Nanocrystallization of a quenched RAFM steel and microstructure evolution during annealing heat treatment

Materials Science & Engineering A 583 (2013) 61–68

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Nanocrystallization of a quenched RAFM steel and microstructure evolution during annealing heat treatment W.B. Liu a, C. Zhang a,n, Z.G. Yang a, Z.X. Xia b a b

Key Laboratory of Advanced Materials of Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Suzhou Nuclear Power Research Institute, Suzhou 215004, China

art ic l e i nf o

a b s t r a c t

Article history: Received 10 February 2013 Received in revised form 10 May 2013 Accepted 23 June 2013 Available online 2 July 2013

Nanocrystalline grains were produced on a quenched steel by means of surface mechanical attrition treatment (SMAT). Nano-grains in the topmost surface and depth-dependent microstructures were characterized by XRD, SEM and TEM. The average grain sizes after SMAT was 6.6 nm by statistical analysis from dark images of TEM. Rapid grain growth was observed after annealing for 5 min at 823 K, and the reasons for this were the release of residual stress and enhanced diffusion rates. The average grain sizes were 42.6 nm, 79.1 nm and 92.8 nm after annealing for 5 min, 30 min and 120 min at 823 K, respectively. The slow grain growth during annealing can contribute to the strong pinning force produced by the dispersive M(C,N) carbides and solute atoms in the present RAFM steel. Diffraction peaks of TaC carbides became weaker after annealing heat treatment from the XRD results, but no M23C6 type carbides were found even after annealing for 120 min. & 2013 Elsevier B.V. All rights reserved.

Keywords: Nanocrystalline materials Surface mechanical attrition treatment Grain growth Residual stress

1. Introduction It is well known that nanocrystalline (NC) materials, with grain sizes typically ranging from 2 to 100 nm, exhibit many excellent properties relative to the coarse-grained counterparts [1]. Lots of studies have been done to investigate the microstructure evolution and mechanical properties of NC materials [2,3]. Severe plastic deformation (SPD) has been proved to be an effective way to refine microstructure of alloys and metals [4]. The surface mechanical attrition treatment (SMAT) [5] is regarded as one of the most important means of SPD, and it has been proved to be a novel and efficient technique to achieve bulk NC materials. In the past decades, a comprehensive understanding of grain refinement mechanism during SMAT [6,7] has been achieved, in which dislocation activities and phase transformations are found to make a key role. It is generally considered that NC material has a large volume fraction of grain boundaries (GBs), which are supposed to have the same structure as the crystallites but showing a density reduction [8,9]. Recent studies indicate that grain growth occurs in NC metals during various SPD [10], including high-pressure torsion [11], uniaxial tension [12] and uniaxial compression [13]. It is generally accepted that the kinetics of grain growth about NC

n

Corresponding author. Tel.: +86 10 62797603; fax: +86 10 62771160. E-mail address: [email protected] (C. Zhang).

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metals and alloys during thermal annealing is to reduce the grain boundary (GB) areas [14]. It is well known that crystal defects and high internal stresses associated with large volume fraction of GBs appear during the drastic deformation. X-ray diffraction (XRD) is a useful tool to analyze the grain size of NC materials [15], but information on TEM observations is more essential [16]. XRD shows the average size of subgrains with small misorientations, typically less than 11 or 21, while the TEM images gives the sizes of the grains featured by high angle GB [17]. And a number of studies have been carried out to measure residual stresses in various materials [18,19]. Materials operating near the fusion reactions will be exposed to a high flux of high temperatures and high energy neutrons [20], and reduced activation ferrite/martensite (RAFM) steels are under consideration for certain nuclear fusion related applications [21]. Lu et al. [22] studied the chromizing process in the SMAT surface layer of low carbon steels, and the results showed that the nanostructures were effectively stabilized by the formation of fine dispersive Cr compound particles. Calculated results [23] showed that all Cr23C6 carbides dissolved after austenitizing at 1253 K, and Cr containing carbides were easy to precipitate when annealing. During annealing process of nanocrystalline, however, the mechanism of microstructure evolution and carbides precipitation and coarsening were still incompletely understood for quenched steels, in which no Cr containing carbides exist. In this work, a nano-structured surface layer with a depthdepended nanocrystalline microstructure was synthesized on a quenched RAFM steel by means of SMAT. And XRD, SEM and TEM

W.B. Liu et al. / Materials Science & Engineering A 583 (2013) 61–68

3. Result 3.1. Microstructure characterization before and after SMAT Fig. 1 shows the XRD profiles of the surface layers before and after SMAT, in which only bcc martensite was detected. The width of XRD peaks became wider and the intensity became weaker after SMAT (Fig. 1(a)). It can be seen that peaks of MC carbides still exist after austenitizing (Fig. 1(b)), but no peaks of M23C6 carbide were detected. Although the peaks of MC carbides became smaller, they did not disappear after SMAT, which indicated the refining or dissolving of MC carbides during SMAT. The diffraction angles (34.941and 40.641) can be further determined as peaks of TaC (111) and TaC (200). And the lattice parameter of TaC (FCC structure) is 0.4440 nm in the present work, which is in the range of 0.4410– 0.4456 nm of TaC in literature [25]. Average grain sizes in the SMAT surface layer were derived from the breadths at half maximum intensity of XRD peaks using the Scherrer–Wilson equation [26], and the measured (200) and (211) peaks were selected. The crystallite size of SMAT-ed sample is 10.2 nm, which obviously demonstrates that the average grain size was effectively refined into nanometer scale.

40

60 Diffraction Angle (2θ)

(220)

(200)

(211)

(110) 20

80

100

before SMAT

34

after SMAT TaC (200)

The material used in the present investigation was a RAFM steel with chemical composition (wt%): 0.091% C, 0.49% Mn, 8.75% Cr, 0.28% Ta, 1.58% W, 0.21% V and balance Fe. Heat treatment included austenitizing at 1253 K for 45 min, followed by water quenching (WQ). The plate sample (∅50
4.0 mm in size) of the quenched steel was submitted to SMAT, the equipment and procedure of SMAT are discussed somewhere else [24]. The ball size was 5 mm in diameter. The vibration frequency of the chamber was 20 kHz, and the plate was treated for 30 min. Samples subjected to SMAT were annealed at 823 K for 5 min, 30 min and 120 min to investigate the microstructure evolution. To avoid being oxidized, all the samples for annealing treatment were sealed in glass tubes under 0.3 atm of argon. X-ray diffraction (XRD) analysis of the surface layer was carried out to measure the phase, grain sizes and residual stress. Step size of 0.021 and range of 2θ from 201 to 1001 were taken to measure the XRD intensity before and after SMAT. Average grain size in the topmost surface was calculated in the step scanning mode, and the bcc Fe (200) and (211) Bragg diffraction peaks were selected. Details of residual stress were discussed later. Cross-sectional observation of the treated sample was performed on scanning electron microscopy (SEM) of type JEOL JSM-4500. The samples were ground and mechanically polished in the standard manner. The polished samples were etched in a particular solution (12 ml alcohol+3 ml hydrochloric acid+1 g ferric trichloride) to obtain the microstructure and morphology of matrix and treated surface layer. Microstructure of the treated surface layer was characterized by transmission electron microscopy (TEM) on JEOL JEM-2011 and Tecnai F20. The TEM samples were obtained by means of cutting, grinding and thinning from the non-treated side at low temperatures.

after SMAT

TaC (111)

2. Experimental

before SMAT

Intensity (CPS)

were used to investigate the microstructure and carbides in the steel. Because of the long-term exposures under high temperatures (above 823 K) when operating near the fusion reactions, the nanostructured RAFM steel was annealed at 823 K for different times to make clear the microstructure evolution, and this would also explore the potential use of nanocrystalline RAFM steel.

Intensity (CPS)

62

36

38 40 Diffraction Angle (2θ)

42

Fig. 1. XRD profile of the surface layers before and after SMAT (a) diffraction peaks between 201 and 1001 and (b) small peaks between 361 and 421.

Cross-sectional SEM observations of the surface layer and matrix are shown in Fig. 2, respectively. Obviously, microstructure in surface layer is distinct from that in the deep matrix. GBs and martensite packets can be clearly seen in the matrix (Fig. 2(a)), and the grain size is 8–10 μm. With increasing depth from the treated surface, gradient nano-submicro-microstructure and obvious deformation bands (Fig. 2(b)) are found due to the decreasing strain and strain rate. Another worth noting information is that no carbides can be obviously seen both in the SMAT layer and in the matrix from the SEM results, which indicated the dissolving of chromium carbide during the austenitizing heat treatment. TEM images of the quenched samples before SMAT were given in Fig. 3. Martensite laths with different directions and GBs/packet boundaries can be clearly seen in the pictures (Fig. 3(a)). There is no retained austenite and carbides observed from the TEM results. The width of martensite laths is 200–300 nm, and fine precipitations and dislocations with high density are also found in the laths (Fig. 3(b)). TEM bright field image obtained from the topmost surface of the SMAT sample and the corresponding dark image taken from the diffractions of bcc (110) are shown in Fig. 4. SAED patterns composed of continuous rings exhibit that these grains are almost nano-grained bcc martensite (Fig. 4(a)). Grains of the original sample have been broken down to equiaxed nano-sized grains and the grains are random in crystallographic orientation (Fig. 4(b)).

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63

Surface

5 μm

5 μm Fig. 2. Cross-sectional SEM images (a) in matrix and (b) in surface layer.

200 nm

500 nm

Fig. 3. TEM images of the quenched samples before SMAT (a) morphology of quenched martensite and (b) morphology of martensite laths.

Volume fraction (%)

30 25 20 15 10 5 0

0 2 4 6 8 10 12 14 16 18 20

Grain size (nm)

50 nm

50 nm

Fig. 4. TEM images of the SMAT steel sample for the top surface layer (a) bright field image and (b) dark field image taken from the diffractions of bcc (110). Inset in (a) shows a SAED pattern, and inset in (b) shows a grain size distribution.

The average grain size by statistical analysis is 6.6 nm (Fig. 4(b)), which is slight smaller than the XRD result (10.2 nm). Another worth noting information is that most of the grains are smaller than 8 nm, although the largest grains are about 20 nm in diameter. The difference might be attributed to the different measuring depths of XRD and TEM. With consideration of the Cu Kα wavelength (0.154056 nm) and its extinction depth in Fe, XRD patterns reflect the structure information from the surface layer of about 5 mm thick [6], while TEM sample can be observed to be less than 0.5 μm thick [27]. As the observation region is nearer to the surface, TEM result is smaller. TEM images of the SMAT steel sample at a depth ∼40 μm are shown in Fig. 5, in which sub-grain boundaries (sub-GB) can be clearly seen. And the grain size is about 150 nm. In the micro-sized

region (Fig. 5(a)), most original grains are found to be subdivided into micro-sized cells in TEM observations. 3.2. Thermal stability In order to investigate the microstructure evolution during annealing heat treatment, the nanocrystalline samples of quenched RAFM steel were annealed at 823 K for 5 min, 30 min and 120 min, respectively. TEM images obtained from the topmost surface of the SMAT sample after annealing at 823 K for 5 min and the corresponding SAED patterns are shown in Fig. 6. The average grain size obtained from statistical analysis from the darken image (Fig. 6(b)) is 42.6 nm. Sun-GB and DDWs seem to be appearing in the grains from the TEM

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50 nm

50 nm

Fig. 5. TEM images of the SMAT steel sample at a depth ∼40 μm (a) bright field image and (b) dark field image taken from the diffractions of bcc (110). Inset in (a) shows a SAED pattern.

Volume fraction (%)

50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 100 Grain size (nm)

100 nm

100 nm

Fig. 6. TEM images of the top surface after annealing at 823 K for 5 min (a) bright field image and (b) dark field image taken from the diffractions of bcc (110). Inset in (a) shows a SAED pattern, and inset in (b) shows a grain size distribution.

Volume fraction (%)

30 25 20 15 10 5

0 20 40 60 80 10 0 12 0 14 0 16 0 18 0 20 0 22 0

0 Grain size (nm)

200 nm

200 nm

Fig. 7. TEM images of the top surface after annealing at 823 K for 30 min (a) bright field image and (b) dark field image taken from the diffractions of bcc (110). Inset in (a) shows a SAED pattern, and inset in (b) shows a grain size distribution.

results. Another worth noting information is the diffraction pattern was still continuous rings after annealing for 5 min. Fig. 7 shows the TEM images obtained from the topmost surface of the SMAT sample after annealing at 823 K for 30 min. The average grain size obtained from statistical analysis from the darken image (Fig. 7(b)) is 79.1 nm. The grain sizes are not

uniform, and abnormal grain growth seems to be appearing in this stage. Larger grains, with diameter about 200 nm, are observed from the TEM results, but the diffraction pattern was almost continuous rings after annealing for 30 min. TEM images obtained from the topmost surface of the SMAT sample after annealing at 823 K for 120 min are given in Fig. 8.

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65

Volume fraction (%)

30 25 20 15 10 5

0 20 40 60 80 10 0 12 0 14 0 16 0 18 0 20 0 22 0

0 Grain size (nm)

200 nm

200 nm

Fig. 8. TEM images of the top surface after annealing at 823 K for 120 min (a) bright field image and (b) dark field image taken from the diffractions of bcc (110). Inset in (a) shows a SAED pattern, and inset in (b) shows a grain size distribution.

And the average grain size obtained from statistical analysis from the darken image (Fig. 8(b)) is 92.8 nm. Another worth noting information is the GBs become straight, and grains with hexagon boundaries seem to be appearing. There seemed to be rings in the SAED patterns, but lots of bright points were on the rings, which implied that some special orientation become stronger.

4. Discussion 4.1. Microstructure evolution during SMAT High-resolution TEM (HRTEM) micrograph and inverse fast Fourier transformation (IFFT) of the top surface layer after SMAT is shown in Fig. 9. Small grains (around 5 nm) with misorientations and different directions can be clearly seen in Fig. 9(a), and the parallels show the different directions. As marked with white ellipses in Fig. 9(b), result of the IFFT picture shows a large number of dislocations and heavy distortion of lattice. The distance of the planes in perfect areas of the IFFT picture is 0.1807 nm, and it is determined to the BCC Fe (110). During plastic deformation dislocation activities are motivated in the original coarse grains. The intersecting dense dislocation walls (DDWs) subdivided the original ferrite grains into finer blocks (or dislocation cells) [28]. More and more dislocations are formed and accumulated with increasing stains. Dislocation rearrangement and annihilation occur in dislocation walls (DWs) at a certain strain level. More dislocation are generated and annihilated in the sub-GB with further increasing of stains. When the balance between dislocation multiplication rate and annihilation rate reached, the increase of strains or duration could not reduce the grain size any longer, and a stabilized grain size is got.

4.2. Residual stress driven grain growth The relationship between residual stress (s) and strain (ε) on the specimen surface under plane stress is given by [29] ∂ðεÞ 2

∂ð sin Ψ Þ

¼

ð1 þ νÞs E

The Bragg equation, λ ¼2d sin θ, where λ is X-ray wave length, d is lattice interplanar spacing and θ is diffraction angle, gives the

following relationship [29]: Δd ¼ cotðθÞΔθ d The strain (ε) can be expressed as [19] ε¼

Δd ¼ cotðθ0 Þðθθ0 Þ d

The residual stress was computed from the formula [19,29] !
  π  E Δð2θÞ cotðθ0 Þ s¼ 2ð1 þ νÞ 180 Δð sin 2 Ψ Þ where ν is Poisson's ratio, E is Young's modulus, θ0 is diffraction angle in a distortion free condition and Ψ is the inclination angle between the diffraction angle plane normal and the sample plane normal. The measurements of residual stress were made on the (211) Bragg diffraction angle of the investigated steel (BCC, a¼ 0.28664 nm) with Cu Kα (λ ¼0.154056 nm) and 2θ¼ 82.331 at angular positions of 2θ from 801to 84.51. The peak position (2θ) was determined from the count data for the angle of incidence, Ψ (Ψ ¼01, 81, 161, 231 and 301). The step size are 0.011, and the duration time were 1 s, 1 s, 2 s, 2 s, 3 s, respectively. In the present work, the residual stress after SMAT is computed to be  453.2 MPa from the known value of E ( ¼200 GPa) and ν (¼ 0.27) [30]. And the value reduced to  319.5 MPa after annealing at 823 K for 5 min. It has been reported that compressive residual stress in the regions near the treated surface was generated during SMAT [31]. And the compressive stresses became tensile at certain depths. And lots of experimental [32] and theoretical models [33] about stress-driven grain growth and migration of grain boundaries in nanocrystalline materials have been systematically studied. Stressassisted microstructural evolution can be explained by the movement of the grains in directions parallel to the GB. And stressdriven grain growth boundary sliding and migration in nanocrystalline metals can result from a shear stress applied tangential to it. In the present work, the compressive stresses generated during SMAT can result in the fast grain growth at the early stage during annealing. 4.3. Microstructure evolution during annealing Some larger grains (about 90 nm) appeared after annealing for 5 min (histogram in Fig. 6(b)), but most of the grains are smaller

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5 nm

1 nm

Fig. 9. HRTEM micrograph of the top surface layer after SMAT (a) highly misoriented nano-grains and (b) IFFT micrograph of the areas marked by the white frame in (a), Inset in (a) shows a Fast Fourier Transformation (FFT) pattern.

50 nm

5 nm

Fig. 10. HRTEM micrograph of the surface layer after annealing at 823 K for 5 min (a) micrograph of the grains and grain boundaries and (b) micrograph of MC carbides, Inset in (b) shows a FFT pattern of the carbide.

than 50 nm. The largest grains are about 200 nm after annealing for 30 min (histogram in Fig. 7(b)), but grains with diameter smaller than 20 nm are also found. And most of the grain sizes are between 60 nm and 120 nm after annealing for 120 min (histogram in Fig. 8(b)), but the largest grains are also smaller than 200 nm. HRTEM micrograph of the surface layer after annealing at 823 K for 5 min is shown in Fig. 10. Grains with subdivided subgrains can be seen clearly. And carbides with hexagon boundaries (about 12 nm) are observed from the HRTEM results. The carbide is precipitated on the GB (Fig. 10(b)), and grows up into one of the two grains besides the boundaries. And the carbide is determined as TaC according to the FFT and IFFT patterns. HRTEM micrograph of the MC carbide in the surface layer after annealing at 823 K for 120 min is given in Fig. 11. It is confirmed that the carbide grows upto about 40 nm in long axis. The distance of the planes in perfect areas of the IFFT picture is 0.2634 nm, and it is determined to the FCC MC (111). According to the XRD results, the average grain sizes of samples annealing for 5 min, 30 min and 120 min are 20.0 nm, 15.3 nm and 18.2 nm, respectively. But the corresponding average grain size calculated from the statistical analysis of TEM images are 42.6 nm, 79.1 nm and 92.8 nm, respectively. The reason for this is originated from the hierarchy of the microstructure of SPD metals [34,35]. The grains with high-angle boundaries are subdivided into subgrains and/or cells during SMAT. The misorientation angle between cells is low (11–21), and XLPA could measure the size of these cells with small orientations. But there is no measurable contrast difference between them in TEM micrographs [16,17], and they can be observed separately only by HRTEM (Fig. 9(a))

20 nm Fig. 11. HRTEM micrograph of the MC carbide in the surface layer after annealing at 823 K for 120 min, inset shows a corresponding FFT pattern of the carbide.

investigations. That is to say, with the increase the grain size when annealing at 823 K, sub-grains with small misorientations in the same grain appeared (Fig. 10(b)). Hence, the grain sizes

WC(100)

67

Anealing for 5 min

composition of TaC phase are given by Bowman [37] after studying the experimental data from others:

Anealing for 30 min

a0 ¼ 4:3007 þ 0:1563X

Anealing for 120 min

where a0 is the lattice parameter of TaC, and X is the ratio of C/Ta. The lattice constant of TaC decreases as the crystal becomes deficient in carbon. When annealing at 823 K, the loss of carbon from TaC results in the decrease of X, and thus decrease the lattice parameter of TaC. And a slight movement of the TaC (111) peak is observed in Fig. 12. Another worth noting information is there are no peaks of Cr23C6 observed even after annealing for 120 min.

Intensity (CPS)

TaC(200)

TaC(111)

W.B. Liu et al. / Materials Science & Engineering A 583 (2013) 61–68

5. Conclusions

34

36

38 40 Diffraction Angle (2θ)

42

Fig. 12. XRD profiles of the surface layers of SMAT samples after annealing for different times.

obtained by XRD were much smaller than that obtained from TEM pictures. 4.4. Inhibition of grain growth by particles The driving force of grain growth during annealing process is the reduction of total area of grain surface according to the graingrowth kinetics in conventional polycrystalline samples. However, the segregated solute atoms in the GB of NC materials induce a drag force on the GB migration [9]. Because of the existence of large number of dispersive M(C,N) carbides and solute atoms in the present RAFM steel, there might exists a strong pinning force on the migration of ferrite GBs, which would hinder the grain growth. The mechanism of the interaction between a second-phase particle and a GB is attraction force, which results from the reduction in GB upon contact between the particle and GB [36]. For a planar GB and a spherical inclusion, the maximum total free energy change of the system can be expressed as [36] ΔG ¼ γ b ð1πr 2 Þ þ 2πrγ ls where γ b is the GB surface tension, r is the radius of inclusion, and γ ls is the line tension of the triple junction. Then the maximum force between the GB and particle is equal to n

f ¼ 2πγ b þ 2πγ ls The force becomes zero for a critical particle size r ncrit ¼

γ γb

ls

Owing to the change of the free energy with different particle sizes, particles with r 4 r ncrit will be wetted by the boundary, while particles with r o r ncrit will be repelled. The particle size to inhibit grain growth of commercial alloys is typically in the range of 10– 50 nm [36]. In the present steel, the size of MC carbides was about 12 nm after annealing for 30 min, and the size became 40 nm after annealing for 120 min. It is possible to infer that the MC carbides in the GB of the present steel hinder the grain growth when annealing at 823 K. XRD profiles of the surface layers of SMAT samples after annealing at 823 K for different time is shown in Fig. 12. The intensity of TaC became weaker After annealing for 30 min, as the time becomes longer. Equations between the lattice parameter and

A gradient microstructure has been produced in the surface layer of a quenched RAFM steel by means of SMAT. The average grain size at the top surface are 10.2 nm (by XRD) and 6.6 nm (by TEM), respectively. Small grains with large misorientations and dislocations in the topmost surface with high density were found in HRTEM results. A rapid grain growth after annealing for 5 min at 823 K and then stable grain growth were observed, and this can be contributed to the release of residual stress from 453.2 MPa to  319.5 MPa after annealing for 5 min. But the average grain size was still within 100 nm after annealing for 120 min. A strong pinning force produced by the dispersive M(C,N) carbides and solute atoms in the present RAFM steel would hinder the grain growth when extend the annealing time. Because of the different test mechanisms, the grain size of annealing samples obtained from XRD were much smaller than that from TEM pictures. MC carbides with size about 12 nm precipitated at the grain boundaries after annealing for 5 min, and grew upto about 40 nm in long axis after annealing for 120 min. During annealing process, intensity of TaC became weaker from the XRD results, but no M23C6 type carbides are found even after annealing for 120 min in the present work.

Acknowledgments The authors are indebted to Dr. Z.B. Wang of Shenyang National Laboratory for Materials Science for the cooperation in the SMAT experiments. Financial support from the National Basic Research Program of China (no. 2011GB108006) and National Natural Science Foundation of China (no. 51071090) is acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

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