• Version
• 117 Downloads
• 3.44 MB File Size
• 1 File Count
• December 4, 2015 Create Date
• December 4, 2015 Last Updated

National Advisory Committee for Aeronautics, Technical Notes - Internal Friction Study of an Aluminum Alloy Containing 2 Weight Percent Copper

A study has been made, by means of low-frequency internal-friction
measurements in both torsional and flexural vibration, of aluminum (Al)
alloy containing h weight percent copper (Cu) during aging. Both
polycrystalline and single-crystal specimens exhibit an initial internal-
friction peak at 175° C (for a frequency of l ops) after solution treat-
ment and quenching. This peak shows all the characteristics of a Zener
relaxation, including strong anisotropy. It falls on aging in a manner
simply related to the decrease in the copper concentration of the matrix.
The peak is also sensitive to the reversion of Guinier—Preston (G.P.) Efl
zones.

The precipitation of the phase 9' is marked by the appearance of a
second peak at 135° C (for a frequency of l cps) and by a rise in the high-
temperature background internal friction. Both these contributions grow
with kinetics shnilar to those for the precipitation of _e'. The second
peak is associated specifically with the presence of the nonequilibrium
phase 6'; it is reduced by the transformation from 9' to a and is
absent in specimens containing only the 6 phase. The high—temperature
background, however, is increased further by this transformation. Possible
causes of the second peak and the background changes are discussed.

Internal friction is often loosely described as the ability of a
solid to damp out vibrations. More strictly, it is a measure of the
vibrational energy dissipated by the operation of specific mechanisms
within the solid. Internal friction arises even at the smallest stress
levels if Hooke's law does not properly describe the static stress-strain
curve of the material. The nonelastic behavior which Zener (ref. 1) has
called anelasticity arises when the strain in the material is dependent on
variables other than stress. A material showing the simplest kind of
anelasticity responds to a suddenly applied stress with an elastic strain
followed by an anelaStic strain which'increases exponentially with time
to a limiting value proportional to the applied stress.