Microstructure Identification Atlas

Look at metal microstructures as seen under the microscope, learn their features, and practise telling them apart. The drawings change a little every time they are drawn.

Every image here is a procedurally drawn schematic, not a micrograph. Which phase looks light and which looks dark follows the typical appearance after etching (for example with nital), but real micrographs differ a lot in colour, contrast and grain shape depending on the etchant, etching time, illumination and magnification. The approximate magnification is the magnification when the drawing is viewed 10 cm wide. In the low-magnification drawings (×100, ×200) the stripes in pearlite only indicate that it is lamellar; real lamellae are usually too fine to resolve at those magnifications. In dark mode the drawings are only dimmed; light and dark are not inverted.

Microstructures

19 microstructures, grouped by how they form. Tap a drawing to open how to identify it.

Slowly cooled and annealed steels

Quenched, isothermally transformed and tempered steels

Cast irons

Solidification structures

Grain shape, grain size and precipitates

How to identify each microstructure

Tap a name to open its schematic, what you see, how it forms and how to tell it from look-alikes.

Slowly cooled and annealed steels

Ferrite (pure iron, very low-carbon steel)Light equiaxed grains outlined by thin dark grain boundaries

Schematic. Approx. magnification ×200 (field width 500 µm)

What you see

Light, angular grains of roughly equal size (equiaxed grains) fill the field, and the grain boundaries show as thin dark lines.
With a little carbon, small dark pearlite patches sit at some grain corners (example: hot-rolled iron with about 0.05% C).

Why it looks light or dark

Atoms are disordered at grain boundaries, so the boundaries are attacked faster than the grain interiors; they become grooves that do not reflect light back and look like dark lines.
With longer etching, the attack rate depends on crystal orientation: grains of the same orientation have the same brightness, so neighbouring grains look slightly different.

Etchants and appearance

Nital (nitric acid in alcohol) reveals ferrite grain boundaries. Picral (picric acid in alcohol) does not reveal ferrite grain boundaries, so use nital to look at grain size.

How it forms (alloy and heat treatment)

Cool or anneal iron with very little carbon (pure iron or very low-carbon steel) from the austenite range. On the Fe–C phase diagram this is the α (BCC) field, which dissolves at most 0.022% C at 727 °C.
Recrystallisation after cold work restores strain-free equiaxed grains.

Properties

Finer grains are harder and stronger (the Hall–Petch relationship). Grain size is expressed as a grain size number (see Grain size on this page). [5]

Look-alikes and how to tell them apart

Austenite (annealing twins): Annealing twins (bands bounded by two straight parallel lines across a grain) are typical of FCC metals such as austenitic stainless steels and copper alloys. Ferrite (BCC) grains normally show none.
Ferrite + pearlite (hypoeutectoid steel): If dark pearlite covers a sizeable fraction of the area, it is a hypoeutectoid steel. A few dark dots only mean a low-carbon ferritic structure.
Duplex grain size (non-uniform grain size): In a duplex grain structure the grain size varies strongly from place to place. Ferrite grains are roughly uniform in size.

Identification clues

Only light, equiaxed polygonal grains
No straight parallel bands (twins) inside the grains
Hardly any dark pearlite

Check with a calculator: Iron–Carbon Phase Diagram Viewer

Practice questions:

Sources

Ferrite + pearlite (hypoeutectoid steel)White ferrite and dark pearlite; more carbon, more dark area

Schematic. Approx. magnification ×200 (field width 500 µm)

Carbon content 0.4 %

Steel with 0.4% C: mass fraction of pearlite by the lever rule 51.2%

What you see

White (light) proeutectoid ferrite grains are mixed with dark islands of pearlite. At higher magnification the pearlite shows lamellae.
The more carbon, the more pearlite. Move the slider below the drawing to redraw it with the fraction given by the lever rule.

Why it looks light or dark

Pearlite is a fine layering of ferrite and cementite; etching leaves steps between the layers, and their shadows make it look dark overall. Proeutectoid ferrite dissolves evenly and stays flat, so it looks light.

Etchants and appearance

Nital shows both the ferrite grain boundaries and the pearlite (the usual choice). Picral resolves pearlite lamellae well but does not reveal ferrite grain boundaries.

How it forms (alloy and heat treatment)

Steel with 0.022–0.76% C cooled slowly in the furnace (annealing) or in air (normalising) from the austenite range (above A3).
Ferrite forms first at the prior austenite grain boundaries; the remaining austenite becomes richer in carbon, and when it reaches 0.76% it transforms to pearlite at 727 °C.
Just below 727 °C the mass fraction of pearlite is (C − 0.022)/(0.76 − 0.022) by the lever rule; for 0.40% C it is about 0.51. The drawing uses this mass fraction as the area fraction.

Properties

Normalising (air cooling) is faster than annealing (furnace cooling), so it gives less ferrite and finer pearlite (example: a 0.35% C steel). [1][2]

Look-alikes and how to tell them apart

Widmanstätten ferrite: Widmanstätten ferrite grows as parallel white wedges from the grain boundaries into the grains. In annealed hypoeutectoid steel the white ferrite forms roughly equiaxed grains.
Pearlite (eutectoid steel): If white ferrite grains are clearly present, it is a hypoeutectoid steel. If almost the whole area is lamellar, it is eutectoid pearlite.
Ductile iron (spheroidal graphite iron): Ductile iron also has white ferrite and dark pearlite, but it contains dark graphite nodules. Steel has no graphite.

Identification clues

White equiaxed grains mixed with dark islands
The dark islands are lamellar at higher magnification
No graphite nodules or flakes

Related terms: Lever rule Eutectoid reaction Pearlite

Check with a calculator: Iron–Carbon Phase Diagram Viewer

Practice questions:

Sources

Pearlite (eutectoid steel)Colonies of thin, aligned lamellae fill the field

Schematic. Approx. magnification ×2,000 (field width 50 µm)

What you see

Alternating layers of ferrite and cementite. The layer direction changes from one colony to the next, giving a fingerprint-like pattern of stripes.
In the same sample, colonies cut obliquely show a wider apparent spacing and those cut nearly perpendicular a narrower one. Colonies too fine to resolve look uniformly dark.

Why it looks light or dark

Etching dissolves ferrite first, so cementite stands slightly proud. Both phases look white in the light microscope, but the cementite layers are thin and the shadows of the steps become dark lines, so pearlite looks dark overall.

Etchants and appearance

Picral hardly reveals ferrite grain boundaries but brings out the cementite clearly, so it is well suited to resolving pearlite lamellae. Pearlite also looks dark after nital.

How it forms (alloy and heat treatment)

Eutectoid steel with about 0.76% C cooled slowly from the austenite range to below 727 °C (γ → α + Fe₃C).
Slower cooling (small undercooling) gives coarse pearlite with wide spacing; faster cooling gives fine pearlite. In air-cooled (normalised) fine pearlite the lamellae are often hard to resolve in the light microscope.

Properties

Finer spacing means harder and stronger pearlite. Hardness ranks fine pearlite > coarse pearlite > spheroidite. [7][8]
The light microscope resolves down to about 500 nm, so very fine pearlite with a spacing of about 50 nm needs an electron microscope. [9]

Look-alikes and how to tell them apart

Eutectic (Pb–Sn and others): A eutectic (such as Pb–Sn) is also lamellar, but its two kinds of layers are similar in thickness. In pearlite the cementite layers are much thinner than the ferrite layers (by the lever rule cementite is (0.76 − 0.022)/(6.70 − 0.022) ≈ 0.11).
Upper bainite: Upper bainite grows as feathery sheaves from grain boundaries, with carbides as broken rows of particles, not long continuous stripes like pearlite.
Ferrite + pearlite (hypoeutectoid steel): If clusters of white proeutectoid ferrite are present, it is a hypoeutectoid steel.

Identification clues

Lamellar stripes over the whole area
Stripe direction changes from colony to colony
No white grains or white network

Related terms: Pearlite Eutectoid reaction

Check with a calculator: Iron–Carbon Phase Diagram Viewer

Practice questions:

Which statement about the eutectoid point of the Fe-Fe₃C phase diagram is correct?

Sources

Proeutectoid cementite + pearlite (hypereutectoid steel)A white cementite network outlines the pearlite grains

Schematic. Approx. magnification ×1,000 (field width 100 µm)

Carbon content 1.2 %

Steel with 1.2% C: mass fraction of proeutectoid cementite by the lever rule 7.4%

What you see

A thin white network (proeutectoid cementite) surrounds grains of lamellar pearlite (the prior austenite grains).
More carbon makes the network thicker. Move the slider below the drawing to redraw it with the thickness given by the lever rule.

Why it looks light or dark

Proeutectoid cementite is hardly attacked by nital or picral and stays white; the steps at the edges of the network show as thin dark lines.

Etchants and appearance

After nital or picral, proeutectoid cementite is white and easy to confuse with white ferrite. Hot alkaline ferricyanide (potassium ferricyanide with potassium hydroxide) turns cementite black and pearlite brown, which tells them apart.

How it forms (alloy and heat treatment)

Steel with more than 0.76% C (hypereutectoid steel) cooled slowly from the austenite range. On crossing the Acm line, proeutectoid cementite forms as a network on the austenite grain boundaries, and the remaining austenite transforms to pearlite at 727 °C.
Just below 727 °C the mass fraction of proeutectoid cementite is (C − 0.76)/(6.70 − 0.76) by the lever rule; for 1.2% C it is about 0.074. The drawing uses this fraction as the area of the network.

Properties

Example: a 1.3% C steel furnace-cooled from 970 °C shows proeutectoid cementite on the grain boundaries and clearly visible pearlite lamellae. [1]

Look-alikes and how to tell them apart

Ferrite + pearlite (hypoeutectoid steel): White ferrite in hypoeutectoid steel forms whole grains with a large area. White proeutectoid cementite in hypereutectoid steel is a thin network along the grain boundaries. If unsure, darken cementite with alkaline ferricyanide.
Precipitates (on grain boundaries and inside grains): In sensitised stainless steel the grain boundaries are dark ditches with no lamellae inside the grains. The hypereutectoid network is white, with pearlite lamellae inside.
Pearlite (eutectoid steel): Without a white grain-boundary network it is eutectoid pearlite.

Identification clues

A thin white network around pearlite grains
The network follows the grain boundaries
Pearlite lamellae inside

Related terms: Pearlite Lever rule

Check with a calculator: Iron–Carbon Phase Diagram Viewer

Practice questions:

When a 1.2 wt% C hypereutectoid steel is slowly cooled from the austenite region, what microstructure is obtained at room temperature?

Sources

Widmanstätten ferriteParallel white wedges grow from the grain boundaries into the grains

Schematic. Approx. magnification ×200 (field width 500 µm)

What you see

A white band of ferrite lies on the prior austenite grain boundaries, and from it many aligned white wedge-shaped plates grow into the grain. Between the plates is dark pearlite.

Why it looks light or dark

Widmanstätten ferrite has little internal structure and etches cleanly white. Bainite, an aggregate of fine plates and carbides, etches dark.

Etchants and appearance

Nital gives a strong contrast between the white plates and dark pearlite.

How it forms (alloy and heat treatment)

When cooling is far from equilibrium, as in as-cast carbon steel (example: 0.40–0.50% C), wedge-shaped ferrite grows from the grain boundaries. Some grows secondarily from a layer of grain-boundary (allotriomorphic) ferrite.
Annealing, normalising or hot rolling removes it.

Properties

Acicular ferrite lowers toughness, so steel with this structure is not recommended for critical components (example: as-cast steel). [3]

Look-alikes and how to tell them apart

Upper bainite: Both grow in parallel from grain boundaries, but Widmanstätten ferrite forms clean white wedges with dark pearlite between them, while upper bainite sheaves are dark as a whole, with rows of carbide between fine plates.
Ferrite + pearlite (hypoeutectoid steel): Ferrite in annealed hypoeutectoid steel is equiaxed, without rows of parallel wedges (normalising can make the ferrite acicular, though).

Identification clues

Parallel white wedges grow into the grains
A white band on the grain boundaries
Dark pearlite between the wedges

Check with a calculator: Iron–Carbon Phase Diagram Viewer

Sources

Spheroidite (spheroidised cementite)Round cementite particles scattered in light ferrite

Schematic. Approx. magnification ×2,000 (field width 50 µm)

What you see

Round cementite particles are spread evenly in a matrix of equiaxed ferrite, with many particles on the ferrite grain boundaries.
No lamellae and no needles.

Why it looks light or dark

Cementite particles are not attacked and stand slightly proud, so the step around each one casts a shadow: they look like dots with a dark rim.

Etchants and appearance

Picral is the standard etchant for spheroidised structures and the distribution of tempering carbides. Combine it with nital to see the ferrite grain boundaries as well.

How it forms (alloy and heat treatment)

Hold pearlite, bainite or quenched steel for a long time just below A1 (727 °C) — spheroidise annealing. Example: a 0.40–0.50% C steel water-quenched from 830–850 °C, held at 710 °C for 72 h and cooled slowly.
Long tempering of quenched steel at 600 °C (days) also coarsens the cementite particles, and acicular ferrite gradually becomes equiaxed.

Properties

Hardness ranks fine pearlite > coarse pearlite > spheroidite; spheroidite is the softest. [4]
A tool steel with about 0.85% C, before heat treatment (ferrite with carbides a few µm in size), is about HV 165. [5]

Look-alikes and how to tell them apart

Tempered martensite: Tempered martensite also contains carbide particles, but the original lath or needle shapes and a darker matrix remain. In spheroidite the ferrite is equiaxed and light, and the particles are round and larger.
Ductile iron (spheroidal graphite iron): Graphite nodules in ductile iron are large enough to see at low magnification and are surrounded by white ferrite rings. Spheroidite particles need high magnification.
Precipitates (on grain boundaries and inside grains): Grain-boundary precipitates line up continuously along the boundaries. Spheroidite particles are spread evenly inside the grains as well.

Identification clues

Round particles scattered all over
No lamellae and no needles
Equiaxed, light ferrite matrix

Related terms: Ostwald ripening

Check with a calculator: Iron–Carbon Phase Diagram Viewer

Practice questions:

When an alloy containing dispersed precipitate particles of different sizes in a matrix is held at high temperature for a long time, which correctly describes Ostwald ripening?

Sources

Quenched, isothermally transformed and tempered steels

Upper bainiteFeathery sheaves growing from grain boundaries, with rows of carbide between plates

Schematic. Approx. magnification ×1,000 (field width 100 µm)

What you see

Sheaves of parallel thin ferrite plates (laths) grow from the prior austenite grain boundaries like feathers.
Cementite particles lie in broken rows between the plates. The light microscope does not resolve individual plates; you see clusters of sheaves.
The drawing shows a sample partly transformed and then quenched: the growing sheaves are dark and the rest is light martensite.

Why it looks light or dark

Etching attacks around the cementite particles, so bainite looks dark overall.

Etchants and appearance

Nital etches it dark. Individual plates and carbides need an electron microscope.

How it forms (alloy and heat treatment)

Cool austenite quickly to just below the pearlite range (usually below 500 °C) and hold it there (isothermal transformation).
If quenching is a little slower than the critical cooling rate, feathery upper bainite may appear only at grain boundaries (example: a 0.35% C steel, water quenched).

Properties

Example: 4340 steel transformed isothermally to mostly upper bainite, 32 HRC. [1]
Its carbides are coarse and lie between the plates, so it is less tough than lower bainite. [4]

Look-alikes and how to tell them apart

Lower bainite: Lower bainite consists of dark needles in random directions; upper bainite grows as parallel sheaves from grain boundaries. Carbide location (between plates in upper bainite, inside the plates at about 60° in lower bainite) needs an electron microscope.
Pearlite (eutectoid steel): Pearlite forms long continuous stripes. Upper bainite has broken rows of carbide particles in feathery sheaves.
Widmanstätten ferrite: Widmanstätten ferrite forms clean white wedges with dark pearlite between them; upper bainite sheaves are dark as a whole.

Identification clues

Parallel sheaves grow like feathers from grain boundaries
Broken rows of dark dots inside the sheaves
Etches dark overall

Related terms: Hardenability

Practice questions:

Sources

Lower bainiteThin dark needles (plates) in random directions

Schematic. Approx. magnification ×1,000 (field width 100 µm)

What you see

Thin dark needles (actually plates) lie in many directions, not parallel to one another (acicular).
In a specimen partly transformed and then quenched, they show as dark needles in a light martensite matrix (as in this drawing).

Why it looks light or dark

Etching attacks around the fine carbides inside the plates, so they etch dark.

Etchants and appearance

After nital they look like dark needles. The carbides inside the plates (parallel rows at about 60° to the plate axis) need an electron microscope.

How it forms (alloy and heat treatment)

Quench austenite to a lower temperature than for upper bainite (typically 200–350 °C) and hold it there.

Properties

Example: 4340 steel transformed isothermally to mostly lower bainite at 280 °C and 330 °C, 50 HRC and 44 HRC. [1]
It has fewer and finer carbides between the plates, so it is tougher than upper bainite. [2]

Look-alikes and how to tell them apart

Tempered martensite: They look very similar in the light microscope. Tempered martensite has carbides in several orientations (Widmanstätten arrays), while lower bainite often has a single orientation (seen in an electron microscope). The heat treatment history is also a clue.
Plate (lenticular) martensite (quenched high-carbon steel): As-quenched plate martensite has thicker plates with a central midrib and white retained austenite around them. Lower bainite needles are thin and dark.
Upper bainite: Upper bainite grows as parallel sheaves from grain boundaries; lower bainite needles point in random directions.

Identification clues

Thin dark needles in random directions
No central midrib
Not parallel sheaves from grain boundaries

Practice questions:

Which correctly describes the difference between a TTT (isothermal transformation) diagram and a CCT (continuous cooling transformation) diagram?

Sources

Lath martensite (quenched low-carbon steel)Regions of fine aligned striations inside the original grains

Schematic. Approx. magnification ×500 (field width 200 µm)

What you see

The boundaries of the original austenite grains (prior austenite grain boundaries) remain, and each grain is divided into regions of fine striations with the same direction. Inside a region, bands of slightly different brightness (blocks) line up.
Individual laths are too fine to resolve in the light microscope.

Why it looks light or dark

As-quenched martensite gives weak contrast with nital; differences in orientation between regions show only as slight differences in brightness.

Etchants and appearance

Etch with nital (contrast is modest). Picral also attacks martensite.

How it forms (alloy and heat treatment)

Steel with less than about 0.6% C quenched from the austenite range faster than the critical cooling rate (water or oil quench). Between about 0.6% and 1% C laths and plates are mixed.

Properties

As-quenched hardness depends almost only on carbon content; alloying changes the depth of hardening, not the hardness. [2]
Martensite reaches its maximum hardness, HRC 66, at 0.8–1.0% C. [1]

Look-alikes and how to tell them apart

Tempered martensite: The shape is very similar, but tempering makes the matrix look darker. As-quenched martensite is lighter with weak contrast.
Upper bainite: Upper bainite grows as feathery sheaves from grain boundaries with rows of dark carbide dots. Lath martensite striations are fine and uniform, and the regions fill the whole grain.
Plate (lenticular) martensite (quenched high-carbon steel): In plate martensite each plate is visible, with a central midrib and white retained austenite.

Identification clues

Prior austenite grain boundaries remain
Grains divided into regions of fine parallel striations
Light overall with weak contrast

Related terms: Martensite Hardenability

Check with a calculator: Iron–Carbon Phase Diagram Viewer

Practice questions:

Sources

Plate (lenticular) martensite (quenched high-carbon steel)Lens-shaped plates with a central midrib crossing a white matrix

Schematic. Approx. magnification ×500 (field width 200 µm)

What you see

Lens-shaped plates lie in many directions, each with a thin line down the middle (midrib). The first large plates cross the whole grain; later ones fill the spaces between and are smaller.
White retained austenite remains between the plates. Microcracks can form where plates hit each other.

Why it looks light or dark

Retained austenite is hardly attacked and stays white; martensite plates look darker.

Etchants and appearance

Etch with nital. In tool steel quenched from too high a temperature, much white structure considered to be retained austenite appears.

How it forms (alloy and heat treatment)

Plates begin to appear above about 0.6% C and dominate above about 1% C. Quench high-carbon steel (such as tool steel) from a high temperature.
Above about 0.4% C, retained austenite starts to remain even after cooling to room temperature.

Properties

Martensite reaches its maximum hardness, HRC 66, at 0.8–1.0% C. With more carbon, retained austenite increases and the hardness does not rise further. [1]
Because of microcracks formed when plates impinge, plate martensite is avoided in most applications. [1]

Look-alikes and how to tell them apart

Lower bainite: Lower bainite needles are thin and dark, with no midrib and no white retained austenite.
Lath martensite (quenched low-carbon steel): Lath martensite is regions of striations too fine to see individually. In plate martensite each plate is visible.
Austenite (annealing twins): Annealing twins in austenite are bands with straight parallel sides, not lenses.

Identification clues

Lens-shaped plates with a central midrib
White retained austenite between plates
Large and small plates mixed

Related terms: Martensite

Check with a calculator: Iron–Carbon Phase Diagram Viewer

Practice questions:

Which statement about the martensitic transformation in steel is correct?

Sources

Tempered martensiteThe martensite shapes remain, but the matrix looks darker

Schematic. Approx. magnification ×500 (field width 200 µm)

What you see

The original lath or plate shapes and the prior austenite grain boundaries remain, but the matrix looks darker than in as-quenched martensite.
The higher the tempering temperature and the longer the time, the larger the carbide particles; the needle shapes gradually disappear and the ferrite approaches equiaxed grains (600 °C for 2 h versus 4 days).

Why it looks light or dark

Tempering decomposes martensite (excess carbon comes out as fine carbides), and the matrix comes to look darker.

Etchants and appearance

Picral is used to see the distribution of carbides in quenched-and-tempered steel. Nital also works.

How it forms (alloy and heat treatment)

Reheat quenched steel to below A1 (727 °C) — tempering. The higher the temperature and the longer the time, the softer the steel.

Properties

Example: 4340 steel oil-quenched and tempered at 200 °C, 52 HRC. [1]
Example: a tool steel with about 0.85% C (SK85) quenched from 780 °C keeps 700 HV or more when tempered up to 200 °C, and is about 400 HV after tempering at 400 °C. [4]

Look-alikes and how to tell them apart

Lath martensite (quenched low-carbon steel): As-quenched martensite is lighter with weak contrast; tempering darkens the matrix.
Lower bainite: They look very similar in the light microscope. Check carbide orientation in an electron microscope (several orientations in tempered martensite, often one in lower bainite) or use the heat treatment history.
Spheroidite (spheroidised cementite): Very long tempering at high temperature approaches spheroidite, but ordinary tempering keeps the original needle or lath shapes.

Identification clues

Martensite striations still visible
Looks darker overall
Prior austenite grain boundaries visible

Related terms: Tempering Martensite

Practice questions:

What is the main purpose of tempering a quenched steel?

Sources

Cast irons

Gray cast iron (flake graphite)Curved black flakes of graphite scattered in a grey matrix

Schematic. Approx. magnification ×100 (field width 1 mm)

What you see

Long thin black graphite flakes are scattered, curving and sometimes branching. Flakes of uniform size in random orientation are type A graphite.
The matrix is pearlite, ferrite or a mixture.

Why it looks light or dark

Graphite is visible without etching. Etching with nital reveals the pearlite matrix.

Etchants and appearance

First look at the shape and distribution of graphite unetched, then reveal the matrix with nital (2%, 10–30 s).

How it forms (alloy and heat treatment)

Cast an iron rich in carbon (above 2.14% C on the Fe–C phase diagram) with a relatively high silicon content (2.5–2.7% Si in one example) and cool it slowly or moderately through the eutectic range; during solidification the eutectic reaction then forms carbon as graphite flakes. With little silicon or fast cooling, graphite does not form readily.
Type D graphite — small flakes or points in the spaces between the prior austenite dendrites — lowers mechanical properties and is undesirable.

Properties

Example from a specification for automotive gray iron: grade G3000 with a pearlitic matrix, HB 187–241; grade G1800 with a ferritic-pearlitic matrix, HB 187 max. [4]
Gray iron has several technologically useful properties: it machines well, melts at a low temperature, resists wear and reproduces the mould shape well. [1]

Look-alikes and how to tell them apart

Ductile iron (spheroidal graphite iron): Round graphite nodules mean ductile iron. Curved black flakes mean gray iron.
Dendrites: A dendrite has a regular trunk with side arms at right angles. Graphite flakes are thin black flakes that curve at random.

Identification clues

Curved black flakes scattered
Pointed ends on the flakes
Graphite visible even unetched

Related terms: Eutectic reaction

Check with a calculator: Iron–Carbon Phase Diagram Viewer

Sources

Ductile iron (spheroidal graphite iron)Black spheres surrounded by white ferrite rings (bull's-eye)

Schematic. Approx. magnification ×100 (field width 1 mm)

What you see

Round black graphite nodules are scattered.
White ferrite surrounds each nodule like a ring (bull's-eye structure), with dark pearlite outside. The matrix is often a mixture of ferrite and pearlite.

Why it looks light or dark

Graphite is black even unetched. After nital, pearlite shows as dark islands and ferrite as the light background.

Etchants and appearance

First check the graphite shape (how round the nodules are) unetched, then reveal the matrix with nital (2%).

How it forms (alloy and heat treatment)

Adding a small amount of magnesium to cast iron melt before casting makes the graphite spherical.
As-cast matrices are often ferrite plus pearlite. Annealing gives a fully ferritic matrix; normalising gives a mostly pearlitic one.

Properties

As cast, strength and hardness rise as the amount of pearlite increases. [3]
Tensile strength is lowest when the matrix is fully ferritic. In JIS G 5502 the three digits after FCD give the tensile strength (N/mm²). [4]

Look-alikes and how to tell them apart

Gray cast iron (flake graphite): Flake-shaped graphite means gray iron.
Ferrite + pearlite (hypoeutectoid steel): The combination of white ferrite and dark pearlite is the same, but black graphite nodules mean cast iron. Steel has no graphite.
Spheroidite (spheroidised cementite): Spheroidite particles are so small that they need high magnification, and they are not surrounded by white rings.

Identification clues

Round black nodules
A white ring around each nodule
Dark pearlite outside the rings

Check with a calculator: Iron–Carbon Phase Diagram Viewer

Sources

Solidification structures

DendritesA tree-like pattern of trunks with side arms at right angles

Schematic. Approx. magnification ×50 (field width 2 mm)

What you see

Trunks (primary arms) with rows of secondary arms at nearly right angles, like tree branches or fern leaves.
The cores of the arms and the spaces between them (the last to solidify) differ in brightness.

Why it looks light or dark

When a casting cools fast, diffusion in the solid cannot keep up, so the composition changes from core to edge (segregation, coring). Regions of different composition etch at different rates, so cores and interdendritic spaces differ in brightness.

Etchants and appearance

In a Cu–4% Sn bronze, alcoholic ferric chloride clearly reveals Sn segregation.

How it forms (alloy and heat treatment)

Casting an alloy, as it solidifies from the liquid. Alloys with a wide solidification range segregate more easily (Cu–4% Sn solidifies over more than 100 °C).
Annealing reduces the segregation and gives an equiaxed grain structure (example: Cu–4% Sn annealed at 700 °C for 2 h).

Properties

Volume contraction during solidification can leave fine porosity (microporosity). [2]

Look-alikes and how to tell them apart

Widmanstätten ferrite: Widmanstätten plates are wedges growing in parallel from grain boundaries, not trunks with side arms.
Gray cast iron (flake graphite): Graphite flakes are thin black flakes, not a regular trunk with arms at right angles.

Identification clues

Side arms at right angles to a trunk
Cores and interdendritic spaces differ in brightness
As-cast structure

Related terms: Columnar grains Shrinkage cavity

Practice questions:

Sources

Eutectic (Pb–Sn and others)Alternating dark and light layers of similar thickness

Schematic. Approx. magnification ×1,000 (field width 100 µm)

What you see

Dark and light layers alternate, and the layer direction changes from one colony to the next.
Where layers are cut obliquely they look like curved lines or dots.

Why it looks light or dark

In the Pb–Sn eutectic, the dark layers are the Pb-rich α phase and the light layers the Sn-rich β phase.
At the eutectic temperature the mass fraction of α is (97.8 − 61.9)/(97.8 − 18.3) ≈ 0.45 by the lever rule, so the two kinds of layers are similar in thickness.

Etchants and appearance

The etchant depends on the alloy (alcoholic ferric chloride in an example of a Bi–Cd eutectic).

How it forms (alloy and heat treatment)

When an alloy of eutectic composition (61.9 wt% Sn for Pb–Sn) cools from the liquid, at the eutectic temperature (183 °C) the liquid changes into two solid phases at once (L → α + β), and the two phases grow cooperatively as layers.
In an off-eutectic alloy (for example Pb–30 wt% Sn), eutectic fills the spaces around grains of primary α formed earlier.

Properties

An alloy of eutectic composition solidifies entirely at a single temperature, the eutectic temperature. [3]

Look-alikes and how to tell them apart

Pearlite (eutectoid steel): Pearlite is a eutectoid formed in solid steel, and its cementite layers are much thinner than its ferrite layers (mass fraction about 0.11). The two phases of the Pb–Sn eutectic have layers of similar thickness.
Upper bainite: Upper bainite is feathery sheaves growing from grain boundaries, not regular alternating layers.

Identification clues

Dark and light layers alternate
Both kinds of layers are similar in thickness
Layer direction changes from colony to colony

Check with a calculator: Binary Phase Diagram Lab (Lever Rule Calculator)

Practice questions:

Sources

Grain shape, grain size and precipitates

Austenite (annealing twins)Bands with two straight parallel sides cross equiaxed grains

Schematic. Approx. magnification ×200 (field width 500 µm)

What you see

Inside the equiaxed grains are bands bounded by two straight parallel boundaries (annealing twins). The bands often run across the whole grain.
Brightness differs from grain to grain.

Why it looks light or dark

Differences in brightness come from different etching rates for different crystal orientations. A twin band has a different orientation from the rest of the grain, so it looks different.

Etchants and appearance

For austenitic stainless steel use glyceregia (a general etch that reveals grain boundaries and twins) or electrolytic 10% oxalic acid. Copper alloys such as brass are etched with ferric chloride solutions or ammonium persulphate.

How it forms (alloy and heat treatment)

Cold work and then anneal an FCC metal (austenitic stainless steel such as SUS304/AISI 304, copper, brass) so that it recrystallises and the grains grow.
In 304, nickel keeps the steel austenitic down to room temperature, so it stays austenite when cooled (quenching does not harden it).

Properties

Austenitic stainless steels cannot be hardened by heat treatment and are softest after rapid cooling from a high temperature; their strength is adjusted by cold work. [4]

Look-alikes and how to tell them apart

Ferrite (pure iron, very low-carbon steel): Ferrite (BCC) grains normally have no annealing twins. Bands with straight parallel sides suggest an FCC metal.
Cold-worked structure (elongated grains): Cold work elongates the grains, bends the twins and produces slip bands. Annealing twins are straight.
Precipitates (on grain boundaries and inside grains): In sensitised stainless steel, electrolytic oxalic acid etching turns the grain boundaries into thick dark ditches.

Identification clues

Bands with two straight parallel sides inside grains
Equiaxed grains
Thin grain boundaries

Related terms: Stainless steel Recrystallization Brass

Practice questions:

Sources

Precipitates (on grain boundaries and inside grains)Grain boundaries outlined by rows of dark particles or ditches, with dots inside grains

Schematic. Approx. magnification ×500 (field width 200 µm)

What you see

When precipitates line the grain boundaries, the boundaries become thick dark lines (ditches). Scattered particles may also appear inside the grains.
Sensitised austenitic stainless steel etched electrolytically in oxalic acid shows a ditch structure, in which grains are completely surrounded by ditches. A step structure (no ditches at the boundaries) is judged not sensitised, and a dual structure (ditches on some boundaries, but no grain completely surrounded) is in between. The drawing shows a ditch structure.

Why it looks light or dark

Precipitates and their surroundings etch at a different rate from the matrix, so the grain boundaries stand out as lines or rows of dots.

Etchants and appearance

Electrolytic 10% oxalic acid (ASTM A262 Practice A) is used to classify step, dual and ditch structures. It also reveals sigma phase and carbides.

How it forms (alloy and heat treatment)

Heating and holding a supersaturated solid solution makes a second phase precipitate on grain boundaries and inside grains.
When austenitic stainless steel is heated to about 500–800 °C (as in a weld heat-affected zone), chromium carbides precipitate on the grain boundaries and the steel becomes sensitised.
The light microscope resolves only features larger than about 500 nm, so the fine precipitates responsible for precipitation hardening are observed with an electron microscope.

Properties

A ditch structure, with grains completely surrounded by ditches, is classed as sensitised and susceptible to intergranular corrosion and intergranular stress corrosion cracking. [1][2]

Look-alikes and how to tell them apart

Austenite (annealing twins): In stainless steel that is not sensitised, grain boundaries are thin lines and annealing twins are visible.
Spheroidite (spheroidised cementite): In spheroidite, round cementite particles are spread evenly inside the grains too, and the boundaries do not become continuous ditches.
Proeutectoid cementite + pearlite (hypereutectoid steel): The network in hypereutectoid steel is white cementite, with pearlite lamellae inside the grains.

Identification clues

Thick dark ditches or rows of particles on grain boundaries
Sparse dots inside the grains
No lamellae or needles inside the grains

Practice questions:

Sources

Duplex grain size (non-uniform grain size)Large grains mixed with clusters of small grains

Schematic. Approx. magnification ×100 (field width 1 mm)

What you see

Grains of very different sizes are mixed: coarse grains scattered among fine ones, fine grains surrounding coarse grains like a chain (necklace structure), or bands of different grain size alternating (often elongated along the working direction).

Why it looks light or dark

Grain boundaries and grain-to-grain differences in brightness are the same as in ordinary equiaxed grains.

Etchants and appearance

Use the usual grain-boundary etchant for the metal (nital for steel).

How it forms (alloy and heat treatment)

When the grain size distribution differs from part to part. It is described either as a random mixture of sizes or as a topological pattern (necklace, bands, variation across the section).

Properties

A single average grain size number does not describe a duplex specimen properly and may even report a size that does not exist in it. ASTM E1181 describes the type and proportions of duplex grain sizes. [1]

Look-alikes and how to tell them apart

Ferrite (pure iron, very low-carbon steel): An ordinary equiaxed structure has roughly uniform grain size that one grain size number can describe.
Cold-worked structure (elongated grains): In a cold-worked structure grains are elongated in one direction. In a duplex structure grains may be equiaxed but differ in size.

Identification clues

Coarse grains mixed with clusters of fine grains
Equiaxed grain shapes
Grain size changes strongly from place to place

Sources

ASTM E1181-02(2023) Standard Test Methods for Characterizing Duplex Grain Sizes (summary)

Cold-worked structure (elongated grains)Grains elongated in the working direction, with parallel slip bands inside

Schematic. Approx. magnification ×200 (field width 500 µm)

What you see

Grains are elongated in the rolling or drawing direction, and parallel lines (slip bands) appear inside them. In places slip bands of different directions cross.
Twins present before deformation become bent (example: brass).

Why it looks light or dark

Slip bands are where deformation has concentrated; they show as thin parallel lines.

Etchants and appearance

Copper alloys are etched with alcoholic ferric chloride; steel with nital.

How it forms (alloy and heat treatment)

Plastic working (rolling, drawing, compression) below the recrystallisation temperature — cold work.
Annealing after working restores strain-free equiaxed grains through recovery and recrystallisation.

Properties

Plastic deformation raises the dislocation density and lowers dislocation mobility, which causes work hardening (about 10¹⁰ m⁻² annealed, about 10¹⁶ m⁻² cold worked). [3]

Look-alikes and how to tell them apart

Duplex grain size (non-uniform grain size): A duplex structure has equiaxed grains, even though their sizes differ.
Austenite (annealing twins): Annealing twins are bands with straight parallel sides. In a cold-worked structure twins are bent, and slip bands are thin parallel lines inside the grains.
Lath martensite (quenched low-carbon steel): Lath martensite striations are aligned region by region inside the original grains, and the grains themselves are not elongated.

Identification clues

Grains elongated in one direction
Parallel slip bands inside the grains
Grain boundaries still visible

Related terms: Work hardening Recovery Recrystallization

Practice questions:

Sources

Grain size (grain size number and mean grain size)

Grain size is expressed by the standard grain size number G. Increasing the number by one doubles the number of grains in the same area (finer grains, larger number). Move the slider to redraw the grains at that grain size and show the conversions.

Standard JIS G 0551 ASTM E112

Grain size number G = 5

The idea of the comparison method

Grains one number apart, drawn at the same magnification. In the comparison method you match the specimen against standard charts like these.

Measurement methods

Comparison method: View the etched specimen at 100× and compare it with a set of standard charts of different grain sizes, choosing the closest number. It is quick, but ratings by one person vary by about ±0.5 and tend to come out somewhat coarser. When comparing at a magnification M other than 100×, add Q = 6.6439 × log₁₀(M/100) to the number read.
Planimetric (Jeffries) method: Count the grains in a known area: grains completely inside the frame count as 1, grains cut by the frame as 1/2, and the corner grains together as 1, giving the number of grains per mm².
Intercept method: Draw test lines and count the points P where they cross grain boundaries. From the number of intersections per millimetre of test line, P_L = P/L, the mean lineal intercept length is ℓ̄ = 1/P_L, which converts to a grain size number.

Conversion formulas

ASTM E112: N = 2^(G−1) (N is the number of grains per square inch at 100×)
ASTM E112: G = 3.321928 × log₁₀ N_A − 2.954 (N_A is the number of grains per mm² at 1×)
ASTM E112, intercept method: G = −6.643856 × log₁₀ ℓ̄ − 3.288 (ℓ̄ is the mean lineal intercept in mm)
JIS G 0551 (ISO 643): m = 8 × 2^G, i.e. G = log₂ m − 3 (m is the number of grains per mm²)

JIS G 0551 follows ISO 643 and defines m = 8 × 2^G grains per mm². ASTM E112 gives 15.50 × 2^(G−1) = 7.75 × 2^G grains per mm², so counting the same structure gives a JIS number about 0.05 lower than the ASTM number. The intercept conversion (ℓ̄ = 0.320 mm at G = 0) is the same in both standards. The grains in the drawing are polygons generated in the plane (a Voronoi tessellation) that continue beyond the frame. The number from the planimetric count scatters more when fewer grains are counted. The conversion of the mean intercept to a grain size number assumes the shapes of plane sections through three-dimensional equiaxed grains, so for these fairly uniform polygons the intercept method can give a slightly higher number.

Sources

Identification practice

Look at the schematic and choose the microstructure. After you answer you get the identification clues and how to tell it from the one you chose. The drawing changes a little every time.

Which microstructure is this?

Show missed microstructures more often

Your record stays in this browser and is never sent anywhere. A link copied before answering lets the recipient solve the same drawing; a link copied after answering opens your choice and the result.

FAQ

Are these images micrographs?

No. They are all procedurally drawn schematics: grain boundaries are a Voronoi tessellation, pearlite and eutectics are layers, martensite is laths or plates, and graphite is simple flakes or spheres, with random variation every time they are drawn. Light and dark follow the typical etched appearance, but real micrographs can differ in colour, contrast and shape.

Why does pearlite look dark?

Etching dissolves ferrite first, so the thin layers of hard cementite stand slightly proud. Both phases look white in the light microscope, but the shadows of the steps become dark lines, and the finer the layers, the darker pearlite looks overall.

When should I use nital and when picral?

Nital reveals ferrite grain boundaries, so use it to look at grain size or the amount of ferrite. Picral hardly reveals ferrite grain boundaries but brings out cementite clearly, so use it for pearlite lamellae and for the distribution of spheroidised or tempering carbides. Combine them to see both.

Can lower bainite and tempered martensite be told apart in the light microscope?

Often not. Both are acicular structures that etch dark; the difference lies in the orientation of the carbides inside the plates (often one orientation in lower bainite, several in tempered martensite), which needs an electron microscope. The heat treatment (isothermal transformation, or quenching followed by tempering) is an important clue.

Where is my practice record stored?

Your streak and the structures you missed are stored only in this browser (localStorage) and are never sent to a server. The question and your answer stay in the URL, so sending the link opens the same question and result.