Science - USA (2020-10-02)

INSIGHTS | PERSPECTIVES

sciencemag.org SCIENCE

CONSERVATION

A boost for

freshwater

conservation

Integrating freshwater and

terrestrial conservation

planning has high returns

By Robin Abell and Ian J. Harrison

S

ystematic conservation planning—

a data-driven process for prioritiz-

ing biodiversity conservation re-

sources—has been strongly biased

over the past two decades toward

terrestrial and marine species and

systems ( 1 ). Freshwater ecosystems, which

are among the most threatened on Earth,

have received less attention. Wetland ex-

tent is estimated to have declined globally

by nearly 70% since 1900 and, on average,

freshwater vertebrate populations declined

by 84% between 1970 and 2016 ( 2 ). There

is an urgent need for prioritizing resources

toward freshwater conservation. On page

117 of this issue, Leal et al. ( 3 ) show that

such prioritizations need not be a zero-sum

game: Integrated cross-realm conservation

planning can, for a negligible reduction

in terrestrial benefits, increase freshwater

benefits up to 600%.

These results are important because, at this

moment, the global conservation community

is setting targets for the next 30 years. The

Convention on Biological Diversity (CBD)

is defining its Post-2020 Global Biodiversity

Framework, to replace the expiring Aichi

Biodiversity Targets. There is an opportunity

with the renewed CBD framework to create

a policy environment and commitments de-

signed to “bend the curve” of freshwater bio-

diversity loss ( 4 ).

Leal et al.’s findings, from their studies in

Pará, Brazil, are of particular relevance to

two targets in the draft framework, which

will be negotiated during the first quarter

of 2021 and finalized at the 15th meeting of

the Conference of the Parties to the CBD,

scheduled for May 2021. One target aims

for a proportion of global land and sea ar-

eas to be under spatial planning, as a pre-

cursor to protecting and restoring natural

ecosystems. The second aims for a propor-

Conservation International, Arlington, VA, USA.

Email: [email protected]

generation industries all require tough
components that are exposed to high tem-
peratures. Superalloys, the best available
option, have an operational limit of around
1100 K ( 2 ). This material constraint affects
the efficiency of potential new technologies
for power generation by limiting operating
temperatures ( 7 ). It is also a serious issue in
aerospace applications. Presently, aircraft
parts that are exposed to hot environments
in engines, or components that will be ex-
posed to the high temperatures caused by
hypersonic travel, require complex and ex-
pensive ceramic thermal barrier coatings to
withstand the service environment ( 8 ).
MoNbTaW, MoNbTaVW ( 9 ), HfNbTaTiZr
( 10 ), and HfMoNbTiZr ( 11 ) are all examples
of MPEAs that display excellent high-tem-
perature strength ( 1 ). However, many of
these systems have limited room-tempera-
ture ductility, which is characteristic of bcc
alloys. The lack of ductility in conventional
bcc systems is related to the mobility of de-
fects (dislocations), but dislocations could
behave differently in bcc MPEAs because
of local variations in composition along
their core ( 2 , 6 ). Local compositional fluc-
tuations are intrinsic to MPEAs, in which
the elements that surround each individual
atom vary (see the figure, right). The high-
temperature strength in MPEA alloys has
been attributed to solid-solution strength-
ening by regions of concentrated solute, the
mobility of certain dislocation structures, or
both effects ( 12 ). Understanding the details
of the dislocation structure and motion is
crucial for a mechanism-guided search for
the best refractory bcc alloys across the im-
mense range of possible compositions.
Wang et al. present a new MPEA MoNbTi
alloy with good room-temperature strength
but considerably lower density than many
of the other refractory MPEA options.
Density is important for transport appli-
cations, especially in rotating parts, where
lower density increases the allowable ser-
vice temperature by decreasing the stress
caused by self-loading. The alloy displays
homogeneous plasticity in microscale ten-
sion tests performed at room temperature
in a scanning electron microscope. To un-
derstand the deformation process, Wang et
al. used a sophisticated experimental setup
that combines microscale mechanical test-
ing and advanced microscopy. They used
a focused ion beam (FIB) to prepare cross
sections underneath nano-indents and
performed in situ deformation on a single-
crystal specimen, again prepared with a
FIB. The tensile axis was aligned with the
[001] direction so that the four 1/2k 111 l–type
Burgers vectors are equally stressed. They
observed unexpected nonscrew dislocation
structures. Multiple slip systems appear to

be operative in addition to those expected

for a bcc alloy at room temperature. Wang

et al. attribute this multiplanar, multichar-

acter dislocation slip to variations in the

glide resistance for dislocations caused by

the atomic-scale chemical fluctuations in

composition along the core of the disloca-

tion. Atomistic simulations show that the

plane that has the lowest stress required

for the movement of dislocations can vary

in this system depending on the local

atomic configuration.

The implication of this finding is that

there are additional pathways for disloca-

tion slip, which is desirable for plasticity

and toughness. This observation explains

the plasticity and supports an explana-

tion for high-temperature strength based

around the slip mechanism instead of solid-

solution strengthening. Deformation was

studied at room temperature, and future

work at high temperature may reveal more

details about the active slip systems.

Activation of additional slip pathways as

a design goal will require renewed effort to

understand how atoms are arranged along

the core of the dislocation. Despite sugges-

tions of local chemical order in MPEAs, ex-

perimental verification has been ambigu-

ous ( 3 ). In Wang et al.’s work, atom probe

tomography suggested that the atoms were

randomly distributed. A smaller number

of species in this alloy, as compared with

many other MPEAs, has reduced the com-

plexity of analysis. Further atom probe

work could be carried out on carefully cho-

sen model MPEAs to minimize overlapping

peaks in data. Last, if bcc MPEA alloys are

truly to rival superalloys for high-temper-

ature use, consideration must be given to

factors beyond the strength, ductility, and

toughness. This includes oxidation resis-

tance, creep strength, fatigue strength, and

routes for manufacture, offering directions

for future research. j

REFERENCES AND NOTES

1. F. Wang et al., Science 370 , 95 (2020).


2. D. B. Miracle, O. N. Senkov, Acta Mater. 122 , 448 (2017).


3. E. P. George, D. Raabe, R. O. Ritchie, Nat. Rev. Mater. 4 ,
   515 (2019).


4. A. Abu-Odeh et al., Acta Mater. 152 , 41 (2018).


5. T. Borkar et al., Acta Mater. 116 , 63 (2016).


6. O. N. Senkov, S. Gorsse, D. B. Miracle, Acta Mater. 175 ,
   394 (2019).


7. Y. Ahn et al., Nucl. Eng. Technol. 47 , 647 (2015).


8. B. Liu et al., J. Mater. Sci. Technol. 35 , 833 (2019).


9. O. N. Senkov, G. B. Wilks, J. M. Scott, D. B. Miracle,
   Intermetallics 19 , 698 (2011).


10. O. N. Senkov et al., J. Mater. Sci. 47 , 4062 (2012).


11. N. N. Guo et al., Mater. Des. 81 , 87 (2015).


12. F. Maresca, W. A. Curtin, Acta Mater. 182 , 235 (2020).

ACKNOWLEDGMENTS

The author thanks M. Griffith for feedback on the manuscript

and acknowledges funding from the Australian Research

Council Future Fellowship (FT180100232).

10.1126/science.abd6587

38 2 OCTOBER 2020 • VOL 370 ISSUE 6512