L Journal of Alloys and Compounds 280 (1998) 240–250 Constitution and hardnesses of the Al–Ir system a ,b a, c * P.J. Hill , L.A. Cornish , M.J. Witcomb a School of Process and Materials Engineering, University of the Witwatersrand, Private Bag 3, WITS, 2050, South Africa b Physical Metallurgy Division, MINTEK, Private Bag X3015, Randburg, 2125, South Africa c Electron Microscope Unit, University of the Witwatersrand Private Bag 3, WITS, 2050, South Africa Received 7 July 1998 Abstract The Al–Ir phase diagram was investigated using optical and scanning electron microscopy, and X-ray diffraction. The results were found to agree with published literature, with the exception of the congruent melting of Al Ir and a previously unreported eutectic 2.7 reaction to form Al Ir and AlIr. The AlIr and Al Ir phases had the widest composition ranges: 47–53 and 23–30 at.% Ir respectively. 2.7 2.7 Mechanical tests, in the form of Vickers hardness tests were used to deduce the fracture toughness of the alloys. It was found that the high Al-content intermetallic compounds were very brittle, and the alloys containing AlIr were tougher. This compound was tougher on the Ir-rich side, especially when there was a small amount of the AlIr1(Ir) eutectic between the AlIr dendrites.  1998 Elsevier Science S.A. All rights reserved. Keywords: Phase diagram; Al–Ir system; Al–Ir intermetallics; Hardness 1. Introduction secondary phase to be studied, with or without any ductilising addition, a thorough knowledge of the relevant Some intermetallic compounds are of interest since they phase diagram is needed. This investigation was thus tend to have higher strength at higher temperatures, and undertaken, together with some exploratory mechanical thus have potential for high temperature applications. This tests. is particularly true for the compounds of the platinum group metals (PGMs). The inherent brittleness of inter- metallic bonding can usually be countered with the pres- 2. Previous work ence of a second phase. This is normally achieved by choosing a composition with a minor second phase com- Esslinger and Schubert [5] first reported the existence of ponent, which is often a eutectic, so that its composite AlIr, and this was confirmed by Schulz et al. [6]. Lattice nature can also be exploited. An alternative method is to parameters of AlIr single crystals were obtained by Axler add small amounts of a ductilising element, such as boron and Roof [7]. Edshammer reported Al Ir to be hexagonal 3 [1], to the single-phase material. The aluminium–iridium in specimens prepared by arc-melting, and cubic at lower system has been selected for the basis of further study temperatures [8]. Edshammer also identified Al Ir in 9 2 since it contains a B2 phase similar to RuAl which was specimens annealed at 6608C [9]. The structure was identified by Fleischer to have good high temperature deduced to be monoclinic and of the type Al Co . No 9 2 strength and reasonable toughness at ambient temperature variation of d-spacings was observed with composition, [2]. Additionally, Ir has been demonstrated to have better suggesting that the Al Ir phase has a limited range. 9 2 creep resistance than Ru [3]. Although iridium oxidises Ferro et al. [10] prepared specimens by induction more easily than any of the other PGMs at low tempera- heating, followed by annealing. They reported the phases: tures, its much improved resistance at temperatures above Al Ir, Al Ir, hexagonal Al Ir, cubic Al Ir and cubic 5 3.75 3 2.7 |12008C has allowed its utilisation as a coating in combus- AlIr. The possibility of another phase between 27 and 33 tion chambers at |22608C [4]. In order for the effect of a at.% Ir was revealed by metallography. The phase width of Al Ir was found to be from 26 to 28 at.% Ir. The 2.7 AlIr1(Ir) eutectic was reported to have the composition * Corresponding author. |70 at.% Ir [10]. 0925-8388 / 98 / $ – see front matter  1998 Elsevier Science S.A. All rights reserved. PII: S0925-8388(98)00748-8