The role of grain boundary character distribution on creep of nickel-16chromium-9iron at 360 degrees C.
Thaveeprungsriporn, Visit
1996
Abstract
The objective of this study is to understand and quantify the role of coincidence site lattice boundary (CSLB) population on creep deformation of Ni-16Cr-9Fe at $360\sp\circ\rm C.$ It is hypothesized that an increase in the CSLB population in the system decreases the annihilation rate of dislocations in the grain boundary leading to an increase in the internal stress and a decrease in the effective stress. The result is a reduction in the creep strain rate. The role of CSLBs in deformation is thus to increase the internal stress by trapping run-in lattice dislocations at the grain boundaries, creating back-stresses on following dislocations, rather than annihilating them as in the case of high angle boundaries (HABs). The hypothesis was substantiated by showing (1) that dislocation absorption kinetics differs substantially between a CSLB and a HAB and (2) that the CSLB fraction strongly affects the internal stress in the solid. Dislocation absorption kinetics were measured by comparing extrinsic grain boundary dislocation (EGBD) density in TEM. Results showed that CSLBs contain an EGBD density which is 3 times higher than HABs at 1.25% strain indicating lower absorption kinetics of CSLBs compared to that in HABs. The observed increase in the yield and flow strength of the CSLB-enhanced sample is attributed to the difference in the EGBD density between CSLBs and HABs. Internal stress was measured by the stress dip test and was found to be ${\approx}25$ MPa higher in the CSLE sample. Steady state creep rates of Ni-16Cr-9Fe crept in $360\sp\circ\rm C$ argon were also found to be strongly affected by the grain boundary character distribution. Increasing the CSLB fraction by approximately a factor of 2 decreases the steady state creep rates by a factor of 8 to 26 in the coarse grain (330 $\mu\rm m)$ samples and a factor of 40 to 66 in the small grain (35 $\mu\rm m)$ samples. The Bailey-Orowan model, which establishes proportionality between creep rate and dislocation annihilation rate, was modified to account for CSLBs. It is postulated that annihilation of EGBDs only occurs at triple lines or quadruple points where at least two HABs intersect. A good correlation was found between model results and data on the dependence of creep rate on CSLB fraction. Comparisons of the CSLB fraction in the system, creep rates and IG cracking reveal that both creep rates and the amount of IG cracking decrease rapidly with increasing CSLB faction. Studies on grain boundary sliding and cavitation of crept coarse grain samples also revealed that HABs are more susceptible to sliding and cavitation than CSLBs. These results provide encouragement that grain boundary can be engineered to tailor the bulk properties of alloys.Other Identifiers
(UMI)AAI9624745
Subjects
Engineering, Nuclear Engineering, Materials Science
Types
Thesis
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