147. DENSIFICATION MECHANISM AND MECHANICAL PROPERTIES OF TUNGSTEN POWDER CONSOLIDATED BY SPARK PLASMA SINTERING

Department: Mechanical & Aerospace Engineering
Faculty Advisor(s): Joanna M. McKittrick

Primary Student
Name: Geuntak Lee
Email: gel003@ucsd.edu
Phone: 858-945-7035
Grad Year: 2017

Abstract
The densification mechanism of tungsten consolidated by spark plasma sintering is revealed by applying the constitutive equation of the continuum theory of sintering at a temperature range from 1600 C to 1800 C and pressure of 60 MPa and 120 MPa. The strain rate sensitivity m is 0.38 to 0.40 and sintering activation energy of about 410.84KJ/mol is obtained. Also, the activation energy for the sintering was obtained by analyzing the densification rate and grain size from the two different temperature holding stage sintering curve, and is 431.04KJ/mol which is the similar activation energy obtained by constitutive equation fitting. Comparison of activation energy for various diffusion path indicates sintering is governed by grain boundary diffusion. By the m and activation energy value, the main mechanism of W compaction can be grain boundary sliding (GBS) controlled by dislocation climb, which dislocation move along the grain boundary. The superplastic behavior of W during the SPS compaction is also supported by EBSD microstructure analysis of the compacted specimens. The texture is removed with increasing holding time. This indicates grain rotation takes place which is the main effect of GBS near the temperature of ~0.5Tm. The density of the compacted pellets reached ~ 81~95% relative density depending on the sintering temperature and holding time. The crystal structure, chemical composition, porosity, grain size, and micro-hardness are evaluated by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy and Vickers micro-hardness machine. Carbon uptake during the spark plasma sintering results in the formation of tungsten carbide at the shell of the compact. Boron nitride is utilized to try to remove the carbon diffusion from the die. The tungsten carbide thickness grows in time and diffusion coefficient of carbon in the tungsten was obtained.

Industry Application Area(s)
Aerospace, Defense, Security | Materials | Sintering

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