME-047 · What is the Charpy impact test?

A notched bar is broken by a swinging pendulum, and the absorbed energy (J) is measured. Testing at several temperatures reveals the ductile-to-brittle transition.

MME-048 · What is fatigue?

Failure under repeated or fluctuating stresses well below the static strength. It proceeds by crack initiation, stable crack growth and final sudden fracture.

MME-049 · What is creep?

Time-dependent plastic deformation under constant load, significant at temperatures above about 0.4 of the absolute melting temperature.

MME-050 · What is the difference between elastic and plastic deformation?

Elastic deformation is recoverable when the load is removed and involves stretching of atomic bonds. Plastic deformation is permanent and occurs mainly by dislocation slip.

MME-051 · What is Poisson's ratio?

The negative ratio of lateral to axial strain under uniaxial load, ν = −εlateral/εaxial. Most metals have ν ≈ 0.3.

MME-052 · What is resilience?

The energy a material absorbs elastically per unit volume, recovered on unloading. The modulus of resilience is Ur = σy²/(2E).

Iron & Steelmaking

MME-053 · What are the raw materials of a blast furnace?

Iron ore (as lump, sinter or pellets), coke and flux (limestone or dolomite). Hot air blast, often oxygen-enriched with injected pulverized coal, enters through the tuyeres.

MME-054 · What is the role of coke in a blast furnace?

It is the fuel, the source of reducing gas (CO), and a permeable structural support for the burden in the lower furnace, where other materials have melted.

MME-055 · What is the role of limestone in a blast furnace?

It calcines to CaO, which combines with acidic gangue (SiO₂, Al₂O₃) to form a fluid slag that also absorbs sulfur.

MME-056 · What is pig iron?

The product of a blast furnace, typically about 4–4.5% C with Si, Mn, P and S. It is brittle and is refined into steel or remelted for cast iron.

MME-057 · What is the basic oxygen (LD) process?

Pure oxygen is blown onto molten pig iron plus scrap in a converter, oxidizing C, Si, Mn and P. The exothermic reactions supply the heat, and a heat takes roughly 15–20 minutes of blowing.

MME-058 · What is an electric arc furnace?

A furnace that melts mainly scrap, DRI or pig iron using arcs from graphite electrodes. It is the main route for recycling steel scrap.

MME-059 · What are killed, semi-killed and rimmed steels?

Killed steel is fully deoxidized (Al, Si), so no gas forms during solidification. Semi-killed is partially deoxidized. Rimmed steel is barely deoxidized, so CO evolution gives a clean rim and a segregated core.

MME-060 · What is continuous casting?

Molten steel flows from a tundish into a water-cooled copper mould, forms a solid shell, and is withdrawn continuously as slabs, blooms or billets. It replaced ingot casting because of higher yield and quality.

MME-061 · What is sponge iron (DRI)?

Iron produced by reducing iron ore in the solid state with gas (CO, H₂) or coal, without melting. It is porous, typically over 85% metallized, and is mainly used as EAF feed.

MME-062 · How are plain carbon steels classified by carbon content?

Low carbon below about 0.25% C, medium carbon about 0.25–0.6% C, and high carbon about 0.6–1.0% C or more.

MME-063 · What are the main effects of common alloying elements in steel?

Cr adds hardenability and corrosion resistance. Ni adds toughness and stabilizes austenite. Mo adds hardenability and creep strength and reduces temper embrittlement. Mn adds hardenability and ties up sulfur. V, Nb and Ti refine grains.

MME-064 · What are the main iron ore minerals?

Hematite (Fe₂O₃, 69.9% Fe), magnetite (Fe₃O₄, 72.4% Fe), limonite/goethite (hydrated iron oxides) and siderite (FeCO₃).

MME-065 · What is secondary (ladle) steelmaking?

Refining in the ladle after primary melting: deoxidation, alloy trimming, desulfurization, vacuum degassing, inclusion control and temperature homogenization, often in a ladle furnace.

Non-Ferrous Metallurgy

MME-066 · What is the difference between brass and bronze?

Brass is a copper–zinc alloy. Bronze is traditionally copper–tin, though the term also covers Cu–Al, Cu–Si and Cu–Be alloys.

MME-067 · Why is aluminium widely used in aircraft?

Its low density (2.7 g/cm³), high specific strength in alloyed tempers, good corrosion resistance and good fabricability make it efficient for airframes.

MME-068 · What do the wrought aluminium alloy series mean?

1xxx is ≥99% Al, 2xxx Cu, 3xxx Mn, 4xxx Si, 5xxx Mg, 6xxx Mg + Si, 7xxx Zn and 8xxx other elements such as Li.

MME-069 · What are the key properties of titanium?

Density about 4.5 g/cm³, high specific strength, excellent corrosion resistance through a TiO₂ film, biocompatibility, and usefulness up to about 500–600 °C in alloys.

MME-070 · What are superalloys?

Ni-, Co- or Fe–Ni-based alloys that retain strength, creep resistance and oxidation resistance at high temperatures. They are used in gas turbine blades, discs and combustors.

MME-071 · Why is copper used for electrical wiring?

It has the highest electrical conductivity of common metals after silver, good ductility, solderability and corrosion resistance at reasonable cost.

MME-072 · What are the key features of magnesium alloys?

They are the lightest structural metals (about 1.74 g/cm³), with good castability and machinability but limited cold formability (HCP) and poor corrosion resistance.

MME-073 · What is duralumin?

An early age-hardenable Al–Cu–Mg–Mn alloy (about 4% Cu). It is the ancestor of the modern 2xxx series such as 2017 and 2024.

MME-074 · What are Babbitt (white) metals?

Bearing alloys based on tin or lead with antimony and copper. Hard intermetallic particles carry the load while the soft matrix conforms and embeds debris.

MME-075 · What is the composition of common solders?

Traditional eutectic solder is 63Sn–37Pb, melting at 183 °C. Lead-free solders are commonly Sn–Ag–Cu (SAC), melting around 217–220 °C.

MME-076 · What are the main uses of nickel and its alloys?

Stainless steels, superalloys, Monel (Ni–Cu) for marine use, Inconel and Hastelloy for heat and corrosion resistance, plating, and batteries.

MME-077 · What are the main uses of zinc?

Galvanizing steel (the largest use), die casting alloys (Zamak), brass and batteries.

MME-078 · What are refractory metals?

Metals with very high melting points: W (3422 °C), Re, Ta, Mo and Nb. They are used in high-temperature furnaces, filaments, rocket nozzles and electronics.

Extractive Metallurgy & Thermodynamics

MME-079 · What is extractive metallurgy?

The science of extracting metals from ores and refining them. Its three main branches are pyrometallurgy (high temperature), hydrometallurgy (aqueous solutions) and electrometallurgy (electrolysis).

MME-080 · What is pyrometallurgy?

Extraction and refining at high temperatures through roasting, smelting, converting and fire refining. Iron, copper, lead and nickel are largely produced this way.

MME-081 · What is hydrometallurgy?

Extraction using aqueous chemistry: leaching the metal into solution, purifying the solution, and recovering the metal by precipitation, cementation or electrowinning. It is used for Au, Zn, Cu oxides and U.

MME-082 · What is electrometallurgy?

Producing or refining metals by electrolysis, in aqueous solutions (Cu, Zn electrowinning and refining) or molten salts (Al, Mg, Na).

MME-083 · What is the difference between an ore and a mineral?

A mineral is a naturally occurring compound with a definite composition and crystal structure. An ore is a rock containing minerals from which a metal can be extracted economically.

MME-084 · What is froth flotation?

A concentration process where finely ground ore is mixed with water and reagents. Collectors make the valuable mineral surfaces hydrophobic, so the particles attach to air bubbles and float into a froth while gangue sinks.

MME-085 · What is the difference between roasting and calcination?

Roasting heats an ore in air to convert sulfides to oxides (e.g. 2ZnS + 3O₂ → 2ZnO + 2SO₂). Calcination heats it in limited or no air to drive off CO₂ or H₂O (e.g. CaCO₃ → CaO + CO₂).

MME-086 · What is smelting?

A high-temperature process in which a charge is melted and chemically reduced or separated into molten metal (or matte) and slag.

MME-087 · What is slag?

A molten mixture of oxides (mainly SiO₂, CaO, Al₂O₃, MgO, FeO) that collects gangue and impurities, protects the metal from oxidation and refines it by absorbing S and P.

MME-088 · What is an Ellingham diagram?

A plot of the standard Gibbs free energy of oxide (or sulfide) formation per mole of O₂ versus temperature. It shows which reductant can reduce a given oxide and at what temperature.

MME-089 · What is the Hall–Héroult process?

Electrolytic production of aluminium from alumina dissolved in molten cryolite (Na₃AlF₆) at about 950–970 °C, using consumable carbon anodes.

MME-090 · What is the Bayer process?

The refining of bauxite to pure alumina by digesting it in hot caustic soda, separating red mud, precipitating Al(OH)₃ and calcining it to Al₂O₃.

Corrosion & Protection

MME-091 · What is corrosion?

The deterioration of a material by chemical or electrochemical reaction with its environment. For metals, it is usually an electrochemical oxidation process.

MME-092 · What are the anodic and cathodic reactions when iron corrodes in water?

Anode: Fe → Fe²⁺ + 2e⁻. Cathode in neutral aerated water: O₂ + 2H₂O + 4e⁻ → 4OH⁻. In acid: 2H⁺ + 2e⁻ → H₂.

MME-093 · What is the difference between uniform and localized corrosion?

Uniform corrosion removes metal evenly over the surface and is predictable. Localized corrosion (pitting, crevice, intergranular, SCC) concentrates attack in small areas and is often more dangerous.

MME-094 · What is pitting corrosion?

Localized attack that forms small, deep cavities, usually where a passive film breaks down, often due to chlorides. It can perforate a component with little overall weight loss.

MME-095 · What is crevice corrosion?

Localized attack within narrow gaps such as under gaskets, washers or deposits, where stagnant solution becomes depleted in oxygen and enriched in acid and chlorides.

MME-096 · What is the galvanic series?

A ranking of metals and alloys by their corrosion potential in a specific environment, usually seawater. Metals higher on the active end corrode preferentially when coupled with nobler ones.

MME-097 · What is cathodic protection?

Making the protected structure the cathode of an electrochemical cell, either with sacrificial anodes (Zn, Mg, Al) or with impressed current from a DC source, so it does not corrode.

MME-098 · What is galvanizing?

Coating steel with zinc, usually by hot dipping in molten zinc at about 450 °C. Zinc acts as a barrier and as a sacrificial anode at scratches.

MME-099 · What is passivation?

The formation of a thin, adherent, protective oxide film that greatly reduces the corrosion rate. Examples are Cr₂O₃ on stainless steel and Al₂O₃ on aluminium.

MME-100 · What are corrosion inhibitors?

Chemicals added in small amounts to an environment to lower the corrosion rate. They work by adsorbing on the surface, forming films, or slowing the anodic or cathodic reaction.

MME-101 · What is stress corrosion cracking?

Cracking caused by the combined action of tensile stress (applied or residual) and a specific corrosive environment, in a susceptible material. Examples are austenitic stainless steel in hot chlorides and brass in ammonia.

MME-102 · How is corrosion rate expressed?

As penetration per year, in mm/y or mils per year (mpy), or as mass loss per unit area per time. 1 mpy = 0.0254 mm/y.

Powder Metallurgy

MME-103 · What is powder metallurgy?

A process that makes parts by compacting metal powders into a shape and sintering them below the melting point to bond the particles.

MME-104 · What are the main steps of powder metallurgy?

Powder production, blending with lubricant and additives, compaction in a die, sintering in a controlled atmosphere, and optional secondary operations such as sizing, machining, impregnation or heat treatment.

MME-105 · How are metal powders produced?

By atomization (water or gas), chemical reduction of oxides, electrolytic deposition, carbonyl decomposition and mechanical milling.

MME-106 · What is sintering?

Heating a powder compact below its melting point, typically 0.7–0.9 of Tm, so that particles bond by atomic diffusion, increasing strength and usually density.

MME-107 · What is a green compact?

A pressed, unsintered powder part. Its green strength comes from mechanical interlocking and cold welding and must be enough for handling before sintering.

MME-108 · What are the advantages of powder metallurgy?

Near-net shape with little scrap, controlled porosity, materials that are hard to melt or alloy (WC–Co, W), good dimensional consistency and low cost in high-volume production.

MME-109 · What are self-lubricating bearings?

Porous sintered bronze or iron bearings impregnated with oil. The oil seeps out under heat and pressure during operation and returns to the pores when it stops.

MME-110 · What is a cemented carbide?

A composite of hard tungsten carbide grains bonded with a ductile cobalt binder, made by liquid-phase sintering. It is used for cutting tools, dies and wear parts.

MME-111 · What is metal injection moulding (MIM)?

Fine metal powder is mixed with a polymer binder, injection-moulded like plastic, debound and then sintered. It makes small, complex, high-density parts.

MME-112 · What is hot isostatic pressing (HIP)?

Applying high temperature and high gas pressure (usually argon) uniformly from all directions to consolidate powder or close internal porosity in castings and additively manufactured parts.

MME-113 · What are apparent density and tap density?

Apparent density is the mass per unit volume of loose powder poured into a container. Tap density is the higher value measured after mechanical tapping settles the powder.

MME-114 · Why is a lubricant such as zinc stearate added to powder?

It reduces friction between particles and against the die walls, giving more uniform density, easier ejection and less die wear. It is burned off early in sintering.

Welding & Joining Metallurgy

MME-115 · What is the difference between fusion and solid-state welding?

Fusion welding melts the base metal at the joint (arc, laser, electron beam). Solid-state welding bonds without melting, using pressure, deformation or diffusion (friction, friction stir, explosive, diffusion bonding).

MME-116 · What is the heat-affected zone (HAZ)?

The base metal next to a fusion weld that did not melt but had its microstructure and properties changed by the welding heat cycle.

MME-117 · What is shielded metal arc welding (SMAW)?

Manual arc welding with a consumable flux-coated electrode (stick). The flux gives gas shielding, slag protection, arc stability and alloying.

MME-118 · What is gas tungsten arc welding (GTAW/TIG)?

Arc welding with a non-consumable tungsten electrode and inert gas shielding (Ar or He), with optional filler. It gives high-quality welds on thin sections and on Al, Ti and stainless steel.

MME-119 · What is gas metal arc welding (GMAW/MIG/MAG)?

Arc welding with a continuously fed consumable wire electrode and a shielding gas: inert for MIG, or active (CO₂ or Ar–CO₂) for MAG. It is productive and easy to automate.

MME-120 · What is submerged arc welding (SAW)?

An automatic process where the arc between a wire electrode and the work is buried under granular flux. It gives high deposition rates and deep penetration on thick plate, mainly in the flat position.

MME-121 · What is the difference between welding, brazing and soldering?

Welding melts the base metal. Brazing uses a filler that melts above 450 °C and soldering a filler that melts below 450 °C. In both, the base metal does not melt and the filler flows into the joint by capillary action.

MME-122 · What is weldability?

The ability of a material to be welded under fabrication conditions into a structure that performs satisfactorily in service, without cracking or unacceptable property loss.

MME-123 · What is carbon equivalent (CE) in welding?

A single number that combines the effects of carbon and alloying elements on hardenability, used to estimate the risk of HAZ hardening and cold cracking and to decide on preheat.

MME-124 · Why is preheat used in welding?

It slows the cooling rate, which reduces hard martensite in the HAZ, lets hydrogen diffuse out, and lowers thermal stresses, reducing the risk of cold cracking.

MME-125 · What are common weld defects?

Porosity, slag inclusions, lack of fusion, incomplete penetration, undercut, overlap, cracks (hot and cold) and excessive spatter or reinforcement.

MME-126 · What is friction stir welding?

A solid-state process where a rotating non-consumable tool with a pin and shoulder plasticizes and stirs the joint material. It is widely used for aluminium because it avoids solidification defects.

Materials Characterization

MME-127 · What are the steps in preparing a metallographic specimen?

Sectioning, mounting, grinding with progressively finer abrasive papers, polishing with diamond or alumina, and etching to reveal the microstructure.

MME-128 · What is nital and what is it used for?

A solution of nitric acid in ethanol, typically 2–5%. It is the standard etchant for carbon and low-alloy steels, revealing ferrite grain boundaries and pearlite.

MME-129 · What is the resolution limit of an optical microscope?

About 0.2 µm, set by the wavelength of visible light and the numerical aperture of the objective. Useful magnification is limited to about 1000–1500×.

MME-130 · What is a scanning electron microscope (SEM)?

An instrument that scans a focused electron beam over a sample and builds an image from emitted secondary or backscattered electrons. It gives high depth of field and resolution down to a few nanometres.

MME-131 · What is Bragg's law?

nλ = 2d sinθ, where λ is the X-ray wavelength, d the interplanar spacing, θ the diffraction angle and n the order. It is the basis of X-ray diffraction.

MME-132 · What is EDS (EDX)?

Energy-dispersive X-ray spectroscopy, usually attached to an SEM. It identifies elements and estimates their concentrations from the characteristic X-rays emitted under the electron beam.

MME-133 · What are the main NDT methods?

Visual inspection, liquid penetrant, magnetic particle, ultrasonic, radiographic and eddy current testing. Acoustic emission and thermography are also used.

MME-134 · How does ultrasonic testing work?

High-frequency sound pulses (typically 1–10 MHz) are sent into a part, and echoes from flaws or the back wall are timed and measured. It detects internal flaws and measures thickness.

MME-135 · What is radiographic testing?

X-rays or gamma rays pass through a part onto film or a digital detector. Voids and inclusions show as density differences, making it good for volumetric weld and casting flaws.

MME-136 · What is dye (liquid) penetrant testing?

A coloured or fluorescent liquid is drawn into surface-breaking cracks by capillary action, the excess is removed, and a developer draws it back out to show the flaw.

MME-137 · What is magnetic particle testing?

A ferromagnetic part is magnetized, and fine iron particles gather at leakage fields caused by surface or near-surface flaws. It works only on ferromagnetic materials.

MME-138 · What is the ASTM grain size number?

A number G defined by n = 2^(G−1), where n is the number of grains per square inch at 100× magnification. A higher G means finer grains.

Composites, Ceramics & Polymers

MME-139 · What is a composite material?

A material made of two or more distinct phases, typically a continuous matrix and a reinforcement (fibres or particles), combined to give properties neither has alone.

MME-140 · What are PMC, MMC and CMC?

Polymer-matrix composites (e.g. CFRP, GFRP), metal-matrix composites (e.g. SiC in Al) and ceramic-matrix composites (e.g. SiC/SiC), classified by matrix type.

MME-141 · What are the common reinforcing fibres?

Glass (E-glass, S-glass), carbon (high strength or high modulus), aramid (Kevlar), boron, and ceramic fibres such as SiC and alumina.

MME-142 · What is a ceramic?

An inorganic, non-metallic material, usually a compound of metals with O, N, C or B, held by ionic and covalent bonds. Ceramics are typically hard, brittle, refractory and electrically insulating.

MME-143 · Why are ceramics brittle?

Ionic and covalent bonds give few mobile dislocations and restrictive slip systems, so they cannot deform plastically to blunt cracks. Fracture is controlled by small flaws.

MME-144 · What is the difference between thermoplastics and thermosets?

Thermoplastics are linear or branched polymers that soften on heating and can be remelted (PE, PP, nylon). Thermosets cure into crosslinked networks that do not melt (epoxy, phenolic).

MME-145 · What is the glass transition temperature (Tg)?

The temperature range where an amorphous polymer changes from a hard, glassy state to a soft, rubbery state as chain segments gain mobility.

MME-146 · What is the degree of polymerization?

The number of repeat units in a polymer chain, equal to the polymer molecular weight divided by the repeat-unit molecular weight.

MME-147 · What is the difference between addition and condensation polymerization?

Addition (chain-growth) polymerization links monomers with double bonds without by-products, as in polyethylene. Condensation (step-growth) links monomers with release of a small molecule such as water, as in nylon and polyester.

MME-148 · What is an elastomer?

A lightly crosslinked polymer above its Tg that can stretch to several hundred percent and recover elastically. Natural rubber, SBR and silicone are examples.

MME-149 · What are common engineering ceramics?

Alumina (Al₂O₃), zirconia (ZrO₂), silicon carbide (SiC), silicon nitride (Si₃N₄), boron carbide (B₄C) and aluminium nitride (AlN).

MME-150 · What is the rule of mixtures for composite modulus?

For loading parallel to continuous fibres, Ec = Vf·Ef + Vm·Em, where V is the volume fraction and E the modulus of the fibre (f) and matrix (m).

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