Transformation-induced fracture toughening in CuAlBe shape memory alloys: A phase-field study

Stress-induced martensitic transformation ahead of a crack tip can relax the stress concentration and produce fracture toughness in shape memory alloys (SMAs). In this manuscript, we utilize a non-isothermal phase-field model (PFM) to study the martensitic transformation induced crack tip toughening in CuAlBe SMA. The force-displacement curve, transformation zone and high stress zone in single crystalline samples show high dependency on the grain orientation with respect to the crack alignment. Comparison between isothermal and non-isothermal simulations reveals that the transformation-induced self-heating decreases the toughening capability by increasing the critical transformation stress. Investigating the high stress zones in CuAlBe and the non-transforming CuAl alloy shows that the toughening is obtained by redistributing the stress concentration far from the crack tip, as the high stress zone follows the moving tip of the transformation zone. Increasing the acuity of the crack tip is found to generate more symmetric martensite wings on both side of the crack axis, and a more localized high stress zone. The polycrystal specimen displays higher toughening due to the internal constraints related to the presence of various grains with difference orientations. Depending on the orientation of the grain inclosing the crack tip, the toughening effect can be lower or higher. Coincidence of the crack tip with a triple junction is found to improve the toughening behavior.