Accepted Manuscript Title: A study on the orientation inheritance in laminated NiAl produced by in situ reaction annealing Author: Yan Du Guohua Fan Lin Geng PII: DOI: Reference:
S0968-4328(16)30006-3 http://dx.doi.org/doi:10.1016/j.micron.2016.01.006 JMIC 2274
To appear in:
Micron
Received date: Revised date: Accepted date:
11-11-2015 25-1-2016 25-1-2016
Please cite this article as: Du, Yan, Fan, Guohua, Geng, Lin, A study on the orientation inheritance in laminated NiAl produced by in situ reaction annealing.Micron http://dx.doi.org/10.1016/j.micron.2016.01.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A study on the orientation inheritance in laminated NiAl produced by in situ reaction annealing Yan Du, Guohua Fan*, Lin Geng School of Materials Science and Engineering, Harbin Institute of Technology, No.92 Xidazhi Street, Harbin, Heilongjiang Province 150001, PR China Corresponding author: Guohua Fan E-mail: [email protected]
Highlights Texture inheritance is newly found in laminated NiAl produced by reaction annealing. There is K-S orientation relationship between parent Ni and product NiAl. Interfacial characterization is used to study phase transformation in Ni-Al system. It is newly found Ni3Al acts as pipeline between parent Ni and product NiAl.
1
Abstract In order to promote the performance of B2 NiAl by texture control of orientation during in situ processing, phase transformation in laminated NiAl with bimodal grain size distribution manufactured by reaction annealing of Ni and Al foils has been studied. It turned out that there existed a Kurdjumov-Sachs orientation relationship (K-S OR) between parent Ni and product NiAl by crystallography analysis according to the electron backscatter diffraction (EBSD) results. The parent Ni did not transform to the product NiAl directly but via the formation of Ni3Al firstly according to the transmission electron microscope (TEM) observation of the interface. This led to a new K-S OR between Ni3Al and NiAl with a small atomic misfit, which made less residual stress generated through the formation of Ni3Al than directly from the parent Ni. Keywords: Laminated NiAl; Reaction annealing; EBSD; Orientation relationship; Texture inheritance
1. Introduction Extensive investigation has identified NiAl with a B2 structure as an attractive material for high temperature applications which possesses high melting temperature, low density, good thermal conductivity and good oxidation resistance (Liu, 1995; Stoloff et al., 2000; Morsi, 2001). Moreover, the crystal orientation of NiAl single crystal strongly influences the deformation and mechanical behavior (Baker, 1995). For instance, texture of NiAl polycrystalline induces plastic anisotropy with being much harder than and (Skrotzki et al., 2001, 2002). Thus, the
2
mechanical properties can be improved if we can control the preferred orientation of NiAl during preparation. Recently, a view of texture inheritance has been discussed in phase transformation process by reactive diffusion of Ti-Ni system (Inoue et al., 2003), and similar result has also been found in Ni-Al(β)/Ni3Al(γ′) two phase alloys obtained by thermomechanical processing (Sakata et al., 2001). Furthermore, electron backscatter diffraction (EBSD) based orientation microscopy has been used to study the orientation relationship between lath martensite and bainite (Miyamoto et al., 2009), and kindred analysis of ferrite and martensite (Suh et al., 2002). Our previous study revealed that dense laminated NiAl sheet could be produced through solid-liquid reaction synthesis between Ni and Al foils when Ni and Al reacted completely with Ni/Al atom ratio of 1:1 (Fan et al., 2014). During the reaction process, NiAl formed through the element atomic diffusion and occupancy. Therefore, there may be orientation relationship between NiAl and Ni. We found orientation relationship existed between Ni and NiAl matrix in our previous study (Wang et al., 2013) on microlaminated TiB2-NiAl composite fabricated by reaction annealing. However, we only assumed the orientation relationship through texture comparison measured by X-ray diffraction (XRD) and did not observe the details during reaction annealing. Therefore, the orientation inheritance of NiAl prepared by in situ reaction method should be investigated in detail to promote further application of NiAl. In this study, through solid-liquid reaction synthesis between stacked Ni and Al foils, pure NiAl sheet consisting of alternate coarse-grained and fine-grained layers was produced. By measuring the orientations of product coarse-grained NiAl and parent Ni after and during the annealing, the orientation relationship between the parent Ni and the product coarse-grained NiAl was established based on EBSD
3
analysis. Moreover, microstructure of the interface between the parent Ni and the product NiAl during processing was studied by transmission electron microscope (TEM). 2. Material and methods Foils of commercial purity Ni and Al with thicknesses of 0.07 mm and 0.1 mm were used as the raw materials to prepare the near-stoichiometric NiAl. Both of the Ni and Al foils were cut into squares with a dimension of 30 × 30 mm2, and treated in an ultrasonic bath of acetone for 5 min. Then, the Ni and Al foils were stacked alternately with Ni at the ends. The laminated NiAl was fabricated by three steps: i) hot pressing diffusion bonding at 500 ˚C under a pressure of 65 MPa for 2 h to obtain laminated Ni–Al composite sheet; ii) reaction annealing at 1200 ˚C for 2 h without pressure to synthesize laminated NiAl; iii) densification at 1100 ˚C for 30 min under a pressure of 30 MPa and furnace cooling to obtain a dense laminated NiAl sheet. The final thickness of the dense laminated NiAl sheet was 2 mm. In order to measure the orientation relationship between parent Ni and product coarse-grained NiAl during the synthesis process, laminated Ni–Al composite sheet was annealed at 1200 ˚C for 5 min. XRD measurements using CuKα radiation were carried out in a diffractometer (Philips X’pert) for phase identification and peak intensity observation of Ni layer. Moreover, microstructure, texture and orientation relationship of NiAl sheet and annealed laminated Ni-Al composite sheet were characterized via EBSD using a Zeiss Supra 35 thermal field emission gun scanning electron microscope (SEM). A detailed study on the interface of annealed laminated Ni-Al composite sheet was conducted by a TEM (Tecnai G2 F30). The thin foil for TEM observation was prepared by an FEI HELIOS Nanolab 600i dual-beam workstation with focused ion beam (FIB). All the
4
observations by SEM, EBSD and TEM were conducted on the normal directiontransverse direction (ND-TD) cross section. 3. Results and discussion 3.1 Preferred orientation analysis of Ni and NiAl After hot pressing, a laminated Ni–Al composite sheet was obtained. Fig. 1(a) shows the SEM backscattering micrograph of laminated Ni–Al composite sheet and the tested plane for XRD measurement. Fig. 1(b) shows the XRD patterns of Ni before (tested plane is normal plane) and after hot pressing, and standard relative diffraction intensity of Ni. The XRD patterns show that a strong {220} peak of Ni appears both before and after hot pressing. The only difference is the relative intensity, which is lower for Ni after hot pressing. This indicated that the texture of raw rolled Ni foils did not change much, i.e. Ni still had a preferred orientation of // ND, which was mainly from brass (Bs) component {110} for rolled fcc metals with medium stacking fault energy like Ni (Ray, 1995). The preferred orientation of Ni did not change much because the hot-pressed temperature was below the recrystallization temperature of Ni (0.4 Tm, Tm(Ni) = 1455 ˚C). Orientation map (inverse pole figure of the ND) of the laminated NiAl synthesized by laminated Ni–Al composite sheet is shown in Fig. 2. The map has been measured using a lattice parameter a = 0.289 nm for NiAl with B2 structure. The laminated NiAl has a strong {111} texture component (23%) tested by EBSD. As shown in Fig. 2(a), the average grain size of coarse-grained NiAl forming at the region of initial Ni layer is around 100 ± 20 μm and the average grain size of fine-grained NiAl forming at the region of initial Al layer is around 20 ± 7 μm. The orientations of them were measured and the crystallographic of both the coarse-grained NiAl and the fine-grained NiAl in inverse pole figures were shown in Figs. 2(b) and 2(c),
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respectively. It is obvious that the grains of coarse-grained NiAl layers most likely have a preferred orientation of // ND, and some others have orientations near // ND. Nevertheless, the grains of fine-grained NiAl layers have random orientation distribution. This demonstrates that the preferred orientation // ND of the laminated NiAl sheet is mainly from the coarse-grained NiAl layers. 3.2 Synthesis mechanism of NiAl As NiAl3 and Ni2Al3 phases decompose at 854 ˚C and 1133 ˚C, respectively, there are only Ni, liquid phase Al(Ni) and the product NiAl existing steadily at 1200 ˚C in this system (the atom ratio of Ni:Al is 1 : 1) according to the Ni-Al binary phase diagram (Okamoto, 1993) and the investigation for the combustion synthesis of NiAl (Zhu et al., 2002; Qiu et al., 2009; Battezzati et al., 1999). The synthesis mechanism which has been briefly discussed in our previous work (Fan et al., 2014) is that the NiAl formed at the region of initial Ni and Al in different ways. In detail, it can be described as following: i) before annealing treatment, Ni and Al are both in solid state; Ni dissolution in liquid Al is expected to occur at 660 ˚C when aluminum melt and then Al(Ni) liquid solution forms; ii) under equilibrium conditions at 1200 ˚C, the saturated solubility of Ni in liquid Al is about 28 at.%; iii) at 1200 ˚C, NiAl phase starts to form between solid Ni and Al(Ni) liquid solution, when the concentration of Ni in liquid Al close to Ni side reaches saturation firstly. As Ni diffuses in NiAl and dissolves in liquid phase Al(Ni), the NiAl layer grows continuously and the content of Ni in liquid phase Al(Ni) increases. As the reaction proceeds, product NiAl on the original Ni side takes the place of Ni and the grains over grow due to the annealing for 2 h. On the original Al side, Ni element dissolves into the liquid phase Al(Ni) continuously, and some NiAl nuclei precipitate from the liquid phase Al(Ni) when the content of Ni reaches to the composition range of NiAl. During the cooling, fine
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equiaxed grains of NiAl form based on the NiAl nuclei. In order to study the phase transformation mechanism from Ni to NiAl during the reaction treatment at 1200 ˚C, the hot-pressed laminated Ni-Al composite sheet was annealed at 1200 ˚C for 5 min and furnace cooled. The resulting sample was characterized by EBSD, and the OIM map (inverse pole figure of the ND), phase map and typical pole figures are shown in Fig. 3. From the OIM map and phase map in Figs. 3(a) and 3(b), it can be found the interface mainly consists of Ni and NiAl. It is necessary to note that a thin layer exists between Ni and NiAl marked by arrows in Fig. 3(b), which has been identified to be Ni3Al according to EBSD Kikuchi pattern and energy dispersive spectroscopy (EDS) analysis. The formation mechanism of Ni3Al may be attributed to two aspects: (i) there is Ni3Al phase close to Ni side at 1200 ˚C in binary phase diagram of Ni-Al. In our experiment, the Ni3Al at the interface was formed by solid diffusion for supersaturated Ni (Al) and ordering phase transformation of Ni (Al); (ii) after further furnace cooling, some Ni3Al nuclei/grains may precipitate/grow from the supersaturated Ni (Al) as shown in Fig. 3(a). It is worth noting that Ni and Ni3Al have almost identical lattice parameters and similar crystal structure, i.e. Ni3Al with a = 0.357 nm and Pm-3m of space group structure and Ni with a = 0.352 nm and Fm-3m. Therefore, during reaction treatment at 1200 ˚C, the Ni3Al may not affect the reaction mechanism and orientation relationship between Ni and NiAl, and similar research results have been reported in Ti-Ni solid phase reaction (Inoue et al., 2003) and Ni-Al(β)/Ni3Al(γ′) two phase alloys (Sakata et al., 2001). As an example, the orientation relationship between the Ni grain (GI) and the neighboring NiAl grain (GII) was analyzed according to {111} and {110} pole figures of both grains. From the pole figures, (-1-11)Ni marked by triangle in Fig. 3(c) and
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(-110)NiAl marked by triangle in Fig. 3(d) almost completely overlap. Meanwhile, (1-10)Ni marked by circle in Fig. 3(c) overlaps with (111)NiAl marked by circle in Fig. 3(d). Thus, the Ni phase and NiAl phase perfectly hold the Kurdjumov-Sachs orientation relationship (K-S OR) of (-1-11)Ni // (-110)NiAl and [1-10]Ni // [111]NiAl. The deviation angle from K-S OR is as small as 4˚. Couples of other Ni grains and their neighboring NiAl grains were also analyzed and similar ORs have been found. This experimental result verifies the theoretical analysis about orientation inheritance of the coarse-grained layers and the preferred orientation // ND of NiAl layer is inherited from preferred orientation // ND of Ni layer. This K-S OR between Ni and NiAl during in situ reaction annealing was discussed in our previous study (Wang et al., 2013) on microlaminated TiB2-NiAl composite, in which the K-S OR was only assumed by analysis of texture of initial Ni and synthesized NiAl without considering the transfer fact of Ni3Al during the reaction annealing. The interface between the parent Ni and the product NiAl is marked by arrows in Fig. 3, which was identified to be Ni3Al according to EBSD and EDS analysis as mentioned above. Thus, the product NiAl does not form directly from Ni but via the formation of Ni3Al firstly. 3.3 Orientation inheritance between NiAl and Ni through Ni3Al In order to study the interface between parent Ni and product NiAl as mentioned in Section 3.2, the interface between Ni and NiAl layers in the laminated Ni-Al composite sheet annealed at 1200 ˚C for 5 min was observed by TEM. The microstructure of the interface is shown in Fig. 4. The selected area electron diffraction (SAED) patterns of the product layers are shown in Fig. 4(b)-(d) as NiAl, NiAl with high density defects, and Ni3Al. As we can see, the interface is clearly separated by four parts, which are parent Ni, Ni3Al, NiAl with high density defects
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and NiAl from right to left as shown in Fig. 4(a). As the SAED patterns presented, all the intermetallic layers show strong ordered structure with superlattice. The NiAl layer next to Ni3Al layer shows complex microstructure with high density defects, and the diffraction pattern as shown in Fig. 4(c) shows complex superlattice structure, which was caused by diffraction deflection due to the formation of product NiAl with different compositions through diffusion. This phenomenon was also found in Ni-Al powder subjected to high energy ball milling (Fan et al., 2015). Considering the formation of Ni3Al, and the K-S OR between the parent Ni and product NiAl, the orientation inheritance between them is actually through Ni3Al. The unit cells with closed packed planes and directions of Ni, Ni3Al and NiAl are illustrated in Fig. 5. The close packed planes and directions in all the Ni, Ni3Al and NiAl, which act as the habit planes and directions in the K-S OR, are also marked colour. Through the thin Ni3Al layer, the interatomic distance and spacing of close-packed planes of Ni (fcc) and NiAl (bcc) became much closer. According to the lattice parameters presented in Section 3.1, the mismatch between Ni3Al and NiAl is only 0.84% according to the K-S OR. The misfit between the intermetallic phases may cause residual stress, which leads to dislocation formation, especially in the NiAl layer with high density defects and the interfaces. The microstructure of synthesized NiAl as demonstrated by Fig. 4(a) is shown in detail in Fig. 6. High-resolution transmission electron microscopy (HRTEM) image of NiAl with high density defects is shown in Fig. 6(a). An area in Fig. 6(a) was selected marked by a square and observed by inverse fast Fourier transform (IFFT) as shown in Fig. 6(b) using diffraction patterns in the inset of Fig. 6(a). It clearly shows dislocations (marked by arrows) and misfit atom planes (marked by dash lines). These defects including vacancy, interstitial defect and antiphase
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boundary (APB) are mainly caused by diffusion in Ni-Al system during annealing and the subsequent cooling. Fig. 6(c) shows dislocations forming at the interface induced by the interatomic distance misfit between the layers. In addition, as the microstructure of the layers is different, the elastic module and the thermal expansion coefficient of the layers may differ. This can also produce dislocations at the interface due to mechanical and thermal residual stress during cooling from 1200 ˚C. Based on the above discussion, a schematic illustration of the orientation inheritance of parent Ni and product NiAl through Ni3Al between them is shown in Fig. 7. Firstly, the parent Ni transforms to Ni3Al based on coherent relationship through diffusion between Ni and Al elements. Then, Ni3Al transforms to NiAl by {111}Ni3Al // {110}NiAl and Ni3Al // NiAl K-S OR, with a misfit value of 0.84%. 4. Conclusions The orientation inheritance in laminated NiAl with bimodal grain size distribution by reaction synthesis of Ni and Al foils has been investigated. The orientation relationship between the parent Ni and the product coarse-grained NiAl has been identified as K-S orientation relationship by crystallography analysis according to EBSD results. The parent Ni does not transform to NiAl directly but via the formation of Ni3Al firstly. Through the thin Ni3Al layer besides Ni layer, NiAl can form based on K-S orientation relationship with a small atomic misfit and less thermal and residual stress than directly from Ni.
Acknowledgments
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The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51571071, 51571070, 51541110), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.
References Baker I., 1995. A review of the mechanical properties of B2 compounds. Mater. Sci. Eng. A 192, 1-13. Battezzati L., Pappalepore P., Durbiano F., Gallino I., 1999. Solid state reactions in Al/Ni alternate foils induced by cold rolling and annealing. Acta Mater. 47(6), 1901-1914. Fan G.H., Wang Q.W., Du Y., Geng L., Hu W., Zhang X., Huang Y.D., 2014. Producing laminated NiAl with bimodal distribution of grain size by solid-liquid reaction treatment. Mater. Sci. Eng. A 590, 318-322. Fan G.H., Geng L., Feng Y., Cui X.P., Yan X.D., 2015. Solid state reaction mechanism and microstructure evolution of Ni-Al powder during high energy ball milling revisited by TEM. Microsc. Microanal. 21, 953-960. Inoue H., Ishio M., Takasugi T., 2003. Texture of TiNi shape memory alloy sheets produced by roll-bonding and solid phase reaction from elementary metals. Acta Mater. 51, 6373-6383. Liu C.T., 1995. Recent advances in ordered intermetallics. Mater. Chem. Phys. 42, 77-86.
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Miyamoto G., Takayama N., Furuhara T., 2009. Accurate measurement of the orientation relationship of lath martensite and bainite by electron backscatter diffraction analysis. Scr. Mater. 60, 1113-1116. Morsi K., 2001. Review: reaction synthesis procssing of Ni-Al intermetallic materials. Mater. Sci. Eng. A 299, 1-15. Okamoto H., 1993. J. Phase Equilibria 14, 257. Qiu X., Liu R., Guo S., Graeter J.H., Kecskes L., Wang J., 2009. Combustion synthesis reactions in cold-rolled Ni/Al and Ti/Al multilayers. Metall. Mater. Trans. A 40, 1541-1546. Ray R.K., 1995. Rolling texture of pure nickel, nickel-iron and nickel-cobalt alloys. Acta Metall. Mater. 43, 3861-3872. Sakata T., Yasuda H.Y., Umakoshi Y., 2001. Control of microstructure and orientation distribution in Ni-Al based (β/ γ′) two phase alloys by thermomechanical processing. Acta Mater. 49, 4231-4239. Skrotzki W., Tamm R., Oertel C.G., Beckers B., Brokmeier H.G., Rybacki E., 2001. Texture induced plastic anisotropy of NiAl polycrystals. Mater. Sci. Eng. A 319-321, 364-367. Skrotzki W., Tamm R., Oertel C.G., Beckers B., Brokmeier H.G., Rybacki E., 2002. Influence of texture and hydrostatic pressure on the room temperature compression of NiAl polycrystals. Mater. Sci. Eng. A 329-331, 235-240. Stoloff N.S., Liu C.T., Deevi S.C., 2000. Emerging applications of intermetallics. Intermetallics 8, 1313-1320. Suh D.W., Kang J.H., Oh K.H., Lee H.C., 2002. Evaluation of the deviation angle of ferrite from the Kudjumov-Sachs relationship in a low carbon steel by EBSD. Scr. Mater. 46, 375-378.
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Wang Q.W., Fan G.H., Geng L., Zhang J., Cui X.P., Pang J.C., Qin S.H., Du Y., 2013. A novel fabrication route to microlaminated TiB2-NiAl composite sheet with {111} texture by roll bonding and annealing treatment. Intermetallics 37, 46-51. Zhu P., Li J.C.M., Liu C.T., 2002. Reaction mechanism of combustion synthesis of NiAl. Mater. Sci. Eng. A 329-331, 57-68.
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Fig. 1. Microstructure of laminated Ni-Al composite: (a) SEM backscattering micrograph of laminated Ni–Al composite sheet; (b) contrastive XRD patterns of Ni before and after hot pressing (tested surface are both perpendicular to the ND).
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Fig. 2. Crystallography of laminated NiAl with bimodal grain size distribution: (a) orientation imaging microscopy (OIM, inverse pole figure colour of the ND) for laminated NiAl sheet; crystallographic of laminated NiAl sheet in inverse pole figures based on the ND for (b) coarse-grained layer and (c) fine-grained layer.
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Fig. 3. Crystallography between both of the Ni and NiAl layers in laminated Ni-Al composite sheet annealed at 1200 ˚C for 5 min and then subjected to furnace cooling: (a) OIM (inverse pole figure color of the ND for both Ni and NiAl); (b) phase map corresponding to (a); {111} and {110} pole figures corresponding to Ni grain GI (c) and NiAl grain GII (d) in (a).
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Fig. 4. Microstructure of the interface between Ni and NiAl layers in laminated Ni-Al composite sheet annealed at 1200 ˚C for 5 min: (a) bright field (BF) image of the interface; (b) SAED pattern of NiAl; (c) SAED pattern of NiAl with high density defects and (d) SAED pattern of Ni3Al.
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Fig. 5. Illustration of unit cells for Ni, Ni3Al and NiAl showing close packed planes and directions in K-S OR.
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Fig. 6. Microstructure of synthesized NiAl (hot-pressed laminated Ni-Al composite sheet annealed at 1200˚C for 5 min): (a) HRTEM image of NiAl with high density defects; (b) IFFT image of the area marked by square using diffraction shown in the inset in (a), showing dislocations and misfit atom planes; (c) BF image of the interface between NiAl layer with high density defects and NiAl layer without high density defects, showing dislocations at the interface induced by the misfit.
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Fig. 7. Schematic illustration of the orientation relationship between parent Ni and product NiAl through the formation of Ni3Al at the interface.
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S0968-4328(16)30006-3 http://dx.doi.org/doi:10.1016/j.micron.2016.01.006 JMIC 2274
To appear in:
Micron
Received date: Revised date: Accepted date:
11-11-2015 25-1-2016 25-1-2016
Please cite this article as: Du, Yan, Fan, Guohua, Geng, Lin, A study on the orientation inheritance in laminated NiAl produced by in situ reaction annealing.Micron http://dx.doi.org/10.1016/j.micron.2016.01.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A study on the orientation inheritance in laminated NiAl produced by in situ reaction annealing Yan Du, Guohua Fan*, Lin Geng School of Materials Science and Engineering, Harbin Institute of Technology, No.92 Xidazhi Street, Harbin, Heilongjiang Province 150001, PR China Corresponding author: Guohua Fan E-mail: [email protected]
Highlights Texture inheritance is newly found in laminated NiAl produced by reaction annealing. There is K-S orientation relationship between parent Ni and product NiAl. Interfacial characterization is used to study phase transformation in Ni-Al system. It is newly found Ni3Al acts as pipeline between parent Ni and product NiAl.
1
Abstract In order to promote the performance of B2 NiAl by texture control of orientation during in situ processing, phase transformation in laminated NiAl with bimodal grain size distribution manufactured by reaction annealing of Ni and Al foils has been studied. It turned out that there existed a Kurdjumov-Sachs orientation relationship (K-S OR) between parent Ni and product NiAl by crystallography analysis according to the electron backscatter diffraction (EBSD) results. The parent Ni did not transform to the product NiAl directly but via the formation of Ni3Al firstly according to the transmission electron microscope (TEM) observation of the interface. This led to a new K-S OR between Ni3Al and NiAl with a small atomic misfit, which made less residual stress generated through the formation of Ni3Al than directly from the parent Ni. Keywords: Laminated NiAl; Reaction annealing; EBSD; Orientation relationship; Texture inheritance
1. Introduction Extensive investigation has identified NiAl with a B2 structure as an attractive material for high temperature applications which possesses high melting temperature, low density, good thermal conductivity and good oxidation resistance (Liu, 1995; Stoloff et al., 2000; Morsi, 2001). Moreover, the crystal orientation of NiAl single crystal strongly influences the deformation and mechanical behavior (Baker, 1995). For instance, texture of NiAl polycrystalline induces plastic anisotropy with being much harder than and (Skrotzki et al., 2001, 2002). Thus, the
2
mechanical properties can be improved if we can control the preferred orientation of NiAl during preparation. Recently, a view of texture inheritance has been discussed in phase transformation process by reactive diffusion of Ti-Ni system (Inoue et al., 2003), and similar result has also been found in Ni-Al(β)/Ni3Al(γ′) two phase alloys obtained by thermomechanical processing (Sakata et al., 2001). Furthermore, electron backscatter diffraction (EBSD) based orientation microscopy has been used to study the orientation relationship between lath martensite and bainite (Miyamoto et al., 2009), and kindred analysis of ferrite and martensite (Suh et al., 2002). Our previous study revealed that dense laminated NiAl sheet could be produced through solid-liquid reaction synthesis between Ni and Al foils when Ni and Al reacted completely with Ni/Al atom ratio of 1:1 (Fan et al., 2014). During the reaction process, NiAl formed through the element atomic diffusion and occupancy. Therefore, there may be orientation relationship between NiAl and Ni. We found orientation relationship existed between Ni and NiAl matrix in our previous study (Wang et al., 2013) on microlaminated TiB2-NiAl composite fabricated by reaction annealing. However, we only assumed the orientation relationship through texture comparison measured by X-ray diffraction (XRD) and did not observe the details during reaction annealing. Therefore, the orientation inheritance of NiAl prepared by in situ reaction method should be investigated in detail to promote further application of NiAl. In this study, through solid-liquid reaction synthesis between stacked Ni and Al foils, pure NiAl sheet consisting of alternate coarse-grained and fine-grained layers was produced. By measuring the orientations of product coarse-grained NiAl and parent Ni after and during the annealing, the orientation relationship between the parent Ni and the product coarse-grained NiAl was established based on EBSD
3
analysis. Moreover, microstructure of the interface between the parent Ni and the product NiAl during processing was studied by transmission electron microscope (TEM). 2. Material and methods Foils of commercial purity Ni and Al with thicknesses of 0.07 mm and 0.1 mm were used as the raw materials to prepare the near-stoichiometric NiAl. Both of the Ni and Al foils were cut into squares with a dimension of 30 × 30 mm2, and treated in an ultrasonic bath of acetone for 5 min. Then, the Ni and Al foils were stacked alternately with Ni at the ends. The laminated NiAl was fabricated by three steps: i) hot pressing diffusion bonding at 500 ˚C under a pressure of 65 MPa for 2 h to obtain laminated Ni–Al composite sheet; ii) reaction annealing at 1200 ˚C for 2 h without pressure to synthesize laminated NiAl; iii) densification at 1100 ˚C for 30 min under a pressure of 30 MPa and furnace cooling to obtain a dense laminated NiAl sheet. The final thickness of the dense laminated NiAl sheet was 2 mm. In order to measure the orientation relationship between parent Ni and product coarse-grained NiAl during the synthesis process, laminated Ni–Al composite sheet was annealed at 1200 ˚C for 5 min. XRD measurements using CuKα radiation were carried out in a diffractometer (Philips X’pert) for phase identification and peak intensity observation of Ni layer. Moreover, microstructure, texture and orientation relationship of NiAl sheet and annealed laminated Ni-Al composite sheet were characterized via EBSD using a Zeiss Supra 35 thermal field emission gun scanning electron microscope (SEM). A detailed study on the interface of annealed laminated Ni-Al composite sheet was conducted by a TEM (Tecnai G2 F30). The thin foil for TEM observation was prepared by an FEI HELIOS Nanolab 600i dual-beam workstation with focused ion beam (FIB). All the
4
observations by SEM, EBSD and TEM were conducted on the normal directiontransverse direction (ND-TD) cross section. 3. Results and discussion 3.1 Preferred orientation analysis of Ni and NiAl After hot pressing, a laminated Ni–Al composite sheet was obtained. Fig. 1(a) shows the SEM backscattering micrograph of laminated Ni–Al composite sheet and the tested plane for XRD measurement. Fig. 1(b) shows the XRD patterns of Ni before (tested plane is normal plane) and after hot pressing, and standard relative diffraction intensity of Ni. The XRD patterns show that a strong {220} peak of Ni appears both before and after hot pressing. The only difference is the relative intensity, which is lower for Ni after hot pressing. This indicated that the texture of raw rolled Ni foils did not change much, i.e. Ni still had a preferred orientation of // ND, which was mainly from brass (Bs) component {110} for rolled fcc metals with medium stacking fault energy like Ni (Ray, 1995). The preferred orientation of Ni did not change much because the hot-pressed temperature was below the recrystallization temperature of Ni (0.4 Tm, Tm(Ni) = 1455 ˚C). Orientation map (inverse pole figure of the ND) of the laminated NiAl synthesized by laminated Ni–Al composite sheet is shown in Fig. 2. The map has been measured using a lattice parameter a = 0.289 nm for NiAl with B2 structure. The laminated NiAl has a strong {111} texture component (23%) tested by EBSD. As shown in Fig. 2(a), the average grain size of coarse-grained NiAl forming at the region of initial Ni layer is around 100 ± 20 μm and the average grain size of fine-grained NiAl forming at the region of initial Al layer is around 20 ± 7 μm. The orientations of them were measured and the crystallographic of both the coarse-grained NiAl and the fine-grained NiAl in inverse pole figures were shown in Figs. 2(b) and 2(c),
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respectively. It is obvious that the grains of coarse-grained NiAl layers most likely have a preferred orientation of // ND, and some others have orientations near // ND. Nevertheless, the grains of fine-grained NiAl layers have random orientation distribution. This demonstrates that the preferred orientation // ND of the laminated NiAl sheet is mainly from the coarse-grained NiAl layers. 3.2 Synthesis mechanism of NiAl As NiAl3 and Ni2Al3 phases decompose at 854 ˚C and 1133 ˚C, respectively, there are only Ni, liquid phase Al(Ni) and the product NiAl existing steadily at 1200 ˚C in this system (the atom ratio of Ni:Al is 1 : 1) according to the Ni-Al binary phase diagram (Okamoto, 1993) and the investigation for the combustion synthesis of NiAl (Zhu et al., 2002; Qiu et al., 2009; Battezzati et al., 1999). The synthesis mechanism which has been briefly discussed in our previous work (Fan et al., 2014) is that the NiAl formed at the region of initial Ni and Al in different ways. In detail, it can be described as following: i) before annealing treatment, Ni and Al are both in solid state; Ni dissolution in liquid Al is expected to occur at 660 ˚C when aluminum melt and then Al(Ni) liquid solution forms; ii) under equilibrium conditions at 1200 ˚C, the saturated solubility of Ni in liquid Al is about 28 at.%; iii) at 1200 ˚C, NiAl phase starts to form between solid Ni and Al(Ni) liquid solution, when the concentration of Ni in liquid Al close to Ni side reaches saturation firstly. As Ni diffuses in NiAl and dissolves in liquid phase Al(Ni), the NiAl layer grows continuously and the content of Ni in liquid phase Al(Ni) increases. As the reaction proceeds, product NiAl on the original Ni side takes the place of Ni and the grains over grow due to the annealing for 2 h. On the original Al side, Ni element dissolves into the liquid phase Al(Ni) continuously, and some NiAl nuclei precipitate from the liquid phase Al(Ni) when the content of Ni reaches to the composition range of NiAl. During the cooling, fine
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equiaxed grains of NiAl form based on the NiAl nuclei. In order to study the phase transformation mechanism from Ni to NiAl during the reaction treatment at 1200 ˚C, the hot-pressed laminated Ni-Al composite sheet was annealed at 1200 ˚C for 5 min and furnace cooled. The resulting sample was characterized by EBSD, and the OIM map (inverse pole figure of the ND), phase map and typical pole figures are shown in Fig. 3. From the OIM map and phase map in Figs. 3(a) and 3(b), it can be found the interface mainly consists of Ni and NiAl. It is necessary to note that a thin layer exists between Ni and NiAl marked by arrows in Fig. 3(b), which has been identified to be Ni3Al according to EBSD Kikuchi pattern and energy dispersive spectroscopy (EDS) analysis. The formation mechanism of Ni3Al may be attributed to two aspects: (i) there is Ni3Al phase close to Ni side at 1200 ˚C in binary phase diagram of Ni-Al. In our experiment, the Ni3Al at the interface was formed by solid diffusion for supersaturated Ni (Al) and ordering phase transformation of Ni (Al); (ii) after further furnace cooling, some Ni3Al nuclei/grains may precipitate/grow from the supersaturated Ni (Al) as shown in Fig. 3(a). It is worth noting that Ni and Ni3Al have almost identical lattice parameters and similar crystal structure, i.e. Ni3Al with a = 0.357 nm and Pm-3m of space group structure and Ni with a = 0.352 nm and Fm-3m. Therefore, during reaction treatment at 1200 ˚C, the Ni3Al may not affect the reaction mechanism and orientation relationship between Ni and NiAl, and similar research results have been reported in Ti-Ni solid phase reaction (Inoue et al., 2003) and Ni-Al(β)/Ni3Al(γ′) two phase alloys (Sakata et al., 2001). As an example, the orientation relationship between the Ni grain (GI) and the neighboring NiAl grain (GII) was analyzed according to {111} and {110} pole figures of both grains. From the pole figures, (-1-11)Ni marked by triangle in Fig. 3(c) and
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(-110)NiAl marked by triangle in Fig. 3(d) almost completely overlap. Meanwhile, (1-10)Ni marked by circle in Fig. 3(c) overlaps with (111)NiAl marked by circle in Fig. 3(d). Thus, the Ni phase and NiAl phase perfectly hold the Kurdjumov-Sachs orientation relationship (K-S OR) of (-1-11)Ni // (-110)NiAl and [1-10]Ni // [111]NiAl. The deviation angle from K-S OR is as small as 4˚. Couples of other Ni grains and their neighboring NiAl grains were also analyzed and similar ORs have been found. This experimental result verifies the theoretical analysis about orientation inheritance of the coarse-grained layers and the preferred orientation // ND of NiAl layer is inherited from preferred orientation // ND of Ni layer. This K-S OR between Ni and NiAl during in situ reaction annealing was discussed in our previous study (Wang et al., 2013) on microlaminated TiB2-NiAl composite, in which the K-S OR was only assumed by analysis of texture of initial Ni and synthesized NiAl without considering the transfer fact of Ni3Al during the reaction annealing. The interface between the parent Ni and the product NiAl is marked by arrows in Fig. 3, which was identified to be Ni3Al according to EBSD and EDS analysis as mentioned above. Thus, the product NiAl does not form directly from Ni but via the formation of Ni3Al firstly. 3.3 Orientation inheritance between NiAl and Ni through Ni3Al In order to study the interface between parent Ni and product NiAl as mentioned in Section 3.2, the interface between Ni and NiAl layers in the laminated Ni-Al composite sheet annealed at 1200 ˚C for 5 min was observed by TEM. The microstructure of the interface is shown in Fig. 4. The selected area electron diffraction (SAED) patterns of the product layers are shown in Fig. 4(b)-(d) as NiAl, NiAl with high density defects, and Ni3Al. As we can see, the interface is clearly separated by four parts, which are parent Ni, Ni3Al, NiAl with high density defects
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and NiAl from right to left as shown in Fig. 4(a). As the SAED patterns presented, all the intermetallic layers show strong ordered structure with superlattice. The NiAl layer next to Ni3Al layer shows complex microstructure with high density defects, and the diffraction pattern as shown in Fig. 4(c) shows complex superlattice structure, which was caused by diffraction deflection due to the formation of product NiAl with different compositions through diffusion. This phenomenon was also found in Ni-Al powder subjected to high energy ball milling (Fan et al., 2015). Considering the formation of Ni3Al, and the K-S OR between the parent Ni and product NiAl, the orientation inheritance between them is actually through Ni3Al. The unit cells with closed packed planes and directions of Ni, Ni3Al and NiAl are illustrated in Fig. 5. The close packed planes and directions in all the Ni, Ni3Al and NiAl, which act as the habit planes and directions in the K-S OR, are also marked colour. Through the thin Ni3Al layer, the interatomic distance and spacing of close-packed planes of Ni (fcc) and NiAl (bcc) became much closer. According to the lattice parameters presented in Section 3.1, the mismatch between Ni3Al and NiAl is only 0.84% according to the K-S OR. The misfit between the intermetallic phases may cause residual stress, which leads to dislocation formation, especially in the NiAl layer with high density defects and the interfaces. The microstructure of synthesized NiAl as demonstrated by Fig. 4(a) is shown in detail in Fig. 6. High-resolution transmission electron microscopy (HRTEM) image of NiAl with high density defects is shown in Fig. 6(a). An area in Fig. 6(a) was selected marked by a square and observed by inverse fast Fourier transform (IFFT) as shown in Fig. 6(b) using diffraction patterns in the inset of Fig. 6(a). It clearly shows dislocations (marked by arrows) and misfit atom planes (marked by dash lines). These defects including vacancy, interstitial defect and antiphase
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boundary (APB) are mainly caused by diffusion in Ni-Al system during annealing and the subsequent cooling. Fig. 6(c) shows dislocations forming at the interface induced by the interatomic distance misfit between the layers. In addition, as the microstructure of the layers is different, the elastic module and the thermal expansion coefficient of the layers may differ. This can also produce dislocations at the interface due to mechanical and thermal residual stress during cooling from 1200 ˚C. Based on the above discussion, a schematic illustration of the orientation inheritance of parent Ni and product NiAl through Ni3Al between them is shown in Fig. 7. Firstly, the parent Ni transforms to Ni3Al based on coherent relationship through diffusion between Ni and Al elements. Then, Ni3Al transforms to NiAl by {111}Ni3Al // {110}NiAl and Ni3Al // NiAl K-S OR, with a misfit value of 0.84%. 4. Conclusions The orientation inheritance in laminated NiAl with bimodal grain size distribution by reaction synthesis of Ni and Al foils has been investigated. The orientation relationship between the parent Ni and the product coarse-grained NiAl has been identified as K-S orientation relationship by crystallography analysis according to EBSD results. The parent Ni does not transform to NiAl directly but via the formation of Ni3Al firstly. Through the thin Ni3Al layer besides Ni layer, NiAl can form based on K-S orientation relationship with a small atomic misfit and less thermal and residual stress than directly from Ni.
Acknowledgments
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The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51571071, 51571070, 51541110), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.
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Miyamoto G., Takayama N., Furuhara T., 2009. Accurate measurement of the orientation relationship of lath martensite and bainite by electron backscatter diffraction analysis. Scr. Mater. 60, 1113-1116. Morsi K., 2001. Review: reaction synthesis procssing of Ni-Al intermetallic materials. Mater. Sci. Eng. A 299, 1-15. Okamoto H., 1993. J. Phase Equilibria 14, 257. Qiu X., Liu R., Guo S., Graeter J.H., Kecskes L., Wang J., 2009. Combustion synthesis reactions in cold-rolled Ni/Al and Ti/Al multilayers. Metall. Mater. Trans. A 40, 1541-1546. Ray R.K., 1995. Rolling texture of pure nickel, nickel-iron and nickel-cobalt alloys. Acta Metall. Mater. 43, 3861-3872. Sakata T., Yasuda H.Y., Umakoshi Y., 2001. Control of microstructure and orientation distribution in Ni-Al based (β/ γ′) two phase alloys by thermomechanical processing. Acta Mater. 49, 4231-4239. Skrotzki W., Tamm R., Oertel C.G., Beckers B., Brokmeier H.G., Rybacki E., 2001. Texture induced plastic anisotropy of NiAl polycrystals. Mater. Sci. Eng. A 319-321, 364-367. Skrotzki W., Tamm R., Oertel C.G., Beckers B., Brokmeier H.G., Rybacki E., 2002. Influence of texture and hydrostatic pressure on the room temperature compression of NiAl polycrystals. Mater. Sci. Eng. A 329-331, 235-240. Stoloff N.S., Liu C.T., Deevi S.C., 2000. Emerging applications of intermetallics. Intermetallics 8, 1313-1320. Suh D.W., Kang J.H., Oh K.H., Lee H.C., 2002. Evaluation of the deviation angle of ferrite from the Kudjumov-Sachs relationship in a low carbon steel by EBSD. Scr. Mater. 46, 375-378.
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Wang Q.W., Fan G.H., Geng L., Zhang J., Cui X.P., Pang J.C., Qin S.H., Du Y., 2013. A novel fabrication route to microlaminated TiB2-NiAl composite sheet with {111} texture by roll bonding and annealing treatment. Intermetallics 37, 46-51. Zhu P., Li J.C.M., Liu C.T., 2002. Reaction mechanism of combustion synthesis of NiAl. Mater. Sci. Eng. A 329-331, 57-68.
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Fig. 1. Microstructure of laminated Ni-Al composite: (a) SEM backscattering micrograph of laminated Ni–Al composite sheet; (b) contrastive XRD patterns of Ni before and after hot pressing (tested surface are both perpendicular to the ND).
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Fig. 2. Crystallography of laminated NiAl with bimodal grain size distribution: (a) orientation imaging microscopy (OIM, inverse pole figure colour of the ND) for laminated NiAl sheet; crystallographic of laminated NiAl sheet in inverse pole figures based on the ND for (b) coarse-grained layer and (c) fine-grained layer.
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Fig. 3. Crystallography between both of the Ni and NiAl layers in laminated Ni-Al composite sheet annealed at 1200 ˚C for 5 min and then subjected to furnace cooling: (a) OIM (inverse pole figure color of the ND for both Ni and NiAl); (b) phase map corresponding to (a); {111} and {110} pole figures corresponding to Ni grain GI (c) and NiAl grain GII (d) in (a).
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Fig. 4. Microstructure of the interface between Ni and NiAl layers in laminated Ni-Al composite sheet annealed at 1200 ˚C for 5 min: (a) bright field (BF) image of the interface; (b) SAED pattern of NiAl; (c) SAED pattern of NiAl with high density defects and (d) SAED pattern of Ni3Al.
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Fig. 5. Illustration of unit cells for Ni, Ni3Al and NiAl showing close packed planes and directions in K-S OR.
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Fig. 6. Microstructure of synthesized NiAl (hot-pressed laminated Ni-Al composite sheet annealed at 1200˚C for 5 min): (a) HRTEM image of NiAl with high density defects; (b) IFFT image of the area marked by square using diffraction shown in the inset in (a), showing dislocations and misfit atom planes; (c) BF image of the interface between NiAl layer with high density defects and NiAl layer without high density defects, showing dislocations at the interface induced by the misfit.
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Fig. 7. Schematic illustration of the orientation relationship between parent Ni and product NiAl through the formation of Ni3Al at the interface.
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