Fracture Resistance and Crack Growth Mechanisms in Functionally Graded Ti–TiB

This paper presents the results of fracture tests and crack path observations for a layered functionally graded material (FGM) consisting of Ti and TiB phases. The composition varied in a nearly linear manner from a TiB-rich layer at the bottom to commercially pure (CP) Ti at the top. Elastic properties of the mixed phase interlayers were measured using nanoindentation testing, demonstrating a linear variation with composition. These results differ significantly from approximations calculated in previous studies using a non-linear rule-of-mixtures approach. Fracture tests were conducted on single edge notch bend [SEN(B)] specimens with the notch aligned orthogonal to the direction of the composition gradient. For this crack orientation, "average" R-curve behavior based on the J-integral was investigated to understand the mechanics of crack growth. The value of J was found to be minimal (less than 1 N/mm) below 47 pct volume fraction of TiB compared to a reported value of approximately 150 N/mm for pure Ti. These results indicate that a steeper transition to high concentrations of the metallic phase is necessary to achieve adequate fracture resistance in this metal/ceramic FGM. Observations on the specimen surface indicate crack path toughening mechanisms of this functionally graded material include crack bridging, branching, and deflection.