Superelastic Niticu Foams Produced By Cast Replication

Due to their unique strain/shape recovery properties, NiTi-based shape memory alloys (SMAs) are currently used in three main areas: (i) biomedical implants, (ii) actuators, and (iii) high damping materials. All three main areas can be improved by adding porosity to NiTi-based SMAs: (i) for implants, bone growth into the pores and reduced stress-shielding due to the low stiffness, (ii) for actuators, improved heat-transfer and response time due to the increased specific area, and (iii) for damping, improved energy absorption from collapse of pores.

Most processing methods used to create porous NiTi are based on powder metallurgy, resulting in often incomplete densification due to the poor sinterability of NiTi. To overcome this problem, we recently demonstrated a processing method in the liquid state by which a porous preform of high-melting space-holder powders is infiltrated with high-melting Zr-, Ni- and Fe-based melts. Here, we report on the use of SrF₂ as a space-holder to create a replicated Ni₄₀Ti₅₀Cu₁₀ foam by the liquid route. While this space-holder is chemically stable in contact with liquid and solid Ni₄₀Ti₅₀Cu₁₀, it can be removed by dissolution in nitric acid at ambient temperature. For the first time, replication casting has been used to create an open-celled NiTi-based SMA foam, avoiding issues associated with the high reactivity and strong contamination tendency of the melt. By casting NiTi-based SMA foams, it is now possible to economically produce porous biomedical implants, actuators, and high damping materials with complex near-net shapes.