Magnesium Alloy - Characteristics

Characteristics

The strength –to-weight ratio of the precipitation-hardened magnesium alloys is comparable with that of the strong alloys of aluminum or with the alloy steels. Magnesium alloys, however, have a lower density and stand greater column loading per unit weight. They are also used when great strength is not necessary, but where a thick, light form is desired. Examples are complicated castings, such as housings or cases for aircraft, and parts for rapidly rotating or reciprocating machines. The strength of magnesium alloys is reduced at somewhat elevated temperatures; temperatures as low as 200F produce considerable reduction in the yield strength.

Despite the active nature of the metal, magnesium and its alloys have good resistance to corrosion. The rate of corrosion is slow compared with rusting of mild steel in the same atmosphere. Immersion in salt water is very dangerous, 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.

Corrosion troubles may be expected even with protective treatment in poorly designed assemblies where moist air is trapped or where rain is allowed to collect. Where such designs are avoided, unpainted magnesium alloy parts that are oily or greasy operate indefinitely with no sign of corrosion. Magnesium alloy parts are usually painted, however, except where the conditions of exposure are mild. To assure proper adherence of the paint, the part is cleaned by grinding, buffing, or blasting, then given an acid dichromate dip.

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 0:1with a constant Y+Gd content of 2.75 mol%. All investigated alloys exhibit remarkable age hardening by precipitation of .BETA. phase with DO19 crystal structure and .BETA. phase with BCO crystal structure, even at aging temperatures higher than 200o Celsius. 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.

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