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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.
