Aluminum precipitation-strengthened

Almost every commercial precipitation strengthened aluminum ahoy contains magnesium as an ahoying element. The Al—Mg system (Fig. 13) has a eutectic at 35% magnesium [7439-95-4] Mg, and 451°C. Maximum soHd solubiHty is 14.9% Mg, and solubiHty decreases to about 0.8% Mg at room temperature. Despite this decreased solubiHty, precipitation strengthening by the mestable P -phase precursor to the equhibrium P phase Al3Mg,... 

Relationship of Composition and Heat Treatment to Environment-Sensitive Cracking of Aluminum Alloys. Those aluminum alloys strengthened by cold working only, particularly the 1000-series alloys, do not develop susceptibility to SCC. The so-called high-strength alloys are strengthened by thermal/mechanical treatments, which result in solid-state precipitation of one or more intermetallic phases that restrict dislocation motion and, hence, increase strength. Their susceptibility to SCC varies extensively with alloy composition and the thermal/mechanical treatment. While susceptibility tends to increase with... 

Aluminum is a low melting point metal, therefore microstructural stability (and consequently creep) is an important issue even for near ambient temperature. Many common structural aluminum alloys are precipitation strengthened at heat treatment temperatures on the order of 423 K, implying that the service temperature must be considerably lower. There are a few aluminum alloys designed for high-temperature applications however, creep makes them unacceptable as materials of construction for contaimnent of pressurized gas at elevated temperature. On the other hand, aluminum alloys are commonly employed at cryogenic temperatures. 

Al—Li. Ahoys containing about two to three percent lithium [7439-93-2] Li, (Fig. 15) received much attention in the 1980s because of their low density and high elastic modulus. Each weight percent of lithium in aluminum ahoys decreases density by about three percent and increases elastic modulus by about six percent. The system is characteri2ed by a eutectic reaction at 8.1% Li at 579°C. The maximum soHd solubiHty is 4.7% Li. The strengthening precipitate in binary Al—Li ahoys is metastable Al Li [12359-85-2] having the cubic LI2 crystal stmcture, and the equhibrium precipitate is complex cubic... 

Quaternary and Higher Alloys. Further additions to commercial aluminum alloys usually are made either to modify the metastable strengthening precipitates or to produce dispersoids. 

The Zn—A1 system permits manipulation of the mechanical properties by suitable heat treatment. The aluminum-rich alpha phase is especially suitable for solution hardening since it can be supersaturated by as much as 30 wt % zinc. Furthermore, both alpha and beta phases can be strengthened by precipitation because of decreasing solute solubiUty with decreasing temperature. 

Non-Heat-Treatable Alloys. Non-heat-treatable aluminum alloys are defined primarily by what they are not. They are not strengthened by second-phase particles and may be better described as non-precipitation-hardening alloys. The non-heat-treatable alloy classes are the Ixxx, 3xxx, and 5xxx alloys. 

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