[001] This invention relates to a high strength castable magnesium alloy, as well as to a method for making such an alloy and articles comprising the alloy.

[002] Background

[003] Magnesium alloys comprising rare earth metals and zirconium (Mg-RE-Zr alloys) are regularly used within aerospace and other specialist applications. In these uses, strength-to-weight ratio is an important factor in the selection of materials. A greater strength-to-weight ratio (i.e. a greater strength in two materials of the same density) is generally desirable. Examples of applications where the strength-to-weight ratio is important include drive train components (gearboxes, housings covers, and similar items), engine components (cases, covers, moving components, and similar) and structural components (panels, body parts, and similar) in both conventional and electric aircraft. Similar material requirements also apply to spacecraft and satellite applications where the total weight of the spacecraft/satellite defines the cost of putting it into space.

[004] Within the family of Mg-RE-Zr alloys, the properties can vary dramatically and there are multiple sub-families. By selection of particular rare earth elements and the addition of small amounts of other elements, certain aspects of alloy performance can be enhanced. There are numerous examples of alloys that fall within the broader family. Generic benefits of various rare earth metals are covered in textbooks (for example, "Light Alloys", 4th Edition, Ian Polmear, page 265, figure 5.17). It is generally accepted that rare earth metals provide increasing strength benefits up to their solid solubility limit within the alloy, and that a further increase in their content then results in a decline in properties.

[005] Examples of such alloys are disclosed in the applicant's earlier US patent no. 7,935,304 B2. This document describes alloys containing: 2 to 4.5% by weight of neodymium; 0.2 to 7.0% of at least one rare earth metal of atomic no. 62 to 71 ; up to 1.3% by weight of zinc; and 0.2 to 0.7% by weight of zirconium; optionally with one or more other minor components. [006] An alloy that falls within the scope of this definition is defined in AMS4429B as having a composition as follows:

* Other rare earths shall principally be the total of cerium, lanthanum, and praseodymium.

[007] Improved castable Mg-RE-Zr alloys have been sought, in particular those having improved strength characteristics.

[008] Statement of invention

[009] This invention relates to a magnesium alloy comprising:

(a) 1.32-1.8 wt% Gd,

(b) 2-3.6 wt% Nd,

(c) 0.55-0.7 wt% Zr,

(d) 0.20-0.40 wt% Zn, and

(d) at least 85 wt% Mg, wherein the ratio of Gd:Nd (wt%) is 0.40-0.63.

[0010] It has been surprisingly found by the inventors that a magnesium alloy having the above composition, and with a Gd:Nd (wt%) ratio of 0.40-0.63, has improved strength, in particular improved Ultimate Tensile Strength (UTS). In some embodiments, the alloy may be a cast magnesium alloy, more particularly a sand cast alloy. In particular, the cast magnesium alloy may have a thickness of 5-350 mm. In some embodiments, the cast magnesium alloy may have a thickness of 25- 350 mm, more particularly 100-350 mm, even more particularly 200-350 mm. In other embodiments, the cast magnesium alloy may have a thickness of 5-100 mm, more particularly 10-100 mm. In the context of the invention, the term "thickness" is used to mean the smallest dimension of the cast magnesium alloy. It has been surprisingly found by the inventors that the alloys of the invention have improved retention of their properties at higher thickness castings. In some embodiments, the alloy may be a wrought magnesium alloy. The wrought alloy may be a forged or extruded alloy.

[0011] In particular, the Ultimate Tensile Strength (UTS) of the magnesium alloy, when sand cast into a 200 mm x 200 mm x 25.4 mm plate and subjected to a T6 ageing process, may be at least 310 MPa as measured according to ASTM B557M- 15. More particularly, the UTS may be at least 315 MPa, even more particularly at least 316 MPa, more particularly at least 320 MPa. In particular, the UTS may be 350 MPa or less.

[0012] In relation to the invention, the term "ageing process" is used to refer to a process in which the magnesium alloy is heated to a temperature above room temperature, held at that temperature for a period of time, and then allowed to return to room temperature (i.e. around 25°C). In particular, the ageing process may be a T6 ageing process. Such processes are known in the art and generally involve heating the magnesium alloy to a temperature of 515°C to 524°C), holding it at that temperature for a period of time (for solution treatment), quenching it (i.e. allowing it to cool to a temperature lower than the temperature to which it has been heated, for example to room temperature (around 25°C)) as required, and reheating the magnesium alloy to a temperature between 200°C to 204°C and then holding it at that temperature for a period of time (for precipitation heat treatment).

[0013] In relation to this invention, the term "alloy" is used to mean a composition made by mixing and fusing two or more metallic elements by melting them together, mixing and re-solidifying them. Thus, in the context of the inventive alloy, any elements mentioned are in their metallic form (and not, for example, present as a salt).

[0014] In particular, the ratio of Gd:Nd (wt%) may be 0.40-0.61. More particularly, the ratio of Gd:Nd (wt%) may be 0.40-0.60. Even more particularly, the ratio of Gd:Nd (wt%) may be 0.40-0.57.

[0015] In particular, the magnesium alloy may comprise 1.35-1.8 wt% Gd, more particularly 1.40-1.75 wt% Gd, even more particularly 1.40-1.70 wt% Gd.

[0016] More particularly, the magnesium alloy may comprise 2.2-3.4 wt% Nd, even more particularly 2.6-3.1 wt% Nd.

[0017] In particular, the magnesium alloy may comprise 0.6-0.7 wt% Zr, more particularly 0.60-0.70 wt% Zr.

[0018] More particularly, the magnesium alloy may comprise at least 90 wt% Mg, even more particularly at least 92 wt% Mg. In some embodiments, the remainder of the alloy may be magnesium and incidental impurities.

[0019] In particular, the magnesium alloy may optionally comprise (i) up to 0.4 wt% of rare earth metals other than Gd and Nd, (ii) up to 0.05 wt% Ag, (iii) up to 0.01 wt% Cu, (iv) up to 0.010 wt% Fe, (v) up to 0.0020 wt% Ni, (vi) up to 0.01 wt% of any other elements, with the remainder being magnesium. The rare earth metals other than Gd and Nd may comprise Ce, La and Pr.

[0020] The term "rare earth metals" is used in relation to the invention to refer to the fifteen lanthanide elements, as well as Sc and Y.

[0021] In particular, the magnesium alloy, when sand cast into a 200 mm x 200 mm x 25.4 mm plate and subjected to a T6 ageing process, may have a mean planar grain size of at least 10 pm, more particularly 10-100 pm, even more particularly 20- 80 pm. Mean planar grain size was measured on a representative section of the magnesium alloy using the Heyn linear intercept method as set out in ASTM E112- 13. In this context, the term "representative section" is used to mean a section that has been selected to represent average conditions within the alloy. Such sections should not normally be taken from areas affected by shearing, burning, or other processes that will alter the grain structure.

[0022] Grain size is known in the art to affect material performance. The Hall-Petch relationship states that UTS can be increased by decreasing grain size. There are other benefits to having a finer grain size. However, grain sizes of less than 10 pm (known as ultrafine) are generally difficult to achieve in the large castings used industrially. Such a large casting would typically have a complex geometry and an average wall thickness of >6 mm. To achieve ultrafine grain sizes in such materials, processes such as forming/working the material or using high solidification rate processes (e.g. atomisation, high pressure die castings) need to be employed. However, these processes are not suitable for many industrial/aerospace components (i.e. the main use of the alloys of the invention), where sand casting is the preferred method of manufacture due to practical considerations. For the inventive alloys, it is therefore desirable to achieve the required properties (e.g. improved UTS) in a material having a grain size greater than 20 pm, and typically less than 100 pm. The grain size of many commercial magnesium alloys, e.g. Mg-AI alloys, has a strong dependency on solidification rate, with an increased cooling rate resulting in a decrease in grain size (see, for example, L.A. Dobrzahski, M. Krol, T. Tahski, Effect of cooling rate and aluminum contents on the Mg-AI-Zn alloys’ structure and mechanical properties, Journal of Achievements in Materials and Manufacturing Engineering 43/2 (2010) 613-633). It has been found by the inventors that the alloys of the invention, when sand cast (i.e. a method having a slow cooling rate), for example with a 25.4 mm wall thickness, can achieve improved (i.e. lower) mean grain size and mechanical properties (e.g. UTS).

[0023] More particularly, the magnesium alloy may comprise more than one crystal phase, i.e. the magnesium alloy may be heterogeneous. In some embodiments, the alloy may comprise a first crystal phase and a second crystal phase. The first crystal phase may differ from the second crystal phase in relation to its crystal structure and/or its composition. In particular, the magnesium alloy, when sand cast into a 200 mm x 200 mm x 25.4 mm plate and subjected to a T6 ageing process, may comprise at least 97 % of a first crystal phase, more particularly at least 98 %, even more particularly at least 98.5 %, more particularly at least 99 %. In this context, the percentage is of the area of a two-dimensional Scanning Electron Microscope (SEM) microstructure image of the magnesium alloy.

[0024] It has been surprisingly found by the inventors that, for the alloys of the invention, having a relatively low Zn content as specified herein increases the first phase content (and decreases the second phase content). This results in the alloys having higher UTS. This is contrary to the general knowledge in this area of technology, where it is generally thought that a higher Zn content results in higher UTS. An example of this may be found in the international specification ASTM B60- 15 which describes ZK41A, ZK51A and ZK61A sand cast alloys. These are Mg-Zn- Zr alloys, and are nominally identical apart from having Zn levels of ~4.3, ~4.6 and ~6% respectively (mid-point in the specification ranges). These alloys have minimum UTS specifications of 200, 234 and 276 MPa respectively, demonstrating how the increased Zn content results in higher UTS values.

[0025] This invention also relates to an aircraft or spacecraft component (e.g. a conventional aircraft or electric aircraft component) comprising the magnesium alloy described above. More particularly, the aircraft component may be an electric aircraft component. The term "aircraft" includes aeroplanes and helicopters. The term "spacecraft" includes satellites. The inclusion of Gd in the alloy, which can provide a neutron absorbing effect, can be beneficial in environments with increased levels of neutrons (e.g. space). In particular, the aircraft component may be a drive train component, an engine component or a structural component. More particularly, the drive train component may be a gearbox, housing or cover. In particular, the engine component may be a case, cover or moving component. More particularly, the structural component may be a panel or body part.

[0026] This invention also relates to a method for producing a magnesium alloy comprising the steps of:

(a) heating Mg, Gd, Nd, Zr and Zn to form a molten magnesium alloy comprising 1.32-1.8 wt% Gd, 2-3.6 wt% Nd, 0.55-0.7 wt% Zr, 0.20-

0.40 wt% Zn and at least 85 wt% Mg, wherein the ratio of Gd:Nd (wt%) is

0.40-0.63,

(b) mixing the resulting molten magnesium alloy, and

(c) casting the magnesium alloy. [0027] In particular, the method may be for producing a magnesium alloy as defined above. Any other required components in the resulting alloy (for example, those listed in the preceding paragraphs describing the alloy) can be added in heating step (a). More particularly, the heating step may be carried out at a temperature of 650°C (ie the melting point of pure magnesium) or more, even more particularly less than 1090°C (the boiling point of pure magnesium). In particular, the temperature range may be 650°C to 850°C, more particularly 700°C to 800°C, even more particularly 750°C to 780°C. More particularly, in step (b) the resulting alloy may be fully molten and/or dissolved.

[0028] More particularly, in step (a) the resulting alloy may be fully molten. In particular, prior to melting in step (a) the alloy components may be present in elemental form or as one or more alloys.

[0029] In particular, in step (c) the casting may comprise pouring the molten magnesium alloy into a mould, and then allowing it to cool and solidify. The mould may be a die mould, a permanent mould, a sand mould, an investment mould, a direct chill casting (DC) mould, or other mould.

[0030] After step (c), the method may comprise one or more of the following additional steps: (d) extruding, (e) forging, (f) rolling, (g) machining.

[0031] This invention will be further described by reference to the following Figures which are not intended to limit the scope of the invention claimed, in which:

Figure 1 shows a graph of Ultimate Tensile Strength (UTS) against the ratio of Gd:Nd (wt%) for the alloys of Examples 1-9,

Figure 2 shows a graph of Ultimate Tensile Strength (UTS) against Zn content (wt%) for the alloys of Examples 10-16

Figure 3 shows the microstructure of the alloy of Example 5 when measured by Scanning Electron Microscopy (SEM),

Figure 4 shows the image of Figure 3 after processing to identify the first and second phases of the alloy,

Figure 5 shows a graph of Ultimate Tensile Strength (UTS) against the area% of a second phase for the alloys of Examples 1-16, and Figure 6 shows a graph of area% of a second phase against the ratio of Gd:Nd (wt%) for the alloys of Examples 1-16.

[0032] Examples

[0033] Magnesium alloy compositions were prepared by combining the components in the amounts listed in Table 1 below. These compositions were then melted by heating them to 750°C to 780°C. The melt was then cast into a 200 mm x 200 mm x 25.4 mm mould and subjected to a T6 ageing process. The Ultimate Tensile Strength (UTS) of the alloys was then tested according to ASTM B557M-15. t Comparative examples

Table 1

[0034] Figure 1 is a graph of UTS against Gd:Nd ratio for the samples in Table 1. This clearly shows that the samples of the invention, i.e. those having the claimed Gd:Nd ratio (wt%), have improved UTS.

[0035] Figure 3 is an SEM image of the optical microstructure of the magnesium alloy of Example 5 (i.e. cast into a 200 mm x 200 mm x 25.4 mm mould and subjected to a T6 ageing process). This shows that the alloy has an approximately equiaxed grain structure. The alloy is almost entirely a single phase, with only a few small regions of a secondary phase. An SEM image of the same alloy is shown in Figure 4. The small amount of secondary phase appears as the white regions in Figure 4. [0036] Further magnesium alloy compositions were prepared by combining the components in the amounts listed in Table 2 below. These compositions were prepared in order to demonstrate the effect of Zn content on the magnesium alloys of the invention. The compositions were melted by heating them to 750°C to 780°C. The melt was then cast into a 200 mm x 200 mm x 25.4 mm mould and subjected to a T6 ageing process. The Ultimate Tensile Strength (UTS) of the alloys was then tested according to ASTM B557M-15.

+ Comparative examples

Table 2

[0037] Figure 2 is a graph of the UTS against Zn content for Examples 10-16 of Table 2. This data shows that magnesium alloys having a Zn content outside of the claimed range exhibit reduced strength properties.

[0038] The percentage of a second phase was measured by generating an optical two-dimensional SEM image of each alloy (i.e. as cast into a 200 mm x 200 mm x 25.4 mm mould and subjected to a T6 heat treatment). The two phases have different atomic compositions, which appear as contrast in a backscattered electron SEM image. By using image processing software the initial SEM image can be converted into a black and white image with the first phase being black and any areas of second phase appearing as white. Software is the used to measure the area of a second phase (white) as a percentage of the total area. This is the type of image shown in Figure 4. At least two images were taken per alloy and an average percentage calculated. These results are shown in Table 3 below.

+ Comparative examples Table 3

[0039] The percentage of second phase values in Table 3 are depicted in Figure 5 against the UTS values for each alloy. The graph in Figure 5 shows a trend of decreasing UTS as the amount of second phase increases.

[0040] A further graph is shown in Figure 6, which plots Gd:Nd ratio against the area percentage of second phase for each of the examples. For ease of reference, the data points have been plotted as circles (Zn values <0.5, i.e. within the range for the alloys of the invention) and squares (Zn values >0.5, i.e. above the range for the alloys of the invention). For the circles (alloys with Zn content within the range of the alloys of invention), for those alloys having a Gd:Nd ratio outside of that required by the alloys of the invention the percentage of the second phase is higher, and thus UTS is lower. The square data points (alloys with Zn content outside of the range of the alloys of invention) show higher second phase percentages, and therefore lower UTS, for alloys having a Gd:Nd ratio within that required by the alloys of the invention but a Zn content above the range required by the inventive alloys.