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Summer 2008 Seminar
Monday, July 21, at 3:30 p.m. Room 184 Watts Hall
Raymond Unocic
PhD Candidate advised by Dr. Michel Mills
Department of Materials Science and Engineering
The Ohio State University
A TEM Study of Creep Deformation Mechanisms in Ni-base Disk Superalloys
Abstract
Ni-base superalloys are used in the hot section of aircraft gas turbine engines since they retain strength and resistance to creep, fatigue, and oxidation at elevated temperature. This study is focused on the use of transmission electron microscopy characterization techniques to investigate creep deformation mechanisms and creep rate-limiting processes in a newer generation Ni-base disk superalloy (ME3/R104). Microtwinning has been found to be a principal deformation mode during creep at intermediate temperatures which is quite surprising to observe in this class of alloys and under creep conditions since twinning is generally considered a low temperature, high strain rate deformation mode. This talk will address the integration of deformation experiments, detailed mechanism characterization, and state-of-the-art modeling techniques aimed at developing a fundamental understanding of the microtwinning process. Based on conventional and high resolution TEM investigations, microtwins have been found to occur via the motion of paired a/6<112> Shockley partial dislocations that shear both the gamma matrix gamma prime precipitates. A novel atomic reordering process occurs in the wake of the shearing partial dislocations to maintain the ordering of the L12 structure of the gamma prime precipitates, which helps to fundamentally explain the temperature and rate dependence of microtwinning under creep conditions within this temperature regime. Experimentally, microtwin nucleation sources have been identified and their evolution into fully developed microtwins as a function of increasing plastic deformation has been explored. The salient microstructural features that lead to the microtwinning mechanism will be addressed. All of these experimental findings have been accounted for in the development of physically based creep models aimed at predicting creep behavior.
Bio
Ray received a B.S. in Metallurgical Engineering from The Ohio State University in 2000 and a M.S. in Materials Science and Engineering from Lehigh University in 2002. He is currently completing his Ph.D. in Materials Science and Engineering at The Ohio State University with Prof. Mike Mills where his research has been focused on the use of advanced materials characterization techniques to elucidate the relationship between microstructure and mechanical behavior of high temperature structural materials.
