The strain variation during the isothermal B2→B19’ martensitic transformation in the quenched or annealed Ni51Ti49 alloy

The strain variation during the isothermal holding under constant stress was studied in the quenched or annealed Ni51Ti49 alloy samples. The isothermal strain variation was found in both samples and this strain was completely recovered on subsequent unloading and heating. This allowed to conclude that the strain variation on holding was caused by the isothermal martensitic transformation. It was found that the maximum value of isothermal strain depended on the alloy heat treatment. This value was equal to 0.5 % in annealed sample and it was equal to 6 % in quenched sample. It was assumed that the formation of the Ni4Ti3 phase during annealing led to a decrease in concentration of substitutional Ni atoms in NiTi phase that were responsible for the isothermal transformation. As a result, the less volume fraction of the martensite formed during holding that supresses the strain variation in annealed samples.

Influence of annealing on the functional properties of the NiTi alloy produced by wire arc additive manufacturing

The influence of the annealing temperature on the recoverable strain variation on cooling and heating under a stress of 200 MPa was studied in the NiTi samples produced by wire arc additive manufacturing. The samples including the Ni-rich NiTi layer in the working length were annealed for 10 hours at various temperature from 450 to 600 °C. It is shown that an increase in annealing temperature leads to non-monontonic variation in the recoverable strain. This is caused by an increase in annealing temperature from 450 to 550 °C increases the volume fraction of Ni4Ti3 precipitates. As a result, the volume fraction of the NiTi phase undergoing the martensitic transformation and recoverable strain decrease. An increase in annealing temperature from 550 to 600 °C leads to a dissolving the Ni4Ti3 precipitates and formation of the Ni3Ti2 precipitates that increases the volume fraction of the NiTi phase and the recoverable strain.

The Effect of the Formation of Superelastic NiTi Phase on Static and Dynamic Corrosion Performance of Ni-P Coating

The addition of superelastic NiTi particles is a great benefit to the toughness of the Ni-P coating. Nonetheless, NiTi nanopowder costs 10 times more than Ti nanopowder. Therefore, in the present study, to reduce the cost, Ni-P-NiTi composite coatings were prepared on AISI 1018 steel substrates by the electroless incorporation of Ti nanoparticles into Ni-P followed by the annealing of Ni-P-Ti coatings. The effect of the formation of a superelastic NiTi phase on static and dynamic corrosion performance was investigated. It was found that the annealed Ni-P-Ti coating (i.e., Ni-P-NiTi coating) has much higher static corrosion resistance than the as-deposited Ni-P coating. The dynamic corrosion rates in the absence of abrasive particles are 10 times higher than the static corrosion rates of the coatings. The dynamic corrosion rates in the presence of abrasive particles are one order of magnitude higher than the dynamic corrosion rates in the absence of abrasive particles. The formation of a superelastic NiTi phase considerably improved the static and dynamic corrosion performance of the Ni-P coating. In the absence of abrasive particles under flowing condition, the dynamic corrosion resistance of the annealed Ni-P-Ti coating (i.e., Ni-P-NiTi coating) is 19 times higher than that of the as-deposited Ni-P coating. In the most aggressive environment (in the presence of abrasive particles), the dynamic corrosion resistance of the annealed Ni-P-Ti coating (i.e., Ni-P-NiTi coating) is four times higher than that of the as-deposited Ni-P coating. The annealed Ni-P-Ti coating (i.e., Ni-P-NiTi coating) can be used in applications where high corrosion resistance is required, especially in an extremely aggressive environment.

Smart 4D-printed implants and instruments

Selective laser melting (SLM) was used to manufacture smart programmed structures with customized properties made of biocompatible NiTi shape-memory alloy. A series of helixes was produced with systematically varied SLM process parameters Laser Exposure Time and Laser Power in order to specifically change the thermo-mechanical material properties of the 3D-structures. This innovation opens up the possibility to adjust the NiTi phase transformation temperature during the manufacturing process. This controllable property determines which of the two crystallographic phases martensite or austenite is present at a certain operating temperature and allows the mechanical properties to be adjusted: martensitic devices are soft and pseudo-plastic due to the shape-memory effect, whereas austenitic structures are pseudo-elastic. In a further step, the SLM process parameters were locally varied within 4Dprinted twin-helixes. As a result, the phases, respectively the mechanical properties of a single component were adjusted at different locations. The ratio of elastic to plastic deformation and the spring constant of the helix can be locally controlled. This allows, for example, the spatio-temporal programming of 3D-printed surgical instruments or implants that are stimuliresponsive.

Effect of Ti Content on the Microstructure and Corrosion Resistance of CoCrFeNiTix High Entropy Alloys Prepared by Laser Cladding

In this paper, CoCrFeNiTix high entropy alloy (HEA) coatings were prepared on the surface of Q235 steel by laser cladding. The microstructure, microhardness, and corrosion resistance of the coatings were studied. The mechanism of their corrosion resistance was elucidated experimentally and by first-principles calculations. The results show that CoCrFeNiTi0.1 adopts a face-centered cubic (FCC) phase, CoCrFeNiTi0.3 exhibits an FCC phase and a tetragonal FeCr phase, and CoCrFeNiTi0.5 adopts an FCC phase, a tetragonal FeCr phase, and a rhombohedral NiTi phase. The FCC phase, tetragonal FeCr phase, rhombohedral NiTi phase, and hexagonal CoTi phase are all observed in the CoCrFeNiTi0.7 HEA. The alloys assume the dendritic structure that is typical of HEAs. Ni and Ti are enriched in the interdendritic regions, whereas Cr and Fe are enriched in the dendrites. With increasing Ti content, the hardness of the cladding layers also increases due to the combined effects of lattice distortion and dispersion strengthening. When exposed to a 3.5 wt.% NaCl solution, pitting corrosion is the main form of corrosion on the CoCrFeNiTix HEA surfaces. The corrosion current densities of CoCrFeNiTix HEAs are much lower than those of other HEAs. As the Ti content increases, the corrosion resistance is improved. Through X-ray photoelectron spectroscopy (XPS) and first-principles calculations, the origin of the higher corrosion resistance of the coatings is connected to the presence of a dense passivation film. In summary, the corrosion resistance and mechanical properties of CoCrFeNiTi0.5 alloy are much better than the other three groups, which promotes the development of HEA systems with high value for industrial application.

Investigation of the Mechanical Behavior of Electroless Ni–P–Ti Composite Coatings

To improve the toughness of Ni–P coatings, NiTi superelastic particles were introduced into the Ni–P matrix through the electroless co-depositing of Ni–P and Ti particles and annealing Ni–P–Ti coatings. The mechanical properties of the coatings were determined through bend testing bilayer specimens and tensile testing the standalone coating. The effects of Ti content and annealing on Young’s modulus, toughness, and fracture strength were investigated. After annealing, the toughness and strength improved considerably. The formation of the superelastic NiTi phase after annealing led to the improvement of toughness and fracture strength of the composite coating through transformation toughening, crack deflection, bridging, and shielding. Different toughening mechanisms interacted with each other and operated together. This contributed to the enhancement of toughness and fracture strength.

Single Particle Erosion Behavior of NiTi-Based Nanolaminates and Superelastic NiTi Monolayer Coatings

Bulk NiTi is used to make parts, such as couplings and bearings, that can be found in many industries such as the automotive, aerospace and medical sectors. Forming and machining bulk superelastic NiTi is a very difficult and costly process; however, applying NiTi as a surface coating will provide an alternate manufacturing method that will minimize machining processes. The objective of this study is to produce a superelastic NiTi-based surface coating that exhibits denting, impact and wear resistance. Superelastic NiTi has been successfully produced through vacuum deposition processes, despite this, there is a lack of a full and comprehensive study on the formation of the NiTi phase during coating development. In this study, the NiTi phase is fabricated through the annealing of sputtered deposited Ti and Ni layers in a coating. To confirm the presence of the intermetallic phases, X-ray diffraction (XRD) and energy dispersive spectrometry (EDS) analysis were performed. The erosion behavior of the coating is evaluated through single particle erosion testing, which resulted in the coatings that contained the NiTi precipitates to exhibit the best damage resistance compared to the other nanolaminates. This indicates that the superelastic NiTi phase increases the resistance to impacting particles. Microstructural evolution and NiTi formation during annealing is discussed and related to the observed damage resistance of the coatings.

Properties of NiTi Shape Memory Alloy Micro-Foils Obtained by Pulsed-Current Sintering of Ni/Ti Foils

Successful one-step manufacturing of micro-foils of NiTi shape memory compound by pulsed-current sintering of nickel and titanium is reported. Sandwich-like starting configurations of Ni/Ti/Ni (ST1, ST4), Ti/Ni/Ti (ST3), and a simple Ni/Ti (ST2) one, were used. XRD and differential scanning calorimetry (DSC) measurements revealed multistep martensitic transformation, much more pronounced for ST1 than for ST2 and ST3. SEM/energy dispersive X-ray spectrometer (EDS) measurements showed the predominant NiTi phase in ST1, ST4, and other intermetallic compounds in addition to it, for ST2 and ST3. The temperature dependence of the electrical resistance for ST4 shows a peak corresponding to the R-phase and a high residual resistivity. The shape memory effect of 100% was obtained for ST1 and ST4, with the temperature range of its recovery dependent on the initial strain. The ST2 and ST3 materials revealed brittleness and a lack of plasticity due to the dominancy of the austenite phase and/or the intermetallic compound content.