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The alloying atoms retained in base metal lattice

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The alloying atoms retained in base metal lattice positions by solution heat treatment 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 treatment temperatures or a low solution temperature can reduce the strength of the heat treated alloy. 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 also disrupts the electronic structure of an alloy and contributes to lower conductivity. The conductivity is not lowered as much if solution heat treatment temperatures are low or if cooling from solution heat treatment temperatures is excessively slow; poor solution heat treatment practices such as these permit too many atoms to come out of solution or form secondary particles. 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 that will allow alloying atom diffusion and coalescence to form microscopic particles of different 330 Electromagnetic Testing F IGURE 10. Influence of metallic additives on conductivity of aluminum. Conductivity, MS·m –1 (%IACS a ) Cr Ti Zn Ni Ag V Mg Si Mn Fe Cu 0 0.1 0.2 0.3 0.4 0.5 0.6 Impurity (percent) 38 (66) 37 (64) 36 (62) 35 (60) 34 (59) Legend %IACS = percentage of International Annealed Copper Standard Ag = silver Cr = chromium Cu = copper Fe = iron Mg = manganese Mn = magnesium Ni = nickel Si = silicon Ti = titanium V = vanadium Zn = zinc
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composition and lattice structure in the metal. The number, size and distribution of the particles are controlled by the time and temperature of the aging process. Temperatures are much lower than those required for solution heat treatment or annealing. Lower temperatures and shorter times result in smaller particle sizes. Higher temperatures favor the formation of fewer but larger particles. 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 over each other, causing plastic deformation. Greatest strengthening usually occurs at a specific range of particle sizes for a particular alloy system. In many cases, aging is performed under conditions designed to provide a specific combination of strength and ductility or corrosion resistance. As aging increases beyond the optimum time or temperature, the particle size increases and gradual softening occurs. When
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  • Fall '19
  • Magnetism, Magnetic Field, Electrical conductivity

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