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Microstructure and high temperature mechanical properties of powder metallurgical Ti-45Al-7Nb-0.3W alloy sheets
Materials and Design 106 (2016) 90–97
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Microstructure and high temperature mechanical properties of powder metallurgical Ti-45Al-7Nb-0.3W alloy sheets Huizhong Li a,b, Yelong Qi a, Xiaopeng Liang a,b,⁎, Zexiao Zhu a, Feng Lv a, Yong Liu b, Yi Yang a a b
School of Materials Science and Engineering, Central South University, Changsha 410083, China State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 19 April 2016 Received in revised form 27 May 2016 Accepted 29 May 2016 Available online 31 May 2016 Keywords: TiAl alloy Microstructure High temperature mechanical property Superplastic deformation Deformation mechanism
a b s t r a c t The microstructure, high temperature mechanical properties and deformation mechanism of powder metallurgical (PM) Ti-45Al-7Nb-0.3W (at.%) alloy sheets were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The tensile test results showed excellent high temperature mechanical properties with a yield stress (YS) of 670 MPa and an ultimate tensile stress (UTS) of 874 MPa at 700 °C and displayed an anomalous strengthening effect, which could be explained by the pinning mechanism of dislocation locks. In addition, the brittle-ductile transition temperature (BDTT) was found at between 800 °C and 850 °C. When deformed below BDTT, high density of dislocations and mechanical twins were observed in the microstructures. However, with increasing deformation temperature above BDTT, kinking of α2/γ lamellar colonies and dynamic recrystallization also occurred. It was noteworthy that the as-rolled sheets displayed superplastic behaviors at 950 °C with initial strain rates of 1 × 10−4 s−1 and 5 × 10−5 s−1. At the same time, severe dynamic recrystallization took place and the grain boundary sliding was improved by β phase, which resulted in an elongation of 243% at the strain rate of 5 × 10−5 s−1. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction TiAl based alloys are considered highly promising high temperature structural materials as a substitute for nickel based alloys because of their attractive properties such as low density, good creep resistance and good high temperature strength etc. [1–4]. TiAl sheets are extremely promising in aerospace applications, including turbine blades, nozzles for helicopters, and back structures for scramjets etc. [5,6]. The high temperature mechanical properties are key performance indexes of TiAl based alloys and there is a continuing need for improving them in order to make those alloys an attractive alternative for conventional nickel based alloys [7,8]. So far, considerable efforts have been devoted to investigating the high temperature mechanical properties of TiAl based alloys. For example, high Nb containing TiAl alloys have been widely studied recently [9–11]. Compared with conventional TiAl alloys, high Nb containing TiAl alloys display remarkably increased mechanical strength from room temperature to 800 °C, which is ascribed to the solution hardening of Nb and microstructural refinement [12,13]. J.D.H. Paul et al. [14] found that Ti-45Al-10Nb alloys and Ti45Al-5Nb alloys had flow stress values of over 800 MPa at room temperature and over 500 MPa at 900 °C in compression process. In
⁎ Corresponding author at: State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China. E-mail address: [email protected] (X. Liang).
http://dx.doi.org/10.1016/j.matdes.2016.05.113 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
comparison to a conventional Ti-47Al-2Cr-0.2Si alloy, they showed a considerable increase in strength. Nevertheless, the high addition of Nb may bring some problems as well, such as poor hot deformability and low room temperature ductility [12,15]. Thus, more studies on high Nb containing TiAl alloys should be carried out. It is generally known that the deformation behavior of TiAl based alloys is very complicated because the microstructure of TiAl based alloys usually consists of multiple phases and structures which result in different deformabilities [16,17]. Therefore, there are numerous mechanisms associated with high temperature properties working simultaneously and synergistically during the deformation [7]. Y.W. Kim et al. developed Ti-45Al-5Nb sheets by pack-rolling through ingot metallurgical (IM) process. And they found that the sheets were strengthened by unstable structures, as well as grain-boundary strengthening and γ phase solution strengthening [18]. Up to now, many studies have been performed on TiAl alloy. And in our previous work, powder metallurgical TiAl sheets have already been produced by hot pack-rolling [19,20]. However, microstructure and high temperature mechanical properties of hot pack-rolled high Nb containing PM TiAl sheets need to be further studied. In this work, the PM Ti-45Al-7Nb-0.3W (at.%) alloy sheets were fabricated by hot pack-rolling. Tensile tests under different conditions were conducted to investigate the microstructure and high temperature mechanical properties of TiAl sheets. Besides, high temperature deformation mechanism and superplastic deformation behaviors at 950 °C were also analyzed in detail.
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2. Experimental procedures The alloy with nominal composition of Ti-45Al-7Nb-0.3W (at.%) was prepared by powder metallurgy. Plates measuring 50 × 40 × 8 mm3 were machined from the hot isostatic pressed billet by electric-discharge machining. Then they were encapsulated in 1.5 mm thick pure Ti cans. The specimens were hot rolled on a hot mill with roller dimension of Ф180 mm × 320 mm. And the rollers were preheated at 250– 300 °C. After heating to the rolling temperature of 1270 °C and holding for 60 min, TiAl sheets were hot pack-rolled with rolling speed of 40 mm/s and a reduction of 10% per pass. And the sheets were held for 5–8 min between each two rolling pass. After rolling, 4 mm thick sheets were obtained and they were then treated at 900 °C for 1 h followed by air cooling. Flat tensile test specimens with a gauge section of 8 mm × 3.4 mm × 2.8 mm were cut in rolling direction from the sheets. High temperature tensile tests at different temperatures ranging from room temperature (20 °C) to 950 °C were conducted on an Instron 3369 universal test machine in air with an initial strain rate of 1 × 10−3 s−1. Superplastic deformation behaviors at 950 °C with initial strain rates of 1 × 10−4 s−1 and 5 × 10−5 s−1 were also investigated. All specimens were held at test temperature for 10 min before testing. Once the tensile specimens broke, they were treated by forced air cooling. So the state of high temperature in the specimens could be kept. For each condition, at least three tests were performed and average values were reported. The grain size of γ phase and α2/γ lamellae was calculated by a software Image-Pro Plus6.0. The phase composition was determined using Cu Kα radiation on a D/Max 2500 X-ray diffractometer. And the deformed microstructures and fracture surfaces of the tensile specimens were analyzed using a Sirion200 scanning electron microscope (SEM). TEM foils were prepared by mechanical polishing and twin-jet electropolishing using a solution of 6% perchloric acid + 34% butanol + 60% methanol at − 20 °C and 25 V, and then observed in a Tecnai G220 microscope operating at 200 kV.
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is distributed at the grain boundaries of γ phase and α2/γ lamellar colonies as can be seen from the picture. 3.2. High temperature mechanical properties The tensile mechanical properties of the as-rolled PM Ti-45Al-7Nb0.3W (at.%) alloy sheets with an initial strain rate of 1 × 10−3 s−1 are presented in Fig. 2. As shown, the variation patterns of yield stress (YS), ultimate tensile stress (UTS) and elongation can be concluded as follows. First, at room temperature, the YS, UTS and elongation of the sheets are 582 MPa, 621 MPa and 2%, respectively. They all increase at elevated temperature. However, the YS and UTS drop sharply with the increasing temperature above 700 °C after exhibiting peak values of 670 MPa and 874 MPa. On the other hand, the elongation of the alloy only reaches to 7.2% at 800 °C, but it rapidly increases to 58.1% at 850 °C, which indicates that the brittle-ductile transition temperature (BDTT) of the sheets is between 800 °C and 850 °C. Eventually, at the temperature of 950 °C, the YS and UTS drop to 307 MPa and 402 MPa, while the elongation value increases to 95%. Fig.3 shows the fracture surfaces of the tensile test specimens tested at room temperature (20 °C), 700 °C, 800 °C, 850 °C, 950 °C, respectively, with an initial strain rate of 1 × 10−3 s−1. It can be seen that the specimens all exhibit brittle fracture modes at the temperature range of 20 ° C–800 °C and the fracture modes are characterized by both the γ phase and α2/γ lamellar colonies. A predominant mode of transgranular fracture with many cleavage planes is observed in Fig.3 (a). While Fig.3 (b) and (d) exhibit delamination along α2/γ lamellar colonies and translamellar cleavage. Intergranular fracture is also observed in Fig.3 (c), which refers to the fracture mode of large γ grains. This is because large deformation takes place at the grain boundaries, then microcracks initiate and propagate along the grain boundaries due to the high stress concentration [20]. Above 850 °C, the specimens exhibit a typical ductile fracture mode. Many dimples can be clearly seen in Fig.3 (e) and (f) and the fracture surfaces are quite severely oxidized, which results in the formation of oxide modules on the surfaces. 3.3. High temperature deformation microstructures
3. Results 3.1. Microstructure of the sheets The XRD pattern and SEM backscattered electron (BSE) image of the as-rolled PM Ti-45Al-7Nb-0.3W (at.%) alloy sheets are presented in Fig. 1. As is shown in Fig. 1 (a), the microstructure of the sheets is mainly composed of γ phase, α2 phase and a small amount of β phase. The SEM image reveals that the sheets show a duplex microstructure with a mean grain size of about 8 μm (Fig. 1 (b)). Meanwhile, the β phase
Fig. 4 shows the SEM BSE images of deformed microstructures after tensile tests at 20 °C, 800 °C, 850 °C and 950 °C, respectively. Compared with the microstructure of as-rolled sheets in Fig. 1 (b), there is not any significant change of microstructures after deformation at 20 °C and 800 °C, as depicted in Fig. 4 (a) and (b). However, when the tensile test temperature increases to 850 °C or even higher, to 950 °C (Fig. 4 (c) and (d)), it can be clearly seen that the β phase becomes more homogeneous than that in the microstructure of as-rolled sheets (Fig. 1 (b)).
Fig. 1. XRD pattern (a) and SEM backscattered electron (BSE) image (b) of the as-rolled Ti-45Al-7Nb-0.3W (at.%) alloy sheets.
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Fig. 2. The relationship between tensile mechanical properties and temperature.
The bright-field TEM images of as-rolled sheets and the tensile test specimens tested at the temperature range of 20 °C–800 °C are shown in Fig. 5. The microstructure of as-rolled sheets mainly consists of α2/γ lamellar colonies and fine γ grains with a low density of dislocations, as Fig. 5 (a) shows. Some annealing twins which were produced during
the rolling and subsequent heat treatment process can be observed in the γ grains. Mechanical twinning is also widely observed in the microstructures of the specimens tested at different temperatures (Fig. 5 (b), (c) and (f)). And it is regarded as an important deformation mechanism because of its capability to enhance the plasticity of the alloy to some extent [15]. Fig. 5 (c) and (d) show that high density of dislocations appear inside the γ grains or pile up on the interfaces of α2/γ lamellar colonies, which may result in high stress concentration of the alloy. Fig. 6 shows the TEM images of the tensile test specimens tested at 850 °C and 950 °C. As is shown in Fig. 6, at high deformation temperature, twining significantly decreases, while the density of dislocations is still very high in α2/γ lamellar colonies. Bending and kinking of the α2/γ lamellar colonies can be observed in Fig. 6 (a) and (b). What is more, some α2/γ lamellar colonies are broken and some new fine grains are formed and located around or within the residual lamellar colonies (Fig. 6 (c), (e) and (f)). These grains are more homogeneous and finer than that in the microstructure of as-rolled sheets (Fig. 5 (a)). Fig. 6 (d) displays the selected area electron diffraction pattern of a fine grain in Fig. 6 (c), which indicates some of the recrystallized grains are β grains. 3.4. Superplastic deformation behaviors at 950 °C As already mentioned above, when the tensile test specimen is tested at 950 °C and initial strain rate of 1 × 10−3 s−1, the elongation can
Fig. 3. fracture surface images of specimens tested at 20 °C (a), (b), 700 °C (c), 800 °C (d), 850 °C (e) and 950 °C (f).
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Fig. 4. SEM images of the tensile test specimens tested at temperatures of 20 °C (a), 800 °C (b), 850 °C (c), and 950 °C (d).
reach to 95%. This indicates the specimen shows a good potential of superplasticity, which is confirmed by the deformation behaviors at 950 ° C with initial strain rates of 1 × 10−4 s−1 and 5 × 10−5 s−1. The tensile specimens deformed to failure at 950 °C with different initial strain rates and their stress-engineering strain curves are presented in Fig. 7. At 950 °C, by decreasing the initial strain rate from 1 × 10−3 s− 1 to 5 × 10−5 s−1, the YS and UTS decrease from 307 MPa and 402 MPa to 196 MPa and 232 MPa, respectively. While the elongation increases from 95% to 243%. The stress-engineering strain curves display a peak stress stage at strain of 20%–30% and then the stress decreases obviously, which implies the strain softening occurs. Fig.8 shows the microstructures and fracture surface of the tensile test specimens tested at 950 °C and 5 × 10−5 s−1. Compared with the microstructures of as-rolled sheets (Fig. 1 (b)) and specimens deformed at initial strain rate of 1 × 10−3 s−1 (Fig. 4), the grains become much smaller after deformation at 950 °C and 5 × 10−5 s−1 (Fig. 8 (a)). It is noteworthy that the β phase in Fig. 8 (a) has become more dispersed. The TEM image of the specimen after superplastic deformation is shown in Fig. 8 (b), in which many refined grains and some residual α2/γ lamellar colonies can be observed. In addition, the cavity morphology of the specimen is illustrated in Fig. 8 (c), which shows many cavities are connected with each other. These cavities mostly are lined up parallel to the tensile direction and some of them are also linked transversely. If the cavities continue to grow up during deformation, the final coalescence would result in fracture. The fracture surface (Fig. 8 (d)) shows that many dimples appear, which is related to the cavities produced during deformation (Fig. 8 (c)).
temperature UTS of 621 MPa and a peak stress of 874 MPa at 700 °C. The theory of Kear-Wilsdorf dislocation locks has been commonly accepted to explain the anomalous strengthening effect of intermetallic compounds [22–25]. As for TiAl based alloy, the anomalous strengthening effect is closely related to the cross-slip of 〈011] superdislocations and 1/2〈110] ordinary dislocations [26]. At room temperature, the critical resolved shear stress (CRSS) of 〈011] superdislocations is lower than that of 1/2〈110] ordinary dislocations [27]. So the glide of 〈011] superdislocations can operate more easily at room temperature. In the process of deformation, the cross-slip of the superdislocations on the {111}planes would operate and the superdislocations could dissociate into some plane defects such as superlattice intrinsic stacking fault (SISF), antiphase boundary (APB) and complex stacking fault (CSF) [28,29]. With the temperature increasing, ordinary dislocation movements could be activated and the plane defects can show pinning effect on those dislocations, which results in immovable dislocation locks. Cross-slip is a typical thermally activated process, so with the temperature increasing, the cross-slip of superdislocations can occur more easily and the pinning effect of the dislocation locks will be more remarkable, which leads to an anomalous strengthening at 700 °C macroscopically. With the further increase of the temperature above 700 °C, the thermal activation is enhanced significantly so that those dislocation locks would be unlocked, in the meantime, dislocation sliding and climbing could be activated again. Thus, the ultimate tensile strength decreases to 771 MPa when the temperature is 750 °C.
4. Discussions
The α2/γ lamellar colonies play an important role in the deformation behaviors. It is generally accepted that in the initial stage, the plastic deformation of TiAl alloy takes place mainly by dislocation movements or mechanical twinning. However, the interfaces of α2/γ lamellar colonies are effective barriers for dislocation movements. So the dislocations will pile up on the interfaces of α2/γ lamellar colonies (Fig. 5 (c), (d)), which requires higher stress to move those dislocations. Therefore, mechanical twinning can be easily activated due to its low energy requirement, which will compensate the lack of dislocation movements [7]. It has
4.1. Anomalous strengthening The anomalous strengthening has been observed in a Nb-free TiAl based alloy Ti-45Al-3Fe-2Mo and a low Nb-containing TiAl based alloy Ti-44Al-6V-3Nb-0.3Y recently [21,22]. The as-rolled high Nb-containing TiAl sheets in the present study have excellent mechanical properties and also display an anomalous strengthening effect with a room
4.2. Deformation of α2/γ lamellar colonies
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Fig. 5. TEM images of as-rolled sheets and the tensile test specimens tested at the temperature range of 20 °C–800 °C: (a) as-rolled sheet; (b) 20 °C; (c) 700 °C; (d) dislocations in (c); (e) selected area electron diffraction pattern (SADP) of γ phase in (c); (f) 800 °C.
been reported that for high Nb containing TiAl alloys, the stacking fault energy (SFE) could be decreased by the alloying element Nb, thus, mechanical twinning could be more easily activated [13,29]. The interfaces of the α2/γ lamellar colonies with high stress concentration can cause the operation of mechanical twinning since the dislocation motion is more difficult, so that the plasticity can be improved during high temperature deformation. As is shown in Fig. 5, mechanical twins are widely observed in the α2/γ lamellar colonies. In Fig. 6 (a) and (d), it can be seen that when the deformation temperature is as high as 850 °C, bending and kinking of the α2/γ lamellar colonies can be easily observed from the picture. However, when the temperature is below 850 °C, this phenomenon can be hardly seen, as shown in Fig. 5. This difference could be ascribed to the reorientation of the α2/γ lamellar colonies which easily occurs at higher temperature. The α2/γ lamellar colonies have three different orientations which lead to different deformabilities: soft orientation, middle orientation and hard orientation [30]. The soft oriented lamellar colonies are easy to be deformed while the hard oriented are not. However, it has been reported [31] that kinking of the hard mode lamellae could convert unfavorably oriented lamellae into favorably oriented one so as to enhance the deformability of α2/γ lamellar colonies. But deformation at low temperature cannot cause reorientation of the lamellar colonies, which means it can only be accomplished at higher temperature.
4.3. Recrystallization behaviors In this study, at lower temperature range (20 °C–800 °C), the authors believe the main deformation mechanisms are dislocation gliding and climbing and mechanical twinning (Fig. 5). At higher temperature range (850 °C–950 °C), the dynamic recrystallization (DRX) plays an important role in the deformation of TiAl alloy, which can be confirmed by Fig. 6 (b), (e), (f) and Fig. 8, where many DRX grains can be observed. When the alloy is deformed at high temperature, the ability for diffusion of atoms, cross-slip of dislocation and migration of grain boundaries are improved, which is beneficial to nucleation and nucleus growth of DRX. These fine DRX grains are located around or within the α2 /γ lamellar colonies, which indicates that the DRX grains have generated accompany with the decomposition of the α2/γ lamellar colonies. During the deformation process, the DRX could be triggered by high density of dislocations and mechanical twins in the α2/γ lamellar colonies. Kim et al. [32] has reported that when the strain was pretty low, the dislocation movements and mechanical twinning were considered to be the main deformation mechanism. Once the strain increased, there would be more dislocations and then subgrains could form, and the low-angle boundaries could turn into high-angle boundaries, then the dynamic recrystallization would occur.
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Fig. 6. TEM images of the tensile test specimens tested at 850 °C and 950 °C: (a), (b), (c) 850 °C; (d) SADP of β phase in (c); (e), (f) 950 °C.
It has been claimed [33,34] that DRX is the main softening mechanism of TiAl alloy. It can absorb the strain energy and reduce the dislocation density to decrease the stress concentration. During the deformation, fine DRX grains can promote grain boundary movement and grain rotation, which results in a more uniform and more harmonious deformation. As a consequence, the ductility of the alloy can be improved by DRX. In the present study, this is confirmed by the high
temperature mechanical properties above BDTT and superplastic deformation behaviors at 950 °C. 4.4. Effect of βphase on deformation The XRD pattern and SEM image in Fig.1 show that there are some β phase in the microstructure of as-rolled sheets, which can also influence
Fig. 7. The tensile specimens deformed to failure (a) and their stress-engineering strain curves. (b) at 950 °C with different initial strain rates.
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Fig. 8. Microstructures and fracture surface of the tensile test specimen tested at 950 °C and 5 × 10−5 s−1: (a) SEM image of the deformed microstructure; (b) TEM image of the specimen; (c) cavity morphology of the specimen; (d) fracture surface image.
the deformation behaviors. At low temperature, the β phase can turn into ordered B2 phase. The B2 phase is hard and brittle and detrimental to the ductility of the alloy at relatively low temperature [15,34], which can explain the low elongation of 2% at room temperature. But the β phase is soft and ductile at elevated temperature. Because the disordered β phase with bcc crystal structure can provide a sufficient number of independent slip systems when deformed at higher temperature [35]. The β phase mainly distributes at grain boundaries of γ phase and α2/γ lamellar colonies. Those β phase can relieve the stress concentration on the interfaces and result in the enhancement of high temperature deformability of the TiAl alloy. T.G. Nieh [36] reported that the fine β grains can reduce the formation of cavities at grain triple junctions. Clemens [37] suggested that the β phase distributed on the grain boundary would promote grain boundary sliding, which was beneficial to the high temperature ductility. Those conclusions are consistent with the results of present study that the tensile test specimens show higher total elongations at higher temperature (850 °C–950 °C). In this research, the large elongation of 243% of the specimen tested at 950 °C and 5 × 10−5 s−1 can be also related to the deformation behavior of β phase. Under the condition of 950 °C and 5 × 10−5 s−1, the soft β phase could be deformed easily, and the low strain rate could ensure that there will be enough time for the DRX process. Once the fine and uniform DRX grains of β phase come into being, these β grains would act as lubricants at the grain boundaries and thus the grain boundary sliding process could proceed more easily, which eventually results in the large elongation of 243%. 5. Conclusions (1) The microstructure of the as-rolled PM Ti-45Al-7Nb-0.3W alloy sheets show a duplex microstructure with some β phase distributed at the grain boundaries.
(2) The sheets exhibit excellent high temperature mechanical properties with a peak UTS of 874 MPa and display an anomalous strengthening effect at 700 °C. In addition, a brittle-ductile transition temperature (BDTT) is found at between 800 °C and 850 °C. (3) The deformation mechanism below BDTT mainly refers to dislocation movements and mechanical twinning. While above BDTT, the deformation mechanism is considered as the combination of dislocation movements, mechanical twinning, bending and kinking of α2/γ lamellar colonies and dynamic recrystallization. (4) The as-rolled sheets display a superplastic behavior at 950 °C. At this point, severe dynamic recrystallization takes place and the grain boundary sliding can be promoted by β phase, which results in an elongation of 243% at the initial strain rate of 5 × 10−5 s−1.
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Microstructure and high temperature mechanical properties of powder metallurgical Ti-45Al-7Nb-0.3W alloy sheets Huizhong Li a,b, Yelong Qi a, Xiaopeng Liang a,b,⁎, Zexiao Zhu a, Feng Lv a, Yong Liu b, Yi Yang a a b
School of Materials Science and Engineering, Central South University, Changsha 410083, China State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 19 April 2016 Received in revised form 27 May 2016 Accepted 29 May 2016 Available online 31 May 2016 Keywords: TiAl alloy Microstructure High temperature mechanical property Superplastic deformation Deformation mechanism
a b s t r a c t The microstructure, high temperature mechanical properties and deformation mechanism of powder metallurgical (PM) Ti-45Al-7Nb-0.3W (at.%) alloy sheets were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The tensile test results showed excellent high temperature mechanical properties with a yield stress (YS) of 670 MPa and an ultimate tensile stress (UTS) of 874 MPa at 700 °C and displayed an anomalous strengthening effect, which could be explained by the pinning mechanism of dislocation locks. In addition, the brittle-ductile transition temperature (BDTT) was found at between 800 °C and 850 °C. When deformed below BDTT, high density of dislocations and mechanical twins were observed in the microstructures. However, with increasing deformation temperature above BDTT, kinking of α2/γ lamellar colonies and dynamic recrystallization also occurred. It was noteworthy that the as-rolled sheets displayed superplastic behaviors at 950 °C with initial strain rates of 1 × 10−4 s−1 and 5 × 10−5 s−1. At the same time, severe dynamic recrystallization took place and the grain boundary sliding was improved by β phase, which resulted in an elongation of 243% at the strain rate of 5 × 10−5 s−1. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction TiAl based alloys are considered highly promising high temperature structural materials as a substitute for nickel based alloys because of their attractive properties such as low density, good creep resistance and good high temperature strength etc. [1–4]. TiAl sheets are extremely promising in aerospace applications, including turbine blades, nozzles for helicopters, and back structures for scramjets etc. [5,6]. The high temperature mechanical properties are key performance indexes of TiAl based alloys and there is a continuing need for improving them in order to make those alloys an attractive alternative for conventional nickel based alloys [7,8]. So far, considerable efforts have been devoted to investigating the high temperature mechanical properties of TiAl based alloys. For example, high Nb containing TiAl alloys have been widely studied recently [9–11]. Compared with conventional TiAl alloys, high Nb containing TiAl alloys display remarkably increased mechanical strength from room temperature to 800 °C, which is ascribed to the solution hardening of Nb and microstructural refinement [12,13]. J.D.H. Paul et al. [14] found that Ti-45Al-10Nb alloys and Ti45Al-5Nb alloys had flow stress values of over 800 MPa at room temperature and over 500 MPa at 900 °C in compression process. In
⁎ Corresponding author at: State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China. E-mail address: [email protected] (X. Liang).
http://dx.doi.org/10.1016/j.matdes.2016.05.113 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
comparison to a conventional Ti-47Al-2Cr-0.2Si alloy, they showed a considerable increase in strength. Nevertheless, the high addition of Nb may bring some problems as well, such as poor hot deformability and low room temperature ductility [12,15]. Thus, more studies on high Nb containing TiAl alloys should be carried out. It is generally known that the deformation behavior of TiAl based alloys is very complicated because the microstructure of TiAl based alloys usually consists of multiple phases and structures which result in different deformabilities [16,17]. Therefore, there are numerous mechanisms associated with high temperature properties working simultaneously and synergistically during the deformation [7]. Y.W. Kim et al. developed Ti-45Al-5Nb sheets by pack-rolling through ingot metallurgical (IM) process. And they found that the sheets were strengthened by unstable structures, as well as grain-boundary strengthening and γ phase solution strengthening [18]. Up to now, many studies have been performed on TiAl alloy. And in our previous work, powder metallurgical TiAl sheets have already been produced by hot pack-rolling [19,20]. However, microstructure and high temperature mechanical properties of hot pack-rolled high Nb containing PM TiAl sheets need to be further studied. In this work, the PM Ti-45Al-7Nb-0.3W (at.%) alloy sheets were fabricated by hot pack-rolling. Tensile tests under different conditions were conducted to investigate the microstructure and high temperature mechanical properties of TiAl sheets. Besides, high temperature deformation mechanism and superplastic deformation behaviors at 950 °C were also analyzed in detail.
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2. Experimental procedures The alloy with nominal composition of Ti-45Al-7Nb-0.3W (at.%) was prepared by powder metallurgy. Plates measuring 50 × 40 × 8 mm3 were machined from the hot isostatic pressed billet by electric-discharge machining. Then they were encapsulated in 1.5 mm thick pure Ti cans. The specimens were hot rolled on a hot mill with roller dimension of Ф180 mm × 320 mm. And the rollers were preheated at 250– 300 °C. After heating to the rolling temperature of 1270 °C and holding for 60 min, TiAl sheets were hot pack-rolled with rolling speed of 40 mm/s and a reduction of 10% per pass. And the sheets were held for 5–8 min between each two rolling pass. After rolling, 4 mm thick sheets were obtained and they were then treated at 900 °C for 1 h followed by air cooling. Flat tensile test specimens with a gauge section of 8 mm × 3.4 mm × 2.8 mm were cut in rolling direction from the sheets. High temperature tensile tests at different temperatures ranging from room temperature (20 °C) to 950 °C were conducted on an Instron 3369 universal test machine in air with an initial strain rate of 1 × 10−3 s−1. Superplastic deformation behaviors at 950 °C with initial strain rates of 1 × 10−4 s−1 and 5 × 10−5 s−1 were also investigated. All specimens were held at test temperature for 10 min before testing. Once the tensile specimens broke, they were treated by forced air cooling. So the state of high temperature in the specimens could be kept. For each condition, at least three tests were performed and average values were reported. The grain size of γ phase and α2/γ lamellae was calculated by a software Image-Pro Plus6.0. The phase composition was determined using Cu Kα radiation on a D/Max 2500 X-ray diffractometer. And the deformed microstructures and fracture surfaces of the tensile specimens were analyzed using a Sirion200 scanning electron microscope (SEM). TEM foils were prepared by mechanical polishing and twin-jet electropolishing using a solution of 6% perchloric acid + 34% butanol + 60% methanol at − 20 °C and 25 V, and then observed in a Tecnai G220 microscope operating at 200 kV.
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is distributed at the grain boundaries of γ phase and α2/γ lamellar colonies as can be seen from the picture. 3.2. High temperature mechanical properties The tensile mechanical properties of the as-rolled PM Ti-45Al-7Nb0.3W (at.%) alloy sheets with an initial strain rate of 1 × 10−3 s−1 are presented in Fig. 2. As shown, the variation patterns of yield stress (YS), ultimate tensile stress (UTS) and elongation can be concluded as follows. First, at room temperature, the YS, UTS and elongation of the sheets are 582 MPa, 621 MPa and 2%, respectively. They all increase at elevated temperature. However, the YS and UTS drop sharply with the increasing temperature above 700 °C after exhibiting peak values of 670 MPa and 874 MPa. On the other hand, the elongation of the alloy only reaches to 7.2% at 800 °C, but it rapidly increases to 58.1% at 850 °C, which indicates that the brittle-ductile transition temperature (BDTT) of the sheets is between 800 °C and 850 °C. Eventually, at the temperature of 950 °C, the YS and UTS drop to 307 MPa and 402 MPa, while the elongation value increases to 95%. Fig.3 shows the fracture surfaces of the tensile test specimens tested at room temperature (20 °C), 700 °C, 800 °C, 850 °C, 950 °C, respectively, with an initial strain rate of 1 × 10−3 s−1. It can be seen that the specimens all exhibit brittle fracture modes at the temperature range of 20 ° C–800 °C and the fracture modes are characterized by both the γ phase and α2/γ lamellar colonies. A predominant mode of transgranular fracture with many cleavage planes is observed in Fig.3 (a). While Fig.3 (b) and (d) exhibit delamination along α2/γ lamellar colonies and translamellar cleavage. Intergranular fracture is also observed in Fig.3 (c), which refers to the fracture mode of large γ grains. This is because large deformation takes place at the grain boundaries, then microcracks initiate and propagate along the grain boundaries due to the high stress concentration [20]. Above 850 °C, the specimens exhibit a typical ductile fracture mode. Many dimples can be clearly seen in Fig.3 (e) and (f) and the fracture surfaces are quite severely oxidized, which results in the formation of oxide modules on the surfaces. 3.3. High temperature deformation microstructures
3. Results 3.1. Microstructure of the sheets The XRD pattern and SEM backscattered electron (BSE) image of the as-rolled PM Ti-45Al-7Nb-0.3W (at.%) alloy sheets are presented in Fig. 1. As is shown in Fig. 1 (a), the microstructure of the sheets is mainly composed of γ phase, α2 phase and a small amount of β phase. The SEM image reveals that the sheets show a duplex microstructure with a mean grain size of about 8 μm (Fig. 1 (b)). Meanwhile, the β phase
Fig. 4 shows the SEM BSE images of deformed microstructures after tensile tests at 20 °C, 800 °C, 850 °C and 950 °C, respectively. Compared with the microstructure of as-rolled sheets in Fig. 1 (b), there is not any significant change of microstructures after deformation at 20 °C and 800 °C, as depicted in Fig. 4 (a) and (b). However, when the tensile test temperature increases to 850 °C or even higher, to 950 °C (Fig. 4 (c) and (d)), it can be clearly seen that the β phase becomes more homogeneous than that in the microstructure of as-rolled sheets (Fig. 1 (b)).
Fig. 1. XRD pattern (a) and SEM backscattered electron (BSE) image (b) of the as-rolled Ti-45Al-7Nb-0.3W (at.%) alloy sheets.
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Fig. 2. The relationship between tensile mechanical properties and temperature.
The bright-field TEM images of as-rolled sheets and the tensile test specimens tested at the temperature range of 20 °C–800 °C are shown in Fig. 5. The microstructure of as-rolled sheets mainly consists of α2/γ lamellar colonies and fine γ grains with a low density of dislocations, as Fig. 5 (a) shows. Some annealing twins which were produced during
the rolling and subsequent heat treatment process can be observed in the γ grains. Mechanical twinning is also widely observed in the microstructures of the specimens tested at different temperatures (Fig. 5 (b), (c) and (f)). And it is regarded as an important deformation mechanism because of its capability to enhance the plasticity of the alloy to some extent [15]. Fig. 5 (c) and (d) show that high density of dislocations appear inside the γ grains or pile up on the interfaces of α2/γ lamellar colonies, which may result in high stress concentration of the alloy. Fig. 6 shows the TEM images of the tensile test specimens tested at 850 °C and 950 °C. As is shown in Fig. 6, at high deformation temperature, twining significantly decreases, while the density of dislocations is still very high in α2/γ lamellar colonies. Bending and kinking of the α2/γ lamellar colonies can be observed in Fig. 6 (a) and (b). What is more, some α2/γ lamellar colonies are broken and some new fine grains are formed and located around or within the residual lamellar colonies (Fig. 6 (c), (e) and (f)). These grains are more homogeneous and finer than that in the microstructure of as-rolled sheets (Fig. 5 (a)). Fig. 6 (d) displays the selected area electron diffraction pattern of a fine grain in Fig. 6 (c), which indicates some of the recrystallized grains are β grains. 3.4. Superplastic deformation behaviors at 950 °C As already mentioned above, when the tensile test specimen is tested at 950 °C and initial strain rate of 1 × 10−3 s−1, the elongation can
Fig. 3. fracture surface images of specimens tested at 20 °C (a), (b), 700 °C (c), 800 °C (d), 850 °C (e) and 950 °C (f).
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Fig. 4. SEM images of the tensile test specimens tested at temperatures of 20 °C (a), 800 °C (b), 850 °C (c), and 950 °C (d).
reach to 95%. This indicates the specimen shows a good potential of superplasticity, which is confirmed by the deformation behaviors at 950 ° C with initial strain rates of 1 × 10−4 s−1 and 5 × 10−5 s−1. The tensile specimens deformed to failure at 950 °C with different initial strain rates and their stress-engineering strain curves are presented in Fig. 7. At 950 °C, by decreasing the initial strain rate from 1 × 10−3 s− 1 to 5 × 10−5 s−1, the YS and UTS decrease from 307 MPa and 402 MPa to 196 MPa and 232 MPa, respectively. While the elongation increases from 95% to 243%. The stress-engineering strain curves display a peak stress stage at strain of 20%–30% and then the stress decreases obviously, which implies the strain softening occurs. Fig.8 shows the microstructures and fracture surface of the tensile test specimens tested at 950 °C and 5 × 10−5 s−1. Compared with the microstructures of as-rolled sheets (Fig. 1 (b)) and specimens deformed at initial strain rate of 1 × 10−3 s−1 (Fig. 4), the grains become much smaller after deformation at 950 °C and 5 × 10−5 s−1 (Fig. 8 (a)). It is noteworthy that the β phase in Fig. 8 (a) has become more dispersed. The TEM image of the specimen after superplastic deformation is shown in Fig. 8 (b), in which many refined grains and some residual α2/γ lamellar colonies can be observed. In addition, the cavity morphology of the specimen is illustrated in Fig. 8 (c), which shows many cavities are connected with each other. These cavities mostly are lined up parallel to the tensile direction and some of them are also linked transversely. If the cavities continue to grow up during deformation, the final coalescence would result in fracture. The fracture surface (Fig. 8 (d)) shows that many dimples appear, which is related to the cavities produced during deformation (Fig. 8 (c)).
temperature UTS of 621 MPa and a peak stress of 874 MPa at 700 °C. The theory of Kear-Wilsdorf dislocation locks has been commonly accepted to explain the anomalous strengthening effect of intermetallic compounds [22–25]. As for TiAl based alloy, the anomalous strengthening effect is closely related to the cross-slip of 〈011] superdislocations and 1/2〈110] ordinary dislocations [26]. At room temperature, the critical resolved shear stress (CRSS) of 〈011] superdislocations is lower than that of 1/2〈110] ordinary dislocations [27]. So the glide of 〈011] superdislocations can operate more easily at room temperature. In the process of deformation, the cross-slip of the superdislocations on the {111}planes would operate and the superdislocations could dissociate into some plane defects such as superlattice intrinsic stacking fault (SISF), antiphase boundary (APB) and complex stacking fault (CSF) [28,29]. With the temperature increasing, ordinary dislocation movements could be activated and the plane defects can show pinning effect on those dislocations, which results in immovable dislocation locks. Cross-slip is a typical thermally activated process, so with the temperature increasing, the cross-slip of superdislocations can occur more easily and the pinning effect of the dislocation locks will be more remarkable, which leads to an anomalous strengthening at 700 °C macroscopically. With the further increase of the temperature above 700 °C, the thermal activation is enhanced significantly so that those dislocation locks would be unlocked, in the meantime, dislocation sliding and climbing could be activated again. Thus, the ultimate tensile strength decreases to 771 MPa when the temperature is 750 °C.
4. Discussions
The α2/γ lamellar colonies play an important role in the deformation behaviors. It is generally accepted that in the initial stage, the plastic deformation of TiAl alloy takes place mainly by dislocation movements or mechanical twinning. However, the interfaces of α2/γ lamellar colonies are effective barriers for dislocation movements. So the dislocations will pile up on the interfaces of α2/γ lamellar colonies (Fig. 5 (c), (d)), which requires higher stress to move those dislocations. Therefore, mechanical twinning can be easily activated due to its low energy requirement, which will compensate the lack of dislocation movements [7]. It has
4.1. Anomalous strengthening The anomalous strengthening has been observed in a Nb-free TiAl based alloy Ti-45Al-3Fe-2Mo and a low Nb-containing TiAl based alloy Ti-44Al-6V-3Nb-0.3Y recently [21,22]. The as-rolled high Nb-containing TiAl sheets in the present study have excellent mechanical properties and also display an anomalous strengthening effect with a room
4.2. Deformation of α2/γ lamellar colonies
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Fig. 5. TEM images of as-rolled sheets and the tensile test specimens tested at the temperature range of 20 °C–800 °C: (a) as-rolled sheet; (b) 20 °C; (c) 700 °C; (d) dislocations in (c); (e) selected area electron diffraction pattern (SADP) of γ phase in (c); (f) 800 °C.
been reported that for high Nb containing TiAl alloys, the stacking fault energy (SFE) could be decreased by the alloying element Nb, thus, mechanical twinning could be more easily activated [13,29]. The interfaces of the α2/γ lamellar colonies with high stress concentration can cause the operation of mechanical twinning since the dislocation motion is more difficult, so that the plasticity can be improved during high temperature deformation. As is shown in Fig. 5, mechanical twins are widely observed in the α2/γ lamellar colonies. In Fig. 6 (a) and (d), it can be seen that when the deformation temperature is as high as 850 °C, bending and kinking of the α2/γ lamellar colonies can be easily observed from the picture. However, when the temperature is below 850 °C, this phenomenon can be hardly seen, as shown in Fig. 5. This difference could be ascribed to the reorientation of the α2/γ lamellar colonies which easily occurs at higher temperature. The α2/γ lamellar colonies have three different orientations which lead to different deformabilities: soft orientation, middle orientation and hard orientation [30]. The soft oriented lamellar colonies are easy to be deformed while the hard oriented are not. However, it has been reported [31] that kinking of the hard mode lamellae could convert unfavorably oriented lamellae into favorably oriented one so as to enhance the deformability of α2/γ lamellar colonies. But deformation at low temperature cannot cause reorientation of the lamellar colonies, which means it can only be accomplished at higher temperature.
4.3. Recrystallization behaviors In this study, at lower temperature range (20 °C–800 °C), the authors believe the main deformation mechanisms are dislocation gliding and climbing and mechanical twinning (Fig. 5). At higher temperature range (850 °C–950 °C), the dynamic recrystallization (DRX) plays an important role in the deformation of TiAl alloy, which can be confirmed by Fig. 6 (b), (e), (f) and Fig. 8, where many DRX grains can be observed. When the alloy is deformed at high temperature, the ability for diffusion of atoms, cross-slip of dislocation and migration of grain boundaries are improved, which is beneficial to nucleation and nucleus growth of DRX. These fine DRX grains are located around or within the α2 /γ lamellar colonies, which indicates that the DRX grains have generated accompany with the decomposition of the α2/γ lamellar colonies. During the deformation process, the DRX could be triggered by high density of dislocations and mechanical twins in the α2/γ lamellar colonies. Kim et al. [32] has reported that when the strain was pretty low, the dislocation movements and mechanical twinning were considered to be the main deformation mechanism. Once the strain increased, there would be more dislocations and then subgrains could form, and the low-angle boundaries could turn into high-angle boundaries, then the dynamic recrystallization would occur.
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Fig. 6. TEM images of the tensile test specimens tested at 850 °C and 950 °C: (a), (b), (c) 850 °C; (d) SADP of β phase in (c); (e), (f) 950 °C.
It has been claimed [33,34] that DRX is the main softening mechanism of TiAl alloy. It can absorb the strain energy and reduce the dislocation density to decrease the stress concentration. During the deformation, fine DRX grains can promote grain boundary movement and grain rotation, which results in a more uniform and more harmonious deformation. As a consequence, the ductility of the alloy can be improved by DRX. In the present study, this is confirmed by the high
temperature mechanical properties above BDTT and superplastic deformation behaviors at 950 °C. 4.4. Effect of βphase on deformation The XRD pattern and SEM image in Fig.1 show that there are some β phase in the microstructure of as-rolled sheets, which can also influence
Fig. 7. The tensile specimens deformed to failure (a) and their stress-engineering strain curves. (b) at 950 °C with different initial strain rates.
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Fig. 8. Microstructures and fracture surface of the tensile test specimen tested at 950 °C and 5 × 10−5 s−1: (a) SEM image of the deformed microstructure; (b) TEM image of the specimen; (c) cavity morphology of the specimen; (d) fracture surface image.
the deformation behaviors. At low temperature, the β phase can turn into ordered B2 phase. The B2 phase is hard and brittle and detrimental to the ductility of the alloy at relatively low temperature [15,34], which can explain the low elongation of 2% at room temperature. But the β phase is soft and ductile at elevated temperature. Because the disordered β phase with bcc crystal structure can provide a sufficient number of independent slip systems when deformed at higher temperature [35]. The β phase mainly distributes at grain boundaries of γ phase and α2/γ lamellar colonies. Those β phase can relieve the stress concentration on the interfaces and result in the enhancement of high temperature deformability of the TiAl alloy. T.G. Nieh [36] reported that the fine β grains can reduce the formation of cavities at grain triple junctions. Clemens [37] suggested that the β phase distributed on the grain boundary would promote grain boundary sliding, which was beneficial to the high temperature ductility. Those conclusions are consistent with the results of present study that the tensile test specimens show higher total elongations at higher temperature (850 °C–950 °C). In this research, the large elongation of 243% of the specimen tested at 950 °C and 5 × 10−5 s−1 can be also related to the deformation behavior of β phase. Under the condition of 950 °C and 5 × 10−5 s−1, the soft β phase could be deformed easily, and the low strain rate could ensure that there will be enough time for the DRX process. Once the fine and uniform DRX grains of β phase come into being, these β grains would act as lubricants at the grain boundaries and thus the grain boundary sliding process could proceed more easily, which eventually results in the large elongation of 243%. 5. Conclusions (1) The microstructure of the as-rolled PM Ti-45Al-7Nb-0.3W alloy sheets show a duplex microstructure with some β phase distributed at the grain boundaries.
(2) The sheets exhibit excellent high temperature mechanical properties with a peak UTS of 874 MPa and display an anomalous strengthening effect at 700 °C. In addition, a brittle-ductile transition temperature (BDTT) is found at between 800 °C and 850 °C. (3) The deformation mechanism below BDTT mainly refers to dislocation movements and mechanical twinning. While above BDTT, the deformation mechanism is considered as the combination of dislocation movements, mechanical twinning, bending and kinking of α2/γ lamellar colonies and dynamic recrystallization. (4) The as-rolled sheets display a superplastic behavior at 950 °C. At this point, severe dynamic recrystallization takes place and the grain boundary sliding can be promoted by β phase, which results in an elongation of 243% at the initial strain rate of 5 × 10−5 s−1.
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