Ricardo Buzolin

The Role of Zn Additions on the Microstructure and Mechanical Properties of Mg–Nd–Zn Alloys

Mg-rare earth alloys have improved yield strength and creep resistance compared to commercial alloys such as AZ91, AM60 or AS21. The influence of Zn additions in Mg–Nd alloys on intermetallic phases and the resulting changes in the macroscopic mechanical behaviour of the materials are not yet fully understood. The aim of this work is to identify the effect of Zn additions on the nature of the intermetallic phases and on the mechanical properties in a Mg–4Nd (wt%) alloy. The addition of 8xa0wt% Zn to a base Mg–4Ndxa0wt% alloy marginally improved the tensile and compression properties at room temperature or at 200xa0°C with the exception of the 0.2% proof stress of Mg–4Nd–8Zn alloys when tensile- or compression-tested at 200xa0°C. Two intermetallic phases, a quasi-binary Mg3(Zn, Nd) and ternary Mg50Nd8Zn42, have been identified by the means of electron microscopy and synchrotron diffraction. These results indicate that there are some discrepancies in the current thermodynamic calculations, suggesting an update to the databases may be required.

The Effect of Zn Content on the Mechanical Properties of Mg-4Nd-xZn Alloys (x = 0, 3, 5 and 8 wt.%)

The mechanical properties of as-cast Mg-4Nd-xZn (x = 0, 3, 5 or 8 wt.%) alloys were investigated both in situ and ex situ in as-cast and solution-treated conditions. The additions of 3 or 5 wt.% Zn in the base Mg-4Nd alloy did not improve yield strength in comparison to the binary Mg-4Nd alloy. Mechanical properties were shown to improve only with the relatively high concentration of 8 wt.% Zn to Mg-4Nd. The change in intermetallic morphology from a continuous intermetallic to a lamella-like intermetallic was the primary reason for the decreased mechanical properties in Mg-4Nd-3Zn and Mg-4Nd-5Zn compared with Mg-4Nd and Mg-4Nd-8Zn. The dissolution of intermetallic at grain boundaries following heat treatment further indicated the importance of grain boundary reinforcement as shown in both in situ and ex situ compression testing. Azimuthal angle-time plots indicated little grain rotation most noticeably in Mg-4Nd, which also indicated the influence of a strong intermetallic network along the grain boundaries.

Intermetallic Phase Characteristics in the Mg–Nd–Zn System

Neodymium, a Rare Earth with low solid solubility in Mg is an ideal alloying element to improve the yield strength and creep resistance cost effectively. The addition of Zn achieves a further improvement; however, its influence on the intermetallic phases in the Mg–Nd–Zn ternary system is not yet fully understood. A Mg-5Nd alloy modified with 3, 5 and 7 wt% of Zn was investigated with in situ synchrotron radiation diffraction during cooling from the molten state to 200 °C in order to investigate the phase-formation and -transformation characteristics of the alloys. The synchrotron diffraction results have been complemented with TEM investigations on the as-solidified samples. The results suggest that Zn has a strong effect on the microstructure by stabilizing the Mg3Nd phase and accelerating the precipitation formation. The experimental results do not fully comply with the theoretical calculations, indicating the necessity of improving the thermodynamic databank for this alloy system.

On the Influence of Solution and Ageing Treatments on the Microstructure of ZK40 Alloys Modified with Ca, Gd, Nd and Y Additions

Abstract As-cast ZK40 alloys, modified with the addition of CaO, Gd, Nd and Y were investigated. Solution heat treatments were performed based on differential thermal analysis results. The unmodified ZK40 alloy exhibited microstructure with nearly no intermetallic compound but with precipitates formed during the solution treatment. The modified ZK40 alloys exhibited a semi-dissolved network of intermetallic compounds along the grain boundaries and zones of intermetallic compounds within the grains. Interestingly, no precipitates were observed immediately next to the grain boundaries. The energy dispersive X-ray spectroscopy line scans showed an enrichment of Zn and Zr in the regions where the precipitates are found, suggesting that they are Zn-Zr precipitates. The ageing behaviour was compared between the as-cast and the solution treated materials and it was found that apart from the ZK40-Gd, ZK40-Nd and ZK40-Y aged at 200 °C after solution treatment, there is no notable ageing response for the investigated alloys.

Phase Formation during Solidification of Mg-Nd-Zn Alloys: An In Situ Synchrotron Radiation Diffraction Study

Mg-4Nd base alloys with Zn additions of 3, 5 and 8 wt % were investigated with in situ synchrotron radiation diffraction during solidification. This method enabled the investigation of phase formation and transformation in the alloys. The diffraction results were supported with TEM observations on the as-solidified samples. The results show the effect of increased Zn addition on stabilizing the Mg3RE phase (RE—rare earth). The experimental results agree only partially with the theoretical calculations indicating the need to improve the existing thermodynamic database on the alloy system.

Effect of the Zn Content on the Compression Behaviour of Mg5Nd(Zn): An In Situ Synchrotron Radiation Diffraction Study

The properties of commercially viable Mg alloys are not sufficient for many of the envisaged applications. The combination of Zn and rare earth metals is one of the most effective ways to enhance the mechanical properties of Mg alloys. In situ synchrotron radiation diffraction is a unique method to investigate the dynamic microstructural processes occurring during deformation. Azimuthal angle–time plots give information on grain structure changes that can be correlated with grain rotation, twinning, recovery and recrystallization. As-cast Mg5Nd, Mg5Nd3Zn, Mg5Nd5Zn and Mg5Nd7Zn alloys were investigated during compression at room temperature, at 200 °C and at 350 °C with a strain rate of 10−3 s−1 until 10% deformation. The results and post mortem metallography were compared. At high temperatures grain rotation and sub-grain formation are active to obtain the final texture, while at room temperature twinning is the dominant deformation mechanism.