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        CoCrFeNi increases strength at cryogenic temperatures
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        CoCrFeNi increases strength at cryogenic temperatures
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        CoCrFeNi increases strength at cryogenic temperatures
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An international team of researchers has discovered that the strength of a CoCrFeNi high-entropy alloy increases at the liquid-helium temperature while the sample maintains excellent ductility. These mechanical properties could make this new alloy useful for cryogenic applications, according to the study published recently in Science China Materials (doi:10.1007/s40843-018-9373-y).

The CoCrFeNi alloy belongs to an emerging class of materials known as high-entropy alloys (HEAs). Compared to traditional alloys such as steel, which mostly consist of one primary metal with small concentrations of additional elements, HEAs are mixtures of several different elements in roughly equal concentrations. HEA research was ignited in the early 2000s with the simultaneous discovery of two different alloys. Today, research continues on HEAs in particular because they exhibit desirable mechanical and structural properties at extreme temperatures. For example, “HEAs usually have different kinds of defects [compared to conventional alloys], such as stacking faults with lower energy, and can be ductile even at very low temperatures,” says Yong Zhang of the University of Science and Technology Beijing. Zhang is corresponding author of the publication.

Zhang and his collaborators performed a series of challenging experiments on CoCrFeNi, assessing the alloy’s strength and ductility down to temperatures of 4.2 K. “Few places in the world can perform these extremely low temperature experiments,” says Peter Liaw of The University of Tennessee, Knoxville, who is a co-author on this publication. When subjected to tensile strength testing at extremely low temperatures, the CoCrFeNi alloy deformed in a slip-stick manner, as demonstrated by jagged or serrated stress–strain curves at 20 K and 4.2 K.

Performing mechanical property measurements at “the liquid-helium temperature is really remarkable,” says Richard LeSar of Iowa State University, “It opens up a different realm for thinking about behaviors in these systems.” LeSar was not involved with this study.

To explain the serrated behavior of the stress–strain curves, images of the CoCrFeNi samples after strain testing were taken using both a transmission electron microscope and a high-resolution scanning transmission electron microscope. The images revealed many small parallel hatched features consistent with the formation of deformation twins, which correspond to boundaries where two different regions of the alloy have shifted relative to one another. In addition to the twinning behavior, a new crystal phase of the CoCrFeNi alloy was also observed.

Tensile properties of the CoCrFeNi alloy at different temperatures. (a) Engineering stress–strain curves and photograph of the dog-bone-shaped samples, before and after tensile tests. The enlarged stress–strain curve at 4.2 K is displayed in the inset and the jagged serrations are clearly observed. (b) Strength and elongation versus temperature for the high-entropy alloy. The ductility reaches a maximum at 77 K. Credit: Science China Materials.

Theoretical analysis of the serrated stress–strain curves revealed an“unstable dynamic process, which was consistent with the instability caused by the phase transition and twinning,” according to Jingli Ren of Zhengzhou University, China. Ren performed the calculations for this study. Both the deformation twinning and phase transition likely contributed to the low-temperature ductility and strength measured in CoCrFeNi HEAs.

Further understanding is needed of how the observed chaotic behaviors in CoCrFeNi contribute to its serrated stress–strain behavior and mechanical properties at low temperatures. According to Liaw, more in-depth analysis and modeling of this and other HEAs are needed to separate the contributions from deformation twinning and phase transition to the strength and ductility seen at the liquid-helium temperature. Such a study could be challenging because the two behaviors often appear simultaneously in some HEAs.

Zhang hopes these results demonstrate the capabilities of HEAs at cryogenic temperatures, which could be used especially as materials for aerospace and nuclear-reactor applications.