Shape Recovery Characteristics of NiTi Foams Fabricated by a Vacuum Process Applied to a Slurry

Abstract

In order to fabricate the NiTi foams with greater than 80% porosity, a vacuum process applied to a slurry was developed. A mixture of elemental Ni and Ti powders was dipped into a solution of 7.5 mass% polyvinyl alcohol, and stirred to make the slurry. The lightly compacted lumps of slurry were then subjected to reduced atmospheric pressure to make foamed compacts. The green foams were debound and sintered under vacuum into NiTi sintered foams with 85% porosity. X-ray analysis showed alloying of NiTi was completed by the sintering at =1100 C. X-ray diffraction analysis and DSC measurement also indicated that the NiTi foams consisted of B2 austenite and B19 0 martensite phases. Measurement of shape recovery strain showed the NiTi foams obtained by this process had the far excellent shape recovery characteristics compared with those of wrought NiTi alloys. Furthermore, repeated compressive deformation and heating greatly increased the shape recovery strains of these high-porosity NiTi foams.

Mechanical Characterization of Superelastic NiTi Nanofoams by Molecular Dynamics Simulations

“…Second, the higher the maximum loading strain, the higher and faster the damage accumulation. This is in line with experimental and our theoretical observations [12,15,17,18,21,30,34,35,37].…”

“…The larger the porosity, the worse the strain recovery [12,15,33]. At larger strains the recovery is decreased but can be enhanced at increased temperatures or with annealing after unloading [35,37]. However, a few studies even find that porous NiTi structures are superior to bulk NiTi in terms of total recovered strain upon unloading [19,37].…”

“…Experimentally, cyclic compressive loading in macroscopic samples was studied with varying number of load cycles [12,15,17,18,21,30,35,37]. After an initial training of 1 to 5 cycles near-perfect superelasticity could be observed for low applied strains.…”