Surface Mechanical Attrition Treatment - Term Paper Example

Surface mechanical attrition treatment (SMAT) of steel: Nano microstructure on surface of bulk sample Presented to [Teacher’s In Partial Fulfillment of the Requirements of [Project/Course name] Dated Abstract Surface mechanical attrition treatment (SMAT) is a recent technique that has evolved to improve on the surfaces of metallic materials based on the strain induced grain refinement. This technique is aimed at improving the overall performance of metals and their alloys without altering the chemical properties of the metal. Steel is refined and a nano structured surface is induced to improve its usability and strengthen its properties in different dimensions. In the resultant deformed material different refinement procedural approaches were investigated together with the mechanisms for the formation of nanocrystallites. Deformation procedure in this material involves dislocation stages, mechanical twinning and the interactions of the mechanical twinning with the dislocations. The performance and properties of the SMAT treated layers were measured and tested by their thickness, hardness, fatigue, tensile strength and wear. The improved properties of the treated surface layer were attributed to the refinement of the strain induced grain. In this work, properties and microstructures of the SMAT treated layers of steel are reviewed Keywords: SMAT treated steel, stress-strain characteristics of SMAT treated alloys, SMAT improves grain properties, grain boundaries and sizes. Contents Abstract 2 1. Introduction 4 1.1. Surface mechanical attrition treatment process 4 1.2. Reduced Material Thickness Due to SMAT 6 2. Deformation of AIS 304 stainless steel through dislocation slip 7 2.0. Mechanical twinning in plastic deformed austenite 7 2.1. Grain refinement process of steel 7 2.1.1. Formation 7 2.1.2. Grain subdivision 8 2.1.3. Formation of nanocrystallites 8 2.2. Low stacking fault of stainless steel 8 3.0. Properties and performance 10 3.1. Hardness 10 3.2. Tensile Strength 11 3.3. Fatigue Properties 12 3.4. Wear and Friction 12 3.5. Diffusion of materials 13 3.5.1. Diffusion mechanisms 14 3.6. Corrosion behavior of steel subject to SMAT 16 4. Conclusion 18 1. Introduction According to Hodgeson, Timokhina and Belad(Hodgson, Timokhina, & Beladi, 2013), Surface mechanical attrition treatment (SMAT) is a new approach that has been developed for the treatment of mechanical surfaces. In this technique a number of balls to be treated are placed in a chamber and are vibrated at a very high frequency by a generator. The sample of the material to be treated is fixed at the upper side of the chamber. It is then repeatedly impacted by the flying balls in order for its surface to be heavily deformed plastically. Beside the conventional use of the plastically deformed surface, it hardens the surface and improves its fatigue and wears properties. SMAT has also been utilized, for example to activate a material surface before nitriding (Lu & Lu, 2004; Z. B. Wang, Lu, & Lu, 2006). SMAT creates multi-layered laminate composites due to its subsequent roll bonding (Caballero, Garcia-Mateo, & Miller, 2014; Lin, J, Wang, & Xue, 2006). With all these practical potential applications, the micro-structural and metallurgical states of the deformed surfaces must be tailored in a reproducible manner and in different ways to form nanostructures with deep hardened zones (Roland, Retraint, Lu, & Lu, 2006; Waltz, Retraint, Roos, & P, 2009). In most cases material failure occur on surfaces such as corrosion, fretting fatigue, fatigue structure and wear (Seto & Matsuda, 2013). These failures remain extremely sensitive to structures (Tao, Zhang, & Lu, 2003) and properties (Micoulaut, Mechkov, Retraint, Viot, & François, 2007) of material surface. To enhance the service lifetime (R et al., 2010) and the global behavior (R et al., 2010) of material optimization of the surface properties and the micro structures SMAT is an effective approach to apply. With an extensive and intensive investigations and research on nanostructured materials in the past, more practical evidences via experiments have proofed this emerging class of materials possess a number of properties whose performances are basically from the old conventional course grained materials (Caballero et al., 2014; X. C. Zhang, Lu, & Shi, 2011). This new set of materials are hard and strong(Sourmail et al., 2013; Tong et al., 2007; Tong, Tao, Wang, Lu, & Lu, 2003) with enhanced physical properties(Capdevila, 2013; Miller, Parish, & Li, 2013), extraordinary tribological properties(Capdevila, 2013; Ma, 2003) and super plastic at low temperature(Branagan et al., 2013; Misra et al., 2012). 1.1. Surface mechanical attrition treatment process To realize a self-Nano-crystallization of a bulk material as the key point (Zing, Lu, & Lu, 2001) a large amount of interfaces and defects are introduced into the surface layers so that the surface micro-structure is changed into a Nano sized crystallites. In another language used by Liu, (G Liu, Lu, & Lu, 2000; G. LIU, WANG, LOU, LU, & LU, 2001), a process of grain refinement into the Nano scale is required in the surface layer as the structure of the coarse grained matrix remains unaltered. It clearly comes out that surface mechanical attrition treatment is the most effective technique in relation to conventional ones to realize self-nano-crystalization on the surface of metallic materials (Lin et al., 2006; Roland, Retraint, Lu, & Lu, 2007; Tong et al., 2007; Tong et al., 2003). Figure 1 below illustrates an experiment of the SMAT process set up. Spherical steel balls possessing smooth surface properties are put in a reflecting chamber which is vibrated by a vibrational generator. The size of the balls is in between1-10 mm in diameter. Figure 1: (a and b) Schematic illustration of the surface mechanical attrition treatment set-up and the repeated multidirectional plastic deformation in the sample surface layer induced by impact of the flying balls(Lin et al., 2006). The chamber has a vibrational frequency of the range 50 Hz TO 20 KHz. Once the balls are resonated in the chamber, the surface to be treated is affected by the large number of balls within a short duration of time. The flying balls move at a velocity of between 1- 20m/s. This velocity depends on the ball size, the distance between the sample surfaces of the ball and the vibrational frequency. (a) (b) Figure 2: Schematic illustration of microstructure characteristics and distributions of strain and strain rate along depth in the surface layer subjected to the SMAT. Based on the velocity of the balls and the measured depth of the pit caused by an individual impact, the strain rate at the sample surface was estimated to be as much as 102–3 s−1(Y. S. Zhang, Han, Wang, & Lu, 2006). Due to the random flying direction of the typical balls within the vibrational chamber, the impact on the sample surface by the directions of the flying balls is random. Each of the impact direction will induce plastic deformation of a very high strain rate on the surface layer of the sample as shown in figure 2b. As a result, the repeated multi directional impact produced at very high rates of strain to the sample surface leads to severe plastic deformation and progressive grain refinement of the entire sample surface down the nanometer regime. 1.2. Reduced Material Thickness Due to SMAT Most SMAT experiments on a number of materials of either alloy or pyre metals, like steel in our case, have shown that nanostructured surface layers can be up to 50µm in thicknesses which may vary from a few nanometers on the top treated surface layer up to about 100nm. Underneath the surface layer, there is the structured layer refined up to about 100µm in thickness. This underneath refined structured layer consists of cells or sub-micrometer sized crystallite separated by either sub boundaries or boundaries (Rivera-Díaz-del-Castillo, Hayashi, & Galindo-Nava, 2013; Zhu, Vassel, Brisset, Lu, & Lu, 2004). Figure 3 represents a detailed micro structural characterization of the SMAT low carbon steel sample. (a) (b) Figure 3:.(a) A typical plane view bright-field TEM image of the top surface layer of the SMAT low carbon steel and (b) Variations of the average grain size and micro strain with the depth from the treated surface determined by means of XRD analysis(Rivera-Díaz-del-Castillo et al., 2013). The deeper layers have deformed coarse grains characterized with various kinds dislocation configuration that include; dislocation cells, dislocation tangles and dislocation walls (Dillien, Seefeldt, Allain, Bouaziz, & Houtte, 2010). For a clear cross sectional view of the SMAT sample, the scheme is shown in figure 2. The refinement layer and the thickness of the nanostructured surface layer of the sample material depend on the material being treated and the parameters involved in processing (Mardiguian, 2010). These processing parameters include the ball size (Dillien et al., 2010), temperature (B & C, 2010) and vibrational frequency (Humbert, Blaineau, Germain, & Gey, 2005; Pantleon, 2008). 2. Deformation of AIS 304 stainless steel through dislocation slip 2.0. Mechanical twinning in plastic deformed austenite AISI 304 stainless steel is an engineering material that is widely used with los stacking fault energies (SFE) of about 17 mJ/m2 and an f.c.c austenite structure. Because it has a lower slip system, its plastic deformation is widely dominated by deformation twinning (Huang, Yang, & Sun, 2007; H. W. Zhang, Hei, Liu, Lu, & Lu, 2003). 2.1. Grain refinement process of steel 2.1.1. Formation This is the formation of the planar dislocation array together with the mechanical twins. It involves slipping of the planes dislocations that are induced by the strain in the austenite phases (Takahashi, Kobayashi, Ueda, Miyazaki, & Kawakami, 2013). This forms regular planar dislocations unlike in other metals and alloys where dislocation cells forms. The reason why dislocation in 304 a steel to arrange themselves in planar arrays on their primary slips planes is because there is a large separation in its partial dislocation (H. W. Zhang et al., 2003). The low stacking fault energies of 304 steel males it difficult for the dislocation to cross slip (Humbert et al., 2005; Pantleon, 2008). 2.1.2. Grain subdivision This is affected by the mechanical twin and the martensitic transformations. Twin boundaries that subdivide the deformed grains are introduced by the formation of the mechanical twins. Two sets of mechanical twins which are accommodated by the plastic deformations are activated hence making the interactions more natural. This produces rhombic blocks (H. W. Zhang et al., 2003). Formation of martensite phases on the top surface layer is due to the interactions of the twins (Huang, Wang, & Yong, 2013; Rolanda, Retraint, Lub, & Luc, 2007). 2.1.3. Formation of nanocrystallites This takes place in the top surface layer due to the high strain and strain rate. These strain properties activate a high density mechanical twins which accommodate the extreme deformations (H. W. Zhang et al., 2003). The twin thickness that is induced is reduced to the nanometer scale thick hence leading to further subdivision of the original grains into nano-scale blocks. This offers the special desired shape. 2.2. Low stacking fault of stainless steel Mechanical twining is favored by materials with low SFE. This is effective at low temperature (G. LIU et al., 2001). This explains why dislocation slip and mechanical twinning dominate the deformation process in steel (H. W. Zhang et al., 2003). Figure 4: A schematic illustration of grain refinement in the materials with plastic deformation accommodated by mechanical twins and dislocations during SMAT(H. W. Zhang et al., 2003). In step 1; high density parallel twins forms in a single direction introducing a large amount of twin boundaries which subdivide the initial coarse grains into matrix alternate or lamellae matrix blocks(Humbert et al., 2005; Pantleon, 2008). In step 2; dislocation in the thin matrix lamellae stack into dislocation walls in order to minimize the strain energy. In step 3; the increasing strain induces evolution of sub grain boundaries from the dislocation walls. This subdivides the matrix alternate or lamellae twin blocks into an equiaxed nanometer sized blocks characterized with mis-orientations. In stainless steel, the twin-twin intersections divides the original coarse grains into rhombic blocks with bordered by large angle boundaries and changed orientations as illustrated in step 2b and 3. Step 4 represents the formation of nanocrystallites whose random orientation requires substantial variation of disorientations in the regular nanometer-sized blocks and the adjacent sub grains in the blocks. Grain rotation and grain boundary sliding may take place due to the mechanisms that are responsible for the random orientation (Seto & Matsuda, 2013). Formation of the random oriented nanocrystallites (Seto & Matsuda, 2013) is as a result of reduction of the grain size. It also affects the grain boundary and grain rotation (Huang, Wang, & Yong, 2013; Huang, Wang, Yong, & Enhancing torsion fatigue behaviour of a martensitic stainless steel by generating gradient nanograined layer via surface mechanical grinding treatment. Materials Science and Technology, 2013; Rolanda et al., 2007) 3.0. Properties and performance 3.1. Hardness Measurement of mechanical performance of SMAT materials samples indicates an increase in tensile strength and hardness (Humbert et al., 2005; Pantleon, 2008). Figure 4 below illustrates uniaxial tensile stress strain curve of 316L steel. It indicates its course grained untreated sample and nanostructured SMAT treated sample surface layer. It is clear from the graph that nanostructured surface samples of stainless steel are highly stronger than the course grained one (CHEN, LU, LU, & LU, 2005). Figure 5:Measured hardness values as a function of depth from the treated surface for the SMAT and subsequent annealing (at 923K for 120 min) Fe samples(CHEN et al., 2005). Nanostructured steel has yield strength of 0.2% offset f as high as 1450MPa. This is 6 times more that the coarse grained sample when compared. It also has a tensile strength of 15550MPa. Nanostructured sample has a decreased plasticity to the elongation failure of up to 3.4%. The exponential (n) of the strain hardening of the SMAT sample is derived by fitting the equation, to the plastic deformation stages of the curve rightfully beyond the yield point. The n value of the coarse grained sample about 0.385 but for the nanostructured sample it is 0.072 which is much smaller. Due to the low efficiency of dislocation storage inside the tiny grains, nanostructured sample lose work hardening during the deformation process. This weak strain hardening indicates that before fracture, accumulation of lattice dislocation occurred during plastic straining. 3.2. Tensile Strength Tensile strength on the SMAT 316 stainless steel was conducted on a 10um nanostructured layer of 1mm in thickness. The SMAT sample shows a very high tensile strength (Capdevila, 2013; Miller et al., 2013). The yield tensile strength is as high as up to 550MPa. The SMAT sample has a 90%increment in yield strength compared to the coarse grained sample of 280MPa. This is because the strain induced nanostructured layer improves the rigidity and strength of the surface layers such that the propagation of initiated defects and cracks are prevented. In addition to the above reason, the top surface layer with high strength and rigidity do block the free movement of dislocations during the plastic deformations (Zing et al., 2001). This prevents formation of slip bands on the sample materials. Figure 6 below shows a tensile strain curve for a SMAT 316 stainless steel sample in comparison with the untreated original sample with same geometry. Figure 6: Tensile test stress-strain curves for the SMAT 316L stainless steel sheet sample (1.5 mm thick) with nanostructured surface layers (of about 15 m thick) on both sides, and for the untreated sample (base material)(Zing et al., 2001). 3.3. Fatigue Properties Tests to ascertain fatigue properties of SMAT samples exhibited a considerable increment of fatigue strength than the untreated sample (Roland et al., 2006). SMAT process on steel of 3mm diameter improved endurance to fatigue by 21%. This increment is obvious for high and low amplitude stress regimes of the SMAT samples. This is because of the high ductility and high yield strength induced by SMAT. Fatigue ignition and propagation is resisted by the surface layer because of the grain refinement of the micro structure and the formation of the greater impressive residual stress. Consequential annealing of the SMAT stainless steel mostly increases the fatigue strength by 5 % (Huang, Wang, & Yong, 2013; Roland et al., 2007). 3.4. Wear and Friction Experiments have shown that wear and friction properties of SMAT treated steel is impressively better that the untreated sample (Capdevila, 2013; Miller et al., 2013). Under normal room temperature the wear and unlubricated frictional properties of SMAT treated carbon steel of a considerable variation in gradient from nanometer to micrometer grain size were investigated (Y. M. Wang et al., 2003). Results indicated the ability to bear loads by the SMAT treated low carbon steel is improves as compared to the untreated sample. As shown in figure 7, wear volume loss of the nanostructured surface layer in the SMAT sample is lower than that of the untreated original sample. Figure 7: Variations of the wear volume loss with load (a) and variations of the coefficient of friction with load (b) for the SMAT and the original low-carbon steel samples(Ma, 2003). The loss of wear vole and frictional coefficients are smaller in the treated sample than the coarse grained sample. The improved wear and friction properties were attributed to the hardened surface layer which reduces the micro cutting and plowing because of surface fatigue fracture degree of plastic removal and under lower load (Seto & Matsuda, 2013). 3.5. Diffusion of materials Nonostructured materials have an enhanced atomic diffusivity compared to conventional polycrystalline materials. This is because of the grain boundaries(GB) that act as fast diffusion channels. This mechanical behavior can be used in upgrading of the traditional technique of surface chemical treatment. This technique includes nitriding process of steel by promoting the diffusion kinetics as well as reduction f the diffusion temperature. Diffusion of materials like chromium and nitrogen can be measured using EDS to determine its depth as shown in figure 8 (K. WANG, TAO, LIU, LU, & LU, 2006). Figure 8: Measured Cr-diffusion depth in various samples (the SMAT sample (▲), the coarse-grained sample (▼), pre-annealed sample A (○) and B (●)) after chromizing at various temperatures(Ma, 2003). From the figure cr-diffusion depth into the SMAT surface at a temperature of 673 is 5.0+/-1.5 um, compared to the course grained at about 1133. This is due to the thicker cromized layer that is formed in a SMAT sample different from the preannealed and course grained sample. 3.5.1. Diffusion mechanisms From a number of studies there are much enhanced diffusivities of cr in the SMAT low carbon steel relative to the corresponding coarse grained samples. Non-equilibrium grain boundary posses a higher free Gibbs energy than the conventional ground boundaries. This energy facilitates diffusion along the grain boundaries by decreasing the formation of defect energy(Tong et al., 2003). In addition to the energy, the existence of enough triple junctions in equilibrium state in the nanostructured surface layer contributes too much enhanced diffusivity. Whenever a grain size is reduced by 10nm , the measured amount of triple junctions can be as high as few volume percents(Tong WP, 2003). These nanostructures surface posses a large volume as compared to the grain boundaries and this is expected to further facilitate the movement of atoms. Results of positron annihilation spectroscopy measurements have been used to demonstrate the existence of plenty of triple junctions of nanostrucetured surface layers. Whenever temperature is not high, the grain boundary diffusion in is the dominant mechanism. Diffusivity in nanostructured materials (Dn) can be simplified (T. S. Wang, Yang, Li, Jin, & Zheng, 2006): where : Grain Boundry Width (~ 1nm) D: Grain Size Db: Diffusion coefficient alongthe grain boundry Grain Boundry diffusion mechanism states that: Where Vb: frequency the atom requires to jump to new point. : defect formation : migration entropies : Corresponding enthalpies : geometrical constant : jump distance : correlation factor of atomic jumps for GB diffusion Substituting second equation into first one gives: Vacancy concentration can be represented as: Combining the above two equations gives Arrhenius relation, Therefore, 3.6. Corrosion behavior of steel subject to SMAT The change of an open circuit sample (OCP) OF UNTREATED SAMPLE OF STEEL measured as a function is shown in figure 9 below(Balusamy, Kumar, & Narayanan, 2010). Figure 9: OCP–time curves of AISI 409 SS in 0.6 M NaCl – untreated and after SMAT using 2, 5 and 8 mm £ balls for various duration of time(Balusamy et al., 2010). The potentio-dynamic polarization curves of untreated samples are shown in figure 10. The corrosion potential and the corrosion current are compiled in table 1 below. Table 1: Corrosion potential (Ecorr) and corrosion current density (I corr) of untreated AISI 409 SS and samples SMATed using 2, 5 and 8 mm £ 316L SS balls for 15, 30 and 45 min(Balusamy et al., 2010). Compared to the untreated sample SMAT samples exhibit a significant decrease in corrosion. Polarization studies have revealed formation of a passive film for this sample as in figure 11. Figure 10: Potentiodynamic polarization curves of AISI 409 SS in 0.6 M NaCl untreated and after SMAT using (a) 2, (b) 5 and (c) 8 mm £ balls for various duration of time(Balusamy et al., 2010). Capacitive behaviour in figure 11, substantiates this observation in the Nyquist Plot. Grain refinement is important in determining corrosion resistance. Grain refinement has been found to increase corrosion rate of steel. 4. Conclusion By means of SMAT technique, a gradient microstructure is obtained in metallic materials based on the conventional coarse grained matrix. This kind of microstructure gradient where nanostructured surface layers are formed offer an advantage to study the grain refined process whose strain induced size range from micrometer to nanometer. In the initial studies different types of mechanisms for the formation of nanocrystallites and grain refinement approaches have been identified in most SMAT materials. Majority of the SMAT materials present unique differences in their deformation modes and intrinsic nature. These modes involve formation of mechanical twinning or dislocation cells/ dislocation interactions with the preformed mechanical twins. The studies also facilitate investigation on the improvement properties on fatigue and wear resistance, tensile and hardness strength of the SMAT sample surfaces. This offer a unique advantage in the engineering fields. 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Generation of nanostructure by SMAT (Surface Mechanical Attrition Treatment) basic concept, different processes and applications Paper presented at the Proceedings of Asian Pacific Conference on Fracture and Strength ’01 and International Conference on Advanced Technology in Experimental Mechanics, Japan Sendai. Read More