The aging-effect-modulated mechanical behavior in U-Nb shape memory alloys through the modified twinning-detwinning process of the α″ phase

Abstract

The deformation behavior in the U-Nb shape memory alloys (SMAs) is significantly sensitive to the aging effect, where the underlying mechanism remains unclear. In the present work, in-situ neutron diffraction is used to investigate simultaneously the macroscopic and microscopic stress-strain behaviors as well as the microstructure evolution in α″-martensitic U-Nb SMAs. The aging-effect-modulated mechanical properties are achieved through the modified twinning-detwinning process. The initial yielding strength is significantly increased from about 330 to 650 MPa at room temperature, while from approximately 440 to 700 MPa at 100 °C. The in-situ observations show that the strength enhancement arises from the aging-induced resistance during the twinning-detwinning process. High resolution neutron diffraction patterns indicate that the α″ phase structure is unchanged after aging. The aging effect is therefore understood by employing the defect migration mechanism, which explains well the enhanced yielding strength and twinning-detwinning resistance. It also suggests that the controllable properties could be realized by engineering the defect migration process and thus tuning the aging effect. These findings might shed some light on materials design and properties controlling for α″-martensitic U-Nb and other SMAs (e.g., Ni-Ti). Keywords: Shape memory alloys, Aging effect, Micromechanism, Twinning-detwinning, Properties tailoring