Formation Of NiTi Research Articles

Multi-material wire arc additive manufacturing of a bio-inspired heterogeneous layered NiTi/Nb/Ti6Al4V structure: Microstructural evolutions and mechanical properties

Obtaining a reliable NiTi–Ti6Al4V(TC4) heterogeneous alloy component exhibiting excellent strength and elasticity via integrating distinct material benefits can provide design flexibility in multifunctional device fabrication. However, the fabrication of this materials couple is challenging due to the development of excessive Ti2Ni intermetallic compound. Inspired by the bioheterogeneous layered structure, Nb interlayer was applied for the wire arc additive manufacturing (WAAM) of NiTi and TC4 to restrict the mixing of alloying elements on both sides. However, the strength of NiTi–Nb-TC4 alloy component was limited by pure Nb characteristics when the NiTi and TC4 was completely separated by the unmolten Nb interlayer. In this work, the tensile strength perpendicular to the interface of NiTi–Nb-TC4 alloy component was enhanced to introduce appropriate amount Ni and Ti in Nb interlayer by adjusting the composite WAAM process at the expense of Ti2Ni. Microstructure evolution, composition change and mechanical properties of composite WAAM NiTi–Nb-TC4 alloy component were investigated. It was shown that the compression strength and fracture strain perpendicular to the interface of WAAM NiTi–Nb-TC4 sample was 1549.0 ± 17 MPa and 22.3 ± 8 %, and the compressive strength can reach 63.6 % and 94.5 % of WAAM NiTi alloy and TC4 alloy, respectively. The composite WAAM NiTi–Nb-TC4 sample maintained 65.3 % recovery rate after ten compression cycles, which confirmed that partial superelasticity was maintained in the WAAM NiTi–Nb-TC4 multilayer structure. This fact further proved that WAAM can be successfully applied to deposit shape memory/superelastic alloys. The use of the Nb interlayer enhanced the tensile strength perpendicular to the interface to 460.7 ± 16 MPa. The ultimate tensile strength perpendicular to the interface of composite WAAM NiTi–Nb-TC4 sample can reach 72.0 % and 60.0 % of WAAM NiTi alloy and TC4 alloy, respectively. The formation of NiTi, TiNb and β-Nb phases in Nb interlayer and the network distribution of Ti2Ni and TiNb phases explained the improved mechanical performance of the composite WAAM NiTi–Nb-TC4 alloy component.

Characterization of NiTi Shape Memory Alloy Sintered at Different Temperatures in Reducing Environment

NiTi has received significant interest as medical implant materials due to its shape memory effect behavior apart from its good biocompatibility and mechanical properties. The formidable challenge of obtaining single phase NiTi from elemental powders via solid state is due to oxidation problem of elemental powders and the oxygen atoms dissolve in NiTi matrix as interstitial impurities forming stable oxygen-rich TiNiOx. This may deterioriate the shape memory behavior of NiTi. This research investigates the use of MgH2 in combination with CaH2 as in-situ reducing agent to eliminate oxidation of the specimen during sintering both at lower and higher sintering temperatures. Here, the effect of sintering temperature on phase formation and transformation behavior of NiTi in reducing environment was studied. The phase formation was characterized by using x-ray diffraction (XRD) where the morphology and elemental analysis were characterized by using the scanning electron microscope (SEM) equipped with EDS. The martensitic transformation behavior was analyzed using differential scanning calorimeter (DSC). The use of MgH2 and CaH2 as reducing agent has a significant influence on the phase formation of NiTi synthesized via solid state especially at 930 °C, where almost single phase NiTi was formed with good transformation behavior. This reducing agent creates a conducive environment for the production of single phase NiTi.

Ball milling for the formation of nanocrystalline intermetallic compounds from Ni-Ti elemental powders

Abstract In the present research work nickel (Ni) and titanium (Ti) elemental powder with an ostensible composition of 50% of each by weight were mechanically alloyed in a planetary high energy ball mill in diverse milling circumstances (10, 20, 30 and 60 h). The inspection exposed that increasing milling time leads to a reduction in crystallite size, and after 60 h of milling, the Ti dissolved in the Ni lattice and the NiTi (B2) phase was obtained. The lattice strain of ball milled mixtures augmented from 0.15 to 0.45 at 60 h milling. With increase in milling time the morphology of pre-alloyed powder changed from lamella to globular. Annealing of as-milled powders at 1100 K for 800 s led to the formation of NiTi (B19′), grain growth and the release of internal strain. The result indicated that this technique is a powerful and highly productive process for preparing NiTi intermetallic compounds with a nano-crystalline structure and appropriate morphology.

The Interface Reaction between Titanium and Iron-Nickel alloys

AbstractThe reaction zones between pure titanium and iron-nickel and pure nickel at 1173 K have been characterized. Two alloys with different initial composition were analyzed. When Ni80Fe20 is used the layer sequence at the reaction interface is: Ni80Fe20 → Ni3Ti → NiTi → NiTi2 →Ti, while for Ni48Fe52 the sequence is: Ni48Fe52 → Ni3Ti → TiFe2 → TiFe → NiTi2 → Ti. The difference is in formation of NiTi, TiFe2 and TiFe phases. The reaction zone remains very thin independently of time due to the fast dissolution rate of Ti in the alloy. The two different elementary chemical interaction processes have been identified in this article, namely the growth of the reaction layers by solid state diffusion and chemical conversion of the compounds by reaction-diffusion in the solid state. The mathematical description combining these processes is presented for description of the diffusion path generated during diffusion process.

Production of Nanostructured Ni-Ti-Ag Alloy by Mechanical Alloying

Because of corrosion resistance and antibacterial effects, shape memory Ni-Ti-Ag alloy can be considered for different biomedical applications. Mechanical alloying is used to produce nanostructured Ni-Ti-Ag alloy from elemental powders. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) are used to characterize the product. Results show that after 1h milling, homogenous distribution of the elements occurs; while no intermetallic compounds is observed. After 3h milling, titanium dissolves in nickel to form amorphous and nanostructured solid phases. Peaks of B2 phase appear in the XRD pattern after the 3h milling of the powder mixture. Sintering of the 3h-milled mixture at 1223 K leads to the formation of NiTi intermetallic compound; while titanium disappears, the content of the element nickel reduces and grain growth takes place.

Mechanochemical assisted synthesis of NiTi intermetallic based nanocomposite reinforced by Al2O3

Abstract Synthesis of NiTi/α-Al 2 O 3 nanocomposite by mechanical alloying (MA) of a powder mixture containing NiO, Al, Ti and Ni was investigated. The structural evaluation of milled and annealed powders was studied by X-ray diffraction, scanning electron microscopy and microhardness measurements. The mechanochemical reaction between NiO and Al took place shortly in a combustion mode leading to increase of vial temperature and formation of NiTi, Al 2 O 3 and a small amount of Ti 2 Ni which transformed to NiTi at further MA times. NiTi/Al 2 O 3 nanocomposite exhibited larger grain size of NiTi, higher microhardness and smaller particle size compared to NiTi. Annealing of the milled powder showed that the products are dependent on the cooling rate. The presence of Al 2 O 3 dispersoids in the NiTi matrix leads to decrease of grain growth and ordering degree of NiTi and finally high thermal stability of nanocomposite during annealing.