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M.D. and B.B. acknowledge support from the French National
Program of Planetology. M.D. carried out part of this work while
visiting the European Space Agency/European Space Research and
Technology Centre and the Leiden Observatory (Netherlands).
K. W. was supported by the National Science Foundation, grant
1518127, and by the Bonus Qualité Recherche of the Observatoire
de la Côte d’Azur (OCA). A.M. acknowledges support from the
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Materials and Methods
Figs. S1 to S7
19 December 2016; accepted 13 July 2017
Published online 3 August 2017
High dislocation density–induced
large ductility in deformed
and partitioned steels
B. B. He,1 B. Hu,2 H. W. Yen,3 G. J. Cheng,3 Z. K. Wang,4 H. W. Luo,2 M. X. Huang1*
A wide variety of industrial applications require materials with high strength and ductility.
Unfortunately, the strategies for increasing material strength, such as processing to
create line defects (dislocations), tend to decrease ductility. We developed a strategy to
circumvent this in inexpensive, medium manganese steel. Cold rolling followed by low-temperature tempering developed steel with metastable austenite grains embedded in a highly
dislocated martensite matrix. This deformed and partitioned (D and P) process produced
dislocation hardening but retained high ductility, both through the glide of intensive mobile
dislocations and by allowing us to control martensitic transformation. The D and P strategy
should apply to any other alloy with deformation-induced martensitic transformation and
provides a pathway for the development of high-strength, high-ductility materials.
Strength and ductility are key mechanical properties of metallic materials for de- veloping energy-efficient and lightweight structural components in a wide variety of industries, including automotive and aero-
space. Unfortunately, improving strength often
results in the degradation of ductility, which is
known as the strength-ductility trade-off (1). Cost
is also an issue, as alloying elements that help
improve both properties, such as cobalt and ti-
tanium, tend to be expensive. Previous efforts
toward resolving this trade-off focused on engi-
neering defects such as grain boundaries (2, 3)
and coherent twin boundaries (4, 5). However,
the strength may reach a limit when grain size
or twin-boundary spacing is reduced to nano-
meters (6–8). Dislocations or line defects are a
different pathway for engineering alloy proper-
ties. In contrast to the above planar defects, the
strength of metallic materials monotonically in-
creases with dislocation density according to
the well-known Taylor hardening law (9). The
problem is that ductility tends to decrease as
the number of dislocations increases. However,
improved choices in alloy composition and clev-
er processing strategies may allow dislocation
engineering to circumvent the strength-ductility
trade-off. In particular, achieving high ductility
in the presence of high dislocation density in an
inexpensive material produced by facile process-
ing routes is desirable for broad industrial appli-
cations at an economic cost.
Severe plastic deformation (SPD) and phase
transformation both generate high dislocation
density in metals. Cold drawing is one SPD that
introduces a very high dislocation density in
pearlitic steel wires (10). Cold drawing and other
SPD methods also may lead to grain refinement
(<100 nm) in metals because of the formation
of high-angle grain boundaries by rearrange-
ment of intensive dislocations (10, 11). By contrast,
the first-order solid-state martensitic transforma-
tion produces a highly dislocated martensite mi-
crostructure without a lot of grain refinement
(~500 nm) in steels (12). Either quenching or
deformation can trigger martensitic transforma-
tion, depending on the alloying content of steels
(13, 14). We show that rolling and low-temperature
tempering produced a high dislocation density
in steel, also enabling a large ductility. In ad-
dition to the high dislocation density, intersti-
tial C atoms partition into and help stabilize the
austenite phase during tempering (13). We define
this thermomechanical treatment as the “deformed
and partitioned (D and P)” process and the cor-
responding steel as D and P steel. The D and P
process could be realized with conventional
processing techniques that are compatible with
existing industrial production lines.
We used a medium Mn steel (10% Mn, 0.47%
C, 2% Al, and 0.7% V, by weight) for the D and P
process. The Mn and C atoms are effective austenite stabilizers (fig. S1). The addition of 2% Al
content is to suppress cementite precipitation
during the tempering process. The addition of
0.7% V is to form intensive nanometer-sized V
carbides, which provided enhanced resistance
to delayed fracture induced by hydrogen embrittlement (15, 16). The D and P steel that we
produced by multiple deformation and annealing
steps (17) possesses a heterogeneous lamella
dual-phase microstructure in which metastable
austenite is embedded in a martensite matrix
(Fig. 1A). We obtained the martensite matrix by
cold rolling (fig. S2) followed by tempering. The
tempered martensite matrix has heterogeneous
grain morphologies and substructures. Large lenticular martensite grains, which constitute most
of the matrix, are mainly decorated with dislocation cells (Fig. 1B). Nevertheless, some large lenticular martensite grains also possess dislocations
1Department of Mechanical Engineering, The University of Hong
Kong, Pokfulam Road, Hong Kong, China. 2School of
Metallurgical and Ecological Engineering, University of Science
and Technology Beijing, Xue Yuan Lu 30, Beijing 100083, China.
3Department of Materials Science and Engineering, National
Taiwan University, Roosevelt Road, Taipei 10617, Taiwan.
4Department of Mechanical and Biomedical Engineering, City
University of Hong Kong, Tat Chee Avenue, Hong Kong, China.
*Corresponding author. Email: firstname.lastname@example.org (M.X.H.);
email@example.com (H. W.L.)