Intermetallics 14 (2006) 603–606 www.elsevier.com/locate/intermet
NiAl–Al2O3 composites produced by pulse plasma sintering with the participation of the SHS reaction A. Michalski *, J. Jaroszewicz, M. Rosin´ski, D. Siemiaszko Department of Materials Science and Engineering, Warsaw University of Technology, 02-507 Warsaw, 141 WoŁoska, Poland Received 1 July 2005; received in revised form 11 September 2005; accepted 19 October 2005 Available online 20 December 2005
Abstract The paper presents the results of examinations of the NiAl–Al2O3 sinters (13, 38 and 55 vol% of Al2O3) produced from a mixture of nickel, aluminum and alumina powders in a single technological process, using the pulse plasma sintering (PPS) method. By subjecting the elemental powders to a PPS process for 900 s, we obtained NiAl–Al2O3 composites of a hardness ranging from 480 HV10 (13% Al2O3) to 680 HV10 (55% Al2O3). The fracture toughness of the sintered materials depended on the amount of the dispersed Al2O3 phase. When examined with a Vickers indenter under a load of 10 kg, the composite containing 13% of Al2O3 showed no cracking. In the composites with 38% Al2O3 and 55% Al2O3 contents, the value of KIC was 9.1 and 8.2 MPam1/2, respectively. q 2005 Published by Elsevier Ltd. Keywords: A. Nickel aluminides; based on NiAl; A. Composites C. Sintering.
1. Introduction NiAl is a potential material for the aircraft and automotive industries, since it has a high melting temperature (1711 K), a low density (5.91 g/cm3) and a good thermal conductivity (75 W/mK). A drawback, which restricts the application range of the NiAl phases, is their low resistance to creep and poor mechanical strength at elevated temperatures. The hightemperature strength of these phases can be improved significantly by reinforcing them with hard ceramic particles, such as TiC, TiB2 or Al2O3 [1,2,3]. An original energy-saving method which is increasingly used for producing ceramic materials and intermetallic phases from elemental powders is Self-propagating high-temperature synthesis (SHS) . The idea underlying the SHS method consists of utilizing the heat generated by the exothermal reaction that proceeds between the reagents during the synthesis process for heating the system to an appropriate temperature, and maintaining this temperature until the reagents fully transform into the reaction products [5,6]. An essential disadvantage of the materials produced by the SHS method is their high porosity . This can be obviated when * Corresponding author. Tel.: C48 226608382; fax: C48 226608705. E-mail address: [email protected]
0966-9795/$ - see front matter q 2005 Published by Elsevier Ltd. doi:10.1016/j.intermet.2005.10.003
the material being synthesized is simultaneously loaded and thereby consolidated. The techniques used for this purpose include e.g. high-temperature isostatic pressing (HIP) , uniaxial pressing , pseudo-hot isostatic pressing PHIP) . In our earlier study , we presented the results of examination of dense NiAl sinters produced from aluminum and nickel powders by the pulse plasma sintering (PPS) method with the participation of the SHS reaction (the method was developed at the Materials Engineering Faculty, Warsaw University of Technology, Poland). The present study is concerned with the NiAl–Al2O3 composites produced from a mixture of Al, Ni and Al2O3 powders using the PPS method. 2. The pulse plasma sintering method The SHS reaction was initiated by high-current electric impulses applied during the PPS process. Under these conditions, the synthesis and sintering of the powder proceed in a specific way, namely, energy of the order of a few kJ is released in the material during a time of several hundred microseconds. This technique of heating combined with a pressure exerted upon the powder mixture permits the SHS ignition temperature to be achieved within the entire powder volume in a very short time. In the PPS method, the powder is heated by the Joule heat at the places where the powder particles contact each other, and by the spark discharges that occur between the particles within the pores present between them. Fig. 1 shows a schematic representation of the apparatus
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Fig. 3. Schematic representation of the process of sintering of the NiAl–Al2O3 composites.
Fig. 1. Schematic representation of the PPS apparatus.
for pulse-plasma sintering. The powder mixture to be sintered is placed in a graphite die, between two punches to which electric energy is delivered from a battery of capacitors (200 mF). The powder is heated by the high-current discharge pulses generated periodically from the capacitor battery, charged, at a maximum, to 10 kV. The duration time of each impulse was several hundred microseconds and the electric current intensity—tens of kA. Fig. 2 shows examples of the pulse current and voltage waveforms during a capacitor discharge. 3. Examination methods The NiAl–Al2O3 (13, 38 and 55 vol%,) composites were prepared using powders of aluminum (99.9% purity, grain size 10 mm), nickel (99%, 5 mm) and Al2O3 (99.7%, !1 mm). The powders were mixed, in the atomic proportions Ni:AlZ1:1 with an addition of Al2O3 powder, in a ball mill for 10 h. Prior
to sintering, the powder was subjected to preparatory consolidation under a pressure of 200 MPa. Resulting samples had a diameter of 8 mm and were 12 mm high. The composites were synthesized and, simultaneously, consolidated in a graphite die under a load of 60 MPa in a vacuum. The temperature on the surface of the graphite die was measured using an Ahlborn IR AMIR 7838-51 pyrometer. During sintering, the temperature and the heating rate of the sample were controlled by controlling the pulse discharge energy and the pulse repetition frequency. Fig. 3 shows schematically the process of fabrication of the NiAl–Al2O3 composites. In the first stage, the powder mixture was heated to a temperature of 600 8C and was maintained at this temperature for 300 s. Then the discharge energy was increased and the sample was heated to the sintering temperature, 1100 8C, and maintained at this temperature for 600 s. Table 1 gives the parameters of the process of fabrication of the NiAl–Al2O3 composites. The phase composition of the sinters was examined in a Philips PW 1140 diffractometer using Co Ka radiation. The microstructure and the chemical composition were analyzed in a HITACHI S3500N scanning electron microscope. The density was measured by the Archimedes method. The hardness and fracture toughness of the sintered samples were determined by the Vickers indentation method with an indentation load of 10 kg on polished cross-sectioned samples. The fracture toughness was calculated according to the formula of Anstis et al.  Table 1 Process parameters of the synthesis of the NiAl–Al2O3 composites
Fig. 2. Examples of the electric current and voltage waveforms during the pulse plasma process, (a) current characteristic, (b) voltage characteristic.
Discharge energy (kJ) Pulse repetition frequency (Hz) Voltage (V) Heating rate (8C/s) Heating time (s) Temperature (8C) Load (MPa) Vacuum (Pa)
2.5 1 5000 10 300 600 60 0.05
8.1 1 9000 7 600 1100 60 0.05
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4. Results and discussion Examinations of the phase composition of the NiAl–Al2O3 composites show that, irrespective of the volumetric share of Al2O3, the composite contains the NiAl and aAl2O3 phases. By way of example, Fig. 4 shows the diffractogram obtained for the NiAl–55 vol%Al2O3 composite. Fig. 5 shows the variation of the density of the sinters as a function of the volumetric content of Al2O3. We can see that the density decreases with increasing Al2O3 content, which is due to the lower density of Al2O3 (3.99 g/cm3) compared to the density of the NiAl (5.91 g/cm3). Irrespective of the volumetric content of Al2O3, the porosity of the composites is below 1%. Fig. 6 show the microstructure of the NiAl–Al2O3 composites with various volumetric contents of the Al2O3 powder particles. Fig. 7 shows how the Al2O3 content affects the hardness of the composite. The hardness of NiAl produced without an addition of Al2O3 is 430HV10, and it increases with increasing volumetric content of Al2O3 to reach 680HV10 at an Al2O3 content of 55 vol%. In the composite with 13 vol% of Al2O3, the only cracks observed are those located close to the Vickers
Fig. 4. XRD patterns obtained for the NiAl–55 vol%Al2O3 composite.
Fig. 6. SEM (SE) image of the NiAl–Al2O3 composites(NiAl is dark and Al2O3 is light regions) with various Al2O3 contents (a)13 vol% (b)38 vol% (c)55 vol%.
Fig. 5. Variation of the density of the NiAl–Al2O3 composite as a function of the Al2O3 content.
indentation, whereas in the NiAl sinter without added Al2O3 and in the composites with 38 and 55% of Al2O3, microcracks have developed from the indentation tips toward the center. Within the composites, the cracks run through the Al2O3 particles, changing the direction of their propagation at the NiAl particles (Fig. 8). The fracture toughness of the composites with 38 and 55% Al2O3 is doubled compared to that of the NiAl sinters produced without the Al2O3 addition. The fracture toughness is 9.1 MPam1/2 in the NiAl/38%Al2O3 composite, 8.2 MPam1/2 in the NiAl/55%Al2O3 composite and 5.4 MPam1/2 in NiAl without added Al2O3.
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a mixture of nickel and aluminum powders with added aAl2O3 particles. The relative density of the composites exceeds 99%. Their hardness increases with increasing Al2O3 content, and reaches a value of 680HV10 at an Al2O3 content of 55 vol%. The fracture toughness in the composites with 38 and 55% of Al2O3 is doubled with respect to that of the NiAl sinters without added Al2O3. Acknowledgements The Minister of Science and Information Society Technologies by Grant 3T08D03426 supported this work. References Fig. 7. Hardness of the NiAl–Al2O3 composites depending on the Al2O3 content.
Fig. 8. Crack observed in the NiAl–55 vol% Al2O3 composite.
5. Conclusions By utilizing the SHS reaction triggered by high-current electric pulses, we produced NiAl–Al2O3 composites from
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