A Deformation Mechanism Map for Incoloy 800H Optimized Using the Genetic Algorithm
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DUCTION
A. Incoloy 800H
INCOLOY 800H is an austenitic stainless steel alloy first developed in the 1950s with a nominal composition of Fe-32Ni-21Cr-1.5Mn-1.0Si-0.4Ti-0.4Al and a carbon content of 0.05 to 0.10 pct.[1,2] It has a solid solution strengthened FeNiCr matrix with additional strengthening from M23 C6 and Ti(C,N) precipitates.[3] 800H is commonly used in high-temperature applications due to its high resistance to corrosion[4,5] and creep.[6,7] In particular, it is used in the petrochemical industry for methane reformer exit tubing.[8,9] It was also short-listed as a candidate for high-temperature components in next generation nuclear reactor systems.[10] B. Creep and Creep Mechanisms Creep is the inelastic deformation of a material under sustained load at elevated temperatures.[11] The temperature at which creep becomes relevant is typically above 0.4 to 0.5 Tm ,[12,13] where Tm is the absolute melting point of the material. The term creep encompasses a number of different high-temperature deformation mechanisms. These mechanisms are often described analytically using the minimum strain-rate (also known
as the secondary creep rate), e_ min , as a function of temperature, T, shear stress, pffiffiffi rs (where r is the uniaxial test stress, and rs ¼ r= 3), and average grain size, d.[14] The creep mechanisms include, but are not limited to, low-temperature power-law creep, where the rate-controlling mechanism is the transport of matter through dislocation cores,[15,16] high-temperature power-law creep, where the rate-controlling mechanism is the climb of dislocations,[17,18] Nabarro-Herring creep, where the rate-controlling mechanism is the diffusion of vacancies through the lattice,[19,20] and Coble creep, where the rate-controlling mechanism is the diffusion of vacancies through grain boundaries.[21] The constitutive equations for the minimum strainrate of low-temperature power-law creep, e_ LT , high-temperature power-law creep, e_ HT , Nabarro-Herring creep, e_ NH , and Coble creep, e_ Co are ac Dc 0 G rs nþ2 Qc exp e_ LT ¼ ALT ½1 bkT G RT
e_ HT
Dl Gb ¼ AHT 0 kT
e_ NH ¼ ANH AARON L. BEARDSLEY, CATHERINE M. BISHOP, and MILO V. KRAL are with the Mechanical Engineering Department, University of Canterbury, Christchurch, New Zealand. Contact e-mail: [email protected] Manuscript submitted April 4, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
e_ Co ¼ ACo
kT
rs G
n exp
Ql RT
½2
2 b rs Ql exp d G RT
½3
3 b rs Qgb exp d G RT
½4
Dl 0 Gb kT
Dgb 0 Gb
where ALT , AHT , ANH , and ACo are constants; ac is the effective cross-sectional area of a dislocation core; Dc 0 , Dl 0 , and Dgb 0 are the pre-exponential diffusion constants for low-temperature power-law creep, high-temperature power-law creep, and Coble creep, respectively; G is the temperature-dependent shear modulus; b is the Burgers vector; k is the Boltzmann constant; n is the power-law exponent; Qc , Ql , and Qgb are the activation energies for low-temperature power-law creep, high-temperature pow
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