Dispersoid strengthening of Al–Cu–Mg P/M alloy utilising transition metal additions R. W. Cooke 1 , R. L. Hexemer, Jr 2 , I. W. Donaldson 2 and D. P. Bishop* 1 The objective of this research was to devise a means by which dispersoid forming transition metals could be incorporated into press and sinter aluminium powder metallurgy (P/M) alloys. Additions of iron and nickel were explored in this context added as either admixed elemental powders or prealloyed additions into the base aluminium powder. Utilising an Al–2?3Cu–1?6Mg– 0?2Sn composition as the base system, elemental additions imparted coarse aluminide phases within the sintered microstructure and diminished the general sintering response of the alloy. The resultant tensile properties of these materials were inferior to those of the unmodified base alloy. Prealloying was much more effective. Using this approach, highly refined distributions of aluminides were achieved without any adverse effects on the compaction or sintering response of the base alloy. A prealloyed addition of 1 wt-% iron was the most effective of those considered as it imparted tangible gains in yield strength and ultimate tensile strength (UTS) to the base alloy. Keywords: Aluminium powder metallurgy, Transition metals, Dispersoid strengthening, Iron, Nickel, Aluminides, Prealloying Introduction Modern aluminium alloys are chemically complex ma- terials that employ multiple alloying additions. Each element is specifically incorporated to enhance material performance. Gains are generally sought in a number of areas such as mechanical properties, corrosion response, and metal formability. Among these, the attenuation of improved mechanical properties is most often targeted. This is accomplished through the inclusion of elements that promote precipitation hardening or dispersoid strengthening. In precipitation hardening, a homoge- nous distribution of exceptionally fine (typically sub- micron) high strength phases/intermetallics is formed in the alloy through an array of different heat treat- ment practices. This network represents a formidable barrier to dislocation movement and as such, prompts a significant increase in mechanical properties. Some of the element groupings commonly added for this purpose includes Cu, Cu/Mg, Cu/Mg/Si, Mg/Si, Zn/Mg and Zn/ Mg/Cu. 1–4 In a dispersoid strengthening scenario, the phase(s) are typically formed during solidification of the starting ingot. Elements commonly adopted for this purpose include Mn, Fe and/or Ni. The dispersoids are con- siderably coarser (typically .5 mm) and more sparsely distributed than the phases formed through precipita- tion hardening. 5 Although this represents a less effective means of strengthening overall, the effect is still con- siderable and can be realised without implementation of expensive heat treatment practices. As such, this par- ticular mechanism is also employed in many aluminium alloys. For example, wrought alloys such as 2014 6 and 6013 7 contain deliberate additions of Mn (y0?8 wt-%) which commonly exists as dispersoids such as Al 12 Mn 3 Si and Al 12 (Mn,Fe) 3 Si in the microstructure. Other alloys such as 2618, 2218 and 8001 include Ni and/or Fe additions on the order of 1 wt-%. Commonly observed dispersoid phases that stem from these additions include Al 7 Cu 2 Fe, Al 7 Cu 4 Ni and Al 9 FeNi. 8 In most cases, the concentrations of transition metals added are relatively dilute (,2 wt-%). The incorporation of dispersoid forming transition metals via aluminium powder metallurgy (P/M) technol- ogy has been studied through certain techniques such as reaction sintering 9,10 and in-situ microfusion. 11 Here, simple binary mixtures of elemental powders are pro- cessed through a press and sinter approach. Relatively high concentrations of transition metals are employed such that the sintered product is typically either a single phase intermetallic or a composite material or sorts involving a pure, soft aluminium matrix coupled with a high volume fraction of an intermetallic dispersoid. In an effort to realise the highest mechanical proper- ties possible, many conventional (wrought/cast) alumi- nium alloys are formulated so as to simultaneously exploit precipitation hardening and dispersoid strength- ening mechanisms. However, this has not been the case with commercially produced press and sinter aluminium P/M alloys. Here, there exists an exclusive reliance on precipitation hardening as the sole strengthening me- chanism. Dispersoid forming elements are merely pre- sent as trace (,0?1 wt-%) impurities and have not been included as deliberate additions. For example, AC2014 is currently the principal alloy employed in the 1 Department of Process Engineering and Applied Science, Dalhousie University, Halifax, NS, Canada 2 GKN Sinter Metals LLC, 3300 University Drive, Auburn Hills, MI, USA *Corresponding author, email Paul.Bishop@dal.ca ß 2012 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 9 March 2011; accepted 11 June 2011 DOI 10.1179/1743290111Y.0000000012 Powder Metallurgy 2012 VOL 55 NO 3 191