Effect of temperature and microstructure on tensile and tensile creep properties of Ti₃SiC₂ in air

Miladin Radovic

doi:10.17918/00007687

The ternary carbide, Ti₃SiC₂, combines some of the best attributes of ceramics and metals. It is stable in inert atmospheres to temperatures above 2200 °C, stiff and yet is readily machinable, oxidation, fatigue and thermal shock resistant and damage tolerant. Thus, Ti₃SiC₂ is good candidate material for high temperature structural application. The aim of this work was to characterize its tensile and tensile creep properties. The mechanical behavior of Ti₃SiC₂ is characterized by a brittle-to-ductile (BTD) transition that is a function of strain rate. Its high strain rate sensitivity (~0.5-0.6) is in the range that is more typical for superplastic materials, although it does not exhibit other attributes of superplasticity. Polycrystalline samples do not exhibit linear elastic behavior in tension even at room temperature. Room temperature loading-unloading tests result in closed hysteresis loops when the stress exceeds ~120 MPa, suggesting that the mechanical response can be described as anelastic (viscoelastic). At high temperatures (1200 °C) intense stress relaxation takes place; cycling loading-unloading tests at high temperature and low strain rates, demonstrate that the samples continue to elongate even during unloading, suggesting that Ti₃SiC₂ deforms viscoplastically. Tensile creep curves exhibit primary, steady state and tertiary regimes. The minimum creep rate can be represented by power law equation with a stress exponent of 1.5 for fine-grained (3-5 [mu]m) samples, and 2 for coarse-grained (100-300 [mu]m) ones. For both microstructures the activation energy for creep is ~450 kJ/mol. The dependence on grain size is quite weak, implying that diffusion creep and/or creep mechanisms based on grain boundary sliding do not play a central role. Results of strain transient dip tests suggest that large internal stresses are developed during creep. Those internal stresses are believed to result in recoverable (anelastic) strains during unloading. The response on small stress decreases in strain-transient creep tests suggests that the steady state creep is recovery controlled. Finally, it is important to note that the mechanical response of Ti₃SiC₂ is quite similar to hexagonal ice. Both materials, if loaded rapidly are brittle, but if loaded slowly are quite plastic. This stems from the fact that both are plastically very anisotropic; deformation occurs overwhelmingly by basal slip. In both cases, stress concentrations, and the rate at which they can be relaxed, dictate the nature of the mechanical response.

Effect of temperature and microstructure on tensile and tensile creep properties of Ti₃SiC₂ in air

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Effect of temperature and microstructure on tensile and tensile creep properties of Ti₃SiC₂ in air

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