Nanoindentation Studies On Precipitation Hardening of Ti-Rich TiNi Shape Memory Ribbons

The precipitation hardening in Ti₅₁Ni₄₉ melt-spinning ribbons prepared by rapid solidification process with thickness ≈ 33μm is investigated by nanoindenter. TEM microstructural observations show large amounts of nano-scale Ti₂Ni precipitates in TiNi matrix with the size of about 20-50nm. R phase transformation can be induced by the coherent strain field surrounding these precipitates, as verified by DSC and XRD results. Nanoindentation tests indicate that, the as-spun ribbon exhibits the highest hardness value and its hardness continuously decreases when the ribbons are aged at 500°C for longer time. The nano-scale Ti₂Ni precipitates can effectively strengthen the TiNi matrix by obvious increment in the hardness. Shape memory effect (SME) test of the ribbon aged at 500°C for 3h shows outstanding reversible strain, 5.10%, under the stress of 98MPa, which suggests that the strengthening effect by nano-scale Ti₂Ni precipitates can significantly promote the shape memory properties in TiNi shape memory ribbons.