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Vital Statistics:
New experiments involving
high-entropy alloys have yielded a multiple-
element material that tests as one of the toughest ever recorded, say
researchers. In addition, the alloy’s toughness—as well as its strength and
ductility—improve at cryogenic temperatures. The new material was
synthesized and tested at the DOE’s Lawrence Berkeley and Oak Ridge
National Laboratories (Berkeley Lab and ORNL).
“We examined CrMnFeCoNi, a high-entropy alloy that contains five
major elements rather than one dominant one,” says Robert Ritchie, a materials
scientist at Berkeley Lab. “Our tests show that despite containing multiple elements
with different crystal structures, this alloy crystallizes as a single phase, face-entered cubic
solid with exceptional damage tolerance, tensile strength above one gigapascal, and fracture
toughness values that are off the charts, exceeding that of virtually all other metallic alloys.”
Success Factors:
Although high-entropy alloys have existed for more than a decade, their quality has only recently become adequate
for scientific study, says Ritchie. Easo George, group leader of ORNL’s Alloy Behavior and Design Group, and his team
combined high-purity elemental starting materials with an arc-melting and drop-casting process to produce
CrMnFeCoNi samples in sheets roughly 10 mm thick. After characterizing samples for tensile properties andmicrostructure, George sent them to Ritchie for fracture and
toughness characterization.
“High-entropy alloys do not derive their properties from a
single dominant constituent or from a second phase,” explains
Ritchie. “The idea behind this concept is that configurational
entropy increases with the number of alloying elements,
counteracting the propensity for compound formation and
stabilizing these alloys into a single phase like a pure metal.”
Tensile strengths and fracture toughness values were measured
for CrMnFeCoNi from room temperature down to 77 K.
Recorded values are among the highest reported for any material.
Results showing that these values increased along with ductility at
cryogenic temperatures are a major departure from the vast majority of metallic
alloys, which lose ductility and become more brittle at lower temperatures. Ritchie and
George explain that the key to the alloy’s cryogenic strength, ductility, and toughness is
a phenomenon callednano-twinning, in which during deformation, atomic
arrangements in adjacent crystalline regions formmirror images of one another.
About the Innovators:
Robert Ritchie is the corresponding author, along with Easo George,
of a paper in Science describing this research, A Fracture Resistant
High-Entropy Alloy for Cryogenic Applications.
What’s Next:
Ritchie notes that the mechanical properties of CrMnFeCoNi and other
high-entropy alloys have yet to be optimized, as they have not been
systematically studied yet. Large-scale studies are just beginning to take
place. Regarding alloy development, one idea being tested out is removing just one or two elements
at a time from the CrMnFeCoNi conglomerate and seeing how that affects the properties. Other
researchers are looking at incorporating refractory metals such as Mo and W to see what happens. Although
commercial applications may be at least a decade away from use as structural materials, Boeing is now conducting a
large study focusing on the corrosion properties of these new and promising alloys.Contact Details:
Robert Ritchie • Lawrence
Laboratory
510.486.5798,
MailStop 62-239, 1 Cyclotron Rd., Berkeley, CA 94720
High-entropy alloys (orange) show an
exceptional combination of toughness
and strength relative to other
materials. Courtesy of Ritchie group.
ADVANCED MATERIALS & PROCESSES • OCTOBER 2014
High-Entropy Metallic Alloys
Specimen
Name:
SucceSS AnAlySiS
Robert Ritchie,
a senior faculty
scientist with
Berkeley Lab and
UC Berkeley, is a
recognized authority
on the mechanical
behavior of materials.
At 77 K, backscattered
electron images taken
in the wake of a
propagated crack
exhibit formation of
pronounced cell
structures resulting
from dislocation
activity that includes
deformation induced
nano twinning.Courtesy
of Ritchie group.