Acoustic Emission in Magnesium and Titanium.

An investigation of the acoustic emission produced during deformation of AZ31B magnesium and alpha-titanium was carried out in order to identify the deformation processes responsible for the emission. Both magnesium and titanium specimens were deformed at constant strain rate while simultaneously measuring the load on the specimen, extension of the sample, RMS voltage, energy rate, event rate, ringdown count rate, signal amplitude, and for some titanium tests, the test temperature. Magnesium exhibited two different types of emission curve. The first consisted of two maxima, the first in the elastic region and the second at the onset of plastic deformation. Data analysis revealed emission in the elastic region to be due to twinning, while the emission association with plastic deformation was found to be due to dislocations. The second emission curve consisted almost entirely of burst emission where each emission burst was observed to coincide with a small drop in stress. This type of emission was found to be due primarily to dislocations. Acoustic emission in titanium was also found to be divisible into two categories. The first was found to occur in specimens where the onset of plastic deformation was followed by flow and hardening. Emission in these tests was composed of a yield emission maximum followed by increasing emission at higher strain. Dislocations were found to be the principal source of acoustic emission in these tests. The second type of emission was found in tests which did not show hardening. No yield emission maximum was observed, and emission was generally found to decrease with increasing strain rate and with decreasing test temperature. This behavior is completely counter to that expected for a dislocation emission source, and emission in these tests was found to be due to twinning. Twinning was observed to be accompanied by burst -type emission in magnesium and titanium, and additionally showed relatively high energy per event ratios. Emission from dislocations was primarily continuous and produced low energy per event ratios.