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Self-propagating high-temperature synthesis of Ni2AlTi

Name
Kirill
Surname
Sidnov
Scientific organization
The National University of Science and Technology MISiS
Academic degree
postgraduate
Position
Materials Modeling and Development Laboratory engineer
Scientific discipline
New materials, Manufacturing technologies & Processes
Topic
Self-propagating high-temperature synthesis of Ni2AlTi
Abstract
We have investigated obtaining of the Ni2AlTi compound using SHS, which is a cheap and environmentally friendly method of this material producing; obtained Ni2AlTi compound. In the present study parameters for obtaining of single-phase samples were established for using of self-propagating high-temperature synthesis in argon atmosphere, where initiation of self-propagating reaction was provided by tungsten wire. The parameters of obtaining more than 90% dense samples using spark plasma sintering and hot pressing were investigated also.
Keywords
Self-propagating high-temperature sythesis, combustion synthesis, NiAl-based alloys, spark plasma sintering, hot pressing
Summary

Self-propagating high-temperature synthesis (SHS) is a method for producing inorganic compounds by exothermic reactions, usually involving salts or pure metals. A variant of this method is known as solid state reactions. Since the process occurs at high temperatures, the method is ideally suited for the production of refractory materials with unusual properties, for example: powders, metallic alloys, or ceramics with high purity, corrosion–resistance at high–temperature or super-hardnessity. Self-propagating high-temperature synthesis (SHS), also known as combustion synthesis (CS), is an energy saving technology to fabricate NiAl-based alloys [1].

The intermetallic nickel aluminide compounds have several exciting features, including low-density, high melting point, excellent corrosion and oxidation resistances, high strength at increased temperature and relatively low cost [2]. For these reasons, these intermetallics are good candidate for diversity of application as elements for airframe and turbines [3]. It was also known that these compounds possess low ductility and tendency for brittle fracture, especially in polycrystalline form, which limited their applications [4]. The alloying, which involves different dopants, such as B, C, Ti, Zr, Nb and Ta, has been used to overcome the problem [5]. 

In this work were used several methods of SHS:

  • direct SHS in argon atmosphere, where initiation of self-propagating reaction was provided by tungsten wire (Fig 1);
  • direct SHS of mechanical activated powder mixture;
  • spark plasma sintering of mechanical activated powder mixture (Fig 2);
  • hot pressing of synthesized material.

Figure 1. Schematic of SHS reactor

Figure 2. Schematic of spark plasma sintering system

The following powders were used to synthesized materials: aluminum (ASD-1); nickel (PNE-1); titanium (PTS-1). The powders were thoroughly mixed to prepare heterogeneous mixtures, which correspond to  required composition (see Table 1). 

Table 1. Compositions of used powder mixtures 

Compound

Metal

Amount of metal, wt.%

Ni2AlTi

Ni

61,1

Al

24,9

Ti

14,0

 

Prepared mixtures were pressed in the uniaxial press dye to form a cylindrical samples (d=15mm; height = 5 mm) to the density 3,4 g/cm3. Fabrication of the intermetallics was performed by using self-propagating high-temperature synthesis method. The basic idea is that the reaction between Al and Ni is extremely exothermic with adiabatic combustion temperature ~1911 K [6]. It means that if one locally (~1 mm3) preheat the mixture to some temperature reaction initiates and propagates in the self-sustained manner along the bulk of the sample, forming desired intermetallic phase. In our case, the samples were inserted into the stainless steel reactor, which was first pumped down to vacuum ~ 10-3 Pa and then filled with argon gas to the pressure of ~ 0.5 MPa. The local preheating of the sample was accomplished with the electrically heated tungsten wire.

In presented work for mechanical activation was used high-energy ball mill «Activator 2S» («Activator», Novosibirsk). For sintering was used spark plasma sintering system «LABOX Model-650».

Also phase composition of prepared samples were studied by x-ray diffraction analysis (DRON-3).

The diffraction pattern of samples obtained by SHS represented on figure 3. It is obvious that able to obtain single-phase samples by the method. However, the samples had a very low density (~35-50%). In order to avoid this, we used the methods following below.

Figure 3. Diffraction pattern of synthesized sample

The next step of researching synthesis of Ni2AlTi was SHS of pretreated by mechanical activation powder mixture. For this operation we have used high-energy planetary ball milling (694 rpm, 2 minutes). Mechanical activation provides a denser samples by the increase contact area between the particles and reduce the distance between the particles of the reaction mixture. The obtained samples had a higher density of around 70% with single-phase composition, such as non-activated samples.

However, this density is not sufficient for the material application. Cylindrical samples (15x6 mm) obtained by the spark plasma sintering of pretreated with mechanical activation reaction mixture. Diffraction pattern of this samples is presented on figure 4. The sintering temperature was 800oC, the dwell time - 10 min, pressure - 40 MPa.

Figure 4. Diffraction pattern of sample obtained by spark plasma sintering

The temperature was then increased and the other parameters remained unchanged. At a temperature of 900oC was able to get single-phase samples (density ~ 90%).Supposedly it is caused by the alpha - beta phase transition in titanium that provides extra warmth over the entire volume of the sample and creates favorable conditions for the synthesis process.

The powder mixture directly synthesized by SHS was used for the densest samples sintering by hot pressing. These samples have a relative density of ~ 93-95 %. Sintering parameters are presented in Table 2.

Table 2. Hot pressing parametrs

Temperature, oС

1200

Pressure, MPa

40

Dwell time, min

10

Heating rate, oC/min

100

 

This confirms the fact that this method allows to obtain a consolidated material suitable for further use.

Сonclusions:

  • We have investigated obtaining of the Ni2AlTi compound using SHS, which is a cheap and environmentally friendly method of this material producing;
  • obtained Ni2AlTi compound;
  • parameters for obtaining of single-phase samples were established for using of self-propagating high-temperature synthesis in argon atmosphere, where initiation of self-propagating reaction was provided by tungsten wire;
  • the parameters of obtaining  more than 90% relatively dense samples using spark plasma sintering and hot pressing were investigated also.

 

References:

[1] "Self-propagated high-temperature synthesis of refractory inorganic compounds", A.G. Merzhanov, I.P. Borovinskaya. Doklady Akademii Nauk SSSR, Vol. 204, N 2, pp. 366-369, May, 1972

[2] D. B. Miracle, R. Darolia, in: Intermetallic Compounds, J. H. Westbrook and R. L. Fleischer (Eds.), Baffins Lane, Chichester, West Sussex, John Wiley & Sons Ltd, 1995, pp. 2,53.

[3] R. Darola, NiAl Alloys for High-Temperature Structural Applications, JOM, 43(3), 1991, 44-49.

[4] A. Bose, B.H. Rabin, R.M. German, Powder Metall, Int., 20, (1988), 25.

[5] I. Baker, P. R. Munroe, Reactive Sintering of Nickel-Aluminide to Near Full Density, J. Met. 40, (1988), 28.

[6] A. Varma, A.S. Mukasyan, Combustion Synthesis of Intermetallic Compounds, in: A. Borisov, L. DeLuca and A. Merzhanov, (Eds.), Self-Propagating High-Temperature Synthesis of Materials, Taylor & Francis, New York, 2002, pp. 1-34.