New model should expedite development of temperature-stable nano-alloys

September 16, 2013 by Matt Shipman
The model correctly predicted the material on the left would not be stable at high temperatures and that the material on the right would retain its nanoscale grain size. The material on the left is Fe-8Ni-4Zr, meaning it is an iron-base alloy with 8 atomic percent nickel and 4 atomic percent zirconium. The grains grew to micron size at 900C. The material on the right is Fe-10Cr-4Zr, meaning it is an iron-base alloy with 10 atomic percent chromium and 4 atomic percent zirconium. This material retained nanoscale grains at 900C. Credit: Mostafa Saber, North Carolina State University

Researchers from North Carolina State University have developed a new theoretical model that will speed the development of new nanomaterial alloys that retain their advantageous properties at elevated temperatures.

Nanoscale materials are made up of tiny crystals, or grains, that are less than 100 in diameter. These materials are of interest to researchers, designers and manufacturers because two materials can have the same but very different mechanical properties depending on their grain size. For example, materials with nanoscale grains can be harder and stronger than chemically identical materials with larger grains.

But widespread use of has long been handicapped by the tendency of nanoscale grains to grow when exposed to elevated temperatures – thereby losing their desired .

This is a problem because creating bulk materials from powdered nanomaterials involves exposure to , and even nanomaterials made using other techniques may be exposed to elevated temperatures. The grains in some nanomaterials can even grow – and lose their desired properties – when exposed to room temperature for an extended period of time.

A team of NC State researchers decided to tackle the problem by exploring a concept that had been discussed in the research community for some time: stabilizing by introducing small amounts of an additional element. The idea is that this additional element would serve as a stabilizing agent, migrating to the – or interfaces between grains – and preventing the grains from growing at elevated temperatures. Implementing that concept had been daunting, since there are thousands of possible combinations of these elements.

To turn that idea into a practical solution, the researchers developed a theoretical model to identify suitable candidates that can be used as stabilizing agents.

The theoretical model focuses on alloys that consist of two elements, such as iron and chromium, then allows users to see what would happen if a third element is added to the mix. If users plug the atomic size and thermodynamic properties of each element into the model, the model predicts the grain size of the alloy at any given temperature.

"This model allows anyone to design alloys in a targeted and effective way without having to resort to a trial-and-error approach," says Dr. Ron Scattergood, a professor of materials science and engineering at NC State and senior author of a paper describing the work. "And our experimental results confirm the accuracy of the model."

"We are already using the model in our investigations into lightweight aluminum alloys and high-temperature alloys for nuclear energy applications," says Dr. Mostafa Saber, lead author of the study and a postdoctoral research scholar in materials science and engineering at NC State.

Explore further: New super strong alloy discovered

More information: The paper, "A Predictive Model for Thermodynamic Stability of Grain Size in Nanocrystalline Ternary Alloys," was published online Sept. 12 in the Journal of Applied Physics.

This work presents a model for evaluating thermodynamic stabilization of ternary nanocrystalline alloys. It is applicable to alloy systems containing strongly segregating size misfit solutes with a significant enthalpy of elastic strain and/or immiscible solutes with a positive mixing enthalpy. On the basis of a regular solution model, the chemical and elastic strain energy contributions are incorporated into the mixing enthalpy [delta]Hmix, and the mixing entropy [delta]Smix is obtained using the ideal solution approximation. The Gibbs mixing free energy [delta]Gmix is minimized with respect to simultaneous variations in grain size and solute segregation parameters. The Lagrange multiplier method is used to obtain numerical solutions for the minimum [delta]Gmix corresponding to an equilibrium grain size for given alloy compositions. The numerical solutions will serve as a guideline for choosing solutes and assessing the possibility of thermodynamic stabilization. The temperature dependence of the nanocrystalline grain size and interfacial solute excess can be evaluated for selected ternary systems. Model predictions are presented using available input data for a wide range of solvent-solute combinations. The model predictions are compared to experimental results for Cu-Zn-Zr, Fe-Cr-Zr and Fe-Ni-Zr alloys where thermodynamic stabilization might be effective.

Related Stories

New super strong alloy discovered

September 8, 2010

( -- International team of researchers has discovered a new super-strength light alloy and had their key findings published in Nature Communications.

It's the fineness of the grind

July 23, 2013

The properties of nanomaterials could be easier to predict in future. Scientists from the Max Planck Institute for Intelligent Systems in Stuttgart have ground metal into continuously finer powders in steps and prepared a ...

Recommended for you

Turbulence in bacterial cultures

November 30, 2015

Turbulent flows surround us, from complex cloud formations to rapidly flowing rivers. Populations of motile bacteria in liquid media can also exhibit patterns of collective motion that resemble turbulent flows, provided the ...

Test racetrack dipole magnet produces record 16 tesla field

November 30, 2015

A new world record has been broken by the CERN magnet group when their racetrack test magnet produced a 16.2 tesla (16.2T) peak field – nearly twice that produced by the current LHC dipoles and the highest ever for a dipole ...

CERN collides heavy nuclei at new record high energy

November 25, 2015

The world's most powerful accelerator, the 27 km long Large Hadron Collider (LHC) operating at CERN in Geneva established collisions between lead nuclei, this morning, at the highest energies ever. The LHC has been colliding ...

'Material universe' yields surprising new particle

November 25, 2015

An international team of researchers has predicted the existence of a new type of particle called the type-II Weyl fermion in metallic materials. When subjected to a magnetic field, the materials containing the particle act ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.