The high-temperature strength of commercial-purity alpha uranium
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I.
INTRODUCTION
THE high-temperature
mechanical properties of uranium have been studied extensively during the past two decades. Numerous factors such as thermomechanical processing, crystallographic texture, grain size, and small alloying additions have affected the high-temperature strength. 1-14Relatively little work, however, has been devoted to studying the effects on mechanical behavior when each of the above factors is varied in a single investigation; thus, it is often necessary to compare properties, such as flow stress levels, obtained by different investigators working under different conditions. Such comparisons can often be misleading. This study investigates the effects on mechanical behavior of varying each of the foregoing factors while other experimental conditions are held constant. In addition, the observed effects are analyzed in light of theoretical mechanisms for high-temperature flow.
mally introduced a grain-size dependence term into the general equation
kkT
D~
/b\P/tr\"
= a~7)~G-)
REVIEW OF THE LITERATURE
A. Constitutive Equations At temperatures above about half the absolute melting point, the mechanical behavior of most pure metals can be expressed using an equation of the form Is 5
D where ~ is the steady-state true strain rate, D = Do exp (-QJRT) is the lattice diffusivity, ~/E is the steady state flow stress divided by Young's modulus, and K is a constant. The diffusivity D is equal to the constant Do (generally cm 2 per second) times the following exponential: negative activation energy divided by the gas constant times absolute temperature. This equation was presented in a slightly different form by Mukherjee et al.16 and correlated with a large amount of experimental data.17 Bird et al. also infor-
R.W. LOGAN, formerly Metallurgist with Lawrence Livermore National Laboratory, Livermore, CA, is now at the University of Michigan, Ann Arbor, MI 48109. Manuscript submitted June 14, 1982.
METALLURGICALTRANSACTIONS A
(-~T)
[2]
where Young's modulus has been replaced with the shear modulus G, kT equals the product of the Boltzmann constant and absolute temperature, b is the burgers vector, d is the average grain size, and A is a dimensionless constant. During the normal climb-controlled dislocation creep observed in most coarse-grained metals (d -> 10 p.m) n = 5, p = 0, and D takes the values for lattice diffusivity as in Eq. [1]. The term T/G on the left side of Eq. [2] results in a slightly different activation energy Q than that given by Eq. [1]. In Eq. [1], the activation energy at constant strain rate is directly proportional to the stress exponent n: 0 ln(o/E) 0 = n
II.
exp
[3]
a(l/r)
Equation [11 supplemented by the (b/d) p term was used to analyze the experimental data generated in this study for two masons. First, the direct proportional relationship between O and n is then obvious from Eq.[3]; this relationship illustrates the effect of uncertainty in the n value on the corresponding activation energy. Second, Eq.[1] has been used previously to analyze high-t
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