DOE researchers develop new super-alloy

Scientists at the Department of Energy's Oak Ridge National Laboratory and the University of Tennessee, Knoxville, have found a way to simultaneously increase an alloy's strength and ductility, a measure of a material's ability to stretch and be hammered thin without breaking.

"A holy grail of structural materials has long been, 'how do you simultaneously enhance strength and ductility?'" said Easo George, principal investigator of the study and Governor's Chair for Advanced Alloy Theory and Development at ORNL and UT. "Defeating the strength–ductility trade-off will enable a new generation of lightweight, strong, damage-tolerant materials."

Ductility measures how much a material can elongate before fracturing and whether that fracturing will be graceful or catastrophic. In even simpler terms, the higher the strength and ductility, the tougher the material.

If structural materials could be made both stronger and more ductile, components of cars, planes, power plants, buildings, and bridges could be built using less material and still come out far more resilient. In addition, lighter-weight vehicles would be more energy-efficient to make and operate, and tougher infrastructure would be more durable.

The Oak Ridge National Lab researchers found that they could increase both strength and ductility by introducing tiny precipitates into its matrix and tuning their size and spacing.

"We put nanoprecipitates into a transformable matrix and carefully controlled their attributes which in turn controlled when and how the matrix transformed," said Ying Yang, co-principal investigator of ORNL who conceived and led the study published in the journal Nature. "In this material, we intentionally induced the matrix to have the capability to undergo a phase transformation."

The alloy contains four major elements – iron, nickel, aluminum, titanium – that form the matrix and precipitates, and three minor elements – carbon, zirconium, and boron – that limit the size of grains, individual metallic crystals.

The precipitates are solids that separate from the metal mixture as the alloy cools.

The research team carefully kept the composition of the matrix and the total amount of nanoprecipitates the same in different samples. However, they varied precipitate sizes and spacings by adjusting the processing temperature and time.

"The strength of a material usually depends on how close the precipitates are to each other," said George. "When you make them a few nanometers [billionths of a meter] in size, they can be very closely spaced. The more closely spaced they are, the stronger the material gets."

While nanoprecipitates in conventional alloys can make them super strong, they also make them very brittle. The team's alloy avoids this brittleness because the precipitates perform a second useful function: by spatially constraining the matrix, they prevent it from transforming during a thermal quench, a quick immersion in water that calls the alloy to room temperature.

It was found that this process increased strength by 20%-90% and elongation (ductility) by 300%.

"Adding precipitates to block dislocations and make materials ultra-strong is well known," added George. "What is new here is that adjusting the spacing of the precipitates also affects phase transformation propensity, which allows multiple deformation mechanisms to be activated as needed to enhance ductility."

The study also revealed a surprising reversal of the normal strengthening effect of nanoprecipitates: an alloy with coarse, widely spaced precipitates. This reversal happens when the nanoprecipitates become so tiny and tightly packed that the phase transformation is essentially shut down during straining of the material, not unlike the transformation suppressed during thermal quenching.

This essentially means that the research team found a way to mimic the way metals "freeze" their molecular structure without submerging it in water and ultimately created a tougher material.

"This research introduces a new family of structural alloys," said Yang. "Precipitate characteristics and alloy chemistry can be precisely tailored to activate deformation mechanisms exactly when needed to thwart the strength–ductility trade-off."

Next, the team plans to investigate the additional factors and deformation mechanisms to identify the combinations that could further improve this new alloy's mechanical properties.

It turns out there is a lot of room for improvement.

"Today's structural materials realize but a small fraction – perhaps only 10% – of their theoretically capable strengths," finished George. "Imagine the weight savings that would be possible in a car or an airplane – and the consequent energy savings – if this strength could be doubled or tripled while maintaining adequate ductility."