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ScienceDirectJ. Mater. Sci. Technol., 2014, -(-), 1e5

Hydrogen-induced Modification in the Deformation and Fracture

of a Precipitation-hardened FeeNi Based Austenitic Alloy

Mingjiu Zhao1)*, Zifeng Guo2), Shenghu Chen1), Hao Liang3), Lijian Rong1)**

1) Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China2) Shougang Research Institute of Technology, Shougang Group, Beijing 100043, China3) Institute of System Engineering, China Academy of Engineering and Physics, Mianyang 621900, China

[Manuscript received October 8, 2013, in revised form November 26, 2013, Available online xxx]

* CorrespFax: þ8** Corres24 23971005-03JournalLimited.http://dx

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Hydrogen-induced modification in the deformation and fracture of a precipitation-hardened FeeNi basedaustenitic alloy has been investigated in the present study by means of thermal hydrogen chargingexperiment, tensile tests, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Itis found that the g0 particles are subjected to the multiple shearing by dislocations during plastic deformation,which promotes the occurrence of the dislocation planar slip. Moreover, the alloy will be enhanced byhydrogen resulting in the formation of strain localization at macroscale. So, the mechanisms of deformationand fracture in the alloy have been proposed in terms of serious hydrogen-induced planar slip at microscalewhich can lead to macroscopic strain localization.

KEY WORDS: Hydrogen; Deformation; Fracture; Precipitation-hardened austenitic alloy

1. Introduction

The precipitation-hardened FeeNi based austenitic alloys,such as A286 (Fee25Nie15Cre1.3Moe2.1Tie0.3Ale0.003B,wt%), are used for a variety of applications in hydrogen envi-ronment, including pumps, valves and other parts which demandhigher strength. These alloys, with yield strength in range of650e800 MPa, are strengthened by an ordered g0-Ni3(Al, Ti)phase (L12 structure) coherently set in an iron-base solid solutiong-matrix. However, these alloys will not only lose about halfductility after hydrogen charging, but also exhibit a change offracture mode from intragranular to intergranular fracture[1,2].Reasons for high hydrogen embrittlement (HE) sensitivity ofprecipitation-hardened austenitic alloys have been studied bymany authors[3e5]. Although the role of second phase has beenexamined, such as g0 phase[2], eta (h)[3] and carbide[5], themechanisms of hydrogen-induced degradation of alloys are still asubject of debate.

onding author. Assoc. Prof., Ph.D.; Tel.: þ86 24 23971985;6 24 23978883; E-mail address: mjzhao@imr.ac.cn (M. Zhao).ponding author. Prof., Ph.D.; Tel.: þ86 24 23971979; Fax: þ868883; E-mail address: ljrong@imr.ac.cn (L. Rong).02/$e see front matter Copyright� 2014, The editorial office ofof Materials Science & Technology. Published by ElsevierAll rights reserved..doi.org/10.1016/j.jmst.2014.03.017

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Concerning about failure process of hydrogen-pre-chargedmaterials, the effect of hydrogen on the deformation and fracturebehavior is important in the understanding of HE ofprecipitation-hardened FeeNi austenitic alloys. Many mecha-nisms proposed for hydrogen-related deformation and fracture,hydrogen-enhanced local plasticity (HELP) mechanism appearto be a viable one[6e9]. The underlying principle in HELPmechanism is that reduced barriers to dislocation motion in thepresence of hydrogen, thereby promoting deformation that oc-curs in a localized region was demonstrated by in situ trans-mission electron microscopy (TEM) observations[8]. Since theseobservations are performed at such a small size scale, it isnecessary to relate them to macroscopic deformation behavior ofmaterials.The present investigation is mainly focused on plastic defor-

mation and fracture process of precipitation-hardened FeeNiaustenitic alloy before and after hydrogen charging. By probingdislocation morphology, slip traces and coarse slip bands, thestudy will reveal the effect of hydrogen at microscale onmacroscopic behavior.

2. Experimental

FeeNi based alloy was prepared in a vacuum inductionfurnace with the nominal compositions as follows: 30Nie15Cre1.3Moe2.0Tie0.26Ale0.3Ve0.25Sie0.002BeFe bal (wt%).The ingot was homogenized at 1433 K for 20 h, then forged androlled into bars with a diameter of 15 mm. The samples were

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Table 1 Tensile properties and the value of RAL of the chargedhydrogen samples

Condition Ultimatestrength (MPa)

0.2% Yieldstrength(MPa)

Totalelongation (%)

RA(%)

RAL

(%)

Without H 1088 725 29.6 64.5With H 1092 730 25.8 39.5 38.8

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subjected to solution treatment at 1253 K for 1 h followed bywater quenching. Then, samples were isothermally peak-aged at1013 K for 8 h followed by air-cooling. Tensile specimens of rodand slice types were produced. The rod specimen was in a gagesection of 5.0 mm in diameter and 25 mm in length and the slicespecimen was in a dog-bone-shape with a width of 3 mm, athickness of 2 mm and a gage length of 25 mm.Hydrogen charging experiment was carried out in an autoclave

with 10 MPa high-purity hydrogen, holding at 573 K for 168 h,following that tensile tests were performed immediately in air atroom temperature by using a Zwick Z050 tensile machine at astrain rate of 1.33 � 10�3 s�1, and the average values of themechanical properties were obtained by measuring three sam-ples. Hydrogen-induced ductility loss can be expressed as fol-lows:

RAL ¼ ðRA� RAHÞ=RA (1)

where RA and RAH are area reduction of uncharged andhydrogen charged samples, respectively. The hydrogencontents in specimens after charging were about 0.0028 wt%.In order to study the slip traces before and after hydrogencharging, interrupted tensile tests to a strain of 10% wereperformed on each pre-polished slice tensile specimen. Thenslip traces of specimen were examined by scanning electronmicroscopy (SEM). Thin foils specimen for TEM analysiswere prepared by double-jet polishing at 253 K in 10%perchloric acid ethanol solution. Characterization of themicrostructures and dislocation configuration was performedon a JEM 2100 TEM with the samples before and afterhydrogen charging. Coarse slip bands on the side surfaces offractured specimen were observed with optical microscopy(OM).

Fig. 1 (a) Optical microstructure of the FeeNi based alloy, (b) TEMmicrograph of g0 precipitates.

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3. Results

3.1. Microstructure, strengthening phase and hydrogen-inducedductility loss

Fig. 1(a) is an optical microstructure of the aged austeniticalloy, and cellular h-Ni3Ti phase, which will result in a markedductility loss as reported elsewhere[1,5], was not observed at thegrain boundary. As depicted in Fig. 1(b), quite a few g0 particlesprecipitated in matrix with a size of about 10e20 nm in diameterwere observed, and their diffraction pattern from [101] is pre-sented in the right upper corner of Fig. 1(b).As shown in Table 1, a marked decrease in the area reduction

is observed after hydrogen charging, which indicates that thenotable ductility loss of the alloy should be resulted fromhydrogen. Fig. 2 is tensile fracture morphology of the alloybefore and after hydrogen charging, and a change of fracturemode from intragranular to intergranular fracture was observed,which is reflected in the high HE sensitivity.

3.2. Dislocation configuration

To explore the reasons for high HE sensitivity, TEM obser-vations on hydrogen-free and pre-charged samples near fracture

Fig. 2 Fractography of the FeeNi based alloy: (a) without hydrogen,(b) with hydrogen.

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Fig. 3 TEM micrographs of dislocation morphology near the fracture:(a) without hydrogen, (b) with hydrogen.

M. Zhao et al.: J. Mater. Sci. Technol., 2014, -(-), 1e5 3

surface were conducted, as indicated in Fig. 3. Both non-chargedand pre-charged specimens exhibit the similar types of disloca-tion configuration, except for a more distinct planar feature in thelatter case (Fig. 3(b)), namely, planar slip of dislocations in thealloy has been promoted by hydrogen, just as reported in purealuminum by Ferreira et al.[8] and in iron single crystal byHwang and Bernstein[10].

3.3. Slip bands and g0 precipitates cut by dislocation

Fig. 4(a) is the SEM image of slip band and g0 precipitatesnear the fracture in the pre-charged specimen and its magnifiedmicrograph is shown in Fig. 4(b). A lot of g0 particles areobserved in the slip bands and some sheared g0 particles areidentified correspondingly, as depicted in Fig. 4(c). Comparedwith the morphology of g0 particles in the non-deformed spec-imen (Fig. 1(b)), a fact can be declared that they have beensubjected to the multiple shearing by dislocations during plasticdeformation because of their ordered atomic structure.

Fig. 4 (a) and (b) SEM images of slip band and g0 precipitates, (c) TEM micrcharged specimen.

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3.4. Hydrogen-induced shear localization

For the non-charged and pre-charged specimens loaded toplastic strain of 10%, their surface slip traces are illustrated inFig. 5. By comparing their differences, two interesting featurescan easily be found. One is that the slip lines are clearly muchstraight in the pre-charged specimen compared with that of theuncharged specimens, and similar phenomena have also beenobserved by other researchers[11,12]. Note that these results ofslip traces are the expected ones which provide an indication ofincreased planar slip of dislocations after hydrogen charging andare in a good consistent with the observations of dislocationmorphology in Fig. 3. The other is that some coarse slip traceswith increased spacing are observed after hydrogen charging(shown by arrows in Fig. 5(b)), which implies that hydrogen-induced heterogeneous slip occurs in pre-charged specimen.Also, similar behaviors, named as strain localization, were foundin some single-phase austenitic steel such as 310S[13] and316L[14]. For example, on the basis of fractography and sliptrace, Abraham and Altstetter[13] proposed that hydrogen pro-motes slip localization evidenced by increased slip step heightsand more intergranular facets. Yagodzinsky et al.[14] studied theeffect of hydrogen on deformation of single crystal austeniticstainless steel, and found that hydrogen enhanced strain locali-zation in the form of shorter and more grouped slip lines, incontrast to uniform ones in an uncharged alloy.Optical micrographs of slip bands near the fracture for the

uncharged and pre-charged specimens are depicted in Fig. 6. Forthe uncharged specimen, much heterogeneous deformation oc-curs by formation of some coarse slip bands within a small re-gion near fracture surface (Fig. 6(a)). These coarse slip bandstypically carry more strains than fine slip bands far away fromfracture and are believed to be preferred sites for macroscopiclocalized shear fracture of alloy[15]. In contrast to the unchargedspecimen, the charged one exhibits a different feature, and fewcoarse slip bands were observed. Instead, there are occasionallysome fine grain-scale slip bands as shown by white arrow inFig. 6(b). In addition, Fig. 6(b) also illustrates large cavitiesahead of crack tip (marked with black arrow), indicating thatfracture initiates and grows along a principle coarse slip band.This provides a strong evidence of the hydrogen-induced shearlocalization in pre-charged specimen.

4. Discussion

While it is generally agreed that hydrogen promotes planarslip of dislocations at microscale, the possibility that it can inturn lead to a significant degradation in ductility is still unclear. Itis required to demonstrate the relationship between dislocationmorphology at microscale and macroscopic failure process. So,

ograph showing g0 particle cut by dislocation near the fracture in the pre-

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Fig. 5 SEM images of slip traces of experimental specimens loaded to10% plastic strain: (a) without hydrogen, (b) with hydrogen.

4 M. Zhao et al.: J. Mater. Sci. Technol., 2014, -(-), 1e5

several results in present study are not worthy and deservediscussion.As mentioned in Section 3, the presence of hydrogen will pro-

mote plastic strain localization in precipitation-hardened FeeNi

Fig. 6 Polished and etched section of samples tested after

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based austenitic alloys. Although the exact mechanism of strainlocalization by hydrogen is still unclear, it has been suggested thatstrain localization can be resulted from a sudden increase inmobiledislocation density within localized deformation bands[16]. Inprecipitation-hardened FeeNi based alloy, since the effectivecross section of the g0 particles is successively reduced aftershearing of the particles observed in our previous study[17], thecritical shear stress for the dislocations to cut through the particlesin an already activated slip plane is lower than others. As a result,slip concentrated on local few slip planes is induced (Fig. 3(a)). Atthe same time, as shown in Fig. 3(b), planar slip of dislocations hasbeen promoted by hydrogen, which will result in increased mobiledislocation densitywithin localized deformation bands. Therefore,the hydrogen-induced strain localization is probably due to seriousslip planarity induced by the combination of effect of shearing g0

particles and hydrogen.It is well known that shear localization, where plastic flow is

concentrated in thin bands, is one of the most common types ofnon-uniform deformation modes. A homogeneous deformationfield often gives way to shear localized deformation. Price andKelly[18] proposed that shear fracture is initiated by characteristiccoarse slip bands. As stated by Gilman[19] and Harren et al.[20],these bands are natural outcomes of the dislocation slip whichmakes plastic flow fundamentally inhomogeneous, and factorsthat influence dislocation mobility are recognized as primaryfactors controlling characterization of bands. In some precipita-tion-hardened alloys, such as AleCu[21] and AleLi[22], thecoarse slip bands occur easily because of particle cutting.Occurrence of the localized deformation within shear bands isunexpectable because it will decrease the ductility of alloy, justas observed in precipitation-hardened AleCu alloy[21]. Note thatthis behavior mentioned above is associated with the bandsforming abruptly and ductility loss.In the present study, the planar slip of dislocations becomes

more distinct after hydrogen charging and deformation is much

tensile test: (a) without hydrogen, (b) with hydrogen.

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M. Zhao et al.: J. Mater. Sci. Technol., 2014, -(-), 1e5 5

more localized. A critical fracture strain will be rapidly exceededin one of coarse slip bands, and crack formation will start earlyalong the slip band that leads to the loss of load bearing capacity,which is manifested by highly localized macroscopic ruptureinitiated by principle coarse slip in the hydrogen-charged alloy,just as theoretical interpretations by Liang et al.[23].

5. Conclusions

(1) At microscale, the g0 particles have been subjected to themultiple shearing by dislocations during plasticdeformation in the FeeNi based austenitic alloy, whichpromotes the occurrence of the dislocation planar slip,and it will be enhanced by hydrogen.

(2) At macroscale, serious dislocation planar slip results in theformation of the strain localization in the alloy afterhydrogen charging.

(3) Under the combination of effect of shearing g0 particles andhydrogen, serious dislocation planar slip occurs at microscaleand results in formation of strain localization at macroscale,which is the main reason for high HE sensitivity in theprecipitation-hardened FeeNi based austenitic alloy.

AcknowledgmentsThe authors are grateful for the National Natural Science

Foundation of China and China Academy of Engineering Physics(NSAF) under grant No. U1230118 and the National NaturalScience Foundation of China (NSFC) under grant No. 51171178.

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