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Materials Science and Engineering A 528 (2011) 7909– 7914

Contents lists available at ScienceDirect

Materials Science and Engineering A

jo ur n al hom epage: www.elsev ier .com/ locate /msea

orphologies and characteristics of deformation induced martensite during lowycle fatigue behaviour of austenitic stainless steel

rpan Dasa,b,∗, S. Sivaprasada, P.C. Chakrabortib, S. Tarafdera

Fatigue & Fracture Group, Materials Science & Technology Division, National Metallurgical Laboratory (CSIR), Jamshedpur 831 007, IndiaDepartment of Metallurgical & Material Engineering, Jadavpur University, Kolkata 700 032, India

r t i c l e i n f o

rticle history:eceived 30 March 2011eceived in revised form 8 July 2011ccepted 9 July 2011vailable online 19 July 2011

a b s t r a c t

Quantification, formation and nucleation micro-mechanisms of deformation induced martensite dur-ing low cycle fatigue behaviour of austenitic stainless steel have been investigated at various strainamplitudes tested at ambient temperature. The evolutionary deformation induced martensite hasbeen quantified through magnetic measurement technique. It has been found that as strain ampli-tude increases, the volume fraction of deformation induced martensite increases. Extensive analytical

eywords:ustenitic stainless steeleformation induced martensiteow cycle fatigueerritescope

transmission electron microscopy studies showed more than one nucleation site for martensitic trans-formation and the transformation micro-mechanisms have been observed to be: � (fcc) → � (hcp), � (fcc)→ �′ (bcc), � (fcc) → deformation twins → �′ (bcc) and � (fcc) → � (hcp) → �′ (bcc).

© 2011 Elsevier B.V. All rights reserved.

ransmission electron microscopy

. Introduction

It is well established that most of the austenitic stainless steelsre unstable and transformed into martensites (i.e. � (hcp) and′ (bcc)) during deformation depending upon their stacking faultnergy, which control the formation of defects [1]. The stackingault energy strongly depends upon the chemistry of the alloynd temperature of deformation [1]. Martensitic transformation,isplacive in nature is believed to be triggered when susceptibleustenitic stainless steel is deformed at temperature below Md30. Its reported that a number of factors, like steel chemistry, deforma-ion mode, strain, stress, strain rate, strain path, grain size, grainrientation and temperature of testing influence the formationf deformation induced martensite [1]. Martensitic transforma-ion micro-mechanisms are well established and connected with aronounced volume increase by 2.57% [2]. Some researchers have

nvestigated the relationship between the extent of ˛′ (bcc) marten-ite and other deformation parameters under uniaxial deformationondition, but the information in this respect is limited for cyclic

lastic deformation processes. Although considerable knowledgease is available on this topic for austenitic stainless steels ineneral, the understanding developed for a particular alloy sys-

∗ Corresponding author at: Department of Metallurgical & Material Engineering,adavpur University, Kolkata 700 032, India. Tel.: +91 9934328051;ax: +91 657 2270527.

E-mail address: dasarpan1@yahoo.co.in (A. Das).

921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2011.07.011

tem under certain deformation conditions cannot be generalisedto other metastable systems. Investigations are, therefore, neces-sary to understand the deformation induced phase transformationin specific cases.

In order to improve the cyclic mechanical performance andfracture resistance of austenitic stainless steels under low cyclefatigue loading condition, it becomes essential to understand therelationship between the deformation factors that influence micro-mechanisms of martensitic transformations, their nucleation sitesand the resulting mechanical behaviour, which are primarily aimedin this study.

2. Experimental

Commercial grade AISI 304LN stainless steel has been used inthe present study. The corresponding Md30 temperature and stack-ing fault energy are calculated to be 2.8 ◦C [3] and 18.9 mJ/m2

[4] respectively. Completely reversed strain controlled low cyclefatigue tests at ten different total strain amplitudes (�εt/2) rang-ing from ±0.35% to ±2.0% were carried out on smooth cylindricalspecimens of 14.00 mm gauge length and 7.00 mm gauge diameter.Tests were conducted at laboratory ambient conditions until com-plete fracture of the specimens in a servo-electric testing machineof ±100 kN load capacity. A triangular waveform with a constant

true tensile strain rate of 2 × 10−2 s−1 was used for cyclic strain-ing. An axial extensometer of 12.5 mm gauge length was attachedto the specimen surface for controlling the test parameter. Speci-mens for transmission electron microscopy study were extracted

7910 A. Das et al. / Materials Science and Engineering A 528 (2011) 7909– 7914

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Fig. 2. The extent of deformation induced martensite as a function of strain ampli-

ig. 1. Peak tensile stress as a function of number of cycles to failure at varioustrain amplitudes at constant strain rate of 0.02 s−1 for 304LN stainless steel testedt room temperature.

y transverse sectioning of the fatigue failed specimens leaving–3 mm distance from the fracture surfaces to avoid the regionsf extensive plastic deformation. Magnetic measurement tech-ique (i.e. Feritescope measurement) has been extensively used forhe quantification of �′ (bcc) martensite (magnetic phase) formeduring strain cycling at various strain amplitudes on the fatigueailed samples throughout their gauge lengths (but not on the frac-ure surfaces). Approximately 30–35 readings were taken for eachatigue failed specimens. The feritescope reading was converted tohe actual martensite content by using Talonen’s correlation [1]. Inhe as received condition, there was no martensite present in thelloy.

Extensive analytical transmission electron microscopy (CM 200)nvestigation has been carried out to understand the martensiticransformation micro-mechanisms and their nucleating sites at anperating voltage of 200 kV under two beam conditions. Selectedrea diffraction patterns have been analysed to calculate the zonexis of the phases present.

. Results and Discussion

It is well established that cyclic deformation alters theicrostructure and causes the instabilities which influence the

yclic plastic performances upon varying the loading conditions.he variation of peak tensile stress with the progression of strainycling is presented in Fig. 1 for various total strain amplitudes�εt/2) studied. It is observed that hardening occurs at the ini-ial stage of cyclic straining and peak tensile stress is reachedithin a maximum number of 10 to 15 cycles depending upon their

mplitudes. Following the hardening stage, continuous softeningccurred in all cases until fracture. At lower strain amplitudes,he degree of the secondary hardening is noted to be insignificant.yclic hardening–softening behaviour has been extensively studied

n numerous materials. It is by now well established that the initialardening results from dislocation multiplication and their mutual

nteractions, interaction of dislocations with solutes and other pre-ipitates [5,6]. Additionally, the increase in slip band density andctivation of number of slip systems have also been reported toear a direct correlation with the strain amplitude dependent cyclicardening behaviour [5]. On the other hand, the softening responsefter the initial hardening results from the generation of additionalobile dislocations and formation of dislocation cell structure of

owered internal stress [5]. Fig. 1 reveals that although the naturef variation of peak tensile stress with the number of cycle is sim-

lar, the degree of cyclic softening is less with increasing strainmplitude. When the strain amplitude is less, the drop in the stressmplitude is high. At higher strain amplitude (i.e. ±0.85%), sec-ndary hardening occurs, which means that the stress amplitude

tude – ferritescope measurement.

increases gradually and it has become more prominent with theincrease in the strain amplitude. This phenomenon is attributed tothe strain amplitude dependent (and stress dependent) martensitictransformation during cyclic straining. Formation of deformationinduced martensite would also influence the dislocation substruc-ture configurations [6].

Deformation induced martensitic transformation in metastableaustenitic stainless steel is known to have an important role on thecyclic stress response under strain-controlled fatigue tests in sucha manner that with the formation of higher amount of marten-site, cyclic softening is not only arrested but may even give rise tosecondary hardening [5] depending upon the grain size and strainamplitude. Present study showing lesser softening with increas-ing strain amplitude inferring that greater amount of martensiteformation occurs during cyclic loading at higher strain amplitude.This has been verified by quantifying the amount of deformationinduced martensite on fatigue failed specimens. On the basis ofcyclic plasticity characteristics of the present alloy and the associ-ated experimental results, dislocation activities and interactionswith deformation induced martensite also controls the micro-mechanisms of cyclic hardening–softening behaviour.

As shown in Fig. 2, it is indeed observed that the extent of defor-mation induced martensite increases with the increase in imposedstrain amplitude. This is in conformity to the other investigationsreported elsewhere [6–10]. Even though there is scatter in the datapoints (Fig. 2), the trend is increasing in nature. The scatter is pri-marily due to the fact that deformation induced martensite does notform uniformly throughout the gauge length of the fatigue speci-men. However, the nature of variation of the martensite volumefraction formed due to cyclic loading is more or less same in mostof the published literatures. It will be worthwhile to refer, in light ofthe above discussion, to the influence of martensitic transformationduring cyclic plasticity, micro-mechanisms of transformation andtheir structural morphologies that have been observed and pro-posed for metastable austenitic stainless steels and multi phasealloys. Grosse et al. [7] reported a linear increase of deformationinduced martensite with the increase in strain amplitude and num-ber of cycles for AISI 321 stainless steel. Raman and Padmanabhan[5] has found a linear relationship between the extent of defor-mation induced martensite measured on the fracture surfaces andthe strain amplitude for AISI 304LN stainless steel in their elegantstudy, a fair correlation between fatigue induced martensite andmonotonic strength enhancement for AISI 304, 304L and 301 stain-less steels have also been investigated by Maier et al. [8]. Sadough

et al. [9] has found that initially deformation induced martensiteformation increases and later there is a tendency towards satura-tion for AISI 304L stainless steel.

A. Das et al. / Materials Science and Engineering A 528 (2011) 7909– 7914 7911

Fig. 3. Cyclically deformed AISI 304LN stainless steel showing the comparison of extent of deformation induced martensite and shear band density at strain amplitude of (a)±0.50% and (b) ±1.00% respectively.

Fig. 4. Evolution of �′ martensite (bcc) and � martensite (hcp) with zone axis of [0 2 3] and [1 2 1 6] respectively in � austenite (fcc) matrix with zone axis of [0 1 4] fromshear band intersection (at strain amplitude of ±0.35%).

Fig. 5. Formation of �′ martensite (bcc) and � martensite (hcp) with zone axis of [1 2 3] and [0 1 1 1] respectively in � austenite (fcc) matrix with zone axis of [1 1 4] at grainboundary triple point (at strain amplitude of ±0.7%).

Fig. 6. Formation of �′ martensite (bcc) with zone axis of [1 2 3] in � austenite (fcc) matrix with zone axis of [1 1 4] at isolated shear band (at strain amplitude of ±0.85%).

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912 A. Das et al. / Materials Science an

Microstructural investigations have been carried out on fatigueailed specimens, revealing inhomogeneous formation of deforma-ion induced martensite. Representative optical images reveal suchnhomogeneous distribution of deformation induced martensitender cyclic plastic deformation and a comparison of its extent ishown in Fig. 3(a-b) at strain amplitudes of ±0.50% and ±1.00%espectively. Other investigators have studied the deformationondition, type and amount of deformation induced martensiteormation in metastable austenitic stainless steels. It is reportedhat deformation induced martensite formation sets in crossing thehreshold plastic strain amplitude of about 0.30% [11,12]. Belowhis threshold value, deformation is governed by slip band forma-ion and � (fcc) → � (hcp) transformation, the micro-mechanisms ofhich depend on initial austenite grain size (initial micro-textures),

teel chemistry and temperature of testing [1]. Baudry and Pineau13] reported that plasticity induced martensitic transformationuring cyclic deformation process occurs through: � (fcc) → � (hcp)

�′ (bcc) transformation sequence and Md30 is about 100 ◦C. Theylso showed that Md plays a decisive role for the morphologicalppearance of �′ (bcc) martensite under cyclic loading condition.n the other hand, Abrassart [14] reported that � (fcc) → � (hcp)

ransformation occurs at temperature above Ms [� (fcc) → �′ (bcc)].inally the extent of �′ (bcc) martensite formation is reported toncrease with increasing strain amplitude because of increasedccumulation of plastic strain [5,6,10]. Formation of greater amountf deformation induced martensite with increasing strain ampli-ude has been found to bear a direct consequence on cyclic strainardening in such a manner that cyclic hardening becomes moreith higher strain amplitude as reported for AISI 304L, SUS 304 –P and 316LN stainless steels [6,7]. In the present study, it has been

ound that the progression of martensitic transformations can be: (fcc) → � (hcp) (Figs. 4, 5, 7A and 7C), � (fcc) → � (hcp) → �′ (bcc)

Figs. 4, 5, 7A and 7C), � (fcc) → �′ (bcc) (Figs. 6, 7A and 7C), and �fcc) → deformation twins → � (hcp) (Fig. 6) and they are primar-ly depending upon the thermodynamical energy (i.e. �GCHEM) ofeformation induced martensite formation [15]. Taking account ofbove discussed studies including our present findings, it is con-luded that the formation of deformation induced martensite inustenitic stainless steel depends upon the chemistry (grade) of theteel through its effect on stacking fault energy, grain size, initialicrotexture, the loading condition (i.e. stress, strain, strain rate),

ocal stress-state and damage concentration. Thus, the total freenergy change of martensitic transformation is the sum of �GCHEMnd �GMECH. Patel and Cohen [16] have described the criterion forhe application of external stress on martensitic transformation.ccording to them, when the external force is acting, the result-

ng effect on the Ms temperature is calculated from the mechanicalork done (i.e. �GMECH) on or by the transforming region as the

esolved shear and normal components of the applied stress arearried through the corresponding transformation strain.

The difference in cyclic stress response observed in the presenttudy is solely due to the formation of varying amount ofeformation induced martensite depending upon the imposedtrain amplitude (i.e. stress). Deformation induced martensite inustenitic stainless steels acts as an elastic reinforcing phase (i.e.omposite kind) which subsequently influences the deformationehaviour as loading continues. According to Mei and Morris [17],everal mechanisms are involved by which deformation inducedartensite influences fatigue crack initiation and propagation

ehaviour. Mangonon and Thomas [2], and Bhadeshia [18] reportedhat martensitic transformation influences incremental strain andhe resulting composite microstructure alters the crack path and

he local mechanical properties. The formation of deformationnduced martensite during cyclic loading at varying strain ampli-udes is accompanied by volume expansion. The compressive stressenerated due to volume expansion and the absorption of energy in

ineering A 528 (2011) 7909– 7914

the process of austenite to martensite transformation delays crackinitiation and propagation by gradual strengthening and tough-ening the material while deformation precedes at varying strainamplitudes [5].

Microstructural characteristics, morphologies and nucleationmicro-mechanisms of deformation induced martensitic trans-formation in cyclically deformed specimens have been studiedthrough extensive analytical transmission electron microscopy.Representative transmission electron microscopy bright fieldimages show the deformation induced martensite nucleation sitesin Figs. 4–7 at various strain amplitudes.

On strain cycling, at shear band intersection, commonly knownmartensite nucleation site, shown in Fig. 4 contains both � (hcp)and ˛′ (bcc) martensites with zone axes of [1 2 1 6] and [2 3 4]respectively in � (fcc) matrix with zone axis of [0 1 4] for cyclicloading at ±0.35% strain amplitude. The present finding as shownin Fig. 4 is in accordance with the well known model for marten-sitic transformation proposed by Mangonon and Thomas [2] andMaxwell et al. [19] as: � (fcc) → � (hcp) → �′ (bcc). Staudham-mer et al. [20] referred this type of martensitic transformation asstress assisted type (i.e. stress assisted martensite). The � (hcp)martensite is formed by overlapping (i.e. regular superposition)stacking faults, and therefore, it is finely dispersed and its mor-phology is heavily faulted. The probability of this transformationincreases with decreasing stacking fault energy. Consequently, itis very difficult to distinguish between single stacking faults, bun-dles of overlapping stacking faults, and faulted or perfect � (hcp)martensite. Therefore, a collective term shear band [21] has oftenbeen used to designate the microstructural features/morphologiesoriginating from the formation and overlapping of stacking faultsin austenitic stainless steels.

Fig. 5 shows that both � (hcp) and �′ (bcc) martensites withzone axes [0 1 1 1] and [1 2 3] respectively nucleate at grain bound-ary triple points in � (fcc) matrix with the zone axis of [1 1 4] forcyclic loading at ±0.70% strain amplitude. This again follows thesequence of stress assisted martensite transformation. Reed andGuntur [22] had shown that the amount of � (hcp) martensite firstincreases with the degree of deformation and then decreases, whilethe �′ (bcc) martensite content at the same time increases con-tinuously. In the present study, we have not quantified � (hcp)martensite, but �′ (bcc) martensite fraction increases with theincrease in strain amplitude (Fig. 2). The morphologies of defor-mation induced that � (hcp) martensite has been observed to beas thin parallel individual striations or packets which extend com-pletely across the grains or as platelets arranged in shear bands.Fig. 6 shows that �′ (bcc) martensite with zone axis of [1 2 3]nucleated in � (fcc) matrix with zone axis of [1 1 4] inside theisolated micro shear band at strain amplitude of ±0.85%. Olsonand Cohen [23,24], Lecroisey and Pineau [25] and Maxwell et al.[19] referred this type of transformation as strain induced type(i.e. strain induced martensite). The �′ (bcc) martensite which hastransformed directly from � (fcc) is observed to be irregular, blockyand sometimes lenticular. Undoubtedly the �′ (bcc) martensitetransformation is closely associated with a strong increase in thework-hardening rate and governs the ductility for austenitic stain-less steel. Kelly [26] reported that �′ (bcc) martensite nucleatedfrom � (hcp) martensite in an austenitic Fe–Cr–Ni–C steel wasformed as laths. Nucleation of �′ (bcc) martensite has frequentlybeen observed at the junction of two � (hcp) martensite bands withthe �′ (bcc) martensite laths parallel to the 〈110〉� direction [27–29].But, some researchers have found the �′ (bcc) martensite nucle-ation to take place also within single shear bands [30–31]. This is

in agreement with our present findings (i.e. Figs. 6 and 7B) also. Ingeneral, the �′ (bcc) – martensite nucleation involves a process bywhich an array of Shockley partial dislocations, i.e., another shearband, can penetrate through the other shear band (i.e. Figs. 4 and 7A

A. Das et al. / Materials Science and Engineering A 528 (2011) 7909– 7914 7913

Fig. 7. Shear band morphologies: (A) evolution of �′ martensite (bcc) and � martensite (hcp) with zone axis of [0 1 1] and [0 1 1 1] respectively in � austenite (fcc) matrixfrom parallel micro shear bands, (B) evolution of �′ martensite (bcc) with zone axis of [0 1 1] in � austenite (fcc) matrix with zone axis of [0 2 2] at isolated micro shear band,( ] and [n ] (at st

avwffie(a[omsumttmug[rv

TM

C) evolution of �′ , martensite (bcc) and � martensite (hcp) with zone axis of [0 1 1o transformation takes place, only � austenite (fcc) matrix with zone axis of [0 0 1

nd 7C). In a polycrystalline steel, the �′ (bcc) martensite contentaries from grain to grain depending on the grain orientation [31],hich is probably attributed to the dependence of the shear band

ormation and �′ (bcc) martensite nucleation on the local stresseld (Fig. 3). Tamura et al. [32], Lecroisey and Pineau [25], Maxwellt al. [19] and Petit et al. [33] distinguished between stress assistedi.e. SAM) and strain induced (i.e. SIM) martensitic transformationsccording to their nucleation sites in their elegant studies. Gey et al.31] found that not all of the 24 possible crystallographic variantsf �′ (bcc) martensite formed when the alloy underwent transfor-ation during tensile deformation. They demonstrated that the

election of particular variants by comparing calculated pole fig-res containing poles from all 24 �′ (bcc) orientations againsteasured pole figures. During deformation induced martensitic

ransformation, variant selection takes place. It has been reportedhat some variants preferentially appeared during the transfor-

ation depending upon the local stress-state. Hence there is noniform distribution of deformation induced martensite in fatiguedauge length due to variation in local stress-state (Fig. 2). Lee et al.

34] derived quantitatively the probability for a nucleation site toeally act as a function of external stress-state for each martensiteariant.

able 1icromechanisms of martensitic transformations during cyclic plastic deformation of 30

Figures SA Mechanism

Fig. 4 ±0.35% Shear band intersection

Fig. 5 ±0.70% Grain boundary triple poi

Fig. 6 ±0.85% Isolated micro shear band

Fig. 7A ±1.60% Shear band intersections

Fig. 7B ±1.60% Isolated shear band

Fig. 7C ±1.60% Shear band intersection

Fig. 7D ±1.60% Isolated shear band

Fig. 7E ±1.60% Isolated shear band

0 1 1] respectively in � austenite (fcc) matrix at shear band intersections, (D and E)rain amplitude of ±1.6%).

Recently the present authors [35] demonstrated in their recentwork that a large amount of published data relating the fractionof deformation induced martensite to plastic strain can in fact bedescribed in terms of the pure thermodynamic effect of appliedstress. The influence of strain is uncertain. Fig. 7 shows the mixedkind of morphologies and characteristics of deformation inducedmartensites’ nucleation sites (i.e. shear bands, shear band intersec-tion, parallel shear bands, etc. This is an interesting feature whichproves that martensite can nucleate at their any nucleation sitesaccording to their favourable interaction energies (i.e. variant selec-tion).

Recently Kundu et al. [36] has observed the correlation betweenthe proportion of the driving force for transformation that isattributed to stress, and the extent of variant selection in their ele-gant study. Table 1 summarizes the types of deformation inducedmartensites formed in the present study and also their nucle-ation sites according to the TEM experiments and selected areadiffraction pattern analysis. Present authors have also shownvarious nucleation sites of deformation induced martensite and

discussed their multiple transformation mechanisms in tensiledeformed AISI 304LN stainless steel at various strain rate conditions[37,38].

4LN stainless steels at various strain amplitudes tested at ambient temperature.

Phases Zone axis

�, �′ , � [1̄ 2 1̄ 6], [0 2 3], [0 1 4]

nt �, �′ , �[

0 1 1̄ 1]

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1 2̄ 3̄]

, [1 1̄ 4̄]

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0 1 1̄ 1]

,[

0 1̄ 1̄]

�′ , � [0 1 1], [1 2 2]�, �′ [0 1 1 1], [0 1 1]� [0 0 1]� [0 0 1]

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914 A. Das et al. / Materials Science an

. Conclusions

Formation of deformation induced martensite increases withncreasing total strain amplitudes. Deformation induced marten-ite does not form uniformly throughout the fatigued gauge length;t depends upon local stress-state conditions and hence variantelection. Transmission electron microscopy results in this presenttudy conclude that the nucleation of deformation induced marten-ite in AISI 304LN stainless steel can simultaneously occur atocations other than intersecting shear bands which is commonlyeferred in literatures. It can form at isolated shear band, shearand intersections, grain boundary triple points, grain boundary-hear band intersection, etc. wherever the interaction energy isavourable. Moreover, under low cycle fatigue condition, multi-le mechanisms such as: � (fcc) → � (hcp), � (fcc) → � (hcp) →′ (bcc), � (fcc) → �′ (bcc) and � (fcc) → deformation twin → �′

bcc) are involved in the formation of deformation induced marten-ite depending upon the thermodynamical energy of martensiticransformation [18].

cknowledgements

The authors express their gratitude to Dr. M. Ghosh, Scientist foris valuable suggestions while carrying out transmission electronicroscopy characterisation. The authors would also like to thank

o all the respected reviewers for their positive and constructiveomments which helped a lot to prepare this manuscript.

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[[[

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