Threshold Stress Creep Behavior of Alloy 617 at Intermediate Temperatures
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UCTION
THE creep behavior alloys deviates from that alloys,[1–3] for which the typically described by a ship: e_ min
of precipitation-strengthened of solid solution single phase minimum creep rate, e_ min , is modified power-law relation-
lb r n /D ; kT l
½1
where D is the diffusion coefficient of the matrix, r is the applied stress, l is the shear modulus, b is the Burgers vector, k is the Boltzmann constant and T is the temperature.[4–6] The stress exponent, n, is accepted to be in the range of 3 to 5 for solid solution alloys and, more specifically, around 5 for nickel[7,8] and nickelchromium alloys.[4,9] In precipitate-strengthened alloys, the value of n and the activation energy, Q, contained in D may be predicted by fitting experimental creep data with Eq. [1]. The values obtained are unrealistically large compared to the range of n values for solid solution alloys and the activation energy for self-diffusion.[10,11] To overcome this, a threshold stress term, r0, is introduced into Eq. [1] to account for the influence JULIAN K. BENZ, LAURA J. CARROLL, JILL K. WRIGHT, RICHARD N. WRIGHT, and THOMAS M. LILLO, Staff Scientists, are with the Idaho National Laboratory, Idaho Falls, ID. Contact e-mail: [email protected] Manuscript submitted October 1, 2013. Article published online March 15, 2014 3010—VOLUME 45A, JUNE 2014
of the precipitates on mobile dislocations and results in the following equation lb r ro n ½2 e_ min / D kT l to describe the minimum creep behavior.[3,12] The value of the threshold stress is dependent on the dominant precipitate bypass mechanism, which may be shearing, Orowan bowing, or climb,[13] and may be determined experimentally by fitting associated creep data. Threshold behavior is commonly observed in c-c¢ superalloys[14] and oxide dispersion-strengthened (ODS) alloys.[3,12] The former system often exhibits a larger volume fraction of semi-coherent precipitates and the latter typically contains relatively fine, incoherent precipitates. There are few alloy systems available that allow the study of low volume fractions (
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