A New Breakthrough in 3D Printing: CrCoNi and High Entropy Alloys With High Fracture Toughness

CrCoNi sits in the middle of the entropy scale. It is roughly equal parts chromium, cobalt and nickel by atomic fraction, which puts its configurational entropy between a conventional alloy and a five-element high entropy alloy. The single-phase face-centered cubic structure is what gives it the combination engineers care about: high strength, good ductility and fracture toughness that does not collapse as the temperature drops.

The reported result that drew attention was fracture toughness measured at 20 K, roughly minus 253 °C. At that temperature most structural metals become brittle. CrCoNi did not. Researchers attributed this to deformation twinning and a transformation-assisted mechanism that keeps dissipating energy ahead of a crack tip instead of letting it run.

For a shop floor, the useful takeaway is narrower. A material that stays tough at cryogenic temperature is a candidate for parts that see thermal cycling and impact: cryogenic valve bodies, superconducting magnet supports, cold-stage hardware, and fracture-critical brackets that cannot be allowed to shatter.

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  Face-centered cubicOne phase, no brittle intermetallic network to crack along.
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  Twinning-dominatedDeformation twinning adds toughness as strain rises.
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  Cryogenic rangeToughness data reported down to 20 K, not just room temperature.

CrCoNi and most equiatomic high entropy alloys print on standard laser powder bed fusion equipment. They are not exotic to the machine. The laser absorbs well into the powder, the melt pool is stable, and the alloy does not demand a heated build plate the way some crack-prone nickel alloys do. Layer thickness of 30 to 60 μm and a laser power in the 200 to 400 W range is a normal starting envelope.

The problem is residual stress, not printability. As-printed CrCoNi carries a large amount of locked-in stress from the rapid thermal cycle. Thin walls and tall unsupported sections will distort or lift off the plate if the scan strategy and support layout are wrong. Stress relief before wire EDM removal from the build plate is standard practice, not optional.

Porosity is the second issue. Because these alloys are usually gas-atomized to tight chemistry control, powder quality drives defect density more than machine settings do. A part that is 99.5 percent dense may still fail a fatigue or fracture test if the remaining porosity sits in a critically stressed region. HIP after printing closes most internal porosity and homogenizes the microstructure, but it also changes the grain structure, so the toughness value from an as-built coupon will not match the HIPed part.

DED and binder jetting are also used with these alloys. DED suits large near-net shapes and repair of worn surfaces. Binder jetting gives lower residual stress and higher build rates, but sintering densification is a separate process step that has to be controlled independently.

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  As-built stressStress relief before plate removal is standard.
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  Powder chemistryAtomization quality drives defect density more than settings.
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  HIP trade-offCloses porosity but changes grain structure and toughness.

A printed CrCoNi part is a near-net blank. As-built surfaces typically sit between Ra 8 and Ra 15 μm, and critical interfaces do not accept that. Sealing faces, bearing bores, threaded holes and mating flanges get machined after printing. This is where most of the tolerance in a printed metal assembly actually comes from.

We machine printed blanks on the same 5-axis centers used for wrought stock. Sixteen simultaneous 5-axis machining centers and a Ø400 mm rotary table handle contoured sealing faces and compound-angle ports in one setup, which matters because re-fixturing a printed part risks distorting it. Holding ±0.005 mm on a machined bore is routine once the blank has been stress relieved.

The practical sequence is: print, stress relief, HIP if the application needs it, remove from plate, machine datums first, then machine functional features from those datums. Skipping the datum step is the most common cause of a printed part that measures wrong on every subsequent operation.

Surface finish targets depend on the feature. Ra 0.8–1.6 μm covers most sealing and sliding surfaces. Ra 0.2–0.8 μm is for optical and vacuum interfaces. As-machined Ra 1.6–3.2 μm is fine for non-critical exterior geometry.

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  As-built roughnessRa 8–15 μm on downward and vertical faces.
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  Datum firstMachine datums, then reference every later feature to them.
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  Sealing facesAlways machined, never left as-printed.

Cost is the first filter. CrCoNi and high entropy alloy powder costs far more than 316L or Ti-6Al-4V, and the powder is often the largest line item in a build. If a part does not need cryogenic toughness or a specific combination of strength and ductility, a standard stainless or titanium grade will do the job for a fraction of the material cost.

Size is the second filter. Most metal powder bed systems build within a few hundred millimeters. A 4,000 mm frame, a long extrusion or a large weldment is not a printing candidate regardless of alloy. Those go to machining or fabrication.

Service temperature is the third. CrCoNi's advantage shows up at low temperature. At elevated temperature, above roughly 600 °C, oxidation and microstructural stability become the limiting factors, and a nickel superalloy such as Inconel is usually the better answer.

Finally, consider whether the property you need can come from geometry instead of material. If a bracket is failing in fatigue, a fillet change or a topology optimization on a cheaper alloy may solve it. Reaching for an exotic printed alloy should be the last step, not the first.

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  CostPowder price dominates the build cost.
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  Build envelopeLarge frames exceed powder bed limits.
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  Hot serviceAbove 600 °C, superalloys win.

Qualification starts with the powder lot. Chemistry, particle size distribution and oxygen content should be documented before the build, because a change in powder supplier can shift the as-built microstructure. Coupons printed from the same lot and the same build as the part are the only honest reference for mechanical properties.

Dimensional inspection comes next. CMM reports on the machined features, plus a first-article report, cover the drawing. For internal features that cannot be measured, CT scanning is the practical option, though it adds lead time and cost.

For fracture-critical parts, mechanical testing on witness coupons is standard: tensile, hardness and, where the application demands it, fracture toughness at the service temperature. We do not run a materials lab in house, so we coordinate testing through qualified third-party labs and pass the reports through with the shipment. Final inspection is 100 percent before shipment, with reports available on request.

Document everything that changed between the coupon build and the production build. Laser power, scan speed, layer thickness, powder reuse count. Those four variables explain most of the variation engineers see between a successful prototype and a production run.

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  Powder lot recordChemistry, PSD and oxygen before the build.
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  Witness couponsSame lot, same build as the part.
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  Process logTrack the four variables that shift properties.