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Effect of powder oxidation on the anisotropy in tensile mechanical properties of bulk Al samples fabricated by spark plasma sintering
Materials Science & Engineering A 764 (2019) 138246
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Effect of powder oxidation on the anisotropy in tensile mechanical properties of bulk Al samples fabricated by spark plasma sintering
T
Lei Caoa, Wei Zenga, Yuehuang Xiea, Jiamiao Lianga,*, Deliang Zhanga,b,** a
Shanghai Key Laboratory of Advanced High-Temperature Materials and Precision Forming, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China b Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang, 110819, China
ARTICLE INFO
ABSTRACT
Keywords: Spark plasma sintering Aluminum Interparticle boundaries Anisotropy Tensile mechanical properties
This paper is a follow-up work reported in reference (L. Cao et al., Mater. Sci. Eng. A. 742 (2019) 305–308.). In the current work, we focused on the effect of the oxidation of Al powder particles on the anisotropy in tensile mechanical properties of bulk Al samples fabricated by spark plasma sintering (SPS). The tensile loading directions were respectively aligned with the transverse and longitudinal directions of materials. The results demonstrate that the oxidation of Al powder particles increases the anisotropy factor (defined as the ratio of the measured mechanical property value in the transverse direction to that in the longitudinal direction) from 1.18 to 2.76 of the elongation to fracture of bulk Al samples. Based on the analysis of the results, it is proposed that if the dominating cracks can initiate and propagate along interparticle boundaries during tensile deformation of a bulk metallic sample fabricated by thermomechanical powder consolidation, the shape of powder particles in the sample would play a major role in controlling its tensile ductility.
1. Introduction Metallic materials often exhibit an anisotropy in mechanical properties due to either the presence of texture [1–5] or other microstructural features [6–8]. As an example, it has recently been reported that the compressive properties of a laminate structured Al-graphene composite sample prepared by hot pressing of a graphene coated Al powder followed by hot rolling shows a significant anisotropy [9]. This work suggests that, for bulk metallic materials prepared by thermomechanical powder consolidation, their unique prior powder particle shapes in conjunction with the microstructural features of the interparticle boundaries (IPBs) can lead to the anisotropy in mechanical properties. Spark plasma sintering (SPS) technique has been widely used to fabricate bulk Al [10,11], Al alloys [12,13] and Al/Al alloy matrix composites [14,15]. This technique involves uniaxial pressing of a powder compact, which readily deforms the powder particles into brick-like shapes. Hence, the fabricated bulk Al may show an anisotropy in mechanical properties as the grain boundaries transformed from IPBs of these deformed powder particles are preferable sites for
cavities during tensile deformation [16]. Our recent work has shown that the microstructures of IPBs in bulk Al samples fabricated by SPS were greatly affected by the oxidation of Al powder particles used. This in turn leads to a dramatic reduction in the elongation to fracture of the samples [16]. It indicates that the microstructural features of the IPBs govern the mechanical properties of bulk samples. If oxidized Al powder particles were used to fabricate bulk Al samples by SPS and the particle shape is anisotropic in relation to the direction of the pressing force used in SPS, then the fabricated samples may show an increased anisotropy in mechanical properties. To our knowledge, the anisotropy in mechanical properties of bulk Al samples fabricated by SPS and the effect of powder oxidation on it have not been reported yet. Hence, the purpose of this work is to fill this missing gap. 2. Experimental procedure The bulk Al samples used in this study were the same as those used in our previous work [16]. Two different types of the samples used were cylinders of 28 mm in diameter and 28 mm in height. One was fabricated by SPS of an as-gas atomized Al powder (referred to as B1 sample)
*
Corresponding author. Corresponding author. Shanghai Key Laboratory of Advanced High-Temperature Materials and Precision Forming, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China. E-mail addresses: [email protected] (J. Liang), [email protected] (D. Zhang). **
https://doi.org/10.1016/j.msea.2019.138246 Received 17 January 2019; Received in revised form 31 July 2019; Accepted 1 August 2019 Available online 02 August 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved.
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investigated in our previous study [16]. The EBSD inverse pole figure (IPF) images (Fig. 2(c) and (d)) of the longitudinal sections in the B1 and B2 samples showed that the grains inside the powder particles were almost equiaxed. Fig. 3 shows the grain size distributions measured from the longitudinal sections of the B1 and B2 samples, and the mean grain sizes were determined to be 14.0 and 15.4 μm, respectively. These values are comparable to the mean grain sizes (16.3 and 17.0 μm) (Fig. 3(b) and (d)) determined by EBSD IPF imaging of the transverse sections of the B1 and B2 samples in Ref. [16]. Fig. 4 shows the EBSD IPFs of the longitudinal and transverse sections of the B1 and B2 samples. These IPFs show that the samples had a texture in both the longitudinal and transverse directions, but the intensity of the texture was fairly low in both directions of the samples. As shown in Fig. 5, the samples regardless of the tensile directions had similar distributions of Taylor factors determined from the EBSD data. Fig. 6 shows the tensile engineering stress-engineering strain curves of the tensile test specimens cut along the transverse and longitudinal directions from the B1 and B2 samples, respectively. Table 1 shows the average values of yield strength (YS), ultimate tensile strength (UTS), tensile strength (TS) at fracture and elongation to fracture of the tensile test specimens. As tabulated in Table 1, for the B1 sample, the YS and UTS in the transverse direction were comparable to those in the longitudinal direction, showing an average value of 30.7 and 76.1 MPa, respectively, while the average elongation to fracture of the former was about 18% higher than that of the latter. The ratio between the value of a mechanical property in the transverse direction to that of the same mechanical property in the longitudinal direction is termed as the anisotropy factor of the mechanical property. Based on the values of the tensile mechanical properties of the B1 sample, its anisotropy factor of the elongation to fracture was determined to be 1.18, which is fairly low. For the B2 sample, the average YS of the tensile test specimens cut along the longitudinal direction was slightly lower than that of the specimens cut along the transverse direction (31.8 MPa vs. 36.7 MPa). The tensile test specimens cut along the transverse direction exhibited mature fracture with an average UTS of 77.7 MPa and an average elongation to fracture of 10.5%. However, the tensile test specimens cut along the longitudinal direction fractured prematurely at a clearly lower average TS of 63.5 MPa and a significantly smaller average elongation to fracture of only 3.8%. The anisotropy factor of the elongation to fracture of the B2 sample was determined to be 2.76. This value is drastically higher than that (1.18) of the B1 sample. Fig. 7(a) and (b) shows the fracture surfaces of the tensile test specimens cut from the B1 sample along the longitudinal and transverse directions, respectively. The image in Fig. 7(b) was already presented in Ref. [16], and it was used here to make a direct comparison. The specimens cut along both directions showed similar dimpled fracture surface morphologies, confirming a ductile fracture mode. It can be seen that the deep pits in the tensile test specimens cut from the longitudinal direction, as indicated by the red arrows shown in Fig. 7(a), were larger than those in the tensile test specimens cut along the transverse direction. This difference can be attributed to the brick-like particle shapes. Fig. 7(c) and (d) shows the longitudinal sections near the fracture surfaces of tensile test specimens cut from the B1 sample along the longitudinal and transverse directions, respectively. The crack paths observed from the two longitudinal sections were similar, favoring a similar fracture behavior. Fig. 8(a) and (b) presents the fracture surfaces of the tensile test specimens cut from the B2 sample along the longitudinal and transverse directions, respectively. The image in Fig. 8(b) presented in Ref. [16] was used here again to make a direct comparison. Again, the tensile test specimens cut from both directions showed similar intergranular fracture surface morphologies, manifesting a brittle fracture mode with the cracks initiating and propagating along IPBs. The pits in the fracture surfaces of the specimens cut along the longitudinal direction were larger than those in the fracture surfaces of the specimens cut along the transverse direction. This difference can also be attributed to the brick-
Fig. 1. Schematic diagrams illustrating (a) the tensile test specimens cut along longitudinal and transverse directions from bulk Al samples, and (b) the view direction to examine the longitudinal sections near the fracture surfaces of the tensile test specimens.
and the other was fabricated by SPS of a further-oxidized Al powder (referred to as B2 sample). The microstructure of bulk Al samples was characterized using scanning electron microscopy (SEM) (FEI NanoSEM 230) and electron back-scattered diffraction (EBSD) (Aztec HKL Max). As the schematic shown in Fig. 1(a), dog-bone-shaped tensile test specimens with a gauge length of 10 mm, a thickness of 2 mm and a width of 3 mm were cut along the transverse and longitudinal directions from the B1 and B2 samples, respectively. Tensile tests were carried out using a ZWICK Z100 testing machine with a strain rate of 5 × 10-4 s-1. Three tensile specimens were tested for each set of data. The fracture surfaces and longitudinal sections near the fracture surfaces of the tensile test specimens were examined using SEM. Fig. 1(b) illustrates the view direction of the longitudinal sections near the fracture surfaces of tensile test specimens. 3. Results As shown in Fig. 2(a) and (b), the powder particles in the B1 and B2 samples exhibited brick-like shapes. The microstructural features of the B1 and B2 samples consisted of grain boundaries (GBs) inside the prior powder particles (termed as Type I GBs), GBs transformed from IPBs in the B1 sample (termed as Type II GBs) and IPBs in the B2 sample. Types I and Ⅱ GBs in the B1 sample and IPBs in the B2 sample have been
Fig. 2. SEM images ((a) and (b)) and EBSD inverse pole figure (IPF) images ((c) and (d)) of the longitudinal sections of the B1 ((a) and (c)) and B2 ((b) and (d)) samples. 2
Materials Science & Engineering A 764 (2019) 138246
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Fig. 3. Grain size distributions of the longitudinal and transverse sections of the B1 and B2 samples: (a) B1-longitudinal; (b) B1-transverse; (c) B2-longitudinal; (d) B2-transverse. The EBSD IPF images were used to obtain statistical data in the figure.
Fig. 4. EBSD IPFs of the longitudinal and transverse sections of the B1 and B2 samples: (a) B1-longitudinal; (b) B1-transverse; (c) B2-longitudinal; (d) B2-transverse (PD: pressing direction; RD: radial direction of the cylindrical bulk Al samples).
like particle shapes in the B2 sample. As shown in Fig. 8(c) and (d), the crack paths in the tensile test specimens cut from both directions exhibited zigzag shaped waves, with the amplitude of the waves associated with the specimens cut along the transverse direction (Fig. 8(d)) being significantly larger than that associated with the specimens cut
along the longitudinal direction (Fig. 8(c)). This difference is consistent with the fact that, for tensile test specimens cut from the B2 sample, the cracks propagated along the IPBs and the Al powder particles had bricklike shapes with their longer sides aligned to the transverse direction.
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Fig. 5. The distributions of Taylor factor values along the tensile directions of the tensile test specimens cut along different directions from the B1 and B2 samples: (a) B1-longitudinal; (b) B1-transverse; (c) B2-longitudinal; (d) B2-transverse. Table 1 The average YS, UTS/TS and elongation to fracture (εf) values of the tensile test specimens cut from the B1 and B2 samples along longitudinal (L) and transverse (T) directions, respectively. Samples
YS/MPa
B1-T B1-L B2-T B2-L
31.0 30.3 36.7 31.8
± ± ± ±
0.9 1.5 2.8 1.3
UTS/TS/MPa
εf/%
76.0 76.2 77.7 63.5
53.3 ± 1.5 45.0 ± 1.3 10.5 ± 0.9 3.8 ± 0.5
± ± ± ±
3.0 2.1 0.3 1.3
[17]. The effect of the texture on the strength can be estimated by the following equation:
= (M
M)
(1)
where M is the Taylor factor (3.06) with no texture, and M is the average Taylor factor along the tensile direction of the material and can be derived using EBSD data [18]. In this study, the average Taylor factor values were calculated to be 3.00 for both transverse and longitudinal directions in the B1 sample (Fig. 5(a) and (b)), and 3.04 and 3.00 for the longitudinal and transverse directions, respectively, in the B2 sample (Fig. 5(c) and (d)). Obviously, Taylor factors vary little in both directions for different types of the fabricated samples. This indicates that the texture has little effect on the transverse and longitudinal yield strengths of bulk Al samples. Additionally, the mean grain sizes in the two directions in one of the studied samples are comparable (14.0 μm vs 16.3 μm for the B1 sample and 15.4 μm vs 17.0 μm for the B2 sample), suggesting comparable grain boundary strengthening
Fig. 6. Tensile engineering stress-engineering strain curves of the tensile test specimens cut along the longitudinal (L) and transverse (T) directions from the B1 and B2 samples, respectively.
4. Discussion It has been well established that the strength of a metallic material can be affected by the texture through changing the Taylor factor which is defined as / , where is the normal stress and is the shear stress 4
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calculated to be 1.22 and 2.76, respectively. The observed weak and strong anisotropy in tensile ductility of the B1 and B2 samples can be attributed to the formation of cavities preferentially at Type II GBs in the B1 sample, the crack nucleation and propagation along IPBs in the B2 sample and the brick-like powder particle shapes in both the samples. As the schematic shown in Fig. 9, due to the brick-like powder particle shapes in bulk Al samples (Fig. 2 (a), 2(b), 8(c) and 8(d)), the average lengths of Type Ⅱ GBs in the B1 sample and IPBs in the B2 sample are clearly shorter in the longitudinal direction than that in the transverse direction. Consequently, it is more difficult for the B1 sample to nucleate cavities at Type II GBs in the longitudinal direction (Fig. 9(b)). Because of a smaller space for the cavities to coalesce during tensile testing along the transverse direction (Fig. 9(b)) than that along the longitudinal direction (Fig. 9(a)), a higher uniform elongation is obtained in the transverse direction than that in the longitudinal direction. As we have shown in our previous study [16], the powder oxidation facilitates the cracks to nucleate and propagate along IPBs, indicating that the IPBs govern the fracture behavior of the B2 sample. It can be envisaged that when the tensile stress is sufficiently large during tensile deformation, microcracks would be encouraged to form along IPBs. As the schematic shown in Fig. 9(d), the clear zigzag shapes of the IPBs that are associated with prior powder particle shapes are seen along the longitudinal direction. This indicates that the formation of the microcracks is more difficult when the tensile loading (tensile stress) is along the transverse direction [22–27]. In this case, a larger amount of plastic deformation is required before a dominating crack nucleates and propagates through the entire sample. In comparison, the much smaller magnitude of the waves is provided by the zigzag shapes of the IPBs along the transverse direction (Fig. 9(c)). Thus, it is easier for the microcracks initiated to unite and form a dominating crack when the tensile stress is along the longitudinal direction. Based on this analysis, it is fairly understandable that more energy is needed to drive the initiation and propagation of the major cracks during tensile deformation in the transverse direction [28]. This will provide a larger amount of elongation prior to fracture, which is in agreement with the tensile curves of the B2 sample shown in Fig. 6.
Fig. 7. Fracture surfaces ((a) and (b)) and longitudinal sections near the fracture surfaces ((c) and (d)) of the tensile test specimens cut from the B1 sample along different directions: (a) B1-longitudinal; (b) B1-transverse [16]; (c) B1longitudinal; (d) B1-transverse. The red arrows in (a) and (b) point to large cavities formed at Type II GBs. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
5. Conclusions In this work, the effect of the powder oxidation on the anisotropy in mechanical properties of bulk Al samples during tensile testing was studied. The loading directions were along the transverse and longitudinal directions of the samples, respectively. The studied samples were fabricated by SPS of both the as-gas atomized (referred to as B1 sample) and further-oxidized (referred to as B2 sample) Al powders. The main conclusions are:
Fig. 8. Fracture surfaces ((a) and (b)) and longitudinal sections near the fracture surfaces ((c) and (d)) of the tensile test specimens cut from the B2 sample along different directions: (a) B2-longitudinal; (b) B2-transverse [16]; (c) B2longitudinal; (d) B2-transverse.
(1) The YS and UTS of the B1 sample do not show an anisotropy during tensile testing in the transverse and longitudinal directions, while its average elongation to fracture presents a weak anisotropy. (2) The powder oxidation leads to an increased anisotropy in tensile mechanical properties of the B2 sample. The anisotropy factors of the UTS/TS and elongation to fracture of the sample increase from 1.00 and 1.18 to 1.22 and 2.76, respectively. (3) For the B1 sample, the anisotropy in tensile mechanical properties is caused by the formation of cavities preferentially at Type II GBs and the alignment of the brick-shaped powder particles in the transverse direction. The powder oxidation makes crack initiate and propagate along IPBs, and this, together with the brick-like powder particle shapes, drastically increases the tensile ductility anisotropy of the B2 sample.
contributions [18–21] from both the transverse and longitudinal directions in the samples here. Hence, the anisotropy is absent for the yield strength in the longitudinal and transverse directions of the B1 (30.3 MPa vs 31.0 MPa) and B2 (31.8 MPa vs 36.7 MPa) samples. As seen in Fig. 6 and Table 1, for the B1 sample, its average elongation to fracture in the transverse direction (53.3%) is 18% higher than that in the longitudinal direction (45.0%). This confirms that the B1 sample shows the anisotropy in tensile ductility. In contrast, owing to the powder oxidation, the B2 sample exhibits stronger anisotropy in both tensile strength and ductility (Fig. 6 and Table 1). The anisotropy factors of the UTS/TS and elongation to fracture of the B2 sample were
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Materials Science & Engineering A 764 (2019) 138246
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Fig. 9. Schematic diagrams illustrating the fracture behavior of the tensile test specimens cut from the B1 and B2 samples along different directions: (a) B1longitudinal; (b) B1-transverse; (c) B2-longitudinal; (d) B2-transverse.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No.51271115).
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of powder oxidation on the anisotropy in tensile mechanical properties of bulk Al samples fabricated by spark plasma sintering
T
Lei Caoa, Wei Zenga, Yuehuang Xiea, Jiamiao Lianga,*, Deliang Zhanga,b,** a
Shanghai Key Laboratory of Advanced High-Temperature Materials and Precision Forming, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China b Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang, 110819, China
ARTICLE INFO
ABSTRACT
Keywords: Spark plasma sintering Aluminum Interparticle boundaries Anisotropy Tensile mechanical properties
This paper is a follow-up work reported in reference (L. Cao et al., Mater. Sci. Eng. A. 742 (2019) 305–308.). In the current work, we focused on the effect of the oxidation of Al powder particles on the anisotropy in tensile mechanical properties of bulk Al samples fabricated by spark plasma sintering (SPS). The tensile loading directions were respectively aligned with the transverse and longitudinal directions of materials. The results demonstrate that the oxidation of Al powder particles increases the anisotropy factor (defined as the ratio of the measured mechanical property value in the transverse direction to that in the longitudinal direction) from 1.18 to 2.76 of the elongation to fracture of bulk Al samples. Based on the analysis of the results, it is proposed that if the dominating cracks can initiate and propagate along interparticle boundaries during tensile deformation of a bulk metallic sample fabricated by thermomechanical powder consolidation, the shape of powder particles in the sample would play a major role in controlling its tensile ductility.
1. Introduction Metallic materials often exhibit an anisotropy in mechanical properties due to either the presence of texture [1–5] or other microstructural features [6–8]. As an example, it has recently been reported that the compressive properties of a laminate structured Al-graphene composite sample prepared by hot pressing of a graphene coated Al powder followed by hot rolling shows a significant anisotropy [9]. This work suggests that, for bulk metallic materials prepared by thermomechanical powder consolidation, their unique prior powder particle shapes in conjunction with the microstructural features of the interparticle boundaries (IPBs) can lead to the anisotropy in mechanical properties. Spark plasma sintering (SPS) technique has been widely used to fabricate bulk Al [10,11], Al alloys [12,13] and Al/Al alloy matrix composites [14,15]. This technique involves uniaxial pressing of a powder compact, which readily deforms the powder particles into brick-like shapes. Hence, the fabricated bulk Al may show an anisotropy in mechanical properties as the grain boundaries transformed from IPBs of these deformed powder particles are preferable sites for
cavities during tensile deformation [16]. Our recent work has shown that the microstructures of IPBs in bulk Al samples fabricated by SPS were greatly affected by the oxidation of Al powder particles used. This in turn leads to a dramatic reduction in the elongation to fracture of the samples [16]. It indicates that the microstructural features of the IPBs govern the mechanical properties of bulk samples. If oxidized Al powder particles were used to fabricate bulk Al samples by SPS and the particle shape is anisotropic in relation to the direction of the pressing force used in SPS, then the fabricated samples may show an increased anisotropy in mechanical properties. To our knowledge, the anisotropy in mechanical properties of bulk Al samples fabricated by SPS and the effect of powder oxidation on it have not been reported yet. Hence, the purpose of this work is to fill this missing gap. 2. Experimental procedure The bulk Al samples used in this study were the same as those used in our previous work [16]. Two different types of the samples used were cylinders of 28 mm in diameter and 28 mm in height. One was fabricated by SPS of an as-gas atomized Al powder (referred to as B1 sample)
*
Corresponding author. Corresponding author. Shanghai Key Laboratory of Advanced High-Temperature Materials and Precision Forming, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China. E-mail addresses: [email protected] (J. Liang), [email protected] (D. Zhang). **
https://doi.org/10.1016/j.msea.2019.138246 Received 17 January 2019; Received in revised form 31 July 2019; Accepted 1 August 2019 Available online 02 August 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved.
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investigated in our previous study [16]. The EBSD inverse pole figure (IPF) images (Fig. 2(c) and (d)) of the longitudinal sections in the B1 and B2 samples showed that the grains inside the powder particles were almost equiaxed. Fig. 3 shows the grain size distributions measured from the longitudinal sections of the B1 and B2 samples, and the mean grain sizes were determined to be 14.0 and 15.4 μm, respectively. These values are comparable to the mean grain sizes (16.3 and 17.0 μm) (Fig. 3(b) and (d)) determined by EBSD IPF imaging of the transverse sections of the B1 and B2 samples in Ref. [16]. Fig. 4 shows the EBSD IPFs of the longitudinal and transverse sections of the B1 and B2 samples. These IPFs show that the samples had a texture in both the longitudinal and transverse directions, but the intensity of the texture was fairly low in both directions of the samples. As shown in Fig. 5, the samples regardless of the tensile directions had similar distributions of Taylor factors determined from the EBSD data. Fig. 6 shows the tensile engineering stress-engineering strain curves of the tensile test specimens cut along the transverse and longitudinal directions from the B1 and B2 samples, respectively. Table 1 shows the average values of yield strength (YS), ultimate tensile strength (UTS), tensile strength (TS) at fracture and elongation to fracture of the tensile test specimens. As tabulated in Table 1, for the B1 sample, the YS and UTS in the transverse direction were comparable to those in the longitudinal direction, showing an average value of 30.7 and 76.1 MPa, respectively, while the average elongation to fracture of the former was about 18% higher than that of the latter. The ratio between the value of a mechanical property in the transverse direction to that of the same mechanical property in the longitudinal direction is termed as the anisotropy factor of the mechanical property. Based on the values of the tensile mechanical properties of the B1 sample, its anisotropy factor of the elongation to fracture was determined to be 1.18, which is fairly low. For the B2 sample, the average YS of the tensile test specimens cut along the longitudinal direction was slightly lower than that of the specimens cut along the transverse direction (31.8 MPa vs. 36.7 MPa). The tensile test specimens cut along the transverse direction exhibited mature fracture with an average UTS of 77.7 MPa and an average elongation to fracture of 10.5%. However, the tensile test specimens cut along the longitudinal direction fractured prematurely at a clearly lower average TS of 63.5 MPa and a significantly smaller average elongation to fracture of only 3.8%. The anisotropy factor of the elongation to fracture of the B2 sample was determined to be 2.76. This value is drastically higher than that (1.18) of the B1 sample. Fig. 7(a) and (b) shows the fracture surfaces of the tensile test specimens cut from the B1 sample along the longitudinal and transverse directions, respectively. The image in Fig. 7(b) was already presented in Ref. [16], and it was used here to make a direct comparison. The specimens cut along both directions showed similar dimpled fracture surface morphologies, confirming a ductile fracture mode. It can be seen that the deep pits in the tensile test specimens cut from the longitudinal direction, as indicated by the red arrows shown in Fig. 7(a), were larger than those in the tensile test specimens cut along the transverse direction. This difference can be attributed to the brick-like particle shapes. Fig. 7(c) and (d) shows the longitudinal sections near the fracture surfaces of tensile test specimens cut from the B1 sample along the longitudinal and transverse directions, respectively. The crack paths observed from the two longitudinal sections were similar, favoring a similar fracture behavior. Fig. 8(a) and (b) presents the fracture surfaces of the tensile test specimens cut from the B2 sample along the longitudinal and transverse directions, respectively. The image in Fig. 8(b) presented in Ref. [16] was used here again to make a direct comparison. Again, the tensile test specimens cut from both directions showed similar intergranular fracture surface morphologies, manifesting a brittle fracture mode with the cracks initiating and propagating along IPBs. The pits in the fracture surfaces of the specimens cut along the longitudinal direction were larger than those in the fracture surfaces of the specimens cut along the transverse direction. This difference can also be attributed to the brick-
Fig. 1. Schematic diagrams illustrating (a) the tensile test specimens cut along longitudinal and transverse directions from bulk Al samples, and (b) the view direction to examine the longitudinal sections near the fracture surfaces of the tensile test specimens.
and the other was fabricated by SPS of a further-oxidized Al powder (referred to as B2 sample). The microstructure of bulk Al samples was characterized using scanning electron microscopy (SEM) (FEI NanoSEM 230) and electron back-scattered diffraction (EBSD) (Aztec HKL Max). As the schematic shown in Fig. 1(a), dog-bone-shaped tensile test specimens with a gauge length of 10 mm, a thickness of 2 mm and a width of 3 mm were cut along the transverse and longitudinal directions from the B1 and B2 samples, respectively. Tensile tests were carried out using a ZWICK Z100 testing machine with a strain rate of 5 × 10-4 s-1. Three tensile specimens were tested for each set of data. The fracture surfaces and longitudinal sections near the fracture surfaces of the tensile test specimens were examined using SEM. Fig. 1(b) illustrates the view direction of the longitudinal sections near the fracture surfaces of tensile test specimens. 3. Results As shown in Fig. 2(a) and (b), the powder particles in the B1 and B2 samples exhibited brick-like shapes. The microstructural features of the B1 and B2 samples consisted of grain boundaries (GBs) inside the prior powder particles (termed as Type I GBs), GBs transformed from IPBs in the B1 sample (termed as Type II GBs) and IPBs in the B2 sample. Types I and Ⅱ GBs in the B1 sample and IPBs in the B2 sample have been
Fig. 2. SEM images ((a) and (b)) and EBSD inverse pole figure (IPF) images ((c) and (d)) of the longitudinal sections of the B1 ((a) and (c)) and B2 ((b) and (d)) samples. 2
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Fig. 3. Grain size distributions of the longitudinal and transverse sections of the B1 and B2 samples: (a) B1-longitudinal; (b) B1-transverse; (c) B2-longitudinal; (d) B2-transverse. The EBSD IPF images were used to obtain statistical data in the figure.
Fig. 4. EBSD IPFs of the longitudinal and transverse sections of the B1 and B2 samples: (a) B1-longitudinal; (b) B1-transverse; (c) B2-longitudinal; (d) B2-transverse (PD: pressing direction; RD: radial direction of the cylindrical bulk Al samples).
like particle shapes in the B2 sample. As shown in Fig. 8(c) and (d), the crack paths in the tensile test specimens cut from both directions exhibited zigzag shaped waves, with the amplitude of the waves associated with the specimens cut along the transverse direction (Fig. 8(d)) being significantly larger than that associated with the specimens cut
along the longitudinal direction (Fig. 8(c)). This difference is consistent with the fact that, for tensile test specimens cut from the B2 sample, the cracks propagated along the IPBs and the Al powder particles had bricklike shapes with their longer sides aligned to the transverse direction.
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Fig. 5. The distributions of Taylor factor values along the tensile directions of the tensile test specimens cut along different directions from the B1 and B2 samples: (a) B1-longitudinal; (b) B1-transverse; (c) B2-longitudinal; (d) B2-transverse. Table 1 The average YS, UTS/TS and elongation to fracture (εf) values of the tensile test specimens cut from the B1 and B2 samples along longitudinal (L) and transverse (T) directions, respectively. Samples
YS/MPa
B1-T B1-L B2-T B2-L
31.0 30.3 36.7 31.8
± ± ± ±
0.9 1.5 2.8 1.3
UTS/TS/MPa
εf/%
76.0 76.2 77.7 63.5
53.3 ± 1.5 45.0 ± 1.3 10.5 ± 0.9 3.8 ± 0.5
± ± ± ±
3.0 2.1 0.3 1.3
[17]. The effect of the texture on the strength can be estimated by the following equation:
= (M
M)
(1)
where M is the Taylor factor (3.06) with no texture, and M is the average Taylor factor along the tensile direction of the material and can be derived using EBSD data [18]. In this study, the average Taylor factor values were calculated to be 3.00 for both transverse and longitudinal directions in the B1 sample (Fig. 5(a) and (b)), and 3.04 and 3.00 for the longitudinal and transverse directions, respectively, in the B2 sample (Fig. 5(c) and (d)). Obviously, Taylor factors vary little in both directions for different types of the fabricated samples. This indicates that the texture has little effect on the transverse and longitudinal yield strengths of bulk Al samples. Additionally, the mean grain sizes in the two directions in one of the studied samples are comparable (14.0 μm vs 16.3 μm for the B1 sample and 15.4 μm vs 17.0 μm for the B2 sample), suggesting comparable grain boundary strengthening
Fig. 6. Tensile engineering stress-engineering strain curves of the tensile test specimens cut along the longitudinal (L) and transverse (T) directions from the B1 and B2 samples, respectively.
4. Discussion It has been well established that the strength of a metallic material can be affected by the texture through changing the Taylor factor which is defined as / , where is the normal stress and is the shear stress 4
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calculated to be 1.22 and 2.76, respectively. The observed weak and strong anisotropy in tensile ductility of the B1 and B2 samples can be attributed to the formation of cavities preferentially at Type II GBs in the B1 sample, the crack nucleation and propagation along IPBs in the B2 sample and the brick-like powder particle shapes in both the samples. As the schematic shown in Fig. 9, due to the brick-like powder particle shapes in bulk Al samples (Fig. 2 (a), 2(b), 8(c) and 8(d)), the average lengths of Type Ⅱ GBs in the B1 sample and IPBs in the B2 sample are clearly shorter in the longitudinal direction than that in the transverse direction. Consequently, it is more difficult for the B1 sample to nucleate cavities at Type II GBs in the longitudinal direction (Fig. 9(b)). Because of a smaller space for the cavities to coalesce during tensile testing along the transverse direction (Fig. 9(b)) than that along the longitudinal direction (Fig. 9(a)), a higher uniform elongation is obtained in the transverse direction than that in the longitudinal direction. As we have shown in our previous study [16], the powder oxidation facilitates the cracks to nucleate and propagate along IPBs, indicating that the IPBs govern the fracture behavior of the B2 sample. It can be envisaged that when the tensile stress is sufficiently large during tensile deformation, microcracks would be encouraged to form along IPBs. As the schematic shown in Fig. 9(d), the clear zigzag shapes of the IPBs that are associated with prior powder particle shapes are seen along the longitudinal direction. This indicates that the formation of the microcracks is more difficult when the tensile loading (tensile stress) is along the transverse direction [22–27]. In this case, a larger amount of plastic deformation is required before a dominating crack nucleates and propagates through the entire sample. In comparison, the much smaller magnitude of the waves is provided by the zigzag shapes of the IPBs along the transverse direction (Fig. 9(c)). Thus, it is easier for the microcracks initiated to unite and form a dominating crack when the tensile stress is along the longitudinal direction. Based on this analysis, it is fairly understandable that more energy is needed to drive the initiation and propagation of the major cracks during tensile deformation in the transverse direction [28]. This will provide a larger amount of elongation prior to fracture, which is in agreement with the tensile curves of the B2 sample shown in Fig. 6.
Fig. 7. Fracture surfaces ((a) and (b)) and longitudinal sections near the fracture surfaces ((c) and (d)) of the tensile test specimens cut from the B1 sample along different directions: (a) B1-longitudinal; (b) B1-transverse [16]; (c) B1longitudinal; (d) B1-transverse. The red arrows in (a) and (b) point to large cavities formed at Type II GBs. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
5. Conclusions In this work, the effect of the powder oxidation on the anisotropy in mechanical properties of bulk Al samples during tensile testing was studied. The loading directions were along the transverse and longitudinal directions of the samples, respectively. The studied samples were fabricated by SPS of both the as-gas atomized (referred to as B1 sample) and further-oxidized (referred to as B2 sample) Al powders. The main conclusions are:
Fig. 8. Fracture surfaces ((a) and (b)) and longitudinal sections near the fracture surfaces ((c) and (d)) of the tensile test specimens cut from the B2 sample along different directions: (a) B2-longitudinal; (b) B2-transverse [16]; (c) B2longitudinal; (d) B2-transverse.
(1) The YS and UTS of the B1 sample do not show an anisotropy during tensile testing in the transverse and longitudinal directions, while its average elongation to fracture presents a weak anisotropy. (2) The powder oxidation leads to an increased anisotropy in tensile mechanical properties of the B2 sample. The anisotropy factors of the UTS/TS and elongation to fracture of the sample increase from 1.00 and 1.18 to 1.22 and 2.76, respectively. (3) For the B1 sample, the anisotropy in tensile mechanical properties is caused by the formation of cavities preferentially at Type II GBs and the alignment of the brick-shaped powder particles in the transverse direction. The powder oxidation makes crack initiate and propagate along IPBs, and this, together with the brick-like powder particle shapes, drastically increases the tensile ductility anisotropy of the B2 sample.
contributions [18–21] from both the transverse and longitudinal directions in the samples here. Hence, the anisotropy is absent for the yield strength in the longitudinal and transverse directions of the B1 (30.3 MPa vs 31.0 MPa) and B2 (31.8 MPa vs 36.7 MPa) samples. As seen in Fig. 6 and Table 1, for the B1 sample, its average elongation to fracture in the transverse direction (53.3%) is 18% higher than that in the longitudinal direction (45.0%). This confirms that the B1 sample shows the anisotropy in tensile ductility. In contrast, owing to the powder oxidation, the B2 sample exhibits stronger anisotropy in both tensile strength and ductility (Fig. 6 and Table 1). The anisotropy factors of the UTS/TS and elongation to fracture of the B2 sample were
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Fig. 9. Schematic diagrams illustrating the fracture behavior of the tensile test specimens cut from the B1 and B2 samples along different directions: (a) B1longitudinal; (b) B1-transverse; (c) B2-longitudinal; (d) B2-transverse.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No.51271115).
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