Magnesium alloy


Magnesium alloys are mixtures of magnesium with other metals, often aluminium, zinc, manganese, silicon, copper, rare earths and zirconium. Magnesium alloys have a hexagonal lattice structure, which affects the fundamental properties of these alloys. Plastic deformation of the hexagonal lattice is more complicated than in cubic latticed metals like aluminium, copper and steel; therefore, magnesium alloys are typically used as cast alloys, but research of wrought alloys has been more extensive since 2003. Cast magnesium alloys are used for many components of modern cars and have been used in some high-performance vehicles; die-cast magnesium is also used for camera bodies and components in lenses.
The commercially dominant magnesium alloys contain aluminium. Another important alloy contains Mg, Al, and Zn. Some are hardenable by heat treatment.
All the alloys may be used for more than one product form, but alloys AZ63 and AZ92 are most used for sand castings, AZ91 for die castings, and AZ92 generally employed for permanent mold castings. For forgings, AZ61 is most used, and here alloy M1 is employed where low strength is required and AZ80 for highest strength. For extrusions, a wide range of shapes, bars, and tubes are made from M1 alloy where low strength suffices or where welding to M1 castings is planned. Alloys AZ31, AZ61 and AZ80 are employed for extrusions in the order named, where increase in strength justifies their increased relative costs.
Magnox, whose name is an abbreviation for "magnesium non-oxidizing", is 99% magnesium and 1% aluminium, and is used in the cladding of fuel rods in magnox nuclear power reactors.
Magnesium alloys are referred to by short codes which denote approximate chemical compositions by weight. For example, AS41 has 4% aluminium and 1% silicon; AZ81 is 7.5% aluminium and 0.7% zinc. If aluminium is present, a manganese component is almost always also present at about 0.2% by weight which serves to improve grain structure; if aluminium and manganese are absent, zirconium is usually present at about 0.8% for this same purpose. Magnesium is a flammable material and must be handled carefully.

Designation

By ASTM specification B951-11, magnesium alloys are represented by two letters followed by two, three, or four numbers and a serial letter. Letters tell main alloying elements as per the table at the right. Numbers indicate respective integer compositions of main alloying elements, from most to least abundant. The serial letter is chosen arbitrarily in order to disambiguate between two alloys with the same designation. Marking AZ91A for example conveys magnesium alloy with roughly 9 weight percent aluminium and 1 weight percent zinc, and the final A means it was the first alloy with this composition at the time of registration. Exact composition should be confirmed from reference standards.
Aluminium, zinc, zirconium, and thorium promote precipitation hardening: manganese improves corrosion resistance; and tin improves castability. Aluminium is the most common alloying element. The numerals correspond to the rounded-off percentage of the two main alloy elements, proceeding alphabetically as compositions become standard. Temper nonstandard designation is usually much the same as in the case of aluminium: using –F, -O, -H1, -T4, -T5, and –T6.
Sand permanent-mold, and die casting are all well developed for magnesium alloys, die casting being the most popular. Although magnesium is about twice as expensive as aluminium, its hot-chamber die-casting process is easier, more economical, and 40% to 50% faster than cold-chamber process required for aluminium. Forming behavior is poor at room temperature, but most conventional processes can be performed when the material is heated to temperatures of. As these temperatures are easily attained and generally do not require a protective atmosphere, many formed and drawn magnesium products are manufactured. The machinability of magnesium alloys is the best of any commercial metal, and in many applications, the savings in machining costs more than compensate for the increased cost of the material. It is necessary, however, to keep the tools sharp and to provide ample space for the chips. Magnesium alloys can be spot-welded nearly as easily as aluminium, but scratch brushing or chemical cleaning is necessary before the weld is formed. Fusion welding is carried out most easily by processes using an inert shielding atmosphere of argon or helium gas. Considerable misinformation exists regarding the fire hazard in processing magnesium alloys. It is true that magnesium alloys are highly combustible when in a finely divided form, such as powder or fine chips, and this hazard should never be ignored. Above, a non-combustible, oxygen-free atmosphere is required to suppress burning. Casting operations often require additional precautions because of the reactivity of magnesium with sand and water in sheet, bar, extruded or cast form; however, magnesium alloys present no real fire hazard.
Thorium-containing alloys are not usually used, since a thorium content of more than 2% requires that a component be handled as a radioactive material, although thoriated magnesium known as Mag-Thor was used in military and aerospace applications in the 1950s. Similarly, uranium-containing alloys have declined in use to the point where the ASTM B275 "G" designation is no longer in the standard.
Magnesium alloys are used for both cast and forged components, with the aluminium-containing alloys usually used for casting and the zirconium-containing ones for forgings; the zirconium-based alloys can be used at higher temperatures and are popular in aerospace.
Magnesium+yttrium+rare-earth+zirconium alloys such as WE54 and WE43 can operate without creep at up to 300C and are reasonably corrosion-resistant.
Trade names have sometimes been associated with magnesium alloys. Examples are:
Magnesium casting proof stress is typically 75–200 MPa, tensile strength 135–285 MPa and elongation 2–10%. Typical density is 1.8 g/cm3 and Young's modulus is 42 GPa. Most common cast alloys are:

Wrought alloys

Magnesium wrought alloy proof stress is typically 160-240 MPa, tensile strength is 180-440 MPa and elongation is 7-40%. The most common wrought alloys are:
Wrought magnesium alloys have a special feature. Their compressive proof strength is smaller than tensile proof strength. After forming, wrought magnesium alloys have a stringy texture in the deformation direction, which increases the tensile proof strength. In compression, the proof strength is smaller because of crystal twinning, which happens more easily in compression than in tension in magnesium alloys because of the hexagonal lattice structure.
Extrusions of rapidly solidified powders reach tensile strengths of up to 740 MPa due to their amorphous character, which is twice as strong as the strongest traditional magnesium alloys and comparable to the strongest aluminium alloys.

Compositions table

Characteristics

Magnesium's particular merits are similar to those of aluminium alloys: low specific gravity with satisfactory strength. Magnesium provides advantages over aluminium, having an even lower density than aluminium. The mechanical properties of magnesium alloys tend to be below those of the strongest of the aluminium alloys.
The strength-to-weight ratio of the precipitation-hardened magnesium alloys is comparable with that of the strong alloys of aluminium or with the alloy steels. Magnesium alloys, however, have a lower density, stand greater column loading per unit weight and have a higher specific modulus. They are also used when great strength is not necessary, but where a thick, light form is desired, or when higher stiffness is needed. Examples are complicated castings, such as housings or cases for aircraft, and parts for rapidly rotating or reciprocating machines. Such applications can induce cyclic crystal twinning and detwinning that lowers yield strength under loading direction change.
The strength of magnesium alloys is reduced at elevated temperatures; temperatures as low as 93 °C produce considerable reduction in the yield strength. Improving the high-temperature properties of magnesium alloys is an active research area with promising results.
Magnesium alloys show strong anisotropy and poor formability at room temperature stemming from their hexagonal close-packed crystal structure, limiting practical processing modes. At room temperature, basal plane slip of dislocation and mechanical crystal twinning are the only operating deformation mechanisms; the presence of twinning additionally requires specific loading conditions to be favorable. For these reasons processing of magnesium alloys must be done at high temperatures to avoid brittle fracture.
The high-temperature properties of magnesium alloys are relevant for automotive and aerospace applications, where slowing creep plays an important role in material lifetime. Magnesium alloys generally have poor creep properties; this shortcoming is attributed to the solute additions rather than the magnesium matrix since pure magnesium shows similar creep life as pure aluminium, but magnesium alloys show decreased creep life compared to aluminium alloys. Creep in magnesium alloys occurs mainly by dislocation slip, activated cross slip, and grain boundary sliding. Addition of small amounts of zinc in Mg-RE alloys has been shown to increase creep life by 600% by stabilizing precipitates on both basal and prismatic planes through localized bond stiffening. These developments have allowed for magnesium alloys to be used in automotive and aerospace applications at relatively high temperatures. Microstructural changes at high temperatures are also influenced by Dynamic recrystallization in fine-grained magnesium alloys.
Individual contributions of gadolinium and yttrium to age hardening and high temperature strength of magnesium alloys containing both elements are investigated using alloys containing different Gd:Y mole ratios of 1:0, 1:1, 1:3, and 0:1 with a constant Y+Gd content of 2.75 mol%. All investigated alloys exhibit remarkable age hardening by precipitation of β phase with DO19 crystal structure and β phase with BCO crystal structure, even at aging temperatures higher than 200 °C. Both precipitates are observed in peak-aged specimens. The precipitates contributing to age hardening are fine and their amount increases as Gd content increases, and this result in increased peak hardness, tensile strength and 0.2% proof stress but decreased elongation. On the other hand, higher Y content increases the elongation of the alloys but results in decreased strength.
Despite its reactivity, magnesium and its alloys have good resistance to corrosion in air at STP. The rate of corrosion is slow compared with rusting of mild steel in the same atmosphere. Immersion in salt water is problematic, but a great improvement in resistance to salt-water corrosion has been achieved, especially for wrought materials, by reducing some impurities particularly nickel and copper to very low proportions or using appropriate coatings.