Compositional Effects of Gd, Y and Ag on Precipitation, Tensile Properties and Corrosion of Mg-Gd-Y-Ag(-Zr) Alloys

Abstract

As the lightest structural material, Mg alloys show great potential as candidates for various structural components in the aerospace, aircraft, automotive and 3C (computer, communication and consumer electronic product) industries. However, difficulties in attaining good strength and ductility in the same material have hindered the wider application of Mg alloys. In this regard, Mg alloys containing rare-earth (RE) elements have attracted widespread research interest for their high strength with acceptable ductility. Mg-Gd(-Y)-Ag alloys are typical examples of high strength Mg-RE alloys: a Mg-15.6Gd-1.8Ag-0.4Zr (wt.%) casting alloy had a yield strength (YS) of 328 MPa, ultimate tensile strength (UTS) of 423 MPa and elongation of 2.6%; while, a Mg-8.5Gd-2.3Y-1.8Y-0.4Zr (wt.%) combined a YS of 268 MPa, UTS of 403 MPa with considerably improved ductility (4.9%, elongation to break). The main reason for the high strength achieved in the Mg-Gd(-Y)-Ag alloys is the combined strengthening effect of prismatic and basal strengthening precipitates. However, detailed characterisation of the strengthening precipitates in this important Mg-Gd(-Y)-Ag alloy system is very limited in the literature to date. Furthermore, the roles of Ag in Mg-Gd and Mg-Y binary alloys are still unclear. The type, atomic structure, orientation and evolution of the precipitates, especially the basal precipitates, in Mg-Gd(Y)-Ag alloys are not unambiguously established. Detailed characterisations of the precipitates in Mg-Gd(-Y)-Ag alloys are critical, since in-depth knowledge of the precipitates in the high strength Mg alloys offers the opportunity to improve our understanding and facilitates rational design the compositions of Mg alloys with improved strength.     In the present study, six alloys with equi-atomic concentration of total alloying elements are designed to study the compositional roles of Gd, Y, Ag in the microstructures and mechanical properties of Mg-Gd-Y-Ag(-Zr) alloys. The addition of Ag to the Mg-Gd binary alloy significantly enhances the age-hardening response. This is due to the formation of a large number of basal precipitates, without obviously sacrificing the prismatic precipitates. The YS, UTS and elongation of the Ag-free counterpart Mg-2.8Gd-0.1Zr (at.%) are 246 MPa, 293 MPa and 0.4%, respectively. In comparison, Mg-2.4Gd-0.4Ag-0.1Zr (at.%) has a YS of 271 MPa, UTS of 414 MPa, and elongation of 2.7%. The YS of Mg-2.4Gd-0.4Ag-0.1Zr was the highest among the six alloys. Contrary to the positive role of Ag in the Mg-Gd binary alloy, the addition of Ag to the Mg-Y binary alloy reduces the age-hardening response. In the peak-aged condition, the microstructure of the Mg-2.4Y-0.4Ag-0.Zr (at.%) is mainly composed of basal precipitates with a limited number of prismatic precipitates. The YS, UTS and elongation of the Ag-free Mg-2.8Y-0.1Zr (at.%) are 237 MPa, 278 MPa and 0.4%, respectively, while those of the Mg-2.4Y-0.4Ag-0.1Zr are 217 MPa, 309 MPa and 2%. The Mg-2.4Y-0.4Ag-0.1Zr has the poorest YS among the six alloys. Increasing the Gd/Y atomic ratio can obviously increase the number density and reduce the size of both prismatic and basal precipitates. A good combination of strength and ductility is achieved in Mg-1.6Gd-0.8Y-0.4Ag-0.1Zr (at.%) which has a Gd/Y atomic ratio of 2:1. It is also noted that for all Ag-containing alloys, improved ductility is achieved in comparison with the Ag-free Mg-Gd(-Zr) and Mg-Y(-Zr) alloys.     Because Mg-2.4Gd-0.4Ag-0.1Zr shows the highest YS with acceptable ductility, microstructures of this alloy were systematically studied using atom probe tomography (APT) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). For the first time, a large number of Ag-rich clusters were observed in the solution-treated Mg-2.4Gd-0.4Ag-0.1Zr using APT. These Ag-rich clusters promote age-hardening kinetics during the early stages of ageing. In the first stage of ageing, the precipitation involves the formation of ordered solute clusters and GP zones. The ordered solute clusters comprise 1-6 solute-rich atomic columns along [0001]α. Among all the types of clusters, hexagonal clusters are the most frequently observed one; this is due to the interconnected hexagonal ring is the most energetically favourable configuration. Three types of GP zones are observed. Zigzag and hexagonal GP zones form as disks on {101 0}α and flat GP zones form as disks on (0001)α. In the peak-aged condition, the microstructure mainly contains β' (base-centred orthorhombic Bravais lattice, a = 0.65 nm, b = 2.27 nm and c = 0.52 nm) precipitates and two types of plate-like basal precipitates ( γ'''and γ''). The γ''' plate is comprised of three atomic planes parallel to (0001)α, with Gd and Ag atoms having an ordered hexagonal distribution in each layer. The γ'' precipitate has an ordered hexagonal structure (a = 0.548 nm, c = 0.417 nm). In the over-aged condition, the precipitation involves the formation of βF' and β1 (fcc, a = 0.74 nm). Contradicting first-principles calculations in the literature, in which the formation of βF' phase is energetically unfavourable in Mg-Gd alloys, well-developed βF' precipitates are observed in Mg-2.4Gd-0.4Ag-0.1Zr. The βF' phase has an orthorhombic Bravais lattice (a = 0.64 nm, b = 1.1 nm and c = 0.52 nm), and this precipitate is always found attached with β' precipitate. Based on experimental observations, the β1 precipitates may form from βF' precipitates. The equilibrium precipitate phases in Mg-2.4Gd-0.4Ag-0.1Zr are β (Mg5Gd, fcc, a = 2.23 nm) and γ (diamond cubic, a = 1.6 nm).     The development of high strength Mg-Gd-Ag(-Zr) alloys thus far has not taken the corrosion issue into consideration. Additions of Ag to the Mg-Gd(-Zr) alloys lead to a significant drop in corrosion resistance. Therefore, a quasi-in-situ STEM/EDS approach was applied to study the corrosion behaviour of Mg-2.4Gd-0.4Ag-0.1Zr and Mg-2.8Gd-0.1Zr upon immersion in 0.01 M NaCl solution. Based on the quasi-in-situ STEM/EDS approach, segregation of O at the interface between the α-Mg matrix and the precipitates were directly observed during corrosion. Besides, Ag-rich particles were found, which was due to the re-deposition of Ag. These results provide solid evidence for the micro-galvanic corrosion. The quasi-in-situ STEM/EDS investigation also suggests that the corrosion of Mg-2.4Gd-0.4Ag-0.1Zr and Mg-2.8Gd-0.1Zr could be characterised by two stages (stage I and stage II). In stage I, the α-Mg matrix dissolves to a small extent; while in stage II, micro-galvanic corrosion is accelerated. Ag addition results in higher tendency for the growth of hydroxide layer which catalyses the cathodic reaction and significantly increases the overall corrosion rate.