Hexagonal Close-packed (HCP)
CN = 12. As shown in Fig. B.5, the stacking sequence is ABCABC. The atoms in the HCP structure are also packed along close-packed planes. Atoms in the HCP planes (called the basal planes) have the same arrangement as those in the FCC close-packed planes. However, in the HCP structure, these planes repeat
every other layer to give a stacking sequence of ABAB. Two lattice parameters, c and a, shown in Fig. B.4, are also needed to describe the HCP unit cell. When c/a = 1.63, the maximum packing efficiency just described is obtained; however, few real HCP structures have this ideal ratio and maximum packing efficiency. The coordination for the BCC structure is 8, which is less than that of the FCC and HCP structures. Since the packing is less efficient in the BCC structure, the closest-packed planes are less densely packed.
The atomic structures of real metals are not perfect but contain defects. One of the most important defects is the line or edge dislocation. Without the presence of dislocations, plastic deformation of metals would be much more restricted. Dislocations create an atomic disruption in the lattice, which makes slippage of the planes that have dislocations much easier, as shown in Fig. B.6. The movement is much like advancing a carpet along a floor by using a wrinkle that is easily propagated down its length. The stress required to cause plastic deformation is orders of magnitude less when dislocations are present than in a dislocation-free perfect crystalline structure.
Dislocations5 do not move with the same degree of ease on all crystallo-graphic planes of atoms and in all crystallographic directions. Ordinarily there
Fig. B-6. Dislocation Movement
Fig. B-6. Dislocation Movement are preferred planes, and in these planes, there are specific directions along which dislocation motion can occur. These planes are called slip planes and the direction of movement is known as the slip direction. The combination of a slip plane and a slip direction forms a slip system. For a particular crystal structure, the slip plane is that plane having the most dense atomic packing, i.e. it has the greatest planar density. The slip direction corresponds to the direction, in this plane, that is most closely packed with atoms, i.e. has the highest linear density.
Since plastic deformation takes place by slip, or sliding, on the close-packed planes, the greater the number of slip systems available, the greater the capacity for plastic deformation. FCC metals have a large number of slip systems (12) and are therefore capable of moderate to extensive plastic deformation. Although BCC systems often have up to 12 slip systems, some of them like steel exhibit a ductile-to-brittle transition as the temperature is lowered due to the strong temperature sensitivity of their yield strength, which causes them to fracture prior to undergoing significant plastic deformation. In general, the number of slip systems available for HCP metals is less than that for either the FCC or BCC metals, and their plastic deformation is much more restricted. The HCP structure normally has only 3-6 slip systems, one fourth to one half the available slip systems in FCC and BCC structures. Therefore, metals with the HCP structure have only moderate to rather poor ductility at room temperature. The HCP metals often require heating to elevated temperatures, where slip becomes much easier, to facilitate forming operations, which is usually required for alloys of magnesium, beryllium, and titanium.
While slip is required to facilitate plastic deformation, and therefore allows a metal to be formed into useful shapes, to strengthen a metal requires increasing the number of barriers to slip and reducing its ability to plastically deform. Increasing the interference to slip, and increasing the strength, can be accomplished by methods such as strain hardening, grain refinement, solid solution strengthening, precipitation hardening, phase transformation, and dispersion hardening. It should be pointed out that not all of these strengthening mechanisms are applicable to all metals. For example, grain size refinement can be used to strengthen steel but is not as nearly effective for aluminum. In addition, several mechanisms can be used for the same metal. For example, solid solution strengthening and precipitation hardening are used in combination for some nickel based superalloys.
As a metal is plastically deformed, new dislocations are created, so that the dislocation density becomes higher and higher. In addition to multiplying, the dislocations become entangled and impede each other's motion. The result is increasing resistance to plastic deformation with increases in the dislocation density. This continual increase in resistance to plastic deformation is known as work hardening, cold working, or strain hardening. Work hardening results in a simultaneous increase in strength and a decrease in ductility. Since the work hardened condition increases the stored energy in the metal and is thermodynamically unstable, the deformed metal will try to return to a state of lower energy. This generally cannot be accomplished at room temperature; elevated temperatures, in the range of 1/2-3/4 of the absolute melting point, are necessary to allow mechanisms, such as diffusion, to restore the lower energy state. The process of heating a work hardened metal to restore its original strength and ductility is called annealing. Metals undergoing forming operations often require intermediate anneals to restore enough ductility to continue the forming operation.
Annealing occurs in three stages: recovery, recrystallization, and grain growth. The first stage is recovery. During recovery, there is a rearrangement of dislocations to reduce the lattice strain energy, along with the annihilation of a number of dislocations. As shown in Fig. B.7, recovery results in a significant relief of internal stresses with only a moderate reduction in strength, a process called stress relieving. During recrystallization, the deformed grain structure is totally replaced by new unstrained grains through a nucleation and growth process, in which new stress free grains grow from nuclei in the deformed matrix. Recrystallization results in a large decrease in strength and a large increase in ductility. After recrystallization, the material can further lower its energy by reducing its total area of grain surface. With extensive annealing, the grain boundaries straighten, small grains shrink and disappear, and larger ones grow. This last stage of annealing, grain growth, is rarely desirable since fine grained structures generally have higher static and fatigue strengths than large grained structures. However, grain coarsening is important in some of the
Work Harden Recovery Recrystallization
>■ Time at Annealing Temperature
>■ Time at Annealing Temperature
Fig. B-7. Cold working and Annealing
Fig. B-8. Solidification Sequence for Metal
Fig. B-8. Solidification Sequence for Metal high temperature super alloys, because large grains resist creep better than small grains due to their higher resistance to creep mechanisms.
As shown in Fig. B.8, the atoms in the grain boundaries are not aligned with the atoms in either crystal and are therefore in a higher energy state than the crystals themselves. In polycrystalline metals, the orientation of the slip planes in adjoining grains is seldom aligned, and the slip plane must change direction when traveling from one grain to another. Reducing the grain size produces more changes in direction of the slip path and also lengthens it, making slip more difficult; therefore, grain boundaries are effective obstacles to slip. Decreasing the grain size is effective in both increasing strength and also increasing ductility, and, as such, is one of the most effective methods of strengthening. Fracture resistance also generally improves with reductions in grain size, because the cracks formed during deformation, which are the precursors to those causing fracture, are limited in size to the grain diameter.
When a metal is alloyed with another metal, either substitutional or interstitial solid solutions (Fig. B.9) are usually formed. Substitutional solid solutions are those in which the solute and solvent atoms are nearly the same size, and solute
Appendix-B A Brief Review of Materials Fundamentals Solute Atom ,-Solute Atom
Fig. B-9. Solid Solutions atoms simply substitute for solvent atoms on the crystalline lattice. Interstitial solid solutions are those in which the solute atoms are much smaller, and fit within the spaces between the existing solvent atoms on the crystalline structure. The insertion of both substitutional and interstitial alloying elements strains the crystalline lattice of the solvent structure. This increase in distortion, or strain, creates barriers to dislocation movement.
Precipitation hardening is used extensively to strengthen aluminum alloys, nickel based superalloys, and PH stainless steels. In precipitation hardening, an alloy is heated to a high enough temperature to take a significant amount of an alloying element into solid solution. It is then rapidly cooled (quenched) to room temperature, trapping the alloying elements in solution. On reheating to an intermediate temperature, the host metal rejects the alloying elements in the form of a fine precipitate that creates matrix strains in the lattice. The fine precipitate particles are barriers to the motion of dislocations and provide resistance to slip.
A portion of a phase diagram for an alloy system that has the characteristics required for precipitation hardening is shown in Fig. B.10. Note that the solvent metal at the left hand edge of the diagram can absorb much more of solute metal at elevated temperature than it can at room temperature. When the alloy is heated to the solution heat treating temperature and held for a sufficient length of time, the solvent metal absorbs some of solute metal. When it is rapidly cooled to room temperature, atoms of the solute metal are trapped in a supersaturated solid solution of the solvent metal. On reheating to an intermediate aging temperature, the supersaturated solution precipitates a very fine phase that acts as barriers to dislocation movement. Note the effects of different aging temperatures shown in Fig. B.10. If the metal is aged at too low a temperature (T1), the desired strength is not obtained and the alloy is said to be underaged. On the other hand, aging at too high a temperature (T4) also results in lower than desired strength and the
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