Studying microstructure in steel subjected to dynamic loading conditions

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An illustration depicting how a material’s microstructure influences its spall strength and damage characteristics. At left: Micrographs of (from top) ferrite-pearlite (FP), martensite (M), and spheroidized 1045 steel (SPH). In summary, the illustration shows that overall damage increased in the order M>SPH>FP in the same material (chemically) as the crystal structure fraction was modified. 

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Los Alamos researchers recently introduced multiple crystal structures, or phases, by heat treating steel — one of the world’s most commonly used materials — to better understand the effect microstructure has on the material’s response to shock loading. Their work revealed how microstructure influences spall strength — the resistance of a material to nucleate voids/damage during shock loading — and the resulting damage characteristics.

Understanding how a material’s microstructure affects its performance is essential to strategically design materials for specific applications. It is especially critical in engineering materials expected to withstand extreme conditions, including those found in aerospace, automotive, and defense applications.

In work selected as an Editor’s Pick in Journal of Applied Physics, the researchers explored the shock response of two plain carbon steels with varying ferrite-pearlite fractions. This alloy is widely used in engineering applications due to its low cost and its balance of strength and ductility. The team’s work revealed that pearlite phase leads to a reduction in the overall spall behavior in such materials and further clarifies the influence of relatively brittle, second-phase particles on dynamic deformation.

In their second publication on this topic, in Acta Materialia, the researchers investigated the spall strength and damage behavior of three common steel microstructures where the crystal structure/phase fraction was again modified using various heat treatments. Their results detail the shock response of each microstructure — including their spall strength and the resulting form of yielding — and the likely cause.

Together, these works highlight how small changes in the microstructure can drastically alter the dynamic response of materials and open up new opportunities to design damage resistant materials through design. 

The work leveraged the Lab’s dynamic materials and materials characterization capabilities, including the 80-mm gas-gun and photon Doppler velocimetry, scanning electron microscopy, and electron backscatter diffraction systems.

Funding and mission

This work, funded by the U.S. Department of Energy, supports the Laboratory’s Materials for the Future capability pillar.

References

“The influence of pearlite fraction on the shock properties of ferrite–pearlite steel microstructures: Insight into the effect of second-phase particles,” Journal of Applied Physics, 131, 115902 (2022); DOI: 10.1063/5.008567. Authors: Virginia K. Euser, Daniel T. Martinez, James A. Valdez, Charles P. Trujillo, Carl M. Cady, David R. Jones and Saryu J. Fensin (Los Alamos National Laboratory).

“The effect of microstructure on the dynamic shock response of 1045 steel,” Acta Materialia, 250, 118874 (2023); DOI: 10.1016/j.actamat.2023.118874. Authors: Virginia K. Euser, Daniel T. Martinez, James A. Valdez, Charles P. Trujillo, Carl M. Cady, David R. Jones and Saryu J. Fensin (Los Alamos National Laboratory).

Technical contact: Saryu Fensin (MPA-CINT)

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A schematic of damage evolution in ferrite-pearlite microstructures with relatively high (top) and low (bottom) pearlite fractions. The illustration clearly shows that pearlite phase is relatively brittle and not only acts as a void nucleation site but also encourages void growth—suggesting that introduction of pearlite phase in a steel should be avoided if a material is meant to be more resistant to damage.