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Effect of a ductility layer on the tensile strength of TiAl-based multilayer composite sheets prepared by EB-PVD
M A TE RI A L S C HA RACT ER I ZA TI O N 95 ( 20 1 4 ) 1 5 7–1 6 3
Available online at www.sciencedirect.com
ScienceDirect www.elsevier.com/locate/matchar
Effect of a ductility layer on the tensile strength of TiAl-based multilayer composite sheets prepared by EB-PVD Rubing Zhanga,⁎, Yaoyao Zhanga , Qiang Liub , Guiqing Chenc , Deming Zhangd a
Department of Mechanics, School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China Beijing Institute of Astronautical Systems Engineering, Beijing 100076, China c Center for Composite Materials, Harbin Institute of Technology, Harbin 150001, China d Beijing General Research Institute of Mining & Metallurgy, Beijing 100044, China b
AR TIC LE D ATA
ABSTR ACT
Article history:
TiAl/Nb and TiAl/NiCoCrAl laminate composite sheets with a thickness of 0.4–0.6 mm and
Received 11 April 2014
dimensions of 150 mm × 100 mm were successfully fabricated by electron beam physical
Received in revised form 19 May 2014
vapor deposition. The microstructures of the sheets were examined, and their mechanical
Accepted 10 June 2014
properties were compared with those of TiAl monolithic sheet produced by electron beam
Available online 11 June 2014
physical vapor deposition. Tensile testing was performed at room temperature and 750 °C, and the fracture surfaces were examined by scanning electron microscopy. Among the
Keywords:
three microlaminate sheets, the TiAl/NiCoCrAl micro-laminate sheet had the best
Vapor deposition
comprehensive properties at room temperature, and the TiAl/Nb micro-laminate sheet
Microstructure
showed the ideal high-temperature strength and plasticity at 750 °C. The result was
Tensile strength
discussed in terms of metal strengthening mechanism.
Scanning electron microscopy
© 2014 Elsevier Inc. All rights reserved.
SEM
1. Introduction Titanium aluminides (TiAl) are the best lightweight hightemperature structural materials for thermal protection systems in aerospace vehicles, such as skin materials, because of their high specific strength, specific stiffness, and good oxidation resistance at elevated temperatures [1,2]. However, polycrystalline TiAl exhibits extreme brittleness at room temperature due to grain boundary fracture. The practical application of TiAl sheets is greatly restricted by its brittleness and shaping problems [3]. TiAl sheets are prepared using different methods, such as the hot rolling of ingots,
⁎ Corresponding author. E-mail address: [email protected] (R. Zhang).
http://dx.doi.org/10.1016/j.matchar.2014.06.010 1044-5803/© 2014 Elsevier Inc. All rights reserved.
powder metallurgy at temperatures over 1100 °C, and rolling monolithic Ti and Al foils with subsequent reaction annealing [4–8]. For example, Chaudhari et al [2] fabricated titanium aluminide sheets with a nearly fully lamellar microstructure using the commonly available rolling and heat treatment. The tensile properties of the titanium aluminide sheets fabricated using the rolling and heating treatments compares fairly well to that of the sheets fabricated through ingot metallurgy processing and powder metallurgy processing in literature. In addition, Cui et al. [8] used the roll bonding and reaction annealing method to fabricate fully dense TiAl-based composite sheets with microlaminated microstructure. The
158
M A TE RI A L S C HA RACT ER I ZA TI O N 95 ( 20 1 4 ) 1 5 7–1 6 3
introduction of TiB2-rich layers significantly refined the lamellar microstructure of the TiAl, and thus, the unique microlaminated TiB2–TiAl composite sheet possesses higher elastic modulus, nanohardness, and temperature strength compared with monolithic TiAl. TiAl sheets have been produced via electron beam physical vapor deposition (EB-PVD), which is an advanced method for fabricating coatings and films [9–11]. However, the treatment does not eliminate the brittleness of the TiAl alloy, which is also a major problem in the use of TiAl sheets as structural materials. The ductile-phase toughening of brittle materials has been widely applied in composites with different ductile reinforcement morphologies, such as particles, fibers, and laminates. The ductile-phase in laminate form has the maximum toughening efficiency for the same volume fraction of the ductile reinforcing phase [12–15]. Over the past two decades, a number of diverse intermetallics have been toughened using various ductile metal laminates. Metal– intermetallic composites (MICs) increase the fracture toughness of metals and the high strength and elastic modulus of intermetallic compounds [16–18]. Wang et al. [18] fabricated Ni/Ni3Al multilayer composites through the reaction synthesis of Nickel (Ni) and Al foils. The effects of temperature on the tensile properties and deformation behavior of Ni/Ni3Al multilayer composites have been systematically investigated. The yield strength of the multilayer composites increased with increasing tensile test temperature, from room temperature to 600 °C. Recent advances in EB-PVD have created new opportunities for preparing multilayer MIC foils. In this study, monolithic TiAl, TiAl/Niobium (Nb), and TiAl/NiCoCrAl microlaminated sheets with thicknesses of 0.4–0.6 mm and 150 mm × 100 mm dimensions were successfully fabricated through EB-PVD. The microstructural and mechanical properties, failure mechanisms, and deformation behavior of the TiAl-based multilayer composites produced through EB-PVD have been reported in previous papers [9,19–21]. The high-temperature tensile properties and deformation behavior of these TiAl-based multilayer composites were investigated in detail.
2. Experimental procedures 2.1. Preparation Monolithic TiAl, TiAl/Nb and TiAl/NiCoCrAl microlaminated sheets were produced via EB-PVD. The ingots of TiAl-based alloys used in this study, which had a nominal composition of Ti–47Al (wt.%), were casted two times in a water-cooling vacuum induction melting furnace. High purity CaF2 (99.7%) and Nb ingots (99.8%) were used in this study. The experimental device is a UE-204 type EB-PVD equipment with a horizontal feed mode. To separate the resulting sheet from the substrate in a convenient manner, a CaF2 ceramic stripper layer of about 20 μm thickness was requisitely deposited on the substrate before the actual evaporation. TiAl and Nb (or NiCoCrAl) ingots were alternately evaporated to produce TiAl/Nb (or TiAl/NiCoCrAl) microlaminates, and the layer thicknesses were controlled through the deposition time. At the same time, as compared with the microlaminates, the
ingot of TiAl was also used as the evaporation source for deposition of the monolithic TiAl foil. The detailed synthesis process and specific equipment can be found in our previous work [9]. After evaporation, the substrate was removed to the load chamber and subsequently extracted from the furnace after cooling to 423 K. At room temperature, the sheets with dimensions of 150 mm × 100 mm × 0.4 mm were obtained via mechanical stripping from the substrate surface. Tensile tests based on GB 6397-86 were conducted on the sheets using an INSTRON-5569 universal material testing machine with a crosshead displacement speed of 0.05 mm/min at room temperature and 750 °C. Before elevated temperature testing, the specimens were kept for 5 min at testing temperature. In this work, five specimens were tested to get an average value. The sheets were degreased ultrasonically in acetone and then pickled in either a 5 vol.% HF + 15 vol.% HNO3 (balance water) solution for 30 s. The microstructure of the TiAl sheet was studied with optical microscopy (OM) under reflecting and polarized light using a Leica DM 2500M microscope. The microstructural features and fractured surfaces of the composite were observed via scanning electron microscopy (SEM, FEI Sirion, The Netherlands) with simultaneous chemical composition analysis via energy dispersive spectroscopy (EDX, EDAX Inc.).
3. Results and discussions The cross-sectional morphologies of the monolithic TiAl, TiAl/Nb, and TiAl/NiCoCrAl microlaminated sheets prepared through EB-PVD are shown in Fig. 1(a)–(c), respectively. Spontaneous delamination alternation of the Ti-rich area and Al-rich area is inside the monolithic TiAl sheet (Fig. 1(a)) [9]. However, the natural stratification in the TiAl layer of the microlaminate sheets is unclear (Fig. 1(b)–(c)). As shown in Fig. 1(b), the TiAl/Nb microlaminated sheet is composed of 22 Nb layers and 23 TiAl layers alternating with uniform distribution. The Nb layer is only 1.0 μm, which is due to the large difference in the saturated Nb vapor pressure and to difficult evaporation. Moreover, Fig. 1(b) shows that the TiAl/NiCoCrAl microlaminated sheet is composed of 11 NiCoCrAl layers and 10 TiAl layers alternating with uniform distribution. The NiCoCrAl layer is thick, and the interlayer spacing is about 40 μm. The outermost layers of the microlaminated sheets are NiCoCrAl layers, which have better oxidation resistance at high temperature. The room temperature stress–strain curves of the monolithic TiAl, TiAl/Nb, and TiAl/NiCoCrAl micro-laminated sheets were determined through tension test (Fig. 2). All three materials have poor plasticity and present no yield phenomena at room temperature, showing brittle fracture without macroscopic plastic deformation. This was determined using the technical features of EB-PVD, such as the loose columnar crystal structure in the coating and high defect density. However, Fig. 2 shows that the strong layer (Nb or NiCoCrAl layer) obviously improves both the strength and plasticity of the TiAl sheet at room temperature. The ultimate tensile strengths and tensile strains of the micro-laminated sheets at room temperature are listed in Table 1. The TiAl/NiCoCrAl micro-laminated sheets have higher strength and plasticity at room temperature than that of the monolithic TiAl sheet, indicating the distinct reinforcement and toughening effect of the NiCoCrAl layer. The tensile strength
M A TE RI A L S C HA RACT ER I ZA TI O N 95 ( 20 1 4 ) 1 5 7–1 6 3
159
Fig. 1 – Micrographs on the cross-section of three sheets: (a) monolithic TiAl sheet; (b) TiAl/Nb microlaminate sheet; and (c) TiAl/NiCoCrAl microlaminate sheet.
and elongation of the TiAl/NiCoCrAl micro-laminated sheet remarkably increased from 384.2 MPa and 0.11% to 547.1 MPa and 0.55%, respectively. The tensile strain of the TiAl/NiCoCrAl micro-laminated sheet increased by nearly 4 times, and its tensile strength significantly increased by 42% compared to that of the monolithic TiAl sheet. In addition, the elongation of the TiAl/Nb micro-laminated sheet increased by nearly 1.5 times, and its strength also increased by 18%. Thus, the micro-laminated sheets have good comprehensive mechanical properties.
Fig. 2 – Tensile stress–strain curves of three micro-laminates sheet at room temperature.
The volume of the strong layer (Nb or NiCoCrAl layer) is less than 25 vol.%. The dislocations in the TiAl layers are deeply restrained by the interfaces. These make the Nb and/or the NiCoCrAl layers as brittle as the TiAl layers, and the micro-laminated sheets simply are composites consisting of two hard phases. The yield strengths of the micro-laminates sheets are expressed as σ 0 ¼ VA σ A þ VB σ B
ð1Þ
where σA and σB are the yield strengths of the strongly constrained TiAl layers and the strong layers (Nb or NiCoCrAl layer), respectively, using σA = 384 MPa, σB-Nb = 920 MPa [22], and σB-NiCoCrAl = 1060 MPa [23]. Calculations from the simple mixing law (Eq. (1)) are only related to the volume fraction of the constituent phases, which is also consistent with the experimental results. The above modeling shows that the strengthening mechanism in the micro-laminated sheets changes from the slide of a single dislocation confined to individual TiAl layers to the load-bearing effect, which is similar to some cases of composites [24]. The ductility of the multilayer composed of a softer phase and a stiffer phase is controlled by the softer phase. At any given strength, the TiAl/NiCoCrAl micro-laminated sheet has higher ductility compared to the TiAl/Nb micro-laminated sheet (Fig. 2). This finding is due to the fracture mechanism in the micro-laminated sheet [25]. The microcrack first nucleates in the brittle TiAl layer, and the further propagation of the microcracks is suppressed by the surrounding ductile NiCoCrAl or Nb layers, resulting in microcrack arrest. The
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M A TE RI A L S C HA RACT ER I ZA TI O N 95 ( 20 1 4 ) 1 5 7–1 6 3
Table 1 – Mechanical properties of TiAl-based sheets. Material
Tensile strength/MPa
Elongation/%
Tensile strength/MPa
Elongation/%
Room temperature: 750 °C TiAl TiAl/Nb TiAl/NiCoCrAl
384.2 453.4 547.1
plastic deformation capability of micro-laminated sheets is heavily dependent on the dislocation movement of the ductile layer. Some dislocations in the ductile layers are movable, and the ductile layers have some deformation capability to prevent the propagation of tiny microcracks. The better the plastic deformation capability of the ductility layers, the more the ductility layers arrest the microcracks. The NiCoCrAl layers have better plastic deformation capability [22,23], and therefore, the TiAl/NiCoCrAl micro-laminated sheets show greater ductility. Fig. 3 shows the SEM images of the tensile fractures of the monolithic TiAl, TiAl/Nb, and TiAl/NiCoCrAl micro-laminates at room temperature. The fracture microstructures of the TiAl/Nb and TiAl/NiCoCrAl micro-laminates are columnar crystal, and no significant difference was observed between the tensile fracture and bending fracture. The fracture of the TiAl/Nb micro-laminates at room temperature is wellarranged, and the continuously distributed clear Nb layer is closely combined with the TiAl layer (Fig. 4(b)). Furthermore, the strong Nb layer deflects cracks and causes some plastic deformations represented by partial quasi-cleavage fracture,
0.11 0.26 0.51
234.9 443.1 310.3
2.93 26.4 72.2
which is a combination of the quasi-cleavage fracture of certain ductility and intergranular brittle fracture. Fig. 3(c) shows that the TiAl layer in the TiAl/NiCoCrAl micro-laminate shows intergranular brittle fracture, whereas the NiCoCrAl layer shows a mixed fracture transgranular and intergranular fracture mechanism, which pertains to the evident ductile fracture mechanism. In addition, some flocculent structures and small crystal grains are found in the crystal boundary of the micro-laminates, indicating the effectiveness of the NiCoCrAl and Nb layers in improving the plasticity and strength of TiAl alloys at room temperature. Fig. 4 shows the stress–strain curves of three TiAl-based alloy foils at 750 °C. The metallic toughening layer significantly improves its high-temperature strength and toughness, exhibiting obvious yield. The elongation of the TiAl/ NiCoCrAl micro-laminate sheet at 750 °C reaches 72.2%, showing typical superplasticity characteristics. The high-temperature strength of the TiAl/Nb micro-laminated sheet reaches 443.1 MPa, almost equal to its strength at room temperature. According to the above data, the TiAl/NiCoCrAl micro-laminates have low BDTT (ductile–brittle transition temperature), and its
Fig. 3 – SEM fractographs of the sheets at room temperature: (a) TiAl; (b) TiAl/Nb; and (c) TiAl/NiCoCrAl.
M A TE RI A L S C HA RACT ER I ZA TI O N 95 ( 20 1 4 ) 1 5 7–1 6 3
Fig. 4 – Tensile stress–strain curves of three micro-laminate sheets at 750 °C.
strength quickly decreases as the test temperature increases, contrary to the TiAl alloy, whose strength slowly decreases, or even increases, as temperature increases [26]. As a result, the TiAl/NiCoCrAl micro-laminated sheet has the most increase of plasticity and highest decrease of strength as the test temperature increases. Nb atoms have high solid solubility in the TiAl matrix of the TiAl/Nb micro-laminated sheet at high temperatures. Therefore, the high Nb content of the Ti–Al–Nb solid solution can induce the orderly distribution of atoms for the TiAl/Nb micro-laminated sheet, which increases its hightemperature strength but decreases its BDTT. Both the plasticity and high-temperature strength of the TiAl/Nb and TiAl/NiCoCrAl micro-laminated sheets simultaneously improved compared with the monolithic TiAl sheet. The typical stress–strain curve of the TiAl alloy sheet at 750 °C shows an absolute brittle fracture without plastic deformation. Therefore, the yield strength of the micro-laminated sheets at 750 °C is also expressed by the simple mixing law. However, the yield strength of the Nb alloy quickly increases as the test temperature increases [27], contrary to the NiCoCrAl alloy whose strength decreases as temperature increases [28]. The calculations are also close to the experimental results. Likewise, the plastic deformation capability of the micro-laminated sheet at high temperatures heavily depends on the dislocation movement of the ductile layer. The TiAl/ NiCoCrAl micro-laminated sheets show greater ductility because the NiCoCrAl layers have better plastic deformation capability at 750 °C [26,27]. Figs. 5–6 show the macro- and micro-fracture morphologies of the monolithic TiAl and the micro-laminated sheets at 750 °C. Fig. 5 shows that the macro-fracture of the TiAl micro-laminate is perpendicular to the tensile axis. Its location also implies that it is formed by internal microscopic defects. The macro-fracture position of the TiAl/NiCoCrAl microlaminated sheet lies in the middle of the testing sample, and is a necking fracture with good plasticity. The macro-fracture of the TiAl/Nb micro-laminated sheet is between the straight brittle fracture and necking fracture. Fig. 6b shows that the hightemperature fracture of the TiAl/Nb micro-laminate mainly contains granular fractures, intergranular fractures, and some
161
Fig. 5 – Tensile macro-fracture morphologies of three sheets at 750 °C: (a) TiAl; (b) TiAl/Nb; and (c) TiAl/NiCoCrAl.
trans-granular dimple fractures surrounding the Nb layer. Compared with the tensile fracture at room temperature (Fig. 2), the TiAl–Nb interfaces are bonded more tightly due to the diffusion of Nb at the interfaces, and the Nb layer is thinned and significantly bent, blunting the micro-crack instead of extending the cleavage. Therefore, the material shows ductile fracture behavior. Fig. 6c shows that the fracture mode of the NiCoCrAl layer in the TiAl/NiCoCrAl micro-laminated sheet presents distinct dimple fracture. The TiAl layer also shows dimple fracture, although it is unclear because of oxidization. These phenomena confirm that the Nb and NiCoCrAl layers decrease the BDTT of the TiAl alloy to less than 750 °C, and significantly improve its high-temperature plasticity.
4. Conclusions The strong metallic layer can effectively improve the hightemperature properties of TiAl sheets. It not only offsets the adverse effects of EB-PVD (e.g. loose structure), but also enables the direct use of the prepared micro-laminate without subsequent processing. Although the TiAl/NiCoCrAl microlaminated sheet has the best comprehensive properties at room temperature (σt = 547.1 MPa, ε = 0.51%), it is inferior because of its poor high-temperature strength caused by the Ni-based alloy. The TiAl/Nb micro-laminate shows ideal hightemperature strength and plasticity (σt = 443.1 MPa, ε = 26.4%). Thus, it has attractive development prospect. Therefore, the micro-laminated sheets have much greater specific strength than the monolithic TiAl sheets.
Acknowledgments This work was supported by the National Natural Science Foundation of China (11102003, 11227801 and 91216301). Support by the Fundamental Research Fund for the Central Universities (2014JBM076) was also acknowledged.
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M A TE RI A L S C HA RACT ER I ZA TI O N 95 ( 20 1 4 ) 1 5 7–1 6 3
Fig. 6 – Micro-fracture morphologies of TiAl-based sheets at 750 °C: (a) TiAl; (b) TiAl/Nb; and (c) TiAl/NiCoCrAl.
REFERENCES [1] Draper SL, Krause D, Lerch B, Locci IE, Doehnert B, Nigam R, Das G, Sickles P, Tabernig B, Reger N, Rissbacher K. Development and evaluation of TiAl sheet structures for hypersonic applications. Mater Sci Eng A 2007;464:330–42. [2] Chaudhari GP, Acoff VL. Titanium aluminide sheets made using roll bonding and reaction annealing. Intermetallics 2010;18:472–8. [3] Mahapatra R, Chou YT, Pope DP. The anomalous flow behavior in single-phase Ti44Al56 single crystals: the effect of deformation history. Mater Sci Eng A 1997;239–240:97–101. [4] Rahaman MN, Dutton RE, Semiatin SL. Fabrication of dense thin sheets of gamma-TiAl by hot isostatic pressing of tape-cast monotapes. Mater Sci Eng A 2003;360:169–75. [5] Wang JN, Xie K. Grain size refinement of a TiAl alloy by rapid heat treatment. Scr Mater 2000;43:441–6. [6] Adams AG, Rahaman MN, Dutton RE. Microstructure of dense thin sheets of γ-TiAl fabricated by hot isostatic pressing of tape-cast monotapes. Mater Sci Eng A 2008;477:137–44. [7] Yang DK, Cizek P, Hodgson P, Cui WE. Ultrafine equiaxed-grain Ti/Al composite produced by accumulative roll bonding. Scr Mater 2010;62:321–4. [8] Cui XP, Fan GH, Geng L, Wang Y, Zhang HW, Peng HX. Fabrication of fully dense TiAl-based composite sheets with a novel microlaminated microstructure. Scr Mater 2012;66:276–9. [9] Zhang DM, Chen GQ, Han JC, Zhang XH, Yao ZZ. Macro-microscopic and phase analysis of TiAl-based alloys sheet fabricated by EB-PVD method. Trans Nonferrous Met Soc China 2007;17:777–82.
[10] Schulz U, Schmücker M. Microstructure of ZrO2 thermal barrier coatings applied by EB-PVD. Mater Sci Eng A 2000;276:1–8. [11] Peng H, Guo HB, He J, Gong SK. Oxidation and diffusion barrier behaviors of double-layer NiCoCrAlY coatings produced by plasma activated EB-PVD. Surf Coat Technol 2011;205:4658–64. [12] Soboyejo WO, Ye F, Chen LC, Bahtishi N, Schwartz DS, Lederich RJ. Effects of reinforcement morphology on the fatigue and fracture behavior of MoSi2/Nb composites. Acta Mater 1996;44:2027–41. [13] Mara NA, Bhattacharyya D, Dickerson P, Hoagland RG, Misra A. Deformability of ultrahigh strength 5 nm Cu/Nb nanolayered composites. Appl Phys Lett 2008;92:231901. [14] Mekky W, Nicholson PS. The fracture toughness of Ni/Al2O3 laminates by digital image correlation I: experimental crack opening displacement and R-curves. Eng Fract Mech 2006;73:571–82. [15] Maximilien EL, Etienne M, Daan HA, Eduardo S, Antoni PT, Robert OR. A novel biomimetic approach to the design of high-performance ceramic–metal composites. J R Soc Interface 2010;7:741–53. [16] Dyer TS, Munir ZA. The synthesis of nickel aluminides by multilayer self-propagating combustion. Metall Mater Trans B 1995;26:603–10. [17] George EP, Liu C, Pope DP. Fracture in Ni3Al: environmental and dopant effects. Phys Status Solidi A 1997;160:517–29. [18] Wang HB, Han JC, Du SY, Northwood DO. Reaction synthesis of Ni/Ni3Al multilayer composites using Ni and Al foils: high-temperature tensile properties and deformation behaviour. J Mater Process Technol 2008;200:433–40.
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[19] Chen GQ, Zhang RB, Zhang DM, Zou H. Study on oxidation property of TiAl/NiCoCrAl microlaminated sheet prepared by EB-PVD. Rare Met Mater Eng 2012;41(1):120–3. [20] Shi GD, Wang Z, Liu MC, Liang J, Wu ZJ. Microstructure and oxidation behavior of NiCoCrAl/YSZ microlaminates produced by EB-PVD. Mater Charact 2011;62(11):1066–70. [21] Zhang RB, Zhang DM, Chen GQ, Wang YS. Microstructure evolution during annealing of TiAl/NiCoCrAl multilayer composite prepared by EB-PVD. Mater Charact 2014. http://dx.doi.org/10.1016/j.matchar.2014.03.015. [22] Lee SW, Horita ZJ. High-pressure torsion for pure chromium and niobium. Mater Trans 2012;53(1):38–45. [23] Chen S, Qu SJ, Han JC. Microstructure and mechanical properties of Ni-based superalloy foil with nanocrystalline surface layer produced by EB-PVD. J Alloys Compd 2009;484:626–30.
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[24] Zhang JY, Zhang X, Liu G, Zhang GJ, Sun J. Scaling of the ductility with yield strength in nanostructured Cu/Cr multilayer films. Scr Mater 2010;63:101–4. [25] Zhang JY, Liu G, Zhang X, Zhang GJ, Sun J, Ma E. A Maximum in ductility and fracture toughness in nanostructured Cu/Cr multilayer films. Scr Mater 2010;62:333–6. [26] Kruml T, Obrtlík K. Microstructure degradation in high temperature fatigue of TiAl alloy. Int J Fatigue 2013. http://dx.doi.org/10.1016/j.ijfatigue.2013.10.005. [27] Huang TW. Influence of cold-working and heat-treating on elastic properties of Nb and Nb-based alloy. Acta Metall Sin 1992;28(1):11–5. [28] Shi GD, Liang J, Chen GQ, Du SY. Comparison of tensile properties between NiCoCrAl/YSZ microlaminates and the monolithic NiCoCrAl foil fabricated by EB-PVD. Mater Lett 2009;63:1665–7.
Available online at www.sciencedirect.com
ScienceDirect www.elsevier.com/locate/matchar
Effect of a ductility layer on the tensile strength of TiAl-based multilayer composite sheets prepared by EB-PVD Rubing Zhanga,⁎, Yaoyao Zhanga , Qiang Liub , Guiqing Chenc , Deming Zhangd a
Department of Mechanics, School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China Beijing Institute of Astronautical Systems Engineering, Beijing 100076, China c Center for Composite Materials, Harbin Institute of Technology, Harbin 150001, China d Beijing General Research Institute of Mining & Metallurgy, Beijing 100044, China b
AR TIC LE D ATA
ABSTR ACT
Article history:
TiAl/Nb and TiAl/NiCoCrAl laminate composite sheets with a thickness of 0.4–0.6 mm and
Received 11 April 2014
dimensions of 150 mm × 100 mm were successfully fabricated by electron beam physical
Received in revised form 19 May 2014
vapor deposition. The microstructures of the sheets were examined, and their mechanical
Accepted 10 June 2014
properties were compared with those of TiAl monolithic sheet produced by electron beam
Available online 11 June 2014
physical vapor deposition. Tensile testing was performed at room temperature and 750 °C, and the fracture surfaces were examined by scanning electron microscopy. Among the
Keywords:
three microlaminate sheets, the TiAl/NiCoCrAl micro-laminate sheet had the best
Vapor deposition
comprehensive properties at room temperature, and the TiAl/Nb micro-laminate sheet
Microstructure
showed the ideal high-temperature strength and plasticity at 750 °C. The result was
Tensile strength
discussed in terms of metal strengthening mechanism.
Scanning electron microscopy
© 2014 Elsevier Inc. All rights reserved.
SEM
1. Introduction Titanium aluminides (TiAl) are the best lightweight hightemperature structural materials for thermal protection systems in aerospace vehicles, such as skin materials, because of their high specific strength, specific stiffness, and good oxidation resistance at elevated temperatures [1,2]. However, polycrystalline TiAl exhibits extreme brittleness at room temperature due to grain boundary fracture. The practical application of TiAl sheets is greatly restricted by its brittleness and shaping problems [3]. TiAl sheets are prepared using different methods, such as the hot rolling of ingots,
⁎ Corresponding author. E-mail address: [email protected] (R. Zhang).
http://dx.doi.org/10.1016/j.matchar.2014.06.010 1044-5803/© 2014 Elsevier Inc. All rights reserved.
powder metallurgy at temperatures over 1100 °C, and rolling monolithic Ti and Al foils with subsequent reaction annealing [4–8]. For example, Chaudhari et al [2] fabricated titanium aluminide sheets with a nearly fully lamellar microstructure using the commonly available rolling and heat treatment. The tensile properties of the titanium aluminide sheets fabricated using the rolling and heating treatments compares fairly well to that of the sheets fabricated through ingot metallurgy processing and powder metallurgy processing in literature. In addition, Cui et al. [8] used the roll bonding and reaction annealing method to fabricate fully dense TiAl-based composite sheets with microlaminated microstructure. The
158
M A TE RI A L S C HA RACT ER I ZA TI O N 95 ( 20 1 4 ) 1 5 7–1 6 3
introduction of TiB2-rich layers significantly refined the lamellar microstructure of the TiAl, and thus, the unique microlaminated TiB2–TiAl composite sheet possesses higher elastic modulus, nanohardness, and temperature strength compared with monolithic TiAl. TiAl sheets have been produced via electron beam physical vapor deposition (EB-PVD), which is an advanced method for fabricating coatings and films [9–11]. However, the treatment does not eliminate the brittleness of the TiAl alloy, which is also a major problem in the use of TiAl sheets as structural materials. The ductile-phase toughening of brittle materials has been widely applied in composites with different ductile reinforcement morphologies, such as particles, fibers, and laminates. The ductile-phase in laminate form has the maximum toughening efficiency for the same volume fraction of the ductile reinforcing phase [12–15]. Over the past two decades, a number of diverse intermetallics have been toughened using various ductile metal laminates. Metal– intermetallic composites (MICs) increase the fracture toughness of metals and the high strength and elastic modulus of intermetallic compounds [16–18]. Wang et al. [18] fabricated Ni/Ni3Al multilayer composites through the reaction synthesis of Nickel (Ni) and Al foils. The effects of temperature on the tensile properties and deformation behavior of Ni/Ni3Al multilayer composites have been systematically investigated. The yield strength of the multilayer composites increased with increasing tensile test temperature, from room temperature to 600 °C. Recent advances in EB-PVD have created new opportunities for preparing multilayer MIC foils. In this study, monolithic TiAl, TiAl/Niobium (Nb), and TiAl/NiCoCrAl microlaminated sheets with thicknesses of 0.4–0.6 mm and 150 mm × 100 mm dimensions were successfully fabricated through EB-PVD. The microstructural and mechanical properties, failure mechanisms, and deformation behavior of the TiAl-based multilayer composites produced through EB-PVD have been reported in previous papers [9,19–21]. The high-temperature tensile properties and deformation behavior of these TiAl-based multilayer composites were investigated in detail.
2. Experimental procedures 2.1. Preparation Monolithic TiAl, TiAl/Nb and TiAl/NiCoCrAl microlaminated sheets were produced via EB-PVD. The ingots of TiAl-based alloys used in this study, which had a nominal composition of Ti–47Al (wt.%), were casted two times in a water-cooling vacuum induction melting furnace. High purity CaF2 (99.7%) and Nb ingots (99.8%) were used in this study. The experimental device is a UE-204 type EB-PVD equipment with a horizontal feed mode. To separate the resulting sheet from the substrate in a convenient manner, a CaF2 ceramic stripper layer of about 20 μm thickness was requisitely deposited on the substrate before the actual evaporation. TiAl and Nb (or NiCoCrAl) ingots were alternately evaporated to produce TiAl/Nb (or TiAl/NiCoCrAl) microlaminates, and the layer thicknesses were controlled through the deposition time. At the same time, as compared with the microlaminates, the
ingot of TiAl was also used as the evaporation source for deposition of the monolithic TiAl foil. The detailed synthesis process and specific equipment can be found in our previous work [9]. After evaporation, the substrate was removed to the load chamber and subsequently extracted from the furnace after cooling to 423 K. At room temperature, the sheets with dimensions of 150 mm × 100 mm × 0.4 mm were obtained via mechanical stripping from the substrate surface. Tensile tests based on GB 6397-86 were conducted on the sheets using an INSTRON-5569 universal material testing machine with a crosshead displacement speed of 0.05 mm/min at room temperature and 750 °C. Before elevated temperature testing, the specimens were kept for 5 min at testing temperature. In this work, five specimens were tested to get an average value. The sheets were degreased ultrasonically in acetone and then pickled in either a 5 vol.% HF + 15 vol.% HNO3 (balance water) solution for 30 s. The microstructure of the TiAl sheet was studied with optical microscopy (OM) under reflecting and polarized light using a Leica DM 2500M microscope. The microstructural features and fractured surfaces of the composite were observed via scanning electron microscopy (SEM, FEI Sirion, The Netherlands) with simultaneous chemical composition analysis via energy dispersive spectroscopy (EDX, EDAX Inc.).
3. Results and discussions The cross-sectional morphologies of the monolithic TiAl, TiAl/Nb, and TiAl/NiCoCrAl microlaminated sheets prepared through EB-PVD are shown in Fig. 1(a)–(c), respectively. Spontaneous delamination alternation of the Ti-rich area and Al-rich area is inside the monolithic TiAl sheet (Fig. 1(a)) [9]. However, the natural stratification in the TiAl layer of the microlaminate sheets is unclear (Fig. 1(b)–(c)). As shown in Fig. 1(b), the TiAl/Nb microlaminated sheet is composed of 22 Nb layers and 23 TiAl layers alternating with uniform distribution. The Nb layer is only 1.0 μm, which is due to the large difference in the saturated Nb vapor pressure and to difficult evaporation. Moreover, Fig. 1(b) shows that the TiAl/NiCoCrAl microlaminated sheet is composed of 11 NiCoCrAl layers and 10 TiAl layers alternating with uniform distribution. The NiCoCrAl layer is thick, and the interlayer spacing is about 40 μm. The outermost layers of the microlaminated sheets are NiCoCrAl layers, which have better oxidation resistance at high temperature. The room temperature stress–strain curves of the monolithic TiAl, TiAl/Nb, and TiAl/NiCoCrAl micro-laminated sheets were determined through tension test (Fig. 2). All three materials have poor plasticity and present no yield phenomena at room temperature, showing brittle fracture without macroscopic plastic deformation. This was determined using the technical features of EB-PVD, such as the loose columnar crystal structure in the coating and high defect density. However, Fig. 2 shows that the strong layer (Nb or NiCoCrAl layer) obviously improves both the strength and plasticity of the TiAl sheet at room temperature. The ultimate tensile strengths and tensile strains of the micro-laminated sheets at room temperature are listed in Table 1. The TiAl/NiCoCrAl micro-laminated sheets have higher strength and plasticity at room temperature than that of the monolithic TiAl sheet, indicating the distinct reinforcement and toughening effect of the NiCoCrAl layer. The tensile strength
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Fig. 1 – Micrographs on the cross-section of three sheets: (a) monolithic TiAl sheet; (b) TiAl/Nb microlaminate sheet; and (c) TiAl/NiCoCrAl microlaminate sheet.
and elongation of the TiAl/NiCoCrAl micro-laminated sheet remarkably increased from 384.2 MPa and 0.11% to 547.1 MPa and 0.55%, respectively. The tensile strain of the TiAl/NiCoCrAl micro-laminated sheet increased by nearly 4 times, and its tensile strength significantly increased by 42% compared to that of the monolithic TiAl sheet. In addition, the elongation of the TiAl/Nb micro-laminated sheet increased by nearly 1.5 times, and its strength also increased by 18%. Thus, the micro-laminated sheets have good comprehensive mechanical properties.
Fig. 2 – Tensile stress–strain curves of three micro-laminates sheet at room temperature.
The volume of the strong layer (Nb or NiCoCrAl layer) is less than 25 vol.%. The dislocations in the TiAl layers are deeply restrained by the interfaces. These make the Nb and/or the NiCoCrAl layers as brittle as the TiAl layers, and the micro-laminated sheets simply are composites consisting of two hard phases. The yield strengths of the micro-laminates sheets are expressed as σ 0 ¼ VA σ A þ VB σ B
ð1Þ
where σA and σB are the yield strengths of the strongly constrained TiAl layers and the strong layers (Nb or NiCoCrAl layer), respectively, using σA = 384 MPa, σB-Nb = 920 MPa [22], and σB-NiCoCrAl = 1060 MPa [23]. Calculations from the simple mixing law (Eq. (1)) are only related to the volume fraction of the constituent phases, which is also consistent with the experimental results. The above modeling shows that the strengthening mechanism in the micro-laminated sheets changes from the slide of a single dislocation confined to individual TiAl layers to the load-bearing effect, which is similar to some cases of composites [24]. The ductility of the multilayer composed of a softer phase and a stiffer phase is controlled by the softer phase. At any given strength, the TiAl/NiCoCrAl micro-laminated sheet has higher ductility compared to the TiAl/Nb micro-laminated sheet (Fig. 2). This finding is due to the fracture mechanism in the micro-laminated sheet [25]. The microcrack first nucleates in the brittle TiAl layer, and the further propagation of the microcracks is suppressed by the surrounding ductile NiCoCrAl or Nb layers, resulting in microcrack arrest. The
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Table 1 – Mechanical properties of TiAl-based sheets. Material
Tensile strength/MPa
Elongation/%
Tensile strength/MPa
Elongation/%
Room temperature: 750 °C TiAl TiAl/Nb TiAl/NiCoCrAl
384.2 453.4 547.1
plastic deformation capability of micro-laminated sheets is heavily dependent on the dislocation movement of the ductile layer. Some dislocations in the ductile layers are movable, and the ductile layers have some deformation capability to prevent the propagation of tiny microcracks. The better the plastic deformation capability of the ductility layers, the more the ductility layers arrest the microcracks. The NiCoCrAl layers have better plastic deformation capability [22,23], and therefore, the TiAl/NiCoCrAl micro-laminated sheets show greater ductility. Fig. 3 shows the SEM images of the tensile fractures of the monolithic TiAl, TiAl/Nb, and TiAl/NiCoCrAl micro-laminates at room temperature. The fracture microstructures of the TiAl/Nb and TiAl/NiCoCrAl micro-laminates are columnar crystal, and no significant difference was observed between the tensile fracture and bending fracture. The fracture of the TiAl/Nb micro-laminates at room temperature is wellarranged, and the continuously distributed clear Nb layer is closely combined with the TiAl layer (Fig. 4(b)). Furthermore, the strong Nb layer deflects cracks and causes some plastic deformations represented by partial quasi-cleavage fracture,
0.11 0.26 0.51
234.9 443.1 310.3
2.93 26.4 72.2
which is a combination of the quasi-cleavage fracture of certain ductility and intergranular brittle fracture. Fig. 3(c) shows that the TiAl layer in the TiAl/NiCoCrAl micro-laminate shows intergranular brittle fracture, whereas the NiCoCrAl layer shows a mixed fracture transgranular and intergranular fracture mechanism, which pertains to the evident ductile fracture mechanism. In addition, some flocculent structures and small crystal grains are found in the crystal boundary of the micro-laminates, indicating the effectiveness of the NiCoCrAl and Nb layers in improving the plasticity and strength of TiAl alloys at room temperature. Fig. 4 shows the stress–strain curves of three TiAl-based alloy foils at 750 °C. The metallic toughening layer significantly improves its high-temperature strength and toughness, exhibiting obvious yield. The elongation of the TiAl/ NiCoCrAl micro-laminate sheet at 750 °C reaches 72.2%, showing typical superplasticity characteristics. The high-temperature strength of the TiAl/Nb micro-laminated sheet reaches 443.1 MPa, almost equal to its strength at room temperature. According to the above data, the TiAl/NiCoCrAl micro-laminates have low BDTT (ductile–brittle transition temperature), and its
Fig. 3 – SEM fractographs of the sheets at room temperature: (a) TiAl; (b) TiAl/Nb; and (c) TiAl/NiCoCrAl.
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Fig. 4 – Tensile stress–strain curves of three micro-laminate sheets at 750 °C.
strength quickly decreases as the test temperature increases, contrary to the TiAl alloy, whose strength slowly decreases, or even increases, as temperature increases [26]. As a result, the TiAl/NiCoCrAl micro-laminated sheet has the most increase of plasticity and highest decrease of strength as the test temperature increases. Nb atoms have high solid solubility in the TiAl matrix of the TiAl/Nb micro-laminated sheet at high temperatures. Therefore, the high Nb content of the Ti–Al–Nb solid solution can induce the orderly distribution of atoms for the TiAl/Nb micro-laminated sheet, which increases its hightemperature strength but decreases its BDTT. Both the plasticity and high-temperature strength of the TiAl/Nb and TiAl/NiCoCrAl micro-laminated sheets simultaneously improved compared with the monolithic TiAl sheet. The typical stress–strain curve of the TiAl alloy sheet at 750 °C shows an absolute brittle fracture without plastic deformation. Therefore, the yield strength of the micro-laminated sheets at 750 °C is also expressed by the simple mixing law. However, the yield strength of the Nb alloy quickly increases as the test temperature increases [27], contrary to the NiCoCrAl alloy whose strength decreases as temperature increases [28]. The calculations are also close to the experimental results. Likewise, the plastic deformation capability of the micro-laminated sheet at high temperatures heavily depends on the dislocation movement of the ductile layer. The TiAl/ NiCoCrAl micro-laminated sheets show greater ductility because the NiCoCrAl layers have better plastic deformation capability at 750 °C [26,27]. Figs. 5–6 show the macro- and micro-fracture morphologies of the monolithic TiAl and the micro-laminated sheets at 750 °C. Fig. 5 shows that the macro-fracture of the TiAl micro-laminate is perpendicular to the tensile axis. Its location also implies that it is formed by internal microscopic defects. The macro-fracture position of the TiAl/NiCoCrAl microlaminated sheet lies in the middle of the testing sample, and is a necking fracture with good plasticity. The macro-fracture of the TiAl/Nb micro-laminated sheet is between the straight brittle fracture and necking fracture. Fig. 6b shows that the hightemperature fracture of the TiAl/Nb micro-laminate mainly contains granular fractures, intergranular fractures, and some
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Fig. 5 – Tensile macro-fracture morphologies of three sheets at 750 °C: (a) TiAl; (b) TiAl/Nb; and (c) TiAl/NiCoCrAl.
trans-granular dimple fractures surrounding the Nb layer. Compared with the tensile fracture at room temperature (Fig. 2), the TiAl–Nb interfaces are bonded more tightly due to the diffusion of Nb at the interfaces, and the Nb layer is thinned and significantly bent, blunting the micro-crack instead of extending the cleavage. Therefore, the material shows ductile fracture behavior. Fig. 6c shows that the fracture mode of the NiCoCrAl layer in the TiAl/NiCoCrAl micro-laminated sheet presents distinct dimple fracture. The TiAl layer also shows dimple fracture, although it is unclear because of oxidization. These phenomena confirm that the Nb and NiCoCrAl layers decrease the BDTT of the TiAl alloy to less than 750 °C, and significantly improve its high-temperature plasticity.
4. Conclusions The strong metallic layer can effectively improve the hightemperature properties of TiAl sheets. It not only offsets the adverse effects of EB-PVD (e.g. loose structure), but also enables the direct use of the prepared micro-laminate without subsequent processing. Although the TiAl/NiCoCrAl microlaminated sheet has the best comprehensive properties at room temperature (σt = 547.1 MPa, ε = 0.51%), it is inferior because of its poor high-temperature strength caused by the Ni-based alloy. The TiAl/Nb micro-laminate shows ideal hightemperature strength and plasticity (σt = 443.1 MPa, ε = 26.4%). Thus, it has attractive development prospect. Therefore, the micro-laminated sheets have much greater specific strength than the monolithic TiAl sheets.
Acknowledgments This work was supported by the National Natural Science Foundation of China (11102003, 11227801 and 91216301). Support by the Fundamental Research Fund for the Central Universities (2014JBM076) was also acknowledged.
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Fig. 6 – Micro-fracture morphologies of TiAl-based sheets at 750 °C: (a) TiAl; (b) TiAl/Nb; and (c) TiAl/NiCoCrAl.
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