Joint contribution of transformation and twinning to the high strength-ductility combination of a FeMnCoCr high entropy alloy at cryogenic temperatures

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 759; pp. 437 - 447
• Main Authors: He, Z.F., Jia, N., Ma, D., Yan, H.L., Li, Z.M., Raabe, D.
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 24.06.2019; Elsevier BV
• Subjects: Alloy development; Alloys; Cryogenic deformation; Cryogenic engineering; Cryogenic temperature; Deformation mechanisms; Dislocations; Ductility; Energy conservation; High entropy alloy; High entropy alloys; Manganese; Martensite; Martensitic transformations; Mechanical behavior; Mechanical twinning; Nickel; Phase transformation; Slip; Stacking fault energy; Strain hardening; Temperature dependence; Tensile deformation; Twinning; Yield strength; Phase transformation; Mechanical behavior; Twinning; Cryogenic deformation; High entropy alloy
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2019.05.057

The microstructure-mechanical property relationships of a non-equiatomic FeMnCoCr high entropy alloy (HEA), which shows a single face-centered cubic (fcc) structure in the undeformed state, have been systematically investigated at room and cryogenic temperatures. Both strength and ductility increase significantly when reducing the probing temperature from 293 K to 77 K. During tensile deformation at 293 K, dislocation slip and mechanical twinning prevail. At 173 K deformation-driven athermal transformation from the fcc phase to the hexagonal close-packed (hcp) martensite is the dominant mechanism while mechanical twinning occurs in grains with high Schmid factors. At 77 K athermal martensitic transformation continues to prevail in addition to dislocation slip and twinning. The reduction in the mean free path for dislocation slip through the fine martensite bundles and deformation twins leads to the further increased strength. The joint activation of transformation and twinning under cryogenic conditions is attributed to the decreased stacking fault energy and the enhanced flow stress of the fcc matrix with decreasing temperature. These mechanisms lead to an elevated strain hardening capacity and an enhanced strength-ductility combination. The temperature-dependent synergy effects of martensite formation, twinning and dislocation plasticity originate from the metastability alloy design concept. This is realized by relaxing the equiatomic HEA constraints towards reduced Ni and increased Mn contents, enabling a non-equiatomic material with low stacking fault energy. These insights are important for designing strong and ductile Ni-saving alloys for cryogenic applications.