Reaction mechanism of combustion synthesis of NiAl

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Abstract

Based on precise temperature measurements during combustion and microstructural analysis of quenched samples, the evolution of reaction of the NiAl combustion synthesis has been studied. The combustion reaction of a multilayer Ni/Al system takes place in a thermal explosion mode under near adiabatic conditions. The experimental results clearly show that the combustion reaction starts right after the melting of Al. From the start to completion, the reaction goes through three stages. In the first stage, the temperature rises from the melting point of aluminum to the decomposition temperature of the intermediate phase NiAl3, i.e. 854 °C. The reaction is the dissolution of nickel in liquid aluminum, with the formation of small amounts of intermediate phases NiAl3 and Ni2Al3 at the solid–liquid interface. In the second stage, the temperature of the system increases from 854 to about 1300 °C. The reaction is still the dissolution of nickel in liquid aluminum solution. However, due to supersaturation, solid NiAl precipitates out at about 1300 °C, generates a great deal of heat and increases the temperature suddenly. The third stage starts at about 1300 °C, and ends at the maximum reaction temperature. The reaction rate of this stage is much higher (two orders higher) than that of first and second stages. The final product, liquid NiAl, forms at this stage.

Introduction

Combustion synthesis of nickel aluminides has been extensively studied in recent years. More than 200 references were found from 1990 to 2000 on self-propagating high-temperature synthesis of nickel aluminides. These investigations were motivated by the potential use of nickel aluminides as high temperature structural materials, by the advantage of combustion synthesis in producing nickel aluminides, and by the need to understand the various aspects of combustion synthesis.

Because of the high rate of the combustion reaction, the understanding of mechanisms and kinetics becomes important for the control of the processes and products. As reviewed recently by Rogachev [1], the available theories and kinetics cannot properly describe the gasless combustion due to a lack of extensive understanding of the microscopic mechanisms involved. Thus, the study of microscopic mechanisms of the combustion reaction is important and timely. However, a mechanistic study is difficult because of: (1) the high rate of reaction; (2) the high temperature at which the reaction takes place; and (3) the lack of direct experimental methods. Most existing ideas about the mechanism of the combustion synthesis are based on the results from indirect experimental observations.

The reaction mechanism of combustion synthesis of NiAl has been repeatedly studied and discussed by many researchers [2], [3], [4], [5], [6], [7], [8], [9]. However, most results are inconclusive or uncertain. For example, the time-resolved X-ray diffraction technique (TRXRD) has been used by four research groups to study the formation process of NiAl since the early 1980s [2], [3], [4], [5]. Three groups studied the same NiAl combustion system, but the conclusions obtained by these three groups are quite different. In addition to the discrepancy of the time delay of NiAl formation, Boldyrev et al. [2] found two unidentifiable intermediate phases while Wong et al. [3] found only one. On the other hand, Rogachev et al. [4] concluded that no intermediate phases appeared and the final product NiAl was the first phase formed. Leaving aside the possibility of different combustion conditions, which might affect the reaction mechanism and cause the disagreement among different researchers, the TRXRD technique itself can not provide solid evidence about reaction mechanisms because of the following limitations:

  • The number of diffraction peaks detected by those researchers is not sufficient to identify the possible intermediate phases. Because of the small angle range of the detection in Refs. [2], [3], only one or two diffraction peaks were detected. It is almost impossible to identify the phases by just one or two peaks due to the overlap of peaks of different phases and the complexities induced by thermal expansion and compositional changes during combustion. In Ref. [4], the detection range was expanded but due to the low intensity of radiation, it is quite possible to miss some peaks induced by small amount of intermediate phases.

  • TRXRD can detect crystalline phases but not liquids or liquid solutions formed during combustion.

  • Due to the high speed of reaction, most events reported by TRXRD actually took place after combustion rather than during the reaction.

By using a high-resolution high-temperature brightness pyrometer, Vol'pe et al. [6] recorded and analyzed the temperature–time profile of Ni–Al reaction systems. Based on the temperature profile and phase diagram, they analyzed the reaction path. However, due to heat losses, temperature measurement errors and/or incomplete reactions, the maximum reaction temperatures they measured were much lower than that calculated and measured by others.

The reaction mechanism of combustion synthesis of NiAl was investigated by Rochgachev et al. [7] and Dey et al. [8] using quenching techniques. Based on the development of microstructures, a ‘reaction coalescence’ mechanism was proposed by Rochgachev et al. [7]. The three characteristic stages of the reaction coalescence are: (1) melting of reactants Ni and Al; (2) small melting droplets coalesce to form large globule; and (3) homogenization of the globule composition. According to the authors, the reaction is just the mixing of liquid Al and Ni. The final product, NiAl, directly forms in the melt. On the other hand, by analyzing the development of microstructures from reacted zone to partially-reacted zone, and to fully reacted zone, Dey et al. [8] concluded that the formation of the NiAl phase during combustion synthesis occurs in several steps. The melting of aluminum is followed by the formation of the Al-rich phases first. The molten Al then reacts with the Al-rich phases and Ni to yield phases which contain higher amounts of Ni, until NiAl forms.

Based on the above brief review, it is seen that the reaction mechanisms during combustion synthesis of NiAl are not clear. In this paper the reaction evolution and the intermediate phases in the combustion synthesis of NiAl were studied by precise temperature measurements and rapid quenching. Multilayers of Ni and Al foils instead of the usual Ni and Al powders were used as the reaction system. The combustion reaction was conducted in a thermal explosion mode near adiabatic conditions. Several thermocouples were used to monitor the temperature and track the reaction path. Meanwhile, a rapid quenching technique coupled with in-situ temperature recording was used to study the evolution of the microstructure. Adiabatic temperatures were calculated as a function of composition during reaction to help identify the intermediate phases.

Section snippets

Sample preparation and combustion synthesis

Foils of 80 μm thick commercially pure nickel (Ni 200, 99.0%Ni min, Midwest Metals, West Lake, OH) and 112 μm thick aluminum (Al 1145, 99.45%Al min, All Foils, Brooklyn Heights, OH) were used to assemble multilayer Ni/Al reaction piles. The overall dimension of the reaction pile was 20 mm×20 mm square and about 6 mm thick after pressing. Before assembling into piles, the foils were cut and cleaned in an ultrasonic bath of acetone for 30 min. After drying, all Ni and Al foils were weighted

Temperature–time profile of combustion reaction

The temperature–time profile of a multilayer Ni/Al reaction pile is shown in Fig. 5. An expanded view of the reaction region is shown in Fig. 6. At the beginning of the experiment the reaction pile was heated by the electric heater at a heating rate of 40 °C min−1. Before the plateau appears around 643–657 °C, the temperature of the sample steadily increases. It indicates that no reaction takes place in this temperature range. The plateau in Fig. 5 represents the melting of aluminum, since the

Conclusions

The combination of precise temperature–time history recording and microstructural analysis of quenched samples provides a powerful method for the mechanistic study of combustion synthesis. For the NiAl combustion reaction system, experimental studies indicate that the reaction starts after about a third of the aluminum is melted. From the beginning to the end, the reaction goes through three stages:

In the first stage the temperature of the system increases from the melting point of aluminum to

Acknowledgements

This research was sponsored by the Division of Materials Sciences, US DOE under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems. We thank John Hunt of the Electron and Optical Microscopy Laboratory in the Center for Materials Research at Cornell University for assistance in the microprobe composition analysis.

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