164 Materials Science and Engineering, A164 (1993) 164-169

On the effect of nitrogen on the dislocation structure of austenitic stainless steel

P. Milliner, C. Solenthaler, P. Uggowitzer and M. O. Speidel Institute of Metallurgy, ETH Ziirich (Switzerland)

Abstract

The strengthening and work hardening characteristics of high nitrogen austenitic steel have been investigated by TEM in order to explain the outstanding mechanical properties, which are very high yield strength and good toughness. It is shown that plastic deformation of those steels always occurs by a combination of planar glide and mechanical twinning. However, the critical stress/strain conditions for the onset of mechanical twinning depend strongly on the actual nitrogen content. Specifically, as the nitrogen content is increased, the onset of deformation twinning is shifted to lower strains and higher stresses, i.e. the more important becomes the contribution of deformation twinning to the total strain. The observed behaviour is explained by the influence of nitrogen on internal friction and stacking fault energy. It is concluded that the high yield strength of cold worked nitrogen-bearing steel is essentially due to tight stackings of twins and stacking faults. Apart from regular structure evolution, inhomogeneous structures are observed, which can be explained in terms of texture formation and the dynamic properties of stacking faults under stress.

1. Introduction

Austenitic high nitrogen steel offers a combination of very attractive engineering properties, namely, high strength, high toughness and high corrosion resistance [1-3]. The nitrogen is present within the f.c.c, matrix exclusively in an interstitial solid solution, i.e. there are no nitrides formed, and has essentially two effects on the structure. First, the nitrogen interstitials introduce strong elastic distortions into the crystal lattice, giving rise to strong solid solution hardening. Second, nitrogen very efficiently lowers the stacking fault energy (SFE)[4, 5].

Typically with nitrogen steel, there is a high work hardening potential to be observed. As an example, Fig. 1 shows the variation of the yield strength depend- ing on nitrogen content and degree of cold work, as observed with AISI 316 L type steel. The difference between the yield points of nitrogen-free and nitrogen- alloyed steel respectively, increases as the deformation degree increases. This variation of strength and work hardening potential cannot be explained straight- forwardly in terms of variations of internal friction, but must be expected to be due to differently evolving microstructures during cold work.

It is the subject of the present research to charac- terize and compare the effect of cold work on the evolution of the microstructure of nitrogen-free and nitrogen-alloyed steel respectively, in order to explain the observed strength properties.

2. Experimental details

The investigated material has been taken from cold drawn wires of AISI 316 L steel (X 2 CrNiMo 18 14 3). Actual deformation degree was 0%, 6%, 21%, 36%, 56% respectively (reduction of cross-section). Actual nitrogen content of the steel was 0.04 wt.% (nitrogen- free steel), 0.25 wt.%, 0.45 wt.%, 0.53 wt.% respec- tively. Tensile test results obtained from cold drawn steel are given in Fig. 1.

a .

E &

n -

J¢

c

m -o

2 0 0 0 ' I ' I J I i [ ' I ' _

• /

lSOO • / /

1 0 0 0

soo r • , . o . o , % ,

, I , I , I ~ I i I ,

0 1 0 2 0 3 0 4 0 5 0 6 0

plast ic deformat ion , e, [%]

Fig. 1. Yield strength of cold worked austenitic steel AISI 316 L, nitrogen-free and nitrogen-alloyed respectively. The observed increase in yield strength corresponding to the shadowed area cannot be explained in terms of increased internal friction but must be owing to differently evolving microstructures.

0921-5093/93/$6.00 © 1993 - Elsevier Sequoia. All rights reserved

P. Miillner et al. / Dislocation structures of austenitic stainless steel 165

Inspection of the microstructure has been done 0y means of conventional TEM using a Philips EM 400 operated at 120 kV. Thin foil discs specimens were used. Specimen preparation was by means of standard techniques; a mixture of acetic acid/perchloric acid 95/5 was used as electrolyte for electropolishing at 15 °C (250 rnA) -1.

f

3. Results

It might be expected that the obviously different strength and work hardening characteristics of nitrogen-free and nitrogen-alloyed steel, respectively, might result from some fundamental difference in dis- location mechanisms and structures. This is not observed, however. It turns out that, nitrogen being present or not, the same two basic mechanisms always control plastic deformation at low temperature: (a) planar glide at low strain, and (b) mechanical twinning at high strain. In an intermediate strain range both mechanisms (a) and (b) cooperate. The crucial dif- ference, however, concerns the critical stress-strain conditions for the onset of multiple planar glide and later for the onset of mechanical twinning.

At the very beginning of the deformation it is ex- clusively planar glide of pairwise moving partial dislocations which controls the evolution of the microstructure. Particular microstructural features are observed: pairs of partial dislocations and their attri- buted stacking faults, planar dipolar dislocation group- ings ("multipoles", Fig. 2), and dislocation pile-ups at grain boundaries. The structures of nitrogen-flee and nitrogen-alloyed steel are quite similar; it may be that in the nitrogen-containing steel dislocation motion is more strictly planar. Multipoles are rare; they become unstable at very low deformation degrees and thus must not be considered to play a major role for final strengthening.

The typical appearance of deformation structures evolving in the low strain region ( t < 5 % ) through multiple planar glide is shown in Figs. 3(a) and (b). The spacings between the criss-crossing active planes are of remarkable regularity. Moreover, it turns out that, referred to the same total strain, the spacings tend to decrease as the nitrogen content of the steel is increased. Thus, in the presence of nitrogen the crystals do not accept the same entity of local displacement per unit area of active plane, and the planarity of glide is much more restrictive than in nitrogen-flee crystals.

At higher deformation degree (e>5%), structure evolution involves twinning as shown in Fig. 4(a). Twinning may feature a multiple character of course, analogous to planar glide. Additionally, second order twinning occurs at the most heavy deformations, i.e.

Fig. 2. Planar dipolar dislocation grouping ("multipole"), as observed in nitrogen-alloyed steel at the onset of planar glide.

structures having already been twinned undergo another, second twinning shear. These twinned struc- tures are generally of a very fine character, especially with the high nitrogen steel variation (Fig. 4(b)), where they appear much more often.

Typically, as the nitrogen content of the steel is increased, the critical conditions for the onset of mechanical twinning are shifted towards lower strains and higher stresses. Thus, nitrogen-free and nitrogen- alloyed steels differ essentially just with respect to the relative contributions of glide and twinning to the total deformation.

Apart from the above-mentioned well-defined--and thus somehow ideal--structures, structure inhomogen- eities are commonly observed. At a given total defor- mation, neighbouring crystals may be in a different stage of structure evolution depending on their dif- ferent crystallographic orientation relative to the axis of applied stress (i.e. relative to the wire axis). So even in the most heavily deformed state, crystals being twinned on all available systems coexist with crystals being twinned on, e.g., one system only, and even

166 P. Milliner et al. / Dislocation structures of austenitic stainless steel

Fig. 3. Glide distribution at 6% plastic deformation: (a) N < 0.04N%, (b) N = 0.53%. The deformation is controlled by multiple planar glide.

coexist with crystals containing no twins at all. These latter crystals, containing just crowded dislocation tangles, typically are aligned with their (100) direction near to the wire axis.

4. Discussion

Nitrogen-alloyed and nitrogen-free steel, respec- tively, undergo basically the same structure evolution. However, there are some significant differences: with a higher nitrogen content (a) the planarity of glide becomes sharper, (b) the deformation mode changes to the next at lower strains, and (c) the structure becomes finer (Le. the distances between active planes and twin lamellae decrease). These observations can be explained by the fact that nitrogen decreases the SFE. The lower the SFE is, the wider the partials separate. Thus, a glide plane of given length will accept fewer dislocations and so can only bear lower strain. Hence, more glide planes have to be activated, the frequency of activated planes is increased, and the distances

between them are correspondingly decreased. Conse- quently second glide systems will be activated earlier.

The transition from planar glide to mechanical twinning is shifted to lower deformation degrees in nitrogen steels. This can be explained as follows. For mechanical twinning, two conditions must be met:

(a) Mechanical twinning is a reaction including extended stacking faults [6]; however, extended stack- ing faults must be available. A low SFE promotes the formation of extended stacking faults and thus sup- ports mechanical twinning;

(b) Mechanical twinning demands high (local) stresses, i.e. a critical shear stress rt must be reached. Twinning occurs when the critical shear stress for glide, rg, rg = r 0 + a G b p 1/2 = r 0 = rFR

is increased up to ft. The effect of nitrogen on rg is twofold. First,

nitrogen increases internal friction r0. This is the contribution of solid solution strengthening. Second, nitrogen decreases the SFE. Both effects give rise to an

P. Mffllner et al. / Dislocation structures of austenitic stainless steel 167

Fig. 4. Microstructures at 56% plastic deformation: (a) N<0.04%; (b) N=0.53%. The deformation is controlled by multiple mechanical twinning.

increase of rg. Whereas the effect of high r0 is obvious, the effect of low SFE is not, because the SFE comes into play much more indirectly over its influence on the local glide distribution, affecting the Frank-Read stress, rFR, as discussed below. However, high nitrogen steels do reach the conditions for mechanical twinning not only earlier but also at more positions so that the struc- ture will develop a finer character.

The general effect of nitrogen is that it changes the glide distribution in such a way that local dislocation density is increased. There are three mechanisms to be distinguished.

First, more active planes are necessary for the same strain, and thus the activation stress for more disloca- tion sources is required.

Second, in the finer structure of nitrogen-alloyed steels more intersections of criss-crossing active planes exist. These intersections involve dislocation reactions of the Lomer-Cottrel l - type creating locks which act as dislocation barriers. At higher strains, twin boundaries form additional strong barriers. Thus, owing to the finer character of the structure, nitrogen-bearing steels contain more dislocation barriers.

high N

- ~ ~ J /low N

cold deformation Fig. 5. Stress/strain conditions of operating deformation mechanisms in nitrogen-free and nitrogen-alloyed steel respec- tively. Nitrogen shifts the deformation mode transitions to higher stresses and lower strains. 1, 2: planar glide, 3, 4: mechanical twinning, 5, 6: second order twinning; 1, 3, 5: primary systems activated, 2, 4, 6: secondary systems activated.

Third, dislocation barriers are more efficient in nitrogen-alloyed steels; to overcome a barrier, a split- up dislocation must pass through totally. As the leading and trailing partials become more widely separated, the

168 P. Milliner et al. / Dislocation structures of austenitic stainless steel

A I \ C E ~ l BC

al =t 1

t± @6D

Fig. 6. Dynamic properties of intrinsic stacking faults under different orientations. (a) and (b) show the situation A with a (111) direc- tion parallel to the wire axis; (c) and (d) show the situation B with a (100) direction parallel to the wire axis. a6', 6'A and BC6', 6'AD are the projections of the partials B6 and 6A on the stress direction for situations A and B respectively. _1 is the line sense. (b) and (d) are the stretched and compressed stacking fault configurations, resulting from the acting stresses o~ and a t on the leading and the trailing partials respectively.

trailing will interact more with the stress fields of neigh- bouring dislocations and be held back. Thus higher stresses are required for passing the barrier. This is especially the case for twin boundaries, where the partials are pinned by the stacking faults.

The shift of deformation mode transitions to lower strains and higher stresses is illustrated in Fig. 5. This scheme agrees with the measured work-hardening behaviour (Fig. 1) and the observed structures.

The observed structure inhomogeneities can be explained as follows.

The different types of structure in adjacent crystals differently oriented are owing to their Schmid factors. For different grains, the required stresses for the tran- sition from one deformation mode to the next are reached at different strains.

The explanation of the twin-free structure of grains with a (100) direction near parallel to the wire axis is somewhat more complex. First, drawing of wires of f.c.c, metals leads to a double fibre texture with (111) and (100) directions parallel to the wire axis [7]. Thus grains with their (100) direction near to the wire axis will be stable during the deformation process.

Second, the split up dislocations are stressed under the same external load in an opposite direction depending on whether a crystal is oriented with (111) or (100), respectively, parallel to the applied stress [8]. Consider a dislocation BA gliding on a d-plane (Fig. 6). The line vector should be parallel to BC. According to the movement rule, the dislocation will move in situa- tion A ( o / / a A , Fig. 6(a)) "downwards" and "upwards" in situation B (o/ /BC/AD, Fig. 6(c)). For the separation of BA into the partials B6 and 6A, the configuration of the included intrinsic stacking fault is shown in Figs.

6(b) and (d). Considering the force acting on the par- tials, which is proportional to the projection of the Burgers vectors on the stress direction, in situation A the intrinsic stacking fault will be stretched whereas in situation B it will be compressed.

This explains the actual observations: the planarity of the glide band structure is less sharp with smaller stacking faults; twins, however, can only be created when extended stacking faults are available.

5. Summary

During deformation two mechanisms are observed: planar glide at lower strains and twinning at higher strains. Both mechanisms start on primary systems fol- lowed by the activation of secondary systems.

Nitrogen influences structural development owing to its decreasing effect on the stacking fault energy and the increasing effect on the internal friction: the planar- ity of glide becomes sharper, the structures become finer, and the transitions from one deformation mode to the next are shifted to lower strains and higher stresses.

These structural changes lead to additional harden- ing effects.

More dislocation sources have to be activated, more dislocation barriers are formed (glide plane intersec- tions as well as twin boundaries), and the efficiency of barriers is enhanced.

Finally the occurrence of grains without twins is explained in terms of texture development and dynamic properties of stacking faults, which are com- pression and expansion under different orientations.

P. Milliner et al. / Dislocation structures of austenitic stainless steel 169

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