EFFECT OF AUSTENITE GRAIN MORPHOLOGY ON VARIANT...

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Iranian Journal of Materials Science & Engineering Vol. 10, Number 2, June 2013

1. INTRODUCTION

In recent years, bulky ultrafine grained (UFG)materials fabricated by severe plasticdeformation with a mean grain size of less than 1µm have been comprehensively studied, due totheir superior mechanical properties [1-5]. Asignificant large area of grain boundaries per unitvolume in UFG materials can affect martensitictransformation behavior as habit plates withinmartensitic structures cannot grow beyond grainboundaries. Previous studies clearly indicatedthat martensitic transformation can besignificantly influenced by austenite grain size[6-11]. Umemoto and Owen [6] demonstratedthat martensite transformation startingtemperature (Ms) decreases with decrease of theaustenite grain size. Maki et al. [7] studied thevariety of martensite morphologies producedafter being transformed from various grain sizesof austenite at different temperatures. Theirresults indicated that the refinement of austenitegrains decreases the Ms, resulting to a changeinmartensite morphology. Although severalstudies have been reported on the austenite grain-

size effects [8-11], but there is no deep systematicstudy of the martensitic transformation fromultrafine-grained austenite.

It is well known that martensite usuallysatisfies Kurdjumov-Sachs (K-S) orientationrelationship with respect to austenite ({111}A //{011}M, <101>A // <111>M, where subscripts Aand M represent austenite and martensite,respectively) [12]. When the K-S relationship ismaintained, 24 equivalent crystallographicvariants of martensite can form from a singlecrystal of austenite. Table 1 summarizes theorientation relationships between the 24crystallographic variants (V1–V24) of martensiteand austenite under K-S orientation relationship.

When plastic deformation is introduced toaustenite before martensitic transformation, the24 variants of martensite do not form equally andsome specific variants are preferentially formed,resulting to formation of transformation texture.This is so-called variant selection. RecentlyChiba et al. [13] reported that variants oflenticular martensite habit planes that wereclosely parallel to compression planes whichwere favorably formed during ausforming of

EFFECT OF AUSTENITE GRAIN MORPHOLOGY ON VARIANTSELECTION OF MARTENSITE TRANSFORMED FROMULTRAFINE-GRAINED AUSTENITEH. R. Jafarian1,* and E. Borhani2* [email protected]: December 2012 Accepted: March 2013

1 School of Metallurgy and Materials Engineering, Iran University of Science and Technology (IUST), Tehran, Iran.

2 Department of Nanotechnology, Nano-Materials Science and Engineering group, Semnan University, Semnan, Iran.

Abstract: In this research, variant selection of martensite transformed from ultrafine-grained (UFG) austenitefabricated by accumulative roll bonding (ARB) process and subsequent annealing was investigated with respecttomorphology of parentaustenitic phase. The results show that the original shape of austenite grain is very effectivefactor in determiningthe preferred variants of martensite transformed from the elongated ultrafine-grained austenitefabricated by 6-cycles via the ARB process. Annealing treatment of the austenitic samples subjected to the 6-cycle ARBprocessed at 873 K for 1.8 ks suppressed the variant selection by changing the morphology of austenite grains fromelongated ultrafine-grains to fully-recrystallized and equiaxed fine-grains.

Keywords: Martensitic transformation, Accumulative roll bonding, Ultrafine-grained austenite, Variant selection,Severe plastic deformation.

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austenite. Lef et al. [14] investigated variantselection of mechanically induced martensitetransformed from metastable austenite. Theyfound that interaction energy between externallyapplied stress and lattice deformation candetermine preferential marteniste variants basedon Patel and Cohen’s theory [15]. Recently, thepresent authors demonstrated that variantselection rule can be changed in severelydeformed UFG austenite depending on amount ofinduced strain [16]. However, variant selectionof martensite in UFG austenite has been rarelystudied. So the aim of this study is to clarify thevariant selection rules of martensite transformedfrom ultrafine-grained austenite with respect to

morphology of austenite grains, i.e., austenitegrain size and shape.2. MATERIALS AND METHODS

An Fe-24Ni-0.3C (wt.%) alloy, of whichchemical composition is shown in Table 2, wasused in the present study. A cast ingot of the alloywas hot-rolled and then cold-rolled to makesheets with thickness of 1 mm. The sheets wereaustenitized at 1173 K for 3.6 ks for obtaining afully recrystallized and coarse-grained austenitewith a mean grain size of 35µm. To obtainultrafine-grained austenite structures, theaustenitized sheets were subjected to

H. R. Jafarian and E. Borhani

Variant Plane parallel Direction parallel

V1 (111)A// (011)M [1̄01]A// [1̄1̄1]M

V2 [1̄01]A// [1̄11̄]M

V3 [011̄]A// [1̄1̄1]M

V4 [011̄]A// [1̄11̄]M

V5 [11̄0]A// [1̄1̄1]M

V6 [11̄0]A// [1̄11̄]M

V7 (1̄11)A// (011)M [01̄1]A// [1̄1̄1]M

V8 [01̄1]A// [1̄11̄]M

V9 [1̄01̄]A// [1̄1̄1]M

V10 [¯̄101̄]A// [1̄11̄]M

V11 [110]A// [1̄1̄1]M

V12 [110]A// [1̄11̄]M

V13 (11̄1)A// (011)M [101̄]A// [1̄1̄1]M

V14 [101̄]A // [1̄11̄]M

V15 [1̄1̄0]A// [1̄1̄1]M

V16 [1̄1̄0]A // [1̄11̄]M

V17 [011]A// [1̄1̄1]M

V18 [011]A// [1̄11̄]M

V19 (111̄)A// (011)M [1̄10]A// [1̄1̄1]M

V20 [1̄10]A// [1̄11̄]M

V21 [0¯̄11̄]A// [1̄1̄1]M

V22 [0¯̄11̄]A// [1̄11̄]M

V23 [101]A// [1̄1̄1]M

V24 [101]A// [1̄11̄]M

Table 1. The 24 crystallographic variants of martensitefor the K-S orientation relationship

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accumulative roll bonding (ARB) process. Figure1 is a schematic illustration showing the principleof the ARB process [3]. In the ARB process, twopieces of the sheets 1 mm thick are stacked afterdegreasing and wire brushing the contactsurfaces. The stacked sheets are roll-bonded by50% reduction in one pass. Then, the roll-bondedsheet is cut into two, stacked to have the initialdimensions before rolling, and the proceduresdescribed above are repeated again. The ARBprocess can apply significant amount of plasticstrain into materials, because the procedures canbe repeated limitlessly. In order to preventdeformation-induced martensitic transformationduring the present ARB process for obtaining fullyaustenitic structures, stacked sheets were held at873 K, at which austenite phase of the studiedalloy is stable, for 0.6 ks and subsequently roll-bonded. The roll-bonded sheets were immediatelycooled in water. The roll-bonding was carried outunder lubrication condition by the use of a two-high mill having a roll diameter of 310 mm.TheARB was repeated up to 6-cycle, so that totalequivalent strain accumulated was 4.8. The 6-

cycle ARB processed specimens were annealed at773, 823 and 873 K for 1.8 ks to obtain UFGaustenite with partly recrystallized or fullyrecrystallized microstructures. The specimenssubjected to the above mentioned treatments werecooled down to 77 K (in liq. N2) to obtainthermally induced martensite.

The microstructures of the specimens before andafter martensitic transformation were characterizedby electron backscatter diffraction (EBSD) in ascanning electron microscope (SEM) equippedwith a field-emission electron gun (Philips XL30S-FEG) operated at 15 kV. EBSD measurementswere carried out with step size of 0.05µm whichwere smaller than the interval of austenite grainboundaries. The sections of the specimen forEBSD observation were polished mechanically andthen electrolytically in a 900mLCH3COOH+100mL HClO4 solution atapproximately 284K with a voltage of 20V. Theobtained EBSD data were analyzed by TSL-OIMAnalysis software Ver. 6.1.3.3. RESULTS AND DISCUSSION

3. 1. Microstructure of Austenite

In Fig. 2, microstructure of austenite processedby ARB is illustrated by grain boundary andphase maps obtained by EBSD measurementsand analysis. In the grain boundary maps, highangle boundaries with misorientation larger than15° are drawn in black lines and low angleboundaries with misorientation between 2° and15° are drawn in red lines. In addition,martensite and austenite phases are shown in pinkand green colors, respectively. The grainboundary map of Fig. 2(a) indicates that thestarting material consists of fully recrystallizedand coarse grained austenite. The phase maps ofFig. 2(a)-(e) demonstrate 100% austenitestructures, indicating that deformation-inducedmartensitic transformations did not occur during

Fe C Ni Si Mn P S O N

bal. 0.29 24.09 0.01 0.07 <0.005 <0.0005 0.0008 0.0006

Table 2. Chemical composition of the alloy studied (wt.%)

Fig. 1. Schematic illustration showing the principle of theaccumulative roll bonding (ARB) [22].

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the ARB process at 873 K. The 1-cycle ARBprocess, corresponding to 50% rolling or anequivalent strain of 0.8, introduced a largeamount of low angle boundaries into theaustenite. Till 2-cycle of ARB process (75%rolling), the microstructures exhibit typicaldeformation microstructures which mainlyinvolve pancake shape austenite grains havinglow angle boundaries (dislocation structures)inside. Increasing the number of ARB process upto 6-cycle causes significant grain refinement asshown in Fig. 2(e).

The grain boundary and phase maps of austeniteARB processed by 6-cycle and subsequentlyannealed at 773, 823 or 873 K for 1.8 ks are shownin Fig. 3. The grain boundary map of the specimenannealed at 773K (Fig. 3(a)) shows that themicrostructure still consists of UFG lamellarstructure, indicating that annealing at 773 K doesnot cause recrystallization of austenite. Grainboundary map of Fig. 3(b) indicates thatrecrystallization occurs partly by increasing theannealing temperature to 823 K. The recrystallizedregions are free of low angle boundaries.Annealing treatment at 873 K results in fullyrecrystallized and equiaxed austenite with meangrain size of 2.5 µm, as shown in Fig. 3(c).Moreover, phase maps superimposed on grainboundary maps show that no phase transformationoccurs during annealing treatments.

Change in austenite grain size with increasingthe number of ARB cycle and subsequentannealing is summarized in Fig. 4. Interceptionmethod was used to measure the grain size (orinterval of boundaries) along normal direction(ND). Here, high angle boundaries were takeninto account for the grain size (dHAGB)measurement. The figure indicates that the grainsize monotonously decrease with increasingstrain (ARB cycle), and UFG austenite withmean grain size of 750 nm was obtained by 6-


Fig. 2. (a)-(e) Grain boundary and phase maps of the starting material, 1-cycle, 2-cycle, 4-cycle and 6-cycle ARB processedspecimens, respectively.

Fig. 3. (a)-(c) Grain boundary and phase maps of thespecimens ARB processed by 6-cycle and annealed at 773,

823 and 873 K respectively.

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cycle ARB process. Annealing treatment ofaustenite ARB processed by 6-cycle result inincreasing of austenite grain size up to 2.5 µm atannealing temperature of 873 K as shown in Fig. 4.3. 2. Variant Selection of Martensite

Figure 5(a) and (b) show an EBSD orientationmap and corresponding {100}A pole figure of the

starting material. The colors in the orientationmap indicates the crystallographic directions ofeach point parallel to TD of the sheet (the normaldirection of the observed plane), according to thestereographic triangle shown in the figure. Theideal <100>A axes of typical texture componentsin f.c.c crystal, i.e., Brass component({110}<112>A), Copper component({112}<111>A), S component ({123}<634>A),

Fig. 4. Change in austenite grain size (boundary interval along ND) with the number of the ARB cycles and subsequentannealing.

Fig. 5. ((a), (b)) EBSD orientation color map and {100} pole figure of austenite before martensitic transformation in thestarting material. ((c), (d)) EBSD orientation color map and {100} pole figure of martensite after martensitic transformation

in the starting material.

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and Cube component ({100}<001>A) are alsoindicated in the {100}A pole figure as circles,triangles, solid rectangles and open rectangles,respectively. The austenite of the starting materialconsists of equiaxed grains with a mean grainsize of 35 µm, and does not have a particulartexture, as shown in Fig. 5(a) and (b). An EBSDorientation map and corresponding {100}M polefigure of martensite in the starting material afterthermally-induced martensitic transformation areshown in Fig. 5(c) and (d). Because theintensities in the pole figure of Fig. 5(d) are veryweak (max. value is 1.594), it can be said thattransformation texture is not developed duringthermal martensitic transformation from thestarting material having coarse-grained austenitewith random texture.

The EBSD orientation map and corresponding{100}A pole figure of austenite after 6 cycles ofARB are shown in Fig. 6(a) and (b). Figure 6(c)and (d) displays the EBSD orientation maps andcorresponding {100}M pole figures of martensitetransformed from the 6-cycle ARB processedspecimen having elongated ultrafine-grainedaustenite. The colors in the orientation mapsagain indicate the crystallographic directions of

each point parallel to TD of the sheets.Preferentially formed variants of martensite canbe determined by comparing the experimentallymeasured {100}M pole figure with thestereographic projection on which <100>M axesof the ideal 24 variants of martensite are plotted.The positions of the ideal <100>M axes for the 24variants of martensite transformed from theaustenite with exact Brass orientation ((011)[2 1̄1]A) are plotted in Fig. 6(d). It reveals that theV6, V16, V19, V21, V22 and V24 are preferentiallyformed during martensitic transformation fromthe 6-cycle ARB processed specimen havingelongated ultrafine-grained austenite.

Kundu [17] demonstrated that variant selectionin thermally-induced martensitic transformationfrom cold-rolled austenite can be predicted frominteraction energy between residual stress in thecold-rolled austenite and shape deformationaccompanying martensitic transformation.However, the authors clearly demonstrated thatthe variant selection in martensitictransformation from the ultrafine-grainedaustenite fabricated by the 6-cycle ARB processis not determined by the interaction energybetween the residual stress inherited by ARB


Fig. 6. EBSD orientation color maps and {100}A pole figures of austenite after 6 cycles of ARB process ((a), (b)) EBSDorientation color maps and {100}M pole figures of martensite transformed from the 6-cycle ARB processed specimen having

elongated ultrafine-grained austenite ((c), (d)). The positions of the ideal <100>M axes for the 24 variants of martensitetransformed from the austenite with exact Brass orientation ((011) [2 1̄1]A) from the austenite are shown by the numbers in

{100}M pole figure of (d). The preferential variants are indicated by red color.

process and shape deformation [16].As explained, the austenite after 6-cycle of

ARB exhibits lamellar structure elongated alongthe rolling direction (RD) of the sheet. Theaverage interval of high angle boundaries alongthe ND of the sheet is about 750 nm. This is atypical ultrafine grained structure fabricated bythe ARB process. Since martensite plate cannotgrow across high angle boundaries of austenite, itseems that the lamellar shape (elongated alongRD) of the ultrafine-grained austenite (Fig. 2(c))affects the variant selection of martensitetransformed from the 6-cycle ARB processedspecimen. On this basis, habit plane orientationsfor the 24 variants of martensiteare plotted on thepole figure of which coordinate systemcorresponds to Brass orientation (RD // [2 1̄1]A,ND // [011]A), as shown in Fig. 7. The pole figureof Fig. 7 reveals that the normal directions ofhabit planes for the preferentially formed variantsof martensite transformed from the 6-cycle ARBprocessed specimen having elongated ultrafine-grained austenite (represented by red color) arenearly parallel to ND (V6, V16) or TD (V19, V21,V22, V24). The martensite plates of which normaldirections of habit planes are parallel to ND orTD can grow along the elongated direction of the

ultrafine-grained austenite in the 6-cycle ARBprocessed specimen. Therefore, it can be

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Fig. 7. A pole figure showing the habit plane orientationfor the 24 variants of martensite. The preferentially formedvariants of martensite transformed from the 6-cycle ARBprocessed specimen having elongated ultrafine-grained

austenite are indicated by red color. The coordinate systemof the pole figure corresponds to the Brass texture (RD =[2 1̄ 1]A, ND = [011]A), which was found in the 6-cycle

ARB processed austenite. The numbers indicate thepositions of the habit plane orientation for the 24 variantsof martensite transformed from the austenite with exact

Brass orientation ((011) [2 1̄ 1]A).

Fig. 8. EBSD orientation color maps and {100}A pole figures of austenite in ((a), (b)) the specimen 6-cycle ARB processedand then annealed at 773 K for 1.8 ks, and ((c), (d)) the specimen 6-cycle ARB processed and then annealed at 873 K for

1.8 ks.

concluded that in the case of the 6-cycle ARBprocessed specimen having elongated ultrafine-grained austenite, austenite grain shapesignificantly affects the variant selection inmartensitic transformation.

Figure 8(a) and (b) shows an EBSD orientationmap and corresponding {100}A pole figure of the6-cycle ARB processed specimen after annealingat 773 K for 1.8 ks. The 6-cycle ARB processedspecimen still consists of elongated ultrafine-grained austenite after annealing at 773 K. Inaddition, Brass texture is kept after annealing at773 K. This is because annealing at 773 K causesno recrystallization but only recovery, asexplained. In contrast, recrystallization occurs byannealing at 873 K for 1.8 ks, resulting in thedevelopment of Cube texture as shown in Fig.8(c) and (d).

Figure 9 (a) and (b) displays the EBSDorientation map and corresponding {100}M polefigure of martensite transformed from theaustenite 6-cycle ARB processed and thenannealed at 773 K for 1.8 ks having elongatedultrafine-grains. The positions of the ideal<100>M axes for the 24 variants of martensite

transformed from the austenite with exact Brassorientation ((011) [2 1̄ 1]A) are plotted in Fig.9(b). The preferentially formed variants ofmartensite transformed from the 6-cycle ARBprocessed specimen after annealing at 773 K areV6, V16, V19, V21, V22 and V24, which arecompletely the same as those transformed fromthe 6-cycle ARB processed specimen (Fig. 6(d)).This fact supports the previous conclusion thatthe variant selection rule is determined byaustenite grain shape, and not by interactionenergy between the residual stress of austeniteand shape deformation, when the austenite haselongated ultrafine-grained structures becausethe residual stress of austenite, which is inheritedduring the 6-cycle ARB process, is released tosome extent by annealing at 773 K.

Figure 9(c) and (d) show an EBSD orientationmap and corresponding {100}M pole figure ofmartensite transformed from the austenite 6-cycle ARB processed and then annealed at 873 Kfor 1.8 ks having equiaxed (recrystallized) fine-grains. Although the austenite 6-cycle ARBprocessed and then annealed at 873 K for 1.8 kshas the strong cube texture (Fig. 8(c)), the

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Fig. 9. EBSD orientation color maps and {100}M pole figures of martensite transformed from ((a), (b))the specimen 6-cycleARB processed and thenannealed at 773 K for 1.8 ks, and ((c), (d)) the specimen 6-cycle ARB processed and thenannealedat 873 K for 1.8 ks.The positions of the ideal <100>M axes for the 24 variants of martensite transformed from the austenitewith exact Brass orientation ((011) [2 1̄1]A) are shown in by the numbers (b). The preferential variants are indicated by red

color.

intensities of martensite in the pole figure of Fig.9(d) are very weak. This indicates that there is novariant selection of martensite transformed fromthe austenite having fully recrystallized andeqiaxed grains even when austenite grain size isvery fine.4. CONCLUSIONS

In this paper, effect of austenite grainmorphology on variant selection of martensitetransformed from the ARB processed specimenshaving elongated ultrafine-grained austenite (the6-cycle ARB processed specimen) and equiaxedfine-grained austenite (the specimen 6-cycleARB processed and annealed at 873 K for 1.8ks), was studied in the Fe-24Ni-0.3C alloy. Theconclusions are summarized as follows:1. The ARB process resulted in ultra-grain

refinement of austenite. The obtainedultrafine grained (UFG) austenite consistedof the lamellar grains elongated along therolling direction (RD) with mean grainthickness of 750 nm after 6-cycle of theARB process. The starting material hadrandom texture, while the 6-cycle ARBprocessed austenite exhibited thedevelopment of strong Brass component.

2. The variant selection in martensitictransformation from the 6-cycle ARBprocessed specimen having elongatedultrafine-grained austenite indicated thatthe elongated grain shape played animportant role for the variant selection ofmartensite, i.e., the martensite plates whichcan grow along the elongated direction ofthe ultrafine-grained austenite werepreferentially formed.

3. Annealing treatment of the 6-cycle ARBprocessed specimen at 773 K for 1.8 ksinduced no recrystallization but onlyrecovery of austenite, resulting in the sametransformation texture and variant selectionrule with those in the 6-cycle ARBprocessed specimen. In contrast, annealingtreatment of the 6-cycle ARB processedaustenite at 873 K for 1.8 ks suppressed thevariant selection by changing the austenitestructure from elongated ultrafine-grains to

fully-recrystallized and equiaxed fine-grains.

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