The use of nanotechnology to improve the bulk and surface properties of steel for structural applications - Nanotechnology in Eco-efficient Construction - page 78

pin the motion of dislocations. Once the grain size is reduced to the nano-
meter range, an inverse Hall-Petch relation can be observed. This can be
credited to the extremely high density of GBs in the microstructure of
nano-grained steel, which leads to 'role-exchange of the grain bulk and the
GB in the deforming mechanism' and 'a transition from intragranular to
GB-mediated deformation' (Weertman, 1993; Song et al. , 1999; Frontán
et al. , 2012). Nonetheless, as reviewed by Lesuer et al. (2010), the tensile
strength of steels generally exhibits a negative linear relationship with the
interparticle spacing, despite the signifi cant variations in their chemical
composition and thermomechanical history (Fig. 5.2). In other words, grain
refi nement has been the main pathway to achieve high strength for steels.
The only exception was a case where a lath martensite nano-structure
greatly enhanced the strength of the steel, thus deviating from this linear
relation (Fig. 5.2).
One potential issue related to microstructure ultrafi ning of steels is that
their heat affected zones (HAZ) tend to exhibit various degrees of embrit-
tlement and localized softening as a result of localized grain coarsening.
This can be addressed by inducing the nucleation of intragranular acicular
ferrite around oxide inclusions, following the oxide-dispersed-strengthened
(ODS) steel technology (Lei et al. , 2007). Another way to refi ne the HAZ

Interparticle spacing D s */nm

10 1 10 2 10 3 10 4

10 4

: 100% Lath Martensite

0.6%C
0.4%C
0.2%C

10 3

0.1%C

￿ ￿ ￿ ￿ ￿ ￿

0.026%C

UHCS 1.8%C & Fe-C Steels
0.15%C and 0.30% C Steels
0.04%C-(Mo, Ti, Mn) Steel
0.89%C, Tempered Martensite
Ball-milled Fe-O Alloys
0.89%C, Quenched Martensite
0.03%C Steel, Quench from 725C
0.6%C Steel, Dynamic Impact
UHCS 1.3%C, Dynamic Impact

10 2

10 1

10 -2 10 -1 10 0 10 1

Interparticle spacing D s */ m

μ

5.2 Tensile strength of steels as a function of interparticle spacing
(adopted from Lesuer et al. , 2010). With kind permission from
Springer Science and Business Media.