WO2010056130A1

WO2010056130A1 - Magnesium based alloys and processes for preparation thereof

Info

Publication number: WO2010056130A1
Application number: PCT/NZ2009/000244
Authority: WO
Inventors: Wei Gao; Hongmei Liu
Original assignee: Auckland Uniservices Ltd
Current assignee: Auckland Uniservices Ltd
Priority date: 2008-11-14
Filing date: 2009-11-12
Publication date: 2010-05-20
Anticipated expiration: 2011-05-14

Abstract

The present invention generally relates to magnesium based alloys, and in particular, to magnesium based alloys containing tin and lead, and processes for the preparation and applications of the magnesium based alloys. The magnesium based alloys of the present invention have been shown to provide good mechanical forming properties at room temperature and may also include one or more additional elements of zinc, aluminium, zirconium, manganese, strontium, calcium, bismuth, antimony, boron, silicon, copper, silver and rare earth elements.

Description

MAGNESIUM BASED ALLOYS AND PROCESSES FOR PREPARATION THEREOF

FIELD

The present invention generally relates to magnesium based alloys. In particular, the present invention relates to magnesium based alloys containing tin and lead, and processes for the preparation and applications of the magnesium based alloys.

BACKGROUND

Magnesium alloys provide excellent mechanical and physical properties, such as low density, high strength-to- weight ratio, good heat and electrical conductivity, good electromagnetic wave-screening ability, and good damping properties. In comparison to steels or aluminium based alloys, magnesium alloys provide many superior properties, particularly with respect to low density and high strength- to-weight ratio, which makes magnesium alloys attractive for applications where weight reduction is critical including applications in aerospace, automotive, transportation, electronic devices and hand tools. For instance, the use of magnesium alloys in forming automobile parts has experienced rapid growth in recent years because of the demand for more light weight and fuel efficient cars. Magnesium alloys can also offer other advantages including superior castability and machinability properties.

However, magnesium metal has a hexagonal close-packed (HCP) crystal structure with a limited number of operative slip systems at room temperature; therefore the ductility and mechanical deformability of magnesium alloys is intrinsically poor at room temperature. The mechanical properties of presently known magnesium based alloys, such as AM60A and AM60B (magnesium alloys containing aluminium and manganese) and AZ91 (magnesium alloy containing aluminium and zinc) , provide relatively low plasticity and elongation levels of, generally, about 3-6% elongation at room temperature, and at best up to about 9-10% elongation. However, the magnesium based alloys having the higher elongation levels are alloys that have undergone specialized further processing or pretreatment , such as mechanical and thermal treatment.

The poor plasticity and cold- forming properties of magnesium alloy results in known magnesium based alloys having a low deformation limit with a large bending radius. At elevated temperatures more slip systems become available and the magnesium alloys become more workable. Thermal- mechanical type deformation processes such as hot-rolling and hot-extrusion have therefore been used to improve deformation properties of some magnesium alloys. Although these processing methods have been used to commercially produce magnesium products such as plates and rods, they present many difficulties and disadvantages. For example, the magnesium alloys have to be protected during the processing by using an inert atmosphere to prevent oxidation at high temperatures, surfaces still require further cleaning steps, and the careful control of the temperature must be employed to avoid burning of the alloys at high temperatures. Consequently, the thermal-mechanical type deformation processing of magnesium based alloys incurs additional costs resulting in products generated that can cost more than twice as much as standard products.

There is therefore a need to provide magnesium based alloys that are mechanically formable at room temperature without the need for pre-treatment or further processing. Manufacturing of products formed from such alloys would, for example, enable the use of a mould and high output stamping process for conversion of magnesium alloy plates into products, such as car parts or mobile phone cases, without the need for further processing of individual casting, machining or surface polishing.

SUMMARY

In an attempt to develop new and alternative magnesium based alloys having advantageous properties, it has now been identified that the incorporation of tin and lead into magnesium based alloys, unexpectedly, provides advantageous properties, especially good mechanical forming properties.

In a first aspect, the present invention provides a magnesium based alloy capable of being mechanically formed at room temperature comprising tin and lead, and optionally one or more elements selected from the group consisting of zinc, aluminium, zirconium, manganese, strontium, calcium, bismuth, antimony, boron, silicon, copper, silver and rare earth elements, and the remainder being magnesium except for incidental impurities.

In a second aspect, the present invention provides a magnesium based alloy containing tin and lead, the alloy having a ductility such that, in use, the alloy can be mechanically formed at room temperature, without pre- treatment, by rolling, extrusion or pressing to a desired shape, or stamping in a mould to form working parts.

In a third aspect, the present invention provides a magnesium based alloy comprising magnesium, tin and lead, wherein the constituents are in the following amounts, by weight :

1-10% tin,

0.1-5% lead,

0-21% in total of one or more elements selected from the group consisting of zinc, aluminium, zirconium, manganese, strontium, calcium, bismuth, antimony, boron, silicon, copper, silver and rare earth elements, and the remainder being magnesium except for incidental impurities .

In one embodiment, the alloy has a level of elongation at room temperature of at least 10%, more preferably at least 15%, and particularly at least 20%. In another embodiment, the amount of lead in the magnesium based alloy is, by weight, less than about 3%, and more preferably less than about 2%.

In another embodiment, the amount of tin in the magnesium based alloy is, by weight, less than about 5%, and more preferably less than about 3%.

In another embodiment, the one or more elements of the magnesium based alloy comprise, by weight, at least one of: 0.1-8% zinc, 0.1-8% aluminium and 0.1-5% zirconium.

In another preferred embodiment, the one or more elements of the magnesium based alloy comprise, by weight, 0.1-8% (in total) of one or more elements selected from manganese, strontium, calcium, bismuth, antimony, boron, silicon, copper, silver and rare earth elements.

In a forth aspect, the present invention provides a magnesium based alloy consisting of, by weight:

1-10% tin,

0.1-5% lead, and the remainder being magnesium except for incidental impurities .

Where appropriate in respect to a magnesium based alloy system containing only tin, lead and magnesium (except for incidental impurities) , the embodiments of the third aspect also apply to the forth aspect of the present invention.

In a fifth aspect, the present invention provides a magnesium based alloy consisting of, by weight:

1-10% tin,

0.1-5% lead, 0-15% in total of at least one other element selected from the group consisting of aluminium, zirconium, strontium, calcium, bismuth, antimony, boron, silicon, copper, silver and rare earth elements, and the remainder being magnesium except for incidental impurities .

In a preferred embodiment of the above aspects, the amount of the at least one other element in the magnesium based alloy is, by weight, 0.1-5%.

The embodiments of the third aspect of the invention described above also apply, where appropriate, to the fifth aspect of the present invention.

In a sixth aspect, the present invention provides a magnesium based alloy containing tin and lead, the alloy having a microstructure comprising dendritic regions that are at least partially defined by grain boundaries and/or inter- dendritic regions, wherein intermetallic precipitates are located at or near the grain boundaries and/or inter- dendritic regions.

In an embodiment of the sixth aspect, the grain boundaries and the inter-dendritic regions each at least partially occupy different regions within the alloy. In a particular embodiment, the grain boundaries and the inter- dendritic regions each form networks that at least partially occupy different regions within the alloy such that the networks partially overlap. In another embodiment, the dendritic regions are generally of similar size.

In another embodiment of the sixth aspect, the intermetallic precipitates contain a higher proportion of lead and/or tin and a lower proportion of magnesium in comparison to the average composition of the alloy.

The embodiments of the first to fifth aspects of the invention described above also apply, where appropriate, to the sixth aspect of the present invention. In an seventh aspect, the present invention provides a process for preparing the magnesium based alloys according to the above described aspects of the invention, wherein the process comprises preparing an alloy composition from the constituents of the alloy and forming the magnesium based alloy.

Preferably, the alloy composition is prepared as a homogeneous composition of the constituents. Preferably, the magnesium based alloy is formed as a cast alloy.

In an eighth aspect, the present invention provides an article composed wholly or partly of the magnesium based alloy according to the aspects of the invention described above .

In a preferred embodiment, the article or a part of the article are formed through a mechanical forming process without pre-treatment . Preferably, the mechanical forming process is at room temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be further described and illustrated, by way of example only, with reference to the accompanying drawings in which:

Figure 1 (comparative prior art example) shows an optical micrograph of a known magnesium alloy AZ91 containing aluminium and zinc;

Figure 2a is a plan and side view showing the shape and size of as-cast test samples formed from magnesium based alloys according to embodiments of the invention;

Figure 2b is a graph showing a plot of stress versus strain for a magnesium based alloy according to an embodiment of the invention;

Figure 3 is a scanning electron micrograph of a magnesium based alloy containing tin and lead, according to an embodiment of the invention, showing magnesium grains and an intermetallic phase at the grain boundaries;

Figure 4a is a scanning electron micrograph of a magnesium based alloy containing tin, lead and zirconium, according to an embodiment of the invention, showing an intermetallic phase at the grain boundaries;

Figure 4b is a scanning electron micrograph of a magnesium based alloy containing tin, lead and zirconium, according to an embodiment of the invention, showing an intermetallic phase at the grain boundaries,-

Figure 5a is a microanalysis of a plate-like form of the intermetallic phase observed in the magnesium based alloy of Figures 4a and 4b;

Figure 5b is a microanalysis of a spherical-like form of the intermetallic phase observed in the magnesium based alloy of Figures 4a and 4b;

Figure 5c is a microanalysis of a star-like form of the intermetallic phase observed in the magnesium based alloy of Figures 4a and 4b;

Figure 6 is an optical micrograph of a magnesium based alloy containing tin, lead, zinc and aluminium, according to an embodiment of the invention; and

Figure 7 is an optical micrograph of a magnesium based alloy containing tin, lead, zinc and aluminium, according to an embodiment of the invention.

DETAILED DESCRIPTION

The magnesium based alloys according to the present invention are suitable for use with conventional mechanical forming processes because the excellent mechanical forming properties of the alloys allow them to be cold formed into complex shapes, preferably without pre-treatment such as mechanical and thermal processing. The present invention is therefore directed to providing magnesium based alloys having good ductility and deformation properties at room temperature. These properties make the magnesium based alloys of the present invention particularly suitable to be mechanical formed at room temperature, such as cold forming including cold-pressing, cold-rolling, cold-extruding and/or cold-stamping into parts or components with complex shapes. Other advantages provided by at least some of the. preferred embodiments of the present invention include the provision of low cost magnesium alloys with improved tensile strength.

In a preferred embodiment, the constituents of the magnesium based alloys of the invention are selected to enable the alloy (1) to be mechanically formed at room temperature without pre-treatment , and (2) to achieve a good combination of high strength and ductility. For example, having sufficient ductility and deformability at room temperature to be capable of undergoing pressing, rolling, extrusion or stamping, without pre-treatment . It will be appreciated that the constituents of the alloys are also selected to enhance or modify various properties of the alloys, in respect to room and elevated temperatures, including tensile strength and creep resistance.

Cold forming typically occurs at room-temperature (around 20°C) so that a protective atmosphere is not required. The magnesium based alloys of the present invention can also be mechanically formed at elevated temperatures such that even better ductility can be achieved. It will be understood that processing of an alloy at elevated temperatures may require the use of a protective atmosphere to prevent oxidation and deterioration of the alloy. The specific temperature that an alloy may require such a protective atmosphere can differ substantially between alloys; for magnesium alloys this may be generally above about 300⁰C, although can be above about 500⁰C, and more typically is between about 300-600⁰C. It will also be appreciated that the magnesium based alloys of the present invention may be mechanically formed at temperatures lower than room temperature (e.g. ~-20°C). The alloys may also be subjected to thermal and/or mechanical treatments to improve properties of strength and ductility.

The magnesium based alloys, according to at least some of the embodiments of the present invention, are also particularly suitable for casting applications because the excellent mechanical forming properties at room temperature allow use of the alloys from an as-cast condition without the need for further pre-treatment . The alloys obtained exhibit excellent fracture resistance and can be used to replace many parts currently made by steel and aluminium, resulting in significant weight and energy savings.

Tin and lead are also relatively inexpensive metals, and the other optional elements of preferred embodiments are either inexpensive or used in low amounts. Therefore, the present invention generally provides a low cost alloy appropriate for use in large scale production.

The terms "mechanically formed" , "mechanical forming process" or like term, refer to an alloy having the property (e.g. ductility) such that, in use, the alloy can undergo a process of being mechanically shaped, deformed or altered at room temperature, preferably without pre-treatment, by rolling, casting, extrusion or pressing to a desired shape, or stamping in a mould, to allow the formation of components or parts. For example, the process may include cold forming comprising cold-pressing, cold-rolling, cold-extruding and/or cold-stamping.

Composition of Magnesium Based Alloys

It has been found that particular advantageous properties are provided by magnesium based alloys comprising between 1-10 wt% tin and 0.1-5wt% lead. Further optional elements can also be added to advantageously modify the properties of the magnesium based alloys, particularly one or more elements selected from the group consisting of zinc, aluminium, zirconium, manganese, strontium, calcium, bismuth, antimony, boron, silicon, copper, silver and rare earth elements .

It will be understood that the magnesium based alloys will principally contain magnesium, and in addition to other components specifically added to modify the properties of the alloy, may also contain incidental impurities. Ideally, the incidental impurity content is zero but it will be appreciated that this is essentially impossible, particularly at an industrial scale of production. Therefore, preferably the impurity content is less than 0.15%, more preferably less than 0.1%, more preferably less than 0.01%, and particularly less than 0.001%.

Table 1 below lists a range of recognised grades of magnesium that are commercially available. It is preferred that a 1st class grade of magnesium (or equivalent) is used in the alloys, although a lower grade may still be employed.

Table 1: Grades of magnesium

In a particular embodiment, the constituents of the magnesium based alloy of the present invention are selected to enable the alloy to be mechanically formable at room temperature without pre-treatment . For example, having sufficient ductility and deformability at room temperature to be capable of undergoing pressing, rolling, extrusion or stamping, without pre-treatment . Preferably, the alloy has a level of elongation at room temperature of at least 10%, more preferably at least 15%, and particularly at least 20%. It will be appreciated that the constituents of the alloys are also selected to enhance or modify various properties of the alloys, in respect to room and elevated temperatures, including tensile strength, creep resistance and corrosion resistance .

It is believed that the incorporation of lead into the alloys is a major contributor to the excellent ductility and observed. Although the incorporation of lead provides the magnesium based alloys with advantageous properties, it is preferable to limit the amount of lead used because of its inherent toxicity. The amount of lead in the magnesium based alloy can be, by weight, less than, about 3%, or less than about 2%, while still providing advantageous properties.

While not wishing to be bound by any theory, the improved ductility provided by the use of lead is believed to arise because (i) lead has a relatively high solubility within the magnesium matrix (41.7 wt% at the eutectic temperature and ~5 wt% at 200⁰C) providing a good solid- solution strengthening effect to the magnesium alloy; (ii) lead enters a intermetallic precipitate of Mg₂Sn in the magnesium based alloys containing tin, which is believed to change the nature and properties of this intermetallic phase and make the alloy more ductile; (iii) the intermetallic precipitates are small and generally uniformly distributed at inter-dendritic regions in the magnesium matrix. It is believed that the combination of these factors contribute to the superior ductility that the magnesium based alloy system containing tin and lead possesses, in addition to the excellent strength of the alloys.

The addition of tin improves the fluidity properties and castability of magnesium alloys. Tin is also required for forming an intermetallic compound of Mg₂Sn, which provides a much higher melting temperature (770°C) compared to, for example, Mg_I7Al₁₂ phase (437°C) in Mg-Al alloys. Therefore, the incorporation of tin into the alloys strengthens the alloys, especially at elevated temperatures. If the alloy contains more than about 10 wt% Sn, the intermetallic precipitates may become too large, decreasing the desired mechanical properties. The amount of tin in the magnesium based alloy, by weight, can be reduced to less than about 5%, or even less than about 3% while still providing advantageous properties to the alloy.

In one particular embodiment, the constituents of the magnesium based alloys can comprise, by weight, at least one of: 0.1-8% zinc (Zn), 0.1-8% aluminium (Al) and 0.1-5% zirconium (Zr) . Further advantageous ranges of the amounts have been identified including 1-5 wt% zinc, 0.5-3.5 wt% aluminium, and 0.5-2.0% zirconium, and particularly 0.6-1.1% zirconium.

The incorporation into the magnesium based alloys of zinc and aluminium, in addition to tin and lead, has also shown particular advantages. The incorporation of zirconium has also shown particular advantages.

Aluminium generally has the effect of solid solution strengthening, which increases the mechanical strength of the alloys at room temperature. However, the aluminium may also provide benefits to the fluidity properties and castability of the alloys. The addition of zinc can improve the strength of the alloy at room temperature and can also aid in overcoming the harmful effect of iron and nickel impurities in the magnesium alloys that may be present. Zirconium can refine the grain size of magnesium alloys to improve strength and increase ductility.

Further elements that can advantageously be added into the magnesium based alloys can include manganese (Mn) , strontium (Sr) , calcium (Ca) , bismuth (Bi) , antimony (As) , boron (B) , silicon (Si) , copper (Cu) , silver (Ag) and rare earth (RE) elements. These elements may be incorporated into the magnesium based alloys in amounts, by weight, of between about 0.1-5% (in total). The addition of these further elements can improve the strength at room and elevated temperatures, creep resistance, and corrosion resistance of the alloys. The rare earth (RE) elements cover elements from the lanthanide and actinide series and group 3 elements including yttrium and scandium.

Heat treatment, including solution treatment at 400~500°C and ageing at 200~350°C, can also be used to further improve the mechanical properties of the present alloys .

In an embodiment, the present invention provides a magnesium based alloy consisting of, by weight: 1-10% tin, 0.1-5% lead, and the remainder being magnesium except for incidental impurities. Although the incorporation of further elements can provide additional advantages, it has been identified that magnesium based alloy systems containing, except for incidental impurities, tin, lead and magnesium, provide advantageous properties including good corrosion resistance and mechanical forming properties at room temperature .

In another embodiment, the present invention provides a magnesium based alloy consisting of, by weight: 1-10% tin, 0.1-5% lead, and 0-21% in total of at least one other element selected from the group consisting of zinc, aluminium, zirconium, manganese, strontium, calcium, bismuth, antimony, boron, silicon, copper, silver and rare earth elements, and the remainder being magnesium except for incidental impurities. In a particular embodiment, the at least one other element is 0-15% in total, and more particularly 0-10% in total .

In another embodiment, the total amount of tin, lead and the at least one other element selected from the group consisting of zinc, aluminium, zirconium, manganese, strontium, calcium, bismuth, antimony, boron, silicon, copper, silver and rare earth elements, is in the range of 1- 30% in total, particularly 1-20% in total, and more particularly 1-15% in total.

The amount of magnesium in the magnesium based alloys is typically in the range of about 65-99%, particularly 75-97%, and more particularly 80-97%. However, it will be appreciated that the amount of magnesium in the alloys can be varied depending on the type and amount of element being added to the alloy.

In another embodiment, the present invention provides a magnesium based alloy consisting of, by weight: 1-10% tin, 0.1-5% lead, and 0-15% in total of at least one other element selected from the group consisting of aluminium, zirconium, strontium, calcium, bismuth, antimony, boron, silicon, copper, silver and rare earth elements, and the remainder being magnesium except for incidental impurities. Particular advantages are provided by the combination of the above components or constituents.

In one particular embodiment, the present invention provides a magnesium based alloy consisting of, by weight: 2.0% tin, 1.0% lead, 0.5% zirconium, and the remainder being magnesium except for incidental impurities. This alloy system has been shown to provide a very high elongation of 22.5% at room temperature in a cast state, with relatively high strength (UTS) of 159.3 MPa. In another particular embodiment, the present invention provides a magnesium based alloy consisting of, by weight: 3.0% tin, 2.0% lead, 1.0% zirconium, and the remainder being magnesium except for incidental impurities. This alloy system has been shown to provide an elongation of 22.7%, with good strength of 155.5 MPa.

Additions of aluminium and zinc to the magnesium-tin- lead alloy system can substantially improve strength while retaining good ductility.

In another particular embodiment, the present invention provides a magnesium based alloy consisting of, by weight: 5.0% tin, 2.0% lead, 2.0% aluminium, 3.0% zinc, and the remainder being magnesium. This alloy system has been shown to provide a high elongation of 20.2%, with good strength of 213 MPa.

In another particular embodiment, the present invention provides a magnesium based alloy consisting of, by weight, 5.0% tin, 2.0% lead, 4.0% aluminium, 3.0% zinc, and the remainder being magnesium. This alloy system has been shown to provide a high elongation of 20.2%, with high strength of 264.3 MPa.

Addition of alloying elements of tin, lead, and other elements selected from the group consisting of zinc, aluminium, zirconium, manganese, strontium, calcium, bismuth, antimony, boron, silicon, copper, silver and rare earth elements, according to various embodiments of the invention described herein, provide magnesium based alloys that present the desired combination of elongation and strength, in particular, high elongation with reasonable strength, or reasonable elongation with high strength, which can be used in various applications. Microstructure of Magnesium Based Alloys

The microstructure of a known as-cast magnesium alloy of AZ91 is shown in Figure 1 and was taken using a standard optical micrograph. It can be seen that the intermetallic compound of β-Mgi₇Ali₂ (with bright contrast) are large in size (often larger than 50 microns) , and continuously- distributed principally along the grain boundaries. The low plasticity of the cast magnesium alloys typically comes from the brittle nature of the intermetallic compound, and the continuous distribution of the compound weakens the alloy at the grain boundaries. Cracks can easily develop and propagate along the grain boundaries, resulting in brittle fracture in testing and service.

Dendritic microstructures may occur in solidified cast alloys, where typically a 2 -phased region occurs consisting of a dendritic (crystalline) phase defined at least in part by grain boundaries and an inter-dendritic or matrix (liquid) phase .

It has been observed that the magnesium based alloys containing tin and lead according to embodiments of the present invention comprise grain boundaries, wherein intermetallic precipitates are located at or near the grain boundaries. The grain boundaries define primary grains that are typically interconnected dendritic networks. The grain boundaries may provide regions in the alloy where primary grains form cell type structures or enclosed regions defined by the grain boundaries. These structures or regions may be of a similar size, for example equiaxed grains. The microstructure for magnesium alloys is typically at least partially dendritic in nature.

Magnesium based alloys containing tin and lead according to embodiments of the present invention, have also been shown to comprise dendritic regions and inter-dendritic regions, wherein intermetallic precipitates are located at or near the inter-dendritic regions.

In one embodiment, the magnesium based alloys comprise grains or dendritic regions that are at least partially- defined by grain boundaries and/or inter-dendritic regions. The grain boundaries and the inter-dendritic regions may each at least partially occupy different regions within the alloy. The grain boundaries and the inter-dendritic regions may each form networks that at least partially occupy different regions within the alloy such that the networks partially overlap. The intermetallic precipitates are generally located at or near the inter-dendritic regions, as opposed to being located continuously along or about the grain boundaries .

The "intermetallic precipitates" generally refer to solid particles or regions formed by the alloy during solidification, or from a solid-state phase transformation during cooling after solidification. It is believed that the intermetallic precipitates generally contain a higher proportion of lead and/or tin and a lower proportion of magnesium with respect to the average composition of the grains, although the precipitates may contain any one of the elements present in the alloy. The precipitates can be present in a spherical, globule, plate or star type form, and are generally between about 0.1-10 microns in diameter, typically 0.5-5 microns in diameter, and more particularly 0.5-3.0 microns in diameter. Other precipitate forms may include nodular, acicular or rod type forms, or regions with less particularised forms. The precipitates are generally uniformly distributed and spaced apart in the inter-dendritic region. It is- believed that these precipitates attribute to the alloys of the present invention the excellent deformation properties observed at room temperature. The precipitates form on cooling of the liquid alloy, which in one embodiment involves allowing the liquid alloy to cool or solidify at an unforced rate under ambient conditions.

The terms "dendrite", "dendritic", "dendritic region" or like terms, generally refer to grains or tree- like structures of solid (or crystalline) magnesium, which includes two or three-dimensional structures with side arms (secondary dendrite arms) , but may include enclosed regions defined by grain boundaries and/or inter-dendritic regions, such as cell type structures. These regions are typically segmented into areas of generally similar size, such as equiaxed grains. During the solidification of cast, the solid magnesium dendrite (or crystalline) grows from one nucleus along a preferential orientation. The diameter of the dendritic regions can be less than about 200 μm, particularly less than about 100 μm, more particularly less than about 50 μm. In a particular embodiment the grains or dendritic regions are between 1 and 20 μm.

The term "inter-dendritic region" generally refers to the liquid phase between the neighbouring magnesium dendrites. Typically, the alloying elements will accumulate at the inter-dendritic (liquid) region due to the lower soluble contents of alloying elements in magnesium dendrite

(solid) than the overall composition of alloy. This soluble content increases with decrease of temperature following solidus line in phase diagram of the alloy system. The growth of magnesium dendrites or grains continues while the alloying elements at inter-dendritic (liquid) regions accumulate further. The intermetallic precipitates formed, particularly through a eutectic solidification, when accumulations of alloying elements at inter-dendritic

(liquid) regions reach a high level. Meanwhile magnesium dendrites keep growing till the end of solidification. During the solidification, the inter-dendritic (liquid) region changes dynamically (in composition, volume and geography) with the growth of magnesium dendrite. In some case, the neighbouring magnesium dendrites may touch each other and form grain boundaries prior to the formation of intermetallic precipitates. Therefore, the magnesium dendrites are defined at least in part by grain boundaries and/or the inter-dendritic liquid phase or region.

After solidification, the inter-dendritic region, a specific microstructural feature of magnesium alloys, can be revealed through appropriate etching of solid alloy sample. This inter-dendritic region (e.g. dark network in Figure 6), comprising intermetallic precipitates and magnesium with higher solute content than magnesium dendrites, corresponds to inter-dendritic liquid at a certain state, particularly at which the eutectic solidification initiates. A vague boundary between inter- dendritic regions and magnesium dendrites typically exists and reflects the transition from dendritic solidification to eutectic solidification.

Preparation of the Magnesium Based Alloys

The magnesium based alloys according to the above described aspects of the invention can be prepared by combining the constituents of the alloy and then forming the magnesium based alloy. The constituents can be simply mixed together to essentially form a homogenous mixture before the alloy is formed. The magnesium based alloys of the present invention are particularly suitable for preparation as a cast alloy produced from a mould.

EXAMPLES

Preparation of Magnesium Based Alloys

A range of magnesium based alloys according to the present invention were prepared as cast alloys (see Table 2 below) to provide specimens for testing mechanical properties. The alloys were prepared in a low carbon steel crucible with the protection of shield gas of hexafluorosilane (SF₆) and nitrogen (N₂) . The raw materials containing the various components of the alloy were heated in the crucible to form a liquid alloy that was stirred to ensure its homogeneity. The liquid alloy was then cast into an iron mould that had been preheated up to 523 K. The casting temperature was 983 K. The cavity dimensions of the iron mould were 20 mm x 120 mm x 140 mm. The samples that were used for mechanical testing were directly cut from the cast plate by spark cutting. Figure 2a shows the size and dimension of the tensile specimens.

The raw materials used in forming the magnesium based alloy specimens comprised pure magnesium (99.95 wt% Mg), pure tin (99.98 wt% Sn), a master alloy containing 61 wt% Sn and 39 wt% Pb, and commercially pure aluminium and zinc (less than 0.2 wt% impurities).

Composition and Properties of Magnesium Based Alloys

The mechanical properties of the magnesium based alloy specimens that were made were also tested. The specimens, referred to as Alloys 1-12, are identified in Table 2 below along with their composition and measured mechanical properties including ultimate tensile strength (UTS) and elongation percentage.

Table 2 : Composition and mechanical properties of specimen Alloys 1-12

All the specimens showed good elongation, with a few of the specimens, namely Alloys 4, 5 and 7-12, showing excellent elongation. Alloy 1 demonstrates that a lower content of tin and lead (2.0 and 1.0 wt% respectively) still provides significantly increased elongation levels at room temperature with 12.9% elongation observed. Alloy 4 demonstrates that the incorporation of a small amount of zirconium (0.5 wt%) with tin and lead (2.0 and 1.0 wt% respectively) provides an improved elongation of over 22% while still maintaining reasonably high tensile strength (160 MPa) . Alloy 7 demonstrates that the combination of aluminium and zinc (2.0 and 3.0 wt% respectively) with tin and lead (5.0 and 2.0 wt% respectively) also provides an improved elongation of over 20% while providing good tensile strength (213 MPa) . Figure 2b shows the stress-strain curve of Alloy 7 recorded during a tensile test at room temperature (2O⁰C) , demonstrating the excellent plasticity of this alloy. Alloy 11 demonstrates the further improved tensile strength (264.3 MPa) with the increasing addition of aluminium while maintaining the good elongations of 20.2%. Generally, tin is more expensive than aluminium and zinc. Adding less tin will reduce the cost of the alloy. Alloy 12, adding less tin and more aluminium, presents similar strength and elongation as alloy 11, but the cost of alloy 12 should be much less than alloy 11.

Alloys 9 and 12 are based on AZ31 (Mg-3.0%Al-l .0%Zn) and AZ61 (Mg-6.0%Al-l .0%Zn) alloys. With addition of Sn and Pb, the normal wrought AZ31 and AZ61 demonstrate enhanced strength and excellent elongations in the cast state. Therefore, the Sn and Pb alloyed AZ31 and AZ61 alloys can be used in the cast state where combination of good strength and ductility is required.

Microstructure of Magnesium Based Alloys

The microstructure of Alloy 2 (containing 3.0 wt% Sn, 2.0 wt% Pb) is shown in Figure 3, and was obtained from a scanning electron microscope. The microstructure shows the formation of an intermetallic phase formed along or about the grain boundaries of the magnesium grains. The intermetallic phase comprises spaced apart particles or precipitates that are generally uniformly distributed along or about the grain boundaries. The precipitates are typically in the form of a spherical or globule, and may include plate and star type forms. Although not wishing to be bound by any theory, it is believed that these small precipitates, which are typically observed to be generally uniformly distributed in the alloy, provide the alloy with its good deformation and ductility properties, allowing the alloy to yield when stressed and to undergo a relatively large deformation without breaking, resulting in the observed high elongation levels. Figures 4a and 4b are scanning electron micrographs showing the microstructure of Alloy 4 (containing 2.0% tin, 1.0% lead and 0.5% zirconium). The microstructures show the formation of magnesium grains and intermetallic precipitates. The intermetallic precipitates are present in essentially three differently shaped forms: a plate or rod- like form (as shown in Figure 4a), a small spherical, ball or globule-like form (as shown in Figure 4b) and a small star-like form (as shown in Figure 4b) .

Figure 5 provides a microanalysis of the forms of intermetallic precipitates observed in Figures 4a and 4b by using an energy dispersive spectroscopy line scanning technique. The three forms of intermetallic precipitates are observed to have different compositions. The plate-like form shown in Figure 4a contains mainly magnesium and tin, and is likely to be substantially comprised of Mg₂Sn composition. The spherical form shown in Figure 4b contains magnesium, tin and a small amount of lead, and is likely to be substantially comprised of Mg₂SnPb composition. The star form shown in Figure 4c is significantly enriched in lead, and may also contain a small amount of zirconium.

Figure 6 shows optical micrographs of the microstructure of alloy 9. A crisscrossing structure of inter-dendritic regions (dark network indicated by arrows) and generally equiaxed grains are observed. Normally, the grain boundaries formed at the last stage of solidification are located in the inter-dendritic regions. This unusual crisscrossing microstructure, which is observed in this embodiment of the invention, has not been reported before. The formation of this specific microstructure facilitates in preventing the distribution of intermetallic precipitates continuously along the grain boundaries which is favoured for the crack propagation and brittle nature of cast alloys, as described in Figure 1. The crisscrossing structure refers to the intercrossing of grain boundaries and inter-dendritic regions (dark network indicated in Figure 6) , or in other words the grain boundaries and inter-dendritic regions each form networks that at least partially occupy different regions within the alloy such that the networks partially overlap. For a dendritic microstructure, dendrites (dendritic regions or grains) are separated by grain boundaries and/or intermetallic precipitates located between two neighbouring grains . The grain boundaries form when dendrites grow larger and touch with neighbouring ones during dendritic growth, or form at the eutectic solidification stage. Generally, grain boundaries are located within the inter- dendritic regions. However, grain boundaries crossing the magnesium dendrites outside the inter-dendritic regions are observed in Figure 6. It is believed that this feature contributes to the ductility improvement of the magnesium alloys. This microstructure can be seen when both grain boundaries and inter-dendritic regions are revealed in one micro- image, such as in Figure 4a and Figure 6.

Figure 7 shows the microstructure of alloy 9. The generally uniform and discrete distribution of intermetallic precipitates contributes to the high ductility of the alloy. A total of 10 wt% alloying elements comprising tin, lead, aluminium, and zinc are added in the alloy. The amount of intermetallic precipitates is much less than AZ91 cast alloys (with total 10 wt% of aluminium and zinc) . The lower amount of intermetallic compounds indicates the higher amount of solid solution of alloying elements in the matrix, which results in the solid solution strengthening of the alloy. The crisscrossing structure shown in Figure 6 is also observed in this alloy. As the intermetallic precipitates generally formed at inter-dendritic region, the crisscrossing grain boundaries with dendritic regions prevent the distribution of intermetallic compounds continuously along the grain boundary. The uniform distribution of intermetallic precipitates is another microstructural factor contributing to the high ductility of the alloy 9, which also presents high tensile strength due to the solid solution of alloying elements and finely distributed intermetallic compounds .

The alloys of the present invention, which have good mechanical forming ability, are expected to have very wide range of applications, particularly in aerospace or airplane parts, car parts such as wheels, car bodies, engine parts and gear boxes, hand tools and electronic devices such as mobile phone and notebook computer parts .

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive .

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Claims

CLAIMS :

1. A magnesium based alloy capable of being mechanically- formed at room temperature comprising tin and lead, and optionally one or more elements selected from the group consisting of zinc, aluminium, zirconium, manganese, strontium, calcium, bismuth, antimony, boron, silicon, copper, silver and rare earth elements, and the remainder being magnesium except for incidental impurities.

2. A magnesium based alloy containing tin and lead, the alloy having a ductility such that, in use, the alloy can be mechanically formed at room temperature, without pre- treatment, by rolling, extrusion or pressing to a desired shape, or stamping in a mould to form working parts.

3. A magnesium based alloy capable of being mechanically formed at room temperature comprising magnesium, tin and lead, wherein the constituents are in the following amounts, by weight:

1-10% tin,

0.1-5% lead,

4. The magnesium based alloy of claim 3, wherein the constituents are in the following amounts, by weight:

1-10% tin,

0.1-5% lead, at least one of 0.1-8% zinc, 0.1-8% aluminium and 0.1-5% zirconium, and the remainder being magnesium except for incidental impurities .

5. The magnesium based alloy of claim 3, consisting of, by weight :

1-10% tin, 0.1-5% lead, and the remainder being magnesium except for incidental impurities .

6. The magnesium based alloy of claim 3, consisting of, by weight :

1-10% tin,

0.1-5% lead,

0-15% in total of at least one other element selected from the group consisting of aluminium, zirconium, strontium, calcium, bismuth, antimony, boron, silicon, copper, silver and rare earth elements, and the remainder being magnesium except for incidental impurities .

7. A magnesium based alloy containing tin and lead, the alloy having a microstructure comprising dendritic regions that are at least partially defined by grain boundaries and/or inter-dendritic regions, wherein intermetallic precipitates are located at or near the grain boundaries and/or inter-dendritic regions.

8. The magnesium based alloy of claim 7, wherein the grain boundaries and the inter-dendritic regions each form networks that at least partially occupy different regions within the alloy such that the networks partially overlap.

9. The magnesium based alloy of claim 7 or claim 8, wherein the precipitates contain a higher proportion of lead and/or tin and a lower proportion of magnesium in comparison to the average composition of the alloy.

10. The magnesium based alloy of any one of claims 7 to 9, wherein the constituents are in the following amounts, by weight :

1-10% tin,

0.1-5% lead,

11. The magnesium based alloy of claim 10, wherein the constituents are in the following amounts, by weight:

1-10% tin,

12. The magnesium based alloy of claim 10, consisting of, by weight :

13. The magnesium based alloy of claim 10, consisting of, by weight :

1-10% tin,

0.1-5% lead,

0-10% in total of at least one other element selected from the group consisting of aluminium, zirconium, strontium, calcium, bismuth, antimony, boron, silicon, copper, silver and rare earth elements, and the remainder being magnesium except for incidental impurities .

14. A process for preparing the magnesium based alloy of any one of claims 1 to 13, wherein the process comprises preparing an alloy composition from the constituents of the alloy and forming the magnesium based alloy.

15. An article composed wholly or partly of the magnesium based alloy according to any one of claims 1 to 13.