MSE Colloquium: Lee Semiatin, An Engineer’s Approach to Modeling of Precipitation in Powder-Metallurgy, Nickel-Base Superalloys

Abstract

The high-temperature-precipitation behavior of a typical powder-metallurgy, gamma/gamma-prime, nickel-base superalloy was determined and used to develop and validate a quantitative fast-acting model. To this end, a series of experiments comprising supersolvus solution treatment followed by continuous cooling at rates typical of those experienced during the manufacture of full-scale components was conducted. The nucleation and growth of secondary-gamma-prime precipitates were deduced via metallography on samples water quenched at various temperatures during the cooling cycle. The observations were interpreted using a fast-acting (spreadsheet) model which incorporated the important aspects of classical, homogeneous-nucleation theory and growth by bulk diffusion. In this regard, particular attention was paid to the determination of model input parameters such as the composition, free energy of formation, and surface energy of the precipitates, and an effective diffusivity; the values so determined contrasted with those from existing thermodynamic and diffusion databases. It was demonstrated that fast-acting-model calculations based on a nickel-chromium pseudo-binary system gave good agreement with measurements of the evolution of precipitate volume fraction, number density, and size during continuous cooling.

Bio

Dr. Lee Semiatin is Senior Scientist (ST), Materials Processing/Processing Science in the Air Force Research Laboratory, Materials and Manufacturing Directorate.  He received a BES in Mechanics from Johns Hopkins University and MS and PhD degrees in Metallurgy and Materials Science from Carnegie Mellon University. 

Dr. Semiatin worked at Battelle Memorial Institute from 1978 to 1991.  Here, he conducted and directed programs for a wide range of government and industry clients.  A large portion of his government-sponsored work was for the Air Force Materials Laboratory and Air Force Office of Scientific Research (AFOSR).  This included basic studies of the workability of difficult-to-process aerospace alloys, the fundamentals of material behavior during deformation processing, and various National Aerospace Plane (NASP) - related programs.  Both the government as well as industrial programs involved a major component of technology transfer and thus working with a wide range of manufacturing companies.

In June 1991, Dr. Semiatin joined the Materials and Manufacturing Directorate as Senior Scientist for Materials Processing/Processing Science.  Under his direction, R&D has been conducted in four major areas:  advanced metallic, intermetallic, and nanocrystalline alloys; conventional titanium, nickel, and aluminum alloys; novel processes; and advanced modeling tools for the prediction of microstructure, texture, and damage evolution during deformation and solidification processing.  The integration of various modeling, characterization, and input-data tools that underlie ICMSE form a key part of current research. These efforts have led to the development of various new forging, extrusion, and rapid heat treatment processes - some of which are utilized on a production basis.  In addition, he consults regularly with a number of manufacturing vendors on material-processing problems which impact Air Force systems. 

Dr. Semiatin has authored/co-authored over 400 journal papers in the area of materials processing. He has also written/edited 18 books/handbooks/conference proceedings, 27 limited distribution reports, and holds 9 patents.