SWELLING AND GAS RELEASE IN A MODEL METALLIC SYSTEM

FAN HSU, Purdue University

Abstract

The swelling and gas release phenomena were studied using pure nickel as a model metallic system. An experimental system was built so that "green" pellets could be sintered at high pressure with a controlled temperature path. High pressure argon was entrapped during sintering, which caused swelling when the ambient high pressure was reduced. Gas release was detected by a quadrupole mass spectrometer. Microstructure evolution during swelling and gas release was examined by scanning electron microscopy. The pore size and form factor distributions were measured by a semi-automatic microcomputer image processing system. The pores were found mostly located and stayed at the grain boundaries. For the first time, the swelling driving force, DF, defined as: DF = p - P - 2(gamma)/r, where p is the internal pore pressure, P is the external pressure, (gamma) is the surface energy, and r is the pore radius, was determined completely from experiments. For the initial swelling stage of low sintering pressures, from 90%TD to 82%TD, a linear relationship was found between the swelling strain rate and the driving force. The activation energy, measured by the temperature cycling technique, was 31 + -2 kcal/mole which is close to the grain boundary diffusion activation energy of 28 (+OR-) 2 kcal/mole. The experimental results supported the grain boundary diffusion controlled cavity growth model proposed by Hull and Rimmer. For the final swelling and gas release stage, abrupt strain rate drop was observed at the peaking of gas release. Higher internal pore pressures had caused earlier gas release at higher densities. The measured amounts of gas release agree well with the calculated values. The swelling strain rates were found to decrease linearly as function of the quantities of released gas.

Degree

Ph.D.

Subject Area

Materials science

COinS