Properties of Nonferrous Metals

Quenching and tempering to produce a martensitic structure is the most effective means of strengthening steels. In the common nonferrous metals, martensite may not occur; and where it does occur, the effect is not as large as in steels. Hence, the other methods of strengthening, which are generally less effective, must be used. The higher strength nonferrous metals often employ precipitation hardening.

For example, consider the strength levels achievable in aluminum alloys, as illustrated by Fig. 3.10. Annealed pure aluminum is very weak and can be strengthened only by cold work. Adding magnesium provides solid-solution strengthening, and the resulting alloy can be cold worked.

Further strengthening is possible by precipitation hardening, which is achieved by various combinations of alloying elements and aging treatments. However, the highest strength available is only about 25% of that for the highest strength steel.

Aluminum is nevertheless widely used, as in aerospace applications, where its light weight and corrosion resistance are major advantages that offset the disadvantage of lower strength than some steels.

We will now discuss the properties of nonferrous metals that are commonly used in structural applications.

Properties of Nonferrous Metals

Aluminum Alloys

For aluminum alloys produced in wrought form, as by rolling or extruding, the naming system involves a four-digit number. The first digit specifies the major alloying elements as listed in Table 1. Subsequent digits are then assigned to indicate specific alloys, with some examples being given in Table 2. The UNS numbers for wrought alloys are similar, except that A9 precedes the four-digit number. Following the four-digit number, a processing code is used, as in 2024-T4, as detailed in Table 1.

For codes involving cold work, HXX, the first number indicates whether only cold work is used (H1X), or whether cold work is followed by partial annealing (H2X) or by a stabilizing heat treatment (H3X). The latter is a low-temperature heat treatment that prevents subsequent gradual changes in the properties. The second digit indicates the degree of cold work, HX8 for the maximum effect of cold work on strength, and HX2, HX4, and HX6 for one-fourth, one-half, and three-fourths as much effect, respectively.

Processing codes of the form TX all involve a solution heat treatment at a high temperature to create a solid solution of alloying elements. This may or may not be followed by cold work, but the material is always subsequently aged, during which precipitation hardening occurs. Natural aging occurs at room temperature, whereas artificial aging involves a second stage of heat treatment. Additional digits following HXX or TX describe additional variations in processing, such as T651 for a T6 treatment in which the material is also stretched up to 3% in length to relieve residual (locked-in) stresses.

The alloy content determines the response to processing. Alloys in the 1XXX, 3XXX, and 5XXX series, and most of those in the 4XXX series, do not respond to precipitation-hardening heat treatment. These alloys achieve some of their strength from solid-solution effects, and all can be strengthened beyond the annealed condition by cold work.

The alloys capable of the highest strengths are those that do respond to precipitation hardening, namely the 2XXX, 6XXX, and 7XXX series, with the exact response to this processing being affected by the alloy content. For example, 2024 can be precipitation hardened by natural aging, but 7075 and similar alloys require artificial aging.

Aluminum alloys produced in cast form have a similar, but separate, naming system. A fourdigit number with a decimal point is used, such as 356.0-T6. Corresponding UNS numbers have A0 preceding the four-digit number and no decimal point, such as A03560.

Titanium Alloys

The density of titanium is considerably greater than that of aluminum, but still only about 60% of that of steel. In addition, the melting temperature is somewhat greater than for steel and far greater than for aluminum. In aerospace applications, the strength-to-weight ratio is important, and in this respect the highest strength titanium alloys are comparable to the highest strength steels.

These characteristics and good corrosion resistance have led to an increase in the application of titanium alloys since commercial development of the material began in the 1940s

Because only about 30 different titanium alloys are in common use, it is sufficient to identify these by simply giving the weight percentages of alloying elements, such as Ti-6Al-4V or Ti-10V2Fe-3Al. Three categories exist: the alpha and near alpha alloys, the beta alloys, and the alpha–beta alloys.

Although the alpha (HCP) crystal structure is stable at room temperature in pure titanium, certain combinations of alloying elements, such as chromium along with vanadium, cause the beta (BCC) structure to be stable, or they result in a mixed structure. Small percentages of molybdenum or nickel improve corrosion resistance; and aluminum, tin, and zirconium improve creep resistance of the alpha phase.

Alpha alloys are strengthened mainly by solid-solution effects and do not respond to heat treatment. The other alloys can be strengthened by heat treatment. As in steels, a martensitic transformation occurs upon quenching, but the effect is less. Precipitation hardening and the effects of complex multiple phases are the principal means of strengthening alpha–beta and beta alloys.

Other Nonferrous Metals

A wide range of copper alloys are employed in diverse applications as a result of their electrical conductivity, corrosion resistance, and attractiveness. Copper is easily alloyed with various other metals, and copper alloys are generally easy to deform or to cast into useful shapes. Strengths are typically lower than for the metals already discussed, but still sufficiently high that copper alloys are often useful as engineering metals.

Percentages of alloying elements range from relatively small to quite substantial, such as 35% zinc in common yellow brass. Copper with approximately 10% tin is called bronze, although this term is also used to describe various alloys with aluminum, silicon, zinc, and other elements. Copper alloys with zinc, aluminum, or nickel are strengthened by solid-solution effects. Beryllium additions permit precipitation hardening and produce the highest strength copper alloys.

Cold work is also frequently used for strengthening, often in combination with the other methods. A variety of common names are in use for various copper alloys, such as beryllium copper, naval brass, and aluminum bronze. The UNS numbering system with a prefix letter C is used for copper alloys.

Magnesium has a melting temperature near that of aluminum, but a density only 65% as great, making it only 22% as dense as steel and the lightest engineering metal. This silvery-white metal is most commonly produced in cast form, but is also extruded, forged, and rolled. Alloying elements do not generally exceed 10% total for all additions, the most common being aluminum, manganese, zinc, and zirconium.

Strengthening methods are roughly similar to those for aluminum alloys. The highest strengths are about 60% as large, resulting in comparable strength-to-weight ratios. The naming system in common use is generally similar to that for aluminum alloys, but differs as to the details. A combination of letters and numbers that identifies the specific alloy is followed by a processing designation, such as AZ91C-T6.

Superalloys are special heat-resisting alloys that are used primarily above 550◦C. The major constituent is either nickel or cobalt, or a combination of iron and nickel, and percentages of alloying elements are often quite large. For example, the Ni-base alloy Udimet 500 contains 48% Ni, 19% Cr, and 19% Co, and the Co-base alloy Haynes 188 has 37% Co, 22% Cr, 22% Ni, and 14% W, with both also containing small percentages of other elements.

Nonstandard combinations of trade names and letters and numerals are commonly used to identify the relatively small number of superalloys that are in common use. Some examples, in addition to the two just described, are Waspaloy, MARM302, A286, and Inconel 718.

Although nickel and cobalt have melting temperatures just below that of iron, superalloys have superior resistance to corrosion, oxidation, and creep compared with steels. Many have substantial strengths even above 750^◦C, which is beyond the useful range for low-alloy and stainless steels.

This accounts for their use in high-temperature applications, despite the high cost due to the relative scarcity of nickel, chromium, and cobalt. Superalloys are often produced in wrought form, and Ni-base and Co-base alloys are also often cast. Strengthening is primarily by solid-solution effects and by various heat treatments, resulting in precipitation of intermetallic compounds or metal carbides.

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