Deformation of Cubic Structures

Properties of Metals DOE-HDBK-1017/1-93 STRAIN When metal experiences strain, its volume remains constant. Therefore, if volume remains constant as the dimension changes on one axis, then the dimensions of at least one other axis must change also. If one dimension increases, another must decrease. There are a few exceptions. For example, strain hardening involves the absorption of strain energy in the material structure, which results in an increase in one dimension without an offsetting decrease in other dimensions. This causes the density of the material to decrease and the volume to increase. If a tensile load is applied to a material, the material will elongate on the axis of the load (perpendicular to the tensile stress plane), as illustrated in Figure 2(a). Conversely, if the load is compressive, the axial dimension will decrease, as illustrated in Figure 2(b). If volume is constant, a corresponding lateral contraction or expansion must occur. This lateral change will bear a fixed relationship to the axial strain. The relationship, or ratio, of lateral to axial strain is called Poisson's ratio after the name of its discoverer. It is usually symbolized by n. Whether or not a material can deform Figure 2 Change of Shape of Cylinder Under Stress plastically at low applied stresses depends on its lattice structure. It is easier for planes of atoms to slide by each other if those planes are closely packed. Therefore lattice structures with closely packed planes allow more plastic deformation than those that are not closely packed. Also, cubic lattice structures allow slippage to occur more easily than non-cubic lattices. This is because of their symmetry which provides closely packed planes in several directions. Most metals are made of the body-centered cubic (BCC), face-centered cubic (FCC), or hexagonal close-packed (HCP) crystals, discussed in more detail in the Module 1, Structure of Metals. A face-centered cubic crystal structure will deform more readily under load before breaking than a body-centered cubic structure. The BCC lattice, although cubic, is not closely packed and forms strong metals. a-iron and tungsten have the BCC form. The FCC lattice is both cubic and closely packed and forms more ductile materials. g-iron, silver, gold, and lead are FCC structured. Finally, HCP lattices are closely packed, but not cubic. HCP metals like cobalt and zinc are not as ductile as the FCC metals. Rev. 0 Page 9 MS-02