A test of grain boundary void growth theories by small angle neutron scattering

Over the past 25 years a large number of theories have been put forth to explain the growth of grain boundary cavities during creep. A number of attempts have been made to verify experimentally various predictions of one or another of these theories. To this end use has been made of transmission, scanning and optical microscopy as well as precision density measurements. Frequently the functional dependence of the time to failure on creep stress or temperature has been investigated in order to determine the role of plasticity in cavitation. While these studies have been very valuable, they have all been somewhat deficient in one or more of the following requirements for accurate experimental determination of the kinetics of void growth: ability to detect voids when they are very small and when they are present in small quantities; ability to separate void nucleation from growth; ability to survey large numbers of voids in the measurements. Recently it has been shown that, for certain model systems, these requirements can be met through the use of small angle neutron scattering (SANS). Voids down to a few nanometers in size can be ''seen'' easily with SANS. Void volume fractions of 10-6 or less can be detected. In the present experiment some 106 to 109 voids contribute to the scattering in each measurement. It is possible to invert the scattering data to obtain size distributions of the voids. A knowledge of the evolution of these distributions as creep proceeds permits accurate testing of the various cavity growth models to be carried out. This paper describes some experimental details of a SANS study of creep cavitation in copper, explains modeling of the results using two theories of cavity growth, and compares the calculated and measured size distributions. A more complete description of the SANS investigation of creep cavitation in copper (including a study of the consequences of prefatiguing) is in preparation