Crystalline oxalic acid aided FeAl intermetallic alloy sintering. Fabrication of intermetallic foam with porosity above 45% Krzysztof Karczewski a , Wojciech J. Stępniowski a,n , Michal Chojnacki b , Stanislaw Jóźwiak a a Department of Advanced Materials and Technologies, Faculty of Advanced Technology and Chemistry, Military University of Technology, Kaliskiego 2 Str., 00-908 Warszawa, Poland b Institute of Physics Polish Academy of Sciences, Division of Physics of Magnetism, Al. Lotników 32/46, 02-668 Warsaw, Poland article info Article history: Received 3 July 2015 Received in revised form 20 July 2015 Accepted 25 September 2015 Available online 28 September 2015 Keywords: Porous intermetallics Sintering Chemical compound assisted sintering Crystalline oxalic acid Fe–Al binary system abstract Intermetallic Fe–Al foam was formed via organic compound assisted sintering. To the starting mixture of iron and aluminum elemental powders various amount of crystalline oxalic acid (COA), (COOH) 2 ·2H 2 O was added as the foaming agent. Due to the decomposition reaction, gaseous products were formed and behaved as the foaming agents. With this approach Fe–Al intermetallic foam was formed and major role of the COA was confirmed by elemental maps. In the pores, remaining traces of carbon were found. It was also found that at 3 wt% of COA additive the greatest porosity equal 48,5% 74,3% was obtained. Above 3 wt% the formed foam was defragmenting. Additionally, to investigate functional application of the formed Fe–Al intermetallic foam, the pressure drop of air during penetration the porous material was examined. It was found that increased porosity of the sintered material allowed to increase the flow of the air through the metallic foam. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Intermetallic alloys from Fe–Al binary system find numerous applications as a typical structural material [1–5] or coating [6–9] due to the high corrosion resistance [6,8] and mechanical prop- erties [1–5]. Fe–Al-based intermetallic alloys are usually manu- factured with classical techniques [1], however recently sintering [4,5],additive methods [10] and spraying techniques [2] are being much more frequently applied. The sintering process employs elemental powders [4,5] and therefore provides opportunity to add modifiers. According to Lazińska et al. modifiers like sodium chloride can be added to the powders prior the sintering [11]. Then, a sintered FeAl inter- metallic alloys is formed with incorporated NaCl. Next, the salt is being flushed out and a porous material is obtained. Also organic compound aided sintering is a quite popular approach of metallic foams formation. According to the literature data, stearic acid [12], PMMA, polyethylene glycol, and stearic acid blends [13,14] are being used as the space vacating agents to form porous metals. Thus, in these cases [12–14] the organic compounds play the same role as NaCl. Previously, we have reported fabrication of organic compound assisted sintering where palmitic acid and cholesteryl myristate were applied as the additives during the sintering [15]. Also in this case a porous FeAl sintered material was formed. In this case a totally different mechanism allowed to form the porous inter- metallic alloy. Due to the combustion reaction of the organic compounds the pressure increased locally during the sintering process and the pores were formed. In this paper we report fabrication of porous FeAl intermetallic alloys where crystalline oxalic acid (COS) was used as the foaming agent. The influence of the foaming agent on the materials mor- phology was studied in details. A method employing combustion assisted sintering allowed us to form FeAl intermetallic foam. 2. Experimental The raw materials used in this study were: Fe powder with the average particle size of 100 mm (99.9% purity), Al powder with the average particle size of o75 mm (99% purity) and crystalline oxalic acid dihydrate (COA, p.a. Chempur). The following compositions were prepared: the reference composition (RC)-Fe–45Al (at%), RC þ 0,5 wt% COA, RC þ 1 wt% COA, RC þ 2 wt% COA and RC þ 3 wt% COA. Subsequently, the powder mixture was consolidated by uniaxial cold pressing under 700 MPa pressure into lasting cylindrical ∅25 mm and 6 mm height pellets. Sintering process was being conducted in the volume con- trolled environmental reactor [15] in argon atmosphere at 700 °C Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters http://dx.doi.org/10.1016/j.matlet.2015.09.104 0167-577X/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. Fax: þ48 261 839 445. E-mail address: wojciech.stepniowski@wat.edu.pl (W.J. Stępniowski). Materials Letters 164 (2016) 32–34