matrix. Consequently, the plastic strain induced
by the glide of these mobile dislocations in the
martensite matrix is estimated to be 6.8%, which
constitutes a large portion of measured Lüders
strain (~7 to 9%) in the D and P steel (Fig. 2). The
high mobile dislocation density in the martensite
matrix accommodates a large plasticity upon
yielding. The catastrophic release and glide of
the mobile screw dislocations result in a low, but
positive, strain-hardening rate during Lüders strain
regime (fig. S6). Such a low but positive strain-hardening rate has been reported in irradiated
metals (29) and w-containing Ti alloys (30), in
which the operation of dislocation channeling
is the governing deformation mechanism.
In addition to the high mobile dislocation
density, the D and P steel had a continuous
transformation-induced plasticity (TRIP) effect
at a large strain during the tensile test (Fig. 3B).
The TRIP effect resulted from the formation of
martensite in the coarse layered austenite grains.
This TRIP effect applied compressive residual
stress to effectively blunt localized plasticity during
tensile straining (fig. S7) (31) and also provided
dynamic strain partitioning between phases and
improved strain hardening (fig. S6) (32). The
formation of the large martensite grains suggests
that coarse layered austenite grains are less stable
than ultrafine austenite grains during uniaxial
tensile deformation (Fig. 3C). Nevertheless, most
of these coarse layered austenite grains only
transform to martensite at strains beyond the
Lüders strain (Fig. 3B), explaining their high
mechanical stability in the D and P steel. We
ascribed the enhancement in the mechanical
stability to the high dislocation density in large
austenite grains (Fig. 1F), where dislocations can
act as barriers for glissile martensite interface
and therefore stabilize austenite grains (19). Moreover, the hard martensite matrix (fig. S8) can
shield austenite from deformation (33), allowing
the austenite to transform at a large strain regime. The austenite grains in the D and P steel
were further stabilized by C partitioning from
martensite (fig. S9) (13) relative to the austenite
grains in deformed steel (fig. S10). In addition to
the effect of C partitioning and relatively higher
Mn content (fig. S4), the high dislocation density
in the D and P steel also controlled release of the
TRIP effect, improving ductility.
The D and P steel also had a twinning-induced
plasticity (TWIP) effect during the tensile test.
We mostly observed the TWIP effect, induced by
the formation of deformation twins, in ultrafine austenite grains (Fig. 3D). The initiation of
nanotwins from phase boundary suggests a high
stress level experienced by lamella austenite
grain (Fig. 3D). Therefore, the TWIP effect also
operated in the large strain regime (fig. S6). The
minor austenite volume fraction (Fig. 1A) of the
small grains means that we expect the TWIP
effect to be much less important than the TRIP
effect, even as deformation twins can accumulate
deformations and improve ductility (4).
We compared the bulk properties of D and P
steel to other high-strength metallic materials
(Fig. 4). The D and P steel exhibits a yield strength
that is 50% higher than that of nanobainite steel
while maintaining a comparable uniform elon-
gation (Fig. 4). Moreover, it exhibits a uniform
elongation that is one order of magnitude larger
than that of commercial maraging steels while
maintaining an equivalent yield strength. Con-
sequently, our D and P steel achieves excellent
tensile properties and defines a new space in the
strength-ductility map (Fig. 4). Despite its dis-
continuous yielding and Lüders strain (Fig. 2),
the ultrahigh yield strength of our D and P steel
makes it a desirable alloy for applications where
yield strength is the main design criterion.
The D and P overcomes the challenge of
creating martensite, which is known to be an
issue for compositionally similar steels that rely
on a quenching and partitioning process (13)
(fig. S5). The high dislocation density in the D
and P steel not only increases the yield strength
by dislocation forest hardening but also enables
a large ductility by the glide of existing mobile
dislocations and by the controlled release of the
TRIP effect. The high dislocation density is the
origin of the inverse strength-ductility trade-off.
We expect that this strategy will be useful in other
systems with similar deformation-induced martensitic transformation mechanisms such as titanium alloys (34). The D and P steel exhibits a low
raw-materials cost as compared to the maraging
steel while maintaining a comparable ultimate
tensile strength (fig. S11). Therefore, by engineering dislocations, we simultaneously alleviate
the economic concerns while achieving ultrahigh
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The authors thank K. Lu for his insightful and constructive
comments on this paper. M.X.H. is grateful for financial support
from the Research Grants Council of Hong Kong (grant nos.
712713, 17203014, and 17255016) and the National Natural
Science Foundation of China (grant no. U1560204). H. W.L. is
grateful for financial support from the National Natural
Science Foundation of China (grant no. U1460203). H. W. Y.
acknowledges the Ministry of Science and Technology of the
Republic of China for providing financial support under
contract MOST-104-2218-E-002-022-MY3 and thanks the
Instrumentation Center at National Taiwan University for
technical support during use of the JSM 7800F PRIME high-resolution scanning electron microscope. The authors also
acknowledge the Shanghai Synchrotron Radiation Facility for
providing the synchrotron XRD facility at beamline no. 14 B. The
present work has pending patents with application numbers
201610455155.3 and PCT/CN2016/096509 and another
awarded patent, number 201410669029.9. M.X.H. supervised
the study. M.X.H., B.B.H., and H. W.L. designed the study. B.B.H.
and B.H. prepared the thermomechanical treatment and the
mechanical tests. H. W. Y., G.J.C., and B.B.H. conducted the
microstructure characterization. B.B.H., M.X.H., H. W.L., and Z.K. W.
analyzed the data. B.B.H. and M.X.H. wrote the paper. The
authors declare no competing financial interests. Data are
available in the manuscript and supplementary materials.
Materials and Methods
Figs. S1 to S14
20 February 2017; accepted 10 August 2017
Published online 24 August 2017
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