Identifier

etd-0826103-140253

Degree

Master of Science (MS)

Department

Mechanical Engineering

Document Type

Thesis

Abstract

IN738LC is a modern, nickel base superalloy utilized at high temperatures in aggressive environments. Durability of this superalloy is dependent on the retention of strengthening by Gamma Prime precipitates. The precipitate growth features in the duplex size (fine and coarse) precipitate distribution was studied by heating the alloy for various times in vacuum at selected temperatures in the range 800 °C to 1100 °C. Increasing the holding times from 1 hour to 100 hours shows joining of the fine particles to form bigger particles, leading to distinct raft patterns. Two different Gamma Prime precipitate growth processes had been observed: merging of smaller precipitates to produce larger ones (in duplex precipitate-size microstructures) that is, both the fine and the coarse precipitate particles grow with time, apparently by the particle movement in the matrix and coalescence – by the particle agglomeration mechanism (PAM), and growth through solute absorption from the matrix (Ostwald Ripening). At 1100 °C the fine particles grew to the size of the coarse particles in about 100 hours and a single coarse size of about 840 nm was obtained. The activation energies for the growth of both the fine and coarse particles decrease with increase in temperature and size of the particles indicating that the coarser particles require less activation energy for the growth than the finer particles. This is attributed to the greater attractive force with which the coarser particles would attract the finer particles, in the overlapping diffusion zone thus requiring less activation energy for the growth. The activation energy for the precipitate growth in the duplex size range also was found to decrease progressively with increasing particle size at a given high temperature.

Date

2003

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Aravamudhan Raman

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