Mechanical Properties at Elevated Temperatures
In projecting applications of magnesium alloys at elevated temperatures,
the tensile and other mechanical properties at the particular service
temperatures must be considered. On account of their relatively low
melting points, below about 1200oF (650oC), the commercial alloys are
necessarily confined to use at only moderately elevated temperatures.
As in the case of aluminum alloys, the safe operating temperatures for
magnesium alloys are far below those of steels.
Depending upon the composition, magnesium alloys begin to melt at a
temperature in the range of about 685o to 1200oF
(360 to 650oC). The
common alloys begin to soften and weaken appreciably on exposure to
temperatures as low as 200oF (95oC). However, some special compositions
have been recently developed which maintain yield and tensile strength
quite well at temperatures up to 400oF (205oC) or higher.
The strength, hardness, and modulus of elasticity of magnesium-base
materials decrease with increasing temperature. Also, the elongation
increases with rising temperature up to just below the melting point
where it drops to nearly zero. As indicated, some magnesium alloys
have been developed recently for use at moderately elevated temperatures.
In several of these the principal alloying ingredient is mischmetal;
other additions may include manganese or zirconium. These compositions
make possible the utilization of magnesium alloys under load at
substantially higher temperatures than formerly.
Creep and stress-rupture data are important in considering
magnesium alloys for various high-temperature applications.
Test results giving these data for a number of alloys are available.
Under conditions where creep may arise (static loading at elevated
temperature), it is useful in design to compare with the yield strength
and tensile strength, for the temperatures of interest, the stress for a
certain deformation or the stress-to-rupture under creep-loading conditions.
Creep data may be used in a comparative and qualitative way.
The usual commercial magnesium alloys of the aluminum-zinc (manganese)
type are relatively stable up to about 300oF (150oC) and may be used for
some applications below that temperature. Solution heat-treated castings
and hard-rolled sheet in the usual alloys are unstable above 300oF (150oC)
and are not suitable for use at elevated temperatures.
As indicated, the ordinary magnesium-base materials used for castings or
for wrought products have comparatively poor strength and poor resistance
to creep at elevated temperatures. Investigations have shown that the
addition of rare-earth metals, in the form of mischmetal, to magnesium
will yield alloys that retain much of their strength at elevated
temperatures and exhibit relatively high resistance to creep over a
wide range of temperature. For brevity, mischmetal is hereafter referred
to as MM and the associated rare-earth metals as RE.
Tests have shown that various zinc-bearing magnesium alloys containing
also, for example, small amounts of zirconium or manganese exhibit good
resistance to creep at elevated temperatures. This refers to compositions
for sand casting. Among these alloys ZK61 and ZM60 may be mentioned. They
have higher creep resistance than AZ92 and AZ63 alloys but lower than the
RE-bearing alloys. Also, these zinc-bearing alloys have good tensile
properties at both room and elevated temperatures.
Investigation has been made to determine which of the elements in MM
contributes the greatest effect in developing high strength and high
resistance to creep, at elevated temperatures in the case of cast magnesium
alloys. This showed that considerably higher properties at elevated
temperatures can be developed by didymium (neodidymium plus praseodidymium)
and by cerium-free MM than by MM. The rating in order of decreasing tensile
and compressive properties at room and elevated temperatures is as follows:
(1) Magnesium-didymium; (2) Magnesium-cerium-free MM; (3) Magnesium- MM ;
(4) Magnesium-cerium; and (5) Magnesium-lanthanum.
Magnesium-didymium alloys do not maintain their superiority in creep
resistance over the other alloys at 500 and 600oF (260 and 320oC).
The highest creep resistance over the entire MM composition range at
these temperatures is exhibited by the cerium-free MM alloys. High
lanthanum alloys have exceptionally good creep resistance at 600oF
In a recent investigation, the attempt was made to develop a magnesium
casting alloy, based on additions of MM plus other metals, with optimum
tensile properties at elevated temperatures and minimum creep.
Of 350 alloys tested, the best properties were obtained with the
following composition: 6 per cent MM, 0.8 manganese, 0.2 nickel, 0.02 per
cent tungsten, and remainder magnesium.
Other recent investigations have shown that additions of thorium to
magnesium yield alloys with the highest creep resistance up to
600oF (320oC) of any magnesium alloy standard to date. Also,
additions of zirconium to thorium-bearing alloys refine the grain
without impairing the creep properties at elevated temperatures.
An investigation was recently carried out to develop a magnesium alloy,
for wrought products, having optimum mechanical properties at elevated
temperatures. Of about 195 alloys tested, the following composition was
found to give the best combination of values: 2 percent MM, 1-1.5 manganese,
0.2 per cent nickel, and remainder magnesium.
The selection of an alloy for applications requiring high creep resistance
must take into consideration the temperature to be encountered as well as
the level of stress. In the use of cast magnesium alloys it has been
suggested that service conditions be divided into three ranges of
temperature. These are as follows: (1) Up to 250oF (120oC);
(2) from 250o to 400oF (120oC- 205oC),
approximately; and (3) above 400oF (205oC)
or perhaps 450oF (235oC).
This division would allow the use of certain
alloys in the lower range, although their creep resistance is relatively
poor at higher temperatures. At the same time, advantage would be taken
of their relatively good mechanical properties at room temperature and
their satisfactory castability. Alloys ZK61 and ZM60 fall into this
category. For the highest temperature range, alloys with the best creep
resistance may be requisite irrespective of their mechanical properties
at the ordinary temperature or their foundry behavior. The ranges of
temperature given also apply in the case of wrought compositions.
The structural or metallographic condition having the maximum resistance
to creep at elevated temperatures is produced by means of suitable heat
treatment. This varies with the alloy composition and the form of the
material (whether cast or wrought). The most suitable conditions for
resisting creep are T2, T6, and T7. These are effected, respectively,
by stabilization of as fabricated (F) products, solution heat treatment
and aging, and solution heat treatment followed by stabilization.
Properties After Heating
Some data are available which show the effect of heating magnesium
alloys to elevated temperatures on their mechanical properties at
Short-time heating at temperatures up to 650oF (345oC),
as may be required in forming sheet or for straightening certain products,
or as may occur during the service life of a part or structure,
effects changes in the room-temperature properties of various
In general, magnesium-alloy castings used in the as-cast (F) or solution
heat-treated (T4) condition gain in yield strength and lose in elongation
on heating for a sufficient period of time in the range up to 650oF (345oC).
Castings in the solution heat-treated and aged (T6) condition lose in
yield strength on such heating.
In nearly all cases, the short-time heating of annealed or hot finished
magnesium-base products at temperatures up to 650oF (345oC) is without
effect on the mechanical properties. The heating of alloy sheet in the
hard-rolled temper (H) to 500-600oF (260-345oC) practically results
in complete annealing and yields the properties of the soft temper (O).
However, this does not apply to M1A-H sheet which softens only slightly
when heated to 600oF (345oC) for not more than a few minutes or to
for not more than 2 hours. Short-time heating of hard-rolled alloy sheet
at 350oF to 400oF (175oC to 205oC), as may be carried out to effect moderate forming
operations, produces properties intermediate between the O and H tempers.
Again, M1A-H sheet is an exception. It is scarcely affected by heating
at 350oF (175oC).
Mechanical properties at low temperatures
In general, the yield strength, tensile strength, and hardness of
magnesium-base alloys increase more or less substantially with decrease
in temperature below zero while the elongation and impact resistance
decrease. Also, the endurance limit is raised appreciably as the
temperature is lowered. These changes apply to both cast and wrought
alloys. The individual effects vary depending upon the alloy composition,
temper, condition (whether cast or wrought), and the subzero temperature.
As concerns the cast alloys, it appears that the T4 temper is better than
the F or T6 temper in behavior at the low temperature of -108oF (78oC).
Thus, it exhibits the greatest increase in strength and hardness and the
least loss of elongation and impact resistance on cooling to the
temperature stated. In addition, the alloys in the T4 temper more
nearly assume their original properties on again attaining room
The wrought alloys in the F temper undergo greater overall changes in
tensile properties than do the cast compositions on cooling
to -108oF (-78oC). Rather remarkable increases in yield strength and
tensile strength are exhibited by the wrought materials on cooling
to -320oF (-195oC).
The modulus of elasticity of magnesium alloys generally increases at
low temperatures. For alloy AZ31 in the form of 3/4-in. round bars,
extruded and cold drawn, the following values have been reported:
At -13oF (-25oC), 6.36 million psi (43850 MPa);
at -108oF (-78oC), 6.83 million psi (47090 MPa);
and at -320oF (-195oC), 7.30 million psi
(50330 MPa). The increase from room temperature to -320oF
(-195oC) was 14.7 per cent.
Some data are available which show the effect of low temperatures on the
fatigue strength of magnesium alloys. In general, the fatigue strength
increases with decreasing temperature. The amount of increase is quite
variable depending upon the composition of the alloy, condition
(whether cast or wrought), number of fatigue cycles, temperature,
and other factors. In one test on forged AZ61 alloy the fatigue
strength (300 million cycles, rotating-beam machine) at -104oF (-75oC)
was 16,000 psi (110 MPa) as against 15,000 psi (105 MPa) at room
The notch-impact resistance of magnesium-base alloys, both cast and wrought,
shows a downward trend as the temperature is lowered. The total change
is markedly variable depending on sundry factors.