Science - USA (2020-10-02)

SCIENCE sciencemag.org

GRAPHIC: KELLIE HOLOSKI/

SCIENCE

METALLURGY

A rival to superalloys at high

temperatures

Slip-pathway activation provides plasticity in a multiprincipal

element alloy with high-temperature strength

essary to determine whether similar stem

cells exist near the human spinal cord central

canal in sufficient numbers ( 11 ). The lumen

of the central canal progressively disappears

during childhood, but cells with stem cell po-

tential can be isolated from the human spinal

cord ( 12 ). It will be necessary to determine

how similar these cells are to mouse ependy-

mal cells and whether their chromatin is sim-

ilarly poised to generate oligodendrocytes.

From a therapeutic viewpoint, efficient

means of inducing gene expression are

needed. Viral delivery systems are under

development that evade immune detec-

tion and can be switched off—an important

safety feature to avoid unwanted prolifera-

tion of cells ( 13 ). Such strategies could be

used to treat any demyelination after spinal

injury or in demyelinating diseases, such as

multiple sclerosis.

Ependymal cells in the mouse spinal cord

have a stem cell potential that sets them

apart from other spinal cell types. For exam-

ple, Llorens-Bobadilla et al. found that astro-

cytes did not produce oligodendrocytes after

forced OLIG2 expression. What else could ep-

endymal cells do? In rats, stem cells derived

from whole spinal cord, including ependymal

cells, formed neurons when transplanted

into the hippocampus, a neurogenic region

of the brain ( 14 ). In anamniotes (salaman-

ders and fishes), spinal ependymal progeni-

tor cells generate neurons after injury in situ

( 12 ). This indicates a potential for neurogen-

esis in spinal stem cells across vertebrates. It

might be informative to determine the nature

of the gene regulatory programs activated by

anamniotes to produce neurons from epen-

dymal cells in the spinal cord and whether

these could be activated by gene therapy in

nonregenerating mammals to contribute to

repair after injury. j

REFERENCES AND NOTES

1. K. Meletis et al., PLOS Biol. 6 , e182 (2008).


2. E. Llorens-Bobadilla et al., Science 370 , eabb8795
   (2020).


3. A. D. Greenhalgh, S. David, F. C. Bennett, Nat. Rev.
   Neurosci. 21 , 139 (2020).


4. M. A. Anderson et al., Nature 532 , 195 (2016).


5. G. Courtine, M. V. Sofroniew, Nat. Med. 25 , 898 (2019).


6. G. J. Duncan et al., Nat. Commun. 9 , 3066 (2018).


7. J. Ruschel et al., Science 348 , 347 (2015).


8. B. T. Lang et al., Nature 518 , 404 (2015).


9. S. Ceto, K. J. Sekiguchi, Y. Takashima, A. Nimmerjahn, M.
   H. Tuszynski, Cell Stem Cell 27 , 430 (2020).


10. S. Mensch et al., Nat. Neurosci. 18 , 628 (2015).


11. D. Garcia-Ovejero et al., Brain 138 , 1583 (2015).


12. C. G. Becker, T. Becker, J.-P. Hugnot, Prog. Neurobiol. 170 ,
    67 (2018).


13. E. R. Burnside et al., Brain 141 , 2362 (2018).


14. L. S. Shihabuddin, P. J. Horner, J. Ray, F. H. Gage, J.
    Neurosci. 20 , 8727 (2000).

ACKNOWLEDGMENTS

Supported by the EU Cofund ERANET NEURON consor-

tium NEURONICHE with contributions from MRC (MR/

R001049/1), Spinal Research, and Wings for Life (C.G.B.) and

by BBSRC project grant BB/R003742/1 (T.B.).

10.1126/science.abe1661

By Julie Cairney

A

lthough conventional alloys are

based mainly on one element, recent

design efforts have focused on mul-

tiprincipal element alloys (MPEAs)

that contain substantial quantities

of several elements. Success with this

approach requires a robust understanding

of the mechanistic origin of MPEA proper-

ties. On page 95 of this issue, Wang et al.

( 1 ) report the deformation behavior in a

promising body-centered cubic (bcc) MPEA

with good room-temperature plasticity and

high strength at the temperatures at which

conventional alloys would soften. They

observed multiplanar, multicharacter dis-

location slip that was not expected in bcc

systems. This property is attributed to vari-

ations in the glide resistance along the core

of dislocations, created by the atomic-scale

fluctuations in composition that are charac-

teristic of MPEAs. This mechanism explains

the plasticity and could be used to guide the

design of new MPEA candidates for high-

temperature applications in aerospace and

power generation.

For centuries, alloy design has involved

taking a base metal and adding small

amounts of other elements to improve the

properties. For example, adding carbon to

iron enhances its strength, creating steel,

and adding yet another element, nickel,

improves its corrosion response, creating

stainless steel. Sophisticated superalloys

have complex compositions that provide

high performance near their melting point,

but they are still based on a primary ele-

ment, usually nickel, cobalt, or iron.

To expand the alloy design space, more

recent efforts have shifted toward the de-

velopment of alloys that contain substantial

quantities of three or more elements (see

the figure). These materials are variously re-

ferred to as MPEAs, complex concentrated

alloys, or high-entropy alloys (a subset of

MPEAs). Some of these new alloys display

unprecedented combinations of strength,

ductility, high-temperature performance, or

functional properties ( 2 , 3 ). Each

new composition can result in the

formation of different phases and

microstructures within the alloy,

which can in turn be altered by

mechanical deformation and heat

treatment. With such an enormous

range of possibilities, traditional

trial-and-error approaches are inef-

fective, and researchers are turning

to computational and combinato-

rial approaches to predict the el-

emental combinations that could

lead to alloys with desirable prop-

erties ( 4 , 5 ). However, for these ap-

proaches to be successful, it is criti-

cal that the alloy design process is

guided by an understanding of the origins

of the specific properties that are desired.

Refractory MPEA alloys, with their ex-

cellent high-temperature strength, show

great promise. They are composed of

combinations of three or more of the ele-

ments chromium (Cr), molybdenum (Mo),

niobium (Nb), vanadium (V), tantalum

(Ta), tungsten (W), hafnium (Hf ), titanium

(Ti), or zirconium (Zr) at nearly equal con-

centrations. The alloys usually have a bcc

crystal structure and, for many of the most

desirable combinations, are single-phase

solid solutions. However, unlike dilute

bcc alloys, many of these alloys show ex-

cellent retention of strength up to 1900 K

( 6 ). Aerospace, petrochemical, and power-

School of Aerospace, Mechanical and Mechatronic

Engineering, University of Sydney, Sydney, Australia. Email:

[email protected]

Conventional

Elements dilute alloy

Multiprincipal

component alloy

A

B

C

D

2 OCTOBER 2020 • VOL 370 ISSUE 6512 37

A more equal union

The schematics represent the atomic structure of conventional

and multifunctional alloys. Conventional alloys are based

mainly on a single element, compared with multicomponent

alloys that contain several elements in similar proportions.