2015 Volume 55 Issue 4 Pages 858-862
Dispersion behavior of intragranular NbC precipitates in Nb added austenitic stainless steel were investigated via nanoscopic characterization in detail, FIB-SEM slice-sectioning tomography, orientation image microscopy, energy-dispersive X-ray spectrometry (EDS), selected area electron diffraction pattern (SAEDP) and transmission electron microscopy (TEM). The heterogeneous dispersion of fine intragranular NbC precipitates were visualized, and in particular, it was found that they were on the {111} slip plane and associated with dislocations.
There are four major strengthening mechanisms available in the metallic system, solid solution strengthening, dispersion strengthening, strain hardening and grain size strengthening. Therefore, intensive studies of microstructures, such as solute of additional elements, dispersion of nanoparticles, dislocations, and grain size, have been carried out for decades to correlate them with the strengthening mechanisms of materials. For the case of dispersion strengthening materials, not only the microstructures, compositions, and morphologies of nanoparticles, but also dispersion manners of nanoparticles play crucial role for the mechanical properties. Furthermore, optimization of dispersed condition of nanoparticles, such as size distribution, inter-particle distances, and volume fraction, leads to the large enhancement of dispersion strengthening.1,2,3)
In the case of steel, the precipitation of microalloying elements play a significant role in controlling the final microstructure and hence the mechanical properties of the final products. Nb is one of the best candidates among these microalloying elements, because both solute Nb atoms and NbC precipitates have a large effect on the microstructure and mechanical properties, from a combination of strengthening mechanisms including solid solution, grain size, dislocation, and precipitation hardening.4,5,6,7) For example, grains can be refined which increases the grain coarsening temperature8,9) and retards the recrystallization by pinning effect of NbC or solute drag effect of Nb.10,11,12) The refinement of grain sizes also leads to the increase of the yield strength and tensile strength.13,14) In addition, precipitation hardenings and increase of flow stress are expected by the presences of NbC.15) The sensitization caused by the formation of M23C6 and simultaneous Cr-depletion region can be suppressed by precipitation of NbC,16) and the lowering C content in solid solution results the improvements of the formability.17)
The precipitation behavior of nano-sized NbC particles has been the subject of intensive studies, and it has been shown that NbC precipitation occurs in a heterogeneous manner, intragranular preferentially at dislocations and at stacking faults to release a part of the elastic strain.18,19,20) Interaction between defects and solute atoms is probably responsible of the heterogeneous precipitation,18) which may cause the segregation of solute atoms on the dislocation and, simultaneous nucleation of precipitates. The defects at grain boundaries can also act as the precipitation sites of NbC.21)
Currently, there are several three-dimensional characterization methods available in materials science and engineering to achieve information of materials; three-dimensional atomic probe at atomic resolution,22,23) (S)TEM tomography at a few nanometer resolution,24,25,26) and FIB-tomography at submicron resolution.27,28) Among these methods, FIB-tomography by dual-beam FIB machine is the most suitable methods to study a large volume for correlating the macroscopic properties with micro/nanostructures since the maximum volume size is almost limitless, x-y resolution is that of the scanning electron microscope (SEM) and z resolution is the slice thickness.27,28,29,30) FIB-tomography consist of the serial sectioning procedure; alternate repetition of milling of thin layers of the x–y planes with the ion beam in the z-direction and imaging those with the electron beam.
In this study, both TEM characterization and FIB-SEM slice-sectioning method was applied on Nb added austenitic stainless steel to examine the dispersion behavior of intragranular precipitates.
The chemical composition of the commercially available Nb added austenitic stainless steel (supplied by Nippon Steel & Sumikin Stainless Steel Corporation, Hikari, Japan) investigated in this paper is listed in Table 1.
| C | Si | Mn | P | Ni | Cr | Mo | Cu | Ti | Nb | Al | N |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 0.031 | 0.37 | 1.54 | 0.023 | 9.77 | 17.4 | 0.1 | 0.15 | 0.005 | 0.443 | 0.021 | 0.015 |
Samples were solution treated at 1573 K for 3.6 ks and stabilized at 1173 K for 10.8 ks in prior to the isothermal ageing at 973 K for 1800 ks, then they were mechanically polished for conventional SEM observation, and electrolytically etched with the mixture of 90% acetic acid and 10% hydrogen peroxide for TEM observations.
2.2. Two-dimensional Microstructural CharacterizationDispersions and structures of precipitates were examined by bright-field TEM and selected area electron diffraction (SAED) patterns, respectively, using TECNAI-G20 (FEI, USA). Elemental distribution maps of intragranular precipitates were also examined by combination of scanning transmission electron microscopy and energy dispersive X-ray spectroscopy (STEM-EDS), ARM-200F (JEOL, Japan).
2.3. Three-dimensional Analyses by FIB-SEM Slice-sectioning TomographyFIB-SEM slice-sectioning was performed by a dual beam focused ion machine (FIB) machine, SMF-1000 (Hitachi High-Tech Science Corp., Japan). The sample was ion-milled by Ga+ beam at an acceleration voltage of 30 kV and ion current of 3 nA, then each cross-sectional image was obtained by the electron beam at an acceleration voltage of 0.5 kV. Orientation imaging microscopy was performed using OIM Data Collection and then analyzed by OIM Data Analysis.
The spatial resolution in depth depends not only on the precision of individual FIB slices but also on the penetration depth of electron. Layers of 20 nm were milled in each step during the serial-sectioning procedure and repeated until 300 successive images were collected. The off-line alignment of images, the volume segmentation, the visualization and the quantification were then performed by commercial software, AVIZO 6.3 (FEI, USA). The drift in the x and y-directions were corrected during off-line data processing by applying least square fitting algorithms to achieve alignment of the images in the stack, as shown in Fig. 1.
A schematic diagram of FIB-SEM slice-sectioning method.
Microstructures of intragranular precipitates were examined by both conventional TEM and analytical TEM. Intragranular precipitates with two different size ranges were found, namely fine precipitates with the average diameter about 15 nm, and coarse precipitates with that in the order of several μm, as seen from a set of low magnified BF-TEM images, Figs. 2(a) to 2(c). Images of fine intragranular precipitates indicated the aligned behavior of precipitates with a particular orientation relationship with the matrix.
A set of low magnified bright-field TEM images (a) to (c), where mixtures of aligned fine intragranular and randomly oriented coarse precipitates are easily seen.
An SAED pattern of fine intragranular precipitates in the matrix, shown in Fig. 3(a), confirmed that they were NbC with face centered cubic structure, as seen from Fig. 3(b).16) In addition, a representative bright-field TEM image of these aligned precipitates showed that fine intragranular NbC precipitates were, in fact, associated with dislocations, as shown in Fig. 3(c).
(a) BF-TEM image with slightly higher magnification and (b) SAEDP of NbC obtained from the dotted encircled region in Fig. 3(a). Figure 3(c) clearly suggests the presences of assocition between dislocations and nanoparticles.
Furthermore, the relationship between the matrix and aligned intragranular particles were examined by electron diffraction analysis. Figures 4(a) and 4(b) show a typical BF-TEM image of aligned intragranular precipitates and an SAED pattern from the same field of view in the [110] beam direction, respectively. A star mark represents (000) spot and the direction from (000) towards (111) spot was found perpendicular to the aligned direction of intragranular precipitates, which was an indicative of themselves on (111). Fig. 4(c) shows a schematic diagram of Thompson’s tetrahedron seeing from [111], presenting the relationship between (111) and <110>. When [110] becomes orthogonal to the sheet by rotating the tetrahedron via [110] axis, the shadowed facet (111) would appear as a line, as seen in Fig. 4(a).
(a) BF-TEM image showing aligned intragranular precipitates and (b) SAEDP from the region shown in (a). Figure 4(c) is showing a representative Thompsons’ tetrahedron presenting the relationship between (111) and <110>.
STEM-ABF images and elemental distribution map of niobium by energy dispersive X-ray spectroscopy were also taken from both fine and coarse intragranular precipitates, as shown in Figs. 5(a) and 5(b) from fine precipitates and Figs. 5(c) and 5(d) from coarse precipitates, respectively. It was confirmed that both fine and coarse intragranular precipitates were indeed NbC.
Two sets of STEM-ABF image and EDS elemental distribution map of Nb, from fine intragranular precipitates, (a) and (b), and from coarse intragranular precipitates, (c) and (d), respectively.
SEM images were taken to achieve images of large field of view with the combination of secondary electrons and backscattered electrons, to optimize the contrast of intragranular precipitates. The surficial information can be achieved from secondary electron image, and compositional information from backscattered electron image, as shown in Fig. 6(a) and in its schematic diagram Fig. 6(b). Two types of intragranular precipitates were observed in the field of view, one aligned on crossing lines with some widths and another one with rod-like or plate-like morphology with random orientation.
(a) shows a typical SEM images taken from the specimen with both secondary electron and backscattered electron, and (b) its schematic diagram.
In addition to these intragranular precipitates, intergranular precipitates with different contrast were also observed.16) Moreover, striations across the sample were occasionally found, known as the curtain effect, probably caused by the uneven chemical composition of the sample, typically at grain boundaries.
A volume, 3.0 μm × 5.0 μm × 6.0 μm, was then reconstructed from the region of interest, the boxed region in Fig. 6(b). It was clearly seen from the three-dimensionally reconstructed volume that fine intragranular precipitates were found on specific crystallographic planes with relatively a large width, about a few hundred nm to almost 1.0 μm with their average width of these as 420 nm, as shown in Fig. 7(a). Moreover, wavy patterns of fine intragranular precipitates were observed when these two planes were seen perpendicularly, as shown in Figs. 7(b) and 7(c). These wavy patterns almost probably reflect the patterns of dislocations, decorated by fine intragranular precipitates. Furthermore, these planes were then indexed as (111) and (111), confirmed by orientation image microscopy, as shown in Fig. 8. Consequently, fine intragranular precipitates were formed on {111} planes at dislocations due to the reaction between solute alloying elements, Nb, and C atoms.
A reconstructed volume (a) and two individual planes (b) and (c).
A reconstructed volume confirming the presences of two planes, primary (111) and secondary (111) confirmed by orientation image mapping.
The average “apparent” angles between primary (111) and secondary (111) were measured as 73.0°, which was slightly larger than the angle between {111} slip planes, 70.5°. It was probably due to that the dislocations on (111) were on different planes, and thus consisting “apparent” planes, as schematically drawn in Fig. 9. Higashida et al. reported a similar phenomenon in Cu system that the angle between primary and secondary slip plane was not exactly 70.5°, but slightly inclined by an angle of less than 5.0°.31)
A schematic diagram of the relationship between the “apparent”, primary (111) and secondary (111) planes.
According to FIB-SEM and TEM, it was found that there were two types of intragranular precipitates available from the sample stabilized at 1173 K for 10.8 ks. Coarse intragranular precipitates were usually found from the solution treated sample with, which did not show particular orientation relationship with the matrix. On the other hand, fine intragranular NbC precipitate were found along dislocations, showing the characteristic wavy arrays of precipitates on specific crystallographic {111} planes. The stress field of edge dislocation might have caused the segregations of solute atoms, Nb, at the dislocation cores in the grain.
Furthermore, C atoms diffusing at high speed would have reacted with Nb atoms at these dislocation cores, and subsequently nucleated and formed fine NbC precipitates on {111}, then yielded the wavy patterns of NbC precipitates as shown in Figs. 7(b) and 7(c). Consequently, dislocations were found decorated by fine intragranular NbC precipitates, similar to the case of copper precipitation on the edge dislocation in silicon.32)
Two variants of the fine intragranular NbC precipitates were found from the reconstructed volume, probably due to themselves being on the primary (111) and secondary (111) planes. Furthermore, the selection of variant should be due to the interaction between dislocation and precipitate strain fields.
Two types of intragranular precipitates were found in Nb added austenitic stainless steel, coarse and fine precipitates, and the ways they dispersed were very different. In particular, fine aligned intragranular precipitates were observed by transmission electron microscopy and they were determined as NbC from selected area diffraction pattern and energy dispersive X-ray spectroscopy. Dispersion behavior of fine aligned intragranular NbC precipitates were examined further by FIB-SEM slice-sectioning and were found on {111} slip planes associated with dislocations. Coarse intragranular NbC precipitates were randomly oriented to the matrix due to their undissolution.
