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Effect of gradient microstructures on strengthening and toughening of AZ31
Materials Science & Engineering A xxx (xxxx) xxx
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Effect of gradient microstructures on strengthening and toughening of AZ31 Maryam Jamalian *, David P. Field School of Mechanical and Materials Engineering, Washington State University, Pullman, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Gradient microstructures Severe shot peening Magnesium Deformation Rule of mixtures
A designed gradient microstructure has the potential to enhance mechanical performance of materials. Severe shot peening was used to develop gradient microstructure in twin-roll cast AZ31. The resulting microstructure has four layers including recrystallized ultra-fine grains, a transient layer, deformed coarse grains containing a high fraction of tension twins, and the as-received structure in the metal interior (coarse grains). Each layer has a distinct feature to control yield/ultimate strength and ductility. Hence, various combinations of them lead to a unique performance. This study establishes a relationship between the microstructure, the hardening mecha nism, and the tensile behavior of the gradient structure. A method was established to estimate the yield strength of the designed structure based on each layer hardness via a modified rule of mixtures. Tensile and micro hardness measurements along with microstructures quantified by electron backscatter diffraction and optical microscopy were utilized to identify strengthening and toughening mechanisms. The optimized structure had a remarkable enhancement in both yield and ultimate tensile strength.
1. Introduction The demand for lightweight metals with high strength and accept able ductility in structural applications is increasing [1–3]. Performance enhancement and cost reduction can be obtained via gradient micro structures [4]. Introducing gradient structures from nano/ultra-fine grains at the surface to micron grain sizes in the center of the metal improves important properties such as fatigue life, strength, and ductility [5,6]. To produce gradient materials, various techniques have been employed such as wire brushing [7], high pressure torsion [8] surface mechanical attrition treatment [9], surface mechanical grinding treatment [10], severe impact loading [11], and various shot peening procedures [12,13]. When it comes to dimensional and geometrical limitations, efficiency, cost, and industrial adaptability, shot peening is a reasonable selection compare to other surface treatments. In this pro cedure, collision forces are applied by spherical shots leading to 1) se vere plastic deformation near surface layers, 2) crystal lattice defects due to shear deformation, 3) an increase in the number of these defects, especially dislocations and twins that are the origin of grain refinement, and 4) twin lamellae which subdivides grains and rotates the crystallite lattice orientations [13–15]. Conventional shot peening with intensified parameters is called se vere shot peening (SSP) and can be used in a controlled manner to optimize the depth of driving force [16]. In this method, gradients of
strain-hardening, compressive residual stresses, and grain size are created from the surface due to large strain and high strain-rates imposed by the SSP procedure [12,17,18]. Gradients in grain size are apparent in the EBSD maps. In this paper, we defined ultra-fine, fine, and coarse grains, to grains with sizes smaller than 4 μm, between 4 μm and 10 μm, and larger than 10 μm, respectively. As a result of this process, three distinct layers were created, 1) defect-free ultra-fine grains with low angle grain boundaries, 2) transient layer containing twins, ultra-fine grains, and deformed coarse grains, and 3) deformed coarse grains containing a high fraction of tension twins [19] as shown in Fig. 1. Previous works demonstrated that refinement of surface grains, gradual changes of deformation twin density and the incompatibility of plastic deformation in different layers, lead to an extra strain hardening region and higher strength for various materials [18,20–24]. A few of these works established the relationship between severe shot peening parameters and initial texture of the microstructure and thickness of the formed layers (UFG-transient and deformed layer) and consequently the tensile behavior [12,13,16,19,25–28]. However, the effect of each layers and different combinations of them on optimizing tensile behavior re mains uninvestigated, especially, the role of the middle coarse grains in controlling failure. Toughening mechanism is considered as final elon gation enhancement and larger hardening region. To this end, for manufacturing a sandwich structure with a different combination of layers, samples with different thickness exposed to the constant SSP
* Corresponding author. E-mail address: [email protected] (M. Jamalian). https://doi.org/10.1016/j.msea.2019.138615 Received 31 August 2019; Received in revised form 24 October 2019; Accepted 30 October 2019 Available online 5 November 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Maryam Jamalian, David P. Field, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2019.138615
M. Jamalian and D.P. Field
Materials Science & Engineering A xxx (xxxx) xxx
sectioned then mechanically ground from 320 to 1200 grade followed by 0.05 μm colloidal silica on a low nap cloth and electropolished in 3:5 phosphoric acid and ethanol solution for 120 s at 1.2 V. Optical micro graphs were taken after etching the sample in a solution of 1:1:7 deionized water, acetic acid, and picric acid for 3–5 s. 3. Results 3.1. Microstructure The graphical maps obtained from EBSD data revealed layers in the microstructure that were created by severe shot peening (Fig. 2). Different types of boundaries such as low-angle grain boundaries (LAGBs) with a misorientation less than about 15∘, high-angle grain boundaries (HAGBs) with a misorientation greater than about 15∘, boundaries with misorientation angles lower than 5∘ are called subgrain boundaries, and twins are those with misorientation about 90∘ about f1210g [30]. In this image, tension twins were presented by black lines. The other colors were used to define boundaries for different mis orientations. Since the red boundaries represent very low mis orientations in the range from 2∘ to 5∘, they can be interpreted as dislocation structures. In the first layer from the surface, dynamic recrystallization occurred due to the large strain/high strain rate and following recovery at low temperature. Thus, near 90% of grains are HAGBs with equiaxed recrystallized ultra-fine grains. Tension twins formed at the lower state of strain and these rotated grains near 90∘ lead to texture weakening. The density of dislocations or subgrain boundaries increased abruptly in the transient layer as a result of incomplete recrystallization and reached to the near 50% of boundaries as shown in Fig. 2-b. A maximum density of twins observed in the deformed coarse grains. Twins appearing in the transient layer increase gradually, reaching a maximum density (0.2% boundaries) and decrease gradually moving towards the center. The main difference between the transient and deformed coarse grain layers was the absence of fine grains and dislo cation cells in the coarse grain layer. Also, in deformed coarse grain layer, the size of grains remained large enough to allow dislocation motion and resulted in desirable elongation before rupture. The abbre viation of each layer is expressed in Table 1 as a reference for the rest of the text. Fig. 3 confirmed the existence of each layer according to the designed structure illustrated in Fig. 4. Microstructure and thickness of each affected layer was controlled by SSP parameters. Thus, these layers were similar to each other for various thicknesses because of fixed SSP parameters. By increasing thickness from 3 mm to 4 mm, the volume fraction of the coarse grain layer doubled with identical gradient layers. Fig. 4 schematically illustrates the existence of each layer according to the samples’ initial thickness and the depth of each gradient layer. Hence, samples with various thickness have a unique combination of layers. According to optical micrographs of treated samples as shown in Fig. 3 UFG, transient, and DCG layers have approximately 0.2, 0.3, and 0.5 mm thicknesses using the processing conditions described above. This means that a sample with an initial thickness of 1 mm only has UFG and transition layers. On the other hand, samples, thicker than 2 mm, contain all 4 layers and afterward, only the volume fraction of CG (layer 4) is a variable which is a function of sample thickness.
Fig. 1. Created layers as a result of severe shot peening.
conditions. The microstructure in each layer remained the same due to fixed SSP parameters. In order to elucidate the relationship between mechanical properties and deformation mechanisms for varying combinations of layers, tensile and microhardness testing along with microstructure characterization. Microstructure was obtained by electron backscatter diffraction (EBSD) and optical microscopy (OM) were conducted on treated samples by fixed SSP parameters with initial thickness ranging from 1 mm to 4 mm. Using the experimental observations, computational analysis was per formed to establish a relation between the mechanical properties of each layer and the global behavior of the sandwich structure. 2. Experimental Specimens were cut via water jet from as-received twin-roll cast (TRC) AZ31 (Mg-3wt%Al-1wt%Zn) sheet according to ASTM standard E2448. The samples were sub-sized tensile specimens with an overall length of 75 mm, the grip width of 25 mm, the fillet radius of 1 mm, and gage section of 25 mm length with 6.2 mm width. The existence of each layer and volume fraction of the coarse grain layer were controlled by the initial sample thickness. Hence, preprocessing was performed to prepare samples with various thickness from 1 to 4 mm. Although the Almen intensity and coverage are well-known parameters for shot peening-based surface treatments. Various SSP parameters would lead to same combination of Almen intensity and coverage. However, SSP parameters have important role in controlling microstructures and me chanical properties [19,29]. These two parameters are a function of pressure, size/shape of shots, exposure duration, etc. Shot mass intensity (SMI) is a surrogate measure for the processing time since it is directly proportional to the peening time. SMI is defined as the total amount of shot hitting the surface with units of grams per square millimeter (g/mm2 ). Shot peening procedures were performed with spherical stainless-steel shot with diameters 3.18 mm (1/8 in) at air pressures of 0.22 MPa and SMI 1.43 g=mm2 , following by 1 h annealing at 150∘C temperature, which results in the optimum tensile performance on TRC-AZ31 [19]. Meanwhile, post annealing was added to recover grain boundaries and to enhance ductility since it is a cold working process that was performed at room temperature. The angle and the distance between the nozzle and the specimens were fixed at about 90∘ and 50 mm, respectively. Mechanical properties were quantified at room temperature by tensile testing performed by UTM-HYD Instron at 10 3 strain rates. The 0.02 yield strength was measured from each stress-strain curve. Vickers hardness was measured along the depth using a micro-hardness tester model MVK-G3 at a maximum load of 0.98 N (100 gf) on the cross-sectional samples. The distance between any two adjacent indents was at least 150 μm to eliminate any possibility of overlapping effects. To enable microstructural analysis, SSP specimens were cross
3.2. Mechanical properties Local effects of gradient structures were evaluated by Vickers hard ness testing. The microhardness variation measured along with the depth of treated samples on a cross-sectioned surface. Hardness was improved due to gradual increase of boundaries such as low/high angle grains and twins in the treated regions. As shown in Fig. 5, the measured Vickers hardness determined gradient change of micro-structure 2
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Materials Science & Engineering A xxx (xxxx) xxx
Fig. 2. EBSD data of gradient structure was produced via severe shot peening: a) image quality map, b) grain boundary map, c) kernel average misorientation, d) orientation map (color-coded inverse pole figure). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
through the thickness for different combinations of layers. The effect of severe shot peening can be seen at about 1 mm depth from the surface wherein the hardness increases from 50 HV in the coarse grains layer to about 105 HV at the UFG layer in the top treated surface. Engineering stress-strain curves for various combinations of layers shown in Fig. 6. Each of these layers has a distinct impact on strength ening and toughening. UFG, high fraction of twins, dislocation density in UFG, and the transient layer control yield/ultimate strength by inhib iting further dislocation motion. Even though the strength was decreased by adding deformed and undeformed coarse grains, their ductility was improved significantly. Since cross-sectional area has a significant effect on elongation, a slimness ratio, K, is defined as the ratio of the initial length and the square root of the cross-sectional area. Experimental data showed that elongation increases with decreasing slimness ratio (i.e. increasing cross-sectional area) [31]. Elongation for various thicknesses of gradient structure and as-received material are illustrated in Fig. 7. As shown in Fig. 7, another toughening mechanism occurred in addition to the role of the slimness factor. Adding an extra layer of coarse grains modifies ductility at approximately the same strength while it is not significant compared with adding the first coarse grain layer. This can be related to the role of interfaces between each layer. Combination of UFG and transient layers improved yield and ul timate strength from 100 MPa and 215 MPa to 200 MPa and 280 MPa, respectively. Adding deformed coarse grains resulted in ultimate strength reduction from 280 MPa to 270 MPa while ductility was enhanced by about 15%. Coarse grain layers led to another 17% ductility enhancement and 10 MPa strength reduction. Doubling the coarse grains layer did not cause a noticeable strength decrease and only enhanced ductility by less than 5%. The summary of tensile properties was expressed in Table 2.
Table 1 Layer definitions. layers combination
abbreviation
Ultra-fine grains Transient (Ultra-fine grains, dislocations, twins) Deformed coarse grains (twins) Coarse grains
UFG T DCG CG
Fig. 3. Optical micrograph of cross-sectioned after treatment by 3.18 mm shot size at a pressure of 0.22 MPa with shot mass intensity 1.43 g/mm2 : a) 1 mm thickness (UFG þ T), b) 2 mm thickness (UFG þ T þ DCG), c) 3 mm thickness (UFG þ T þ DCG þ CG). 3
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Fig. 4. Schematic combination of layers for a) 1 mm thickness layer (UFG þ T), b) 2 mm thickness layer (UFG þ T þ DCG), c) 3 mm thickness layer (UFG þ T þ DCG þ CG), d) 4 mm thickness layer (UFG þ T þ DCG þ CG þ CG).
Fig. 6. Tensile behavior with various combination of different layers after treatment by 3.18 mm shot size at a pressure of 0.22 MPa with shot mass in tensity 1.43 g/.mm2
Fig. 5. Micro-hardness-depth profiles after treatment by 3.18 mm shot size at a pressure of 0.22 MPa with shot mass intensity 1.43 g/mm2 for various combi nations of different layers.
3.3. Fracture Fracture surface morphologies were investigated to observe alter ation of fracture appearance by the existence of each layer. As illustrated in Fig. 8, pure shear fracture at 45∘ to the tensile axis converted to cup and cone fracture by adding deformed and undeformed coarse grains. In the two first layers, cracks propagated by shear at 45∘ to the tensile axis while in the coarse grains crack growth was near 90∘ with respect to the applied load. In all combinations, little evidence of necking was observed that can be related to the nature of the HCP structure. Secondary electron (SE) images perpendicular to the fracture surface provided more detail about failure procedures (Fig. 9). As shown, there was a smooth surface in UFG regions that looked like brittle fracture. By moving from the edge, dimples started to appear and the size of them increased gradually as the coarse grain regions experienced fracture. 4. Discussion Superior combination of strength and ductility have been achieved as a consequence of gradients in statistically stored dislocations (SSD) and geometrically necessary dislocations (GND) which were generated as a
Fig. 7. Relationship between slimness ratio and elongation for gradient struc ture and as-received structure. 4
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combination of gradient layers. For instance, a combination of two volumes of coarse grain layers with 2 mm thickness specimens led to 4 mm thickness sample. With respect to these combinations, the esti mated overall yield strength was represented in Table 3. Rules of mixture do not generally consider the mechanical in compatibility between the layers. As expected, this method cannot predict overall yield strength with high accuracy. While as shown here and in previous works, 3D stress states in tension is generated as a result of compressive residual stresses that are created via cold work surface treatment [28,32]. 3D stress is generated since applied stress first ex ceeds the yield strength of the softer region while the harder area is still deformed elastically [33]. Consequently, compressive stress is gener ated to keep coherency. In the other word, this 3D stress states are necessary to keep a consistent stress between layers. Initial stress state and the stress state evolution are the main reasons of extraordinary strengthening and ductility enhancement. Hence, the effect of them should be considered in the calculation. Moering [33] showed that yield stress is proportional to the hardness in each layer of gradient structure as illustrated in Eq. (2).
Table 2 Summary of tensile properties for various layer combinations. layer combination
yield strength
ultimate strength
ductility
toughness
σy (MPa)
σu (MPa)
εf (%)
K (MPa m0:5 )
CG UFG þ T UFG þ T þ DCG
102.26 200.4 167.78
215.32 277.25 271.48
– – 15%
22.83 29.65 32.52
UFG þ T þ DCG þ CG
153.79
225.96
17%
36.46
UFG þ T þ DCG þ CG þ CG
143.16
256.85
5%
37.37
result of the SSP procedure [19,21]. Both types of dislocations are generated during plastic deformation. SSDs are generally considered to be mobile dislocations and these contribute the maximum to deforma tion in the material. Unfortunately, it is difficult to measure these properly. GNDs are formed due to geometrical constraints of lattices which result in internal plastic strain gradients. These are generally considered to be immobile, and are trapped in the lattice, causing lattice curvature and grain fragmentation. As shown in the KAM map of Fig. 2-c, GNDs are detectable by measuring misorientation from adja cent points. This surface treatment created gradient microstructure with distinct layers such that each of them has unique features and properties. The failure characteristic is dependent on the features and the interac tion among these layers. In order to calculate the effect of gradient layers on the overall mechanical response, in the first step, the rule of mixtures was applied by using Eq. (1) as follows: X σ y ¼ νi σyi (1)
(2)
σ y ¼ KH ν
Table 3 Calculated by applying Eq. (1) to obtain yield strength from tensile testing. layer combination
CG UFG þ T þ DCG UFG þ T þ DCG þ CG UFG þ T þ DCG þ CG þ CG
where νi is the volume fraction of each gradient layer and σ yi is their corresponding yield strength. As shown in the results section of this paper, changing the initial thickness of the plates led to various
sample thickness
experimental (MPa)
calculated
error
(mm)
σye (MPa)
σyc (MPa)
eσyc (%)
1 2 3
102.26 167.78 153.79
– – 145.94
– – 5.10
4
143.16
135.02
5.95
Fig. 8. Optical graph of fracture: a) 1 mm thickness (UFG þ T), b) 2 mm thickness (UFG þ T þ DCG), c) 3 mm thickness (UFG þ T þ DCG þ CG), d) 4 mm thickness (UFG þ T þ DCG þ CG þ CG), e) 4 mm thickness-normal to SSP surface.
Fig. 9. SE image of 3 mm thickness (UFG þ T þ DCG þ CG) half of thickness. 5
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Table 4 Calculated by applying Eq. (3) to obtain yield strength from tensile testing.
where K is a constant, σy is yield strength, H is the micro-hardness and ν is the volume fraction of each hardened layer. Thus, the proportion of yield strength of gradient layer to initial structure can be calculated as: P P σygl K νi Hi νi Hi ¼ ¼ (3) σ yc g KHcg Hcg
layer combination
CG UFG þ T UFG þ T þ DCG UFG þ T þ DCG þ CG UFG þ T þ DCG þ CG þ CG
Eq. (3) was applied to Vickers hardness data that measured along the thickness (Fig. 5). The result is shown in Table 4. It this method, distinct layers were not considered but the hardness at each indentation was to determine theoretical yield strength. The distance between each indent is used as volume fraction. Thus, there were more intermediate areas with potential stress incompatibilities between them that have not been considered. Thus, the error increased compared to the previous calculation for which we have considered only two layers. To overcome this problem, a new term as a function of hardness in two adjacent layers was added to Eq. (3). This term included transverse stresses and interactions between layers as expressed in Eq. (4). The additional part was added to estimate the effect of stress incompatibility by considering the differential between them. Since the inequality be tween the layers always results in a positive effect on strengthening, the square of the difference was calculated. Furthermore, to normalize the values to the measured hardness, the square of the hardness difference is divided by the average of two adjacent measured hardness values. The form of the equation was selected to best fit the data, thus Eq. (4) is a phenomenological adjustment to the rule of mixtures. P σygl νi Hi X ðHi Hi 1 Þ2 � � ¼ þ (4) σ yc g Hcg Hi þHi 1
sample thickness
experimental (MPa)
calculated
error
(mm)
σye (MPa)
σyH (MPa)
eσyH (%)
1 1 2 3
102.26 200.93 167.78 153.79
– 177.91 147.44 134.14
– 11.45 12.12 12.77
4
143.16
125.88
12.31
Table 5 Calculated by applying Eq. (4) to obtain yield strength from tensile testing. layer combination
CG UFG þ T UFG þ T þ DCG UFG þ T þ DCG þ CG UFG þ T þ DCG þ CG þ CG
sample thickness
experimental (MPa)
calculated
error
(mm)
σye (MPa)
σyMH (MPa)
eσyH (%)
1 1 2 3
102.26 200.93 167.78 153.79
– 199.77 167.05 155.00
– 0.57 0.43 0.79
4
143.16
144.93
0.95
and it can be estimated by using the rule of mixtures. The obtained yield strength from each layer and hardness distribution provided us with near 5% and 12% error, respectively. However, considering in compatibility between the layers and modifying the rule of mixtures decreased the estimation error to less than 1%.
2
Adding the second term to enrich Eq. (3) can cover the strengthening as a result of mechanical incompatibility between early-plastic and stable elastic regions. As seen by the results in Table 5, this simple modification results in estimates of the total yield strength with less than 1% error. For justifying the toughening mechanism, we should note that the coarse grains layer was capable of plastically deforming due to the possibility of dislocation motion. Moreover, coarse grains have a higher tolerance for crack growth. At the beginning of the deformation, coarse grains started plastic deformation first while the other areas were in the elastic regime. Strain partitioning and stress transfer between layers and the consequential back-stress strain-hardening were attributed to ductility enchantment. Therefore, adding deformed and undeformed coarse grain layers can improve ductility near 15%. However, adding another layer of CG was not as effective as the first one since it did not add an extra interface layer. However, it increased the area that cracks grew perpendicular to the applied load. As this coarse grain region in creases further, the tensile and fracture performance will approach that of the TRC alloy. Fatigue strength will likely be enhanced due to surface compression, but measurement of fatigue performance is beyond the scope of the present study.
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5. Conclusions Due to the SSP process, the gradient layers of UFG, transient, deformed, and undeformed grains were created in the as-received bimodal structure. The first two layers from the surface contained a high fraction of low and high angle grain boundaries and dislocation pile-ups. Hence, they contributed mostly to strength enhancement. Whereas deformed and undeformed micro grains layers impacted the failure process and increased the ductility near 30%. These two interior layers can convert pure shear fracture to semi ductile cup and cone fracture. Meanwhile, requirements for strain compatibility between the layers resulted in a higher value of strengthening and toughening. The yield strength was obtained from the created gradient structure 6
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Effect of gradient microstructures on strengthening and toughening of AZ31 Maryam Jamalian *, David P. Field School of Mechanical and Materials Engineering, Washington State University, Pullman, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Gradient microstructures Severe shot peening Magnesium Deformation Rule of mixtures
A designed gradient microstructure has the potential to enhance mechanical performance of materials. Severe shot peening was used to develop gradient microstructure in twin-roll cast AZ31. The resulting microstructure has four layers including recrystallized ultra-fine grains, a transient layer, deformed coarse grains containing a high fraction of tension twins, and the as-received structure in the metal interior (coarse grains). Each layer has a distinct feature to control yield/ultimate strength and ductility. Hence, various combinations of them lead to a unique performance. This study establishes a relationship between the microstructure, the hardening mecha nism, and the tensile behavior of the gradient structure. A method was established to estimate the yield strength of the designed structure based on each layer hardness via a modified rule of mixtures. Tensile and micro hardness measurements along with microstructures quantified by electron backscatter diffraction and optical microscopy were utilized to identify strengthening and toughening mechanisms. The optimized structure had a remarkable enhancement in both yield and ultimate tensile strength.
1. Introduction The demand for lightweight metals with high strength and accept able ductility in structural applications is increasing [1–3]. Performance enhancement and cost reduction can be obtained via gradient micro structures [4]. Introducing gradient structures from nano/ultra-fine grains at the surface to micron grain sizes in the center of the metal improves important properties such as fatigue life, strength, and ductility [5,6]. To produce gradient materials, various techniques have been employed such as wire brushing [7], high pressure torsion [8] surface mechanical attrition treatment [9], surface mechanical grinding treatment [10], severe impact loading [11], and various shot peening procedures [12,13]. When it comes to dimensional and geometrical limitations, efficiency, cost, and industrial adaptability, shot peening is a reasonable selection compare to other surface treatments. In this pro cedure, collision forces are applied by spherical shots leading to 1) se vere plastic deformation near surface layers, 2) crystal lattice defects due to shear deformation, 3) an increase in the number of these defects, especially dislocations and twins that are the origin of grain refinement, and 4) twin lamellae which subdivides grains and rotates the crystallite lattice orientations [13–15]. Conventional shot peening with intensified parameters is called se vere shot peening (SSP) and can be used in a controlled manner to optimize the depth of driving force [16]. In this method, gradients of
strain-hardening, compressive residual stresses, and grain size are created from the surface due to large strain and high strain-rates imposed by the SSP procedure [12,17,18]. Gradients in grain size are apparent in the EBSD maps. In this paper, we defined ultra-fine, fine, and coarse grains, to grains with sizes smaller than 4 μm, between 4 μm and 10 μm, and larger than 10 μm, respectively. As a result of this process, three distinct layers were created, 1) defect-free ultra-fine grains with low angle grain boundaries, 2) transient layer containing twins, ultra-fine grains, and deformed coarse grains, and 3) deformed coarse grains containing a high fraction of tension twins [19] as shown in Fig. 1. Previous works demonstrated that refinement of surface grains, gradual changes of deformation twin density and the incompatibility of plastic deformation in different layers, lead to an extra strain hardening region and higher strength for various materials [18,20–24]. A few of these works established the relationship between severe shot peening parameters and initial texture of the microstructure and thickness of the formed layers (UFG-transient and deformed layer) and consequently the tensile behavior [12,13,16,19,25–28]. However, the effect of each layers and different combinations of them on optimizing tensile behavior re mains uninvestigated, especially, the role of the middle coarse grains in controlling failure. Toughening mechanism is considered as final elon gation enhancement and larger hardening region. To this end, for manufacturing a sandwich structure with a different combination of layers, samples with different thickness exposed to the constant SSP
* Corresponding author. E-mail address: [email protected] (M. Jamalian). https://doi.org/10.1016/j.msea.2019.138615 Received 31 August 2019; Received in revised form 24 October 2019; Accepted 30 October 2019 Available online 5 November 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Maryam Jamalian, David P. Field, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2019.138615
M. Jamalian and D.P. Field
Materials Science & Engineering A xxx (xxxx) xxx
sectioned then mechanically ground from 320 to 1200 grade followed by 0.05 μm colloidal silica on a low nap cloth and electropolished in 3:5 phosphoric acid and ethanol solution for 120 s at 1.2 V. Optical micro graphs were taken after etching the sample in a solution of 1:1:7 deionized water, acetic acid, and picric acid for 3–5 s. 3. Results 3.1. Microstructure The graphical maps obtained from EBSD data revealed layers in the microstructure that were created by severe shot peening (Fig. 2). Different types of boundaries such as low-angle grain boundaries (LAGBs) with a misorientation less than about 15∘, high-angle grain boundaries (HAGBs) with a misorientation greater than about 15∘, boundaries with misorientation angles lower than 5∘ are called subgrain boundaries, and twins are those with misorientation about 90∘ about f1210g [30]. In this image, tension twins were presented by black lines. The other colors were used to define boundaries for different mis orientations. Since the red boundaries represent very low mis orientations in the range from 2∘ to 5∘, they can be interpreted as dislocation structures. In the first layer from the surface, dynamic recrystallization occurred due to the large strain/high strain rate and following recovery at low temperature. Thus, near 90% of grains are HAGBs with equiaxed recrystallized ultra-fine grains. Tension twins formed at the lower state of strain and these rotated grains near 90∘ lead to texture weakening. The density of dislocations or subgrain boundaries increased abruptly in the transient layer as a result of incomplete recrystallization and reached to the near 50% of boundaries as shown in Fig. 2-b. A maximum density of twins observed in the deformed coarse grains. Twins appearing in the transient layer increase gradually, reaching a maximum density (0.2% boundaries) and decrease gradually moving towards the center. The main difference between the transient and deformed coarse grain layers was the absence of fine grains and dislo cation cells in the coarse grain layer. Also, in deformed coarse grain layer, the size of grains remained large enough to allow dislocation motion and resulted in desirable elongation before rupture. The abbre viation of each layer is expressed in Table 1 as a reference for the rest of the text. Fig. 3 confirmed the existence of each layer according to the designed structure illustrated in Fig. 4. Microstructure and thickness of each affected layer was controlled by SSP parameters. Thus, these layers were similar to each other for various thicknesses because of fixed SSP parameters. By increasing thickness from 3 mm to 4 mm, the volume fraction of the coarse grain layer doubled with identical gradient layers. Fig. 4 schematically illustrates the existence of each layer according to the samples’ initial thickness and the depth of each gradient layer. Hence, samples with various thickness have a unique combination of layers. According to optical micrographs of treated samples as shown in Fig. 3 UFG, transient, and DCG layers have approximately 0.2, 0.3, and 0.5 mm thicknesses using the processing conditions described above. This means that a sample with an initial thickness of 1 mm only has UFG and transition layers. On the other hand, samples, thicker than 2 mm, contain all 4 layers and afterward, only the volume fraction of CG (layer 4) is a variable which is a function of sample thickness.
Fig. 1. Created layers as a result of severe shot peening.
conditions. The microstructure in each layer remained the same due to fixed SSP parameters. In order to elucidate the relationship between mechanical properties and deformation mechanisms for varying combinations of layers, tensile and microhardness testing along with microstructure characterization. Microstructure was obtained by electron backscatter diffraction (EBSD) and optical microscopy (OM) were conducted on treated samples by fixed SSP parameters with initial thickness ranging from 1 mm to 4 mm. Using the experimental observations, computational analysis was per formed to establish a relation between the mechanical properties of each layer and the global behavior of the sandwich structure. 2. Experimental Specimens were cut via water jet from as-received twin-roll cast (TRC) AZ31 (Mg-3wt%Al-1wt%Zn) sheet according to ASTM standard E2448. The samples were sub-sized tensile specimens with an overall length of 75 mm, the grip width of 25 mm, the fillet radius of 1 mm, and gage section of 25 mm length with 6.2 mm width. The existence of each layer and volume fraction of the coarse grain layer were controlled by the initial sample thickness. Hence, preprocessing was performed to prepare samples with various thickness from 1 to 4 mm. Although the Almen intensity and coverage are well-known parameters for shot peening-based surface treatments. Various SSP parameters would lead to same combination of Almen intensity and coverage. However, SSP parameters have important role in controlling microstructures and me chanical properties [19,29]. These two parameters are a function of pressure, size/shape of shots, exposure duration, etc. Shot mass intensity (SMI) is a surrogate measure for the processing time since it is directly proportional to the peening time. SMI is defined as the total amount of shot hitting the surface with units of grams per square millimeter (g/mm2 ). Shot peening procedures were performed with spherical stainless-steel shot with diameters 3.18 mm (1/8 in) at air pressures of 0.22 MPa and SMI 1.43 g=mm2 , following by 1 h annealing at 150∘C temperature, which results in the optimum tensile performance on TRC-AZ31 [19]. Meanwhile, post annealing was added to recover grain boundaries and to enhance ductility since it is a cold working process that was performed at room temperature. The angle and the distance between the nozzle and the specimens were fixed at about 90∘ and 50 mm, respectively. Mechanical properties were quantified at room temperature by tensile testing performed by UTM-HYD Instron at 10 3 strain rates. The 0.02 yield strength was measured from each stress-strain curve. Vickers hardness was measured along the depth using a micro-hardness tester model MVK-G3 at a maximum load of 0.98 N (100 gf) on the cross-sectional samples. The distance between any two adjacent indents was at least 150 μm to eliminate any possibility of overlapping effects. To enable microstructural analysis, SSP specimens were cross
3.2. Mechanical properties Local effects of gradient structures were evaluated by Vickers hard ness testing. The microhardness variation measured along with the depth of treated samples on a cross-sectioned surface. Hardness was improved due to gradual increase of boundaries such as low/high angle grains and twins in the treated regions. As shown in Fig. 5, the measured Vickers hardness determined gradient change of micro-structure 2
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Fig. 2. EBSD data of gradient structure was produced via severe shot peening: a) image quality map, b) grain boundary map, c) kernel average misorientation, d) orientation map (color-coded inverse pole figure). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
through the thickness for different combinations of layers. The effect of severe shot peening can be seen at about 1 mm depth from the surface wherein the hardness increases from 50 HV in the coarse grains layer to about 105 HV at the UFG layer in the top treated surface. Engineering stress-strain curves for various combinations of layers shown in Fig. 6. Each of these layers has a distinct impact on strength ening and toughening. UFG, high fraction of twins, dislocation density in UFG, and the transient layer control yield/ultimate strength by inhib iting further dislocation motion. Even though the strength was decreased by adding deformed and undeformed coarse grains, their ductility was improved significantly. Since cross-sectional area has a significant effect on elongation, a slimness ratio, K, is defined as the ratio of the initial length and the square root of the cross-sectional area. Experimental data showed that elongation increases with decreasing slimness ratio (i.e. increasing cross-sectional area) [31]. Elongation for various thicknesses of gradient structure and as-received material are illustrated in Fig. 7. As shown in Fig. 7, another toughening mechanism occurred in addition to the role of the slimness factor. Adding an extra layer of coarse grains modifies ductility at approximately the same strength while it is not significant compared with adding the first coarse grain layer. This can be related to the role of interfaces between each layer. Combination of UFG and transient layers improved yield and ul timate strength from 100 MPa and 215 MPa to 200 MPa and 280 MPa, respectively. Adding deformed coarse grains resulted in ultimate strength reduction from 280 MPa to 270 MPa while ductility was enhanced by about 15%. Coarse grain layers led to another 17% ductility enhancement and 10 MPa strength reduction. Doubling the coarse grains layer did not cause a noticeable strength decrease and only enhanced ductility by less than 5%. The summary of tensile properties was expressed in Table 2.
Table 1 Layer definitions. layers combination
abbreviation
Ultra-fine grains Transient (Ultra-fine grains, dislocations, twins) Deformed coarse grains (twins) Coarse grains
UFG T DCG CG
Fig. 3. Optical micrograph of cross-sectioned after treatment by 3.18 mm shot size at a pressure of 0.22 MPa with shot mass intensity 1.43 g/mm2 : a) 1 mm thickness (UFG þ T), b) 2 mm thickness (UFG þ T þ DCG), c) 3 mm thickness (UFG þ T þ DCG þ CG). 3
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Fig. 4. Schematic combination of layers for a) 1 mm thickness layer (UFG þ T), b) 2 mm thickness layer (UFG þ T þ DCG), c) 3 mm thickness layer (UFG þ T þ DCG þ CG), d) 4 mm thickness layer (UFG þ T þ DCG þ CG þ CG).
Fig. 6. Tensile behavior with various combination of different layers after treatment by 3.18 mm shot size at a pressure of 0.22 MPa with shot mass in tensity 1.43 g/.mm2
Fig. 5. Micro-hardness-depth profiles after treatment by 3.18 mm shot size at a pressure of 0.22 MPa with shot mass intensity 1.43 g/mm2 for various combi nations of different layers.
3.3. Fracture Fracture surface morphologies were investigated to observe alter ation of fracture appearance by the existence of each layer. As illustrated in Fig. 8, pure shear fracture at 45∘ to the tensile axis converted to cup and cone fracture by adding deformed and undeformed coarse grains. In the two first layers, cracks propagated by shear at 45∘ to the tensile axis while in the coarse grains crack growth was near 90∘ with respect to the applied load. In all combinations, little evidence of necking was observed that can be related to the nature of the HCP structure. Secondary electron (SE) images perpendicular to the fracture surface provided more detail about failure procedures (Fig. 9). As shown, there was a smooth surface in UFG regions that looked like brittle fracture. By moving from the edge, dimples started to appear and the size of them increased gradually as the coarse grain regions experienced fracture. 4. Discussion Superior combination of strength and ductility have been achieved as a consequence of gradients in statistically stored dislocations (SSD) and geometrically necessary dislocations (GND) which were generated as a
Fig. 7. Relationship between slimness ratio and elongation for gradient struc ture and as-received structure. 4
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combination of gradient layers. For instance, a combination of two volumes of coarse grain layers with 2 mm thickness specimens led to 4 mm thickness sample. With respect to these combinations, the esti mated overall yield strength was represented in Table 3. Rules of mixture do not generally consider the mechanical in compatibility between the layers. As expected, this method cannot predict overall yield strength with high accuracy. While as shown here and in previous works, 3D stress states in tension is generated as a result of compressive residual stresses that are created via cold work surface treatment [28,32]. 3D stress is generated since applied stress first ex ceeds the yield strength of the softer region while the harder area is still deformed elastically [33]. Consequently, compressive stress is gener ated to keep coherency. In the other word, this 3D stress states are necessary to keep a consistent stress between layers. Initial stress state and the stress state evolution are the main reasons of extraordinary strengthening and ductility enhancement. Hence, the effect of them should be considered in the calculation. Moering [33] showed that yield stress is proportional to the hardness in each layer of gradient structure as illustrated in Eq. (2).
Table 2 Summary of tensile properties for various layer combinations. layer combination
yield strength
ultimate strength
ductility
toughness
σy (MPa)
σu (MPa)
εf (%)
K (MPa m0:5 )
CG UFG þ T UFG þ T þ DCG
102.26 200.4 167.78
215.32 277.25 271.48
– – 15%
22.83 29.65 32.52
UFG þ T þ DCG þ CG
153.79
225.96
17%
36.46
UFG þ T þ DCG þ CG þ CG
143.16
256.85
5%
37.37
result of the SSP procedure [19,21]. Both types of dislocations are generated during plastic deformation. SSDs are generally considered to be mobile dislocations and these contribute the maximum to deforma tion in the material. Unfortunately, it is difficult to measure these properly. GNDs are formed due to geometrical constraints of lattices which result in internal plastic strain gradients. These are generally considered to be immobile, and are trapped in the lattice, causing lattice curvature and grain fragmentation. As shown in the KAM map of Fig. 2-c, GNDs are detectable by measuring misorientation from adja cent points. This surface treatment created gradient microstructure with distinct layers such that each of them has unique features and properties. The failure characteristic is dependent on the features and the interac tion among these layers. In order to calculate the effect of gradient layers on the overall mechanical response, in the first step, the rule of mixtures was applied by using Eq. (1) as follows: X σ y ¼ νi σyi (1)
(2)
σ y ¼ KH ν
Table 3 Calculated by applying Eq. (1) to obtain yield strength from tensile testing. layer combination
CG UFG þ T þ DCG UFG þ T þ DCG þ CG UFG þ T þ DCG þ CG þ CG
where νi is the volume fraction of each gradient layer and σ yi is their corresponding yield strength. As shown in the results section of this paper, changing the initial thickness of the plates led to various
sample thickness
experimental (MPa)
calculated
error
(mm)
σye (MPa)
σyc (MPa)
eσyc (%)
1 2 3
102.26 167.78 153.79
– – 145.94
– – 5.10
4
143.16
135.02
5.95
Fig. 8. Optical graph of fracture: a) 1 mm thickness (UFG þ T), b) 2 mm thickness (UFG þ T þ DCG), c) 3 mm thickness (UFG þ T þ DCG þ CG), d) 4 mm thickness (UFG þ T þ DCG þ CG þ CG), e) 4 mm thickness-normal to SSP surface.
Fig. 9. SE image of 3 mm thickness (UFG þ T þ DCG þ CG) half of thickness. 5
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Table 4 Calculated by applying Eq. (3) to obtain yield strength from tensile testing.
where K is a constant, σy is yield strength, H is the micro-hardness and ν is the volume fraction of each hardened layer. Thus, the proportion of yield strength of gradient layer to initial structure can be calculated as: P P σygl K νi Hi νi Hi ¼ ¼ (3) σ yc g KHcg Hcg
layer combination
CG UFG þ T UFG þ T þ DCG UFG þ T þ DCG þ CG UFG þ T þ DCG þ CG þ CG
Eq. (3) was applied to Vickers hardness data that measured along the thickness (Fig. 5). The result is shown in Table 4. It this method, distinct layers were not considered but the hardness at each indentation was to determine theoretical yield strength. The distance between each indent is used as volume fraction. Thus, there were more intermediate areas with potential stress incompatibilities between them that have not been considered. Thus, the error increased compared to the previous calculation for which we have considered only two layers. To overcome this problem, a new term as a function of hardness in two adjacent layers was added to Eq. (3). This term included transverse stresses and interactions between layers as expressed in Eq. (4). The additional part was added to estimate the effect of stress incompatibility by considering the differential between them. Since the inequality be tween the layers always results in a positive effect on strengthening, the square of the difference was calculated. Furthermore, to normalize the values to the measured hardness, the square of the hardness difference is divided by the average of two adjacent measured hardness values. The form of the equation was selected to best fit the data, thus Eq. (4) is a phenomenological adjustment to the rule of mixtures. P σygl νi Hi X ðHi Hi 1 Þ2 � � ¼ þ (4) σ yc g Hcg Hi þHi 1
sample thickness
experimental (MPa)
calculated
error
(mm)
σye (MPa)
σyH (MPa)
eσyH (%)
1 1 2 3
102.26 200.93 167.78 153.79
– 177.91 147.44 134.14
– 11.45 12.12 12.77
4
143.16
125.88
12.31
Table 5 Calculated by applying Eq. (4) to obtain yield strength from tensile testing. layer combination
CG UFG þ T UFG þ T þ DCG UFG þ T þ DCG þ CG UFG þ T þ DCG þ CG þ CG
sample thickness
experimental (MPa)
calculated
error
(mm)
σye (MPa)
σyMH (MPa)
eσyH (%)
1 1 2 3
102.26 200.93 167.78 153.79
– 199.77 167.05 155.00
– 0.57 0.43 0.79
4
143.16
144.93
0.95
and it can be estimated by using the rule of mixtures. The obtained yield strength from each layer and hardness distribution provided us with near 5% and 12% error, respectively. However, considering in compatibility between the layers and modifying the rule of mixtures decreased the estimation error to less than 1%.
2
Adding the second term to enrich Eq. (3) can cover the strengthening as a result of mechanical incompatibility between early-plastic and stable elastic regions. As seen by the results in Table 5, this simple modification results in estimates of the total yield strength with less than 1% error. For justifying the toughening mechanism, we should note that the coarse grains layer was capable of plastically deforming due to the possibility of dislocation motion. Moreover, coarse grains have a higher tolerance for crack growth. At the beginning of the deformation, coarse grains started plastic deformation first while the other areas were in the elastic regime. Strain partitioning and stress transfer between layers and the consequential back-stress strain-hardening were attributed to ductility enchantment. Therefore, adding deformed and undeformed coarse grain layers can improve ductility near 15%. However, adding another layer of CG was not as effective as the first one since it did not add an extra interface layer. However, it increased the area that cracks grew perpendicular to the applied load. As this coarse grain region in creases further, the tensile and fracture performance will approach that of the TRC alloy. Fatigue strength will likely be enhanced due to surface compression, but measurement of fatigue performance is beyond the scope of the present study.
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5. Conclusions Due to the SSP process, the gradient layers of UFG, transient, deformed, and undeformed grains were created in the as-received bimodal structure. The first two layers from the surface contained a high fraction of low and high angle grain boundaries and dislocation pile-ups. Hence, they contributed mostly to strength enhancement. Whereas deformed and undeformed micro grains layers impacted the failure process and increased the ductility near 30%. These two interior layers can convert pure shear fracture to semi ductile cup and cone fracture. Meanwhile, requirements for strain compatibility between the layers resulted in a higher value of strengthening and toughening. The yield strength was obtained from the created gradient structure 6
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