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National Advisory Committee for Aeronautics, Technical Notes - Quantitative Treatment of the Creep of Metals by Dislocation and Rate Process Theories

An equation for the steady-state rate of creep has been derived.
by applying the theory of dislocations to the creep of pure metals;
lJEhe form of this equation is in good agreement with empirical equa-
tions describing creep rates. rJ'.'he theory was also used. to predict
the dependence of steady-state rate of creep on physical constants
of the material and good agreement was obtained with data in the
literature for pure annealed metals. @he rate of creep was found to
decrease with increasing modulus of rigidity} 5311s fact suggests
that one of the requirements for a heatoresisting alloy is that its
matrix be a metal that has a high modulus of rigidity and therefore
a high modulus of elasticity.

II'he development of the gas turbine in combination with the Jet
as a power plant for military aircraft has focused attention on the
need for heat-resisting alloys. (One of the criteria used to evaluate
heat-resi sting alloys is creep resistance» that is, the resistance
to plastic deformation over a period of time. Current evaluation of
creep resistance is acccmplished by creep tests in which creep curves
(elongation plotted against time) are obtained at constant stress and
temperature. A typical creep curve is shown in figure 1. lI'he initial
stay, in which the slope of the curve or rate of creep is rapidly
decreasing, is commonly designated the primary or transient stage;
secondary or steady-state creep refers to the straight-line portion
of the creep curve. After a sufficient length of time, the rate of
creep increases in the region designated the tertiary stage. In
certain cases, the rate of creep continuously increases and cannot
be divided into these three stages (reference 1(a)). The steady-
state rate of creep is generally the criterion by which the creep
resistance of heat-resisting alloys is expressed. Because this rate
rapidly increases with temperature, creep becomes an important factor
in limiting safe-operating temperatures.