Hardening and microstructural reactions in high-temperature equal-channel angular pressed Mg–Nd–Gd–Zn–Zr alloy

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Abstract

The use of high-temperature (330 °C) equal-channel angular pressing (ECAP) is demonstrated to promote precipitation of a fine and uniform dispersion of the FCC β1 phase in an Mg–Nd–Gd–Zn–Zr alloy. Significantly, this process induces a hardening reaction in the alloy, where isothermal ageing at this temperature leads only to softening. The evolution of microstructure is characterized using transmission electron microscopy and scanning electron microscopy. The nucleation and growth of precipitates during the high-temperature ECAP are discussed. This research highlights a new approach to engineer precipitate microstructures via the application of severe plastic deformation so as to extend the property space of high-temperature Mg alloys.

Introduction

Magnesium alloys containing rare earth (RE) elements have received significant recent attention because they exhibit a strong age-hardening response and good creep resistance [1], [2], [3], [4], [5], [6]. In particular, the RE-modified Mg alloys exhibit improved high-temperature performance when compared with conventional Mg alloys and are able to withstand service temperatures up to ∼250 °C [7], [8]. Already, these materials are being used in the aircraft industry [7] and the high-temperature properties of these alloys have generated significant interest as potential materials for jet engine applications [1]. A wide range of Mg-RE alloys have now been investigated to explore their age-hardening response and precipitation sequences [1], [2], [3], [4], [5], [6], [7], [8], [9]. The Mg–Nd–Gd–Zn–Zr alloys seem particularly attractive because they exhibit a stronger hardening response with lower overall RE levels [5]. Two modes of precipitation reaction have been reported in these alloys, depending on the ageing temperature. Below ∼200 °C, ageing produces a high strengthening effect due to the formation of a fine and uniform dispersion of β″ precipitates (DO19 hexagonal; a = b = 0.64 nm and c = 0.52 nm [5]) and β′ precipitates (base-centred orthormbic; a = 0.640 nm, b = 2.223 nm, c = 0.521 nm) [5]. The metastable β″ and β′ phases are unstable at ageing temperatures above 300 °C and the β1 phase (FCC; space group Fm3¯m, a = 0.73 nm [6]) precipitates directly from the solid solution, instead. It has been a challenge to optimize the dispersion of fine β1 precipitates because these precipitates exhibit low nucleation rates and rapid growth during conventional ageing treatments at 300 °C. There is great interest to develop new approaches to boost the nucleation and restrict the growth of β1 precipitates in Mg-RE alloys in order to improve the alloy strength at high temperatures.

Severe plastic deformation (SPD) is well known to be effective in modifying microstructure and creating a unique balance of properties. Equal-channel angular pressing (ECAP) is an attractive SPD method because it can produce ultrafine-grained microstructures and has potential for scale-up and mass production [10], [11], [12]. As might be expected, initial studies have demonstrated that low-temperature SPD leads to more pronounced grain refinement than the high-temperature variant and most SPD experiments tend to be conducted at lower temperatures [10], [11], [12], [13], [14], [15], [16], [17]. It is also well known that ultrafine-grained microstructures often exhibit poor stability at higher temperatures [18], [19], [20], [21] and are therefore not amenable to high-temperature service applications. High-temperature SPD as an alternative can be attractive if it can successfully modify precipitate microstructure for better strengthening effect in Mg-RE alloys. To our knowledge, there are no reports yet available on the effects of high-temperature SPD on the hardening and microstructure in Mg-RE alloys and the opportunity for dynamic modification of the precipitation processes. More recently, there has been growing interest in using SPD to control precipitation microstructures so as to achieve a combination of strengthening from both grain refinement and precipitation hardening [22], [23], [24] and SPD was recently demonstrated as effective in modifying precipitate microstructures in Al–Zn–Mg–Cu alloys [25].

In this research, we investigate the use of ECAP to modify high-temperature precipitate microstructure in order to strengthen Mg-RE alloys. A two-step ECAP procedure was devised to refine the precipitate microstructure of an Mg–Gd–Nd–Zn–Zr alloy and improve its strength for high-temperature applications. Careful microscopy experiments were performed to characterize the resultant multi-scale microstructures of the Mg-RE alloy. This paper presents a detailed overview of the evolution of precipitate microstructure and for the first time address the nucleation and growth of precipitates in the alloy during ECAP processing at 330 °C.

Section snippets

Experimental material and procedures

The Mg–2.58Nd–1.80Gd–0.60Zn–0.15Zr (wt.%) alloy was prepared from high purity Mg (99.9 wt.%), Zn (99.9 wt.%), Nd (99.9 wt.%), and Mg–28Gd (wt.%) and Mg–33Zr master alloys in an electrical resistance furnace under the protection of an anti-oxidizing flux before being cast into a sand mould to form rods of 10 mm diameter and 200 mm length. These as-cast rods were cut into short billets having lengths of 50 mm. Solution heat treatment was performed on the billets in a salt bath at 520 °C for 18 h,

Hardening responses of an Mg–Nd–Gd–Zn–Zr alloy processed by different routes

Fig. 1 shows that processing by isothermal ageing at 330 °C produces a very different hardening response in the as-quenched (AQ) Mg–Nd–Gd–Zn–Zr alloy from processing by ECAP at the same temperature. The ageing treatments at 330 °C up to 90 min did not produce any significant strengthening effect in the alloy, indicating that the alloy had a poor hardening response during ageing at the temperature. In contrast, a marked hardening effect from an initial value of 60 to a value of 73 Hv was evident in

Precipitation in Mg–Nd–Gd–Zn–Zr alloy during isothermal ageing at 330 °C

The high-temperature ageing at 330 °C led to precipitation of β1 and this is consistent with a precipitation sequence of SSSS  β1  β reported in a similar alloy aged at 300 °C [5]. The number density of these β1 precipitates showed no clear decrease during ageing from 30 to 90 min, but the length of those large β1 platelets increased during the time. Precipitation of hexagonal γ′ was identified during the isothermal ageing treatment, Fig. 4. These metastable precipitates have been widely reported in

Summary and conclusions

  • 1.

    High-temperature ECAP is an effective route to modify precipitate microstructure to obtain precipitation strengthening in the Mg–Nd–Gd–Zn–Zr alloy, although it has no significant effect on the precipitation sequence in forming β phase, i.e. SSSS  β1  β, but the precipitation of γ′ was suppressed under the ECAP.

  • 2.

    High-temperature ECAP effectively promotes the nucleation of dense β1 precipitates, resulting in a marked strengthening effect, i.e. a ∼19% increase of hardness above that of the

Acknowledgements

The authors are grateful for scientific and technical input and support from the Australian Microscopy & Microanalysis Research Facility (AMMRF) node at The University of Sydney, and the Australian Research Council with the ARC Centre of Excellence for Design in Light Metals. The assistance of Dr. Bulent Gun and Dr. Yanyan Sun on sample preparation and SEM analysis and the assistance of Dr. Xiaolin Wu and Mr. Jizhong Li on ECAP processing experiments are greatly acknowledged.

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