Measurement of changes in microstructures in metals using doppler-broadened positron annihilation spectroscopy

Abstract

The purpose of this research is 1) to develop Doppler-broadened positron annihilation (DBPA) technique as a non-destructive testing (NDT) technique to evaluate the changes in microstructures in materials and 2) to apply the technique to evaluate sensitization in stainless steels. The developed Doppler-broadened positron annihilation (DBPA) measurement system consists of a 59 mu Ci Na22 positron source, a Canberra GC-1020 HP (Ge) detector in conjunction with a gamma ray spectrometer with the use of two independent units of a biased amplifier and a 4K MCA, 2 energy ranges of 358-567 keV and 1131-1337 keV with a channel factor resolution of about 50 eV/channel was achieved and a computer unit to collect the information. To compensate and/or correct for any statistical errors caused by instrument instability and experimental variations, the compensating technique was proposed and found to satisfactorily reduce the standard deviation to be within +_2%. The developed DBPA measurement was applied to evaluate dislocation density in copper specimens and sensitization condition in stainless steels specimens. It was found that triangle S of copper specimens increases with deformation until about 6.6% when the triangle S begins to decrease. This technique was applied to evaluate sensitization in stainless steels and it was found that triangle S increases as the sensitization time increases although the triangle S at 16 hr seems to slightly decrease after 8 hr. The increasing in the S parameter of sensitized specimen is believed to be related to the precipitation of chromium carbides or chromium depletion at the grain boundaries. Preliminary results indicate that this technique may be used as a non-destructive testing (NDT) technique for measuring the changes in microstructure in metals.