Materials Science and Engineering A 445–446 (2007) 336–340 Order hardening of Pt 8 X alloys S. Nxumalo, M.P. Nzula 1 , C.I. Lang ∗ Centre for Materials Engineering, Department of Mechanical Engineering, University of Cape Town, Private Bag, Rondebosch 7701, South Africa Received 19 April 2006; received in revised form 3 September 2006; accepted 14 September 2006 Abstract Platinum containing around 11.1 at.% chromium or vanadium undergoes an ordering transformation upon heat treatment, resulting in the formation of ordered domains with the Pt 8 Ti superlattice structure. The formation of these ordered domains, which are dispersed in a disordered matrix, results in an increase in hardness. Hardness is observed to be sensitive to domain size, with a maximum hardness at around 5 nm, if the ordered volume fraction is high. © 2006 Elsevier B.V. All rights reserved. Keywords: Ordering; Hardness; Electron diffraction 1. Introduction When platinum is alloyed with an element from group 4 to 6 of the periodic table (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo or W), a Pt 8 X ordered structure can be formed of which Pt 8 Ti is the pro- totype [1]. More generally, when a solute element from group 4 to 6 of the periodic table is added to a host metal from group 10 (platinum, palladium or nickel), an A 8 B ordered structure can occur [2,3]. Although the crystallography and thermodynamic stability of the A 8 B ordered structure have been the subject of considerable interest, to date little attention has been paid to the effect of the formation of the A 8 B ordered structure on the properties of these alloys. An exception is the nickel–tantalum system, for which the effect of A 8 B type ordering on flow stress has been studied [4]. The effect of the formation of Pt 8 X on the mechanical properties of platinum alloys has not previously been investigated. In this paper we report the results of a study of the effect of the formation of Pt 8 Cr and Pt 8 V on the hardness of platinum–chromium and platinum–vanadium alloys, respec- tively. It is well established that an ordering transformation can result in an increase in hardness and strength [5–9]; however, several different models for strengthening due to ordering have ∗ Corresponding author. Tel.: +27 216503675; fax: +27 216897571. E-mail address: clang@ebe.uct.ac.za (C.I. Lang). 1 Present address: Advanced Materials Division, Mintek, 200 Hans Strijdom Drive, Randburg 2121, South Africa. been proposed so it is difficult to generalise regarding the mech- anism. A change in strength may arise from long-range order [6,8], involving a complete transformation of the disordered alloy to a superlattice structure; or from a partial transforma- tion to a mixed state containing ordered domains in a disordered or short-range ordered matrix [10,11]; or from a change to a statistical preference for unlike nearest neighbours, statistical short-range order (SSRO) [12]. Irani and Cahn [13] suggested that all order strengthening models should contain the central concept of a maximum flow stress at an intermediate degree of order, as this is the most common factor observed experi- mentally. More recent work on order strengthened alloys has focussed on alloys which are strengthened by the presence of coherent ordered precipitates in a disordered matrix [14], such as the nickel-based superalloys and aluminium–lithium alloys. In the platinum–chromium system, the ordered structure Pt 8 Cr was first observed by Nzula et al. [15]. The ordering trans- formation occurred at temperatures below 500 ◦ C and appeared to require the presence of excess vacancies, which were intro- duced by deforming the alloy prior to heat treatment. The Pt 8 V ordered structure was first observed by Schryvers et al. [16], upon heat treating a previously water quenched sample at 700 ◦ C. Nxumalo and Lang [17] showed that Pt 8 V is thermody- namically stable below 810 ◦ C, and can form upon heat treatment of either quenched or deformed samples. Post-deformation heat treatment of both Pt 10 at.% Cr and Pt 11 at.% V was reported to result in formation of nanometer-scale ordered domains, of Pt 8 Cr and Pt 8 V, respectively, in a deformed matrix; a fully ordered alloy was not observed [15,17]. A mixed state of order 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.09.048