The Max Phases: Ductile, Machinable Ternary Carbides And Nitrides For High Temperature And Other Applications

Abstract:

The MAX phases – layered ternary carbides and nitrides with the general formula Mn+1AXn (MAX), where n=1, 2, or 3, M is an early transition metal, A is an A-group element (mostly IIIA and IVA), and X is either C or N – represent a new class of solids that posses an unusual, and sometimes unique, combination of properties that makes them good candidate materials for high temperature applications. Like ceramics, they are elastically stiff, have relatively low thermal expansion coefficients, good thermal and electrical conductivities, and are resistant to chemical attack. However, mechanically they cannot be more different: they are relatively soft (Vickers hardness 2–8 GPa) and most readily machinable, thermal shock resistant, and damage tolerant. Moreover, some are fatigue and creep, and oxidation resistant. This talk summarizes over 14 years of research and our current understanding of the mechanical properties of the MAX phases in general, and Ti3SiC2 and Ti2AlC in particular as two of the most studied MAX phases. At higher temperatures, they go through a brittle-to-plastic transition. At room temperature, they can be compressed to stresses as high as 1 GPa and fully recover upon removal of the load, while dissipating 25% of the mechanical energy. Special emphasis will be given to their non-linear kinking behavior and underling incipient kink band (IKB) mechanism. Since the MAX phases are potentially good candidate materials for high temperature structural applications, their elastic, stress-strain and creep behavior at elevated temperatures (up to 1400 oC) will also be reviewed.

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