The fracture of field-ion microscope specimens

Abstract

The premature failure of field-ion microscope specimens, either during preparation or in the microscope, has been investigated using scanning and transmission electron microscopy. Specimen materials were tungsten, a tungsten-rhenium-hafnium-carbon alloy, molybdenum, a molybdenum TZM alloy, iridium, a nimonic alloy, and titanium carbide. Four distinct kinds of endform have been found. Many specimens show evidence for classical brittle failure while others appear to have failed in a ductile manner. In some cases it appears that local heating or even melting has occurred. When only a small amount of material is removed in the fracture event, an almost perfect field-evaporation endform can result. Some materials have been found to fail in more than one way according to the prior treatment of the specimen. The behaviour is correlated with the mechanical stresses induced by the electric field in the field-ion microscope and during specimen preparation. The causes and avoidance of failure are discussed.