MAGNESIUM ALLOY AND METHOD FOR PRODUCING SAME

The invention relates to a magnesium alloy.

The invention also relates to a method for producing a magnesium alloy.

Particularly due to their low density and good mechanical properties, magnesium alloys constitute frequently used structural alloys or structural materials, especially in the field of the automotive industry and aircraft industry. It is known that a ductility of magnesium alloys can be improved by adding lithium (Li), wherein a shift from a hexagonal crystal system to a body-centered cubic crystal system typically takes place in the magnesium alloy as the lithium amount increases. This is associated with an increased number of slip planes, whereby the occurrence of a markedly improved ductility as the lithium amount increases can be explained. However, this approach can be accompanied by a reduction in the strength and corrosion resistance of the magnesium alloy, so that other alloying elements, such as aluminum or zinc for example, are often added in order to lessen these disadvantages and to normally achieve at least moderate strengths and corrosion resistances.

This is addressed by the invention. The object of the invention is to specify a magnesium alloy that has a high strength, in particular a high compressive strength, and a good deformability.

It is also an object of the invention to specify a method for producing a magnesium alloy of this type.

According to the invention, the object is attained with a magnesium alloy comprising, in particular being made from, (in at %)

15.0% to 70.0% lithium,

greater than 0.0% aluminum,

magnesium and production-related impurities as a remainder,

wherein a ratio of aluminum to magnesium (in at %) is 1:6 to 4:6.

The basis of the invention is the finding that, with an aforementioned alloy composition of a magnesium alloy having a corresponding amount of lithium (Li) as well as a mandatory amount of aluminum (Al) in a specific, aforementioned aluminum-to-magnesium ratio range, a micro-scale microstructure or a fine, in particular fine lamellar, microstructure forms in the magnesium alloy. The theoretical foundation of these characteristics is considered to be a eutectic transformation of the magnesium alloy which occurs at an aforementioned ratio of aluminum to magnesium. The fine-scale microstructure is accompanied by a high strength, in particular a high compressive strength, wherein a good deformability of the magnesium alloy is ensured at the same time at corresponding aforementioned amounts of lithium in the magnesium alloy. In this case, the orientation composition or orientation line in the phase diagram is in particular a ratio of aluminum to magnesium (in atomic percent, abbreviated by at %) of approx. 3:6, since a particularly homogeneous fine-scale or homogeneous fine lamellar microstructure or morphology is found at this ratio. In a range encompassing this ratio, above all at an aluminum-to-magnesium ratio (in at %) of 1:6 to 4:6, the fine, in particular fine lamellar, microstructure or morphology is also found in a varyingly pronounced degree, which is accordingly normally associated with varyingly pronounced magnitudes of strength, in particular a magnitude of compressive strength, as well as deformability or ductility of the magnesium alloy. Because of these special morphological characteristics in the stated composition range, a formation of a magnesium alloy that has both a high strength, in particular compressive strength, and also a good deformability is thus enabled.

Advantageously, it is provided that the magnesium alloy comprises (in at %) 30.0% to 60.0%, in particular 40% to 50%, lithium. As a result, a pronounced strength and a particularly pronounced deformability can be achieved. This is likely due in particular to a combination of finely structured morphology and a transformation to a body-centered cubic crystal system in the stated lithium range. A high strength and also a high deformability emerge particularly strongly if the magnesium alloy comprises (in at %) 45% to 50%, in particular 45% to 48%, lithium.

Typically, the magnesium alloy comprises (in at %) greater than 0.05%, in particular greater than 0.1%, normally greater than 1% aluminum.

A structural alloy with high usability can be achieved if the magnesium alloy is embodied as a magnesium-based alloy. In accordance with notation typically used in practice, magnesium-based alloy thereby denotes a magnesium alloy which, taking the alloy amounts thereof in percentage by weight (wt %) as a basis, contains magnesium as the main element or as the largest alloy amount. Especially in combination with the amounts of lithium stated, in particular those stated above, a feasible structural alloy with very high strength properties and pronounced deformability can be achieved.

It has been shown that a, in particular lamellar, microstructure with a high degree of fineness can be achieved if the ratio of aluminum to magnesium (in at %) is 1.2:6 to 4:6, in particular 1.4:6 to 4:6, preferably 1.5:6 to 4:6. It is beneficial to a pronounced fineness or a fine, in particular lamellar, microstructure if the ratio of aluminum to magnesium (in at %) is 1.8:6 to 3.5:6, in particular 2:6 to 3.5:6, preferably 2.5:6 to 3.5:6. A particularly high strength, in particular compressive strength, can thus be achieved. This is particularly true at an aluminum-to-magnesium ratio (in at %) of 2.8:6 to 3.3:6, preferably approximately 3:6, at which a very homogeneous fine morphology or microstructure is obtainable. For this purpose, it is particularly advantageous if the magnesium alloy is (in at %) 30.0% to 60.0% lithium and a ratio of aluminum to magnesium (in at %) is 2.5:6 to 3.5:6, in particular 2.8:6 to 3.3:6, preferably approximately 3:6. A particularly pronounced homogeneity can also be achieved if the magnesium alloy thereby comprises (in at %) 40.0% to 60.0% lithium.

It should be understood that the stated ratios of aluminum to magnesium carry corresponding uncertainties such as those which are typical in a production of alloys, in particular where casting processes are used, and, accordingly, cannot be interpreted as fully exact values; rather, they are subject to a conventional rounding scheme that is useful in practice, as is expediently applied by a prudent person skilled in the art in the field of alloy production, in particular where casting processes are used, in order to produce a corresponding magnesium alloy.

It has proven effective if the magnesium alloy comprises greater than 0.0 to 3.0 wt %, in particular greater than 0.0 to 2.0 wt %, preferably greater than 0.0 to 1.5 wt %, calcium (Ca). In this manner, an improved corrosion resistance of the magnesium alloy can be achieved. Typically, a lower limit of content ranges, in particular those mentioned above, for calcium is greater than 0.05 wt %. In particular, a reduced oxidation tendency of the magnesium alloy can thus be realized, advantageously typically in that a stable oxidation layer forms on a surface of the magnesium alloy. Additionally, with an aforementioned amount of calcium, a grain-refining effect in the magnesium alloy can be utilized or achieved so that a high stability of the fine-scale microstructure is achievable and a strength of the magnesium alloy can be further increased. Both a high oxidation resistance and also an increased strength or a stabilization of the strength properties can be achieved if the magnesium alloy comprises 0.5 wt % to 1.0 wt % calcium. When calcium is present in the magnesium alloy, the effects stated above are in particular based on a formation of CaO. Accordingly, it can be specifically provided that calcium is added to the magnesium alloy as an alloy amount or contained in the magnesium alloy at least partially, in particularly predominantly, preferably completely, in the form of CaO. A homogeneous distribution of calcium or CaO in the magnesium alloy can thus be facilitated. It is thus particularly advantageous if the magnesium alloy comprises CaO with the amounts stated above for calcium.

For a reduction of an oxidation tendency, it is beneficial if the magnesium alloy comprises greater than 0.0 to 3.0 wt %, preferably 1.0 wt % to 2.0 wt %, rare earth metals, in particular yttrium (Y). Typically, a lower limit of content ranges, in particular those mentioned above, for rare earth metals, above all yttrium, is greater than 0.05 wt %. Here, a formation of Y₂O₃occurring in the magnesium alloy is of particular relevance. Accordingly, it can be specifically provided that yttrium is added to the magnesium alloy as an alloy amount or contained in the magnesium alloy at least partially, in particular predominantly, preferably completely, in the form of Y₂O₃. It is thus advantageous if the magnesium alloy comprises Y₂O₃with the amounts stated above for yttrium.

An oxidation tendency can be reduced in particular if both calcium, in particular in the form of CaO, and also rare earth metals, in particular yttrium, preferably in the form of Y₂O₃, are respectively contained in the magnesium alloy according to the aforementioned content ranges, wherein in particular calcium at greater than 0.0, in particular greater than 0.05 wt % to 1.5 wt %, and yttrium at 1.0 wt % to 2.0 wt % have proven effective.

A particularly pronounced corrosion resistance can be achieved if the magnesium alloy contains calcium and rare earth metals, in particular yttrium, wherein a total amount of calcium and rare earth metals, in particular yttrium, is greater than 0.0, in particular greater than 0.05 wt % to 3.0 wt %, preferably 1.0 wt % to 2.5 wt %.

It is advantageous if a compressive strength of the magnesium alloy, in particular at room temperature, is at least 300 MPa, in particular at least 350 MPa, preferably at least 380 MPa, particularly preferably at least 400 MPa. This can be achieved with an alloy composition provided according to the invention for the magnesium alloy as a result of the finely structured microstructure thereof, in particular according to a production of the magnesium alloy by casting. Preferably, the aforementioned values apply to a maximum compressive strength, specifically for a compressive yield point or compressive yield strength, of the magnesium alloy. Advantageously, the compressive strength or maximum compressive strength, or compressive yield point or compressive yield strength, of the magnesium alloy can be at least 410 MPa, specifically at least 430 MPa. This can typically be feasibly achieved with a heat treatment, in particular as described below.

It has been shown that the magnesium alloy has a good aging capacity, wherein a strength, in particular compressive strength, and/or a deformability of the magnesium alloy can be further optimized, or preferably increased, by heat treatment of the magnesium alloy. It is therefore advantageously provided that a specific compressive strength, in particular a maximum specific compressive strength, of the magnesium alloy, in particular at room temperature, in an aged state is at least 300 Nm/g, in particular at least 330 Nm/g, preferably at least 350 Nm/g. The aged state thereby denotes a state of the magnesium alloy after a completed heat treatment of the magnesium alloy. Boundary conditions of the heat treatment that are beneficial thereto are further explained below, in particular as part of a method for producing a magnesium alloy, acid can be applied accordingly.

The stated material characteristics for the magnesium alloy, primarily values for compressive strength or specific compressive strength, are thereby based in particular on a room temperature that typically lies between 20° C. and 25° C., normally approximately 20° C.

It has been shown that a particularly high strength, in particular compressive strength, and advantageously high deformability can be achieved if the magnesium alloy comprises 18.0 wt % to 24.0 wt %, in particular 18.0 wt % to 22 wt % lithium and 15.0 wt % to 30.0 wt %, in particular 16.5 wt % to 28.0 wt % aluminum. Here, it has also been shown that, with an additional amount of calcium, a hardness of the magnesium alloy, in particular as part of a heat treatment that is carried out, can be optimized or can be set in a targeted manner. For this purpose, it is advantageous if the magnesium alloy also comprises calcium at greater than 0.0, in particular greater than 0.05 wt %, to 2.5 wt %, in particular 0.1 wt % to 2.0 wt %, preferably 0.3 wt % to 1.5 wt %. It is thus possible not only to influence or improve a corrosion resistance or oxidation tendency using calcium specifically in this content range of lithium and aluminum, but also to influence a hardness of the magnesium alloy. This becomes apparent in particular when the magnesium alloy comprises 18.0 wt % to 22 wt % lithium and 16.5 wt % to 28.0 wt % aluminum, particularly prominently at 0.1 wt % to 2.0 wt %, in particular at 0.3 wt % to 1.5 wt %, calcium. During a heat treatment, the hardness typically increases as a duration of heat treatment increases, so that a hardness of the magnesium alloy can be set as a function of a duration of the heat treatment. It is beneficial to a high hardness if a heat treatment between 200° C. and 450° C. has a heat treatment duration of greater than 1 hour, in particular greater than 3 hours. Specifically, an easily manageable and easily processable composition or magnesium alloy can be obtained if the magnesium alloy comprises 20 wt % lithium and 15.0 to 30.0 wt %, in particular 16.5 wt % to 28.0 wt %, particularly preferably 18.0 wt % to 0.26.0 wt %, aluminum. This is especially true if calcium is also contained in the magnesium alloy as stated above.

The mechanical properties of the magnesium alloy can be optimized for a specific intended application through additions of other alloying elements. For a fine-tuning of a strength, in particular the compressive strength, of the magnesium alloy, it is beneficial if the magnesium alloy comprises 3.0 wt % to 10.0 wt % zinc. An optimization of the compressive strength, in particular without notably limiting a deformability, can be achieved if the magnesium alloy comprises 7.0 wt % to 10.0 wt % zinc. Alternatively or cumulatively to zinc, it is beneficial thereto if the magnesium alloy comprises 2.0 wt % to 10.0 wt %, preferably 3.0 wt % to 7.0 wt %, silicon.

A method for producing a magnesium alloy according to the invention normally involves starting materials of the magnesium alloy being mixed and, proceeding from a liquid or semi-liquid phase, being cooled. The magnesium alloy according to the invention, or a feedstock, semi-finished product, or element having or being made from the magnesium alloy, can easily be produced by means of typical casting processes, for example using mold casting processes, die-casting processes, continuous casting processes or permanent mold casting processes. It has proven advantageous in particular if the production of the magnesium alloy according to the invention comprises a heat treatment in order to optimize a microstructure or morphology of the magnesium alloy with regard to a strength, in particular compressive strength, or deformability.

The other object of the invention is attained with a method for producing a magnesium alloy according to the invention, wherein a heat treatment of the magnesium alloy is carried out in order to optimize or increase a strength, in particular compressive strength, and/or deformability of the magnesium alloy. It has been shown that, through a heat treatment of the magnesium alloy, a strength, in particular compressive strength, and deformability of the magnesium alloy can be further optimized or increased so that they can be set in a targeted manner in particular, preferably such that they are adjusted for an intended, application of the magnesium alloy.

It should be understood that the method according to the invention may be embodied correspondingly or analogously to the features, advantages, implementations, and effects that are described, in particular as described above, within the scope of a magnesium alloy according to the invention. The same also applies to the magnesium alloy according to the invention with regard to a described method according to the invention, in particular as described below, and to the individual treatment steps or production steps thereof.

It is beneficial to a pronounced increase in strength, in particular compressive strength, if the heat treatment is carried out at a temperature greater than 200° C., in particular between 200° C. and 450° C., for more than 20 minutes, in particular more than 1 hour. A heat treatment at a temperature between 250° C. and 400° C., preferably between 270° C. and 350° C., has proven particularly suitable for a pronounced increase in strength, in particular compressive strength. Here, it is advantageous if the heat treatment is carried out for more than 1 hour (h), preferably between 1 hour and 10 hours, particularly preferably between 1 hour and 6 hours, in order to efficiently set the strength. A heat treatment between 300° C. and 350° C., preferably between 320° C. and 340° C., for 2 hours to 5 hours has proven particularly efficient for a sustained increase in strength with a simultaneous optimization of a deformability of the magnesium alloy. It should be understood that, in principle, a longer heat treatment duration can also be common; however, the heat treatment durations stated above have been shown to be particularly feasible with regard to a time-efficient optimization of the mechanical properties.

A feedstock, semi-finished product, or element is advantageously realized having, in particular being made from, a magnesium alloy according to the invention or such that it is obtainable using a method according to the invention for producing a magnesium alloy according to the invention. In accordance with the explanations, features, and effects of the magnesium alloy according to the invention or a magnesium alloy produced using a method according to the invention, a feedstock, semi-finished product, or element formed with a magnesium alloy also has an advantageously high strength, in particular compressive strength, and good deformability.

Additional features, advantages, and effects follow from the exemplary embodiments described below. In the drawings which are thereby referenced:

FIG. 1 shows a schematic phase diagram illustration for Mg—Li—Al, in which composition ranges of the magnesium alloy according to the invention are indicated;

FIG. 2 shows a yield stress diagram of multiple magnesium alloy specimens from a magnesium alloy according to the invention;

FIG. 3 and FIG. 4 show scanning electron microscope images of a magnesium alloy specimen from a magnesium alloy according to the invention at different magnifications;

FIG. 5 shows a yield stress diagram of magnesium alloy specimens from a magnesium alloy according to the invention after completed heat treatments;

FIG. 6 shows a yield stress diagram of magnesium alloy specimens from a further magnesium alloy according to the invention after completed heat treatments;

FIG. 7 shows a hardness diagram of magnesium alloy specimens from a magnesium alloy according to the invention.

FIG. 1 shows a schematic phase diagram illustration (in at %) for magnesium-lithium-aluminum (Mg—Li—Al) according, to a typical ternary phase diagram design, wherein composition ranges or content ranges of alloy amounts of a magnesium alloy according to the invention are indicated. In the phase diagram illustration, an orientation composition of an Mg—Li—Al alloy with an aluminum-to-magnesium ratio (in at %) of approx. 3:6 is depicted as dash-dotted line A, since in accordance with a finding on which the invention is based, a particularly homogeneous, fine-scale, in particular fine lamellar, microstructure or morphology is found in a content range of 15.0 at % to 70.0 at % lithium at this ratio of aluminum to magnesium. In a range encompassing this ratio, indicated by an aluminum-to-magnesium ratio (in at %) of 1:6 to 4:6, this fine-scale or finely structured microstructure is furthermore found in a varyingly pronounced degree and explains an advantageously high strength, in particular compressive strength, and good deformability of the magnesium alloy in this range. A composition range (in at %) of 15.0% to 70.0% lithium and an aluminum-to-magnesium ratio (in at %) of 1:6 to 4:6 are illustrated clearly in FIG. 1 by a quadrangle depicted by a solid line, denoted by reference numeral 1. A pronounced strength and particularly pronounced deformability are found in particular in a composition range (in at %) of 30.0% to 60.0% lithium and an aluminum-to-magnesium ratio (in at %) of 1:6 to 4:6. This composition range is illustrated in FIG. 1 by a quadrangle depicted by a dashed line, denoted by reference numeral 2.

In the course of a development of the magnesium alloy according to the invention, series of tests were conducted with different alloy compositions of magnesium alloys, in particular of alloy compositions correspondingly defined according to the invention. Below, characteristics of magnesium alloy specimens fabricated from Mg-20% Li-15% Al-1% Ca-0.5% Y (in wt %) and Mg-20% Li-24% Al-1% Ca-0.5% Y (in wt %) are presented as being representative of the aforementioned composition ranges. The magnesium alloy specimens were produced by means of permanent mold casting, wherein in particular magnesium alloy specimens having a cylindrical shape, with a diameter of 5 mm and a length of 10 mm, were fabricated. The magnesium alloy specimens were subjected to compression tests at room temperature, approximately 20° C. and yield curves which depict a yield stress, in MPa, as a function of a degree of deformation, in %, were calculated as a result.

FIG. 2 shows a yield stress diagram with yield curves as a result of compression tests at room temperature using magnesium alloy specimens fabricated from Mg-20% Li-15% Al-1% Ca-0.5% Y (in wt %). Yield curves of magnesium alloy specimens immediately following a production of the magnesium alloy specimens (as cast) are illustrated, shown in FIG. 2 as solid lines, denoted by reference numeral 3. In addition, yield curves of magnesium alloy specimens after a completed heat treatment (aged) of the magnesium alloy specimens are illustrated, shown in FIG. 2 as dashed lines, denoted by reference numeral 4. For this purpose, magnesium alloy specimens were subjected to a heat treatment at 330° C. for 3 hours, and yield curves were then calculated by means of compression tests. A clear influence of the heat treatment on the compressive strength and deformability of the magnesium alloy specimens is evident, as a result of which there is the potential to set the compressive strength, and deformability in an optimized manner using heat treatment, in particular for an eventual intended application.

FIG. 3 and FIG. 4 show scanning electron microscope images of the magnesium alloy specimens fabricated from Mg-20% Li-15% Al-1% Ca-0.5% Y (in wt %) at different magnifications. Evident are, on the one hand, light grain boundary phases (in whitish-gray) that were identified as Al—Ca and, on the other hand, pronounced fine crystalline structures or morphologies in a region surrounded by the grain boundary phases, in particular in a center section of said region, or in the interior of the mixed crystal phase, clearly evident in FIG. 4 in particular. Also identifiable is a markedly different fine structure, in particular in the proximity of the grain boundary phases.

FIG. 5 shows a yield stress diagram with yield curves as a result of compression tests at room temperature using magnesium alloy specimens fabricated from Mg-20% Li-15% Al-1% Ca-0.5% Y (in wt %), wherein magnesium alloy specimens were examined after completed heat treatments at different heat treatment temperatures. Yield curves of magnesium alloy specimens which were subjected to a heat treatment at 270° C. for 4 hours are illustrated, depicted in FIG. 5 as dashed lines, denoted by reference numeral 5, and yield curves of magnesium alloy specimens which were subjected to a heat treatment at 330° C. for 4 hours, depicted in FIG. 5 as solid lines, denoted by reference numeral 6. Evident is a pronounced influence of heat treatment temperatures on the mechanical properties of the magnesium alloy specimens, wherein a heat treatment temperature of 330° C. compared to a lower heat treatment temperature of 270° C. leads to a pronounced improvement in the compressive strength, there also being a very good deformability of the magnesium alloy specimens at the same time.

FIG. 6 shows a yield stress diagram with yield curves as a result of compression tests at room temperature using magnesium alloy specimens fabricated from Mg-20% Li-15% Al-1% Ca-0.5% Y (in wt %), wherein magnesium alloy specimens were examined after completed heat treatments at different heat treatment temperatures. Yield curves of magnesium alloy specimens which were subjected to a heat treatment at 270° C. for 4 hours are illustrated, depicted in FIG. 6 as dashed lines, denoted by reference numeral 7, and yield curves of magnesium alloy specimens which were subjected to a heat treatment at 330° C. for 4 hours, depicted in FIG. 6 as solid lines, denoted by reference numeral 8. Here, analogously to the result illustrated in FIG. 5, a pronounced influence of heat treatment temperatures on the mechanical properties of the magnesium alloy specimens is once again found, wherein a heat treatment temperature of 330° C. compared to a lower heat treatment temperature of 270° C. leads to an improvement in the compressive strength, there also being a good deformability of the magnesium alloy specimens at the same time.

FIG. 7 shows a hardness diagram as a result of Vickers hardness tests at room temperature, approximately 20° C., using magnesium alloy specimens fabricated from Mg-20% Li-15% Al-1% Ca-0.5% Y (in wt %), wherein magnesium alloy specimens were examined after completed heat treatments with different heat treatment durations. 330° C. was used as a heat treatment temperature. In the hardness diagram, mean values of Vickers hardnesses (HV 0.1) from multiple measurements are respectively shown as a function of different heat treatment durations t, from 0 minutes (min) to 300 minutes, of the magnesium alloy specimens. Evident is a successive increase in the hardness with a heat treatment duration, wherein a high hardness can be achieved in particular at a heat treatment duration of more than 60 minutes. With regard to the image depictions shown in FIG. 3 and FIG. 4, these characteristics can potentially be explained by a diffusion of calcium into the inner region of the mixed crystal phase.

A magnesium alloy according to the invention thus advantageously exhibits both a high strength and also a good deformability, both of which can be optimized, or preferably increased, by means of heat treatment in particular. Specifically, there is also the possibility of optimizing, or setting in a defined manner, a hardness of the magnesium alloy. The magnesium alloy according to the invention, or an element having or being made from the magnesium alloy according to the invention, thus offers the potential to realize, preferably such that they suit a purpose, robust and resilient components, especially structural components, in particular in the automotive industry, aircraft industry, and/or space industry.

MAGNESIUM ALLOY AND METHOD FOR PRODUCING SAME

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