Which materials show a ductile-to-brittle transition?

Body-centred-cubic metals — notably ferritic and carbon steels — show a sharp transition to brittle cleavage as temperature drops. Face-centred-cubic metals such as austenitic stainless steels, aluminium and copper generally do not, which is why austenitic grades are chosen for cryogenic service.

How is the transition temperature defined?

There is no single definition. Common criteria are an absorbed-energy level (e.g. 27 J or 40 J on the Charpy curve), a fracture-appearance transition temperature (FATT, the temperature for 50% shear/ductile fracture), or a lateral-expansion value. Always state which criterion a quoted DBTT uses.

Does refining grain size help?

Yes — and uniquely so. Grain refinement is the one strengthening mechanism that simultaneously raises yield strength and lowers the transition temperature, improving both strength and low-temperature toughness. Most other strengthening routes raise DBTT.

Fracture · guide

The ductile-to-brittle transition

A steel that is tough at room temperature can shatter like glass in the cold. The ductile-to-brittle transition is behind some of history's most infamous fractures, and it governs material choice for anything operating at low temperature. Here is what controls it.

The transition curve

Plot Charpy impact energy against temperature for a ferritic steel and you get an S-curve: a high, ductile upper shelf, a steep transition, and a low, brittle lower shelf where failure is by cleavage. The drop happens because cleavage fracture stress is roughly temperature-independent while the yield strength rises as it gets colder — below the crossover, the steel reaches its cleavage stress before it can yield.

Defining a transition temperature

Because the transition is a band, not a point, design codes pin it with a criterion: an energy level (commonly 27 J or 40 J), a fracture-appearance transition temperature (FATT, 50% ductile fracture), or a lateral expansion. A quoted DBTT is meaningless without saying which.

These are correlative impact criteria. For a quantitative, transferable fracture toughness in the transition, the Master Curve (ASTM E1921) indexes the toughness by a reference temperature T₀ with statistical bounds.

What shifts the transition

Toward brittle (higher DBTT): carbon and phosphorus, coarse grain size, higher strain rate, thicker section (more constraint/triaxiality), and neutron irradiation in reactor steels. Toward ductile (lower DBTT): grain refinement, nickel additions, and lower interstitial content. Manganese and a low sulfur/phosphorus chemistry generally help.

cold + coarse grain + high strain rate + triaxiality ⇒ cleavage

Open the calculatorDBTT / transition-curve tool →Fit the Charpy transition curve, read the 27 J / 40 J / FATT transition temperatures, and link to the Master Curve toughness.

Designing around it

The rule is simple: keep the minimum design metal temperature safely above the transition with margin, or choose a material that has no transition in the service range (austenitic stainless, aluminium, nickel alloys for cryogenic duty). Account for section thickness and dynamic loading, both of which push the effective transition up.

Frequently asked

Which materials show a ductile-to-brittle transition?: Body-centred-cubic metals — notably ferritic and carbon steels — show a sharp transition to brittle cleavage as temperature drops. Face-centred-cubic metals such as austenitic stainless steels, aluminium and copper generally do not, which is why austenitic grades are chosen for cryogenic service.
How is the transition temperature defined?: There is no single definition. Common criteria are an absorbed-energy level (e.g. 27 J or 40 J on the Charpy curve), a fracture-appearance transition temperature (FATT, the temperature for 50% shear/ductile fracture), or a lateral-expansion value. Always state which criterion a quoted DBTT uses.
Does refining grain size help?: Yes — and uniquely so. Grain refinement is the one strengthening mechanism that simultaneously raises yield strength and lowers the transition temperature, improving both strength and low-temperature toughness. Most other strengthening routes raise DBTT.

References

ASTM E23, "Standard Test Methods for Notched Bar Impact Testing of Metallic Materials."
ASTM E1921, "Reference Temperature T₀ for Ferritic Steels in the Transition Range" (Master Curve).
T.L. Anderson, "Fracture Mechanics: Fundamentals and Applications," CRC Press.
J.F. Knott, "Fundamentals of Fracture Mechanics," Butterworths.

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