ISIJ International
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Physical Properties
Effect of Nitrogen Addition on the Stacking-Fault Energies in Si-added Austenitic Stainless Steel
Yasuhito KawaharaRyo TeranishiChikako TakushimaJun-ichi HamadaKenji Kaneko
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2021 Volume 61 Issue 3 Pages 1029-1036

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Abstract

Austenitic stainless steels have superior room temperature and high temperature strengths, strongly influenced by stacking-faults in the steel microstructure. Nitrogen addition makes substantial contribution to room temperature and high temperature strengths, so it is essential to consider the effect of nitrogen on the stacking-fault energies (SFE) to understand strength mechanism of the steel and to enhance the strength. In this study, SFE were measured by weak-beam TEM method, and deformation mechanisms of nitrogen-added austenitic stainless steels at room temperature and at high temperature (1173 K) were discussed in terms of SFE in Si-added austenitic stainless steel (Fe-19 wt%Cr-13 wt%Ni-0.05 wt%C-3 wt%Si-x wt%N). Nitrogen addition resulted in the decrease of SFE, which changed dislocation structures at room temperature and at 1173 K. At room temperature, nitrogen addition resulted in dislocation localization, and at 1173 K, all samples formed the sub-grain structure, caused by the dislocation recovery. It was revealed that the increase of nitrogen content resulted in the increase of the dislocation density in the sub-boundary, which indicates that the decrease of SFE contributes to the high temperature strength.

1. Introduction

Austenitic stainless steels have been extensively used for constructions and automobiles1) because they have superior properties of corrosion resistance,2) heat resistance3,4) and weldability.5) Addition of alloying elements into the steels improves their properties, and among the elements, nitrogen suppresses the crevice corrosion6) and increases the room temperature strength,7) as well as the high temperature strength.8,9) Recently, austenitic stainless steels are more likely to be used at higher temperature for the improvement of energy efficiency, thus superior strength at high temperature are required. Nitrogen addition has been recognized to cause the solid solution strengthening mechanisms such as, size effect,10) I-S effect11) and decrease of stacking-fault energy (SFE).12,13) In the case of austenitic stainless steels, stacking faults strongly contribute to the high temperature strength because they have low SFE.12,13) Therefore, characterization of the effect of nitrogen addition on SFE leads to the elucidation of the high temperature strengthening mechanism by nitrogen addition.

Several researchers have reported that there is a correlation between nitrogen content and SFE to understand the origin of the superior strength of nitrogen-added austenitic stainless steels. Ojima et al.14) and Yonezawa et al.15) calculated SFE directly by measuring the width of dissociated dislocations using transmission electron microscopy (TEM). Mosecker et al.16) evaluated SFE in Cr–Mn stainless steels by computational thermodynamics. Schramm et al.17) and Saenarjhan et al.18) measured SFE in Cr–Ni, Cr–Ni–Mn and Mn–Ni stainless steels by X-ray diffraction (XRD). SFE of austenitic stainless steels have been measured through these methods, as summarized in Table 1. Although TEM is the most suitable way to measure SFE among them due to the direct observation of microstructures, the relationship between nitrogen content and SFE is still ambiguous.14,15)

Table 1. Stacking-fault energies of various stainless steels.
Ref.C (wt%)Si (wt%)Mn (wt%)Ni (wt%)Cr (wt%)N (wt%)SFE (mJ/m2)Method
This study0.053.20.813.519.50.0133.5TEM
0.053.10.813.519.50.0919.8
0.053.10.813.619.80.1913.9
Schramm17)0.0250.30.828.2818.3118XRD
0.0740.321.6411.8518.0234
0.0470.561.7318.824.794
0.0550.531.413.0117.1578
0.0270.438.757.11210.3165
0.0340.139.556.4820.30.2641
0.0410.45.1712.3421.5764
0.911.2915.74.121
Saenarjhan18)0.010.3314.84.115.40.018.9XRD
0.020.2815.34.015.40.1013.6
0.020.2615.24.115.00.2019.1
0.010.2714.94.014.90.3124.7
Ojima14)0.0150.431.312.9517.350.019314.2TEM
0.0480.330.819.825.10.02330.9
0.0480.310.8420.1250.32542.9
0.0194.1623.09122.3

In this study, weak-beam TEM was applied to investigate the width of dissociated dislocations in order to measure the SFE.19) The influence of nitrogen addition on SFE in Si-added austenitic stainless steel (Fe-19 wt%Cr-13 wt%Ni-0.05 wt%C-3 wt%Si-x wt%N) was characterized by TEM. In addition, the influence of nitrogen addition was discussed on dislocation structures at room temperature and at high temperature, considering SFE.

2. Experimental

2.1. Samples

Composition of the steel used in this study is listed in Table 2. 20 kg of Si-added austenitic stainless steel with 0.01, 0.09 and 0.19 wt% of nitrogen was prepared by vacuum melting, forged to 55 mm at 1523 K and ground to 40 mm. The samples were then heated at 1423 K for 3.6 ks and heat-rolled to 6 mm. The heat-rolled samples were annealed at 1423 K for 60 s in an air atmosphere and air-cooled. This cooled samples were cold-rolled again to 2 mm, and then annealed at 1423 K for 60 s in air and air-cooled. The samples with nitrogen addition of 0.01, 0.09 and 0.19 wt% were indexed as 0.01 N, 0.09 N and 0.19 N, respectively.

Table 2. Chemical composition of specimens used in this study (wt%).
No.FeCSiMnMoPSNiCrN
0.01 NBal0.053.20.80.10.030.000713.519.50.01
0.09 NBal0.053.10.80.10.030.000713.519.50.09
0.19 NBal0.053.10.80.10.030.000713.619.80.19

2.2. Tensile Tests at Room Temperature

The cold-rolled samples were subjected to the tensile test at room temperature. Tensile test specimens were JIS 13B test pieces, which had widths of 50 mm between the evaluation points. The tensile test pieces were prepared from the cold-rolled annealed sample so that the tensile directions were parallel to the rolling direction. Tensile tests were performed at room temperature with a stress increase rate of 10 MPa/sec (strain rate: 8.0 × 10−4/sec), and after 0.2% proof stress, a tensile rate of 25 mm/min (strain rate: 8.3 × 10−3/sec). TEM samples were prepared from 5% pre-strained tensile specimens, tensiled at a rate of 10 MPa/sec. Note that the strain was measured by a clip-type extensometer.

2.3. Tensile Tests at High Temperature

The cold-rolled samples were subjected to the tensile test at high temperature. Tensile test specimens were 10 mm wide and had lengths of 35 mm between the evaluation points. The tensile test pieces were sampled from the cold-rolled annealed sample so that the tensile directions were parallel to the rolling direction. In the high-temperature tensile test, the temperature was raised to 1173 K at a heating rate of 100 K/min in an air atmosphere, and after holding for 10 min, tensile was performed at 0.3%/min (strain rate: 5.0 × 10−5/sec). After 0.2% proof stress, the test was performed at a tensile speed of 3 mm/min (strain rate: 1.4 × 10−3/sec). TEM samples were prepared from 5% pre-strained tensile specimens, tensiled at a rate of 0.3%/min and air-cooled to room temperature at a cooling rate of about 180 K/min. Note that the TEM samples were exposed to 1173 K for 17 minutes, and the strain was measured by a differential transformer-type extensometer.

Thin TEM foils were fabricated by a conventional twin-jet method using 5 mol%HClO4-95 mol%CH3COOH solution at room temperature with a current of 25 mA and a voltage of 30 V.

2.4. TEM Observation

2.4.1. The Measurement of the Width of Dissociated Dislocations

TEM observations of the samples deformed to 5% strain at room temperature were carried by JEM-2100HC (JEOL, Japan) operated at 200 kV. The width of dissociated dislocations was measured by the weak-beam method.19) Electron beam direction was set to about [111] of the matrix (γ-Fe), and {220} diffracted spots were intentionally excited by tilting the samples. The SFE values were calculated using Eq. (1).20)   

SFE= G b p 2 (2-ν) 8πΔ(1-ν) ( 1- 2νcos2β 2-ν ) (1)
where Δ is the dissociated dislocation width, β is the angle between the dislocation line and the Burgers vector of the perfect dislocation, G is the shear modulus, bp is the magnitude of the Burgers vector of the partial dislocations and ν is the Poisson ratio. G and ν of the austenitic stainless steel with similar composition (Fe-17.5 wt%Cr-13.8 wt%Ni-0.6 wt%Mn) were 75 GPa and 0.33, respectively.21) The actual values of the dissociation width (Δ) were calculated from the experimental width (Δobs) according to Eqs. (2) and (3).22)   
a i =- s g [ g* 2π { b ip + b ipe 2(1-ν) } ] (2)
  
Δ obs = { Δ 2 + ( a 1 + a 2 ) 2 a 1 2 a 2 2 + 2( a 1 + a 2 ) a 1 a 2 Δ- 4Δ a 1 } 1/2 (3)
where sg is the deviation parameter, g* is the magnitude of the diffraction vector, bip is the magnitude of the i-th Burgers vector of the partial dislocations and bipe is the edge component of the bip.

2.4.2. The Observation of Dislocation Structure at Room and High Temperature

The TEM observation of dislocation structure was carried by JEM-1300NEF (JEOL, Japan) operated at 1250 kV, and the TEM samples were deformed to 5% strain by room temperature and high temperature tensile tests. The dislocation density was also measured in the high temperature tensile samples by Ham’s method,23) and the Ω-type electron spectrometer was used for the observation of the thick film areas. The sample thickness was calculated by the Log-ratio method using the electron energy loss spectrum (EELS).24) The mean free path of inelastic scattered electrons of Fe at 1250 kV was 250 nm.25) Electron beam direction was set about [011] of matrix (γ-Fe), and {111} spots were excited by tilting the samples.

3. Results and Discussion

3.1. The Relationship between the Content of Nitrogen and the SFE

Figures 1(a) to 1(c) show sets of the weak-beam images of dissociated dislocations observed under several excited conditions. The contrasts of dissociated dislocation were disappeared by altering the diffraction conditions, and Burgers vector of perfect dislocations was determined. Nitrogen addition resulted in changing dislocation configuration from tangle to planar with the same Burgers vector.

Fig. 1.

A series of weak-beam TEM observation of (a) 0.01 N, (b) 0.09 N and (c) 0.19 N deformed at room temperature.

The SFE values of 0.01 N, 0.09 N and 0.19 N were measured and summarized in Fig. 2, as 14–43 mJ/m2, 12–29 mJ/m2, 9.0–18 mJ/m2, respectively. It was confirmed that the nitrogen addition resulted in the increase of the width of dissociated dislocation, so that the decrease of SFE. There are two methods available to remove the influences of dislocation interactions.26) One is to measure the SFE from the slightly deformed samples, another one is to measure the SFE from extended-nodes.26) Nitrogen addition resulted in the increase of edge dislocation components, which indicates the suppression of the movements of edge dislocations. Nevertheless, TEM observation is limited to small areas, so it is necessary to study the effect of nitrogen on dislocation components from relatively larger areas.

Fig. 2.

Width of a dissociated dislocation as a function of the tilt angle between the Burgers vector and the dislocation lines (a) 0.01 N, (b) 0.09 N and (c) 0.19 N. (Online version in color.)

Figure 3 shows the relationship between nitrogen content and SFE. Average SFE values of 0.01 N, 0.09 N and 0.19 N were 33.5 mJ/m2, 19.8 mJ/m2 and 13.9 mJ/m2, respectively. Conventionally, SFE has been expressed as a monotonic function, and relationships were proposed by several researchers as Eqs. (4), (5), (6), (7).   

SFE(mJ/ m 2 )=25.7+2[wt%Ni]+410[wt%C] -0.9[wt%Cr]-77[wt%N] -13[wt%Si]-1.2[wt%Mn] (Pickering27)) (4)
  
SFE(mJ/ m 2 )=34+1.4[wt%Ni]-1.1[wt%Cr]-77[wt%N] (Schramm et al.17)) (5)
  
SFE(mJ/ m 2 )=-7.1+2.8[wt%Ni] +0.49[wt%Cr]+2.0[wt%Mo] -2.0[wt%Si]+0.75[wt%Mn] -5.7[wt%C]-24[wt%N] (Yonezawa et al.15)) (6)
  
SFE(mJ/ m 2 )=5.53-0.16[wt%Cr] +1.40[wt%Ni]+17.10[wt%N] (Ojima et al.14)) (7)
Fig. 3.

The relationship between nitrogen content and stacking-fault energy.

In order to correlate the relationship between the nitrogen content and the SFE, Eqs. (4), (5), (6) are plotted additionally as broken lines on Fig. 3. 0.01 N has similar SFE value to the one in the case of Eq. (6). Present work shows SFE is decreased by about 107 mJ/m2 by addition of 1 wt% of nitrogen. The effect of nitrogen content on SFE in the present work is larger than that of Eq. (6) and close to that of Eq. (5), which indicates that the relationship between nitrogen content and SFE is dependent on the Mo content and independent of the Si content. Wakita et al.10) reported that the addition of both Mo and nitrogen improved the creep properties by suppressing the nitride precipitation. This is because Mo interacts with nitrogen, which results in the decreases of the effect of nitrogen on SFE.

On the other hand, Ojima et al.14) reported that nitrogen addition increased the SFE. This contradiction is probably caused by the correction of Δobs. Figure 4 shows the prior- and post-correction of the dissociation width of this study, where differences between Δobs and Δ become larger when dislocations contain more screw components. In fact, there are numbers of reports about the relationship between nitrogen and SFE. For example, the SFE decreased significantly with nitrogen addition, but SFE decreased slightly when the nitrogen addition exceeded a certain level.28) The influences of nitrogen on SFE of Cr–Mn and Cr–Mn–Ni steels were also reported.29,30,31) These reports showed that the effect of nitrogen changed significantly depending on the alloy composition and temperature. In order to clarify the relationship between nitrogen content and SFE, it is necessary to consider the interaction between elements32) and the influence of the electron density distribution at the Fermi level.29)

Fig. 4.

The corrected width of dissociated dislocations (a) before and (b) after. (Online version in color.)

3.2. The Effect of Nitrogen on the Tensile Properties

Figure 5 shows the stress-strain curve at room temperature where improvements of 0.2% proof stress and tensile strength could easily be recognized, as summarized in Table 3. In particular, the increase of tensile strength from 0.09 N to 0.19 N is almost three times higher than those from 0.01 N to 0.09 N, expected to be caused by the decrease of SFE. Furthermore, the decrease of uniform elongation of 0.19 N is probably caused by the stress concentration due to the planarization of dislocation.33)

Fig. 5.

S-S curves in RT tensile test about 0.01 N, 0.09 N and 0.19 N.

Table 3. Mechanical properties of the samples.
0.2% yield point (MPa)Tensile strength (MPa)Uniform elongation (%)Local elongation (%)Total elongation (%)
RT0.01 N25166364.65.6770.3
0.09 N30669563.25.5468.7
0.19 N38078252.911.364.2
HT0.01 N438216.687.2103
0.09 N651057.6888.896
0.19 N851344.3363.668

Figure 6 shows the stress-strain curves at 1173 K where improvements of 0.2% proof stress and tensile strength were also recognized, as also summarized in Table 3. Figure 6(a) shows the stress-strain curve in the total strain range, and Fig. 6(b) shows that up to 3% strain range. Previous studies reported that the high temperature strength was greatly improved with nitrogen addition to 19 wt%Cr-13 wt%Ni-3.3 wt%Si steel,34) whose composition was close to that of the present work. On the other hand, as shown in Fig. 6(b), the high temperature yield-drop phenomenon was observed in all samples, which indicated the occurrence of the solute dragging effect.35,36,37,38) Nitrogen addition increased the upper yield point and the amount of yield-drop, which indicated that the nitrogen addition contributed to the solute dragging effect. The nitrogen addition extended the softening time to reach the steady state, which suggested the suppression of the dislocation recovery. Furthermore, when the strain was increased at around 0.5%, work hardening values were found to increase with the increase of nitrogen content. The work hardening values of 0.01 N, 0.09 N and 0.19 N were calculated as 38 MPa, 44 MPa and 54 MPa, respectively. This suggests that the activation volume calculated by Eq. (8) decreases with the nitrogen addition.39)   

v*=kT ln( γ ˙ 2 γ ˙ 1 ) τ 2 - τ 1 (8)
where v* as the activation volume, k as the Boltzmann constant, T as the temperature,   γ ˙ i as the stress rate, and τi as the shear stress. From Eq. (8), the activation volume of 0.01 N, 0.09 N and 0.19 N were calculated as 1.42 nm3(30.7 b3), 1.23 nm3(26.4 b3) and 1.00 nm3(21.5 b3), respectively. It has been reported that the nitrogen addition decreased the activation volume in the case of room temperature creep of austenitic stainless steel,40) which indicated an increase of the thermal stress. Considering the decrease of SFE, nitrogen addition is supposed to decrease the activation volume. For these reasons, dislocation planarization decreases the activation volume because it increases the thermal stress by the local stress concentration.40)
Fig. 6.

S-S curves in 1173 K about 0.01 N, 0.09 N and 0.19 N (a) all, (b) up to 3% strain.

3.3. The Effect of Nitrogen on the Dislocation Structure in Terms of SFE

Figure 7 shows the dislocation structure observed in the samples deformed at the room temperature. As shown in Fig. 7, dislocations were localized on the slip plane with the increase of nitrogen content. The dislocation localization on the slip plane was confirmed in other stainless steels with low-SFE, which exhibited high work hardening and elongation.41) Nitrogen addition contributes to the dislocation localization because of the decrease of SFE. In addition, considering Section 3.2., the short range ordering (SRO) also contributes to the dislocation localization.42) In summary, the combination between SFE and SRO contributes to the dislocation localization. The contribution of SRO is indicated by the fact that the curved dislocations were hardly observed in 0.19 N.

Fig. 7.

Dislocation structures of (a) 0.01 N, (b) 0.09 N and (c) 0.19 N deformed at room temperature.

Figure 8 shows the dislocation structure deformed at 1173 K and the relationship between nitrogen content and the dislocation density, which formed a sub-grain structure. Nitrogen addition increased the total dislocation density by the increase of the dislocation density in the sub-boundary (ρsubboundary), while the dislocation density in the sub-grain (ρsubgrain) were remained constant, caused by the dislocation recovery. The following mechanism are proposed to explain this dislocation recovery.38)

Fig. 8.

Dislocation structures of (a) 0.01 N, (b) 0.09 N and (c) 0.19 N deformed at 1173 K and (d) Dislocation density.

1) Slipping dislocations interact with other dislocations and terminate.

2) Dislocation climbs and reacts with other dislocations.

3) Sub-boundary formed at the place where dislocation reacted, which suppressed the motion of dislocations.

Therefore, the dislocation recovery mainly occurs in the sub-boundary, and the dislocation slip mainly occurs in the sub-grain. Addition of nitrogen content decreases the dislocation climbing rate because the dislocation density in the sub-boundary increased. The dislocation climbing rate is calculated by Eq. (9).38)   

v c = v 0 C j ΩG kT σ G ( SFE Gb ) 2 D l b 2 (9)
where vc as the dislocation climbing rate, v0 as the constant, k as the Boltzmann constant, T as the temperature, Ω as the volume of atoms, G as the shear modulus, σ as the tensile stress, D1 as the diffusion rate of matrix, Cj as the density of dislocation jog, SFE as the stacking-fault energy, b as the magnitude of the Burgers vector. Equation (9) shows the decrease of SFE contributes to the decrease of vc, which results in the increase of dislocation density in the sub-boundary.38)

In addition, planar dislocation arrays were observed near the grain boundaries of the 0.19 N deformed at high temperature. This fact supports the stress concentration expected in Section 3.2. Figure 9 shows the planar dislocation arrays observed in 0.19 N deformed at room temperature and high temperature. In 0.19 N sample, the planar dislocation array was observed in whole region at room temperature, while that was observed only near the grain boundary at high temperature. In addition, planar dislocation arrays were found aligned at room temperature, but not at high temperature, which indicated that the cross-slip frequency occurred at high temperature. SFE of austenitic stainless steels increases as the increase of temperature,43) which indicates the cross-slip of the planar dislocation frequently occurs at high temperature. In the future, it is necessary to analyze quantitatively the effect of nitrogen addition on the SFE at 1173 K to discuss the high temperature deformation mechanism.

Fig. 9.

Dislocation planarization of 0.19 N deformed at (a) room temperature and (b) 1173 K. (Online version in color.)

4. Conclusion

In this study, microstructural analysis was performed by TEM for the characterization of the effect of nitrogen addition on the SFE of Si-added austenitic stainless steel. During the TEM experiments, the influence of nitrogen addition (0.01, 0.09, 0.19 wt%) on the SFE was examined by the weak-beam method. Then, in terms of the SFE, the effect of nitrogen addition on the deformation mechanism at room temperature and 1173 K was discussed. The findings obtained are shown below.

(1) Nitrogen addition to Si-added austenitic stainless steel decreased SFE.

(2) Nitrogen addition to Si-added austenitic stainless steel improved the tensile properties at room temperature and 1173 K.

(3) In samples deformed at room temperature, nitrogen addition resulted in the dislocation localization on the slip plane, caused by the decrease of SFE and the formation of SRO.

(4) In samples deformed at 1173 K, all samples formed sub-grain structure, caused by the dislocation recovery. Nitrogen addition increased the dislocation density in the sub-boundary, caused by the decrease of SFE.

(5) In 0.19 N, the planar dislocation array was observed in whole region at room temperature, while that was observed only near the grain boundary at 1173 K. The cross-slip frequency occurred at 1173 K because SFE of austenitic stainless steel increased as the increase of temperature.

References
 
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