In a recent work, we have reported outstanding strength and work hardening exhibited by a metastable high entropy alloy (HEA), Fe42Mn28Co10Cr15Si5 (in at. %), undergoing the strain-induced martensitic transformation from metastable gamma austenite (γ) to stable epsilon martensite (ε). However, the alloy exhibited poor ductility, which was attributed to the presence of the brittle sigma (σ) phase in its microstructure. The present work reports the evolution of microstructure, strength, and ductility of a similar HEA, Fe38.5Mn20Co20Cr15Si5Cu1.5 (in at. %), designed to suppress the formation of σ phase. A cast and then rolled plate of the alloy was processed into four conditions by annealing for 10 and 30 min at 1100 °C and by friction stir processing (FSP) at tool rotation rates of 150 and 400 revolutions per minute (RPM) to facilitate detailed examinations of variable initial grain structures. Neutron diffraction and electron microscopy were employed to characterize the microstructure and texture evolution. The initial materials had variable grain size but nearly 100% γ structure. Diffusionless strain induced γ→ε phase transformation took place under compression with higher rate initially and slower rate at the later stages of deformation, independent on the initial grain size. The transformation facilitated part of plastic strain accommodation and rapid strain hardening owing to a transformation-induced dynamic Hall-Petch-type barrier effect, increase in dislocation density, and texture. The peak strength of nearly 2 GPa was achieved under compression using the structure created by double pass FSP (150 RPM followed by 150 RPM). Remarkably, the tensile elongation exhibited by the alloy was nearly 20% with fracture surfaces featuring a combination of ductile dimples and cleavage.
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Effects of microstructure on the mechanical properties of Ti2AlC in compression
This study investigates the effect of microstructure, specifically the grain size and TiAlx impurity, on the compressive strength and hysteretic behavior of Ti2AlC at room temperature. Given the plate-like nature of the MAX phase grains, the length and thicknesses of over 100 grains for each microstructure were measured. A Hall-Petch like relationship between compressive strength and the grain length was observed, but not such a relationship was observed with the grain thickness. Results from cyclic compression testing in combination with resonant ultrasound spectroscopy show that room temperature mechanical response of Ti2AlC can be divided into four stress regions regardless of the variation in grain size and/or amount of impurities. The grain size effect on the transition stresses for stress regions was also investigated. It was found that all transition stresses, between the different stress regions, also follow different Hall-Petch-type relationships.
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- Award ID(s):
- 1729350
- NSF-PAR ID:
- 10073596
- Date Published:
- Journal Name:
- Acta materialia
- Volume:
- 143
- ISSN:
- 1359-6454
- Page Range / eLocation ID:
- 130-140
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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