The M n +1AX n , or MAX, phases are layered, hexagonal, early transition-metal carbides and nitrides, where n = 1, 2, or 3 "M" is an early transition metal, "A" is an A-group (mostly groups 13 and 14) element, and "X" is C and/or N. In every case, near-close-packed M layers are interleaved with layers of pure group-A element with the X atoms filling the octahedral sites between the former ( Figure 1.1 a–c). The M6X octahedra are edge-sharing and are identical to those found in the rock salt structure. The A-group elements are located at the center of trigonal prisms that are larger than the octahedral sites and thus better able to accommodate the larger A atoms. The main difference between the structures with various n values ( Figure 1.1 a–c) is in the number of M layers separating the A layers: in the M2AX, or 211, phases, there are two; in the M3AX2, or 312, phases there are three; and in the M4AX3, or 413, phases, there are four. As discussed in more detail in later chapters, this layering is crucial and fundamental to understanding MAX-phase properties in general, and their mechanical properties in particular. Currently, the MAX phases number over 60 ( Figure 1.2 ) with new ones, especially 413s and solid solutions, still being discovered.
Figure 1.1 Atomic structures of (a) 211, (b) 312, and (c) 413 phases, with emphasis on the edge-sharing nature of the MX6 octahedra.
Figure 1.2 List of known MAX phases and elements of the periodic table that react to form them.
Most of the MAX phases are 211 phases, some are 312s, and the rest are 413s. The M group elements include Ti, V, Cr, Zr, Nb, Mo, Hf, and Ta. The A elements include Al, Si, P, S, Ga, Ge, As, Cd, In, Sn, Tl, and Pb. The X elements are either C and/or N.
Thermally, elastically, and electrically, the MAX phases share many of the advantageous attributes of their respective binary metal carbides or nitrides: they are elastically stiff, and electrically and thermally conductive. Mechanically, however, they cannot be more different: they are readily machinable – remarkably a simple hack-saw will do ( Figure 1.3 ) – relatively soft, resistant to thermal shock, and unusually damage-tolerant. They are the only polycrystalline solids that deform by a combination of kink and shear band formation, together with the delaminations of individual grains. Dislocations multiply and are mobile at room temperature, glide exclusively on the basal planes, and are overwhelmingly arranged either in arrays or kink boundaries. They combine ease of machinability with excellent mechanical properties, especially at temperatures 1000 °C. Some, such as Ti3SiC2 and Ti4AlN3, combine mechanical anisotropy with thermal properties that are surprisingly isotropic.
Figure 1.3 One of the hallmarks of the MAX phases is the ease with which they can be machined with (a) a manual hack-saw and (b) lathe.
As discussed in this book, this unusual combination of properties is traceable to their layered structure, the mostly metallic – with covalent and ionic contributions – nature of the MX bonds that are exceptionally strong, together with M–A bonds that are relatively weak, especially in shear. The best characterized ternaries to date are Ti3SiC2, Ti3AlC2, and Ti2AlC. We currently know their compressive and flexural strengths and their temperature dependencies, in addition to their hardness, oxidation resistance, fracture toughness and R-curve behavior, and tribological properties. Additionally, their electrical conductivities, Hall and Seebeck coefficients, heat capacities (both at low and high temperatures), elastic properties and their temperature dependencies, thermal ex