ARTICLES nature materials | VOL 2 | FEBRUARY 2003 | www.nature.com/naturematerials 107 S lip by dislocation motion is the prevalent micromechanism of plastic deformation in almost all crystalline materials. It is widely recognized that crystalline materials that exhibit a large multiplicity of easy glide slip systems (for example, face-centred and body-centred cubic metals) demonstrate significant ductility, and that dislocation activity in these materials is irreversible, that is, it is not possible, in general, to return the material to its initial microstructural state. It is also well known that materials that exhibit limited amounts of slip are generally brittle (for example, ceramics), which hinders their use in a number of engineering applications. Most known crystalline materials fall into one of the two categories described above. We have encountered a class of materials that exhibit extensive slip, but on a limited number of easy glide slip systems. This new class of layered ternary compounds, with a general formula of M n+1 AX n (where n = 1 to 3, M is an early transition metal,A is an A-group element, and X is C and/or N) is now referred to as MAX phases in the literature 1–4 .There are roughly 50 M 2 AX phases 2 ; three M 3 AX 2 (Ti 3 SiC 2 , Ti 3 GeC 2 and Ti 3 AlC 2 (ref. 3)) and one M 4 AX 3 , Ti 4 AlN 3 (ref. 4). In previous work 1,5–16 , we have reported on the exceptional thermal shock resistance and damage tolerance shown by these materials. In this paper, we report unique characteristics of the mechanical response of Ti 3 SiC 2 —perhaps the most significant ones observed to date in this interesting class of materials—in simple compression cyclic tests conducted at room and at higher temperatures. It is strongly believed that the unique characteristics of the mechanical behaviour of Ti 3 SiC 2 reported in this paper can be attributed largely to the following known facts about this material: (i) Basal slip, and only basal slip, is operative at all temperatures 11,12 . (ii) Because they are confined to the basal planes, dislocations arrange themselves either in arrays (pile-ups) on the same slip plane, or in walls (tilt and twist boundaries) normal to the arrays 11,12 . Dislocation interactions, other than orthogonal, are difficult and unlikely to occur. Hence dislocations can move back and forth reversibly and extensively.(iii) Because of their high c /a ratios, twinning is unlikely, and has never been observed. Instead, deformation occurs by a combination of glide and the formation of kink bands within individual grains 11–13,15 . The kink bands mentioned above were first observed 17 in Zn single crystals loaded parallel to their basal planes. Kinking is distinct from slip Dislocation-based deformation in crystalline solids is almost always plastic. Here we show that polycrystalline samples of Ti 3 SiC 2 loaded cyclically at room temperature, in compression, to stresses up to 1 GPa, fully recover on the removal of the load, while dissipating about 25% (0.7 MJ m –3 ) of the mechanical energy. The stress–strain curves outline fully reversible, rate-independent, closed hysteresis loops that are strongly influenced by grain size, with the energy dissipated being significantly larger in the coarse-grained material. At temperatures greater than 1,000 °C, the loops are open, the response is strain-rate dependent, and cyclic hardening is observed. This hitherto unreported phenomenon is attributed to the reversible formation and annihilation of incipient kink bands at room-temperature deformation. At higher temperatures, the incipient kink bands dissociate and coalesce to form regular irreversible kink bands. The loss factor for Ti 3 SiC 2 is higher than most woods, and comparable to polypropylene and nylon. The technological implications of having a stiff, lightweight machinable ceramic that can dissipate up to 25% of the mechanical energy per cycle are discussed. Fully reversible, dislocation-based compressive deformation of Ti 3 SiC 2 to 1 GPa M. W. BARSOUM* †1 , T. ZHEN* 1 , S.R. KALIDINDI 1 , M. RADOVIC 2 AND A. MURUGAIAH 1 1 Department of Materials Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USA 2 Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA *These authors contributed equally to this work † e-mail: barsoumw@drexel.edu Published online: 26 January 2003; doi:10.1038/nmat814 © 2003 Nature Publishing Group