Magnesium Alloys Properties on Elevated Temperatures

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Magnesium Alloys Performance at Elevated Temperatures: Properties and Applications

The mechanical properties of magnesium-based materials—including strength, hardness, and modulus of elasticity—decrease as temperature rises. Simultaneously, elongation increases with temperature until approaching the melting point, where it rapidly diminishes to nearly zero. Recent metallurgical advancements have yielded specialized magnesium alloys specifically engineered for moderately elevated temperature applications. These innovative compositions enable the practical utilization of magnesium alloys under load at substantially higher temperatures than previously possible.

When considering magnesium alloys for elevated temperature applications, engineers must carefully evaluate tensile and other mechanical properties at the intended service temperatures. Due to their relatively low melting points (below approximately 1200°F or 650°C), commercial magnesium alloys are necessarily limited to applications at only moderately elevated temperatures. Similar to aluminum alloys, the safe operating temperatures for magnesium alloys remain significantly below those of steels.

Depending on their specific composition, magnesium alloys begin to melt at temperatures ranging from approximately 685°F to 1200°F (360°C to 650°C). Traditional magnesium alloys begin to soften and weaken appreciably when exposed to temperatures as low as 200°F (95°C). However, recently developed specialized compositions maintain yield and tensile strength remarkably well at temperatures up to 400°F (205°C) or higher.

Creep Resistance in Magnesium Alloys

Understanding Creep Behavior

When designing with magnesium alloys for elevated temperature applications, creep and stress-rupture data become critical performance indicators. Under conditions where creep may occur—static loading at elevated temperatures—engineers must compare yield strength and tensile strength against the stress for specific deformation or stress-to-rupture under creep-loading conditions. Creep data provides valuable comparative insights that inform design decisions for high-temperature applications.

Standard commercial magnesium alloys containing aluminum-zinc (with manganese) remain relatively stable up to approximately 300°F (150°C) and may serve effectively in applications below this threshold. However, solution heat-treated castings and hard-rolled sheet products in conventional alloys become unstable above 300°F (150°C), making them unsuitable for elevated temperature applications.

Advanced Alloy Compositions for Enhanced Creep Resistance

Traditional magnesium-based materials used for castings or wrought products demonstrate comparatively poor strength and creep resistance at elevated temperatures. Research has shown that incorporating rare-earth metals, particularly in the form of mischmetal (MM), produces magnesium alloys that retain significant strength at elevated temperatures and exhibit remarkably high resistance to creep across wide temperature ranges.

Testing reveals that various zinc-bearing magnesium alloys containing small amounts of zirconium or manganese demonstrate good creep resistance at elevated temperatures, particularly in sand casting compositions. Notable among these are ZK61 and ZM60 alloys, which exhibit higher creep resistance than AZ92 and AZ63 alloys, though still lower than rare-earth-bearing (RE) alloys. These zinc-bearing compositions also maintain favorable tensile properties at both room and elevated temperatures.

Research investigating which elements in mischmetal contribute most significantly to developing high strength and creep resistance has yielded important insights. Considerably higher properties at elevated temperatures can be achieved with didymium (neodidymium plus praseodidymium) and cerium-free MM compared to standard MM. The ranking of magnesium alloys in order of decreasing tensile and compressive properties at both room and elevated temperatures is:

• Magnesium-didymium
• Magnesium-cerium-free MM
• Magnesium-MM
• Magnesium-cerium
• Magnesium-lanthanum

Interestingly, magnesium-didymium alloys do not maintain their superior creep resistance over other alloys at 500°F and 600°F (260°C and 320°C). At these higher temperatures, cerium-free MM alloys demonstrate the highest creep resistance across the entire MM composition range. High lanthanum alloys exhibit exceptionally good creep resistance at 600°F (320°C).

Recent Developments in High-Temperature Alloys

Recent metallurgical investigations have focused on developing magnesium casting alloys with optimum tensile properties at elevated temperatures and minimum creep. Among 350 alloys tested, superior properties were achieved with a composition of 6% mischmetal, 0.8% manganese, 0.2% nickel, 0.02% tungsten, with magnesium comprising the remainder.

Additional research has revealed that thorium additions to magnesium yield alloys with the highest creep resistance (up to 600°F/320°C) of any standard magnesium alloy to date. Furthermore, zirconium additions to thorium-bearing alloys refine the grain structure without compromising creep properties at elevated temperatures.

For wrought products requiring optimal mechanical properties at elevated temperatures, extensive testing of approximately 195 alloys identified the following composition as providing the best combination of performance characteristics: 2% mischmetal, 1-1.5% manganese, 0.2% nickel, with magnesium comprising the remainder.

Temperature Range Considerations for Alloy Selection

When selecting magnesium alloys for applications requiring high creep resistance, both the operating temperature and stress levels must be carefully considered. For cast magnesium alloys, service conditions can be divided into three temperature ranges:

1. Up to 250°F (120°C)
2. From 250°F to 400°F (120°C to 205°C), approximately
3. Above 400°F (205°C) or perhaps 450°F (235°C)

This classification enables the use of certain alloys in the lower temperature range, even if their creep resistance diminishes at higher temperatures. Alloys such as ZK61 and ZM60 fall into this category, offering the advantage of good room-temperature mechanical properties and satisfactory castability. For the highest temperature range, alloys with superior creep resistance may be essential, regardless of their room-temperature mechanical properties or foundry characteristics. These temperature ranges apply to both cast and wrought compositions.

The optimal structural or metallographic condition for maximum creep resistance at elevated temperatures is achieved through appropriate heat treatment, which varies according to alloy composition and material form (cast or wrought). The most favorable conditions for resisting creep are T2, T6, and T7, achieved respectively by stabilization of as-fabricated (F) products, solution heat treatment and aging, and solution heat treatment followed by stabilization.

Effects of Temperature Exposure on Magnesium Alloys

Properties After Heat Exposure

Understanding how elevated temperature exposure affects the mechanical properties of magnesium alloys is crucial for applications involving thermal cycling or high-temperature service. Available data reveal significant insights into how heating magnesium alloys to elevated temperatures impacts their subsequent room-temperature performance characteristics.

Short-term heating at temperatures up to 650°F (345°C)—whether for sheet forming, component straightening, or during normal service conditions—produces measurable changes in the room-temperature properties of various magnesium alloys. These changes vary based on alloy composition and initial temper condition.

In general, magnesium-alloy castings in the as-cast (F) or solution heat-treated (T4) condition experience increased yield strength but decreased elongation when heated for sufficient periods at temperatures up to 650°F (345°C). Conversely, castings in the solution heat-treated and aged (T6) condition typically demonstrate reduced yield strength following similar heat exposure.

For wrought products, the effects of heat exposure follow different patterns. Nearly all annealed or hot-finished magnesium-base products maintain their mechanical properties when subjected to short-term heating at temperatures up to 650°F (345°C). However, when alloy sheet in the hard-rolled temper (H) is heated to 500-600°F (260-345°C), it essentially undergoes complete annealing, resulting in properties similar to the soft temper (O).

An important exception is M1A-H sheet, which demonstrates remarkable thermal stability, softening only slightly when heated to 600°F (345°C) for less than a few minutes or to 500°F (260°C) for less than 2 hours. When hard-rolled alloy sheet is heated at moderate temperatures of 350-400°F (175-205°C)—as might occur during forming operations—the resulting properties fall between the O and H tempers. Again, M1A-H sheet proves exceptional, showing minimal property changes when heated at 350°F (175°C).

Mechanical Properties at Low Temperatures

The behavior of magnesium alloys at subzero temperatures presents another important performance dimension. As temperature decreases below zero, magnesium-base alloys generally exhibit increasing yield strength, tensile strength, and hardness, while elongation and impact resistance decrease. The endurance limit also rises appreciably at lower temperatures. These changes affect both cast and wrought alloys, though the magnitude varies with alloy composition, temper, condition, and the specific subzero temperature.

For cast alloys, the T4 temper demonstrates superior low-temperature performance compared to F or T6 tempers at -108°F (-78°C). T4 temper alloys show the greatest increase in strength and hardness while experiencing the least reduction in elongation and impact resistance. Additionally, alloys in the T4 temper more completely recover their original properties upon returning to room temperature.

Wrought alloys in the F temper undergo more significant changes in tensile properties than cast compositions when cooled to -108°F (-78°C). These materials exhibit remarkable increases in yield and tensile strength when cooled to the extreme temperature of -320°F (-195°C).

The elastic properties of magnesium alloys also improve at low temperatures. For alloy AZ31 in 3/4-inch round bars (extruded and cold drawn), the modulus of elasticity increases progressively as temperature decreases:

• At -13°F (-25°C): 6.36 million psi (43,850 MPa)
• At -108°F (-78°C): 6.83 million psi (47,090 MPa)
• At -320°F (-195°C): 7.30 million psi (50,330 MPa)

This represents a substantial 14.7% increase in modulus from room temperature to -320°F (-195°C).

Fatigue performance of magnesium alloys generally improves at reduced temperatures. The fatigue strength increases as temperature decreases, though the magnitude varies with alloy composition, condition, cycle count, and temperature. In one test of forged AZ61 alloy, the fatigue strength (at 300 million cycles on a rotating-beam machine) increased from 15,000 psi (105 MPa) at room temperature to 16,000 psi (110 MPa) at -104°F (-75°C).

In contrast to these improvements, notch-impact resistance of both cast and wrought magnesium-base alloys demonstrates a declining trend as temperature decreases. The total change varies significantly depending on numerous metallurgical and testing factors.

Conclusion

The temperature-dependent properties of magnesium alloys reveal both challenges and opportunities for engineering applications. Recent developments in alloy composition—particularly those incorporating rare-earth metals, thorium, and zirconium—have significantly expanded the temperature range in which magnesium alloys can effectively operate. These advancements enable the utilization of magnesium's lightweight advantages in more demanding thermal environments than previously possible.

When selecting magnesium alloys for specific applications, engineers must carefully consider the complete temperature profile the component will experience—including elevated temperatures during service, potential thermal processing, and exposure to low temperatures. The optimal alloy selection balances composition, temper, and processing to achieve the required performance characteristics across the entire operating temperature range.

As metallurgical research continues, further improvements in high-temperature magnesium alloys can be anticipated, potentially expanding their application into even more demanding thermal environments while maintaining their fundamental advantages in lightweight structural design.