The Effect of Applied Stress of the Decomposition Cu-15Ni-8Sn Spinodal Alloy
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THE EFFECT OF APPLIED STRESS OF THE DECOMPOSITION Cu-15Ni-8Sn SPINODAL ALLOY S. AHN AND T. TSAKALAKOS Department of Mechanics and Materials Science, College of Engineering, Rutgers University, New Brunswick, New Jersey 08854 ABSTRACT The effect of stress on spinodal decomposition in Cu-15Ni-8Sn alloy was investigated at various temperatures and stresses. As the aging temperature approached the coherent spinodal temperature about 500'C, the anisotropy among the three equivalent modulations of the stress aged specimens became pronounced. The wavelength of the [0011 modulation along the tensile stress axis was smaller than that of the [100] or [010] modulation perpendicular to the stress axis. TEM diffraction patterns also showed the anisotropy effect of the applied stress to the composition modulation. A theoretical model based on micromechanics concepts was employed to explain the experimental results. INTRODUCTION It has been known that when a solid-solution is quenched into a region inside the spinodal it decomposes into a structure which is thought to be composed of stationary superposed composition plane waves [1]. It has also been reported that the elastic energy resulting from the coherency strains plays an important part in the thermodynamics, kinetics and especially morphology of the decomposed structure [2]. In most cubic systems the elastic energy is minimum for plane waves, and as a result it develops the morphology composed only of superimposed plane waves. In the present paper, Cu-15Ni-8Sn polycrystalline alloys, which is very well known as a spinodal system [3,4], is investigated to show the anisotropy effect that the applied stress induces in the morphology of the spinodal structure. According to the theoretical predictions [5], a finite stress field applied during the aging can purturb an elastic energy equivalence in three plane waves and change the morphology of the decomposed structure. Electron microscopy observations are presented as a direct evidence of anisotropy effect of the applied stress to the morphology. REVIEW OF THE THEORY The linear theory of diffusion developed by Cahn [1] predicts that the amplitude of a fluctuation with wave vector k as a function of time is given by c(k,t) = c(k,o)eR(k)t
(1)
where R(k) is the amplification factor given by R(k) = -(M/Nv)k
2
{f"+Y(n)+2Kk 2 1
(2)
Here M is the atomic mobility, Nv is the number of atoms per unit volume, Y(A) is the Fourier elastic energy function produced by the internal stress, K is the gradient energy coefficient, and f" = 9Cf/Dc2. The maximum of R(k) in most cubic crystal occurs for k parallel to three directions and its magnitude is given by k km
Mat.
Res. Soc. Symp.
Proc. Vol.
21 (1984)
-f"+Y 4K
Published by Elsevier Science Publishing Co..
(3)
Inc.
540
Tsakalakos [5] has shown that the amplification factor under applied stress field is R(k) = -(M/Nv)k2{f"+Y(n)+YO(n)+2Kk
2
}
(4)
which differs from that derived by Cahn by the term Ya(fl) introduced by applied stress. The stress interaction function Y1(;) was c
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