T. Zhen

Spherical Nanoindentations and Kink Bands in Ti 3 SiC 2

We report for the first time on load versus depth-of-indentation response of Ti 3 SiC 2 surfaces loaded with a 13.5 μm spherical tipped diamond indenter up to loads of 500 mN. Using orientation imaging microscopy, two groups of crystals were identified; one in which the basal planes were parallel to, and the other normal to, the surface. When the load-penetration depth curves were converted to stress-strain curves the following was apparent: when the surfaces were loaded normal to the c axis, the response at the lowest loads was linear elastic—well described by a modulus of 320 GPa—followed by a clear yield point at approximately 4.5 GPa. And while the first cycle was slightly open, the next 4 on the same location were significantly harder, almost indistinguishable, and fully reversible. At the highest loads (500 mN) pop-ins due to delaminations between basal planes were observed. When pop-ins were not observed the indentations, for the most part, left no trace. When the load was applied parallel to the c axis, the initial response was again linear elastic (modulus of 320 GPa) followed by a yield point of approximately 4 GPa. Here again significant hardening was observed between the first and subsequent cycles. Each cycle resulted in some strain, but no concomitant increase in yield points. This orientation was even more damage tolerant than the orthogonal direction. This response was attributed to the formation of incipient kink bands that lead to the formation of regular kink bands. Remarkably, these dislocation-based mechanisms allow repeated loading of Ti 3 SiC 2 without damage, while dissipating significant amounts of energy per unit volume, W d , during each cycle. The values of W d measured herein were in excellent agreement with corresponding measurements in simple compression tests reported earlier, confirming that the same mechanisms continue to operate even at the high (≈9 GPa) stress levels typical of the indentation experiments.

Creep rupture induced silica-based nanofibers formed on fracture surfaces of Ti 3 SiC 2

Ti3SiC2 is a representative compound of the Mn+1AXn (or MAX) phases, where M is an early transition metal, A is a group-A element, X is carbon and/or nitrogen, and n 1–3. By now it is fairly well established that Ti3SiC2 possesses good electrical conductivity (4.3 × 10 −1 m) and thermal conductivity (39.9 W/mK), is relatively soft (HV 4 GPa), machinable, damage tolerant, and resistant to thermal shock. Because of its good high-temperature mechanical properties and oxidation resistance, it is a promising candidate for high-temperature structural applications. For such applications, it is paramount to understand its creep response, which has been reported elsewhere. The purpose of this present article, however, is not to report on its creep behavior but to report on interesting features observed on fracture surfaces of samples that were crept at 1300 °C. Coarse-grained (grain size 30–50 m) Ti3SiC2 cylinders (9.8 mm in diameter and 31 mm long) were loaded under a compressive stress of 100 MPa at 1300 °C in air until rupture (for details see Ref. 8). The creep strain at rupture was ∼4%. The sample failed in a shear-like manner, with the fracture surface plane forming an angle of ∼45° to the loading axis. The fracture surface was observed with an FEI-XL30 field emission scanning electron microscope (SEM) equipped with an energydispersive spectrum (EDS) analyzer. Figure 1 shows an SEM micrograph of multiple nanofibers on the fracture surface. A higher magnification micrograph is shown in the inset. The nanofibers have a more or less circular cross-section, and appear as if they were spun from a melt. Their diameter is ∼250 nm, and their lengths vary from a few to over 50 m. Figure 2 is an SEM micrograph of another set of nanofibers, where again the diameter is ∼250 nm. The lower inset magnifies the root of the nanofiber, which is reminiscent of a tree root. The upper inset shows a node in the nanofiber that became charged in the SEM as it was scanned with the electron beam at a slow scan speed, indicating that it most likely is an electric insulator. To determine the chemistries of various features shown in Figs. 1 and 2, individual nanofibers and faceted particles were retrieved from the fracture surfaces and affixed on a conductive carbon tape for EDS analysis. Figures 3(a) and 3(b) show the EDS spectra (with SEM micrographs as insets) of a single nanofiber (a) and a faceted particle (b) taken from the fracture surface, respectively. The quantitative results from the three spectra were summarized in the respective insets in Fig. 3. Excluding the strong C signal, contributed by the carbon tape, it is obvious that the nanofiber contains high concentrations of Si and O, and negligible Ti [Fig. 3(a)]. The faceted particle, on the other hand, contains a high concentration of Ti and O and negligible Si. The reason for the higher O content recorded is not clear at this time, but could be due to the O concentration in the carbon tape adhesive. Nevertheless, on the basis of previous oxidation results, we believe the nanofibers have an SiO2 chemistry and the faceted particles are most probably rutile or TiO2. The EDS spectrum in Fig. 3(c), taken on the plate-shaped grain in Fig. 2 directly on the fracture surface, indicates that the Ti3SiC2 grains were also oxidized. During the first few minutes of oxidation of Ti3SiC2 in air at high temperatures, a thin, ∼600 nm, most probably amorphous, silicon oxide layer forms. Traces of this layer are sometimes found later in the oxidation process. Address all correspondence to this author. e-mail: z.m.sun@aist.go.jp; barsoumw@drexel.edu DOI: 10.1557/JMR.2005.0381