The Effect of Zinc and Gadolinium on the Precipitation Sequence and Quench Sensitivity of Four Mg‐Nd‐Gd alloys

Abstract

Magnesium rare earth alloys show an excellent age hardening response, due to the decreasing solid solubility of rare earth elements with decreasing temperature-a critical requirement for age-hardening. It is known that the poor casting quality of binary Mg-Zn alloys can be improved with the addition of rare earth elements. [1] Additions of gadolinium to yttrium and neodymium-based systems have been shown to improve the mechanical properties of the alloy. [2] Previous researchers [3] showed that a 4 wt%Zn-1.5 wt%Ce misch metal alloy formed the same rod and disk-shaped precipitates as a Mg-9 wt%Zn alloy. The precipitation sequence for a Mg-2.8 wt%Nd-1.3 wt%Zn alloy was shown to be sssslow temperature reaction-c''-c. [4] The 'low temperature reaction' was either the formation of GP zones or short range atomic ordering between 50 and 150°C. The c'' precipitates formed as plates on the (0001) Mg planes between 200°C and 225°C, and were also formed during short ageing times at 250°C and 325°C. The c precipitates had a rod-like morphology and grew in the <1010> Mg and <2110> Mg directions, between 200°C and 325°C, after initial precipitation of c''.The Mg-3 wt%Nd(Zn) system has been investigated. [5] Upon ageing at 200°C, additions of 0.5 wt% Zn to the binary alloy Mg-3 wt%Nd increased the time to reach peak-hardness and delayed over-ageing. In the peak-aged condition the binary alloy precipitates were plate-shaped with habit planes parallel to the magnesium matrix prism planes. In comparison, the ternary alloy containing 1.35 wt%Zn aged under the same conditions had plate-shaped precipitates formed parallel to the (0001) Mg planes. The authors concluded that the decrease in hardness associated with the 1.35 wt%Zn alloy was due to the orientation of the plate-shaped precipitates that provided a low barrier to dislocation motion through the matrix Work has been carried out to determine the effect of zinc additions of 0 wt%, 0.5 wt%, 1 wt% and 2 wt% on a Mg-6 wt%Gd-0.6wt %Zr alloy. [6] The ageing response of the four alloys at 250°C was noted to be similar for the zinc-free and 0.5 wt% zinc alloys. The alloys containing 1 wt% and 2 wt% zinc had an enhanced ageing response. It was noted that at peak hardness at 250°C the zinc-free alloy contained sparse, coarse, plate-shaped precipitates on the prism planes of the matrix. The 1 wt% and 2 wt% zinc alloys contained plateshaped precipitates which formed on the basal planes of the magnesium matrix. These were identified as being the same as those found in Mg-RE-Zn alloys. [5,7] The research described in the present paper contributed to the development of Elektron 21 undertaken by Magnesium Elektron. The alloy was developed for use for specialist autosport and aerospace applications. ExperimentalAlloy production and solution heat treatment were carried out at Magnesium Elektron. The alloys were cast into sand moulds to form plates 20 cm × 20 cm × 3 cm. The alloy compositions are listed below:(1) Mg-2.7 wt%Nd-0.007 wt%Zn-0.35 wt%Gd-0.48 wt%Zr (2) Mg-2.8 wt%Nd-0.6 wt%Zn-0.29 wt%G...

The Microstructure Evolution of a High Zr‐Containing WE Magnesium Alloy Through Isothermal Semi‐Solid Treatment

“…The XRD results, shown in Figure 2, indicate to the co-existence of Mg 24 Y 5 and Zr phase in the initial as-cast microstructural. The microstructure also contains fine dispersed Mg 12 NdY precipitates, [10,29] arrowed in Figure 1a. However, these could not be during EDS and XRD analyses.…”

“…Yttrium (Y) with high solubility in magnesium is considered to be one of the most effective RE elements in improving mechanical properties of the alloys. [10,12] The corresponding fundamental literature implies that the RE-containing precipitates are highly thermally stable. [6] It is worth mentioning that the Mg-Y binary alloy exhibits poor creep and corrosion properties at high temperatures, particularly above 300°C.…”

“…Therefore, the precipitation hardening is possible and expected. [10,12] The corresponding fundamental literature implies that the RE-containing precipitates are highly thermally stable. [13] As such, they may assist in improving the creep resistance, oxidation resistance, and the high temperature strength.…”

Effects of Quenching Rate on the Microstructures and Mechanical Properties of the Heat Treatable Mg-4.2Y-2.3Nd-1.0Gd-0.6Zr Magnesium Alloy

“…And in magnesium alloys, L.B. Gill found that the Mg-Nd-Gd-Zn-Zr alloys have quench sensitivity, especially when the content of element Zn is 1.3 wt% in the work [6]. D. Wu also investigated the effect of quenching rate on the microstructures and mechanical properties of the Mg-2.7Nd-0.6Zn-0.5Zr (wt.%) alloy, and found that in the air quenching sample large basal precipitates have formed in the matrix which results in low tensile strength [7].…”

Crystalline Structure of Metastable Phases in Mg-Nd Alloy Containing Zn and Zr

“…Mg-RE alloys show good castability, high strength at elevated temperatures and good creep resistance [1][2][3][4]. Additional alloying elements in Mg-RE alloys were used to achieve higher mechanical properties [5,6]. While these systems have been extensively investigated, they are not fully understood and their potentialities have not been exhausted.…”