Time: 10:00 a.m.
Place: Materials Day 2013, Ruhr-Universität Bochum, Bochum, Germany
An equiatomic CoCrFeMnNi high-entropy alloy, which crystallizes in the face-centered cubic crystal structure, was produced by arc melting and drop casting. The drop-cast ingots were homogenized, cold rolled and recrystallized at various temperatures to establish different grain sizes between 4~155 μm. The single-phase character of the alloy was confirmed by x-ray diffraction, and scanning and transmission electron microscopy. Quasi-static tensile tests were performed at an engineering strain rate of 10-3 s-1 at temperatures between 77 and 1073 K. It is found that the yield and ultimate tensile strength, as well as the elongation to fracture, all increase when the test temperature is lowered; this trend is most pronounced when the tests are conducted below room temperature. In order to identify the reasons for this unusual behavior, transmission electron microscopy investigations were conducted on samples that were deformed at 77, 293 and 873 K to various strains between 2~40%. It was concluded that nanoscale deformation twinning, which was observed only after straining at 77 K, might account for the increase in the ultimate tensile strength and elongation to fracture at cryogenic temperatures through a “dynamic Hall-Petch effect” and the concomitant higher work hardening rates. The nature of the temperature dependent yield strength, which is not seen in pure FCC metals, cannot be explained by our microstructural observations and may be an inherent solution hardening effect.