Nav: Home

How interstitial ordering affects high-strength steels

May 14, 2020

For the fundamental basic composition of any steel - iron and carbon - the concentration and ordering of carbon atoms and their interaction with the iron host lattice in martensitic steels was analysed by a team of scientists from the Max-Planck-Institut für Eisenforschung (MPIE) and the Ruhr-Universität Bochum (RUB). The scientists examined the mechanisms of collective interstitial ordering in Fe-C steels and determined how anharmonicity and segregation affect the ordering mechanism and consequently, the material's performance. Their recent findings were published in Nature Materials.

Where carbon atoms go

„When carbon atoms enter the iron host lattice of martensitic steels, they diffuse between the iron atoms and do not take over the iron atoms' positions in the host lattice. Nevertheless, they create strain fields influencing the whole lattice. Understanding the mechanism of the resulting interstitial ordering is a key to designing ultra-high performance steels as they gain their strength from the martensite formation, thus, from the collective interstitial ordering", explains Dr. Tilmann Hickel. Hickel is head of the group „Computational Phase Studies" at the MPIE and was the main supervisor of Dr. Xie Zhang, the first author of the publication. Each interstitial atom, due to its size and chemical interaction with atoms of the host lattice, creates a local strain field that displaces its neighbouring host atoms away from their original lattice positions. „Imagine inserting a wooden stick into sand at the beach and watching how the stick displaces the grains of sand surrounding it. The same happens when we add carbon to the iron host lattice. The carbon interstitials, find their way through the host lattice, order in energetically favourable places and distort and harden the previous structure", explains Hickel. A high concentration of interstitials leads to ordering/disordering phenomena and lattice distortions, thus influencing the steels' bulk performance.

The research team identified two components that influence the interstitial ordering. The first one results from the anharmonicity caused by the strain fields in the Fe lattice. „Due to this anharmonicity, the critical C concentration for an order-disorder transformation is decreased. To understand the displacement of the Fe atoms at different distances, we must consider the anharmonic contribution in the first neighbour position of a C interstitial", explains Dr. Jutta Rogal from the Interdisciplinary Centre for Advanced Materials Simulation of the Ruhr-Universität Bochum.

The second component that influences the interstitial ordering is the segregation of C to extended defects. This segregation takes place at low C concentrations and is suppressed at high C concentrations due to a lowering of the C chemical potential in ordered martensite. The chemical potential of C in Fe-C martensite gradually increases with increasing C concentration until 0.8 at.% are reached. Then it rapidly decreases due to the order-disorder transition.

Order-disorder transition

Both components, the level of anharmonicity and the segregation behaviour, are decisive for the order-disorder transition. „An unexpected outcome of the study was that it is not sufficient to analyse only the arrangement of the carbon atoms in bulk. Rather, a strong competition between the carbon concentration in the bulk and its segregation to extended defects occurs. Only with this insight it was possible to gain a comprehensive understanding of the order-disorder transition. This competition decreases with an increasing concentration of carbon interstitials, as extended defects can incorporate interstitials only to a limited amount. The exact concentration depends on the density of the defects. In our calculations and confirmed by experiments, disordered martensite is triggered by a carbon concentration in the range between 0.8 at.% and 2.6 at.%. Above 2.6 at.% ordered martensite is formed, which provides a superior strength to steels. Below 0.8 at.%, carbon atoms segregate to dislocations in grain boundaries", explains Professor Jörg Neugebauer, director of the department Computational Materials Design at the MPIE. The theoretical calculations were confirmed by transmission electron microscopy and atom probe tomography measurements performed at the Ruhr-Universität Bochum.

In general, the exact critical C concentration depends on the microstructure of the material and the binding energy between C and a specific extended defect. The shown critical concentration range of 0.8 at.% and 2.6 at.% is not universal, but depends on the sample and its extended defects. However, the critical concentrations can be precisely calculated if a) the exact binding energy between C and the extended defect, and b) the maximum C concentration that can be included by the extended defect, are known. The MPIE and RUB team showed the decisive role anharmonicity and segregation play regarding the mechanism of interstitial ordering, using the Fe-C alloys as a model for other relevant systems. Including anharmonic effects into order-disorder phase transitions provides a new level of predictive materials modelling, paving the way to designing ultra-high-performance steels.

The research was supported by the German Research Foundation within the DFG-ANR project C-TRAM.

Original publication

Xie Zhang, Hongcai Wang, Tilmann Hickel, Jutta Rogal, Yujiao Li, Jörg Neugebauer: Mechanism of collective interstitial ordering in Fe-C alloys. In: Nature Materials, 2020, DOI: 10.1038/s41563-020-0677-9

Press contact

Yasmin Ahmed Salem
Press and Public Relations Officer
Max-Planck-Institut für Eisenforschung
Tel.: +49 211 6792 722

Ruhr-University Bochum

Related Carbon Articles:

Can wood construction transform cities from carbon source to carbon vault?
A new study by researchers and architects at Yale and the Potsdam Institute for Climate Impact Research predicts that a transition to timber-based wood products in the construction of new housing, buildings, and infrastructure would not only offset enormous amounts of carbon emissions related to concrete and steel production -- it could turn the world's cities into a vast carbon sink.
Investigation of oceanic 'black carbon' uncovers mystery in global carbon cycle
An unexpected finding published today in Nature Communications challenges a long-held assumption about the origin of oceanic black coal, and introduces a tantalizing new mystery: If oceanic black carbon is significantly different from the black carbon found in rivers, where did it come from?
First fully rechargeable carbon dioxide battery with carbon neutrality
Researchers at the University of Illinois at Chicago are the first to show that lithium-carbon dioxide batteries can be designed to operate in a fully rechargeable manner, and they have successfully tested a lithium-carbon dioxide battery prototype running up to 500 consecutive cycles of charge/recharge processes.
How and when was carbon distributed in the Earth?
A magma ocean existing during the core formation is thought to have been highly depleted in carbon due to its high-siderophile (iron loving) behavior.
New route to carbon-neutral fuels from carbon dioxide discovered by Stanford-DTU team
A new way to convert carbon dioxide into the building block for sustainable liquid fuels was very efficient in tests and did not have the reaction that destroys the conventional device.
How much carbon the land can stomach with more carbon dioxide in the air
Researchers from 28 institutions in nine countries succeeded in quantifying carbon dioxide fertilization for the past five decades, using simulations from 12 terrestrial ecosystem models and observations from seven field carbon dioxide enrichment experiments.
'Charismatic carbon'
According to the Intergovernmental Panel on Climate Change (IPCC), addressing carbon emissions from our food sector is absolutely essential to combatting climate change.
Extreme wildfires threaten to turn boreal forests from carbon sinks to carbon sources
A research team investigated the impact of extreme fires on previously intact carbon stores by studying the soil and vegetation of the boreal forest and how they changed after a record-setting fire season in the Northwest Territories in 2014.
Can we still have fun if the UK goes carbon neutral?
Will Britain going carbon neutral mean no more fun? Experts from the University of Surrey have urged local policy makers to put in place infrastructure that will enable people to enjoy recreation and leisure while keeping their carbon footprint down.
Could there be life without carbon? (video)
One element is the backbone of all forms of life we've ever discovered on Earth: carbon.
More Carbon News and Carbon Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Making Amends
What makes a true apology? What does it mean to make amends for past mistakes? This hour, TED speakers explore how repairing the wrongs of the past is the first step toward healing for the future. Guests include historian and preservationist Brent Leggs, law professor Martha Minow, librarian Dawn Wacek, and playwright V (formerly Eve Ensler).
Now Playing: Science for the People

#565 The Great Wide Indoors
We're all spending a bit more time indoors this summer than we probably figured. But did you ever stop to think about why the places we live and work as designed the way they are? And how they could be designed better? We're talking with Emily Anthes about her new book "The Great Indoors: The Surprising Science of how Buildings Shape our Behavior, Health and Happiness".
Now Playing: Radiolab

The Third. A TED Talk.
Jad gives a TED talk about his life as a journalist and how Radiolab has evolved over the years. Here's how TED described it:How do you end a story? Host of Radiolab Jad Abumrad tells how his search for an answer led him home to the mountains of Tennessee, where he met an unexpected teacher: Dolly Parton.Jad Nicholas Abumrad is a Lebanese-American radio host, composer and producer. He is the founder of the syndicated public radio program Radiolab, which is broadcast on over 600 radio stations nationwide and is downloaded more than 120 million times a year as a podcast. He also created More Perfect, a podcast that tells the stories behind the Supreme Court's most famous decisions. And most recently, Dolly Parton's America, a nine-episode podcast exploring the life and times of the iconic country music star. Abumrad has received three Peabody Awards and was named a MacArthur Fellow in 2011.