Structure and Properties of Large Diameter Hot Rolled NiTi Bars for Seismic Applications

Shape memory alloys have gained attention from civil engineering community over the past decade due to increasing needs for high damping materials to enhance buildings and bridges in active earthquake zones. While seismic design and modeling have been proposed and evaluated, shape memory alloys have not been optimized for these unique applications. Data on this class of materials made for seismic applications remains limited.

For this study, large diameter NiTi bars were manufactured through vacuum induction melting and vacuum arc remelting followed by thermal-mechanical processing. A set of mechanical engineering properties were chosen to evaluate the NiTi bars for seismic design. The as-rolled NiTi bars show good strength and ductility; however they do not exhibit superelastic properties at ambient temperature to meet the requirement for seismic design of SMA-reinforced structures. Further heat treatment at low temperature was introduced and resulted in superelasticity though a significant residual strain persisted. A systematic study on heat treatment was performed and excellent superelastic properties were achieved through optimization of manufacturing process and proper annealing treatment. Microstructure characterization and phase transformation tests confirmed the effect of heat treatment on the mechanical behavior.