Solid solution creep behavior of Sn- x Bi alloys
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as 2Q CE εz ss 5 sinh exp RT E RT
[1]
Equation [1] describes steady-state creep where at low strain rates there is linear stress dependence and at high strain rates there is an exponential stress dependence. The transition in creep behavior occurs at a critical, breakaway stress, sc 5 E/a. This stress is compared to the breakaway stresses proposed by Friedel and by Cottrell and Jaswon. There is good agreement at low solute concentrations to the breakaway stress proposed by Friedel, but sc is significantly lower than the breakaway stress predicted by Cottrell and Jaswon. Several observations suggest that for Sn-xBi alloys, dislocation climb is the rate-limiting mechanism in the nonlinear region. First, the stress sensitivity of the steady-state strain rate data is similar to that of pure Sn, where dislocation climb is known to be the rate-limiting mechanism. Second, primary creep is observed throughout the tested stress range. Third, incremental additions of Bi decrease the steady-state creep rates, even though Bi has a higher diffusivity in Sn than Sn by self-diffusion.
I.
INTRODUCTION
TIN (Sn) has played a historic role in furthering the understanding of creep phenomena. In 1936, single-crystal Sn was used by Chalmers[1] to determine the exact nature and characteristics of the yield point of a material. Chalmers sought to determine whether there actually exists an exact yield point for materials, or whether its measured value is merely a function of the accuracy of the measurement technique. What he found was that creep occurred at all stresses. Additionally, two types of creep were observed: ‘‘microcreep’’ at strain rates under 1 3 1028 s21 and ‘‘macrocreep’’ at higher creep rates. The microcreep rate increased linearly with stress, while the macrocreep rate was found to increase with stress raised to an exponent greater than one. In 1948, Cottrell,[2] in a historic report to the London Physical Society, developed the theory that creep is a result of slow dislocation movement, citing Chalmers’ microcreep data for single crystals of Sn as supporting evidence. In 1966, Harris et al.[3] examined the influence of Pb additions on the microcreep rates of Sn single crystals and confirmed the Cottrell–Jaswon[4] theory of microcreep in the presence of solute atmospheres. As D. MITLIN, formerly Undergraduate Student, Department of Materials and Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, is Graduate Student, Center for Advanced Materials, Lawrence Berkeley National Laboratory, and the Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. C.H. RAEDER, formerly Graduate Student, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, is Senior Process Engineer, Advanced Micro Devices, Austin, TX 78741. R.W. MESSLER, Jr., Associate Professor of Materials Science and Engineering and Director of Materials Joining, is with Rensselaer Polytechnic Institute. Manuscript submitted December 14, 1994. METALLUR
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