It is shown that electron microfractography can be applied successfully to the study of high temperature, high strain rate fracture. Although the fracture surfaces exhibited some oxidation and thermal etching, the quality of the fractographs was adequate to identify the operative fracture modes for a variety of alloys and test conditions. Three alloys were studied: (1) a nickel base superalloy (Inconel 718), (2) an austenitic stainless steel (AISI 304), and (3) a low carbon steel (AISI 1008). High strain rate, hot tension tests were carried out in vacuum or in argon after direct heating to the test temperature and after cooling from a reheating temperature typical of actual hot rolling operations.
The hot ductility, as indicated by the reduction in area at fracture, is described in terms of various temperature-ductility ranges which cover incipient melting as well as normal hot working temperatures. Electron fractography was combined with metallographic observations to obtain a more detailed description of this hot ductility behavior. A variety of fracture modes were observed depending on the alloy and test conditions with examples of transgranular shear rupture, intergranular shear rupture, and brittle intergranular decohesion. Despite the high deformation temperatures, some of the fracture surfaces exhibited areas of cleavage and brittle interphase separation resulting from crack initiation at second phase particles. A description of the specific fracture modes in each temperature-ductility range is given for each alloy, and the reasons for the observed variations in hot ductility are discussed.