Strengthening of Metals
The strength of metals and alloys can be modified through various combinations of cold working, alloying, and heat treating. As discussed in the previous section, the ability of a crystalline material to plastically deform largely depends on the ability for dislocation to move within a material. Therefore, impeding the movement of dislocations will result in the strengthening of the material. For example, a microstructure with finer grains typically results in both higher strength and superior toughness compared to the same alloy with physically larger grains. In case of grain size, there may also be tradeoff between strength and creep characteristics. Other strengthening mechanisms are achieved at the expense of lower ductility and toughness. There are many strengthening mechanisms, which include:
- Solid Solution Strengthening (alloying)
- Work Hardening (Cold Working)
- Precipitation Hardening
- Grain Refinement
- Transformation Hardening
Solid Solition Strengthening – Alloying
Atoms of different elements dissolved in the matrix phase can lead to its strengthening by solid solution strengthening. High-purity metals are almost always softer and weaker than alloys composed of the same base metal. Increasing the concentration of the impurity results in an attendant increase in tensile and yield strengths. The solute may incorporate into the solvent crystal lattice substitutionally, by replacing a solvent particle in the lattice, or interstitially, by fitting into the space between solvent particles. This impose lattice strains on surrounding atoms resulting in a lattice strain field. Even small amounts of solute can affect the electrical and physical properties of the solvent. Steel, probably the most common structural metal, is a good example of an alloy. It is an alloy of iron and carbon, with other elements to give it certain desirable properties. Adding a small amount of non-metallic carbon to iron trades its great ductility for the greater strength. Adding a small amount of non-metallic carbon to iron trades its great ductility for the greater strength. For non-ferrous alloys, manganese and magnesium are examples of elements added to aluminum for the purpose of solid solution strengthening.
Pure iron is too soft to be used for the purpose of structure, but the addition of small quantities of other elements (carbon, manganese or silicon for instance) greatly increases its mechanical strength. Alloys are usually stronger than pure metals, although they generally offer reduced electrical and thermal conductivity. Strength is the most important criterion by which many structural materials are judged. Therefore, alloys are used for engineering construction. The synergistic effect of alloying elements and heat treatment produces a tremendous variety of microstructures and properties.
- Carbon. Carbon is a non-metallic element, which is an important alloying element in all ferrous metal based materials. Carbon is always present in metallic alloys, i.e., in all grades of stainless steel and heat resistant alloys. Carbon is a very strong austenitizer and increases the strength of steel. In fact, it is the principal hardening element and is essential to the formation of cementite, Fe3C, pearlite, spheroidite, and iron-carbon martensite. Adding a small amount of non-metallic carbon to iron trades its great ductility for the greater strength. If it is combined with chromium as a separate constituent (chromium carbide), it may have a detrimental effect on corrosion resistance by removing some of the chromium from solid solution in the alloy and, as a consequence, reducing the amount of chromium available to ensure corrosion resistance.
- Chromium. Chromium increases hardness, strength, and corrosion resistance. The strengthening effect of forming stable metal carbides at the grain boundaries and the strong increase in corrosion resistance made chromium an important alloying material for steel. The resistance of these metallic alloys to the chemical effects of corrosive agents is based on passivation. For passivation to occur and remain stable, the Fe-Cr alloy must have a minimum chromium content of about 11% by weight, above which passivity can occur and below which it is impossible. Chromium can be used as a hardening element and is frequently used with a toughening element such as nickel to produce superior mechanical properties. At higher temperatures, chromium contributes increased strength. The high-speed tool steels contain between 3 and 5% chromium. It is ordinarily used for applications of this nature in conjunction with molybdenum.
- Nickel. Nickel is one of most common alloying elements. About 65% of nickel production is used in stainless steels. Because nickel does not form any carbide compounds in steel, it remains in solution in the ferrite, thus strengthening and toughening the ferrite phase. Nickel steels are easily heat treated because nickel lowers the critical cooling rate. Nickel based alloys (e.g., Fe-Cr-Ni(Mo) alloys) alloys exhibit excellent ductility and toughness, even at high strength levels and these properties are retained up to low temperatures. Nickel also reduces thermal expansion for better dimensional stability. Nickel is the base elements for superalloys, which are are a group of nickel, iron–nickel and cobalt alloys used in jet engines. These metals have excellent resistance to thermal creep deformation and retain their stiffness, strength, toughness and dimensional stability at temperatures much higher than the other aerospace structural materials.
- Molybdenum. Found in small quantities in stainless steels, molybdenum increases hardenability and strength, particularly at high temperatures. The high melting point of molybdenum makes it important for giving strength to steel and other metallic alloys at high temperatures. Molybdenum is unique in the extent to which it increases the high-temperature tensile and creep strengths of steel. It retards the transformation of austenite to pearlite far more than it does the transformation of austenite to bainite; thus, bainite may be produced by continuous cooling of molybdenum-containing steels.
- Vanadium. Vanadium is generally added to steel to inhibit grain growth during heat treatment. In controlling grain growth, it improves both the strength and toughness of hardened and tempered steels. The size of the grain determines the properties of the metal. For example, smaller grain size increases tensile strength and tends to increase ductility. A larger grain size is preferred for improved high-temperature creep properties. Vanadium is added to promote abrasion resistance and to produce hard and stable carbides which being only partly soluble, release little carbon into the matrix.
- Tungsten. Produces stable carbides and refines grain size so as to increase hardness, particularly at high temperatures. Tungsten is used extensively in high-speed tool steels and has been proposed as a substitute for molybdenum in reduced-activation ferritic steels for nuclear applications. The addition of about 10% of tungsten and molybdenum in total maximises efficiently the hardness and toughness of high speed steels and maintains those properties at the high temperatures generated when cutting metals. Tungsten and molybdenum are interchangeable at an atomic level and both promote resistance to tempering which gives improved tool cutting performance at higher temperatures.