Effect of grain orientation on the compressive response of highly oriented MAX phase Ti₃SiC₂

The MAX phases comprise of a group of layered ternary carbides that exhibit unique mechanical properties which bridge the gap between their metal and ceramic constituents. To study the effects of the global grain orientation, Ti, Si and TiC powders were hot pressed to synthesize highly oriented bulk Ti₃SiC₂. X-ray diffraction (XRD) was used to verify the grain orientation and a Lotgering factor of 0.87 with respect to the c-axis was obtained. Prepared Ti₃SiC₂ samples have been compressed in two orientations, loading along the c-axis (∥c-axis) and perpendicular to the c-axis (⊥c-axis) at 10⁻³ s⁻¹ using a standard load frame and at 10² s⁻¹ using a Kolsky (split-Hopkinson) bar. The average compressive strength along the ⊥c-axis orientation was 761 MPa under quasi-static conditions and 987 MPa under dynamic loading, exhibiting a 30% increase on average. The ∥c-axis orientation exhibited no rate dependence in compressive strength; however both orientations exhibited an increase of strain at failure under dynamic conditions by over 0.5%, on average. The orientation-dependent failure behavior at different strain rates were examined using high-speed imaging and 2D digital image correlation (DIC) during loading and via scanning electron microscopy (SEM) post-mortem. Results indicate that the ⊥c-axis fracture surface exhibited a mixture of transgranular and intergranular cracks, kink bands and delaminations, whereas ∥c-axis was limited to a combination of intergranular and transgranular cracks. Such fracture distinctions due to the availability (or lack thereof) for kink band formation appear to be responsible for the anisotropic compressive behavior.