Materials Science and Engineering A
481–482 (2008) 36–39
Properties offine-grained steels generated by
displacive transformation
H.K.D.H.Bhadeshia∗
University of Cambridge,Materials Science and Metallurgy,Pembroke Street,Cambridge CB23QZ,UK
Received6June2006;received in revised form6November2006;accepted6November2006
Abstract
It has been possible in recent times to make large quantities of steels in which the controlling scale is20nm or ,comparable to that of carbon nanotubes.The mechanical properties of such steels are abnormal.For example,in some cases the ductility vanishes as the strength increases,whereas in others the ductility almost entirely consists of uniform plastic strain.Some of the steels also can tolerate large fractions of brittle phases before fracture.These and other aspects of strong,nanostructured steels are critically assessed to arrive at a hypothesis which rationalises the odd observations.
© 2007 Elsevier B.V. All rights reserved.
Keywords:Mechanical properties;Fine scale;Martensite;Bainite
1.Introduction
It has been possible for some time,to produce iron in which the space-filling crystals are just20atoms wide[1,2].These samples were prepared from the vapour phase followed by con-solidation.Although of limited engineering value,work of this sort inspired efforts to invent methods of making large quantities of steels with similarlyfine grain structures.Many of the results have been disappointing in that the steels tend to lack ductility [3].In contrast,ductile bainitic steels have been produced in which the controlling scale of the ferrite crystals is20–40nm [4].The purpose here is to explain these contradictory obser-vations and in the process,propose a theory for the strain to fracture for the novel bainitic steels.We begin by considering why ductility is lost when extremelyfine grain structures are induced into ordinary metallic materials.
2.Fine grains and diminished work-hardening capacity
Modern technologies allow steels to be made routinely and in large quantities with grain sizes limited
to a minimum of about1␮m by recalescence effects[5,6].Limited processes, generally involving severe thermomechanical processing,have
∗Tel.:+441223334301;fax:+441223334567.
E-mail address:hkdb@cam.ac.uk.been developed to achieve nanostructured ferrite grains in steel, with a size in the range20–100nm.Experiments indicate that the Hall–Petch equation holds down to some20nm,confirming that enormous strengths can be achieved by refining the grain size.The equation begins to fail at grain sizes less than about 20nm,possibly because other mechanisms of deformation,such as grain boundary sliding,begin to play a prominent role.The volume fraction V B of material occupied by the boundaries is given by
V B 2a¯
L(1) where¯L is the mean lineal intercept defining the grain size and a is the thickness of the boundary layer.Clearly,the fraction of atoms located at the grain surfaces becomes very large as the crystal size reaches minute scales,facilitating diffusional processes such as grain sliding(Fig.1).
Although the nanostructured steels are strengthened as expected from the Hall–Petch equation,they
tend to exhibit unstable plasticity after yielding(Fig.2)[3,7].The plastic insta-bility occurs in both tension and in compression testing,with shear bands causing failure in the latter case.It is as if the capac-ity of the material to work harden following yielding diminishes. The consequence is an unacceptable reduction in ductility as the grain size is reduced in the nanometer range.At veryfine grain sizes,the conventional mechanisms of dislocation multiplication
0921-5093/$–see front matter© 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.11.181
H.K.D.H.Bhadeshia/Materials Science and Engineering A 481–482 (2008) 36–39
37
Fig.1.The volume fraction of grain boundary as a function of the grain size. fail because of the proximity of the closely spaced boundaries. It then becomes impossible to accumulate dislocations during deformation.Grain boundaries are also good sinks for defects. This would explain the observed inability of nanostructured materials to work harden.
The difficulty that veryfine grains have in deforming by a dislocation mechanism is highlighted in recent experiments[8] where mosaics of minute crystals of ferrite were forced to deform in shear.Instead of the expected lattice-invariant deformation, the crystals underwent a shear transformation into austenite. 3.Austenite and enhanced work-hardening capacity
The loss of work hardening capacity infine-grained ferritic-steels can in principle be compensated for by introducing retained austenite into the microstructure.Plastic deformation can induce the austenite to transform into harder martensite.The resulting enhancement of ductility is a complex combination of the increase in work hardening capacity and the transforma-tion strains due to the formation of martensite.These effects are often lumped together and described as transformation-induced plasticity(TRIP)[9].Note that TRIP is not unique to marten-site.Bainite[10]and Widmanst¨a tten ferrite[
11]also exhibit the phenomenon given their similar shape deformations.These transformations all are ,they are accompanied by a shape deformation which is an invariant-plane strain with a large shear component and hence show pronounced transformation-induced plasticity under appropriate circumstances.
To appreciate the role of retained austenite it is necessary to distinguish between two kinds of steels—those which are ini-tially in a fully austenitic state and others in which the retained austenite is a minor phase.The latter category is of particular relevance because it represents cheap alloys,often referred to as the TRIP-assisted steels[12,13].In these steels,the aver-age carbon concentration is low(0.15wt.%)but the austenite becomes enriched with 1wt.%C by the partitioning of car-bon when other phases grow.It therefore becomes stable at room temperature without the use of expensive solutes.
The transformation strain due to the formation of martensite in common TRIP-assisted steels makes only a minor contribu-tion to the overall ductility.This is partly because the fraction of retained austenite tends to be<0.2but also because the exploitation of transformation strain requires variant selection [14,15]
On the other hand,the martensite that forms is very hard and through composite effects,raises the work hardening coef-ficient of the entire microstructure[16,17].It is this which makes a major contribution to the extent of uniform ductility achieved by the composite microstructure.This conclusion is consistent with recent work by Jacques et al.[18]who argue that in cer-tain steels containing only a small amount of retained austenite, the composite effect gives uniform elongation which is supe-rior compared with commercial alloys with larger quantities of austenite.
To summarise,retained austenite makes an important contribution in enhancing the work-hardening rate during defor-mation,and can be exploited in the context of nanostructured steels,as will be described below.
4.Nanostructured bainite
An unconventional,carbide-free steel has recently been invented which on close examination is found to contain
bainitic-Fig.2.Loss of ductility as the grain dimensions are dramatically reduced.The grain size is indicated in micrometers adjacent to each tensile curve.(a)Aluminium alloy and(b)iron alloy.Courtesy of Tsuji et al.[7].
38H.K.D.H.Bhadeshia/Materials Science and Engineering
A 481–482 (2008) 36–39
Fig.  3.Fe–0.98C–1.46Si–1.89Mn–0.26Mo–1.26Cr–0.09V(wt.%),trans-formed at200◦C for5d.Transmission electron micrograph[4,19,20].The inset is of a carbon nanotube at the same magnification,courtesy of Ian Kinloch. ferrite plates as thin as20nm,separated by carbon-enriched films of retained austenite[4,19–23].This is the hardest ever bainite,which can be manufactured in bulk form,without the need for rapid heat treatment or mechanical processing.Many details have already been published but the structure is illus-trated in Fig.3.It is important to note that it consists only of two phases,slender plates of bainitic ferrite in a matrix of carbon-enriched austenite.We now proceed to discuss its mechanical behaviour.generated
5.Nanostructured bainite:mechanical properties
The hardness of the nanostructured bainite can be as high as690HV,with tensile strengths in excess of2200MPa,com-pressive strength in excess of3000MPa,ductility in the range of 5–30%and K IC values up to45MPa m1/2.The original sources for these values can be found in a recent review[4].The highest strength is achieved by forming bainite at the lowest transfor-mation temperatures.
The reason for the high strength is well-understood from the scale of the microstructure and the details of the compositions and fractions of the phases.However,the stress versus strain behaviour is intriguing in many respects.There are two examples shown in Fig.4and corresponding details are summarised in Table1.
The gradual yielding is as expected given the transformation plasticity and indeed the defect density of the microstructure Table1
T I,V␥,σY andσUTS stand for isothermal transformation temperature,the volume fraction of retained austenite,the0.2%proof and ultimate tensile strengths, respectively[24]
T I(◦C)V␥σY(GPa)σUTS(GPa)Elongation(%) 2000.17  1.41  2.267.6
3000.21  1.40  1.939.4
4000.37  1.25  1.7
27.5Fig.  4.Fe–0.79C–1.56Si–1.98Mn–0.24Mo–1.01Cr–1.51Co–1.01Al(wt.%). True and engineering stress–strain curves.(a)Bainite generated by transfor-mation at200◦C.(b)Bainite generated by transformation at300◦C.Data from [24].
generated by displacive transformation[15].It is striking to see in Fig.4that virtually all of the elongation is uniform,with hardly any necking.Indeed,the broken halves of each tensile specimen could be neatlyfitted together.It is not clear what determines the fracture strain.
It is now possible to estimate the change in the austenite content as a function of plastic strain and the driving force for martensitic transformation in TRIP steels[25].Fig.5shows the expected variation in V␥with strain for the three cases listed in Table1.Also plotted are points which define in each case the strain at which the tensile samples failed.A prominent fea-ture is that they all fail when the retained austenite content
is Fig.5.Calculated variation in the fraction of austenite as a function of plastic strain for the samples listed in Table1.Data adapted from[24].
H.K.D.H.Bhadeshia/Materials Science and Engineering A 481–482 (2008) 36–3939
reduced to about10%.An experimental study by Sherif[26]on an aluminium-free alloy which is otherwise identical to the steel considered here,is consistent with this conclusion.His X-ray studies also indicated that tensile failure in nanostructured bai-nite occurs when the retained austenite content is diminished to about10%.
This observation can be understood if it is assumed that fail-ure occurs when the austenite,which is the toughest of all the phases present,becomes geometrically ,it loses per-colation,leading to fracture.Garboczi et al.have developed a numerical model for the percolation threshold when freely overlapping objects(general ellipsoids)are placed in a matrix [27].Since the austenite is subdivided roughly into the form of plates by the bainite,it can be represented by oblate ellip-soids with an aspect ratio r of between about1/10and1/100. The percolation threshold is then found to be p c 1., 0.127≥p c≥0.0127.This is consistent with the observation that tensile failure occurs when V␥ 0.1.
This inference is in one sense surprising since the transforma-tion of austenite leads to the formation of very hard,untempered, high-carbon martensite which should be highly susceptible to fracture.The carbon concentration of the retained austenite before it transforms is between1and2wt.%.However,a study of Fig.5shows that large amounts of this potentially brittle martensite(7–27%)can be tolerated i
n the microstructure before the fraction of austenite reaches the percolation threshold and fracture actually occurs.The question then arises as to why this is the case.
The solution to this query lies in the fact that the tendency of the martensite to crack in a mixed microstructure of austenite and martensite depends on its absolute size[28].In these mixtures, it is more difficult to crackfine martensite.It is thefine scale of the retained austenite in the nanostructured bainitic steels that permits the martensite to be tolerated without endangering their mechanical properties.
Afine plate size makes it difficult to transfer load on to the martensite when the composite mixture is strained.It has also been demonstrated that there is an exaggerated tendency to form long plates of martensite,which are most prone to cracking, when the austenite grain size is coarse[28].
6.Summary
Steels with very closely spaced grain boundaries generally suffer from a lack of ductility due to the loss of work hard-ening capacity.This problem can be remedied by introducing retained austenite in the microstructure.The strain-or stress-induced martensitic transformation of this austenite enhances the work-hardening coefficient,making it possible to get substantial ductility in nanostructured bainitic
steels.However,the amount of austenite must then be above the percolation threshold,which is estimated to be about10vol.%.
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