Effects of Thermal Expansion Mismatch Stresses on th Room-Temperature Fracture of Boron Carbid

Calculation of apparent fracture energies (γ_a) using the applied failure stress and fractographically determined flaw sizes (C) in B₄ C shows γ_a decreasing with C , once C < 100 μm. This is attributed to increasing contributions of microstructural stresses (σ_i) due to thermal expansion anisotropy ( 1 × 10⁻⁶°C⁻¹). Extrapolation of this σ_i contribution to C ∼ G (the grain size) and calculation from maximum thermal expansion mismatch are in reasonable agreement. e.g., giving σ∼ 1000 MPa. Strength-mirror size data also show deviations at higher strengths-smaller mirror sizes consistent with both the occurrence and estimated level of such microstructural stress contributions to failure.