The Role of Dislocation Climb across Particles at Creep Conditions in 9 to 12 Pct Cr Steels
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STEAM pressure and temperature are critical parameters in determination of power plant efficiency, and creep strength and oxidation resistance are limiting material properties when trying to achieve higher efficiency. Thus, martensitic 9 to 12 pct Cr steels, such as P92, have been subject to extensive research within both European cooperations (COST) and national projects. During the latter part of last century, new 9 to 12 pct Cr steels have been developed by controlling the amount of alloying elements, causing precipitation of stable particles, and improving the creep strength of the material. These obstacles interfere with moving dislocations and provide an important strengthening mechanism.[1] With increasing temperature, the climb mobility of dislocations increases and dislocations can climb across particles, thus reducing the particle hardening. Using the Orowan equation would overestimate the particle strengthening, because only some of the dislocations bow around particles. Brown and Ham first suggested a model for local climb where the dislocation segment is in contact with the particle.[2] The model has been extended for different particle geometries, such as spherical particles by Shewfelt and Brown.[3] A dislocation climb model by Lagneborg[4] allows the dislocation to relax as it climbs across particles and to avoid the sharp dislocation bend where the dislocation leaves the glide plane. This type of model is denoted general climb. Srolowitz et al.[5] first suggested that the threshold stress at particle climb HANS MAGNUSSON, Graduate Student, and ROLF SANDSTRO¨M, Professor, are with the Materials Science and Engineering and Brinell Centre, Royal Institute of Technology, 100 44, Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted March 7, 2007. Article published online September 1, 2007. 2428—VOLUME 38A, OCTOBER 2007
arises instead due to the dislocation attachment to particles. The particle and matrix interface produces an attractive dislocation position and a breakaway stress is needed. Some experimental transmission electron microscopy (TEM) micrographs confirm this theory.[6] Previous attempts to estimate the minimum threshold stress strongly vary with the assumptions about climb mechanisms and particle parameters, as explained by Blum et al.[7] Suggested values of the ratio of threshold stress, rth, to Orowan stress, rOro, at general climb are about[7] 0:004
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