ISSN 1061-3862, International Journal of Self-Propagating High-Temperature Synthesis, 2007, Vol. 16, No. 1, pp. 12–17. © Allerton Press, Inc., 2007.
Time-Resolved X-ray Diffraction Study of SHS-produced NiAl and NiAl–ZrO2 Composites D. Tingauda, L. Stuppflera, S. Parisb, D. Vrelc, F. Bernardb, C. Penota, and F. Nardoua a Université
de Limoges, SPCTS, UMR 6638 CNRS, 87060 Limoges Cedex, France de Bourgogne, ICB, UMR 5209 CNRS, 21078 Dijon, France c Université Paris XIII, LIMHP, UPR 1311, 93430 Villetaneuse, France e-mail:
[email protected]
b Université
Received December 22, 2006
Abstract—NiAl and NiAl–ZrO2 intermetallic composites were prepared from pressed elemental powders by SHS in a dynamic vacuum. The effect of diluent properties (such as conductivity and grain size) on the parameters of the SHS reaction has been investigated by time-resolved X-ray diffraction (TRXRD). By decreasing the reactivity of the green mixture, these nonreactive additives not only slow down the velocity of combustion front propagation but can also give rise to local melting of the product. Synchrotron radiation was also used to elucidate the reaction mechanism. It has been found that the combustion synthesis of NiAl is triggered by aluminum melting and proceeds via the formation of two transient intermetallic phases, NiAl3 and Ni2Al3. In the final product, only the NiAl phase has been detected. Keywords: intermetallics, NiAl–ZrO2 composite, SHS, time-resolved X-ray diffraction PACS numbers: 81.05.Je, 81.05.Mh, 81.16.Be, 81.20.Ka DOI: 10.3103/S1061386207010025
ture [5]. Among ceramic-type diluents that can be used for the purpose, yttrium–partially stabilized zirconia (PSZ) is promising for high intermetallic matrix reinforcement over a wide temperature range because of its intrinsic characteristics and specific allotropic transformation. Our previous work [6], carried out in the mode of thermal explosion in a hot press, has highlighted the important role played by these nonreactive particles during germination growth mechanisms and their influence on the mechanical properties of the product. In this study, the SHS of nickel aluminide (in the presence/absence of NiAl) and NiAl–PSZ composites was investigated by TRXRD to define more precisely the effect of these diluents on the parameters of the SHS reaction and its mechanism.
INTRODUCTION Nickel aluminide is an interesting material for hightemperature structural applications because of its low density (5.95 g cm–3), high melting point (1638°C), high thermal conductivity (70 W m–1 K–1), and excellent corrosion resistance. These properties make it particularly competitive compared to nickel-based superalloys [1]. Moreover, in the form of a single crystal, this intermetallic has already replaced the above alloys, especially in the industrial-scale production of turbine blades [2]. However, the implementation of NiAl is still severely restricted because working with this material at room temperature causes its brittle fracture due to low ductility. Most of the conventional processing methods used for the fabrication of metal alloys, such as hot rolling or casting, were employed for the production of NiAl. Since this compound exhibits a high negative enthalpy of formation (∆H from –59 to –72 kJ mol–1, depending on its stoichiometry) [3], it can also be produced by SHS (also called combustion synthesis). The advantages of this process include the possibility of obtaining complex or metastable phases and highpurity products with low energy and equipment requirements [4]. During SHS, the materials undergo high stress because of violent thermal shocks. So, to prevent samples from cracking, a thermal regulator, which can be either the final product or ceramic particles (such as Al2O3, SiC, ZrO2), is usually added to the green mix-
EXPERIMENTAL The combustion synthesis was performed (under dynamic vacuum) on cold-pressed powder samples placed in a reaction chamber. The compacts were prepared from Al and Ni powders containing (or not containing) a diluent (thermal regulator) in amounts up to 10 vol %. Three different thermal regulators were used: NiAl, ZrO2–6 wt % Y2O3 (6PSZ), and ZrO2–8 wt % Y2O3 (8PSZ). These mixtures were homogenized in a mixer with helicoidal movement for 2 h. Cylindershaped compacts, with a diameter of 10 mm and a height of 10 mm, were prepared by uniaxial pressing (200 MPa). Their green density ranged between 67 and 12
