Materials Science Forum Vols. 490-491 (2005) pp 625-630 © (2005) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.490-491.625
Online: 2005-07-15
Generation of nanostructures on 316L stainless steel and its effect on mechanical behavior T. Roland 1, D. Retraint 1, K. Lu 2, J. Lu 1,a 1
2
LASMIS, University of Technology of Troyes, 10000, Troyes, France Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, PR China a corresponding author,
[email protected]
keywords : nanostructured materials, mechanical properties, mechanical attrition treatment.
Abstract Improved mechanical behavior of surface nanostructured metallic materials produced by means of a surface mechanical attrition treatment (S.M.A.T) is investigated experimentally. Based on microscopic observations and residual stress measurements, factors leading to the high strength and yielding are discussed. The effects due to treatment, as compressive residual stresses, are in that way studied for a better understanding of their influence on the global mechanical response of the nanostructured material. In regards of this, a simple way to increase the ductility of such a nanostructured material is also presented. 1. Introduction It’s well known that materials with small grain size have their mechanical properties improved according to the hall-petch relation [1,2]. However, depending on the material and the synthesis process used, nanocrystallization has usually a tendency for reducing ductility at room temperature of these materials compared to their coarse-grained ones. In the past few years, several kinds of techniques have been developped to produce bulk nanocrystalline materials [3, 4, 5, 6]. However, due to limitation of each of techniques, preparation of ideal bulk nanocrystalline samples still remain a hard task. So, much effort has been concentrated on producing nanostructured surface layers on various materials with use of techniques involving severe plastic deformation mechanisms. The Surface Mechanical Attrition Treatment (S.M.A.T) is one of these recently developed techniques [7]. To date, this technique has been successfully applied in achieving surface nanocrystallization (SNC) in a variety of classical materials including pure metals and alloys [8, 9]. In this work, nanostructured surface layer will be presented on a 316L stainless steel whose intrinsic mechanical performances are quite low. It is shown that S.M.A treament allows to reach a grain refinement mechanism with grain size varying in the nanometer scale, from 10 nm to 150 nm over a thickness of a few to about 50 µm. Afterwards, mechanical properties of so treated specimens are studied through tensile tests. Because the S.M.A.T can introduce important compressive residual stress field formed on a thickness larger than the nanostructured layer, the present study takes into account its influence on the global mechanical behavior of the materials, and through it, points out a way to maintain important ductility of this nanostructured alloy. Till now, preliminary experimental works indicated that performances and tribological properties of materials could be enhanced by S.M.A.T induced surface nanocrystalization, but no information is available on the consequence of the mechanical effects due to treatment on the global response of the treated materials [10]. So, one of the main purpose of this study is to give some basic knowledge on the underlying mechanisms responsible for the reinforced properties of materials submited to S.M.A.T.
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2. Sample and Experimental procedure 2.1 Sample preparation The studies were carried out on a commercial grade 316L stainless steel. Tensile samples of 1mm and 0.5 mm thickness were subjected to S.M.A treatment in order to achieve a nanocrystalline surface layer on both sides. The geometry of the sample is shown in figure 1. The as-received samples had an initial grain size of, on average, 10 to 50 µm.
Fig 1. Illustration of the geometry of the tensile sample
2.2 S.M.A treatment Figure 2 shows the typical device used to transform the surface layer of the sample into a nanostructured one. The general system is schematically illustrated in figure 3. The key idea is to create a nanocrystalline surface layer from superficial structure by introducing random and severe plastic deformations [11]. shot Reflecting chamber
Booster
sonotrode
sonotrode
Converter
Electrical generator
Fig 2. Photograph of the S.M.A machine
Fig 3. Schematic illustration of the S.M.A.T setup
To this aim, a generator delivrates a very high frequency electrical signal (20 KHz) to a converter that brings it over to a mechanical vibration. This mechanical vibration is then amplified and communicated to a reflecting chamber, where shot is used to peen the surface of the specimen. Because of the high frequency of the system the entire surface is peened many times with repeated multidirectional loading. As a result, several plastic deformation mechanisms are sollicited and conduct to a refined surface structure to the nanometer scale without changing the chemical composition. In this work, treatment time was chosen to be 5, 15 or 30 min and, as shot, steel balls of 3 mm in diameter were used. A little study showed that with these parameters almen intensity is of about 0.72 mmA. Considering the impact effect of the treatment, as previous work, evaluation of the microhardness (Hv5) has been realized with two different intensities and for several treatment times. Figure 4 shows the results obtained. From these, it appears that under treatment the surface of the specimen hardens quickly and can reach more than twice its initial value. This hardening effect saturates all the more quick as the amplitude of vibration of the system is higher.
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Fig 4. Microhardness of materials after treatment with (a) a vibration amplitude of 25µm and (b) of 50 µm and for different treatment times.
After such a treatment, microstructure was examined by means of transmission electron microscope and scanning electron microscope. 3. Results and discussions 3.1 Microstructural characterization of nanostructured 316 L Stainless steel Transmission electron microscopy observations were carried out on a CM20 philips transmission electron microscope with a maximum operating voltage of 300kV. The plan view from different depths were obtained first by mechanically polishing the corresponding surface from the untreated side untill the sample reaches about 30 µm thickness and then thinned by means of Ar ion-thinning with proper incident angles. After the S.M.A treatment, the sample was transformed into surface nanostructured layer. Previous works [9] point out a nanocrystalline surface layer of about 10 µm with the similar experimental conditions as used here, and an average grain size of 10 nm at the top surface layer. Figure 5 shows a TEM micrograph of the surface layer. Nanoscale grains with highly random crystallographic orientations can be observed. Treated surface
50 nm Fig 5. Dark field TEM image showing the top surface layer after treament
25 µm
Fig 6. SEM cross section observation of the 316L S.M.A treated sample
SEM observations (figure 6) of the cross section show several clues of plastic deformations mechanisms consisting of multiple slip bands still presents far from the top surface, untill about 100 µm deep. Created during treament because of the multidirectional and repeated loading, these multiple slip bands suggest that grain refinement can be associated with shear deformation induced by localized stress. And they especially suggest that grain refinement and severe plastic deformation reach a rather thick surface layer.
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3.2 Tensile behavior 316L stainless steel specimens were submitted to the treatment for different times. Tensile tests were carried out to determine the mechanical properties of the treated and untreated materials. An instron 4484 material testing machine was used to measure the strain-stress relationship. The experimental results are shown in figure 7. 900
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Treatment σe (Mpa) σr (Mpa) time 5 min 546.5 734.7 15 min 655.8 756.8 30 min 725.1 789.4 *average of 5 tensile samples
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Fig 7. Comparison of tensile properties between SMATed 316L for different times and untreated one
Table 1. Evolution of the Mechanical properties as a function of the treatment time.
In all the cases, curves for the treated materials are higher than that for the untreated material. So, S.M.A treament may effectively enhanced the strength of materials without considerable degradation of ductility as seen in table 1. Tensile tests performed at room temperature on treated 316L stainless steel for 15 min show a real improvement of its mechanical properties. The yield stress rising from 280 MPa to about 655 MPa (increase of 134 %) while significant ductility is still remaining (30%). The ultimate stress rises of 25%. To explain the high increase of the yield stress for the treated samples, a concept has been developed. Focused on 316L stainless steel the process can be described as follow. First, nanostructured surface acts as a strong protective layer that prevent cracks from being initiated and propagated. Secondly, because nanostructured layer has a high rigidity and strength, movements of dislocations are arrested in front of the surface layer and complete slip bands can’t form. Figure 8 shows a schematic illustration of this phenomenom supported by S.E.M image of the materialhardening mechanism with a sample stretched under a normal stress of 700 MPa [12]. The white band located vertically on the left is the nanostructured surface layer, while the refined structure layer is adjacent to it from the right. (b)
(a) Slip bands stopped by the nanostructured layer
Nanostructured layers
Fig 8. (a) Schematic illustration of the hardening process due to nanostructured layer and (b) S.E.M photo shows that slips are arrested in front of the nanostructured layer
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Experiments based on SEM observations indicated that for the untreated sample, as soon as the applied stress reaches 320 MPa, slip bands appear on the surface of the sample. However, no slip band is present on the treated sample for this level of loading. About ductility, usually nanostructures are strong but fragile materials. Here, fracture profiles have shown a combined fragile-ductile process. In fact, from the center of the sample to its nanostructured egdes, the mechanism seems to be ductile and to become fragile. During tensile test, pile-up of dislocations forms in the larger central part (base material and refined layer) of the treated sample and then they are somewhat enclosed into the sample by the nanostructured layer, causing material-hardening and the increase of the yield stress of the overall material. 3.3 Measurements and Study of Residual stress Because the S.M.A treament can result in a compressive residual stress field on the surface of the material, for a complete understanding of global mechanical tensile behavior of the surface nanostructured materials, it was interesting to take their effects into account. The moiré interferometry technique [13] combined with a drilling device was used to measure the in-depth residual stress distribution in a S.M.A treated sample (figure 9a). Figure 9b shows the overall values along the tensile sample for 316L stainless steel. One can see that compressive residual stress goes beyond the elastic limit of the base material and this over a important thickness compared with that of the nanostructured layer. A maximum value was obtained at 50 µm deep, in the region of the refined layer, with –500 MPa. The compressive residual stresses are equilibrated by tensile residual stress located in the centered region with approximately 100 MPa. With a reduced tensile sample thickness, this value must be much higher. (b)
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Fig 9.(a) Optical and mechanical setup for moiré interferometry and hole-drilling method (b) Residual stresses observed after 15 mn of SMAT for 316L stainless steel
3.4 Improvement of ductility of Surface nanocrystallized material induced by S.M.A.T With the known distribution of the residual stresses, ductility of the nanostructured material has been improved through simple relaxing method described here after. Because the residual stresses are positive in the centered part of the plastically graded material, the key idea to improve the ductility of nanostructured material induced by S.M. A treatment was to relax the stress at the middle of the sample to postpone the moment where the cracks appear. To do so, during tensile test, at a load level closed to the yield point, just above, load was removed and then applied again. This has for effect to relax the stress inside the material and
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by this way to give it back some more possibility of deformation. This manner of relaxing the residual stresses, allowed us to recover not less than 10% of ductility in comparison with the unrelaxed nanostructured material. Figure 10 shows results obtained with a stainless steel of 0.5 mm thickness treated for 15 min. After relaxation of the inside stresses ductility is increased of 25%. Results presented here are from an average of 5 tensile samples. Not relaxed
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Fig 10. Recovery of ductility after relaxation of residual stresses
4. Conclusions First in this paper, nanostrutured surface layer has been developped on 316L stainless steel by means of S.M.A.T. From the view of surface engineering, the gradual variation of plastic properties, if properly controlled, can lead to significantly improvements in the behaviour of structures. So, motivation of the present work came from tensile tests of these surface nanocrystallized samples. From the results of the tension test, it can be seen that behavior of surface nanocrystallized samples shows significant difference from the untreated one. Based on experiments, this difference is contributed to that the residual stresses and the grain refinement have significant effects on tension results. To study this effect systematically, mechanism of improvement of properties is explained through a hardening model of the treated material. At the same time, considering shake-down of residual stresses in the plastically graded materials, an experiment has been developped to recover good ductility with this nanostructured materials. References [1] H. Gleiter, Prog. Mater. Sci. 33 (1988) 223. [2] K. Lu, Mater. Sci. Eng. (1996) 161. [3] R. Birringer, H. Gleiter, H.P. Klein, Phys. Lett. A102 (1984) p365 [4] K. Lu, J.T. Wang, W.D Wei, J. Appl. Phys 69 (1991) p522 [5] R.Z Valiev, R.K Islamgaliev, I.V alexandrov, Prog. Mater. Sci 45 (2000) 103. [6] U. Erb, A.M El-Sherik, G. Palumbo, K.T Aust, Nanostructured Mater. 2 (1993) p383 [7] K. Lu, J. Lu, J. Mater. Sci. Technol. 15 (1999) 193 [8] G. Liu, J. Lu, K. Lu, Mater. Sci. Eng. A286 (2000) 91 [9] N.R Tao, M.L Sui, J. Lu, K. Lu, Nanostructured Mater. 11 (1999) p433 [10] Z.B Wang, X.P Yong, N.R Tao, S. Li, G. Liu, J. Lu, K. Lu, Acta Metall. Sinica 37 (2001) 1251 [11] french patent FR2812284, FR 01022482 [12] J. Lu, K. Lu, comprehensive struct. integrity 8 (2003), p495 [13] M. Ya, Y. Xing, F. Dai, K. Lu, J. Lu, Surf. Coat. Technol. 68 (2003) 148
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Generation of Nanostructures on 316L Stainless Steel and Its Effect on Mechanical Behavior 10.4028/www.scientific.net/MSF.490-491.625 DOI References [5] R.Z Valiev, R.K Islamgaliev, I.V alexandrov, Prog. Mater. Sci 45 (2000) 103. doi:10.1016/S0079-6425(99)00007-9 [3] R. Birringer, H. Gleiter, H.P. Klein, Phys. Lett. A102 (1984) p365 doi:10.2307/2872944
