Annealing Effects On Mechanical Properties

T.O. 33B-1-1 4-75 During the annealing of alloys, the temperature is selected sufficiently high to permit the alloying atoms to migrate readily. However, this selected temperature is sufficiently below that of maximum solubility to favor the formation of separate particles and compounds by the alloying atoms. Slow cooling from the annealing temperature encourages even more alloying atoms to move from their random position in the base metal lattice to aid in the growth of larger secondary compounds. 4.7.1.12 Annealing Effects On Mechanical Properties. Annealing removes many of the obstacles to plastic flow, such as interacting dislocations and the numerous individual alloying atoms ind fine particles that normally resist plastic deformation. These processes generally result in metals of lower strength and greater ductility after annealing. 4.7.1.13 Annealing Effects On Conductivity. The annealing process reduces obstacles to electron flow. Therefore, annealing improves the conductivity of a metal. Increased annealing times favors more complete diffusion and greater coalescence and growth of particles with associated increase in conductivity. 4.7.1.14 Solution Heat Treating. The minimum number of alloying atoms will occupy lattice sites of the base metal when a temperature slightly below melting point is reached. In interstitial solid solutions, the maximum number of atoms will occupy interstatial positions. As temperatures are lowered, the atoms of many alloying elements will tend to diffuse together and form separate compounds or regions with a different lattice. If the metal is cooled sufficiently rapidly, the atoms do not have time to diffuse and are held in their original lattice positions (retained in solution). The process is called solution heat treating. Any delay in rapid cooling (delayed quench) or a slow rate of cooling will permit an increased amount of diffusion and reduce the number of alloying atoms held in solution. 4.7.1.15 Solution Heat Treating Effects On Mechanical Properties. The alloying atoms retained in base metal lattice positions by solution heat treating present obstacles to dislocation movement. The resistance to plastic deformation increases the strength of the metal. In many instances, more than one alloying element contributes to the higher strength of alloys. Slow rates of cooling from solution heat treating temperatures or too low a solution heat treating temperature can reduce the strength of the heat treated alloy. 4.7.1.16 Solution Heat Treating Effects On Conductivity. The distortion and stresses established by the substitution of alloying atoms for those of the base metal reduce the conductivity of the metal. The greater the number of solute atoms of a specific material, the greater the reduction in conductivity. The presence of lattice vacancies caused by solution heat treating also disrupts the electronic structure of an alloy and contributes to lower conductivity. The conductivity is not lowered as much if solution heat treat temperatures are low or cooling from solution heat treat temperatures is excessively slow. Poor solution heat treat practices such as these permit too many atoms to come out of solution or form secondary particles. 4.7.1.17 Precipitation Heat Treatment. If an alloy has been solution heat treated to retain atoms in the same lattice occupied at high temperature, properties can be further modified by a precipitation or aging treatment. During a precipitation treatment, an alloy is heated to a temperature which will allow alloying atom diffusion and coalescence to form microscopic particles of different composition and lattice structure within the metal. The number, size, and distribution of the particles is controlled by the time and temperature of the aging process. Temperatures are much lower than those required for solution heat treating or annealing. Lower temperatures and shorter times result in smaller particle sizes. Higher temperatures favor the formation of fewer but larger particles. 4.7.1.18 Precipitation Treatment Effects On Mechanical Properties. Precipitation or aging treatments are generally designed to increase the strength of alloys, particularly the yield strength. The strengthening is accomplished by the formation of small particles of different composition and lattice structure from the original lattice. The small particles provide obstacles to the movement of dislocations in which planes of atoms slip one over the other causing plastic deformation. Greatest strengthening usually occurs at a specific range of particle size for a particular alloy system. In many cases, aging is performed under conditions designed to