Niobium Wires as Space Holder and Sintering Aid for Porous NiTi

Abstract

Several processing methods exist to create porous NiTi (nitinol) that fulfills engineering applications (e.g., as dampener or actuator) or biomedical requirements (e.g., as implant). [1] Porous NiTi has an unusual combination of properties associated with its porous structure (i.e., low density, low stiffness, high permeability) and the alloy itself (i.e., high corrosion resistance, high strength, excellent biocompatibility, and shape recovery), making it uniquely suited as bone implant. An attractive strategy for producing porous NiTi is based on the combination of temporary space-holders with powder metallurgy methods to achieve control over pore structure and fraction. The most commonly used temporary space-holders for NiTi are non-metallic substances such as polymer, saccharose, [2] NaCl, [2,3] urea, and NH 4 CO 3 , [4] which evaporate during or after the powder consolidation process. These pore-forming space-holders can produce > 30% macroporosity with a 100-400 mm size range optimal for osseo-integration; however, the final microstructure often also displays microporosity with micron-to submicron size due to the Kirkendall effect or insufficient powder densification, which tends to weaken the alloy. Many consolidation techniques such as hot isostatic pressing [3] or the use of elemental Ni and Ti powders [1] have been applied to reduce microporosity, with a trade-off in expense or undesirable intermetallic phases, respectively. In our recent studies [5,6] Nb was identified as a densification enhancer in porous NiTi:mixed NiTi/Nb powders react to create a small volume fraction of wetting eutectic liquid which fills the microporosity between the remaining NiTi powders while NaCl is used as a space-holder to create macropores. The NiTi-Nb solid alloy present after solidification was reported as biocompatible [7] with good damping behavior. [8] In the present work, a novel fabrication strategy for porous NiTi-Nb is demonstrated, based on, but distinct from, our earlier method using NaCl. [5,6] We use here niobium in discontinuous wire form as both space-holder and densification enhancer for NiTi powders. Metallic space-holders have been used previously to create porous alloys, but they have never, to our knowledge, been used also as densification enhancer. For instance, NiTi powders were densified with Mg powders space-holders which evaporated during consolidation [9] and Ti, [10] Ti-6Al-4V [11] or NiTi powders [12] were densified with steel wires which were removed electrochemically to create elongated pores.The Nb wires in the present work were expected to completely react with the surrounding NiTi powders to form a eutectic liquid which then wicked through available capillary channels within the NiTi powder compact to densify it, while leaving the previously occupied wire spaces empty as elongated pores. One of the advantages of the NiTi-Nb system for such a process is the ability to create this transient eutectic liquid at temperatures below both the NiTi and Nb melting points, which reduces production cost...

Active Cooling of a Microvascular Shape Memory Alloy‐Polymer Matrix Composite Hybrid Material

“…[39,40] Bansiddhi et al used Nb wires as space holders in NiTi to create channels during sintering as the Nb reacted with the NiTi to create a eutectic liquid that wicked into the spaces in the NiTi. [41] Aydogmus et al used magnesium (Mg) particles as a sacrificial material to fabricate porous titanium alloys and evaporated the particles during pressureless sintering. [42,43] Modeled on this work, Mg wires were used in the present work as the sacrificial material to create vascular channels in the NiTi SMA.…”

Phase transformation and deformation behavior of NiTi-Nb eutectic joined NiTi wires

“…Bansiddhi and Dunand2223 used liquid NiTi-Nb eutectic phase to bond NiTi powder to create porous NiTi. As metallurgical bonding at contact point improves both strength and stiffness, depositing brazing paste at contact point and dipping woven structure in powder slurry are generally required during specimen preparation2425.…”

Review and perspectives: shape memory alloy composite systems

“…It is clear that, with the increase in volume fraction of SiC particle, the elongation of the composites decreased, while the yield stress and elastic modulus of the composites increased. Based on the previous studies, the influence of the volume fraction of SiC particle on the mechanical properties of the SiC particle reinforced aluminum matrix composites was the same [28,29]. The enhancement mechanism of the SiC particulate was mainly due to the high strength and high modulus [30].…”