In situ synthesis of NiAl–NbB 2 composite powder through combustion synthesis Ali Akbar Shokati a,⇑ , Nader Parvin a , Naser Sabzianpour a , Mohammad Shokati b , Ali Hemmati a a Department of Mining and Metallurgical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran 1591634311, Iran b Faculty of Mechanical Engineering, K.N. Toosi University of Technology, P.O. Box: 19395-1999, Tehran, Iran article info Article history: Received 21 June 2012 Received in revised form 23 July 2012 Accepted 4 August 2012 Available online 16 August 2012 Keywords: Coating materials Composite materials High-temperature alloys Intermetallics Chemical synthesis Microstructure abstract Synthesis of a novel NiAl matrix composite powder reinforced with 0–40 wt.% NbB 2 by combustion syn- thesis in thermal explosion mode was investigated. The elemental powders of Ni, Al, Nb, and amorphous boron were used as starting material. For all compositions final products consisted of only the NiAl and NbB 2 phases. Coarser NbB 2 with a relatively uniform distribution in NiAl matrix was formed with rising NbB 2 content. Microhardness of NiAl considerably increased from 377 ± 13 HV 0.05 to 866 ± 81 HV 0.05 for NiAl with 40 wt.% NbB 2 . High microhardness, proper size and distribution of NbB 2 in NiAl matrix make it a good candidate as precursor for thermal spray application. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction NiAl intermetallic has received noticeable attention as an attractive material for high temperature applications on account of its fascinating characteristics such as good thermal conductivity, high Young’s modulus, low density, high melting temperature and excellent oxidation and corrosion resistance at elevated tempera- tures [1,2]. Despite its attractive properties, some of its inherent shortcomings severely limit the structural application of NiAl, which include poor fracture toughness at ambient temperature, absence of low brittle-to-ductile transition temperature (BDTT) and insufficient strength at elevated temperatures [3]. The draw- backs of NiAl can be dramatically improved by adding ceramic reinforcements such as TiC, TiB 2 , Al 2 O 3 , ZrB 2 and CeO 2 particles [4–11]. It has been reported that composite strengthening can noticeably increase room temperature fracture toughness [4,12] and also improved compressive strength of NiAl at both ambient and elevated temperatures [13,14]. NiAl matrix composites rein- forced with ceramic particulates seem to offer a promising candi- date in corrosive, erosive, oxidation or wear condition, particularly at elevated temperatures [9,15,16]. As a result of mentioned properties, NiAl matrix composite powder is suitable as a precursor powder for oxidation and thermal barrier coatings [8,9]. Currently, NiAl matrix composites have been fabricated through a variety of novel procedures such as high pressure reaction sinter- ing [4,5], mechanical alloying [17,18], hot isostatic pressing [19], combustion synthesis [7,8] etc. In case of synthesis of composite materials, it has been well established that combustion synthesis has considerable benefits, including less processing time, low en- ergy consumption and in situ synthesis of matrix and reinforce- ments component. Literature review reveals that no work has been focused on the synthesis of NiAl–NbB 2 by the combustion synthesis method. The aim of the present investigation is to examine the feasibility of in situ synthesis of NiAl matrix composite powder with 0– 40 wt.% NbB 2 reinforcement by thermal explosion reaction of Ni– Al–Nb–B system. The effect of NbB 2 content on the mechanical properties and microstructure of final products were also investi- gated in this paper. 2. Experimental procedure In this study elemental powder mixtures including Ni (purity >99%, particle size <10 lm), Al (purity >99%, particle size <30 lm), Nb (purity >99%, particle size <15 lm), and amorphous boron (purity >95%, particle size <1 lm), were used as the raw materials. Morphologies of all elemental powders are presented in Fig. 1. The molar proportions of Al/Ni and Nb/B were fixed to 1:1 and 1:2 according to stoi- chiometric reactions (1), (2), respectively. The powders were homogenized in a ball mill. The resulting blends were then uniaxially cold compacted to form cylindrical pellets of 11 mm diameter and 10 mm height with a relative density of 0.75. Ther- mal explosion synthesis was done in a horizontal tube furnace under an atmo- sphere of high purity argon (99.99%). The compacted pellets were placed on graphite crucibles in the furnace. The reactant pellets were heated uniformly at a 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.08.024 ⇑ Corresponding author. Tel.: +98 (21)64542955; fax: +98 (21)66405846. E-mail addresses: ali_shokati@aut.ac.ir (A.A. Shokati), nparvin@aut.ac.ir (N. Parvin), naser_sabz@yahoo.com (N. Sabzianpour), mohammad_shokaty14@yahoo. com (M. Shokati), ali19he@gmail.com (A. Hemmati). Journal of Alloys and Compounds 549 (2013) 141–146 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom