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has been suggested that this latter parameter can encompass both the K and Onet descriptions of creep crack growth with the reservation that variations in specimen geometry may influence the results. Although this latter approach is concerned with the concept of the rate of change of energy release rate during crack propagation, it has been shown that by measuring the displacement of specimens, direct proportionality with the parameter C* is obtained, 1~although there may be difficulties in distinguishing between C* and (6) control of crack growth if the range of crack growth is unduly restricted. It must also be pointed out however that recent work has suggested that K and C* may characterize opposite situations, ~2dependent on the relationship between the creep zone size and the specimen geometry. There is thus, at the present time, no single parameter which can be used under different circumstances to predict either creep crack growth rates or the life of cracked components on a continuum basis. A further problem also exists concerning the influence of the long term development of creep damage in a component before a macroscopic crack propagates into such a damaged region. This damage can take two forms: either the microstructural aging of the material or, the creation of a general background level of cavitation damage which can assist the subsequent macroscopic crack propagation. Little work has been carried out on this problem, and one of the purposes of the present paper is to show that prior cavitation damage increases creep crack growth rates and lowers rupture times. However any discussion of the influence of prior damage on macroscopic creep crack growth must lead to a detailed consideration of the fine scale nature of such damage and also to a study of the microscopic nature of creep crack growth. It is now established that macroscopic creep crack growth occurs by a continuous process of cavity nucleation and growth, followed by cavity interlinkage/coalescence leading to true crack propagation. A number of models13 17have recently been proposed on a microscopic basis in an attempt to predict creep crack growth on the basis of the growth of cavities. The major problem with such models is the dearth of information which directly relates macroscopic crack growth rates to detailed measurements of cavity sizes and spacings

!SSN 0360-2!33,81/0211-0173500.75/0 M E T A L L U R G I C A L TRANSACTIONS A 9 1981 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME

VOLUME 12A, FEBRUARY 1981--173

from the same specimens. The second purpose of this paper is therefore to give detailed measurements of cavitation damage and use these for a consideration of some models for creep crack growth in the sense of correlating the macroscopic and microscopic data from the same specimens. EXPERIMENTAL A commercial 0.5 pct Cr, 0.5 pct Mo, 0.25 pct V steel of composition given in Table I was given a full solution treatment of 1/2 h at 1533 K with an air cool, followed by a tempering treatment of 24 h at 953 K. This g