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Microstructure and tensile properties of hot fogred high Nb containing TiAl based alloy with initial near lamellar microstructure
Materials Science & Engineering A 642 (2015) 16–21
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Microstructure and tensile properties of hot fogred high Nb containing TiAl based alloy with initial near lamellar microstructure S.Z. Zhang a,n, C.J. Zhang a, Z.X. Du b, Z.P. Hou a, P. Lin a, Y.Y. Chen c a
School of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China School of Materials Science and Engineering, Inner Mongolia University of Technology, Hohhot 010051, PR China c National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 16 May 2015 Received in revised form 21 June 2015 Accepted 22 June 2015 Available online 27 June 2015
High Nb containing TiAl based alloy with a nominal composition of Ti–44Al–8Nb–0.2W–0.2B–Y (in at%) exhibiting near lamellar microstructure was fabricated. Cylindrical blanks were quasi-isothermal forged at 1275 °C for 0.05 s 1, with a total reduction of 70%. The as-forged blanks were furnace-cooled (FCed) or air-cooled (ACed). The microstructure of the as-forged alloy mainly consists of remnant lamellar colonies (RL) and broken-down (BD) area caused by inhomogeneous deformation. Lamellar orientation and α2 lath thickness strongly affect the deformation of lamellar colonies. In the RL structure, the α2 phase s are remain in lath states and the γ laths recrystallized. The Blackburn relationship between the previous α2 lath and the γ lath in the RL structure is disrupted. Meanwhile, stress induced α-γ phase transformation generates the optimal perfect Blackburn relationship between the parent α2 lath and newborn γ grains. Cooling rate significantly affects the as-forged microstructure, thereby influencing the bulging rate of γ phase boundaries into α2 phases. Cooling rate also influences the tensile properties of the alloy. & 2015 Elsevier B.V. All rights reserved.
Keywords: EBSD Mechanical characterization Intermetallics Thermomechanical processing Recrystallization Orientation relationships
1. Introduction Titanium aluminide (TiAl) alloys have been considered for application as high temperature structural materials due to their combination of high melting point, low density, high specific strength, low diffusivity, good resistance against oxidation and corrosion, and high ignition resistance [1,2]. Especially, high Nb containing TiAl based alloy has improved tensile strength, creep strength as well as oxidation resistance than the ordinary TiAl alloy, has been focused on in recent studies [3–5]. However, Nb addition leads to a significant primary segregation of the alloying elements in traditional high Nb containig TiAl based alloy [6,7]. High Nb containing TiAl alloy with low Al content solidifies via the body centered cubic (bcc) β-phase, resulting in significant grain refinement of the casting, and reducing element segregation [8–10]. Moreover, B addition further refined grain size and reduced the segregation in β interdendritic region [11,12]. Currently, the microstructure of the as-cast high Nb containing TiAl based alloy is mainly composed of α2 phases, small lamellar colonies, and β phases along the grain boundaries, because of β-phase solidification and B addition [12]. However, the deformation behavior of the microstructure is not entirely consistent with that of the n
Corresponding author. E-mail address: [email protected] (S.Z. Zhang).
http://dx.doi.org/10.1016/j.msea.2015.06.066 0921-5093/& 2015 Elsevier B.V. All rights reserved.
traditional high Nb containing TiAl based alloy, cooling rate after hot forging affects the wrought microstructure and mechanical properties. The present work investigated the deformation behavior, microstructure evolution, and tensile properties of hot forged high Nb containing TiAl based alloy. The aim of this study was to clarify the deformation behavior and the effects of cooling rate on microstructure and mechanical properties of this high Nb containing TiAl based alloy with low Al content and solidifying via β-phase.
2. Experimental Cast ingot of the alloy Ti–44Al–8Nb–0.2W–0.2B–Y (in at%) with a diameter of 220 mm and a length of around 500 mm was fabricated by a Vacuum Arc Remelting (VAR) furnace, and subsequently followed by hot isostatic pressing (HIP) at 1300 °C and 130 MPa for 3 h under argon atmosphere. Cylindrical blanks with dimensions of ∅60 mm 100 mm were cut from the cast ingot by electric-discharge machining, and then canned in 304 stainless steel tubes with a thickness of 10 mm. The canned blank was firstly quasi-isothermal forged perpendicularly to the cylinder axis at 1275 °C and 0.05 s 1, with an engineering strain of 50%, then the canned blank with half reduction was annealing treated at 1275 °C for about 30 min, after that a secondary quasi-isothermal forging was carried out at 1275 °C and 0.05 s 1 still. The total
S.Z. Zhang et al. / Materials Science & Engineering A 642 (2015) 16–21
reduction of canned blank is about 70%. After forging, the canned plates were furnace cooled (FCed) or air cooled (ACed) in order to study the effects of cooling rate. After forging, the forged billets were sectioned along the compression axial. Microstructural samples were taken with an image plane parallel to the deformation axis of forged specimens and mechanical polished. The microstructure was characterized by scanning electron microscopy (SEM) using backscattered electrons (BSE). For SEM a Quanta 200FEG field-emission scanning electron microscope was used. Electron backscatter diffraction (EBSD) analyses were performed using a Hikari camera (EDAX) mounted in the FEI Quanta 200FEG scanning electron microscope, and specimens for EBSD were prepared by electropolishing at 19 V and 45 °C in a solution of 600 ml methanol, 300 ml n-butanol and 60 ml perchloric acid. All EBSD data were analyzed using IOM 5.31 software from EDAX-TSL. The room temperature tensile tests were conducted on Instron 5569 universal test machine in air with nominal strain rate of 4.0 10 5 s 1. The room temperature tensile specimens had a gauge section of 6 2 mm2 cross-section and 18 mm gauge length. High temperature tensile tests were performed at a nominal strain rate of 1.0 10 5 s 1 using an Instron 5500R testing machine. The high temperature tensile specimens with a gauge section of 5 2 mm2 cross-section and 20 mm gauge length were tested at temperatures ranging from 650 °C to 900 °C.
3. Results After HIP, the microstructure of this as-cast Ti–44Al–8Nb– 0.2W–0.2B–Y alloy was observed by SEM in BSE mode (Fig. 1). The alloy presents a uniform near lamellar microstructure and consists of relatively small lamellar colonies (average size of 80 mm), γ and α2 grains, a small amount of borides with a curvy morphology, and Al2Y in bulk particles. As the contrast of SEM in BSE mode is strongly affected by atomic number (‘Z’ number), the γ phase appears dark, the α2 phase exhibits an intermediate brightness and the Al2Y and borides are the brightest phases in BSE mode. The mixture of α2 and γ phases around the lamellar colonies with nearly no β (or B2) phase is found in this alloy, that is other than the B containing TiAl alloys solidifying solely via the β phase [13– 16]. This difference could be due to the high HIP temperature in α þ γ phase field that improves the diffusion of β-stabilizing elements, namely, Nb and W. As shown in Fig. 1, the alloy contain more α2 phases and thicker lamella spacing than that of traditional high Nb containing TiAl alloy, which reduces the blocking of interface to dislocation movement and is in favor of dynamic recrystallization (DRX) of coarsening γ laths [10,17–23]. The microstructure of the as-forged Ti–44Al–8Nb–0.2W–0.2B– Y alloy was illustrated by SEM in BSE mode and OM in Fig. 2. The forging axis is parallel to the length of pages. After hot forging, the material shows a fibrous microstructure at low magnification consisting of remnant lamellar colonies (RL) and broken down (BD) area, showing in Fig. 2a and b. The detailed analyses of the BD microstructure are shown in high magnification images shown in Fig. 2c and d. This BD area is a duplex microstructure, mainly consists of α2 phase in massive morphology or interrupted laths and recrystallized γ grains. The RL structures are aligned perpendicular to the forging axis and severely bent, seeing in Fig. 2a and b. In the RL structures, α2 laths are retained in their original states and γ laths are broken down and recrystallized (Fig. 2e and f), particularly in regions consisting of coarse α2 laths. And in the regions containing thin α2 laths (marked by arrows in Fig. 2e), α2 laths and γ laths are broken and recrystallized. Compared with the FCed microstructure, the ACed microstructure contains more α2 phases in the BD area, illustrating that cooling rate influenced the
17
Fig. 1. Microstructure of as-cast Ti–44Al–8Nb–0.2W–0.2B–Y alloy at HIPed state.
α2 phase content significantly in BD area. While the RL structures of FCed sample and ACed blank have no essential difference. The EBSD analysis results of the RL structure are shown in Fig. 3. In Fig. 3a, grain boundary orientation angles between 2° and 5° are marked with red lines, green lines indicate the orientation angles between 5° and 15° (angles below 15° were defined as lowangle grain boundaries), and blue lines denote the high-angle grain boundaries which have angles larger than 15°. And in Fig. 3c, phase overlapped with IQ map, α2 phase is represented by maroon, and γ phase is in dark green. Compared with the SEM results, the EBSD maps clearly show that γ laths in the RL structure are broken into small grains and recrystallized, whereas α2 laths remain in their original state. This phenomenon is further elucidated in Fig. 3b. Pole figures of different grains or subgrains were developed by EBSD technique to understand the orientation relationship between different grains. In Fig. 3c, the analyzed grains and the corresponding pole figures are serially numbered from one to nine. The orientation relationship between the α2 and γ laths in the optimal lamellar structure is the so called Blackburn and orientation relationship (i.e. (0001)α // (111)γ −
−
<1120 >α // < 110>γ ) [24,25]. However, the pole figures from the adjacent α2 and γ laths (subgrains) exhibit large misorientation from the perfect Blackburn orientation relationship (e.g. γ subgrains 3 and 4 and α2 lath 5), and even the absence of Blackburn orientation relationship (e.g. γ subgrains 8 and 9 and α2 lath 5). This result suggests that lamellae are heavily deformed during hot forging. The perfect Blackburn orientation relationship is observed between grains 1 and 2, as well as between grains 5 and 6. The γ grains with perfect Blackburn orientation relationship to their adjacent α2 laths are nucleated and grown in the α2 laths (e.g. γ grains 1 and 6 grown in the α2 laths 2 and 5). The FCed and ACed microstructure were analyzed by EBSD technique, as shown in Fig. 4. Compared with the FCed microstructure (Fig. 4f), the ACed microstructure contains many small grains surrounding larger grains (Fig. 4b). The corresponding grain size distribution histograms are shown in Fig. 4d and h. The grain size distribution of the ACed microstructure is asymmetrical, and the area fraction of grains with size larger than 4 μm is about 15%. Meanwhile, the grain size distribution of the FCed microstructure is symmetrical and the largest grain size is smaller than 3 μm. In Fig. 4c and g, red color represents the α2 phase, green color the γ
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S.Z. Zhang et al. / Materials Science & Engineering A 642 (2015) 16–21
Fig. 2. Microstructures of as-forged Ti–44Al–8Nb–0.2W–0.2B–Y alloy following different cooling rate: (a), (b) are OM images; (c), (d), (e), (f) are SEM images; (a), (c), (e) are the air cooled microstructures, respectively; (b), (d), (f) are the furnace cooled microstructures, respectively.
phase, and yellow color the B2 phase. Compared with the FCed microstructure, the ACed microstructure presents larger volume fraction of the α2 phase and smaller curvature of the γ phase grain boundaries. EBSD results further reveals that the volume fraction of the α2 phase in the ACed microstructure is about 23.7% and is approximately 4 times high than that in the FCed microstructure. Regardless of the FCed or ACed microstructure, the volume fraction of the α2 phase in as-forged alloy is lower than that of as-cast alloy. Fig. 5 shows the tensile properties of the as-forged Ti–44Al– 8Nb–0.2W–0.2B–Y alloy at different temperatures. As shown in Fig. 5a, the room temperature tensile strength (UTS) of the ACed and FCed microstructures can increase to 930 MPa and 885 MPa, respectively. The ACed structure presents a relatively higher tensile strength, whereas the tensile elongation (δ) of the as-forged Ti–44Al–8Nb–0.2W–0.2B–Y alloy does not increase., The difference of tensile strength between the ACed microstructure and the
FCed microstructures remarkably increases and reaches about 85 MPa when the tensile temperature is increased to 650 °C. The difference is still evidents but only about 15 MPa, as the temperature is increased to 900 °C. The difference is lessened with further increase in tensile temperature. Yield strength (YS) is also present, but the gap is considerably small at 750 °C, Moreover, no difference exists in tensile elongation (δ) between the ACed microstructure and FCed microstructures.
4. Discussion 4.1. RL area RL structure is an important component of the hot worked TiAl alloy. As illustrated by Fig. 2 and Fig. 3, the γ lath is broken and recrystallized, and the α2 laths, particularly coarse α2 laths still in
S.Z. Zhang et al. / Materials Science & Engineering A 642 (2015) 16–21
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Fig. 3. EBSD images of as-forged Ti–44Al–8Nb–0.2W–0.2B–Y alloy following air cooled process: (a) IQþ grain boundary map; (b) grain map; (c) phase þIQ map. The pole figure (1–9) are corresponded to the numbered grains or subgrains in (c) respectively. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
their prior states. The plastic behavior of the lamellar colonies is strongly affected by the angle φ between the lamellar boundaries
and the loading axis [22]. The yield stress is the highest when the loading axis is perpendicular to lamellar boundaries. In this
Fig. 4. EBSD images of BD structure in as-forged Ti–44Al–8Nb–0.2W–0.2B–Y alloy following different cooling rate: (a), (e) IQ map; (b), (f) grain map; (c), (g) phase map; (d), (h) gain size distribution. (a), (b), (c), (d) are the air cooled structures, and (e), (f), (g), (h) are the furnace cooled structures. (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. 5. Tensile properties of as-forged Ti–44Al–8Nb–0.2W–0.2B–Y alloy at different temperature: (a) room temperature and (b) high temperature.
direction, the α2 phase is deformed in its hardest slip mode, and deformation must propagate through the lamellar interfaces, then this lamellar colony is in its hard orientation and will be remnant as RL structure [3–10,10–26]. Thus, the α2 phase significantly affects the RL structure. The lamellar colony in hard orientation, which consists of coarse α and thin γ laths can be almost completely preserved [22]. In this study, the as-cast Ti–44Al–8Nb– 0.2W–0.2B–Y alloy contains lot of coarse α2 laths, which generates more RL structure than traditional TiAl alloys. As shown in Fig. 2 and Fig. 3, γ laths in the RL structure are recrystallized already. Moreover, the remnant lamellar structure in hot worked Ti–48Al– 2Cr alloy has been reported to exhibit recrystallization of γ lamellae and spheroidization of α2 laths in regions with high local strain, whereas α2 laths in the other regions are retained at their prior states [27]. Zhang reported that the dislocation density in fine γ laths is considerable lower than that in coarse laths, and the extent of deformation and recrystallization of relatively coarse γ laths is higher than that of fine γ lath [22]. Moreover, the stacking fault energy of α2 phase is higher than that of γ phase [10,10–28]. At this point, the α phase tends to undergo dynamic recovery (DRV) during deformation in the α þ γ phase field, whereas DRX is the main deformation mechanism of the γ phase. This phenomenon causes the α2 phase to remain in its prior state and γ phase recrystallization. Coarse γ laths and thin α2 laths in lamellar colonies should be obtained to achieve effective recrystallization. After hot forging (Fig. 3), a certain misorientation or absence of the perfect Blackburn orientation relationship is detected between the previous adjacent α2 lath and γ lath; this is the result of severe inhomogeneous deformation of lamellae during hot forging. In fact, γ laths are converted into subgrains (or grains) with different misorientations relative to their prior orientations, whereas α2 laths remain in their original orientation. A perfect Blackburn orientation relationship is also detected in Fig. 3 by the corresponding pole figure. But it should be noted that the perfect Blackburn orientation relationship between the parent α2 lath and newborn γ grains is further observed in area where the γ phase is nucleated and grown in the α2 laths. The precipitation of the γ phase from the α2 phase can be attributed to stress induced phase transformation [29]. It has been revealed that the HCP-FCC structure change (α-γ transformation) could be brought about if −
a/3 < 1010> Shockley partials travel on alternate basal planes of −
the HCP phase. Hence, gliding of the a/3 < 1010> partial plays an important role in α-γ transformation [30]. In this way, external
stress could induce α-γ phase transformation during hot forging of the Ti–44Al–8Nb–0.2W–0.2B–Y alloy. This phenomenon has also been reported by other scholars [3,31,32].
4.2. Cooling rate The as-forged microstructural morphology and phase composition are significantly affected by cooling rate. As the ACed specimen has a relatively fast cooling rate, fine grains cannot combine and grow into big ones, and the substructure in deformed grains cannot be completely eliminated. The ACed specimen demonstrates a wider grain size distribution and finer grains than the FCed blanks. The curvature of the grain boundaries between the γ phase and the adjacent α phase suggests that the phase (grain) boundary bulging (Fig. 4c and g) is the mechanism underlying the growth of γ phases into the α phase after hot forging (during furnace and air cooling process). Meanwhile, solid state phase transformation is controlled by diffusion which is strongly affected by cooling rate [33,34]. The higher the cooling rate, the more difficult the diffusion, and then α-γ solid phase transformation is inhibited. It is the reason why ACed specimen contains more α2 phases than that of FCed specimens. The progress of boundary bulging can lead to the reduction of α2 grains (Fig. 4) and the breakdown of α2 laths [22–35]. Cooling rate nearly does not affect tensile elongation, but sharply affects the tensile strength. With the increase of cooling rate, the residual stress and dislocation density of the ACed specimen are higher than that of the FCed material, i.e., the ACed specimen has a better work hardening effect than that of FCed blank. During furnace cooling, softening mechanism, such as static recovery, static recrystallization and meta dynamic recrystallization, can release work hardening, and finally reduce tensile strength [5,22,36]. However, the difference of room temperature tensile strength between the ACed blank and the FCed specimen is not as high as that of the alloy tensiled at 650 °C, because room temperature elongation of this as-forged high Nb containing TiAl based alloy is insufficient. With the tensile temperature increase, the tensiled specimen of ACed can be softened during tensile process and the work hardening effect releases obviously, and the difference of tensile strength between these two cooling rate specimens falls.
S.Z. Zhang et al. / Materials Science & Engineering A 642 (2015) 16–21
5. Conclusion An ingot of Ti–44Al–8Nb–0.2W–0.2B–Y alloy with near lamellar microstructure was fabricated. The microstructure of the as-forged Ti–44Al–8Nb–0.2W–0.2B–Y alloy, the influence of cooling rate on the as-forged microstructural morphology and phase composition and tensile properties of the alloy are discussed. The main conclusions are summarized as follows: 1. The microstructure of the hot forged Ti–44Al–8Nb–0.2W–0.2B– Y alloy mainly consists of remnant RL and BD area. The BD area is a duplex microstructure, and composed of recrystallized γ grains and α2 phase in massive morphology or interrupted laths. 2. In the RL structure, α2 laths are retained in their prior states and γ laths are recrystallized. Coarse α2 laths are difficult to be deformed. The perfect Blackburn relationship of α2 lath and γ laths in the RL structure is disrupted by severe inhomogeneous deformation. The γ phase is nucleated and grown in the α2 laths, resulting in the perfect Blackburn relationship, caused by stress induced α-γ phase transformation. 3. The as-forged microstructure is significantly affected by cooling rate. The ACed microstructure contains lots of small grains surrounding large grains, and presents larger volume fraction of the α2 phase and smaller curvature of γ phase grain boundaries. 4. Phase (grain) boundary bulging is the main mechanism of γ phases growing into α2 phases, which is strongly affected by cooling rate. The tensile strength of the ACed specimen is higher than that of the FCed blank, because of high cooling rate improves the work hardening effect.
Acknowledgments This work was financially supported by the State Key Laboratory for Advanced Metal and Materials Foundation (Nos. 2014ZD06 and 2013-ZD06), the Special/Youth Foundation of Taiyuan University of Technology (No. 2013T004) and the Qualified Personnel Foundation of Taiyuan University of Technology (No: tyutrc201342a).
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Microstructure and tensile properties of hot fogred high Nb containing TiAl based alloy with initial near lamellar microstructure S.Z. Zhang a,n, C.J. Zhang a, Z.X. Du b, Z.P. Hou a, P. Lin a, Y.Y. Chen c a
School of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China School of Materials Science and Engineering, Inner Mongolia University of Technology, Hohhot 010051, PR China c National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 16 May 2015 Received in revised form 21 June 2015 Accepted 22 June 2015 Available online 27 June 2015
High Nb containing TiAl based alloy with a nominal composition of Ti–44Al–8Nb–0.2W–0.2B–Y (in at%) exhibiting near lamellar microstructure was fabricated. Cylindrical blanks were quasi-isothermal forged at 1275 °C for 0.05 s 1, with a total reduction of 70%. The as-forged blanks were furnace-cooled (FCed) or air-cooled (ACed). The microstructure of the as-forged alloy mainly consists of remnant lamellar colonies (RL) and broken-down (BD) area caused by inhomogeneous deformation. Lamellar orientation and α2 lath thickness strongly affect the deformation of lamellar colonies. In the RL structure, the α2 phase s are remain in lath states and the γ laths recrystallized. The Blackburn relationship between the previous α2 lath and the γ lath in the RL structure is disrupted. Meanwhile, stress induced α-γ phase transformation generates the optimal perfect Blackburn relationship between the parent α2 lath and newborn γ grains. Cooling rate significantly affects the as-forged microstructure, thereby influencing the bulging rate of γ phase boundaries into α2 phases. Cooling rate also influences the tensile properties of the alloy. & 2015 Elsevier B.V. All rights reserved.
Keywords: EBSD Mechanical characterization Intermetallics Thermomechanical processing Recrystallization Orientation relationships
1. Introduction Titanium aluminide (TiAl) alloys have been considered for application as high temperature structural materials due to their combination of high melting point, low density, high specific strength, low diffusivity, good resistance against oxidation and corrosion, and high ignition resistance [1,2]. Especially, high Nb containing TiAl based alloy has improved tensile strength, creep strength as well as oxidation resistance than the ordinary TiAl alloy, has been focused on in recent studies [3–5]. However, Nb addition leads to a significant primary segregation of the alloying elements in traditional high Nb containig TiAl based alloy [6,7]. High Nb containing TiAl alloy with low Al content solidifies via the body centered cubic (bcc) β-phase, resulting in significant grain refinement of the casting, and reducing element segregation [8–10]. Moreover, B addition further refined grain size and reduced the segregation in β interdendritic region [11,12]. Currently, the microstructure of the as-cast high Nb containing TiAl based alloy is mainly composed of α2 phases, small lamellar colonies, and β phases along the grain boundaries, because of β-phase solidification and B addition [12]. However, the deformation behavior of the microstructure is not entirely consistent with that of the n
Corresponding author. E-mail address: [email protected] (S.Z. Zhang).
http://dx.doi.org/10.1016/j.msea.2015.06.066 0921-5093/& 2015 Elsevier B.V. All rights reserved.
traditional high Nb containing TiAl based alloy, cooling rate after hot forging affects the wrought microstructure and mechanical properties. The present work investigated the deformation behavior, microstructure evolution, and tensile properties of hot forged high Nb containing TiAl based alloy. The aim of this study was to clarify the deformation behavior and the effects of cooling rate on microstructure and mechanical properties of this high Nb containing TiAl based alloy with low Al content and solidifying via β-phase.
2. Experimental Cast ingot of the alloy Ti–44Al–8Nb–0.2W–0.2B–Y (in at%) with a diameter of 220 mm and a length of around 500 mm was fabricated by a Vacuum Arc Remelting (VAR) furnace, and subsequently followed by hot isostatic pressing (HIP) at 1300 °C and 130 MPa for 3 h under argon atmosphere. Cylindrical blanks with dimensions of ∅60 mm 100 mm were cut from the cast ingot by electric-discharge machining, and then canned in 304 stainless steel tubes with a thickness of 10 mm. The canned blank was firstly quasi-isothermal forged perpendicularly to the cylinder axis at 1275 °C and 0.05 s 1, with an engineering strain of 50%, then the canned blank with half reduction was annealing treated at 1275 °C for about 30 min, after that a secondary quasi-isothermal forging was carried out at 1275 °C and 0.05 s 1 still. The total
S.Z. Zhang et al. / Materials Science & Engineering A 642 (2015) 16–21
reduction of canned blank is about 70%. After forging, the canned plates were furnace cooled (FCed) or air cooled (ACed) in order to study the effects of cooling rate. After forging, the forged billets were sectioned along the compression axial. Microstructural samples were taken with an image plane parallel to the deformation axis of forged specimens and mechanical polished. The microstructure was characterized by scanning electron microscopy (SEM) using backscattered electrons (BSE). For SEM a Quanta 200FEG field-emission scanning electron microscope was used. Electron backscatter diffraction (EBSD) analyses were performed using a Hikari camera (EDAX) mounted in the FEI Quanta 200FEG scanning electron microscope, and specimens for EBSD were prepared by electropolishing at 19 V and 45 °C in a solution of 600 ml methanol, 300 ml n-butanol and 60 ml perchloric acid. All EBSD data were analyzed using IOM 5.31 software from EDAX-TSL. The room temperature tensile tests were conducted on Instron 5569 universal test machine in air with nominal strain rate of 4.0 10 5 s 1. The room temperature tensile specimens had a gauge section of 6 2 mm2 cross-section and 18 mm gauge length. High temperature tensile tests were performed at a nominal strain rate of 1.0 10 5 s 1 using an Instron 5500R testing machine. The high temperature tensile specimens with a gauge section of 5 2 mm2 cross-section and 20 mm gauge length were tested at temperatures ranging from 650 °C to 900 °C.
3. Results After HIP, the microstructure of this as-cast Ti–44Al–8Nb– 0.2W–0.2B–Y alloy was observed by SEM in BSE mode (Fig. 1). The alloy presents a uniform near lamellar microstructure and consists of relatively small lamellar colonies (average size of 80 mm), γ and α2 grains, a small amount of borides with a curvy morphology, and Al2Y in bulk particles. As the contrast of SEM in BSE mode is strongly affected by atomic number (‘Z’ number), the γ phase appears dark, the α2 phase exhibits an intermediate brightness and the Al2Y and borides are the brightest phases in BSE mode. The mixture of α2 and γ phases around the lamellar colonies with nearly no β (or B2) phase is found in this alloy, that is other than the B containing TiAl alloys solidifying solely via the β phase [13– 16]. This difference could be due to the high HIP temperature in α þ γ phase field that improves the diffusion of β-stabilizing elements, namely, Nb and W. As shown in Fig. 1, the alloy contain more α2 phases and thicker lamella spacing than that of traditional high Nb containing TiAl alloy, which reduces the blocking of interface to dislocation movement and is in favor of dynamic recrystallization (DRX) of coarsening γ laths [10,17–23]. The microstructure of the as-forged Ti–44Al–8Nb–0.2W–0.2B– Y alloy was illustrated by SEM in BSE mode and OM in Fig. 2. The forging axis is parallel to the length of pages. After hot forging, the material shows a fibrous microstructure at low magnification consisting of remnant lamellar colonies (RL) and broken down (BD) area, showing in Fig. 2a and b. The detailed analyses of the BD microstructure are shown in high magnification images shown in Fig. 2c and d. This BD area is a duplex microstructure, mainly consists of α2 phase in massive morphology or interrupted laths and recrystallized γ grains. The RL structures are aligned perpendicular to the forging axis and severely bent, seeing in Fig. 2a and b. In the RL structures, α2 laths are retained in their original states and γ laths are broken down and recrystallized (Fig. 2e and f), particularly in regions consisting of coarse α2 laths. And in the regions containing thin α2 laths (marked by arrows in Fig. 2e), α2 laths and γ laths are broken and recrystallized. Compared with the FCed microstructure, the ACed microstructure contains more α2 phases in the BD area, illustrating that cooling rate influenced the
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Fig. 1. Microstructure of as-cast Ti–44Al–8Nb–0.2W–0.2B–Y alloy at HIPed state.
α2 phase content significantly in BD area. While the RL structures of FCed sample and ACed blank have no essential difference. The EBSD analysis results of the RL structure are shown in Fig. 3. In Fig. 3a, grain boundary orientation angles between 2° and 5° are marked with red lines, green lines indicate the orientation angles between 5° and 15° (angles below 15° were defined as lowangle grain boundaries), and blue lines denote the high-angle grain boundaries which have angles larger than 15°. And in Fig. 3c, phase overlapped with IQ map, α2 phase is represented by maroon, and γ phase is in dark green. Compared with the SEM results, the EBSD maps clearly show that γ laths in the RL structure are broken into small grains and recrystallized, whereas α2 laths remain in their original state. This phenomenon is further elucidated in Fig. 3b. Pole figures of different grains or subgrains were developed by EBSD technique to understand the orientation relationship between different grains. In Fig. 3c, the analyzed grains and the corresponding pole figures are serially numbered from one to nine. The orientation relationship between the α2 and γ laths in the optimal lamellar structure is the so called Blackburn and orientation relationship (i.e. (0001)α // (111)γ −
−
<1120 >α // < 110>γ ) [24,25]. However, the pole figures from the adjacent α2 and γ laths (subgrains) exhibit large misorientation from the perfect Blackburn orientation relationship (e.g. γ subgrains 3 and 4 and α2 lath 5), and even the absence of Blackburn orientation relationship (e.g. γ subgrains 8 and 9 and α2 lath 5). This result suggests that lamellae are heavily deformed during hot forging. The perfect Blackburn orientation relationship is observed between grains 1 and 2, as well as between grains 5 and 6. The γ grains with perfect Blackburn orientation relationship to their adjacent α2 laths are nucleated and grown in the α2 laths (e.g. γ grains 1 and 6 grown in the α2 laths 2 and 5). The FCed and ACed microstructure were analyzed by EBSD technique, as shown in Fig. 4. Compared with the FCed microstructure (Fig. 4f), the ACed microstructure contains many small grains surrounding larger grains (Fig. 4b). The corresponding grain size distribution histograms are shown in Fig. 4d and h. The grain size distribution of the ACed microstructure is asymmetrical, and the area fraction of grains with size larger than 4 μm is about 15%. Meanwhile, the grain size distribution of the FCed microstructure is symmetrical and the largest grain size is smaller than 3 μm. In Fig. 4c and g, red color represents the α2 phase, green color the γ
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Fig. 2. Microstructures of as-forged Ti–44Al–8Nb–0.2W–0.2B–Y alloy following different cooling rate: (a), (b) are OM images; (c), (d), (e), (f) are SEM images; (a), (c), (e) are the air cooled microstructures, respectively; (b), (d), (f) are the furnace cooled microstructures, respectively.
phase, and yellow color the B2 phase. Compared with the FCed microstructure, the ACed microstructure presents larger volume fraction of the α2 phase and smaller curvature of the γ phase grain boundaries. EBSD results further reveals that the volume fraction of the α2 phase in the ACed microstructure is about 23.7% and is approximately 4 times high than that in the FCed microstructure. Regardless of the FCed or ACed microstructure, the volume fraction of the α2 phase in as-forged alloy is lower than that of as-cast alloy. Fig. 5 shows the tensile properties of the as-forged Ti–44Al– 8Nb–0.2W–0.2B–Y alloy at different temperatures. As shown in Fig. 5a, the room temperature tensile strength (UTS) of the ACed and FCed microstructures can increase to 930 MPa and 885 MPa, respectively. The ACed structure presents a relatively higher tensile strength, whereas the tensile elongation (δ) of the as-forged Ti–44Al–8Nb–0.2W–0.2B–Y alloy does not increase., The difference of tensile strength between the ACed microstructure and the
FCed microstructures remarkably increases and reaches about 85 MPa when the tensile temperature is increased to 650 °C. The difference is still evidents but only about 15 MPa, as the temperature is increased to 900 °C. The difference is lessened with further increase in tensile temperature. Yield strength (YS) is also present, but the gap is considerably small at 750 °C, Moreover, no difference exists in tensile elongation (δ) between the ACed microstructure and FCed microstructures.
4. Discussion 4.1. RL area RL structure is an important component of the hot worked TiAl alloy. As illustrated by Fig. 2 and Fig. 3, the γ lath is broken and recrystallized, and the α2 laths, particularly coarse α2 laths still in
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Fig. 3. EBSD images of as-forged Ti–44Al–8Nb–0.2W–0.2B–Y alloy following air cooled process: (a) IQþ grain boundary map; (b) grain map; (c) phase þIQ map. The pole figure (1–9) are corresponded to the numbered grains or subgrains in (c) respectively. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
their prior states. The plastic behavior of the lamellar colonies is strongly affected by the angle φ between the lamellar boundaries
and the loading axis [22]. The yield stress is the highest when the loading axis is perpendicular to lamellar boundaries. In this
Fig. 4. EBSD images of BD structure in as-forged Ti–44Al–8Nb–0.2W–0.2B–Y alloy following different cooling rate: (a), (e) IQ map; (b), (f) grain map; (c), (g) phase map; (d), (h) gain size distribution. (a), (b), (c), (d) are the air cooled structures, and (e), (f), (g), (h) are the furnace cooled structures. (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. 5. Tensile properties of as-forged Ti–44Al–8Nb–0.2W–0.2B–Y alloy at different temperature: (a) room temperature and (b) high temperature.
direction, the α2 phase is deformed in its hardest slip mode, and deformation must propagate through the lamellar interfaces, then this lamellar colony is in its hard orientation and will be remnant as RL structure [3–10,10–26]. Thus, the α2 phase significantly affects the RL structure. The lamellar colony in hard orientation, which consists of coarse α and thin γ laths can be almost completely preserved [22]. In this study, the as-cast Ti–44Al–8Nb– 0.2W–0.2B–Y alloy contains lot of coarse α2 laths, which generates more RL structure than traditional TiAl alloys. As shown in Fig. 2 and Fig. 3, γ laths in the RL structure are recrystallized already. Moreover, the remnant lamellar structure in hot worked Ti–48Al– 2Cr alloy has been reported to exhibit recrystallization of γ lamellae and spheroidization of α2 laths in regions with high local strain, whereas α2 laths in the other regions are retained at their prior states [27]. Zhang reported that the dislocation density in fine γ laths is considerable lower than that in coarse laths, and the extent of deformation and recrystallization of relatively coarse γ laths is higher than that of fine γ lath [22]. Moreover, the stacking fault energy of α2 phase is higher than that of γ phase [10,10–28]. At this point, the α phase tends to undergo dynamic recovery (DRV) during deformation in the α þ γ phase field, whereas DRX is the main deformation mechanism of the γ phase. This phenomenon causes the α2 phase to remain in its prior state and γ phase recrystallization. Coarse γ laths and thin α2 laths in lamellar colonies should be obtained to achieve effective recrystallization. After hot forging (Fig. 3), a certain misorientation or absence of the perfect Blackburn orientation relationship is detected between the previous adjacent α2 lath and γ lath; this is the result of severe inhomogeneous deformation of lamellae during hot forging. In fact, γ laths are converted into subgrains (or grains) with different misorientations relative to their prior orientations, whereas α2 laths remain in their original orientation. A perfect Blackburn orientation relationship is also detected in Fig. 3 by the corresponding pole figure. But it should be noted that the perfect Blackburn orientation relationship between the parent α2 lath and newborn γ grains is further observed in area where the γ phase is nucleated and grown in the α2 laths. The precipitation of the γ phase from the α2 phase can be attributed to stress induced phase transformation [29]. It has been revealed that the HCP-FCC structure change (α-γ transformation) could be brought about if −
a/3 < 1010> Shockley partials travel on alternate basal planes of −
the HCP phase. Hence, gliding of the a/3 < 1010> partial plays an important role in α-γ transformation [30]. In this way, external
stress could induce α-γ phase transformation during hot forging of the Ti–44Al–8Nb–0.2W–0.2B–Y alloy. This phenomenon has also been reported by other scholars [3,31,32].
4.2. Cooling rate The as-forged microstructural morphology and phase composition are significantly affected by cooling rate. As the ACed specimen has a relatively fast cooling rate, fine grains cannot combine and grow into big ones, and the substructure in deformed grains cannot be completely eliminated. The ACed specimen demonstrates a wider grain size distribution and finer grains than the FCed blanks. The curvature of the grain boundaries between the γ phase and the adjacent α phase suggests that the phase (grain) boundary bulging (Fig. 4c and g) is the mechanism underlying the growth of γ phases into the α phase after hot forging (during furnace and air cooling process). Meanwhile, solid state phase transformation is controlled by diffusion which is strongly affected by cooling rate [33,34]. The higher the cooling rate, the more difficult the diffusion, and then α-γ solid phase transformation is inhibited. It is the reason why ACed specimen contains more α2 phases than that of FCed specimens. The progress of boundary bulging can lead to the reduction of α2 grains (Fig. 4) and the breakdown of α2 laths [22–35]. Cooling rate nearly does not affect tensile elongation, but sharply affects the tensile strength. With the increase of cooling rate, the residual stress and dislocation density of the ACed specimen are higher than that of the FCed material, i.e., the ACed specimen has a better work hardening effect than that of FCed blank. During furnace cooling, softening mechanism, such as static recovery, static recrystallization and meta dynamic recrystallization, can release work hardening, and finally reduce tensile strength [5,22,36]. However, the difference of room temperature tensile strength between the ACed blank and the FCed specimen is not as high as that of the alloy tensiled at 650 °C, because room temperature elongation of this as-forged high Nb containing TiAl based alloy is insufficient. With the tensile temperature increase, the tensiled specimen of ACed can be softened during tensile process and the work hardening effect releases obviously, and the difference of tensile strength between these two cooling rate specimens falls.
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5. Conclusion An ingot of Ti–44Al–8Nb–0.2W–0.2B–Y alloy with near lamellar microstructure was fabricated. The microstructure of the as-forged Ti–44Al–8Nb–0.2W–0.2B–Y alloy, the influence of cooling rate on the as-forged microstructural morphology and phase composition and tensile properties of the alloy are discussed. The main conclusions are summarized as follows: 1. The microstructure of the hot forged Ti–44Al–8Nb–0.2W–0.2B– Y alloy mainly consists of remnant RL and BD area. The BD area is a duplex microstructure, and composed of recrystallized γ grains and α2 phase in massive morphology or interrupted laths. 2. In the RL structure, α2 laths are retained in their prior states and γ laths are recrystallized. Coarse α2 laths are difficult to be deformed. The perfect Blackburn relationship of α2 lath and γ laths in the RL structure is disrupted by severe inhomogeneous deformation. The γ phase is nucleated and grown in the α2 laths, resulting in the perfect Blackburn relationship, caused by stress induced α-γ phase transformation. 3. The as-forged microstructure is significantly affected by cooling rate. The ACed microstructure contains lots of small grains surrounding large grains, and presents larger volume fraction of the α2 phase and smaller curvature of γ phase grain boundaries. 4. Phase (grain) boundary bulging is the main mechanism of γ phases growing into α2 phases, which is strongly affected by cooling rate. The tensile strength of the ACed specimen is higher than that of the FCed blank, because of high cooling rate improves the work hardening effect.
Acknowledgments This work was financially supported by the State Key Laboratory for Advanced Metal and Materials Foundation (Nos. 2014ZD06 and 2013-ZD06), the Special/Youth Foundation of Taiyuan University of Technology (No. 2013T004) and the Qualified Personnel Foundation of Taiyuan University of Technology (No: tyutrc201342a).
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