Materials Science and Engineering A 515 (2009) 26–31
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A Mg–Al–Nd alloy produced via a powder metallurgical route C.J. Bettles ∗ , M.H. Moss, R. Lapovok ARC Centre of Excellence for Design in Light Metals, Monash University, Clayton, Victoria, Australia
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Article history: Received 6 February 2009 Received in revised form 8 February 2009 Accepted 26 March 2009 Keywords: Magnesium Powder metallurgy Rare earth elements
a b s t r a c t A Mg–5 wt.%Al–2 wt.%Nd alloy has been prepared by a powder metallurgical route using a blend of two dissimilar alloy powders. The initial consolidation of the powders was achieved through a single equal channel angular extrusion pass at 150 ◦ C. After heat treatment at temperatures between 420 ◦ C and 530 ◦ C, it was possible to produce a microstructure that consisted of a uniform distribution of Al3 Nd and Al11 Nd3 precipitates in a magnesium matrix. These precipitates displayed distinct orientation relationships with the matrix. The size and shape of the precipitates depended on the heat treatment temperature and time. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Rare Earth (RE) additions to magnesium, either alone or in conjunction with aluminium, have become increasingly popular in recent years for applications at elevated temperatures where they impart improved creep resistance. Whilst several creep resistant alloys have recently been developed for both low pressure/gravity and high pressure die cast (HPDC) processing routes, the most popular remains AE42, which is solely a HPDC alloy and has limited creep resistance above 150 ◦ C. Being a HPDC alloy, there are limited opportunities to use heat treatment to improve the elevated temperature behaviour. The improvements, such as they are, in the creep properties of these Mg–Al–RE alloys over the Mg–Al–Zn alloys such as AZ91 are linked to the formation of Al–RE phases, primarily Al11 RE3 in AE42 [1]. However, the deleterious Mg17 Al12 phase may also form during solidification if there is excess aluminium in the alloy, or as a transformation product after prolonged exposure to elevated temperatures during service. This latter effect is particularly prevalent in alloys based on cerium-based mischmetal, but can be alleviated by increasing the Nd level at the expense of the La, in which case the primary intermetallic phase becomes the more stable Al2 RE [2]. An alternative net shape process for smaller components, based on powder metallurgy and heat treatment, is ‘interference hardening’. This process, which alloys the elements together in the solid state, was first described by Busk and Leontis in 1950 and involved the high strain ratio simultaneous extrusion of Mg–0.3 wt.%Zr and
∗ Corresponding author. Fax: +61 3 99054940. E-mail address:
[email protected] (C.J. Bettles). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.03.073
12 wt.% Mg–Al eutectic powder particles [3]. The extrusion of the separated elements resulted in a fully dense microstructure that, upon heat treatment, transformed into a Mg–Al matrix containing a continuous distribution of AlZr3 precipitates. If this alloy had been processed via a molten route the aluminium would have formed massive insoluble intermetallics with the zirconium during solidification, resulting in a microstructure that would be impossible to fully homogenise, assuming heat treatment was possible, and therefore the uniform distribution of precipitates throughout the matrix would be unattainable. The principles of interference hardening and the criteria for the selection of appropriate powder compositions have been described elsewhere [4]. This method requires the different species of metal powder to be highly deformed plastically during consolidation in order to ensure good interparticle bonding, and hence the formation of adequate diffusion paths. Indeed, the patent based on this method recommends extrusion ratios in excess of 30:1 (equivalent to an imposed strain of 3.4), which leads to some severe restrictions on the final size of any component that may be produced by this method [5]. Equal Channel Angular Pressing (ECAP) is an alternative technique that provides high strains, of the order of 1.15, on the billet per pass. A major advantage of this technique is that the strain can be imposed without a change in the cross sectional dimensions, and hence multiple passes may be undertaken without dimensional change to further accumulate strain. In addition, the strain induced is uniform across the cross section of the billet while the required high extrusion ratio and friction in conventional extrusion lead to nonuniform strain distribution within the extruded billet [6–8]. This technique has previously been shown to adequately consolidate both titanium and aluminium powders [9–11] and the authors have recently demonstrated that dense magnesium powder compacts are possible [12].
C.J. Bettles et al. / Materials Science and Engineering A 515 (2009) 26–31
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Table 1 Compacted and heat treated densities of billets, produced with either Al or Mg17 Al12 powder. Sample 1 2 3 4 5 * 6 7 8
9 *
Al form
Compacted density (g cm−3 )
Relative density (%)
Heat treatment
Density after heat treatment (g cm−3 )
Al Al Al Al Al Al Mg17 Al12 (coarse) Mg17 Al12 (coarse)
1.811 1.808 1.81 1.813 1.775 1.806 1.775 1.78
100 99.8 99.9 100 98 99.7 98.8 99.1
– 2 h at 530 ◦ C 4 h at 530 ◦ C 8 h at 530 ◦ C 24 h at 420 ◦ C 24 h at 420 ◦ C – 2 h at 420 ◦ C 4 h at 420 ◦ C 24 h at 420 ◦ C
– 1.517 1.538 1.544 1.733 1.64 – 1.766 1.771 1.751
Mg17 Al12 (fine)
1.766
98.3
Relative density after heat treatment (%) 83.8 84.9 85.2 95.7 90.5 97.5 97.8 96.7
All samples processed with a single ECAP pass, except in this case, which had 4 passes.
Interference hardening opens up possibilities for the microstructural manipulation of several magnesium alloy compositions that would otherwise be impossible to precipitation harden due to the thermal stability of the solidification compounds and/or the selection of the casting process. One such alloy system is that based on Mg–Al–RE compositions. In this paper, preliminary results, from a Mg–Al–Nd alloy, demonstrating the viability of this technique are reported. 2. Experimental Two target alloy compositions were investigated: Mg–5 wt. %Al–2 wt.%Nd and Mg–3.7 wt.%Al–2 wt.%Nd. The majority alloying element, which was the aluminium, was added in two forms: Pure aluminium powder (
