Precipitation Hardening Effects On Conductivity

T.O. 33B-1-1 4-76 provide a specific combination of strength and ductility or corrosion resistance. As aging increases beyond the optimum time or temperature, particle size increases and gradual softening occurs. When material has been aged for an excessive time or at too high a temperature, it is said to be over-aged. 4.7.1.19 Precipitation Hardening Effects On Conductivity. The removal of foreign atoms from the parent lattice during precipitation hardening removes much of the distortion of the electron distribution in the lattice. This action favors the movement of electrons through the metal and results in higher conductivity. As increased amounts of foreign atoms are removed from solution and particle growth occurs during over-aging, conductivity continues to increase. 4.7.1.20 Measurement Of Mechanical Properties. The most common method of determining the strength of metals is by means of a tensile test. In the tensile test, a specimen is cut from the metal to be tested, machined to a specified configuration, and tested to failure by application of a known tensile force. The stress at which a known amount of plastic deformation occurs and the breaking stress can then be determined. Many other destructive type tests can be performed to establish such properties as impact resistance, notch sensitivity and fatigue strength. All of these methods require destroying a section of the part to be tested and involve considerable time and expense 4.7.1.21 Hardness Testing. An approximate measure of strength of metals may be established by hardness testing. Hardness is usually determined by the resistance of a metal to penetration by a rounded or pointed indenter pressed into the surface with a known static force. Measurement of hardness is based on the depth of penetration of the indenter or the plane area of the indentation. For many metals, correlation has been established between hardness and tensile strength. Hardness supplies no information regarding ductility although portable hardness testers are available, access and geometry often limit their use. 4.7.1.22 Conductivity And Mechanical Properties. The same variables of chemical composition, heat treatment, and metal working which determine the mechanical properties of a metal also establish its electrical conductivity and magnetic permeability. As a result, correlation has been obtained between electrical conductivity and mechanical properties. This correlation does not mean that the conductivity value of a metal will reliably measure its mechanical properties. However, for some metals, deviation of the measured conductivity from a specified conductivity range or excessive variation in conductivity within a given part or specimen indicates a probable deviation in properties. This deviation may be detrimental to the performance of the metal and requires additional engineering investigation using hardness testing and other forms of testing to determine the magnitude of the deviation and disposition of the parts. 4.7.1.23 Requirement For Application. Application of conductivity measurement for correlation with mechanical properties requires a clearly defined difference in conductivity ranges between the various alloys, tempers, or heat treatments involved. Differences in conductivity and/or permeability exist between alloys of many metals including aluminum, copper, magnesium, steel, and titanium. Not all alloys in each system are separable because of overlapping conductivity ranges. Even when overlapping conductivity ranges for two materials occur, separation of the two is possible if it is known that in the particular lot of material being tested, one material has Conductivity in the upper end of the total range and the second material is in the lower end of the range. Some metals have clearly defined differences in conductivity or permeability between the standard heat treat tempers. This situation exists for most structural aluminum alloys, many magnesium alloys, some copper alloys, and various steels. Little or no difference in conductivity is noted between the various heat treat conditions of titanium alloys with one or two exceptions.