Intermetallics 10 (2002) 377–389 www.elsevier.com/locate/intermet

Investigations of the formation mechanism of nanostructured NbAl3 via MASHS reaction V. Gauthiera,*, F. Bernarda, E. Gaffetb, D. Vrelc, M. Gailhanoud, J.P. Larpina a

Laboratoire de Recherches sur la Re´activite´ des Solides, UMR 5613 CNRS, Universite´ de Bourgogne- BP 47870, F21078 Dijon Cedex, France b Groupe Nanomate´riaux, UMR 5060 CNRS, Universite´ de Technologie de Belfort-Montbe´liard, F90010 Belfort, France c Laboratoire d’Inge´nierie des Mate´riaux et des Hautes Pressions, UPR 1311 CNRS, Universite´ Paris nord, F93430 Villetaneuse, France d Laboratoire d’Utilisation du Rayonnement Electromagne´tique, UMR 130 CNRS/CEA/MENRT, Universite´ Paris sud, F91405 Orsay, France Received 2 January 2002; accepted 15 January 2002

Abstract The nanostructured NbAl3 intermetallic compound was synthesized using the mechanically-activated self-propagating hightemperature synthesis (MASHS) technique. This process results from the combination of two steps: a short duration ball-milling of a pure elemental Nb+3Al powder mixture followed by a self-propagating high-temperature synthesis (SHS) reaction induced by the Nb+3Al reaction exothermicity. Synchrotron time-resolved XRD coupled with a 2D infrared camera were used to investigate the structural and thermal evolutions during the SHS reaction, and to study in situ the mechanism of NbAl3 formation. The influence of the incoming heat flux and the mechanical activation effect on the phase transformation kinetics induced by the SHS process were studied. Owing to the temporal resolution of 100 and 200 ms between two consecutive diffraction patterns and IR images, respectively, it was shown that solid niobium reacts with liquid aluminium via a heterogeneous nucleation reaction, to form the NbAl3 compound by successive combustion fronts. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Niobium aluminides; A. Nanostructured intermetallics; B. Phase transformations; C. Mechanical alloying and milling; C. Powder metallurgy, including consolidation

1. Introduction The niobium aluminide NbAl3 is a potential material for high-temperature applications, such as for turbine blades in aircraft engines or stationary gas turbines, because of its strength at high temperature, high melting point and low density [1]. However, some of its properties still need to be improved, such as its insufficient ductility at room temperature [2,3] and somewhat unsatisfactory oxidation resistance at temperatures above 700  C [4–7]. Conventional processing techniques used to synthesize coarse grained NbAl3, by a combination of casting, powder grinding and consolidation by hot pressing, have been investigated [8–10]. An alternative method, variously known as combustion synthesis, reactive sintering or self-propagating high-temperature synthesis * Corresponding author. Tel.: +33-3-80-39-61-58; fax: +33-3-8039-61-32. E-mail addresses: [email protected] (V. Gauthier), fbernard@ u-bourgogne.fr (F. Bernard), [email protected] (E. Gaffet), vrel@ limhp.univ-paris13.fr (D. Vrel).

(SHS), has also been used to produce microcrystalline NbAl3 [11–14]. In this technique, the exothermal reaction between the reactant powders is initiated by an external heat source and becomes self-sustaining to yield the final product, without requiring any additional energy. The SHS process saves time and energy since this technique proceeds in seconds or minutes compared with hours for conventional processing routes [15–17]. Demand for novel materials with superior properties for aerospace applications has prompted research for synthesizing nanocrystalline materials. Numerous papers [18–20] have reported many unique properties associated with nanocrystalline materials, such as the enhanced ductility and diffusivity compared with ordinary materials. These unique characteristics of nanocrystalline materials have obviously been attributed to the atomistically controlled ultrafine grains [21–23]. Nanomaterials have indeed the potential of improving the performance characteristics in such applications as composite polymers, catalysts, filtration systems and transmission media [20]. Unfortunately, until now the relatively limited number of experimental data on

0966-9795/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(02)00010-9