Stacking fault energy decrease in austenitic stainless steels induced by hydrogen pairs formation

Stacking fault energy decrease in austenitic stainless steels induced by hydrogen pairs formation

Scripta Materialia, Vol. 39, No. 8, pp. 1145-1149, 1998 Elsevier Science Ltd Copyright © 1998 Acta Metallurgica Inc. Printed in the USA. All rights re...

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Scripta Materialia, Vol. 39, No. 8, pp. 1145-1149, 1998 Elsevier Science Ltd Copyright © 1998 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/98 $19.00 1 .00

Pergamon

PII S1359-6462(98)00285-1

STACKING FAULT ENERGY DECREASE IN AUSTENITIC STAINLESS STEELS INDUCED BY HYDROGEN PAIRS FORMATION J.D. Hermida* *Departmento de Materiales, C.A.C-CNEA, 1650 San Martı´n, Argentina

A. Roviglione** **Department de Ingenierı´a Meca´nica y Naval, Faculty de Ingenierı´a, Uba, Argentina (Received March 23, 1998) (Accepted June 30, 1998)

Introduction The decrease of the Stacking Fault Energy (SFE), induced by hydrogen in austenitic stainless steels, was always invoked to explain the formation of e-martensite at room temperature during cathodic charging of hydrogen [1,2]. Pontini and Hermida [3] measured by XRD a reduction of 37 pct of the SFE of an AISI 304 steel at room temperature, in the presence of only 274 ppm of hydrogen. However, the nature of this phenomenon is still unknown. Recently, Obiol et al. [4], using the Atoms Superposition and Electron Delocalization-Molecular Orbital (ASED-MO) method, calculated the binding energy for H-H pair formation in the faulted zone of an FCC iron matrix. It was shown that, the H-H pair formation is more likely to occur along directions connecting octahedral interstices of the HCP stacking sequence and that are normal to the {111} planes. The binding energy found was 25.75 eV, being this value significantly larger than the corresponding one for vacuum: 24.75 eV). In this work, an explanation of the SFE decrease is developed on the basis of this previous result.

H-H Pairs Formation As it was mentioned in the Introduction, the addition of a small content of hydrogen dramatically affects the physical property of the system. The reduction of 37 pct of the SFE allows the nucleation of the e-martensite at room temperature and low deformations (see next section), which is not possible without hydrogen. The question that remains unanswered is: in what way hydrogen decreases the SFE. One possibility could be the obvious one: hydrogen lowers so much the free energy of the e-martensite that it becomes the stable phase at room temperature. However, in our case, the system iron-hydrogen is a very diluted solution that it would likely behave as an ideal solution. So, both phases (austenite and e-martensite) would show a similar variation in their free energies as a function of molar solute fraction. 1145

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Figure 1(a): Six iron atoms of a (111) plane. (b): Projection of Fig. 1a on (2# 20) plane.

Therefore, searching for a more acceptable answer to the question mentioned above, we propose that the formation of H-H pairs [4] could be the responsible for the SFE decrease. The phenomenon can be interpreted as an energetically convenient response of the system generating a larger faulted zone to favor spontaneous H-H pairs formation by chemical reaction. During faulting, the slip on {111} planes cause close neighbor octahedral interstices in g phase, occupied by hydrogen atoms, to become close neighbor octahedral interstices in the hcp phase, which is the most favorable position for H-H pairs formation. Crystallographic Description We shall use two figures to make this concept clearer. In Fig. 1a, six atoms of a [111] plane are shown. They are named as B according to the traditional stacking sequence ABC of an FCC structure. The positions of the atoms of layers A and C, below and above the plane of the drawing, respectively, are also shown. The “y” axis is coincident with the [1# 1# 2] direction and “x” axis with the [1# 10] direction. The numbers on “x” axis indicate a sequence of (2# 20) planes normal to the (111) plane. In Fig. 1b, a projection on the (2# 20) plane #0 is shown. Layer B is labeled as zero, two layers A are labeled as -I and II, respectively, and layer C as I. The superscripts represent the (2# 20) plane to which the atoms belong to. For instance, A(1,3) represent two atoms of layer A belonging to planes #1 and #3

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that are coincident on the projection. Octahedral interstices are indicated with a letter O. The subscripts indicate the positions at which they are located. For instance, between layers -I and 0, interstices OC(1,3) are at positions C and belong to planes #1 and #3. We shall analyze what happens to the positions of these interstices when a stacking fault is produced. Let us suppose that layer C (I) is the fault plane. When atoms move to A positions, interstices below this plane, indicated as O(0,2,4) become to be located at any one of C positions: OC(1,3) and OC(2). The A interstices above layer I, initially labeled as OB(1,3) and OB(0,2), will also be located at C positions. All these “movements” of the interstices are indicated by arrows. Then, if a hydrogen atom is initially occupying one interstice below layer I, e.g.: OA(2), there will be three interstices that can occupy after faulting: OC(1), OC(2) or OC(3). For each one, there are three interstices above layer I, initially at B positions (the three B positions surrounding each C positions, see Fig. 1a), that can be occupied by hydrogen atoms, in such a way that, after faulting, it could happen that a pair of interstices OC(1), OC(2) or OC(3), one on each side of layer I, be occupied by hydrogen atoms. But, among these nine possibilities of having such a pair, there are only six independent, because three (numbered as (1), (2) and (3) in Fig. 1a) are shared by two groups. SFE Decrease Calculation So, the expression for the SFE decrease could be given by the binding energies released during the H-H pairs formation EF(H) 2 EF 5 rH-H EH-H

(1)

where rH-H is the number of pairs formed per unit area of the faulted plane and EH-H is the binding energy of each pair. It is of interest to obtain an estimation of the pairs density to compare the result of equation (1) with the experimental value of SFE decrease from the work by Pontini and Hermida [3] EFH 2 EF 5 21.12 3 1022 Joule/m2. The hydrogen weight fraction is cH 5 274 3 1026 It can be transformed in an atomic fraction given by NH 55.85 3 274 3 1026 5 5 1.53 3 1022 NFe 1 2 274 3 1026 This value represents the probability of finding a hydrogen atom per iron atom. In a FCC lattice there are equal number of octahedral interstices and atoms. So, the probability of finding an interstice occupied by a hydrogen atom is given by the same value pHi 5 1.53 3 1022 5 1/65.36 ' 1/65 which means that, among 65 interstices, one could be occupied by a hydrogen atom. As it was already mentioned, there are six possibilities for each interstice occupied by a hydrogen atom to find another hydrogen in a favorable position for the formation of an H-H pair after faulting. Therefore, the probability of finding two hydrogen atoms in favorable positions can be expressed by pH-H 5 i

1 6 3 5 1.42 3 1023 65 65

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Then, the concentration of these pairs is given by cH-H 5 1.43 3 1023 3 (274/2) 3 1026 5 0.19 ppm To obtain the pairs density over the faulted planes some unity conversions must be made. In an austenite cell with lattice parameter ag 5 3.59 3 10210 m there are eight {111} planes, whose total area is 219 A111 m2 8 5 8.93 3 10

In 1 m3 of austenite there are N 5 2.16 3 1028 cells So, as the probability of finding a faulted plane is a 5 0.004 [3], the total faulted area per cubic meter of austenite is A111F 5 7.71 3 107 m2F/mg3 and, as the austenite density is rg 5 8.02 3 106 g/m3, the following expression holds

rH-H (mol H-H/m2F) 5 cH-H (ppm) 3

8.02 3 106 gFe/m3 7.71 3 107 m2F/mg3

which drives to

rH-H 5 2 3 1028 mol H-H/m2 Now, the SFE decrease can be evaluated using equation [1] and remembering that, according to the work by Obiol et al. [4], EH-H 5 25.75 eV 5 25.55 3 105 Joule/mol H-H Therefore, EHF 2 EF 5 21.11 3 1022 Joule/m2 which is in remarkable agreement with the experimental value: 21.12 3 1022 Joule/m2.

e-Martensite Constancy Following the procedures of hydrogen charging and rolling described in [3], several thickness reductions: 2, 3, 5, 10 and 15 pct were performed by rolling at room temperature, immediately after charging. The volume fraction of e-martensite was measured by XRD using CoKa and the Direct Comparison Method described by Cullity [5]. To avoid texture effects, averages of the ratios between measured and theoretical intensities for four peaks, {10.0}, {10.1}, {10.2} and {10.3}, were determined. No traces of a9-martensite were found. In Fig. 2, a plot of the volume fraction versus thickness reduction is shown. As can be seen, a large amount of e-martensite is formed from the very beginning of deformation. After one year at room temperature, the measurement of the volume fractions was repeated and the same values were obtained.

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Figure 2. Volume fraction of e-martensite as a function of thickness reduction, which was performed after hydrogen charging.

After such a long period, hydrogen cannot be retained in the sample and, at least, a partial reversion to austenite was expected. A possible explanation for this abnormal constancy of the e-martensite could be the H-H pairs formation. There exist no doubt that much more energy is needed to remove H-H pairs than individual hydrogen atoms by diffusion. Consequently, if the pairs are formed, they would act as impediments to the back transformation. Conclusions The SFE decrease in austenitic stainless steels could be explained by H-H pairs formation in the faulted zone. The large tendency of hydrogen atoms to bond would find a very favorable situation in the neighbor octahedral interstices of the HCP phase. The enormous binding energy would be the explanation for the large SFE decrease produced by such an small hydrogen pairs concentration. Acknowledgments We are very grateful to Adriana Pontini for her permission to use the results of e-martensite constancy, included in her Doctoral Thesis. References 1. 2. 3. 4. 5.

M. L. Holtzworth and M. R. Louthan Jr., Corrosion. 24, 110 (1968). A. Inoue, Y. Hosoya, and T. Masumoto, Trans. ISIJ. 19, 170 (1979). A. E. Pontini and J. D. Hermida, Scripta Mater. 37, 1831 (1997). E. Obiol, L. Moro, A. Roviglione, J. D. Hermida, and A. Juan, J. Phys. D. Appl. Phys. 31, 1 (1998). B. Cullity, Elements of X-Ray Diffraction, Addison-Wesley Publishing Co., New York (1969).