Mechanical Properties of Magnesium and Magnesium Alloys

Sommario:

As with metal manufactures in general, the tensile and other properties of the magnesium materials depend upon the composition, condition (whether cast or wrought), details of fabrication, heat treatment, and other factors. For the same composition, some mechanical properties vary considerably with the type of product.

Magnesium-base products are available in a wide range of mechanical properties. As with metal manufactures in general, the tensile and other properties of the magnesium materials depend upon the composition, condition (whether cast or wrought), details of fabrication, heat treatment, and other factors. For the same composition, some mechanical properties vary considerably with the type of product. Accordingly, it is important to define the kind of material as well as the alloy composition. Also, the direction in which the test specimen is taken in relation to the direction of fabrication and the thickness of the product are factors which influence strength and other properties.

In engineering design, the more important mechanical properties which ordinarily may come under consideration may include the following:

  • tensile yield and ultimate strength,
  • elongation,
  • shear strength,
  • compressive yield strength,
  • hardness,
  • impact resistance, and
  • endurance limit.

This refers to values at the ordinary temperature. For some applications, properties at subzero or elevated temperatures may be important.

The mechanical properties of magnesium alloys are determined in accordance with ASTM procedures where applicable. Properties of specification and commercial grades will meet the minimum specified values established by the principal producers. Minimum properties represent values of specification and commercial grade alloys when such values have been established. In cases where they have not been established, the minima are given only as suggested values for design purposes.

As is the case with other nonferrous metals, magnesium exhibits no definite yield point. In accordance with ASTM procedure, the yield strength of magnesium alloys is defined as the stress at which the stress-strain curve deviates 0.2 percent from the modulus line (the so-called 0.2 percent offset point).

The compressive yield strength of magnesium alloys is defined as the stress necessary to produce 0.2 per cent permanent set. In the case of cast magnesium alloys the compressive yield strength is substantially the same as the tensile yield strength. However, for most wrought magnesium alloys the compressive yield strength is 60 to 70 percent of the tensile yield strength. Compressive properties are determined on specimens having a slenderness ratio (1/g) of 12.

Casting Alloys. Tensile and other mechanical properties of the cast alloys are determined on separately poured test bars conforming to standard ASTM procedure. Specifications may require that specimens cut from sections of industrial castings have tensile ultimate and yield strengths that are at least 75 percent of those determined on separately cast bars and elongation at least 25 percent. For small grain size in both bars and castings, the strength values of sections up to 1/2 in. thick may be about 90 percent of the minimum test-bar values.

Wrought Alloys. As maybe noted, some alloys are produced in several wrought forms while others find application for one kind of manufacture only, for example, extrusions or forgings.

The mechanical properties of wrought magnesium alloys are determined on specimens cut from the actual manufactures - extrusions, forgings, or rolled products. Specimens are usually cut in a direction parallel to the direction of working or predominant flow of metal. They may also be cut transversely or at 45° to show the directional properties.

The properties given for sheet generally apply to material in the range of thickness from 0.020 to 0.250 in. Flat rolled material which is 0.251 in. and upwards in thickness is defined as plate. The mechanical properties of strip cut from sheet are the same as those of the sheet, and the properties of coiled strip approximate those of sheet. Properties for extrusions are specified separately for solid shapes (including bars and rods), for hollow shapes, and for tubing.

In the case of forgings, sections worked from extruded stock 3 in. in diameter or less usually exhibit tensile and compressive yield strengths greater than the minimum specification values. The mechanical properties of forgings made from thicker stock which is subjected to substantial movement of metal in the processing may also exceed the specified minima, but ordinarily by lesser margins than do small forgings. However, thick, heavy forgings made from cast billets or extruded stock 6 to 8 in. thick may have properties considerably below the average because of the difficulty of effecting enough mechanical work during either extrusion or forging. Actually, the properties obtainable in a particular forging may be varied appreciably by variations in the production practice.

Directional Properties. In the case of wrought magnesium alloys, the mechanical properties may vary appreciably depending upon the degree of working and the direction of metal flow. The lowest tensile properties usually occur in the "across-grain" direction (that is, perpendicular to the direction of working) and in sections that have been worked the least. Thus, relatively poor properties may appear in thick sections and in those subjected to the least amount of flow in the processing. It is to be noted, however, that the longitudinal properties of sheet are normally a little lower than the transverse properties.

For specification purposes, the tensile properties of sheet and plate in magnesium-base materials are ordinarily determined on specimens cut parallel to the direction of rolling, that is, longitudinal specimens. As indicated just above, properties in this direction (with but few exceptions) are slightly lower than those of transverse specimens. Likewise, in the case of extrusions, the tensile properties are normally determined on longitudinal specimens. Generally, they have properties higher than those of transverse specimens. In forgings, the tensile properties are determined on specimens taken in the direction of maximum metal flow. These properties are usually higher than properties determined in other directions of the forging.

In wrought magnesium alloys, it has been found that high yield strength in tension is generally accompanied by low yield strength in compression, and vice versa. For example, an extruded shape has a high tensile yield strength but a low compressive yield strength in longitudinal specimens; in transverse specimens this relation may often be the opposite. A similar condition has been found to exist in forgings.

Yield Strength Ratios. The ratio of compressive yield strength to tensile yield strength in magnesium materials may be influenced markedly by various factors including alloy composition, heat treatment, condition (whether cast or wrought), and directional effects. It has been pointed out that for cast magnesium alloys the compressive yield strength is about the same as the tensile yield strength. For wrought alloys, however, the former may be considerably lower.

The yield strengths of wrought magnesium alloys normally vary as a function of the direction of metal flow. In longitudinal specimens the yield strength in compression is generally about 60 to 70 percent of the yield strength in tension.

Modulus of Elasticity. For magnesium and its commercial alloys the modulus of elasticity in tension and compression is about 45 GPa. The modulus of elasticity in shear, or modulus of rigidity, is about 16 GPa. Poisson`s ratio is 0.35. In the case of Ml type magnesium alloys the modulus of elasticity has been reported as somewhat lower than 45 GPa, the value of 38 GPa having been given. The modulus of elasticity decreases with rising temperature.

Cyclic cold working to stresses exceeding the yield strength in both tension and compression reduces the modulus of elasticity of magnesium alloys. Values approximating 27.5 GPa have been observed. Such cyclic cold working, probably in the form of roll straightening, is believed to be responsible for the lower modulus values of magnesium sheet and extruded shapes which have occasionally been reported.

Endurance Limit. Considerable data are available on the fatigue strength of magnesium alloys. As is the case with metals in general, these materials are sensitive to notches and other stress-raisers which have the effect of reducing substantially their endurance limits. Unsoundness (micro shrinkage), which greatly lowers the tensile strength of cast magnesium alloys, reduces their endurance limit to similar extent.

Impact Resistance. The standard Charpy and Izod impact tests as carried out on notched bars serve to measure the relative notch sensitivity of magnesium alloys under impact loading. Similar impact tests on unnotched bars give much higher values and may be adapted to indicate resilience or toughness. Resilience is the capacity of a material to absorb shock within the elastic range. It is inversely proportional to the modulus of elasticity. Since magnesium-base materials have relatively low modulus of elasticity, they have high unit resilience. Toughness is determined by the strength and ductility. It is measured by the area under the stress-strain curve. An approximation for toughness value may be made by multiplying the tensile strength by the elongation.

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