MICROSTRUCTURAL EVOLUTION OF A F138 AUSTENITIC STAINLESS STEEL AFTER

MICROSTRUCTURAL ANALYSIS OF COPPER THIN FILMS FOR CHARACTERIZATION OF
MICROSTRUCTURAL EVOLUTION OF A F138 AUSTENITIC STAINLESS STEEL AFTER





Microstructural evolution of a F138 austenitic stainless steel after deformation by ECAP and HPT

Microstructural evolution of a F138 austenitic stainless steel after deformation by ECAP and HPT







A.M. Kliauga1*, V.L. Sordi 2, S. Dobatkin3

1 Department of Production Engineering, Federal University of São Carlos, Campus Sorocaba, Rod. João Leme dos Santos km 110, 18052-780 Sorocaba, Brazil - [email protected]

2 Department of Materials Engineering, Federal University of São Carlos,

3 A.A. Baikov Institute of Metallurgy and Materials Science, Russian Academy of Science,









Abstract: A F138 austenitic stainless steel was solution heat treated, deformed by equal-channel angular pressing (ECAP) at 25, 100, 200, and 300oC. The equivalent strain was ~0.7 per pass and the applied equivalent strain varied from 0.7 to 4.2. The same material was also deformed by high pressure torsion (HPT) at 300 and 480oC, applying 6GPa pressure and 5 turns; the equivalent strain was ~ 4.5 at r/2 and ~5.2 at the vicinity of the disk edge.. Microstructure evolution was observed by transmission electron microscopy (TEM) electron back-scattered diffraction (EBSD) and X –ray diffraction. The effect of severe plastic deformation was studied at 25 and 300oC: at 25oC further deformation led to the formation of grain subdivision inside deformation bands and the onset of new grains formation after 2 ECAE passes. The deformation at 300 and 400oC up to 6 passes lead to the formation of recrystallized grains of the order of 100 nm size.

< and {311} peaks are plotted against the theoretical strain for the ECAP samples. The deformation at 300 oC produced lower accumulated strain in the crystalline lattice, since the peaks for this temperature are less distorted than those of the samples deformed at room temperature.


MICROSTRUCTURAL EVOLUTION OF A F138 AUSTENITIC STAINLESS STEEL AFTER

Figure 2 Austenite peak broadening depending on the equivalent strain for ECAP deformed samples: {220} and {311} planes and deformation temperatures ( room temperature = RT and 300 oC).


During SPD in medium to high stacking fault energy (SFE) metals (e.g. Cu, Ni and Al) the microstructure evolves through the formation of dislocation cells [7]. In contrast, in low SFE metals (such as austenitic stainless steel, brass, Al–Mg alloys, and Cu–Al alloys) deformation leads to twins and second-generation microbands rather than dislocation cells [8]. Deformation twinning transforms the homogeneously deformed fcc structures into a fine laminar structure, which will be further divided by the twin-matrix layers and intersections of twins. This behavior accelerate the formation of nanograins with uniform distribution. The SFE is very sensitive to the chemical composition and also increases with the temperature, so that for medium SFE values (20-30 mJ/m2) the deformation mechanism – twinning or dislocation glide - can be controlled by changing the temperature.

In this work the undeformed samples showed coarse grains containing recrystalization twins. The sample deformed by one ECAP pass at room temperature presented a large number of deformation twinning (Figure 3a). As the temperature increased to 100 and 200 oC the amount of grains with deformation twins decreased (Figures 3b and 3c) and for deformation at 300 oC only few deformation twins were observed; the grains elongate in the shear direction of the ECAP die (figure 3d).

Figure 4 shows the results EBSD analysis on the samples 1x RT, 1x 300C and HPT 300C. At room temperature original grains are subdivided into regions of same crystal orientation separated by high angle twin boundaries. Some deformation bands with higher misorientation are concentrated at the original grain boundaries and with increasing deformation steps become more preeminent across the grains. For the deformed samples at 300C 1x ECAE and HPT only deformation bands with an initial grain subdivision (for the HPT sample) could be observed with gradual misorientation increase in the regions where deformation was more concentrated.


MICROSTRUCTURAL EVOLUTION OF A F138 AUSTENITIC STAINLESS STEEL AFTER

Figure 3 : Optical images showing the microstructures of samples: a) one ECAP pass at room temperature; b) one pass at 100 oC c) one pass at 200 oC; and d) one ECAP pass at 300 oC.



a) MICROSTRUCTURAL EVOLUTION OF A F138 AUSTENITIC STAINLESS STEEL AFTER b) MICROSTRUCTURAL EVOLUTION OF A F138 AUSTENITIC STAINLESS STEEL AFTER c) MICROSTRUCTURAL EVOLUTION OF A F138 AUSTENITIC STAINLESS STEEL AFTER

Figure 4 EBSD analysis of the samples a) ECAP 1x RT; b) ECAP 1x 300 oC and c) HPT at 300 oC (transition region at the beginning of the severe plastic deformation). Inverse pole figures showing local misorientations, image quality showing dark regions of lower diffraction intensity that corresponds do sites of increased deformation and grain boundaries; image quality maps with identified grain low and high angle boundaries.


The volume fraction of twins in the first ECAP pass at room temperature was 60 ± 10 % measured form TEM images. After 2 passes several new microstructural features as localized shear bands, with intercepted twins, grain fragmentation (figure 5a) were observed. Between 2 and 4 passes the extend of submicron size microstructure increased. At 300 oC and after 4 passes (figure 5b) the microstructure is composed of localized shear bands with internal dense dislocation walls, but the diffraction pattern does not indicate high angle misorientation inside the bands. After 6 passes dislocation cell rotation and grain subdivision started (figure 5c). After HPT (figure 5d) the microstructure is characterized by both elongated and equiaxed grains which are separated by sharp grain boundaries as shown in the micrograph, and the diffraction pattern indicate a great misorientation between individual grains. At 480 oC only a small number of crystals exhibit deformation twins; grains are equiaxed and smaller than 100 nm (figure 5e).


MICROSTRUCTURAL EVOLUTION OF A F138 AUSTENITIC STAINLESS STEEL AFTER

Figure 5 TEM images of samples: a) ECAP 2x at room temperature; b) ECAP 4x at 300 oC; c) 6x at 300 oC d) HPT at 300 oC and e) HPT at 480 oC.


Summary and conclusions



Deformation of the F138 austenitic stainless steel at room temperature and at 300 and 480 oC lead to two different deformation mechanisms: twinning and dislocation glide. The formation of submicron size grains was observed at the HPT deformed samples at 300 and 480 oC and its formation was mainly associated to the dislocation glide process.


Acknowledgements

The authors thank FAPESP (Fundação de Apoio à Pesquisa no Estado de São Paulo) for financial support.


References

  1. Langdon T.G. Processing by sever plastic deformation: Historical developments and current impact. Materials Science Forum v. 667-669: p. 9, 2011.

  2. Iwahashi Y.; Wang J.; Horita Z.; Nemoto M.; Langdon T.G. Scripta Materialia, v 35: p. 143, 1996.

  3. Zhilyaev AP, Nurislamova GV, Kim B-K, Baró MD, Szpunar JA, Langdon TG. Experimental parameters influencing grain refinement and microstructural evolution during high-pressure torsion. Acta Materialia, v 51,nr 3, p. 753-765, 2003.

  4. Greger M.; Kander L,; Kocich R. Structure and low-cycle fatigue of steel AISI 316 after ECAP Archives of Materials Science Engineering, v31: p. 41-44, 2008.

  5. Litovchenko I.Y.; Tyumentsev, A.N.T.; Shevchenko N.V.; Korzinov A.V. Evolution of Structural and Phase States at Large Plastic Deformations of an Austenitic Steel 17Cr–14Ni–2Mo. The Physics of Metals and Metallography ,v.112: p.412-423 ,2011.

  6. Scheriau S.; Zhang Z.; Kleber S.; Pippan R. Deformation mechanisms of a modified 316L austenitic steel subjected to high pressure torsion. Materials Science Engineering A, v.528: p.2776-2786, 2011.

  7. Liu, F.; Zhang Y.; Wang J.T. Microstructure Evolution of Pure Nickel up to a High Strain Level during Equal-Channel Angular Pressing. Materials Science Forum, v.667-669, p.319-,2011.

  8. Zhang Y.; Tao N.R.; Lu K. Effects of stacking fault energy, strain rate and temperatureon microstructure and strength of nanostructured Cu–Al alloys subjected to plastic deformation, Acta Materialia v.59 :p. 6048-6058, 2011.









Tags: after deformation, 316 after, steel, after, evolution, austenitic, stainless, microstructural