High-Temperature Cyclic Fatigue-Crack Growth in Monolithic Ti3SiC2 Ceramics

identical slip planes) and walls or kink boundaries. Thus, in addition to regular slip, mechanisms for ambient temperature The cyclic fatigue behavior of reactive hot-pressed Ti3SiCz plastic deformation in Ti3SiCz are thought to involve the ceramics are examined at temperatures from ambient to 1200°C readjustment of local stress and strain fields from kink band with the objective of characterizing the high-temperature (boundaries) formation, buckling and delamination of individual mechanisms controlling crack growth, Comparisons are made of grains, the delamination and associated damage being contained two monolithic Ti3SiCz materials with fine(3-10 pm) and coarseby the kink boundaries (6). The delaminations typically occur at grained (70-300 pm) microstructures. Results indicate that the the intersection of the walls and arrays and result in the &, fatigue thresholds are not substantially changed between 250 annihilation of the latter (6). It iS this containment Of damage that and 1 1000~; however, there is a s h q decrease in mth at 12000~ is believed to be the major source of damage tolerance in Ti3SiC,; (above the “ductile-brittle” transition temperature), where in fact, the plastic behavior in general is unusual for carbides and significant high-temperature deformation and damage are first is believed to be due to its layered structure and the metallic apparent. Of the two microstructures, the coarse-grained Ti3SiCz nature of the bonding (6). exhibits substantially higher cyclic-crack growth resistance at both with Such intrinsic deformation and toughness properties, Ti3S1C2 ambient and elevated temperatures. This results from an clearly offers Some Potential for many structural applications; enhanced effect in the coarser grained microstruc~e of crack however, if this is to be realized, it iS important that some bridging in the crack wake from both grains and lamellae within aSSeSSment be made Of its crack-growth behavior, Particularly at grains, and from the correspondingly more tortuous crack path. elevated temperatures. It is the objective of the present paper to examine the subcritical crack-growth characteristics of Ti3SiC2 Introduction under cyclic loads, in both fineand coarse-grained microstructural conditions and at temperatures from ambient to 120ooc, with the objective of defining the salient damage and Crack-tiP shielding n~chanisms that govern crack advance. The polycrystalline ternary carbide, Ti3SiCz, exhibits a surprising combination of properties for a ceramic; for example, it displays high toughness (Kc > 8 MPadm) and a ratio of hardness (-4 GPa) to elastic modulus (-320 GPa) more typical of a ductile material. Background Processed by reactive hot-pressing techniques from powders of Ti, sIc and graphite, T ~ ~ s ~ c ~ also shows a range of inelastic The fracture toughness and cyclic fatigue-crack growth behavior deformation modes not typically Seen in cermics at room of monolithic Ti3SiCz at ambient temperatures was first temperature (1-6), including grain bending, grain buckling, and characterized by Gilbert et al. (7) in both fine(3-10 Pm) and significant amounts of basal slip. Indeed, in general Ti3SiCz coarse-grained (50-200 Pm) Conditions. FatiWe-crack growth appears to be one of the most damage tolerant of all nonthresholds, M t h , Were found to be as high as 6 and 9 MPadm, transforming monolithic ceramics (1-7). respectively, in the fine and coarse-grained structures; In Ti3SiCz, basal plane dislocations are mobile and multiply at corresponding fracture toughnesses, K, were measured (at the room temperature. They are confined to two orthogonal peak of the R-curve) at, respectively, 9.5 and 16 MPadm. The directions: basal plane arrays (wherein the dislocations exist on high toughness (the K, value for the coarse-grained structure is Fatigue L Fracture Behavior of High Temperature Materials Edited by P.K. Liaw TMS (The Minerals, Metals & Materials Society), 2000