SPEAKER:
Professor Michel W. Barsoum, Distinguished Professor in the Department of Materials Science and Engineering at Drexel University
ABSTRACT:
In this, the second of a five part series, I will focus on the effects of grain size and composition on the mechanical properties of the MAX phases at room temperature. Because every basal plane is a potential slip plane, active at all temperatures, together with the, predominantly, metallic character of the bonding in these phases, their room temperature mechanical properties are quite unique. With c/a ratios range that range from 4 to 8, non-basal dislocations or twins have ever been implicated in their deformation, which implies that only 2 independent basal slip systems are operative at all times. It follows that the MAX phases are plastically – not but necessarily elastically – extremely anisotropic. Instead, like most other layered materials such as graphite, mica, etc., kink band formation is their preferred mode of deformation under compression.
The ramifications of this fact are far reaching and used to explain much of the response of the MAX phase to stress. It explains, among other observations, their exceptional damage tolerance, fracture toughness values of the order of 12 to 15 MPa√m, and the large anisotropy between their compressive and tensile strengths. When loaded in compression, spontaneously and fully reversible stress-strain curves are observed. Some MAX phases can be compressed to stresses as high as 1 GPa and fully recover upon removal of the load, while dissipating 25% of mechanical energy. Solids that behave in this fashion have been classified as kinking non-linear elastic, KNE. A sufficient condition for a solid to be KNE is plastic anisotropy. The ubiquity and implications of KNE solids will be highlighted.