There is growing evidence that bainitic ferrite which retains a substantial amount of carbon in solid solution does not have cubic symmetry. We provide additional data on a different nanostructured bainitic steel to support this evidence, based on synchrotron X-ray diffraction experiments. The data are consistent only with a displacive transformation mechanism for bainite.We would like to thank Rolls-Royce plc and the Engineering and Physical
Sciences Research Council for their support during this project.This is the final version of the article. It first appeared from Maney Publishing via http://dx.doi.org/10.1179/1743284714Y.000000069

Bhadeshia, HKDH

Dippel, AC

Hulme-Smith, CN

Lonardelli, I

Peet, MJ

There is growing evidence that bainitic ferrite which retains a substantial amount of carbon in solid solution does not have cubic symmetry. We provide additional data on a different nanostructured bainitic steel to support this evidence, based on synchrotron X-ray diffraction experiments. The data are consistent only with a displacive transformation mechanism for bainite.QC 20180604</p

Hulme-Smith, Christopher

Peet, Mathew James

Lonardelli, I.

Dippel, Ann Christin

Bhadeshia, Harshad Kumar Dharamshi Hansraj

Academic Archive On-line (KTH, Sweden)

Further evidence of tetragonality in bainitic ferrite

Peet, Mathew

Dippel, A. C.

Bhadeshia, H. K. D. H.

Hulme-Smith, C. N.

DESY

Publikationer från KTH

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 There is growing evidence that bainitic ferrite which retains a substantial amount of carbon in solid solution does not have cubic symmetry. We provide additional data on a different nanostructured bainitic steel to support this evidence, based on synchrotron X-ray diffraction experiments. The data are consistent only with a displacive transformation mechanism for bainite. We would like to thank Rolls-Royce plc and the Engineering and Physical
Sciences Research Council for their support during this project.This is the final version of the article. It first appeared from Maney Publishing via http://dx.doi.org/10.1179/1743284714Y.000000069

Apollo (Cambridge)

SHORT COMMUNICATIONFurther evidence of tetragonality in bainiticferriteC. N. Hulme-Smith*1, M. J. Peet1, I. Lonardelli2, A. C. Dippel3 andH. K. D. H. Bhadeshia1There is growing evidence that bainitic ferrite which retains a substantial amount of carbon in solidsolution does not have cubic symmetry. We provide additional data on a different nanostructuredbainitic steel to support this evidence, based on synchrotron X-ray diffraction experiments. Thedata are consistent only with a displacive transformation mechanism for bainite.Keywords: Tetragonal bainitic ferrite, Nanostructured bainite, Synchrotron, X-ray diffractionIntroductionIt has been demonstrated that large concentrations ofcarbon remain in solid solution within defect-freeregions of bainitic ferrite;1–3 this evidence has come tolight because of the discovery of nanostructured bainitewhich forms at exceptionally low temperatures.4–6 Thecarbon concentrations observed in solution requireexplanation because the bainite forms at temperatureswhere the ‘excess carbon’ ought to partition into theresidual austenite given the paraequilibrium phasediagram between body-centred cubic ferrite and auste-nite. The process of carbon diffusion is certainly notlimited by the lack of thermal activation in the time andtemperature domain where bainite forms.7A possibility suggested by first-principles calculations,and the fact that the bainite forms initially by adiffusionless, displacive transformation mechanism, isthat the symmetry of the ferrite is not cubic. In steels, theBain strain8 would then necessarily leave the ferrite in atetragonal state since randomly distributed carbon atomsin austenite would effectively be ordered into just one ofthe three sub-lattices of octahedral sites in the ferrite (e.g.Refs. 9–11). The first-principles calculations12 then sug-gested that the equilibrium solubility of carbon in thetetragonal ferrite that is in contact with austenite wouldincrease significantly, thus explaining the reluctance ofcarbon to partition into the austenite. The tetragonalferrite should eventually relax towards the cubic form asprocesses associated with tempering take hold, but at arate slower than expected with the cubic form.In this scenario, it is possible to detect tetragonality inthe bainitic ferrite containing the excess carbon. The firstevidence for this was reported in nanostructured bainitecharacterised using synchrotron X-rays13 in a steel ofcomposition Fe–0?84C–2?26Mn–1?78Si–1?55Co–1?47Cr–0?25Mo–0?11V (wt-%). In an independent study, differentthermal expansion coefficients measured along the unit-cell axes for ferrite have been found in a lower carbonalloy. This is only possible with a non-cubic unit-cell.Similarly, other X-ray diffraction data and latticeimaging using high resolution microscopy are consistentwith residual tetragonality in the bainitic ferrite lattice.14The purpose of the present work is to supplement theevidence with new data on a different steel capable oftransforming into nanostructured bainite, currently part ofa separate study. The chemical composition is Fe–1?037C–3?89Si–1?97Mn–0?24Mo–1?43Al (wt-%). Material washomogenised and samples spark machined to 3 mmdiameter rods. These rods were sealed in quartz ampulesfilled with argon and austenitised at 1050uC for 30 min.While hot, the samples were transferred to a low tem-perature oven capable of controlling the temperature to aprecision of ¡0?1. Once in the oven, the quartz ampuleswere smashed and the sample left to air-cool. Twoisothermal transformation heat treatments were con-ducted, at 200uC for 10 days, and at 300uC for one day.Under these conditions, bainitic ferrite is expected to formwith the maximum dissolved carbon content. Sampleswere finally turned on a lathe to a diameter of 2 mm tomaximise X-ray transmission and improve temporalresolution; using optical microscopy, no plastic deforma-tion could be detected at the surface to a depth of ,5 mm.The experiment was performed on the P02?1 beam line atDESY, Hamburg, Germany. The X-rays (wavelength50?2069 A˚) were measured by an area detector and recordedas a TIFF with 32-bit dynamic range.It was found during analysis that it was impossible toachieve a satisfactory fit using Rietveld refinement ifcarbides were included and therefore only austeniteand ferrite were included in analysis. During Rietveldanalysis, the background function (fifth order polyno-mial), incident X-ray intensity, austenite phase fractionand for both phases: lattice parameter(s), crystallite sizeand microstrain were refined. All other parameters werefixed. The phase fraction of ferrite was set to be theresidue of the sample. Microstrain and crystallite sizewere assumed to be isotropic.The goodness-of-fit for each refinement was assessedby using the weighted profile R-factor Rwp. Rwp is1Materials Science and Metallurgy, University of Cambridge, UK2Materials Engineering and Industrial Technologies, University of Trento,Italy3Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany*Corresponding author, email cns28@cam.ac.uk 2015 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteMORE OpenChoice articles are open access and distributed under the terms of the Creative Commons Attribution License 3.0Received 21 August 2014; accepted 7 October 2014DOI 10.1179/1743284714Y.0000000691 Materials Science and Technology 2015 VOL 31 NO 2254defined in equation 1, where xi is the observed intensityat the ith data point, yi is the calculated intensity, wi isequal to the inverse of the square of the standarduncertainty in the observed intensity s2 xi½  {1~ xið Þ{1and the summations are taken over all data points usedin the refinement.16,17 Equation 1 shows that fits bettermatching the data have lower Rwp.Rwp~Pwi(yi{xi)2Pwix2i" #1=2(1)Results and discussionThe results for the sample transformed at 200uC aregiven in Fig. 1. Figure 1a shows the evolution of theaustenite lattice parameter and weight fraction duringheating and Fig. 1b shows the ferrite lattice parametersand c/a parameter ratio. The equivalent results for thesample transformed at 300uC are given in Fig. 2. Notethat the initial expansion of the austenite latticeparameter is as the sample is heated to 500uC with verylittle change in its weight fraction. The reduction inlattice parameter (ac) occurs as the austenite decom-poses. The reduction in lattice parameter at constanttemperature implies that carbon leaves solid solutionwithin the austenite.18,19 At the temperatures beingconsidered, carbon is less stable in solid solution inferrite than austenite, the carbon lost from the austenitecannot enter the ferrite and the only possibility is thatthe carbon precipitates as carbides.The Rwp values for the body-centred cubic (BCC) andbody-centred tetragonal (BCT) ferrite for each sampleare given in Fig. 3. It is evident that the tetragonal latticeprovides a much better fit to the bainitic ferrite unit cell.Austenite fitted well to the face-centred cubic cell,suggesting that there is no geometrical distortion fromthe two-dimensional detector that might give rise topeak asymmetry in the case of ferrite.The c/a ratio for the as transformed alloy is1?0099¡0?0002 and 1?0086¡0?0003 for the samplestransformed at 200 and 300uC respectively. The greaterc/a ratio is expected for the lower temperature because thebainitic ferrite is known to retain more carbon in solidsolution when transformed at lower temperatures.1,5The values for c/a ratio found here are some-what greater than those reported previously for a 0?8wt-%C nanostructured bainite transformed at similartemperatures.15 This is consistent with the currentalloy’s higher bulk carbon content since the partitioningof carbon following diffusionless transformation wouldtake longer to achieve.7 It may also be seen that thevolume fraction of austenite begins to fall after y0?15 hfor the sample transformed at 200uC whereas the phasefractions of the sample transformed at 300uC do notchange until 0?4 h, despite being exposed to the sameheat treatment. This is expected, as the lower thetransformation temperature, the more carbon enrichedthe retained austenite becomes. This leads to a greaterdriving force for carbide precipitation and more rapiddecomposition.1 Results of Rietveld reﬁnement on sample transformedat 200uC: sample was tempered at 500uC for time indi-cated on horizontal axis; dashed lines represent ¡uncertainty in each reﬁned parameter quoted by MAUD2 Results of Rietveld reﬁnement on sample transformedat 300uC: dashed lines represent ¡ uncertainty in eachreﬁned parameter quoted by MAUDHulme-Smith et al. Further evidence of tetragonality in bainitic ferriteMaterials Science and Technology 2015 VOL 31 NO 2 255It is observed in both the 200- and 300uC-transformedsamples that both a and c ferrite lattice parametersincrease during heating. Both lattice parametersdecrease slightly at the same time of the austenite phasefraction change. Subsequently, the a-parameter stabi-lises close to the maximum value. The c-parametergradually decreases towards the a-parameter as carbonslowly escapes the ferrite and the lattice becomes morecubic. Long term heating would result in the c-parameter apporaching the a-parameter, as the latticetends towards the body-centred cubic structure.Conclusions1. The combination of atom probe data showingexcess carbon in solid solution within the bainitic ferrite,and the observed reduced symmetry of the bainiticferrite unit cell, along with thermodynamic and kineticdata support the theory that the initial stages of thebainite transformation do not involve diffusion.2. The results provide further support for thehypothesis that the reluctance of carbon to partitionfrom the bainitic ferrite into austenite is due to itsincreased solubility in tetragonal ferrite.3. Previous work1,5 has shown that the carbon that isretained in solution in bainitic ferrite increases as thetransformation temperature is reduced. This is expectedas diffusion becomes slower at lower temperatures. Thepresent work shows that the same effect can be achievedby increasing the average concentration of carbon inthe steel. These interpretations are all consistent withtheory.7AcknowledgementsWe would like to thank Rolls-Royce plc and theEngineering and Physical Sciences Research Councilfor their support during this project. Parts of thisresearch were carried out at the light source PETRA IIIat DESY, a member of the Helmholtz Association(HGF).References1. F. G. Caballero, M. K. Miller, C. Garcia-Mateo, J. Cornide andM. J. Santofimia: ‘Temperature dependence of carbon super-saturation of ferrite in bainitic steels’, Scr. Mater., 2012, 67, 846–849.2. E. Pereloma, H. Beladi, L. Zhang and I. Timokhina:‘Understanding the behavior of advanced high-strength steelsusing atom probe tomography’, Metall. Mater. Trans. A, 2012,43A, 3958–3971.3. F. G. Caballero, M. K. Miller, C. Garcia-Mateo and J. Cornide:‘New experimental evidence of the diffusionless transformationnature of bainite’, J. Alloys Compd, 2013, 577, S626–S630.4. F. G. Caballero, H. K. D. H. Bhadeshia, K. J. A. Mawella, D. G.Jones and P. Brown: ‘Very strong, low–temperature bainite’,Mater. Sci. Technol., 2002, 18, 279–284.5. F. G. Caballero, M. K. Miller and C. G. Mateo: ‘Openingpreviously impossible avenues for phase transformation in inno-vative steels by atom probe tomography’, Mater. Sci. Technol.,2014, 30, 1034–1039.6. C. Garcia-Mateo, T. Sourmail, F. G. Caballero, V. Smanio, M.Kuntz, C. Ziegler, A. Leiro, E. Vuorinen, R. Elvira and T. Teeri:‘Nanostructured steel industrialisation: plausible reality’, Mater.Sci. Technol., 2014, 30, 1071–1078.7. H. K. D. H. Bhadeshia: ‘Bainite in steels’, in ‘Phase transforma-tions ’87’, (ed. G. W. Lorimer), 309–314; 1988, London, Institute ofMetals.8. E. C. Bain: ‘The nature of martensite’, Trans. AIME, 1924, 70, 25–46.9. E. Honda and Z. Nishiyama: ‘On the nature of the tetragonal andcubic martensites’, Sci. Rep. Tohoku Imperial Univ., 1932, 21, 299–331.10. J. W. Christian: ‘Tetragonal martensites in ferrous alloys – acritique’, Mater. Trans. JIM, 1992, 33, 208–214.11. H. K. D. H. Bhadeshia: ‘Carbon in cubic and tetragonal ferrite’,Philosoph. Magaz., 2013, 93, 3714–3715.12. J. H. Jang, H. K. D. H. Bhadeshia and D. W. Suh: ‘Solubility ofcarbon in tetragonal ferrite in equilibrium with austenite’, Scr.Mater., 2012, 68, 195–198.13. C. N. Hulme-Smith, I. Lonardelli, A. C. Dippel and H. K. D. H.Bhadeshia: ‘Experimental evidence for non-cubic bainitic ferrite’,Scr. Mater., 2013, 69, 409–412.14. F. G. Caballero, M. Miller, H. W. Yen, J. A. Jimenez, C. G.Mateo, L. M. Rivas and J. R. Yang: ‘Carbon supersaturation andtetragonal bainitic ferrite in nanocrystalline bainitic steels’, Proc.143rd Annual Meeting and Exhibition, San Diego, CA, USA, 2014,TMS, unpublished.15. C. N. Hulme-Smith, I. Lonardelli, M. J. Peet, A. C. Dippel andH. K. D. H. Bhadeshia: ‘Enhanced thermal stability in nanos-tructured bainitic steel’, Scr. Mater., 2013, 69, 191–194.16. B. H. Toby: ‘R factors in Rietveld analysis: how good is goodenough?’, Powder Diffract., 2012, 21, 67–70.17. R. A. Young (ed.): The Rietveld method’; 1993, Oxford, OxfordUniversity Press.18. A. Saha Podder and H. K. D. H. Bhadeshia: ‘Thermal stability ofaustenite retained in bainitic steels’, Mater. Sci. Eng. A, 2010, A527,2121–2128.19. A. S. Podder, I. Lonardelli, A. Molinari and H. K. D. H.Bhadeshia: ‘Thermal stability of retained austenite in bainitic steel:an in situ study’, Proc. Royal Soc. A, 2011, 467A, 3141–3156.20. J. G. Speer, E. de Moor and A. J. Clarke: ‘Critical assessment:quenching and partitioning’, Mater. Sci. Technol., 2014, DOI:http://dx.doi.org/10?1179/1743284714Y.0000000628.3 Rwp for BCT and BCC ferrite (smaller Rwp means betterﬁt): a transformed at 200uC and tempered at 500uC forperiods indicated on horizontal axis; b as a but trans-formed at 300uCHulme-Smith et al. Further evidence of tetragonality in bainitic ferrite256 Materials Science and Technology 2015 VOL 31 NO 2

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Further evidence of tetragonality in bainitic ferrite

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