Creep cavitation in a NiCr steel
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I.
INTRODUCTION
THE creep fracture of engineering structural components subject to use at ambient temperatures under low applied stresses often takes place along grain boundaries. The rupture process includes nucleation and growth of creep cavities, coalescence of cavities into microcracks, and interlinkage of microcracks to form a macroscopic crack and propagation of the macroscopic crack across the components, leading to the ultimate fracture. Among these, creep cavitation is usually the ratecontrolling step and thereby has been the subject of extensive study for the past two decades. Now, it is well established that cavities nucleate preferentially on grain boundary triple junctions, precipitates, or ledges where high local stresses are induced during the grain boundary sliding process and that cavities grow by diffusion of atoms from the cavity wall into neighboring grain boundaries or by creep of surrounding materials, t~-~~ In materials such as 347 S.S., t231 a-Fe, t221 Cu alloy, and NIMONIC* 80A, tl~l cavities are found to nucleate con*NIMONIC is a trademark of Inco Alloys International, Inc., Huntington, WV.
tinually throughout the rupture process, and the continual nucleation of cavities must be considered for the accurate assessments of rupture times and their stress dependencies. [11-~6,38]However, as noted by Raj, t~Sj it is difficult to predict nucleation rate of cavities reasonably using the classical nucleation theory, which generally shows high stress sensitivities, ta,17,18J Recently, it has been observed in some metals that the relation Ngb = heo "m
[1]
holds over varying stress ranges of creep data. [19-27]Here, Ngo, e, A, and tr denote cavity number per unit grain boundary area, creep strain, the proportionality constant,
H.C. CHO, Graduate Student, and JIN YU, Professor, are with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Seoul, Korea. I.S. PARK, Manager, is with the Technical Center, Daewoo Heavy Industries, Inchun, Korea. Manuscript submitted October 25, 1990. METALLURGICAL TRANSACTIONS A
and stress, respectively, and m is a constant often equal to zero. When Eq. [1] is differentiated with respect to time (denoted as (')) and identification is made of Ngb and ~ as steady-state nucleation rate (I~) and creep rate (~s), respectively, we have N~b=Is
=,~
m= ~
m+.
[2]
where Norton's creep equation (~ = Bo ~) is utilized. Cavity nucleation formulas based on Eq. [2] are combined with growth formulas, which provide reasonably accurate estimates of rupture times (ty) and their stress dependencies. [1,18,22,28,3~ (More descriptions on the method are given in Section II.) However, as in a-Fe tz21 and NIMONIC 80A, [z4] the Ngb vs e relation is not always well defined and the power-law creep equation does not properly express the strain rate of a material which shows significant primary and tertiary creep. From a practical point of view, the most serious problem is associated with the difficulties of measuring Ngb and of opening gra
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