Unlocking how microstructure drives phase changes in austenitic steels

C Cai, G Spartacus, A Das, KS Shanks, W Mu, P Hedström

How does grain size shape the way advanced steels respond to extreme conditions? Using high-energy X-ray diffraction at the FAST beamline, researchers uncovered how the parent austenite grain size in stainless steel can either suppress or accelerate the transformation to martensite. The findings reveal new pathways for tuning microstructure to improve the performance of these widely used structural materials.

Schematic illustration of the in situ tensile loading SXRD data experimental setup.

Schematic illustration of the in situ tensile loading SXRD data experimental setup at the FAST beamline. 

What is the discovery? 

Austenitic stainless steels combine high strength and toughness, making them useful for automotive applications. These steels can undergo a crystallographic phase transformation from austenite to martensite, triggered either through cooling below the martensitic start temperature or by mechanical deformation. This martensitic transformation (MT) impacts the material’s mechanical performance. The tendency to undergo MT is known to be influenced by the parent austenite grain size (PAGS), but the details of this correlation are not fully understood. Here, researchers from the KTH Royal Institute of Technology and Luleå University of Technology in Sweden used high-energy x-ray diffraction at CHESS’ Forming and Shaping Technology (FAST) beamline to investigate the influence of PAGS on MT in Fe-18Cr-12Ni. These measurements showed that smaller PAGS suppresses MT in samples flash-cooled to cryogenic temperatures, but promotes MT during room-temperature mechanical deformation. Based on this, the team was able to draw conclusions about the impact of PAGS on martensite nucleation vs growth rate. 

Spatial maps showing grain morphology and orientation, along with histograms of grain sizes, for three Fe-18Cr-12Ni alloys (labelled A, B, and C) with increasing PAGS, achieved by increasing the annealing temperature during sample fabrication.
Spatial maps showing grain morphology and orientation, along with histograms of grain sizes, for three Fe-18Cr-12Ni alloys (labelled A, B, and C) with increasing PAGS, achieved by increasing the annealing temperature during sample fabrication.

 

Plots
Left: diffraction patterns from samples A, B, and C after flash-cooling in liquid nitrogen. Samples A and B are fully austenitic after cooling, whereas C – with the largest PAGS – shows a mix of austenitic (γ) and martensitic (α’) peaks. Right: martensite fraction as a function of strain for samples A, B, and C as measured via in-situ x-ray diffraction during mechanical loading of un-cooled samples. Here, increased PAGS leads to lower martensitic content at elevated strain levels.

Why is this important? 

Material microstructure is known to influence mechanical properties and behavior, but the details of these structure/property/performance relationships can be challenging to elucidate. This study provides new insight into the impact of grain size – a parameter that can be tuned during material processing – on phase transformation pathways in austenitic steels, paving the way for improved control over the performance of this industrially-important material. 

Why did this research need CHEXS? 

The high-intensity, high-energy x-rays available at CHEXS beamlines are a uniquely powerful tool for probing material microstructure and micromechanical response in situ during mechanical deformation. In particular, this study relied on a combination of capabilities available specifically at the FAST beamline: 1) high-intensity, >50 keV x-rays capable of penetrating through the millimeter-scale, high-Z samples, 2) large-area detectors capable of capturing information over a large swath of reciprocal space, to facilitate efficient detection of different crystallographic phases 3) the RAMS2 mechanical load frame, which enabled in situ mechanical testing. 

How was the work funded? 

The Center for High Energy X-ray Sciences (CHEXS), The U.S. National Science Foundation, NSF MPS/BIO/ENG (DMR-1829070) Swedish Research Council (BR), grant number 2022-04900 Competence Centre Neutron and X-ray science for industrial Technology transitions (NEXT), Vinnova, grant number 2023-00553

Reference

On the contrasting effects of parent austenite grain size on athermal and deformation-induced martensitic transformation in an Fe-18Cr-12Ni (wt. %) alloy

C Cai, G Spartacus, A Das, KS Shanks, W Mu, P Hedström

Materialia (2025); https://doi.org/10.1016/j.mtla.2025.102602