Fracture of Brittle Nonferrous Metals

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.

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.

The Fracture of Magnesium

The effect of temperature and grain size on the fracture strengths of high-purity magnesium and of a Mg-2wt % Al alloy was examined. These data reveal that each material exhibits two characteristically different types of fracture behavior. Over the higher range of temperatures the fracture stress decreases in a manner that parallels the flow-stress temperature relationship, and the fracture is of a ductile type. Over the lower range of temperatures the true fracture stress is independent of temperature but highly dependent upon grain size. As the grain size is decreased the fracture stress increases, and the temperature of transition to the low temperature fracture behavior range is reduced. The addition of aluminum to magnesium has the important effect of increasing the fracture stress at a given grain size as well as reducing the transition temperature.

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.

The Fracture of Titanium

Titanium alloys can be grouped into three categories according to the predominant phase or phases in their microstructure. In pure titanium the α-phase, an HCP structure, is stable to 1625°F (885°C), and the BCC β-phase is stable from 1625°F to the melting point of 3130°F (1740°C).

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.

The Fracture of Beryllium

Beryllium can be melted and cast but, as the castings are brittle and difficult to machine, practically all the beryllium used in space, nuclear, and other applications is made from beryllium powders and undergoes powder metallurgy processing. The powder is usually fabricated by the mechanical commutation of vacuum-cast ingots of magnesium-reduced beryllium pebbles or electrolytically produced beryllium flakes. Each of the powder particles is enveloped by BeO, which tends to hinder grain growth during processing.

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.

Summary

1. Commercially available group Va refractory metals (V, Nb, Ta) are considerably more ductile and have considerably lower transition temperatures than commercially available group VIa refractory metals (Cr, Mo, W). This results from the fact that the solubility of impurities in the Va metals is much higher than in the VIa metals. Consequently, at the impurity contents that normally exist in commercial materials, impurity atoms and/or second phase particles are segregated at grain boundaries in VIa metals. These lower the plastic work expended in microcrack formation γ_m, and hence lower the toughness.
2. Oxygen is a particularly strong embrittling agent for molybdenum and tungsten; nitrogen and sulfur are particularly harmful in wrought chromium. Hydrogen is the most effective embrittling agent for Va metals.
3. After impurity content, grain size is the most important variable affecting the ductility and toughness of Va and VIa refractory metals. In general, ductility and toughness are increased by grain refinement.
4. Cold or warm working and alloying provide the most efficient means of increasing the ductility and toughness of refractory metals. The beneficial effects of working are thought to result from the production of a fibrous microstructure, which permits splitting and crack tip blunting when toughness is evaluated in the working direction. Alloying can improve ductility and toughness in VIa metals by lowering the tendency for impurity segregation at grain boundaries or by getting out the impurities.
5. The fracture behavior of the HCP metals beryllium, magnesium, and titanium, together with that of some of their alloys, has been reviewed. Brittle fracture at low temperatures is common to these HCP metals and, in both titanium and beryllium, interstitial elements such as oxygen influence this behavior.
6. In each of these metals the decrease in the number of modes with decreasing temperature is a common feature involved in low ductility fractures.