Mechanisms of Ordered Gamma Prime Precipitation in Nickel Base Superalloys Page: 84
View a full description of this dissertation.
Extracted Text
The following text was automatically extracted from the image on this page using optical character recognition software:
Therefore gaussians are in close agreement with the frequency distributions of the two
generations of y'. A third population of y' precipitates is seen in the EFTEM images; however, it
is difficult to quantify this third generation using EFTEM which is lot smaller than primary and
secondary populations. It has been shown and reported that secondary and tertiary generations of
y' have much higher number density as compared to primary; however, the above distribution
also suggest during each nucleation burst which occurs over a range of temperature most number
precipitates nucleate at lower temperature (highest peaks of distribution) giving rise to close to
exponential distribution, which otherwise would be closer to a normal distribution if all
precipitates in each generation nucleated at a single temperature with almost similar sizes. A
dashed line is shown in the figure which represents the third y' generation (tertiary); however, as
mentioned above the third population cannot be quantified using EFTEM, hence shown as a line.
The distribution also shows that the size difference between primary and secondary y' is more
compared to secondary and tertiary y'. In other words, the size range of tertiary and secondary y'
is closer as compared to primary y'. This could be due the diffusivity of elements that are high
very high at high temperature (steep part of curve) and decrease exponentially at lower
temperatures and becomes almost constant at lower temperatures. After the first burst of
nucleation due to continuous decrease in temperature and hence continuous decrease in
diffusivities of elements, the primary y' precipitates do not grow to reach their thermodynamic
equilibrium volume fraction and have a tendency to grow but are limited by the kinetic factors
involved. Similar effect is experienced by the successive generations of y' which is further
complicated by the continuous change in composition of y that would have an effect on the
thermodynamic driving force for nucleation of different generations of y'.84
Upcoming Pages
Here’s what’s next.
Search Inside
This dissertation can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Dissertation.
Singh, Antariksh Rao Pratap. Mechanisms of Ordered Gamma Prime Precipitation in Nickel Base Superalloys, dissertation, May 2011; Denton, Texas. (https://digital.library.unt.edu/ark:/67531/metadc67949/m1/96/: accessed May 4, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; .