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National Advisory Committee for Aeronautics, Report - Influence of Alloying Upon Grain Boundary Creep

In a previous investigation upon grain~boundary creep in
bicrystals of pure aluminum (ref. 1) it was shown that dis-
placement at the grain boundary begins only after an induc-
tion interval and then proceeds in a spasmodic sequence of
surges, alternating with periods of rest. The overall dis-
placement is an approximately linear function of the cube
root of the time and increases with temperature according
to the Arrhenius relationship, and, over modest ranges of
stress, its logarithm is a linear function of the stress. The
direction of movement is that of the mam‘mum resolved
shear stress in the plane of the grain boundary, irrespective
of crystal orientation; but the overall rate of displacement
is highly sensitive to crystal orientation, increasing with the
angular difference between the active slip systems in the
conjugate crystal. Yielding occurs in' a zone of metal of
substantial depth adjacent to the grain boundary and is
obviously heterogeneous in character.

This mode of behavior was explained upon the basis of a
slip-recovery process modeled after that which has been
employed by McLean (ref. 2) to account for steady-state
creep in polycrystalline metals. It was postulated that
transgranular slip causes an accumulation of bending energy
adjacent to the grain boundary, which accumulation results

in an early and localized recovery in a narrow band all along
the boundary. The metal, thus softened, yields preferenti-
ally and the direction of yielding is parallel to that of the
grain boundary. When yielding parallel to the grain
boundary has ceased, owing to a loss of coordination of the
slip-recovery cycles in the affected zone, there still remains
in progress the same transgranular yielding that was present
initially. This plastic motion again builds an accumulation
of energy at the grain boundary, resulting in another occur-
rence of coordinated recovery and a new wave of displace—
ment parallel to the grain boundary.

Based upon this view of the mechanism of grain-boundary
gliding, it was postulated that the addition of alloying
elements should afiect the process most markedly in those
respects relating to the occurrence of recovery. It was
hoped that the observation of the details of spasmodic
gliding in an alloy series would shed 'further light upon the
fimction of alloying elements in depressing the creep rate.