The high temperature and/or high strength-to-weight requirements of aerospace structures, advanced propulsion systems, high-speed aircraft and deep submergence vessels have stimulated the development of certain nonferrous metals that can be used in these applications.
From the point of view of catastrophic fracture, the most important and interesting of the nonferrous metals are the BCC refractories, high strength aluminum alloys and the HCP metals, magnesium, beryllium, and titanium. Although low strength aluminum, copper- and nickel-base alloys are used extensively as structural materials, these FCC metals are quite ductile and tough in the absence of extreme environments and need not to be discussed here.
In addition to the failures that arise because of extreme environmental conditions, e.g., high operating temperatures and corrosive environments, or fatigue, there are also brittle failures that occur in some of these metals because of their inherent low ductility and toughness. These failures can cause problems in fabrication and in certain service applications e.g., at the low temperatures that exist in outer space.
From the point of view of catastrophic fracture, the most important and interesting of the nonferrous metals are the BCC refractories, high strength aluminum alloys and the HCP metals, magnesium, beryllium, and titanium. Although low strength aluminum, copper- and nickel-base alloys are used extensively as structural materials, these FCC metals are quite ductile and tough in the absence of extreme environments and need not to be discussed here.
Cleavage is the only mode of unstable fracture in the BCC refractory and low strength HCP metals, so that the effect of composition and microstructure on toughness and ductility can be described by variations in impact and tensile-transition temperatures; low-energy tear is the primary mode of unstable fracture in the high strength alloys so that their variations in toughness appear as variations in G1c, DWTT energy, Cv(max), and tensile ductility.
As in the case of steels, alloy content and processing conditions affect the toughness of nonferrous metals by affecting their microstructures, which, in turn, determine their toughness.
In polycrystalline pure magnesium and in commercial magnesium alloys a significant number of microcracks are initiated at a strain which is about equal to one-half of the total strain to fracture. A significant amount of the total plastic deformation therefore occurs in the microcracks grow and link up in the final stages of fracture. The ductility of these alloys increases abruptly with increasing testing temperature at temperatures slightly above ambient. This ductility rise results from a microstructural instability, that is, recovery and recrystallization during straining, of the metals at the testing temperature.
At room temperature, failure of polycrystalline high-purity magnesium occurs on various crystallographic planes of high index as well as by an intergranular mechanism. Fracture at low temperatures is also intergranular along with complex transcrystalline modes.
The addition of lithium to magnesium alloy has an interesting effect on low-temperature fracture behavior. The stress for prismatic slip is reduced relative to that for basal slip, and thereby the low-temperature ductility increased. The greater facility for slip is also reflected in a decrease in strain-hardening behavior as barriers to plastic deformation can be more easily circumvented. In magnesium alloys of high lithium content (14.5%), fracture even at 78°K occurs by necking, and there is no leveling of the fracture stress as in the case of pure magnesium.
Certain alloying additions stabilize the α-phase. Among these are aluminum and the interstitial elements carbon, hydrogen, nitrogen, and oxygen. Most alloying elements, such as chromium, columbium, copper, iron, manganese, molybdenum, tantalum, and vanadium stabilize the β-phase to the extent that a mixed α-β-phase or an entirely β-phase alloy can persist down to room temperature.
Some elements, notably tin and zirconium, behave as neutral solutes in titanium and have little effect on the transformation temperature, acting as strengtheners of the α-phase. The single-phase α-alloys are not heat-treatable, but both the α-β-alloys and the β-alloys can be strengthened by heat treatment. The yield strengths of these alloys range from 30,000 psi for commercially pure titanium up to 250,000 psi for certain α-β-alloys.
One group of elements that affects the low-temperature fracture behavior, especially of notched specimens, are the interstitials. Because of the deleterious effect of interstitials, extra-low interstitials grades (ELI) are available for use where structural reliability at low temperatures is of concern.
Hydrogen, which can embrittle titanium, is soluble to a negligible extent in beryllium and has no effect on the transition from ductile to brittle behavior. Oxygen is able to embrittle beryllium, but there is no quantitative measure of the effect.
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