Civil Engineering Reference
In-Depth Information
(e.g., El of 15%; impact toughness of 200 J at
20°C) and high-temperature
creep resistance (e.g., at 923K, time-to-rupture 2 orders of magnitude higher
than conventional creep-resistant steels), etc. There are multiple mecha-
nisms underlying the outstanding strength-ductility synergy of nanocom-
posite steels. Often the toughness is introduced by the lath or lamella
structure or high percentage of high-angle GBs; ductility and formability
are introduced by the retained
−
; and strength is
enhanced by the nano-grains, nano-precipitates of carbides, nitrides, or
carbonitrides, or other nano-inclusions.
One recent advance in this fi eld is the use of dynamic plastic deformation
(DPD) followed by annealing to produce SS and alloy steels with superior
strength-ductility synergy, such as TS of 1 GPa, El of 27%, and very high
work-hardening rates (Liu
et al.
, 2010; Lu
et al.
, 2012; Yan
et al.
, 2012). These
nanocomposite steels feature a unique single-phase hierachical microstruc-
ture with
γ
fi lms or tempered
α
′
grains and low dislocation density. Their mechanical behavior is 'elastically
homogeneous but plastically heterogeneous'. It was hypothesized that the
NT boundaries act as slip planes while resisting dislocation motion. Figure
5.3 presents typical bright-fi eld transmission electron microscopy (TEM)
images of 316 SS after DPD and annealing, showing the early-stage static
recystallization (SRX) in shear bands between NT bundles. The SRX grains
and NT bundles introduce a great amount of ductility into the steel bulk,
while enhancing its strength simultaneously (Yan
et al.
, 2012).
∼
20 vol% nanotwin (NT) bundles embedded in micro- and nano-
γ
5.3.2 Nanotechnology to improve mechanical properties
of steel surface
Nanotechnology has also been employed to enhance the mechanical prop-
erties of the steel surface layer, by achieving the desirable fi nely crystalline
microstructure of steel or by modifying its chemical composition and mor-
phology at the nano- or micro-scale (Lo
et al.
, 2009). Table 5.2 summarizes
recent research on this subject, involving various classes of steel: carbon
steel, SS, alloy steel, etc. A variety of processing approaches have been
investigated for producing a nanocomposite surface on steels, typically via
surface SPD or SPD coupled with TMT. While there tends to be a gradient
microstructure from the treated surface to the bulk of steel, there is also a
high level of diversity in the resulting surface microstructure, ranging from
nano-grains of
α
or
α
and cementite, to nano-scale retained
γ
grains embed-
ded in the fi ne bainite and
α
′
, to net-shape pearlite along the nano-grains
of
, to nano-twinned ultrafi ne crystals. These nano-modifi ed steel surfaces
feature novel mechanical properties, characteristic of considerable improve-
ments in strength (e.g., TS by 91% and fatigue strength by 13%), hardness
(by 100%), wear resistance (by 97%), and fatigue strength (by 25%), and
α
Search WWH ::
Custom Search