Structure Evolution and Residual Elastic Stresses in a Ti-Ni-Cu-Co Glassy/Crystalline Phase Alloy

Recently, nanocrystalline Ti-Ni-Cu-Co system alloys were found to have excellent stability of superelastic behaviour for elastocaloric applications. One of the factors limiting implementation of TiNi-based alloys in microelectromechanical systems is residual elastic stresses. In this work, the structure formation and stress evolution in the Ti50Ni22Cu22Co6 glassy/crystalline ribbon are studied on heating. The as-cast alloy has a glassy structure in the middle part of the ribbon while the surface crystals of textured B2 phase are found. However, due to the sharp [100] texture and B2 phase stabilizing effect, the only partial recrystallization takes place on subsequent heating. The x-ray diffraction studies indicated that the elastic stresses in the B2 phase reach a value of similar to 750 MPa near the surface plane. After isothermal annealing at crystallization temperature, the level of residual stresses reduced by a factor of 2. The origin of elastic stresses induced in the glassy/crystalline matrix is discussed.

Automated summary

Nanocrystalline Ti-Ni-Cu-Co system alloys exhibit excellent stability of superelastic behavior, but residual elastic stresses remain an issue in their implementation in microelectromechanical systems. By studying the structure formation and stress evolution, it was found that elastic stresses in the B2 phase can be reduced by half through isothermal annealing.