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Al explosively welded clads
Materials Characterization 129 (2017) 242–246
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Texture transformations near the bonding zones of the three-layer Al/Ti/Al explosively welded clads
MARK
R. Chulista,⁎, D.M. Fronczeka, Z. Szulcb, J. Wojewoda-Budkaa a b
Institute of Metallurgy and Materials Science, Polish Academy of Sciences, 25 Reymonta St., 30-059 Cracow, Poland High Energy Technologies Works ‘Explomet’, 100H Oswiecimska St., 45-641 Opole, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords: Aluminum Titanium Joining Explosive welding Texture EBSD
Al/Ti/Al explosively welded clads were analyzed by electron backscatter diffraction. Particular attention was drawn to texture evolution across the Al/Ti/Al interfaces. Significant changes in texture within the joined metals were conducted with the place of detonation of explosive material. The so-called base (lower) Al clad is characterized by a typical rolling texture exhibiting copper and brass components, while the flyer (upper) clad, located closer to the detonation wave front, transformed to a shear texture with rotated cube and {111} 〈110〉 components. The shear deformation also changes substantially the texture of Ti producing a much higher twin density. The observed changes unveil the effect of explosion on the microstructure of the joined metals and are discussed with respect to deformation mode and further annealing process.
1. Introduction Recently, extensive research has been focused on the explosive welding (EXW) method since it allows fabrication of multilayer materials such as Al/Ti [1–3], Al/Cu [4], Ni/Al [5,6], steel/Cu/Ti/Al [7] or Ti/Nb/Cu/steel [8] for various industrial applications. Additionally, successful attempts to manufacture biocompatible materials for the medical applications were also performed [9,10]. They can be manufactured using parallel [11,12] or inclined [13] geometry. EXW is classified as a cold forming process [14], therefore, the joined material is assumed to retain its pre-bond properties. However, extreme conditions such as high pressure, high velocity at the collision site and locally high temperature may influence not only the interface of two metals/alloys to be welded, but also the neighboring areas [15,16]. Unlike the so-called cold welding methods, where no melted or liquid phases are expected to occur, the interface obtained using EXW technique appears to have a relatively large nanocrystalline regions of a peninsula-like or island-like morphology [16–18]. It strongly suggests that EXW process exhibits locally non-equilibrium conditions deviating from the chemical equilibrium at solidification front [19–21]. Such metastable conditions favor a large variety of intermetallics being far from equilibrium [22]. Beside equilibrium and non-equilibrium intermediate phases also quasicrystals and amorphous phases can be observed in the interface zone [23–26]. Moreover, a typical solidification microstructure composed of columnar and equiaxed grains can be found at the interface in the state after EXW [11,17]. For all these
⁎
Corresponding author. E-mail address: [email protected] (R. Chulist).
http://dx.doi.org/10.1016/j.matchar.2017.05.007 Received 7 February 2017; Received in revised form 25 April 2017; Accepted 8 May 2017 Available online 08 May 2017 1044-5803/ © 2017 Elsevier Inc. All rights reserved.
reasons the location and strength of explosion seems to play a crucial role for the bonding process as well as the further accompanying heat treatment. Physical, mechanical or chemical properties of single grains vary with the crystallographic direction. Thus, a strong texture imparts significant anisotropy of physical properties [27]. On the other hand the preferred crystallographic orientation or texture of polycrystalline material may act as a good indicator for plastic deformation [28,29]. Grains have a tendency to group around typical texture components which is characteristic for a particular material and deformation mode. With a view to this, it is the main aim of this paper to correlate the texture and microstructural changes across the A1050/Ti Gr.2/A1050 samples with the place of detonation. It also gives an insight into the plastic deformation mode which takes place during EXW. The three layer system was chosen for the sake of clarity and to show significant changes between the so-called lower and the upper interfaces. 2. Material and Methods A1050 and Ti Gr. 2 clads with the dimension of 150 mm × 240 mm × 1 mm and 150 mm × 240 mm × 0.8 mm were used, respectively. The chemical composition of the alloys is given in Table 1. The initial plates were cold rolled. Such a three-layer, parallel arrangement was explosively welded in air atmosphere by the Explomet Company. As the base (lower) plate A1050 clad was applied while Ti Gr. 2 and A1050 (upper) were used for the flyer plates. Between the
Materials Characterization 129 (2017) 242–246
R. Chulist et al.
Table 1 Chemical composition of the A1050 and Ti Gr. 2 alloys. Alloy
A1050 Ti Gr. 2
Chemical composition [wt%] Fe
O
C
N
H
Si
Zn
Cu
Mg
Mn
Ti
< 0.4 < 0.3
– < 0.25
– < 0.1
– < 0.03
– < 0.015
< 0.25 –
< 0.07 –
< 0.06 –
< 0.05 –
< 0.05 –
< 0.05
Fig. 1. BSE images of the upper and lower interfaces of the Al/Ti/Al clad.
A1050 flyer plate and explosive material a silicon interlayer was placed in order to prevent the erosion of explosion (experimental set-up is given in [3], Fig. 1). Subsequently, samples with the dimensions of 6 mm × 12 mm × 2.8 mm were cut with respect to the rolling coordinate system from the central part of the three-layered sandwich. Abrasive papers (1000, 2000 and 5000) and diamond polishing pastes (6 μm, 3 μm, 1 μm and 0.25 μm) mixed with alumina were applied for surface preparation. The samples were then processed to obtain a proper surface quality for EBSD measurements using electropolishing process with the following parameters: electrolyte Struers A3, voltage = 35 V, T = 283 K, sample temperature = 77 K, polishing time = 15 s. Investigation of microstructure and texture were performed using a FEI Quanta 3D FEG scanning electron microscope equipped with the TSL EBSD system. The orientation distribution function (ODF) and pole figures were calculated using TSL software. The Euler angles were used in Bunge notation [30]. To account best for the orthorhombic symmetry of rolling and simple shear, the crystal and sample reference systems have been defined as X || rolling/shear direction (RD/SD), Y || normal direction (ND) and Z || transverse direction (TD). Concerning the specific texture components both for A1050 (copper, brass, rotated cube and shear components) and for Ti Gr. 2 (mainly basal texture) ODF and pole figures for A1050 and Ti Gr. 2 were chosen, respectively. Such a texture representation allowed for easy comparison of the so-called upper and lower sides of the materials.
Unlike the upper plate the lower one seems to be only slightly affected by EXW since only few grains with rotated cube orientations are observed in the immediate vicinity of the lower interface. This large distinction can be clearly seen in Fig. 2, where the brass (turquoise) and copper (magenta) components are mainly observed in the lower A1050 plate, while the upper one is dominated by rotated cube (green) and {111} 〈110〉 (orange/brown) shear components (color coding according to Fig. 3). In both A1050 clads, a wavy alignment of elongated grains can be observed, however, the wave character is much more pronounced within the upper A1050 plate, Fig. 2. The texture evolution of both A1050 plates can be understand using ODF. The information contained in the ODF section at φ2 = 45° shows a drastic change of texture between the upper and lower part. The initial copper and brass components change to rotated cube and {111} 〈110〉 components, respectively. The copper component is rotated about 15° around the TD direction (〈110〉) towards the rotated cube component. The same holds for the brass component, however, the rotation axis (TD) is parallel to 〈111〉 for this orientation. Both shear components slightly deviate from the ideal positions in positive φ1 direction which may be due to EXW causing a highly intense and rapid deformation. It should be mentioned that the rotation axis in a shear process is perpendicular to the shear plane and shear direction for the given orientation (for shear plane normal and shear direction indication see Fig. 3). EXW changes the initial texture and microstructure of the Ti Gr. 2 plate as well. The shape of titanium grains is more equiaxed and a high number of twins can be indicated. The EBSD map revealed strongly twinned microstructure with two split basal poles for both lower and upper part of Ti Gr. 2 plate, Figs. 4 and 5. The texture of the lower part resembles that of the initial one with a symmetric distribution of two basal poles, while the texture of the upper part is more asymmetric with one dominating component adjacent to ND, Fig. 5. The same tendency can be observed plotting compression and tension twins for both Ti Gr. 2 regions. The lower part exhibits a comparable fraction of the tension and compression twins (48/52 area fraction), whereas the upper part is overshadow with compression twins (21/79), Fig. 5. Since the plane strain deformation (rolling) can be expressed as the Tucker compression-tensile state [40] it seems to be consistent with the twin activity. On the other hand, explosion promotes compression twins due to high pressure during explosion causing permanent deformation at the interface. In summary, it can be concluded that comparing the regions nearby the upper and lower interfaces of A1050 and Ti Gr. 2 plates significant
3. Results and Discussion Fig. 1 presents a SEM image of the A1050/Ti Gr. 2/A1050 interfaces in the state obtained directly after EXW. It is clearly showed that no discontinuities, pores or impurities were observed along the bonding. This confirms the fact that during explosion an air jetting strips away all contaminations and impurities cleaning the metal surfaces. As a result a good bonding could be manufactured [31–33]. The initial plates were cold rolled containing a typical rolling texture. The texture of A1050 plates is characterized by the {112} 〈111〉 copper and {110} 〈112〉 brass components [34–37] while Ti Gr. 2 forms a texture with the basal spread poles tiled away from the normal direction (ND) towards the transverse direction (TD) [38,39]. A close examination of texture after EXW reveals significant changes in the upper regions located closer to the detonation place. Especially, a strong texture transition took place in the upper A1050 plate, where the initial rolling texture transforms into a pure shear texture yielding mainly rotated cube and {111} 〈110〉 components, Figs. 2 and 3. 243
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Fig. 2. EBSD texture component maps of the upper and lower A1050 plates showing the distribution of the main texture components with the color coding according to the key figure given in Fig. 3. The particular components are determined with 10° deviation.
indicates a heavy deformation. The stored plastic and elastic energy which has been accumulated in the surface regions may be released in annealing process being driving force for the recrystallization and grain growth processes. This conclusion can be translated into the interface development during heat treatment, especially the growth kinetics of intermetallic phases, since by analogy, the stored energy is much higher in the upper than in the lower interface. Thus, analyzing the regions close to the bonding zones may provide information not only on the operating deformation mode during deformation but also may give some information about type and degree of deformation of the bonded interfaces. Especially, it seems to play a crucial role for analyzing
differences in texture and microstructure can be observed. The texture of so-called lower regions, located farther from the detonation site, remains practically unchanged compared to that of initial one. Only the microstructure is slightly affected since a higher number of twins or wavy shape grain arrangements can be revealed close to the interface. The situation is completely different with regard to the upper plates. The A1050 grains are characterized by elongated shape parallel to the detonation direction. Similar observation was reported in works of Akbari Mousavi et al. [17,41] and Zamani et al. [42]. Moreover, a higher twin concentration can be detected in the upper part of Ti Gr. 2 plate as well. The strong texture transformation in these regions also
Fig. 3. ODF section at φ2 = 45° of the upper and lower A1050 plates after EXW. Key figure gives the ideal components of rolling and shear components of fcc metals. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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Fig. 4. EBSD All Euler maps of the upper and lower part of Ti Gr. 2 plate showing the distribution of compression (green) and tension (red) twins. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
4. Conclusions Studying the texture evolution across the A1050/Ti Gr. 2/A1050 interfaces it can be concluded that joining by plastic deformation in EXW has a shear character. As a result shear deformation changes the initial brass/copper type texture of the upper A1050 plate to a typical shear texture yielding rotated cube and {111} 〈110〉 components while the texture of the lower A1050 plate remains practically unchanged. The EXW process causes a strong asymmetry of the basal poles for the upper part of Ti Gr. 2 resulting in a higher number of compression twins compared with that of the lower part. These strong distinctions should be especially considered if a post heat treatment is performed. Acknowledgments The authors thank to High Energy Technologies Works ‘Explomet’ (Opole, Poland) for providing good quality A1050/Ti Gr. 2/A1050 clads. Samples were examined in the Accredited Testing Laboratories at the Institute of Metallurgy and Materials Science of the Polish Academy of Sciences in Cracow. References Fig. 5. (0001) and (10–10) pole figures of the Ti Gr. 2 after EXW for the upper and lower part.
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Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Texture transformations near the bonding zones of the three-layer Al/Ti/Al explosively welded clads
MARK
R. Chulista,⁎, D.M. Fronczeka, Z. Szulcb, J. Wojewoda-Budkaa a b
Institute of Metallurgy and Materials Science, Polish Academy of Sciences, 25 Reymonta St., 30-059 Cracow, Poland High Energy Technologies Works ‘Explomet’, 100H Oswiecimska St., 45-641 Opole, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords: Aluminum Titanium Joining Explosive welding Texture EBSD
Al/Ti/Al explosively welded clads were analyzed by electron backscatter diffraction. Particular attention was drawn to texture evolution across the Al/Ti/Al interfaces. Significant changes in texture within the joined metals were conducted with the place of detonation of explosive material. The so-called base (lower) Al clad is characterized by a typical rolling texture exhibiting copper and brass components, while the flyer (upper) clad, located closer to the detonation wave front, transformed to a shear texture with rotated cube and {111} 〈110〉 components. The shear deformation also changes substantially the texture of Ti producing a much higher twin density. The observed changes unveil the effect of explosion on the microstructure of the joined metals and are discussed with respect to deformation mode and further annealing process.
1. Introduction Recently, extensive research has been focused on the explosive welding (EXW) method since it allows fabrication of multilayer materials such as Al/Ti [1–3], Al/Cu [4], Ni/Al [5,6], steel/Cu/Ti/Al [7] or Ti/Nb/Cu/steel [8] for various industrial applications. Additionally, successful attempts to manufacture biocompatible materials for the medical applications were also performed [9,10]. They can be manufactured using parallel [11,12] or inclined [13] geometry. EXW is classified as a cold forming process [14], therefore, the joined material is assumed to retain its pre-bond properties. However, extreme conditions such as high pressure, high velocity at the collision site and locally high temperature may influence not only the interface of two metals/alloys to be welded, but also the neighboring areas [15,16]. Unlike the so-called cold welding methods, where no melted or liquid phases are expected to occur, the interface obtained using EXW technique appears to have a relatively large nanocrystalline regions of a peninsula-like or island-like morphology [16–18]. It strongly suggests that EXW process exhibits locally non-equilibrium conditions deviating from the chemical equilibrium at solidification front [19–21]. Such metastable conditions favor a large variety of intermetallics being far from equilibrium [22]. Beside equilibrium and non-equilibrium intermediate phases also quasicrystals and amorphous phases can be observed in the interface zone [23–26]. Moreover, a typical solidification microstructure composed of columnar and equiaxed grains can be found at the interface in the state after EXW [11,17]. For all these
⁎
Corresponding author. E-mail address: [email protected] (R. Chulist).
http://dx.doi.org/10.1016/j.matchar.2017.05.007 Received 7 February 2017; Received in revised form 25 April 2017; Accepted 8 May 2017 Available online 08 May 2017 1044-5803/ © 2017 Elsevier Inc. All rights reserved.
reasons the location and strength of explosion seems to play a crucial role for the bonding process as well as the further accompanying heat treatment. Physical, mechanical or chemical properties of single grains vary with the crystallographic direction. Thus, a strong texture imparts significant anisotropy of physical properties [27]. On the other hand the preferred crystallographic orientation or texture of polycrystalline material may act as a good indicator for plastic deformation [28,29]. Grains have a tendency to group around typical texture components which is characteristic for a particular material and deformation mode. With a view to this, it is the main aim of this paper to correlate the texture and microstructural changes across the A1050/Ti Gr.2/A1050 samples with the place of detonation. It also gives an insight into the plastic deformation mode which takes place during EXW. The three layer system was chosen for the sake of clarity and to show significant changes between the so-called lower and the upper interfaces. 2. Material and Methods A1050 and Ti Gr. 2 clads with the dimension of 150 mm × 240 mm × 1 mm and 150 mm × 240 mm × 0.8 mm were used, respectively. The chemical composition of the alloys is given in Table 1. The initial plates were cold rolled. Such a three-layer, parallel arrangement was explosively welded in air atmosphere by the Explomet Company. As the base (lower) plate A1050 clad was applied while Ti Gr. 2 and A1050 (upper) were used for the flyer plates. Between the
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Table 1 Chemical composition of the A1050 and Ti Gr. 2 alloys. Alloy
A1050 Ti Gr. 2
Chemical composition [wt%] Fe
O
C
N
H
Si
Zn
Cu
Mg
Mn
Ti
< 0.4 < 0.3
– < 0.25
– < 0.1
– < 0.03
– < 0.015
< 0.25 –
< 0.07 –
< 0.06 –
< 0.05 –
< 0.05 –
< 0.05
Fig. 1. BSE images of the upper and lower interfaces of the Al/Ti/Al clad.
A1050 flyer plate and explosive material a silicon interlayer was placed in order to prevent the erosion of explosion (experimental set-up is given in [3], Fig. 1). Subsequently, samples with the dimensions of 6 mm × 12 mm × 2.8 mm were cut with respect to the rolling coordinate system from the central part of the three-layered sandwich. Abrasive papers (1000, 2000 and 5000) and diamond polishing pastes (6 μm, 3 μm, 1 μm and 0.25 μm) mixed with alumina were applied for surface preparation. The samples were then processed to obtain a proper surface quality for EBSD measurements using electropolishing process with the following parameters: electrolyte Struers A3, voltage = 35 V, T = 283 K, sample temperature = 77 K, polishing time = 15 s. Investigation of microstructure and texture were performed using a FEI Quanta 3D FEG scanning electron microscope equipped with the TSL EBSD system. The orientation distribution function (ODF) and pole figures were calculated using TSL software. The Euler angles were used in Bunge notation [30]. To account best for the orthorhombic symmetry of rolling and simple shear, the crystal and sample reference systems have been defined as X || rolling/shear direction (RD/SD), Y || normal direction (ND) and Z || transverse direction (TD). Concerning the specific texture components both for A1050 (copper, brass, rotated cube and shear components) and for Ti Gr. 2 (mainly basal texture) ODF and pole figures for A1050 and Ti Gr. 2 were chosen, respectively. Such a texture representation allowed for easy comparison of the so-called upper and lower sides of the materials.
Unlike the upper plate the lower one seems to be only slightly affected by EXW since only few grains with rotated cube orientations are observed in the immediate vicinity of the lower interface. This large distinction can be clearly seen in Fig. 2, where the brass (turquoise) and copper (magenta) components are mainly observed in the lower A1050 plate, while the upper one is dominated by rotated cube (green) and {111} 〈110〉 (orange/brown) shear components (color coding according to Fig. 3). In both A1050 clads, a wavy alignment of elongated grains can be observed, however, the wave character is much more pronounced within the upper A1050 plate, Fig. 2. The texture evolution of both A1050 plates can be understand using ODF. The information contained in the ODF section at φ2 = 45° shows a drastic change of texture between the upper and lower part. The initial copper and brass components change to rotated cube and {111} 〈110〉 components, respectively. The copper component is rotated about 15° around the TD direction (〈110〉) towards the rotated cube component. The same holds for the brass component, however, the rotation axis (TD) is parallel to 〈111〉 for this orientation. Both shear components slightly deviate from the ideal positions in positive φ1 direction which may be due to EXW causing a highly intense and rapid deformation. It should be mentioned that the rotation axis in a shear process is perpendicular to the shear plane and shear direction for the given orientation (for shear plane normal and shear direction indication see Fig. 3). EXW changes the initial texture and microstructure of the Ti Gr. 2 plate as well. The shape of titanium grains is more equiaxed and a high number of twins can be indicated. The EBSD map revealed strongly twinned microstructure with two split basal poles for both lower and upper part of Ti Gr. 2 plate, Figs. 4 and 5. The texture of the lower part resembles that of the initial one with a symmetric distribution of two basal poles, while the texture of the upper part is more asymmetric with one dominating component adjacent to ND, Fig. 5. The same tendency can be observed plotting compression and tension twins for both Ti Gr. 2 regions. The lower part exhibits a comparable fraction of the tension and compression twins (48/52 area fraction), whereas the upper part is overshadow with compression twins (21/79), Fig. 5. Since the plane strain deformation (rolling) can be expressed as the Tucker compression-tensile state [40] it seems to be consistent with the twin activity. On the other hand, explosion promotes compression twins due to high pressure during explosion causing permanent deformation at the interface. In summary, it can be concluded that comparing the regions nearby the upper and lower interfaces of A1050 and Ti Gr. 2 plates significant
3. Results and Discussion Fig. 1 presents a SEM image of the A1050/Ti Gr. 2/A1050 interfaces in the state obtained directly after EXW. It is clearly showed that no discontinuities, pores or impurities were observed along the bonding. This confirms the fact that during explosion an air jetting strips away all contaminations and impurities cleaning the metal surfaces. As a result a good bonding could be manufactured [31–33]. The initial plates were cold rolled containing a typical rolling texture. The texture of A1050 plates is characterized by the {112} 〈111〉 copper and {110} 〈112〉 brass components [34–37] while Ti Gr. 2 forms a texture with the basal spread poles tiled away from the normal direction (ND) towards the transverse direction (TD) [38,39]. A close examination of texture after EXW reveals significant changes in the upper regions located closer to the detonation place. Especially, a strong texture transition took place in the upper A1050 plate, where the initial rolling texture transforms into a pure shear texture yielding mainly rotated cube and {111} 〈110〉 components, Figs. 2 and 3. 243
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Fig. 2. EBSD texture component maps of the upper and lower A1050 plates showing the distribution of the main texture components with the color coding according to the key figure given in Fig. 3. The particular components are determined with 10° deviation.
indicates a heavy deformation. The stored plastic and elastic energy which has been accumulated in the surface regions may be released in annealing process being driving force for the recrystallization and grain growth processes. This conclusion can be translated into the interface development during heat treatment, especially the growth kinetics of intermetallic phases, since by analogy, the stored energy is much higher in the upper than in the lower interface. Thus, analyzing the regions close to the bonding zones may provide information not only on the operating deformation mode during deformation but also may give some information about type and degree of deformation of the bonded interfaces. Especially, it seems to play a crucial role for analyzing
differences in texture and microstructure can be observed. The texture of so-called lower regions, located farther from the detonation site, remains practically unchanged compared to that of initial one. Only the microstructure is slightly affected since a higher number of twins or wavy shape grain arrangements can be revealed close to the interface. The situation is completely different with regard to the upper plates. The A1050 grains are characterized by elongated shape parallel to the detonation direction. Similar observation was reported in works of Akbari Mousavi et al. [17,41] and Zamani et al. [42]. Moreover, a higher twin concentration can be detected in the upper part of Ti Gr. 2 plate as well. The strong texture transformation in these regions also
Fig. 3. ODF section at φ2 = 45° of the upper and lower A1050 plates after EXW. Key figure gives the ideal components of rolling and shear components of fcc metals. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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Fig. 4. EBSD All Euler maps of the upper and lower part of Ti Gr. 2 plate showing the distribution of compression (green) and tension (red) twins. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
4. Conclusions Studying the texture evolution across the A1050/Ti Gr. 2/A1050 interfaces it can be concluded that joining by plastic deformation in EXW has a shear character. As a result shear deformation changes the initial brass/copper type texture of the upper A1050 plate to a typical shear texture yielding rotated cube and {111} 〈110〉 components while the texture of the lower A1050 plate remains practically unchanged. The EXW process causes a strong asymmetry of the basal poles for the upper part of Ti Gr. 2 resulting in a higher number of compression twins compared with that of the lower part. These strong distinctions should be especially considered if a post heat treatment is performed. Acknowledgments The authors thank to High Energy Technologies Works ‘Explomet’ (Opole, Poland) for providing good quality A1050/Ti Gr. 2/A1050 clads. Samples were examined in the Accredited Testing Laboratories at the Institute of Metallurgy and Materials Science of the Polish Academy of Sciences in Cracow. References Fig. 5. (0001) and (10–10) pole figures of the Ti Gr. 2 after EXW for the upper and lower part.
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