Journal of NanomaterialsVolume 2011 (2011), Article ID 934801, 6 pagesdoi:10.1155/2011/934801

Research Article

Combustion Synthesis of Large Bulk Nanostructured Ni₆₅Al₂₁Cr₁₄ Alloy

Jiqiang Ma, Jun Yang, Qinling Bi, Licai Fu, Yonghai Kang, and Weimin Liu

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, ChinaGraduate University of Chinese Academy of Sciences, Beijing 100039, China

Received 11 January 2011; Accepted 17 February 2011

Academic Editor: Zhi Li Xiao

Copyright © 2011 Jiqiang Ma et al. Jiqiang Ma et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

A large bulk nanostructured Ni₆₅Al₂₁Cr₁₄ alloy with dimensions of Φ 100 mm × 6 mm was produced by combustion synthesis technique followed with rapid solidification. The Ni₆₅Al₂₁Cr₁₄ alloy was composed of γ′-Ni₃Al/γ-Ni(Al, Cr) eutectic matrix and γ-Ni(Al, Cr) dendrite. The eutectic matrix consisted of 80–150 nm cuboidal γ′-Ni₃Al and 2–5 nm γ-Ni(Al, Cr) boundary. The dentrite was comprised of high-density growth twins with about 3–20 nm in width. The nanostructured Ni₆₅Al₂₁Cr₁₄ alloy exhibited simultaneously high fracture strength of 2200 MPa and good ductility of 26% in compression test.

1. Introduction

Nanostructured materials exhibited enhanced strength, hardness, ductility at low temperature, wear-resistance, and other unusual properties in comparison with corresponding coarse counterparts [1–5]. Nanostructured materials can be produced by several methods, including consolidation of nanopowder by pressing and sintering, electrodeposition, devitrification of amorphous, and severe plastic deformation [6, 7]. However, bulk nanostructured materials obtained by the above methods often suffer from porosity, contamination, weak bonding, and small dimensions. Furthermore, extortionate equipment and enormous energy have to be used in the above methods usually. Specially, the size of the prepared nanostructured materials is very limited. Consequently, it is of great interest to develop new processes that are convenient, low in cost, and capable of being scaled up for tailoring the nanostructured materials. Because the process is inexpensive and can obtain large bulk materials with good mechanical properties [8–13], combustion synthesis (CS) could become a promising method to produce bulk nanostructured materials.

Nickel-aluminium-based intermetallic compounds have been extensively studied because of their high melting points, relatively low densities, high strength, as well as good corrosion and oxidation resistances [14–21]. The chromium alloying not only plays a solid solution strengthening role for the γ-Ni matrix and γ′ precipitates but also improves corrosion and oxidation resistances by creating a native, adherent oxide film [21, 22]. Moreover, the chromium alloying can also reduce brittleness at temperatures of 600–800°C in oxidizing atmosphere [23].

In this study, a bulk nanostructured Ni₆₅Al₂₁Cr₁₄ alloy has been synthesized successfully by CS technique associated with rapid solidification by designing an aluminothermic reaction route. The obtained bulk nanostructured Ni₆₅Al₂₁Cr₁₄ alloy exhibits simultaneously high strength and large ductility.

2. Experimental

Nickel, aluminum, chromium sesquioxide, and chromium trioxide powders were weighed according to the stoichiometry of the aluminothermic reaction (1) which is characterized by releasing a large amount of exothermic heat. The powders were dry-mixed for 8 hours with Al₂O₃ spheres in a stainless steel jar, and then 800 g of the mixed powders were cold-pressed on a copper substrate which was 170 mm in the diameter and fixed in a copper tube as a mold under a uniaxial pressure of 30 MPa. The substrate and tube were cleaned by ethanol before use. Characteristics of the reactant powders are given in Table 1.

Table 1: Properties of the raw material powders.

Powder
Size (mesh)
Purity (wt.%)
Impurity

Ni
200
>99.5
Zn, Pb, Mn, Si

Al
100–200
>99
Fe, Si, Cu, H₂O

CrO₃
—
>99
Cl⁻, SO₄  2−, Na, Al, Fe

Cr₂O₃
—
>99
Cl⁻, SO₄  2−, Fe

Five grams of Al, S, and MnO₂ powder mixture in mass ratio of 1 : 1: 1 was pressed into a pellet. The pellet was then put on top of the pressed reactant powders as an igniter. The chemical reaction of the igniter can be initiated at about 260°C, which results in an instantaneous release of a large amount of heat that ignites the aluminothermic reaction.

The copper mold with the reactants was placed into a reactor as described in the literature [8]. After being purged twice by argon gas, the reactor was heated continuously after introducing 4 MPa argon gas. The reaction of the igniter was started under 4 MPa argon gas when the reactor reached 260°C. The released heat ignited the aluminothermic reactants and the synthesis reaction was subsequently finished in a few seconds. The products were kept in the reactor under argon gas pressure to cool.

After cooling to room temperature, the products were taken out from the reactor. The obtained gray Ni₆₅Al₂₁Cr₁₄ alloy was about 100 mm in diameter and 8 mm thick as shown in the photograph in Figure 1. The black Al₂O₃ product was on the top of the target product (the density of the alloy is measured to be 7.07 g/cm³), which separated naturally from the desired Ni₆₅Al₂₁Cr₁₄ alloy and was removed easily by hand.

Figure 1: Photograph of the as-produced Ni₆₅Al₂₁Cr₁₄ alloy sample.

The polished cross section of the Ni₆₅Al₂₁Cr₁₄ alloy was examined with X-ray diffractometry (XRD, Philips, X'Pert-MRD) using a Cu Kα radiation source. The scanning electron microscope (SEM, JSM-5600LV) and field emission SEM (JSM-6701F) were used for microstructure observation from the cross sections of the Ni₆₅Al₂₁Cr₁₄ sample. Thin foils for TEM observation were prepared by twin-jet polishing using a solution of 20 ml sulphuric acid and 80 ml ethanol at a voltage of 60 V and were examined by using a transmission electron microscope (TEM, JEM-1200EX) operated at 120 KV.

The samples for compression test were machined into cylindrical bars with dimensions of Φ3 mm × 5 mm, and the surfaces were polished with 1000-grit emery paper. Quasistatic compression tests at a strain rate of 3 × 10⁻⁴ s⁻¹ were conducted at room temperature on a universal tester (Adamel, French). To minimize friction effect, the specimen-die interfaces were lubricated with graphite. Vickers microhardness of the Ni₆₅Al₂₁Cr₁₄ sample was measured using an MH-5 microhardness tester under a load of 300 g and a dwell time of 10 s. The sample density was measured by Archimedes method using a balance with a sensitivity of 0.1 mg.

3. Results and Discussion

The chemical reaction of the igniter is initiated at about 260°C and instantaneously releases large heat, which ignites the reaction of the reactants. Then combustion wave of the reaction propagates from top to bottom of the reactant compact and the reactants transform to Ni-Al-Cr alloy and Al₂O₃ melt where the combustion wave has passed. The adiabatic temperature () of the combustion reaction at 260°C is approximately calculated to be about 2700°C by the method as described in the literature [8, 24]. The products of the Ni₆₅Al₂₁Cr₁₄ and Al₂O₃ are in superheat liquid state but nearly do not vaporize during the processing because their boiling point rises to above 4500°C under the 4 MPa gas pressure by calculation based on the Clausius-Clapeyron equation and Trouton rule as described in supporting information of the literature [8]. Since liquid Al₂O₃ is immiscible with liquid Ni₆₅Al₂₁Cr₁₄, it separates automatically and lifts to the top owing to its low density [8]. The liquid Ni₆₅Al₂₁Cr₁₄ is deposited on the copper substrate surface.

Figure 2(a) shows a typical X-ray diffraction pattern of the Ni₆₅Al₂₁Cr₁₄ alloy, which indicates that the alloy consists of γ′-Ni₃Al and γ-Ni(Al, Cr) solid solution phases and the crystallographic structures of the alloy have a highly preferred orientation of (200) face. The XRD peaks are significantly broadened, indicating that the alloy has small grain/domain sizes. Scherrer formulation was used to quantitatively analyze the XRD broadenings; the averaged grain size is calculated to be 24.97 nm, and the lattice parameter is calculated to be 0.3550 nm. The contamination phases from igniter (Al₂O₃, MnSx) are mainly dissolved in byproduct Al₂O₃ slag and are very small quantity, so they are not detected by XRD test. Although not shown here, the outgrowth black Al₂O₃ on top of the Ni₆₅Al₂₁Cr₁₄ alloy is mainly composed of α-Al₂O₃ phase by the XRD analysis.

(a)

(b)

(c)

(d)

Figure 2: XRD pattern of the Ni₆₅Al₂₁Cr₁₄ alloy (a); SEM secondary electron image of the Ni₆₅Al₂₁Cr₁₄ alloy (b) and FSEM secondary electron images of the matrix (c) and the dentrite (d).

The SEM secondary electronic image of the Ni₆₅Al₂₁Cr₁₄ alloy is shown in Figure 2(b). It can be seen that the Ni₆₅Al₂₁Cr₁₄ alloy is a composite structure of micrometer scaled dendrites dispersed in a matrix. The dendrite occupies about 50% in volume according to the statistic of the SEM observations. Figures 2(c) and 2(d) show the fine microstructures of the matrix and dendrite by the FESEM secondary electronic images, respectively. It can be seen that the matrix is composed of 80–150 nm cubic γ′ precipitated phase and 2–5 nm boundary γ phase (Figure 2(c)). The dendrites consist mainly of 50–100 nm lamellar or fiber γ phase and a minor cubic γ′ phase (Figure 2(d)). The bright field TEM images of the matrix and dendrites and corresponding selected area diffraction patterns (SAED) are presented in Figure 3. Figure 3(a) indicates that the matrix is composed of roughly 80–150 nm cubic crystallites and 2–5 nm boundary γ phase, which are in good agreement with the result of FESEM. The corresponding SAED pattern shows discontinuous rings, indicating that many different orientations among the grains. By TEM images and corresponding SAED patterns, it can be further found that the nanolamellar and fiber in the dendrites consist of about 3–20 nm high dense twins (see Figure 3(b)). It has been proved that both nano/ultrafine eutectic structure [9, 25–27] and nanotwinned structure [28, 29] could enhance strength and ductility of metallic materials.

(a)

(b)

Figure 3: Bright field TEM images of the matrix (a) and dentrite (b) and corresponding SAED patterns.

The formation of the special composite nanostructure is attributed to three factors [30]. First, the contaminants (Al₂O₃, MnSx, etc.) introduced from reactants and igniter dissolve in the metallic melt under the very high degree of superheating (°C), and hence there are no heterogeneous nucleation sites available for liquid to crystallize on. Metallic liquid without nucleation sites for crystallization has a high undercooling degree and low crystallization onset temperature for solidification, which results in a large number of small homogeneous nuclei being simultaneously formed during solidification; Second, owing to the high viscosity of the metallic liquid at low crystallization onset temperature, the nuclei growth determined by diffusion of Ni, Al, and Cr atoms is slow. Third, the time of nuclei growth is short because of the high cooling rate resulting from the copper substrate quenching.

Figure 4(a) shows a typical compressive engineering stress-strain curve of the nanostructured Ni₆₅Al₂₁Cr₁₄ alloy. The yield strength is about 750 MPa, which is comparable to a nickel-based superalloy [31]. Impressive fracture strength of 2200 MPa and plastic strain of 26% was reached in compression test, respectively. The nanostructured Ni₆₅Al₂₁Cr₁₄ alloy exhibits a continuous stress increase with increasing strain, indicating continuous strain hardening until failure. Figure 4(b) shows lots of dimples with a diameter of 10–20 μm in the fracture surface from the compression test. Although large numbers of slip bands were observed in the fractured profile surface (Figure 4(c)), the local deformation may be impeded by the dendrites which means local stress concentration in compressive deformation [32], which can explain the high strain hardening ability of the nanostructured Ni₆₅Al₂₁Cr₁₄ alloy. On the other hand, nanotwin boundaries are much more stable against migration than conventional grain boundaries [28, 29]. Twin strengthening has been obtained in the nanostructured Ni₆₅Al₂₁Cr₁₄ alloy. So, in our experiment the nanostructure with dendrites composite and high density nanotwins is responsible for high fracture stress and good ductility in the compression test.

(a)

(b)

(c)

Figure 4: Typical compressive engineering stress—strain curve (a), SEM micrographs of the fractured surface (b), and fractured profile (c) of the Ni₆₅Al₂₁Cr₁₄ alloy.

4. Conclusion

The large bulk nanostructured Ni₆₅Al₂₁Cr₁₄ alloy is obtained by using combustion synthesis technique associated with rapid solidification. The nanostructured Ni₆₅Al₂₁Cr₁₄ alloy samples are characterized by SEM, XRD, and TEM. The as-produced large bulk nanostructured Ni₆₅Al₂₁Cr₁₄ alloy exhibits simultaneously high compressive fracture strength (2200 MPa) and good ductility (~26%).

Acknowledgments

This work was supported by the National Natural Science Foundation of China (50801064) and the National 973 Project of China (2007CB607601).

References

K. S. Kumar, H. Van Swygenhoven, and S. Suresh, “Mechanical behavior of nanocrystalline metals and alloys,” Acta Materialia, vol. 51, no. 19, pp. 5743–5774, 2003.

M. A. Meyers, A. Mishra, and D. J. Benson, “Mechanical properties of nanocrystalline materials,” Progress in Materials Science, vol. 51, no. 4, pp. 427–556, 2006.

Y. Wang, M. Chen, F. Zhou, and E. Ma, “High tensile ductility in a nanostructured metal,” Nature, vol. 419, no. 6910, pp. 912–915, 2002.

E. Hosseini and M. Kazeminezhad, “Nanostructure and mechanical properties of 0-7 strained aluminum by CGP: XRD, TEM and tensile test,” Materials Science and Engineering A, vol. 526, no. 1-2, pp. 219–224, 2009.

Y. S. Yang, J. G. Bae, and C. G. Park, “Nanostructure and mechanical properties of heavily cold-drawn steel wires,” Materials Science and Engineering A, vol. 508, no. 1-2, pp. 148–155, 2009.

N. Tao and K. Lu, “Dynamic plastic deformation (DPD): a novel technique for synthesizing bulk nanostructured metals,” Journal of Materials Science and Technology, vol. 23, no. 6, pp. 771–774, 2007.

I. V. Alexandrov and R. Z. Valiev, “Developing of SPD processing and enhanced properties in bulk nanostructured metals,” Scripta Materialia, vol. 44, no. 8-9, pp. 1605–1608, 2001.

P. La, J. Yang, D. J. H. Cockayne, W. Liu, Q. Xue, and Y. Li, “Bulk nanocrystalline FeAl-based material prepared by aluminothermic reaction,” Advanced Materials, vol. 18, no. 6, pp. 733–737, 2006.

J. Yang, J. Ma, W. Liu, Q. Bi, and Q. Xue, “Large-scale Fe-C nanoeutectic alloy prepared by a self-propagating high-temperature synthesis casting route,” Scripta Materialia, vol. 58, no. 12, pp. 1074–1077, 2008.

L. Li, Q. Bi, J. Yang, L. Fu, L. Wang, S. Wang, and W. Liu, “Large-scale synthesis of Al-Cu-Fe submicron quasicrystals,” Scripta Materialia, vol. 59, no. 6, pp. 587–590, 2008.

L. Fu, J. Yang, Q. Bi, and W. Liu, “Enhanced ductility of dendrite-ultrafine eutectic composite FeB alloy prepared by a self-propagating high-temperature synthesis,” Advanced Engineering Materials, vol. 11, no. 3, pp. 194–197, 2009.

J. Yang, P. La, W. Liu, and Y. Hao, “Microstructure and properties of Fe₃Al-Fe₃AlC0.5 composites prepared by self-propagating high temperature synthesis casting,” Materials Science and Engineering A, vol. 382, no. 1-2, pp. 8–14, 2004.

P. La, Y. Wei, R. Lv, Y. Zhao, and Y. Yang, “Effect of Mn element on microstructure and mechanical properties of bulk nanocrystalline Fe₃Al based materials prepared by aluminothermic reaction,” Materials Science and Engineering A, vol. 527, no. 9, pp. 2313–2319, 2010.

H. Hou, Y. Zhao, and Y. Zhao, “Simulation of the precipitation process of ordered intermetallic compounds in binary and ternary Ni-Al-based alloys by the phase-field model,” Materials Science and Engineering A, vol. 499, no. 1-2, pp. 204–207, 2009.

P. Peng, D. W. Zhou, J. S. Liu, R. Yang, and Z. Q. Hu, “First-principles study of the properties of Ni/NiAl interface doped with B or P,” Materials Science and Engineering A, vol. 416, no. 1-2, pp. 169–175, 2006.

J. Bonneville and C. Coupeau, “Quantitative atomic force microscopy analysis of slip traces in Ni₃Al yield stress anomaly,” Materials Science and Engineering A, vol. 483-484, pp. 87–90, 2008.

Y. Ma and A. J. Ardell, “Chemical diffusion in hypostoichiometric Ni₃Al from data on coarsening of Ni-Al solid solution precipitates,” Materials Science and Engineering A, vol. 516, no. 1-2, pp. 259–262, 2009.

L. N. Wang, Y. Liu, J. J. Yu, Y. Xu, X. F. Sun, H. R. Guan, and Z. Q. Hu, “Orientation and temperature dependence of yielding and deformation behavior of a nickel-base single crystal superalloy,” Materials Science and Engineering A, vol. 505, no. 1-2, pp. 144–150, 2009.

A. Hibino, S. Matsuoka, and M. Kiuchi, “Synthesis and sintering of Ni₃Al intermetallic compound by combustion synthesis process,” Journal of Materials Processing Technology, vol. 112, no. 1, pp. 127–135, 2001.

L. Y. Sheng, W. Zhang, J. T. Guo, Z. S. Wang, V. E. Ovcharenko, L. Z. Zhou, and H. Q. Ye, “Microstructure and mechanical properties of Ni₃Al fabricated by thermal explosion and hot extrusion,” Intermetallics, vol. 17, no. 7, pp. 572–577, 2009.

N. S. Stoloff, “Physical and mechanical metallurgy of Ni₃Al and its alloys,” International Materials Reviews, vol. 34, no. 4, pp. 153–183, 1989.

H. Choe and D. C. Dunand, “Synthesis, structure, and mechanical properties of Ni-Al and Ni-Cr-Al superalloy foams,” Acta Materialia, vol. 52, no. 5, pp. 1283–1295, 2004.

T. Czeppe and S. Wierzbinski, “Structure and mechanical properties of NiAl and Ni₃Al-based alloys,” International Journal of Mechanical Sciences, vol. 42, no. 8, pp. 1499–1518, 2000.

J. Ma, J. Yang, Q. Bi, and W. Liu, “Preparation of an ultrafine-grained Fe-40Al intermetallic compound,” Acta Metallurgica Sinica (English Letters), vol. 23, no. 1, pp. 50–56, 2010.

J. Das, R. Theissmann, W. Löser, and J. Eckert, “Effect of Sn on microstructure and mechanical properties of Ti-Fe-(Sn) ultrafine eutectic composites,” Journal of Materials Research, vol. 25, pp. 943–956, 2010.

J. Eckert, J. Das, G. He, M. Calin, and K. B. Kim, “Ti-base bulk nanostructure-dendrite composites: microstructure and deformation,” Materials Science and Engineering A, vol. 448-451, pp. 24–29, 2007.

K. B. Kim, J. Das, W. Löser, M. H. Lee, D. H. Kim, S. K. Roy, and J. Eckert, “Microstructural comparison of ZrNbCuNiAl nanostructure-dendrite composites produced by different casting techniques,” Materials Science and Engineering A, vol. 448–451, pp. 747–751, 2007.

L. Lu, X. Chen, X. Huang, and K. Lu, “Revealing the maximum strength in nanotwinned copper,” Science, vol. 323, no. 5914, pp. 607–610, 2009.

L. Lu, Y. Shen, X. Chen, L. Qian, and K. Lu, “Ultrahigh strength and high electrical conductivity in copper,” Science, vol. 304, no. 5669, pp. 422–426, 2004.

H. Hu, Metal Solidification Principle, China Machine Press, Beijing, China, 2000.

G. Mi, Z. Li, S. Tian, X. Zhao, Z. Luo, S. Zeng, and Q. Li, “Microstructure and tensile properties of spray-deposited IC6 alloy,” Journal of Materials Science and Technology, vol. 14, no. 2, pp. 143–146, 1998.

H. S. Kim and Y. Estrin, “Strength and strain hardening of nanocrystalline materials,” Materials Science and Engineering A, vol. 483-484, pp. 127–130, 2008.