CommunicationSlip System Activity During CyclicPlasticity

ARPAN DAS

The present investigation discusses about the occurrenceand activity of slip systems in low-cycle fatigue defor-mation of metastable austenite (fcc) at various totalstrain amplitudes (Det) under ambient temperature,where the rate of deformation is kept constant. It hasbeen found that with the increase in strain amplitude,the activity of the single slip system decreases and that ofthe multiple slip system increases.

DOI: 10.1007/s11661-014-2295-0� The Minerals, Metals & Materials Society and ASMInternational 2014

Significant amount of research work has been carriedout on the cyclic plastic behavior of metastable austen-itic stainless steels since the introduction of these alloysinto industrial installation. Type AISI 304LN stainlesssteel is one that is metastable and paramagnetic.Metastable austenitic stainless steels undergo deforma-tion-induced martensitic (DIM) transformation fromaustenite c (fcc) into e (hcp) and a¢ (bcc) martensitesunder monotonic and cyclic deformation at ambienttemperature. Austenite to DIM transformation isbelieved to be triggered when the susceptible austeniticstainless steels are deformed at temperatures belowMd30. It was reported that a number of factors, e.g., steelchemistry, stress state, stress, strain, strain rate, grainsize, initial crystallographic microtexture, and tempera-ture of deformation influence the formation of DIM.[1,2]

The present author also demonstrated that a largeamount of published data relating the fraction of DIMto plastic strain, in fact, can be described in terms of thepure thermodynamic effect of applied stress.[2]

These alloys are generally employed in applicationsranging from the pharmaceutical instruments to pipingcomponents in the nuclear power plant industriesbecause of their excellent combination of mechanicalproperties and corrosion resistance. In many applica-tions, these steels are often submitted to cyclic mechan-ical loading. To improve the fatigue life of a component,a better understanding of the cyclic behavior is requiredat many different scales. The identification of microm-echanics of cyclic plastic damage on the microstructuralscale is of prime importance. Martensitic transformation

micromechanics and the deformation-induced disloca-tion substructures were discussed thoroughly for thesame material by the present author elsewhere.[3,4] Thisarticle aims to go further in the analysis of these cyclicplastic deformation micromechanisms. The occurrenceand activity of slip systems were investigated for all thedeformed specimens at various strain amplitudes. Thisstudy was achieved by combining field emission gun–electron backscattered diffraction (FEG-EBSD) analysiswith scanning electron microscopy (SEM).In order to test the hypothesis, many strain-controlled

low-cycle fatigue (LCF) experiments were performedunder room temperature by simply varying the totalstrain amplitudes (Det) at constant strain rate exten-sively. The extent of DIM was also systematically variedthrough the variation of strain amplitudes of LCF testswhere initially there was no martensite present for thealloy. Such a scheme was implemented on metastableAISI 304LN austenitic stainless steel. The chemicalcomposition (in wt pct) of the material was: C 0.03, Mn1.78, Si 0.65, S 0.02, P 0.034, Ni 8.17, Cr 18.73, Mo 0.26,Cu 0.29, N 0.08, and balance Fe. The Md30 temperatureof the material at which 50 pct of the austenitetransforms to martensite at a true strain of 0.30,calculated from the equation of Angel,[5] was found tobe 275.8 K (2.8 �C). Initial austenite grain size of thematerial was found to be approximately 70 lm. Thetensile properties of AISI 304LN stainless steel at fivedifferent strain rates were determined through tensiletests of round bar cylindrical solid specimens, asdiscussed elsewhere.[6,7] In the present investigation,extensive magnetic measurements were carried out tomeasure DIM content in specimens tested at varioustotal strain amplitudes. Specimens for FEG-EBSDexperiments were extracted by transverse slicing of theLCF-failed specimens leaving 2 to 3 mm distance fromthe fracture surfaces to avoid the regions of excessiveplastic deformation. Those thin slices were convention-ally polished (metallography) and electropolished inCH3COOH : HClO4 = 9 : 1 (electrolyte) at a constantvoltage of 22 V and maintaining the sample anode andstainless steel bowl–cathode for 70 seconds with con-stant stirring. The temperature of the steel bath wasmaintained at 258 K (�15 �C) throughout. In general,the pattern indexing quality in EBSD was very high(> 98 pct) and consistent for all the frames scanned. TheEBSD technique was extensively employed for obtainingthe crystallographic information of the austenite grainsand the associated Schmid factors at various strainamplitudes. The parameters that were kept constant forall the EBSD investigations are as follows: scan area(3509 200 lm) magnification (5009) and step size(0.50 lm). Several frames (6–9) were scanned for eachspecimen. Only austenite grains were considered,because the extent of a¢ (bcc) martensite was signifi-cantly less for all the strain amplitudes.The details of the cyclic plastic behavior of the current

material were reported by the present author else-where.[3,4] In these two references, cyclic plasticitybehavior at ambient temperature was demonstratedthrough understanding of the solid-state martensitictransformation of the present metastable austenitic

ARPAN DAS, Scientist, is with the Fatigue & Fracture Group,Materials Science & Technology Division, CSIR – National Metal-lurgical Laboratory, Jamshedpur 831 007, Jharkhand, India. Contacte-mail: dasarpan1@yahoo.co.in

Manuscript submitted October 1, 2013.Article published online April 15, 2014

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stainless steel and the evolution of dislocation cellsubstructures at various strain amplitudes. It would beworth discussing now the slip system activity in thedeformed austenite grains at all the strain amplitudes;this activity is responsible for the cyclic plastic responseof this steel in the present context.

Figure 1 shows that with the increase in strain ampli-tude, DIM content increases drastically. The crackdensity measured on the fatigue fractured surfaces alsoincreases with the increase in strain amplitude, which isexpected. Baudry and Pineau[8] concluded in their pioneerstudy that both the initiation and propagation of fatiguecracks are accelerated by the formation of a¢ (bcc)martensite under strain-controlled LCF deformation.Both the initiation and propagation of fatigue cracks areaccelerated by the formation of deformation-induced a¢(bcc)martensite. The increase in crack propagation rate inthe presence of a¢ (bcc)martensite is strongly related to theincrease in the cyclic plastic stress response associatedwith the formation of DIM. The quantification and themicromechanisms of martensitic transformation duringcyclic plasticity behavior of the present material werereported in detail by the present author elsewhere.[3,4,9]

There is a strong correlation between DIM measured onthe fatigued gage length surfaces with the crack densitymeasured on the fatigued fracture surfaces at variousstrain amplitudes (Figure 1). It is to be noted that both areincreasing with the increase in strain amplitudes. Adetailed discussion related to the magnetic response ofall the specimens cyclically deformed at various strainamplitudes is made elsewhere.[9]

Severity in cyclic deformation influences the slip ortwinning mode and the subsequent crack initiation andpropagation behavior,[10] which will be discussed later.Conversely, the cyclic deformation, which alters themicrostructure, can cause the instabilities influencing thefatigue performance of the material upon varying thestrain amplitudes, which was demonstrated by Ramanand Padmanabhan[10] in their elegant study. Figure 2(a)shows the typical slip features and the morphologies ofDIM after the LCF test. The activated slip systems aremarked in red. Cyclic fracture features consist of differ-entially oriented fatigue ductile striations (Figure 2(c))

with different amount of secondary cracks (Figure 2(b))throughout the fracture surfaces. The crack is found topropagate along the shear bands at the interfacebetween the shear band and other adjacent low strainregion shown by an arrow. Hence, the fatigue crackpropagates along the direction perpendicular to theloading axis.It is well known that there are 12 slip systems, {111}h110i, in fcc materials. The cyclic plasticity energies areprimarily accumulated in the austenitic grains accordingto their severity in deformation (i.e., imposed strainamplitudes). Straight slip lines have been observed duringthe first few cycles for all the specimens. This is inaccordancewith the observation ofEl Bartali et al.[11,12] intheir elegant studies. Austenitic grains that consist of slipmarkings are relatively well distributed, and few of thesurface morphologies remain rough (Figure 2). The slipsystem is activated when associated slip markings areobserved at the specimen surface. According to Zhangand Wang,[13] the activated slip systems near the grainboundaries can be 2 to 8, indicating that the slipdeformation becomes complicated due to the strainincompatibility by the grain boundary under cyclic plasticdeformation. The work of Gough[14] shows that a metaldeforms under cyclic strain by slip on the same atomicplanes and in the same crystallographic directions as inunidirectional strain. Whereas the unidirectional defor-mation slip is usually widespread throughout all thegrains, in fatigue, some grains will show slip lines whileother grains will give no evidence of slip. Slip lines aregenerally formed during the first few thousand cycles ofstress. Successive cycles produce additional slip bands,but the number of slip bands is not directly proportionalto the number of cycles of stress. Therefore, the occur-rence of slip during LCF deformation does not mean thata crack will form. This was done according to the study ofEl Bartali et al.[11,12]

The surface of martensite plates is characterized ascomplex and very rough (Figure 2). The austenite grainswhere the martensite plates were present were found tobe highly strained, and the intersection of shear bandswas found frequently. Present author showed that thenon-uniform distribution of martensite indicatesthe in homogeneous nature of deformation on themicroscale.[15] It was determined that there was avariation of slip markings at the fatigue surfaces depend-ing upon the strain amplitude imposed. The slip systemwas activated when the slip marks were observed, and theactivity was found to be different due to the variation inimposed strain amplitude and the strain accumulationduring cycling. FEG-EBSD measurements were per-formed extensively for all the fatigued failed samples toidentify the slip system activated in the deformed austen-ite grains. Figure 3 shows the relative frequency of theSchmid factors calculated for all the potential slip systemsin the deformed austenite grains (about 500 grains) atvarious strain amplitudes. In most of the deformedaustenite grains, one slip system was activated with aSchmid factor of around 0.48. The secondary slip system,when activated, had a Schmid factor higher than 0.355.The slip system with a Schmid factor smaller than 0.27was never activated. All the curves shown in Figure 3 are

Fig. 1—Extent of martensite measured on fatigued gage length sur-faces and crack density measured on the fracture surfaces as a func-tion of total strain amplitude.

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precisely overlapped, confirming the slip system activitycriteria in the deformed austenite grains.

The curves represented in Figure 3 show two slopechanges, at 0.34 and 0.42, which define three differentSchmid factor domains. In the first domain (0.275 £m £0.285), no slip system is activated for all the strainamplitudes. In the second domain (0.355 £m £ 0.425),multiple slip systems are activated for all strain ampli-tudes. In the third domain (0.435 £m £ 0.505), only oneslip system is activated and it was found to be 0.47 forall the strain amplitudes. In the present study, the extentof DIM was substantially less for all the strain ampli-tudes, hence it was not considered.

According to El Bartali et al.,[11,12] identification of slipsystems in bcc structures is more complex than in the fccphase because of the much higher number of possible slipsystems and the easy cross-slip. Figure 4 shows theSchmid factor domain distribution with the imposedstrain amplitudes. It has been investigated that no slipsystems are activated in the first domain of the Schmidfactor for all the strain amplitudes. In the second domainof the Schmid factor, the activity (relative frequency) ofmultiple slip systems increases with the increase in strainamplitudes. In the third domain of the Schmid factor,there is a drastic decrease in single slip system activity(relative frequency) with the increase in strain amplitude.

Fig. 2—(a) Differentially oriented martensite plates in the austenite matrix; red lines are the slip lines with different Schmid factors. (b) Secondarycracks and striation in the fatigue fracture surface (strain amplitude: ±1.0 pct). (c) Magnified view of the fatigue striations (Color figure online).

Fig. 3—Schmid factor distribution for the as-received and cyclicallydeformed specimens at various strain amplitudes under ambienttemperature.

Fig. 4—Schmid factor domains (i.e., slip system activities) as a func-tion of strain amplitude after LCF deformation of AISI 304LNstainless steel. AR—as received.

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It appears that there is systematic slip system activityvariation of the deformed austenite at various strainamplitudes. Austenite grains are more responsible fordetermining the cyclic plastic behavior of metastableaustenitic stainless steels at all strain amplitudes. Withthe increase in strain amplitude, the activity of a singleslip system decreases drastically; on the other hand, withthe increase in strain amplitude, the multiple slip systemactivity increases.

The author acknowledges the support and encour-agement of Dr. S. Srikanth, Director, CSIR–NML.The author expresses his sincere gratitude to Dr. S.Tarafder, Chief Scientist, for his fruitful suggestions;to Dr. S. Sivaprasad for his assistance with themechanical characterization; and to Dr. S.K. Das forhis assistance with the electron microscopy.

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Mater. Sci. Technol., 2011, vol. 27, pp. 366–70.3. A. Das, S. Sivaprasad, P.C. Chakraborti, and S. Tarafder: Mater.

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Tarafder: Mater. Sci. Eng. A, 2008, vol. 486, pp. 283–86.7. A. Das and S. Tarafder: Int. J. Plast., 2009, vol. 25, pp. 2222–47.8. G.BaudryandA.Pineau:Mater.Sci.Eng.A, 1977, vol. 28, pp. 229–42.9. A. Das: J. Magn. Magn. Mater., 2014, vol. 361, pp. 232–42.10. S.G.S. Raman and K.A. Padmanabhan: Mater. Sci. Technol.,

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