Optimizing strength and ductility of austenitic stainless steels throughequal-channel angular pressing and adding nitrogen element

F.Y. Dong, P. Zhang, J.C. Pang, D.M. Chen, K. Yang, Z.F. Zhang n

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

a r t i c l e i n f o

Article history:Received 27 May 2013Received in revised form24 July 2013Accepted 27 August 2013Available online 4 September 2013

Keywords:Austenitic stainless steelsEqual-channel angular pressing (ECAP)Deformation twinsStrengthDuctility

a b s t r a c t

Two austenitic stainless steels were processed by equal-channel angular pressing to systematicallyinvestigate the influences of alloying nitrogen and severe plastic deformation on the strength andductility. With increasing the number of ECAP pass and adding nitrogen element, the density ofdeformation twins increased. It is shown that the two steels exhibit a similar general trend that thestrength increases and the ductility decreases with increasing the deformation strain, but an enhancedstrength–ductility synergy can be achieved by adding nitrogen element.

& 2013 Published by Elsevier B.V.

1. Introduction

Austenitic stainless steels are one group of the currently favoredstructural multifunctional materials considered for many applica-tions due to its attractive combination of excellent ductility andformability paired with high strength, stability of its austeniticstructure, and superb corrosion and irradiation resistance [1–3].However, their lower yield strength is a major drawback, whichlimits their technological applications. Microstructural refinement isan effective approach for strengthening, which can be induced byplastic deformation such as cold rolling (CR) [4], equal channelangular pressing (ECAP) [5], high-pressure torsion (HPT) [6] anddynamic plastic deformation (DPD) [7]. Among the techniques above,ECAP is superior to the other techniques in permitting the applica-tion of a large amount of strain without a significant change ofsample cross section, control over the development of grain mor-phology and texture, and ease of process. Although microstructuralrefinement can result in a record-high strength, ductility and work-hardening are decreased considerably due to the inability to accu-mulate dislocations with a saturation of dislocations [8,9]. Thiscritical shortcoming of ultrafine-grained/nanostructure (UFG/NS)materials has restricted their practical applications, although severalstrategies have been proposed to improve their ductilities [10–13].

Except for microstructural refinement, alloying is another con-ventional strengthening mechanism by the interactions between

solution atoms and moving dislocations, which could potentiallybe beneficial to the promotion of ductility [14]. Thus, a simultaneousincrease in strength and ductility was achieved recently inUFG/NS Cu and Cu–Al/Cu–Zn alloys processed by ECAP or HPTthrough appropriate tailoring of the stacking fault energy (SFE)[15–19]. Therefore, tailoring the SFE by alloying design may be autilizable approach to upgrade the mechanical properties of UFG/NCmaterials.

Accordingly, some experiments were conducted on 316L and316LN steels with chemical compositions listed in Table 1 using theECAP to explore the influences of alloying and the SPD processing onstrength and ductility. Thus, we expect to gain a relatively homo-geneous microstructure in UFG/NC 316L(N) steels through the ECAPtechnique and critically discuss several questions in this study: howdoes the nitrogen element affect the microstructural evolution duringECAP? Can the strength and ductility be simultaneously improved byadding the nitrogen content in 316L steels? These essential issues aremeticulously elucidated through systematic investigation and detailedanalysis.

2. Experimental procedure

The 316L and 316LN steels were produced by vacuum inductionmelting and electro-slag remelting. The nitrogen element in 316LNsteel was added through the intermediate alloy of CrN in the processof melting. The as-received steels have been hot-forged and solution-heat-treated in a quartz tube under vacuum at 1100 1C for 4 h follo-wed by water quenching. Before the ECAP processing, some round

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0921-5093/$ - see front matter & 2013 Published by Elsevier B.V.http://dx.doi.org/10.1016/j.msea.2013.08.056

n Corresponding author. Tel.: þ86 24 23971043.E-mail address: zhfzhang@imr.ac.cn (Z.F. Zhang).

Materials Science & Engineering A 587 (2013) 185–191

cylindrical billets (8 mm in diameter and 45mm in length) were cutfrom the sheet by means of electrical discharge machining (EDM).

The ECAP procedure was performed using a die fabricated fromtool steel with two channels intersecting at an inner angle of 1201and an outer angle of 301, accordingly yielding an effective strainof �0.628 for each pass. All the rods coated with a MoS2 lubricantwere processed at room temperature (RT).

Microstructures of the as-ECAP and the annealed samples werecharacterized by using transmission electron microscopy (TEM, FEITecnai F20) operated at 200 kV. Thin foils for TEM observations,cut from the Z plane in the center of the pressed rods using sparkcutting, were mechanically ground to about 50 μm thick and thenthinned by a twin-jet polishing method in an electrolyte consistingof 8% perchloric acid and 92% alcohol at �15 1C.

Tensile specimens with a dog-bone shape were cut into a gaugelength of 8 mm, with a width of 2 mm and a thickness of 1 mmfrom the processed billets along the extrusion direction of ECAP. Inthe absence of any effects of the microstructure inhomogeneity onthe tensile behavior, the specimens for comparation are selected inthe same place through the transverse section. Tensile tests werecarried out at RT with an Instron 5982 testing machine operatingat a strain rate of 1�10�3 s�1 for the as-received and ECAP-pro-cessed specimens.

3. Results and discussion

3.1. Microstructure observations

The typical microstructures of the two materials after 1- and 4-passes are displayed in Fig. 1. After ECAP for one pass, themicrostructures mainly consist of elongated cells separated bygeometrically necessary boundaries (GNBs), with a high density ofdislocations in the two materials (Fig. 1(a) and Fig. 2(a and b)).Moreover, in some grains, some deformation twins can be found,as illustrated in Fig. 1(b and c) and Fig. 2(c). Substantially differentmicrostructural features were observed in different areas of thesamples. This can be explained by the orientation dependence ofthe deformation mechanisms (slip versus twinning). After ECAPfor 4 passes, more deformation twin lamellas and some shearbands are found to coexist for both 316L and 316LN steels, asshown in Fig. 1(d–f) and Fig. 2(d–f), in which the arrows point tosome shear bands. It is indicated that with an increase in thenumber of ECAP pass and the addition of nitrogen atoms, thedensity of deformation twins is obviously increased as illustratedin Fig. 3. The microstructure evolution can be mainly attributed tothe stress state during ECAP and the effects of nitrogen additionand will be discussed in detail below.

Recently, it is shown that the nucleation of deformation twinsin austenitic steels strongly depends on the SFE and applied stresslevel [20]. Byun [21] reported that during deformation of 316stainless steel, a variety of deformation structures was produceddepending on the applied stress level: (1) dislocation tangles weredominant at low equivalent stress of 400 MPa; (2) isolated stack-ing faults smaller than about 1 μmwere formed in the stress rangefrom about 400–600 MPa; and (3) twin bands became dominant

at stress exceeding 600 MPa at RT. In the present work, during thefirst pass of ECAP through the die at RT steady extrusion occurredat a relatively low applied normal stress, which may not exceedthe critical stress for twinning (st) for most grains. The appliednormal stress increases with the increase of the ECAP pass.Therefore, deformation twins develop most rapidly after the firstpass of ECAP through the die. The density of twins increases withthe increase of the ECAP pass because the applied normal stress inmost grains exceeds the critical stress st.

When the material strength is increased such as by the additionof interstitial atoms or by applied high strain level, deformationtwinning will occur because of the effect of stress on the partialdislocation separation [21]. Meanwhile, the propensity of twiningis also promoted by solutes raising the shear stress in the slipplane, reducing SFE and increasing the planarity of slip. It wasargued that the nucleation stress for twinning is directly propor-tional to the SFE [22]. The SFE in austenitic stainless steels of 300series varies broadly from low to high values: 9.2–80.7 mJ/m2 [23].Depending on the chemical composition, the SFE of the two steelscan be estimated by the following empirical relation [24]:

SFE ½mJ m�2� ¼ 25:7þ2Niþ410C�0:9Cr�77N�13Si�1:2Mn ð1Þ

From the alloy chemistry (Table 1), Eq. (1) yields a medium SFEof 34.8 mJ/m2 for the 316L steel and 25.8 mJ/m�2 for the 316LNsteel under investigation, respectively. The decrease in SFE leadsto a decrease in the twinning stress and thus to the formationof deformation twins at early stage of deformation in the316L(N) steels [22], which is similar to the results in Cu–Al andCu–Zn alloys [15–19].

Therefore, on the one hand, the introduction of N atom maylead to an interaction between dislocations and solute atomsbecause they can reduce the dislocation mobility and limitsdynamic recovery [25], which plays a crucial role in annihilatingdislocations and rearranging them in a lower energy configurationof cell walls. Whereas, on the other hand, the addition of Nelement into the austenitic (fcc) matrix lowers the SFE and therebyrestricts the occurrence of cross-slip [25,26]. Both the addition ofsolute atoms and decrease in the SFE may significantly enhancethe propensity to deformation twinning through restricting thedislocation motion to form the planar-type dislocation configura-tion or a decrease in twinning stress [22].

3.2. Tensile properties

Having deciphered the effects of ECAP and N element on themicrostructures and the corresponding deformation mechanisms, itis necessary to investigate their mechanical properties with respectto these microstructures. Typical tensile engineering stress–straincurves of the 316L(N) steels subjected to different passes of ECAPand the as-received CG sample are presented in Fig. 4(a) and (b). Itcan be seen that the ECAP processing can significantly improve thetensile strength in both 316L and 316L(N) steels, but there is anobvious loss in ductility. The mechanical properties, including yieldstress (YS), ultimate tensile strength (UTS), uniform elongation (UE)and total elongation (TE) as a function of the number of ECAP passesare summarized in Fig. 4(c and d) and Table 2. In order to reveal thework hardening behavior of the present samples, the engineeringstress–strain curves in Fig. 4(a and b) were converted into the truestress–strain curves. Work-hardening rate Θ vs. true strain e (a) andvs. true stress are shown in Fig. 5.

On the macroscale, the Considère criterion governs the onset oflocalized deformation in tension:

Θ¼ ds=dεrs ð2Þ

Table 1Chemical compositions of the 316L and 316LN stainless steel.

MaterialComposition (wt%)

Cr Ni Mo C N Si Mn S P

316L 17.2 14.61 2.16 0.01 0.01 0.24 1.28 0.005 0.007316LN 15.3 12.54 2.50 0.01 0.14 0.21 1.48 0.003 0.008

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where s and ε are the true stress and true strain, respectively. Thiswell-known criterion also predicts that the as-processed UFG/NCalloys will lose the strain hardening quickly once yielding occurs,enhancing the propensity for plastic instability in the early stage ofplastic deformation.

On the other hand, it is noted that the mechanical properties of thetwo steels exhibit a similar trend, which is consistent with those ofthe ECAPed or HPTed Cu–Al and Cu–Zn alloys [15–19]. It is seen thatthe YS and UTS tend to increase linearly with the increase of ECAPpass, as well as an obvious drop in the uniform elongation. A shortuniform elongation stage could reasonably be expected from thelimited hardenability which is commonly observed after SPD, in accor-dance with Considère criterion for the loss of macroscopic stabilityand the onset of necking. Not in line with early investigations of Cualloys, plastic instability with the onset of necking did not occur duringthe early stage of tensile deformation in the two 316L(N) steels afterone-pass ECAP processing. The difference may originate from the large

microstructural heterogeneity and strain localization, facilitating thenucleation of shear bands for Cu alloys using ECAP die with 901 sharpinner corner. By contrast, the loading conditions of the 316L(N) steelsare less severe after processing using die with 1201 inner corner. Withfurther deformation the amounts of shear bands slowly increase andapproximately reach saturation for specimens, which will lead tobroken by a number of visible shear cracks at top of the billets whensubjected to five or more passes of ECAP [16,27]. The UE of theprocessed 316L(N) steels shows a slight decrease with the number ofECAP pass, which can be attributed to a larger decrease in the Θ as thestress (and strain, respectively) increases [28]. A high Θ is crucial forhigh UE because it can help delay localized deformation [29]. Besides,the elongation to failure is low and nearly remains constant from 10%to 30% over all passes in the two ECAPed steels.

The strength–ductility synergy is revealed in Fig. 6 by plotting UEvs UTS for the ECAPed samples, showing a typical trade-off trendbetween strength and elongation. Meanwhile, the effects of adding

Fig. 1. Microstructures of as-ECAPed 316L for (a–c) one pass and (d–f) four passes: (a) elongated dislocation boundaries, (b–d) deformation twins, (e and f) shear bands.

F.Y. Dong et al. / Materials Science & Engineering A 587 (2013) 185–191 3

nitrogen element and SPD processing on the mechanical propertiesare also displayed in this figure. In close-packed structures, the SFEdetermines the extent of dislocation dissociation, which influencesthe ease of cross slip and subsequent microstructure [15]. The lowSFE can effectively restrict the dislocation activities due to the largewidth of the stacking fault. As a result, cross slip, as an essentialdynamic recovery mechanism, is more difficult in low-SFE metals,which makes the strain hardening of metals more rapid [25]. Also,the lower is SFE, the higher density of twins is. According to theresults by Shen et al. [30] both the strength and ductility increasewith decreasing twin lamellae thickness. Our results show thesame appealing combination of strength and elongation to failure,although the improved ductility has not been confirmed in thepresent study, since the short uniform deformation stage is stillbelow expectations. Nevertheless, a greater capacity to deform in the

necking was convincingly demonstrated by careful measurements ofthe area reduction after tensile fracture which is the same as that byUeno et al. [31].

The superior strength–ductility synergy with adding nitrogenin the as-received and the ECAPed 316L(N) steels may be attrib-uted to several factors. In terms of strength, the higher strengthwith adding nitrogen may arise primarily from two factors. First,the introduction of nitrogen atoms leads to solution strengtheningand the strength increment introduced by adding 0.14% N atomsinto Fe matrix is estimated at �100 MPa. Secondly, the SFE doesnot only affect the microstructure of the steels, but affect theirmechanical properties as well. The decrease of SFE makes itdifficult for the full dislocation to cross slip or climb when itencounters a barrier, which hinders the dislocation recovery viacross slip and climb. In addition, although the grain size after ECAP

Fig. 2. Microstructures of as-ECAPed 316LN for (a–c) one pass and (d–f) four passes: (a) elongated dislocation boundaries, (b) planar dislocation bands, (c) twinsintersections .(d) deformation twins, (e) secondary twins, and (f) shear bands.

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slightly decreases, a high density of deformation twins was formedand the twin density increases with adding nitrogen. As alreadynoted, twin boundaries may provide significant strengtheningeffect by confining dislocation glide and blocking the propagationof slip bands [11,30]. Besides, the ductility is also improved byadding nitrogen due to the consequent increase in the strainhardening rate. The effective strain hardening is controlled by adislocation storage (hardening) component and a dynamic recov-ery (softening) component related to dislocation annihilation

process. The decrease of SFE and development of profuse twinsand SFs suppress the dynamic recovery and hinder the annihila-tion of dislocations [13,16]. Therefore, the localized deformation orplastic instability can be delayed, thereby promoting the tensilestrength and ductility. At the same time a large number of twinboundaries as well as nanometer thick twin/matrix (T/M) lamellaeprovide ample room for dislocation slip and storage (Fig. 1(c)) [11].In addition, TBs are much more hardenable than conventionalhigh-angle grain boundaries. Therefore, twins are effective inincreasing the dislocation storage capacity, which can be used topromote the strain hardening, and improve the ductility effec-tively. With more twins and SFs produced during ECAP processingto accommodate the plastic deformation, thus the Θ is therebyenhanced and localized deformation is delayed to increase the UEwith addition of nitrogen element.

Fig. 3. Illustration of the twin density map of the 316L(N) steels with an increase inthe number of ECAP pass and the addition of nitrogen element.

Fig. 4. Tensile engineering stress–strain curves and of 316L (a) and 316LN (b) steels subjected to ECAP. The mechanical properties, as a function of the number of ECAP pass(c and d).

Table 2Mechanical properties of 316L and 316LN stainless steel subjected to differentpasses of ECAP.

ECAP passes 0 1 2 3 4

316L YS (0.2%) (MPa) 166 493 738 900 1021UTS (MPa) 585 469 807 938 1037UE (%) 68.8 17.1 1.91 1.49 1.32TE (%) 76.4 30.9 14.3 11.1 9.91

316LN YS (0.2%) (MPa) 268 621 783 924 1112UTS (MPa) 623 759 868 990 1151UE (%) 54.8 10.4 5.76 1.92 1.73TE (%) 75.2 31.5 17.1 13.6 11.2

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4. Conclusion

With the increase in the number of ECAP pass and the additionof nitrogen element, the density of deformation twins increases.It is indicated that the strength and ductility can be simultaneouslyimproved by adding nitrogen in 316L(N) steels, which can beattributed to the much easy formation of deformation twins, SFsandmicroscale shear bands, and their interactions. Combined with the

previous results, it can be concluded that increasing the density ofdeformation twins during deformation by alloying design may be autilizable approach to achieve simultaneous amelioration of strengthand ductility of UFG/NS materials.

Acknowledgments

The authors would like to thank M. X. Yang, Q. Q. Duan forsamples preparation and the tensile tests. This work was financiallysupported by the National Natural Science Foundation of China(NSFC) under Grant nos. 50890173, 50931005, 51301179, 51331007and the National Basic Research Program of China under Grant no.2010CB631006 and No. 2012CB619101.

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