Design workflow.(A to C) Schematics of the fcc stability and phase prediction via thermodynamic modeling. (A) Co, Cr, Fe, Mn, and Ni are mixed into the HEA, where element contents are in the range of 0 to 40 at %. (B) The phase at the homogenization temperature (1200°C) and room temperature are predicted via thermodynamic modeling. We only select alloys with a single fcc phase at 1200°C. If the Gibbs free energy of fcc is lower than the hcp at room temperature, then the alloy is labeled as an fcc single-phase alloy; otherwise, it is an fcc + hcp dual-phase alloy. (C) The Gibbs free energy difference between fcc and hcp for all compositions are compared with two reference alloys and are categorized into less stable fcc and more stable fcc. The alloys with the compositionally complementary elemental pairs that produce the maximum change of kernel density are identified, and seven alloys with a different fcc stability are chosen for further calculations. Because all designed alloys have a lower fcc stability than Ref-1 (Co20Cr20Fe20Mn20Ni20), they will form martensite or twins after cold rolling. (D) Deformation mechanism prediction via DFT. USFE, ISFE, UMFE, and UTFE are unstable stacking fault energy, intrinsic stacking fault energy, unstable martensite fault energy, and unstable twin fault energy, respectively. We propose that if the energy barrier for martensite (hcp) formation (UMFE-ISFE) is smaller than the energy barrier for twinning (UTFE-ISFE), then the alloy is a TRIP alloy; otherwise, the alloy is TWIP dominant. (E and F) Design validation by experiments. (E) Schematics of the sample preparation, testing, characterization, and statistical analysis. (F) Evaluation of the design metrics. Designed alloys are shown as predicted secondary deformation mechanism and phases. More than 80% of the designed alloys have higher hardness compared with the reference HEA (Ref-2: Co10Cr10Fe40Mn40). EBSD, electron backscatter diffraction; SEM, scanning electron microscope. Credit: Science Advances (2022). DOI: 10.1126/sciadv.abo7333
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Design workflow.(A to C) Schematics of the fcc stability and phase prediction via thermodynamic modeling. (A) Co, Cr, Fe, Mn, and Ni are mixed into the HEA, where element contents are in the range of 0 to 40 at %. (B) The phase at the homogenization temperature (1200°C) and room temperature are predicted via thermodynamic modeling. We only select alloys with a single fcc phase at 1200°C. If the Gibbs free energy of fcc is lower than the hcp at room temperature, then the alloy is labeled as an fcc single-phase alloy; otherwise, it is an fcc + hcp dual-phase alloy. (C) The Gibbs free energy difference between fcc and hcp for all compositions are compared with two reference alloys and are categorized into less stable fcc and more stable fcc. The alloys with the compositionally complementary elemental pairs that produce the maximum change of kernel density are identified, and seven alloys with a different fcc stability are chosen for further calculations. Because all designed alloys have a lower fcc stability than Ref-1 (Co20Cr20Fe20Mn20Ni20), they will form martensite or twins after cold rolling. (D) Deformation mechanism prediction via DFT. USFE, ISFE, UMFE, and UTFE are unstable stacking fault energy, intrinsic stacking fault energy, unstable martensite fault energy, and unstable twin fault energy, respectively. We propose that if the energy barrier for martensite (hcp) formation (UMFE-ISFE) is smaller than the energy barrier for twinning (UTFE-ISFE), then the alloy is a TRIP alloy; otherwise, the alloy is TWIP dominant. (E and F) Design validation by experiments. (E) Schematics of the sample preparation, testing, characterization, and statistical analysis. (F) Evaluation of the design metrics. Designed alloys are shown as predicted secondary deformation mechanism and phases. More than 80% of the designed alloys have higher hardness compared with the reference HEA (Ref-2: Co10Cr10Fe40Mn40). EBSD, electron backscatter diffraction; SEM, scanning electron microscope. Credit: Science Advances (2022). DOI: 10.1126/sciadv.abo7333
Sometimes, in creating an alloy out of multiple metals, defects and structural instability can occur in the material. Now, researchers at the University of Pittsburgh Swanson School of Engineering are harnessing those imperfections to make the material stronger while retaining its flexibility.
The investigators are designing metastable alloys that can overcome the well-known trade off between strength and ductility, revealing a strategy that can create alloys suited to a broad range of applications.
"Our work is showing how we can include intentional flaws in an alloy to make it stronger while retaining the ductility, or flexibility, of the material," said Wei Xiong, assistant professor of mechanical engineering and materials science, whose Â鶹ÒùÔºical Metallurgy and Materials Design Laboratory led the study. "The techniques we are developing can be used to make materials fit for earthquake construction, naval ships, aerospace, nuclear energy, or even transportation for oil or hydrogen—all applications where a strong but flexible material is crucial."
This study looks at two mechanisms for metastability engineering that can be used to create strong, ductile alloys: transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP). TRIP and TWIP use changes in the microstructure that occur under pressure, which cause some defects in the material, to form purposeful defects that improve the strength.
"You can think of the strength and ductility of a material like plastic versus glass. Plastic is much more ductile and flexible: It is not as strong, but you can bend it with your hands," explained Xiong. "Glass is stronger than plastic, but it's also much less flexible and will break if you try to bend it. This is the trade off that we are trying to overcome with alloys—something that has both strength and ductility."
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To conduct their study, Xiong worked with lead author Xin Wang, graduate student in the Â鶹ÒùÔºical Metallurgy and Materials Design Laboratory, as well as researchers at the the Illinois Institute of Technology and Northwestern University.
The CALPHAD modeling supported by density functional theory calculations by the team provides fundamental knowledge that can be applied to developing metastable alloys with TRIP/TWIP for enhanced strength-ductility synergy. It also can be applied to concentrated alloys, like steel and nickel.
"We want to understand the unstable microstructure so we can predict the instability, and then we can use the defects to further increase strength and elongation," said Wang. "The resulting material is then self-strengthening—deform it, and it actually gets stronger."
The paper was published in Science Advances.
More information:
Xin Wang et al, Design metastability in high-entropy alloys by tailoring unstable fault energies, Science Advances (2022).