While Al and some Ti are consumed during oxidation, the resulting non-stoichiometric Ti 2AlC phase retains its hexagonal crystal structure 16.
3D KINK CRACKED VERSION CRACK
Cracks in Ti 2AlC can be fully repaired due to the formation of fine-grained Al 2O 3 and some TiO 2 within the crack gap upon high temperature oxidation 4, 15. The self-healing behavior is due to oxidation reactions creating products that bond well to the crack faces and fill cracks with strong reaction product 5. The space group of Ti 2AlC is P6 3/mmc with lattice parameters of a = 3.04 Å and c = 13.60 Å. The octahedral Ti 2C layers are interrupted by layers of pure Al, which forms a Ti 2C-Al-Ti 2C-Al layered structure. The crystalline unit cell of Ti 2AlC contains two sub units 11. In this work monolithic Ti 2AlC MAX phase material was studied. In contrast to many other ceramics, MAX phase materials are tough and therefore damage tolerant and also easily machinable 14. Further, dislocations can multiply and glide on the basal planes of the hexagonal lattice 13, while plastic deformation of polycrystalline MAX phase material typically occurs by a combination of kink and shear band formation, together with the delamination of lamellar grains 7.
Their static strength is maintained up to high temperatures, above which creep becomes the limiting factor 7, 11, 12. The high thermal conductivity also makes these ceramics thermal shock resistant. Due to its layered structure, a MAX phase material exhibits a unique combination of mechanical, thermal and electric properties 7, 8, 9, 10, 11, 12. These ceramics are composed of layered compounds with a M n+1AX n configuration 7 where M is an early transition metal, A is most commonly a group IIIA or IVA element (typically Al or Si) and X is either C or N. Recently, a new class of ternary ceramics, known as MAX-phase metallo-ceramics, was found to have the unique ability to fully and sometimes even repeatedly, heal cracks in a completely autonomous manner when exposed for sufficiently long times to intended high use temperatures in an oxidative gaseous environment 2, 3, 4, 5, 6. without the need to introduce discrete ‘foreign’ healing entities) self-healing ceramics would be ideally suited. For applications that require structural integrity at high temperatures, intrinsic (i.e. The application of these so called ‘self-healing’ materials has the potential to drastically increase the durability and reliability of structural components. In recent years, new types of engineering materials have been developed that can repair internal crack and creep damage autonomously using healing mechanisms based on the physico-chemical nature of the material 1. Our results demonstrate that healed cracks can have sufficient mechanical integrity to make subsequent cracks form elsewhere upon reloading after healing. For the first time, the rate and position dependence of crack repair in pristine Ti 2AlC material and in previously healed cracks has been quantified. Coupling a specialized thermomechanical setup to a synchrotron X-ray tomographic microscopy endstation at the TOMCAT beamline, we captured the temporal evolution of local crack opening and healing during multiple cracking and autonomous repair cycles at a temperature of 1500 K. An attractive feature of this material is its capacity for the autonomous healing of cracks when operating at high temperatures. The Ti 2AlC MAX phase possesses attractive thermomechanical properties even beyond a temperature of 1000 K. MAX phase materials are emerging as attractive engineering materials in applications where the material is exposed to severe thermal and mechanical conditions in an oxidative environment.