Microstructural characterization of Mg-Y-Zn based alloys

Magnesium alloys containing rare-earth (RE) elements have received considerable attention during the past two decades due to their potential in achieving high strength and excellent creep resistance. Most of rare-earth containing magnesium alloys are precipitation hardenable, such as Mg-Y-Nd, Mg-Gd, Mg-Gd-Nd/Y, or the alloys based on them. The microstructural evolutions in these alloys have been relatively well established. In comparison, ternary Mg-Y-Zn alloys exhibit little age-hardening response, even though an ultrahigh strength of 600 MPa has been achieved in these alloys when they are produced by rapid solidification processes. The unique microstructures of the Mg-Y-Zn alloys have attracted much attention in recent years. The microstructures characteristic of the Mg-Y-Zn alloys usually include a range of phases with long-period stacking ordered (LPSO) structures. It should be noted that these LPSO phases are also commonly observed in a range of other alloy systems, including Mg-Gd-Zn, Mg-Gd-Y-Zn, Mg-Dy-Zn, Mg-Ho-Zn, Mg-Er-Zn, and Mg-Y-Cu. However, work to date on the detailed microstructural characterization of the Mg-Y-Zn alloys is very limited. The detailed atomic structures and structural relationships of most commonly observed LPSO phases remain to be unambiguously established. There is still lack of in-depth characterization of precipitates and the microstructural evolution in the Mg-Y-Zn alloys. It is noted that the characterization of the microstructure in the Mg-Y-Zn alloys is of considerable importance since it offers the potential to improve our understanding of a group of Mg alloys with similar microstructures. Therefore, the current investigation involves systematic examination of the microstructural evolution of the Mg-Y-Zn alloys subjected to appropriate heat treatments, detailed characterization of the crystal structures of intermetallic phases, nano-scale precipitates, and identification of planar defects exhibited in the microstructure. Furthermore, the present studies also involve exploring the potential of enhancing the age-hardening response of the Mg-Y-Zn alloys by appropriate additions of Ag elements and characterizing the precipitate microstructures. Specifically, the age-hardening response of these alloys has been monitored by measurements of their bulk Vickers hardness as a function of isothermal ageing time and temperature. The detailed characterization of microstructures has been carried out using conventional transmission electron microscopy (TEM), high-angle annular dark-field transmission scanning electron microscopy (HAADF-STEM), three dimensional atom probe (3DAP) and computer simulation. The principal aim of this investigation is to improve understanding of the precise role of Zn and Ag on the age-hardening response, microstructural evolution and the structure-property relationships of the Mg-Y alloys. Hardness measurements in an Mg-8Y-2Zn-0.6Zr (wt.%) alloy indicated little or no age-hardening response aged at 200°C and 250°C. This poor response to ageing is due to the relatively high fraction of retained intermetallic particles that formed during solidification and the small amount of precipitation of fine-scale precipitates inside individual magnesium grains. The intermetallic particles or precipitates in the as-cast and as-quenched Mg-8Y-2Zn-0.6Zr (wt.%) alloys usually exhibit 18R and/or 14H LPSO structures. They are both made of building blocks that have a hexagonal structure (γ' phase). Each building block has an ABCA-type stacking sequence of the close-packed planes, with B and C layers rich in Y and Zn atoms. The unit cell of the 18R structure comprises three building blocks that generate shears in the same direction, while a single unit cell of the 14H structure consists of two such building blocks that have opposite shears. The 18R structure is determined as an ordered base-centred monoclinic structure (a = 1.112 nm, b = 1.926 nm, c = 4.689 nm, β = 83.25°) and that the stacking sequence of the close-packed planes is ABABCACACABCBCBCABA. It has a composition of Mg10Y1Zn1. The orientation relationship between the 18R and α-Mg matrix is that (001)18R // (0001)α and [010]18R // <11-20>α. The 18R structure is observed predominantly in the as-cast samples. It is not thermodynamically stable at 500°C and is gradually replaced by the 14H structure after prolonged heat treatments at this temperature. The 14H structure has an ordered hexagonal structure (a = 1.112 nm, c = 3.647 nm). The stacking sequence of the close-packed planes is ABABCACACACBABA. The composition of the 14H structure is Mg12Y1Zn1. The orientation relationship of the 14H with α-Mg matrix is that (0001)14H // (0001)α and <0-110>14H // <-1-120>α. Besides the γ', 18R and 14H, small ribbon-like stacking faults also present in the microstructures of the Mg-8Y-2Zn-0.6Zr (wt.%) alloy in the early stage of solid-solution heat treatment. They are identified as intrinsic I2 type stacking faults bounded by two Shockley partial dislocations, which is further confirmed by computer simulation. These stacking faults show structural relationship with the building blocks of the 18R and 14H phases. Systematic additions of Ag to an Mg-6Y-1Zn-0.6Zr (wt.%) alloy led to a reduced volume fraction of retained intermetallic particles in the microstructure after solid-solution heat treatments and promoted the formation of fine-scale precipitates at the expense of coarse intermetallic particles during the following ageing process. When the Ag concentration in the alloy was over 2wt.%, the alloy exhibited a remarkable precipitation-hardening response during isothermal ageing at 200°C. The microstructures in ternary Mg-Y-Zn alloys were modified by micro-alloying additions of Ag. A third phase, designated δ, appeared in the as-cast microstructure of an Mg-6Y-2Ag-1Zn-0.6Zr (wt.%) alloy in addition to the 18R intermetallic particles and the 14H precipitates. It has a diamond-cubic symmetry (Fd-3m) with a lattice parameter of a = 1.59 ± 0.01 nm. The δ particles often have a substructure of twins on {111} planes. This phase can be fully dissolved after solution heat treatment at 500°C. The higher supersaturation of solute atoms in magnesium grains of the as-quenched samples results in a denser distribution of fine-scale γ′′ precipitate plates during the ageing process which contributes to the age-hardening response of Mg-6Y-2Ag-1Zn-0.6Zr (wt%) alloys. In the early stage of ageing at 200°C, G.P. zones form in the microstructure of the Mg-6Y-2Ag-1Zn-0.6Zr (wt.%) alloy. With continued ageing at 200°C, the G.P. zones are replaced by γ′′ precipitate plates. The denser distribution of the γ′′ precipitates results in the remarkable age-hardening response of the Mg-6Y-2Ag-1Zn-0.6Zr (wt.%) alloy. The γ′′ phase has a hexagonal Bravais lattice with lattice parameters of a = 0.556 nm and c = 0.450 nm. It forms as (0001)α plates with a thickness of a single unit cell height. The orientation relationship between γ′′ and α-Mg phases is such that (0001)γ′′ // (0001)α and [10-10]γ′′ // [2-1-10]α. It has a composition of approximately Mg10Y1Ag1. The γ'' precipitate plates tend to form in stacks when they are exposed to longer ageing time at 200°C. The separation distance of two adjacent plates in individual stacks is quite small compared to the plate diameter/length itself and it is irregular. After secondary ageing at temperatures 300°C and above, the γ'' phase is observed to be replaced by the γ' and 14H phases. Based on these experimental observations, the precipitation sequence in the Mg-6Y-2Ag-1Zn-0.6Zr (wt.%) alloys can be summarized as: α-Mg (S.S.S.S.) → G.P. zone → γ′′ → γ' → 14H+δ.