Grain boundary segregation of P and its interaction with Cr and Mo in Ni-base alloys

Tian, Jinsen (2019). Grain boundary segregation of P and its interaction with Cr and Mo in Ni-base alloys. University of Birmingham. Ph.D.

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The behaviour of phosphorus (P) segregation to grain boundary during cooling and aging and its interaction with chromium (Cr) and molybdenum (Mo) in Ni-base alloys have been investigated using transmission electron microscopy.
P segregation takes place during cooling and aging. The segregation width was several nanometres, much wider than the grain boundary thickness. This indicated a non-equilibrium mechanism. The effect of grain boundary misorientation angle and grain size on the segregation level has been investigated. The grain boundary P concentration increased with increasing misorientation angle until about 45°. A further increase in the grain boundary misorientaion angle resulted in a decline of the segregation level. Compared with random high angle grain boundaries, special grain boundaries displayed much lower segregation. This can be related to the free volume dependence on the misorientation angle. When the effective time during cooling is shorter than a certain critical time, increasing grain size reduced the grain boundary segregation level because of the higher mass transfer coefficient close to the grain boundary in samples with smaller grain size.
Segregation level during cooling at different rates and after aging for different times was obtained. A critical cooling rate and aging time existed where the grain boundary P concentration reached a maximum, indicating a non-equilibrium segregation pattern. Calculations based on Faulkner’s [1, 2] and Xu’s [3] and Wu’s [4] theory were conducted and compared with the experimental results. As regard the effect of grain size, the results based on Wu’s theory were consistent with our experimental results.
The elemental interaction between P and Cr or Mo was investigated. On samples without P addition, no segregation of Cr and Mo was observed after cooling and aging due to the low binding energy between vacancies and Cr or Mo. With P addition, P segregated to the grain boundary while Cr and Mo were depleted at the grain boundary. With increasing P concentration, the grain boundary Cr and Mo concentrations decreased in a linear manner due to the repulsive relative interaction coefficient between P and Cr or Mo in Ni-base alloys.

Type of Work: Thesis (Doctorates > Ph.D.)
Award Type: Doctorates > Ph.D.
Licence: All rights reserved
College/Faculty: Colleges (2008 onwards) > College of Engineering & Physical Sciences
School or Department: School of Metallurgy and Materials
Funders: Other
Other Funders: China Scholarship Council, University of Birmingham
Subjects: T Technology > TN Mining engineering. Metallurgy


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