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National Advisory Committee for Aeronautics, Report - Plastic Deformation of Aluminum Single Crystals at Elevated Temperatures

This report describes the results of a comprehensive study of
plastic deformation of aluminum single crystals over a wide
range of temperatures. The results of constant—stress creep
tests have been reported for the temperature range from 400° to
900° F. For these tests, a new capacitance-type extensometer
was designed. This unit has a range of 0.30 inch over which
the sensitivity is very nearly linear and can be varied from as
low a sensitivity as is desired to a maximum of 10 microinches
per millivolt with good stability.

In constant—load creep tests at temperatures up to 1,100° F,
four additional slip planes (the (100), (311), (110), and (211))
have been observed, besides the customary (111) plane. The
(311) slip plane has been observed above 300° F; the (100),
above 600° F; the (211), above 700° F; and the (110), only at
1,000° F.

The stress—strain curve has been determined in constant-load-
rate tests at temperatures of 82°, 500°, and 1,100° F for single
crystals of two purities, 9999+ percent and 99.95 percent
ahunuuun.

Experiments were carried out to investigate the eject of small
amounts of prestraining, by two diferent methods, on the creep
and tensile properties of these aluminum single crystals. Two
highrresolution X-ray techniques used to detect and to follow
the strain in these experiments showed a definite polygonization
of the crystalline lattice during creep.

Light microscopy has been employed to observe the details
of the process of kinking. Electron microscopy has been used
with various replica techniques to investigate the complex nature
of the slip bands resulting from plastic deformation at elevated
temperatures. From these observations, it has been concluded
that plastic defamation takes place predominantly by slip
which is accompanied by the mechanism of kinking and
polygonization.

INTRODUCTION

This investigation has been directed toward the problem
of determining the mechanisms of creep. This required that
a number of different approaches be employed in attacking
the problem. Reliable creep data had to be obtained over
a wide range of temperatures. Then perturbations or small
changes had to be introduced into the specimens to study
their effects on the deformation characteristics. Examples

of such perturbations are a small change in purity or a small
amount of prestraining. In addition, it was necessary to
observe the changes in the specimens resulting from plastic
defamation. This can be done on the surface by light mi-
croscopy, or, on an even finer scale, by electron microscopy.
The deformation can be studied below the surface by high-
resolution X-ray diffraction methods.