Ni-rich NiTi Research Articles

First-principles investigation on the adsorption and dissociation of O2 and H2O molecules on the Ni-rich TiNi alloy surface

The excellent corrosion resistance of Ti-Ni binary alloy highly depends on the thin passive films spontaneously formed on the surface. For the Ni-rich 60NiTi, the TiNi3 phase would coexist with the B2 TiNi matrix phase after the final heat treatment, affecting the formation of passive films. In this study, the adsorption and dissociation of O2 molecule on the TiNi (110) and TiNi3 (110) surface are investigated by the DFT method to have an insight into the distinction of oxidation behavior between TiNi and TiNi3 phase. Subsequently, the interaction of H2O molecule with the most stable O2 adsorption configuration is further studied in order to clarify the atomistic evolution process when the oxidized surface is exposed to the wet air and erosion solution. The calculated results implied that the O2-TiNi/H2O system exhibits the physisorption nature of the H2O molecule. However, the H2O molecule, when interacted with the O2-TiNi3 configuration, can be dissociated into OH group and H atom. These then were captured to create the Ti-O, Ni-O and H-O bond. This study may provide the useful details involved in the formation of oxide/alloy interface on the Ni-rich TiNi alloy as well as the interaction of oxide/alloy with the water solution.

Microstructural and Mechanical Response of NiTi Lattice 3D Structure Produced by Selective Laser Melting

Nowadays, additive manufacturing (AM) permits to realize complex metallic structural parts, and the use of NiTi alloy, known as Nitinol, allows the integration of specific functions to the AM products. One of the most promising designs for AM is concerning the use of lattice structures that show lightweight, higher than bulk material deformability, improved damping properties, high exchange surface. Moreover, lattice structures can be realized with struts, having dimensions below 1 mm—this is very attractive for the realization of Nitinol components for biomedical devices. In this light, the present work regarded the experimental characterization of lattice structures, produced by selective laser melting (SLM), by using Ni-rich NiTi alloy. Differential scanning calorimetry (DSC), electron backscatter diffraction (EBSD), and compression testing were carried out for analyzing microstructure, martensitic transformation (MT) evolution, and superelasticity response of the SLMed lattice samples. The lattice microstructures were compared with those of the SLMed bulk material for highlighting differences. Localized martensite was detected in the nodes zones, where the rapid solidification tends to accumulate solidification stresses. An increase of martensitic transformation temperatures was also observed in lattice NiTi.

Thermal activation of stress-induced martensitic transformation in Ni-rich Ti-Ni alloys

We performed compression tests at low temperatures under various strain rates for Ni-rich Ti-Ni alloys. Thermal activation theory for the habit plane motion with a thermal activation energy of ~0.5 eV successfully reproduced both the examined temperature and strain-rate dependences of the transformation stress hysteresis. This indicates that the thermal activation process of the habit plane motion underlies the dramatic increase in the transformation hysteresis at lower temperatures in this system. These findings are critical to elucidate the physical impact of the thermal activation nature on the transformation hysteresis and to consider the feasibility of low-temperature operation of shape-memory functions.

Assessment of Small-Scale Deformation Characteristics and Stress-Strain Behavior of NiTi Based Shape Memory Alloy Using Nanoindentation

The study is focused on appreciating the micro-mechanisms that govern the deformation behavior of a Ni-rich NiTi alloy at the sub-micron scale. This alloy undergoes reversible stress-induced-martensitic-transformation leading to pseudoelasticity. Correspondingly, significant strain recoverability is noted. In this investigation, nanoindentation is utilized as the primary experimental tool. Various parameters including the indenter tip configuration, size and applied load levels are varied systematically. It is observed that compared to the most commonly used sharp Berkovich indenter tip, blunt spherical tip imposes reduced amount of strain and strain-gradient within the indentation volume. This opens up the window for systematically varying the sequential deformation modes in NiTi. Further in-depth analysis revealed that optimum combination of indenter tip configuration, size and applied load level is prerequisite to appreciate the pseudoelastic mechanism in NiTi. Most importantly, a tailored and simplified protocol is formulated for converting nanoindentation load-displacement response to corresponding indentation-stress-strain behavior for pseudoelastic alloy. These indentation-stress-strain curves featuring the signature deformation characteristics of NiTi alloy are realized without utilizing any specialized experimental modes. In a nutshell, this study highlights the importance of choosing adequate nanoindentation parameters for assessing the functional characteristics of pseudoelastic NiTi system at sub-micron scale. Accordingly, overall localized deformation behavior of this unique alloy is appreciated.

Mechanical and elastocaloric effect of aged Ni-rich TiNi shape memory alloy under load-controlled deformation

This study investigated the elastocaloric effect of Ni-rich Ti49.6Ni50.4 shape memory rods under compression. As-received rods, rods aged at 300 °C for 1 h, and rods aged at 275 °C for 125 h were investigated to reveal the effects of aging on the elastocaloric properties. These samples were first trained under constant stress of 1200 MPa or 1500 MPa, which decreased the B19′ transformation temperature and stabilized the R-phase transformation. Associated with the changes in transformation temperatures, the pseudoelasticity performance and elastocaloric effect of the samples decayed with increasing training times and then became stable when the training time exceeded a certain value. The sample aged at 275 °C for 125 h, which was most hardened by the precipitation hardening effect, exhibited better stability during the training process due to less plastic deformation and thus retained an obvious elastocaloric cooling effect. Because the precipitation treatment broadened the transformation range of the sample aged at 275 °C for 125 h by introducing a two-stage B19’→R→B2 reverse transformation, the elastocaloric cooling effect of 7 °C could be utilized at a relatively large temperature range of −20 °C to 100 °C.

Effects of Ni and Cr addition on the wear performance of NiTi alloy

Development of hard materials for structural and bearing applications is a challenging task due to complexities in materials processing. Nickel-based intermetallics are a choice for the wear resistance applications. In this study, in order to examine the effects of Ni and Cr addition on the wear performance of Ni-rich alloys, 60Ni, 55Ni-5Cr, 50Ni-5Cr, and 50Ni-10Cr (at.%) alloys were developed through arc melting technique. The aim was to conduct a microstructural and phase analysis study and their effects on hardness and wear behavior. Samples were solution treated at 900 °C for 5 h under vacuum followed by equilibrium cooling. Microstructural and phase analysis revealed the presence of B2 cubic NiTi austenite along with Ni-rich hexagonal Ni3Ti and Cr-rich tetragonal Cr3Ni2 phases throughout the microstructure of alloys. The Cr developed phase was not present in binary composition because of Cr absence. Ternary compositions possessed higher hardness and wear resistance than the binary alloy. Hardness and wear resistance of 50Ni-5Cr and 50Ni-10Cr were comparable with tool steels and conventional 440C martensitic stainless steel bearing material; this high hardness and wear resistance was attributed to the strong solid solution strengthening and precipitation hardening effects of Cr along with the combined effect of Ni-rich Ni3Ti and NiTi austenite phases. Improvement in hardness and wear resistance were correlated with the Cr addition in the developed alloys.

Evolution of crystallographic orientation, precipitation, phase transformation and mechanical properties realized by enhancing deposition current for dual-wire arc additive manufactured Ni-rich NiTi alloy

Ni-rich NiTi alloys were deposited using the in-situ alloying wire arc additive manufacturing (WAAM) method, with varying deposition currents from 80 A to 120 A. The effects of deposition current on the crystal orientation, precipitation, phase transformation and mechanical properties of the WAAM-deposited NiTi alloys were investigated. The results show that increasing the deposition current during the WAAM process would result in noticeable coarsening of B2 grain and an increased volume fraction of high angle grain boundaries (HAGBs). Also, the texture intensity gradually decreased with increasing deposition current. The fabricated components are dominated by the B2 phase with quantities of Ni4Ti3 precipitates in all samples. When increasing the deposition current during the WAAM process, the size of Ni4Ti3 precipitates generally increased and gradually decomposed into a stable Ni3Ti phase which could be detected in the sample produced at 120 A. Furthermore, all of the characteristic phase transformation temperatures increased with the deposition current while the ultimate tensile strength dropped from 927.9 MPa to 613.8 MPa and elongation reduced from 8.7 % to 5.6 %. The cyclic loading-unloading tests revealed that similar trends for the evolution of irreversible strain (εir), recoverable strain (εre), recovery ratio, and elastic energy storage efficiency (η) during cycling were obtained in all samples processed with different deposition currents. The highest εre of 3.2 % and the highest recovery ratio of 53.9 % were obtained in the sample processed with 80 A at an applied stress of 700 MPa for ten cycles. The change of mechanical properties with varying deposition current is due to a combination of factors including precipitation hardening effect, grain refinement effect, and crystal orientation. These results can be useful for optimizing WAAM process parameters to fabricate NiTi components with acceptable structural properties.

Effect of low-energy proton on the microstructure, martensitic transformation and mechanical properties of irradiated Ni-rich TiNi alloy thin films

Effect of low-energy proton on the microstructure, martensitic transformation and mechanical properties of irradiated Ni-rich TiNi alloy thin films

Effect of Hf solute addition on the phase transformation behavior and hardness of a Ni-rich NiTi alloy

Hf substituted NiTi alloys are crucial for high temperature shape memory applications owing to their martensitic phase transformation temperatures around 100°C. For further high temperature applications above this limit, a balanced combination of high phase transformation temperatures, high strength and low thermal hysteresis is the foremost criterion. Here, we show that substitution of an array of Hf concentrations (5–25 at.%) to Ni50.3Ti49.7 alloy leads to a wide range of phase transformation temperatures from sub-zero to 280°C. Our studies establish complete solid solution solubility of Hf in the Ni50.3Ti49.7 alloy up to 25 at.% Hf. As a result, the hardness Ni50.3Ti49.7 alloy increases linearly with Hf concentration on account of solid solution strengthening effect. We further show that the Ni50.3Ti29.7Hf20 alloy exhibits least thermal hysteresis combined with high phase transformation temperatures and hardness amongst the compositions investigated. Our study paves the way for designing Ni-rich NiTiHf high temperature shape memory alloys, with operating temperatures up to 250°C and thermal hysteresis as low as 30°C.

Fabrication of step functionally graded NiTi by laser heating

This paper investigates the application of localized laser heating to produce a step pseudoelastic behavior of a cold worked Ni-rich Ti-50.9 at%Ni alloy. The localized heating was done using a custom-made laser heating apparatus. The aim is to create step graded martensitic phase transformation plateaus with large stress window and good recoverability. A cold-worked NiTi wire was locally heated using laser at 400 °C for 10 min, 475 °C for 30 min and 525 °C for 20 min along its length. These temperature and duration settings were carefully chosen based on isothermal heat treatment of the same Ti-50.9 at%Ni wire. Under tensile load, the wire exhibited deformation in a sequence of stress plateaus (step deformation), starting from the segment heated at the highest temperature and ending at the segment heated at the lowest temperature. The step gradient stress plateau produced 310 MPa and 270 MPa stress windows on the forward and reverse transformations, respectively. The residual strain on the recovery was about 1%. This novel heat treatment technique provides the possibility to produce a step pseudoelastic behavior of Ni-rich NiTi alloys to improve the mechanical responses of this smart material in actuator applications.

Martensite stabilisation effect in Ni-rich NiTi shape memory alloy with different structure and martensitic transformations

The aim of the present work was to study the martensite stabilisation effect in Ni51Ti49 alloy with different structures and martensitic transformations. The quenched Ni51Ti49 alloy underwent the B2→B19’ transformation, whereas the B2→R→B19’ transformation occurred in the annealed sample. The martensite stabilisation effect was studied after deformation in the B19’ state in both samples, and in the R state in the annealed sample. It was found that despite the sample structure, no plastic strain appeared during deformation up to 7%, however, the martensite stabilisation effect was observed in both the quenched and annealed samples. If deformation occurred due to the reorientation of the R phase, the martensite stabilisation effect was negligible and did not exceed 1 °C. If the stress induced B19’ phase appeared during deformation in the R state, the martensite stabilisation effect occurred. It was found that the shift in the reverse transformation temperatures on heating of the pre-deformed sample, depended on the structure of the B19’ phase. It was assumed that the damage to the interface coherency resulted in the appearance of an additional energy barrier that influenced the finish temperature of the reverse transformation. Furthermore, it was believed that the martensite reorientation during deformation changed the stored elastic energy, and the shift in the start temperature of the reverse transformation was due to the change in the stored elastic energy and loss in the interface coherency.

Wire and arc additive manufacturing of a Ni-rich NiTi shape memory alloy: Microstructure and mechanical properties

Wire and Arc Additive Manufacturing (WAAM) was used for fabrication of NiTi parts using a commercialy available Ni-rich NiTi wire as the feedstock material. The as-built parts are near fully austenitic at room temperature as confirmed by differential scanning calorimetry, X-ray diffraction and superelastic cycling. The as-built microstructure changed from collumnar, in the first deposited layers, to equiaxed in the last deposited ones as a result of the different thermal cycle conditions. This is the first work where WAAM NiTi parts exhibit superelastic behavior under tensile conditions, highlighting the potential use of the technique for the creation of parts shaped in a complex manner based on this material and process. The potential to use WAAM for deposition of advanced functional materials is demonstrated.

Surface Integrity of Ni-Rich NiTi Shape Memory Alloy at Optimized Level of Wire Electric Discharge Machining Parameters

The concern of this experimental work is to study the surface integrity aspects such as surface morphology, three-dimensional surface topography, recast layer, phase analysis, and micro-hardness for Ni55.95Ti44.05 shape memory alloy at the optimized level of wire electric discharge machining parameters. A mathematical model was developed for surface roughness and material removal rate considering servo voltage, pulse on time, wire tension, wire feed rate, and pulse off time using response surface methodology technique. In order to obtain the optimized parameters, multi-objective optimization technique grey relation analysis was utilized. The adequacy of the developed model was also checked by analysis of variance. At optimal parameters setting, i.e., pulse on time 123 µs, pulse off time 58 µs, servo voltage 50 V, wire tension 3 N, and wire feed rate 5 m/min, maximum material removal rate (8.223 mm3/min) and minimum surface roughness (1.93 µm) were achieved. Surface characteristics of machined surface divulge the presence of discharge craters, debris, molten droplets, micro-voids, spherical nodules, and cracks. A recast layer of thickness 19 µm with approximately 21% of foreign elements was deposited on the machined surface at optimized parameters, whereas the micro-hardness of the outer machined surface was found to be increased approximately 1.98 times as compared to micro-hardness of bulk material. X-ray diffraction analysis shows the presence of the following compounds on the machined surface NiTi, Ni4Ti3, Ti4O3, Cu5Zn8, Ni(TiO3), and NiZn.

Microstructural effect on the thermomechanical behavior of aged Ni-rich NiTi SMA

In this study, we investigate the microstructural effect on the thermomechanical behavior of an aged 50.6 at% Ni NiTi shape memory alloy. The thermal and structural characterizations at zero-stress were performed by means of Differential Scanning Calorimetry (DSC) and x-ray Diffraction (XRD) analyzes, supported by Scanning Electron Microscopy (SEM) observations. It was reported that increasing the aging temperature induces a significant change in microstructural features and thermal properties such as transformation process, transformation temperatures and transformation rate. The precipitation phase kinetic as function of aging temperature was also discussed. In order to understand the influence of the observed microstructural changes on the NiTi mechanical behavior, the stress-induced martensitic transformation was studied through an isothermal compression tests. The hardness and the superelastic stress-strain loops were strongly affected by the microstructural behavior. The correlation between thermal and mechanical behaviors was carried out by the construction of the critical stress-temperature diagram. Two different transformation processes were selected to study the effect of the Ti3Ni4 precipitation phase on the global NiTi thermomechanical behavior. The obtained results can be very useful to predict the occurrence of the shape memory and superelastic properties for an aged Ni-rich NiTi alloy.

Microstructural characterization and quantitative phase analysis of Ni-rich NiTi after stress assisted aging for long times using the Rietveld method

The present work was aimed to quantitatively analyze the phases in the Ni-rich NiTi shape memory alloy which was aged at different temperatures of 470, 500 and 530 °C for 11, 38, 196 and 536 h. The aging treatment was done while applying various stress values. The phase transformation behavior of prepared specimens was analyzed using differential scanning calorimetry (DSC) and X-ray diffraction (XRD) at ambient temperature and 75 °C. The weight percent of the different phases was measured by the Rietveld refinement method using MAUD software. The quantitative phase analysis and transmission electron microscopy (TEM) images show that the amount of Ni4Ti3 precipitates is increased in the specimens aged under stress. Furthermore, the Ni4Ti3 metastable precipitates are transformed to Ni3Ti2 and Ni3Ti stable phases after a long period aging under 120 MPa for 536 h and the various variants of precipitates are not affected by applied stress after long periods of aging.

Tribologically induced amorphization in the subsurface of aged Ni-rich TiNi alloy during dry sliding

The microstructure modification due to friction energy transformation in near-surface zone is crucial to the tribological performance of materials. To study the surface and subsurface response of materials exhibiting stress-induced martensitic transformation to repeated sliding, we have investigated the quite novel microstructure changes induced during the dry sliding of an aged Ti-51.5 at.% Ni alloy against the GCr15 spheres. In the process of reciprocating sliding, the B2 austenite primarily transforms into amorphous phase in the worn subsurface area. Remarkably, B2 clusters of nanocrystals below 10 nm are randomly embedded in the amorphous ribbon in the topmost region whereas the medium-range-ordered debris in the order of 3–5 nm are retained in the deeper layer. More importantly, it can be found that large number of nanocrystalline phase and amorphous structure coexist in the strip at a depth about 2.5 μm. Additionally, XRD patterns of the worn tracks show the appearance of B19’, being different from the B2 in the original microstructure. The martensitic transformation therefore plays a crucial role in accommodating associated plastic deformation for the formation of an amorphous layer. The distinctive elementary mechanisms identified here will be essential for the future modeling of the tribologically deformed layer of the alloys.

Effect of Ni content and Hf addition on the unlubricated wear performance of Ni-rich NiTi alloys

Due to the combination of high hardness, high strength, excellent corrosion resistance, superelasticity and low elastic modulus, Ni-rich NiTi alloys have recently been considered as a promising alloy system for using in bearing and related applications. In this study, in order to examine the effects of the Ni content and Hf addition on the unlubricated wear performance of the Ni-rich NiTi alloys, both 52Ni, 55Ni, 52Ni-5Hf and 55Ni-5Hf (at%) alloys were fabricated. The microhardness test results indicate that 55Ni and 55Ni-5Hf samples exhibited higher hardness than the 52Ni and 52Ni-5Hf samples, respectively, this is attributed to the higher volume of the Ni4Ti3 phases in the higher Ni-rich alloys. In addition, the Hf addition further improves the hardness of the alloys. The wear test results also confirm the superior wear performance of the 55Ni-5Hf alloys than that of the 55Ni and 52Ni-5Hf alloys. The SEM observations from the worn surfaces demonstrated the same wear mechanisms of these tested alloys, which are mainly abrasive wear. Hence, it can be deduced that both the higher Ni content and Hf addition are beneficial to obtain better wear performance.

A hidden single-stage martensitic transformation from B2 parent phase to B19′ martensite phase in an aged Ni51Ti49 alloy

The aged Ni-rich NiTi shape memory alloys (SMAs) exhibit the multi-stage martensitic transformation (MMT), which has important influences on functional properties and practical applications of the NiTi SMAs. A hidden single-stage martensitic transformation from B2 parent phase to B19′ martensite phase is found in an aged Ni51Ti49 alloy, which happens concurrently with a commonly observed two-stage martensitic transformation B2–R–B19′ (R: martensite phase) and actually composes one stage of a multi-stage martensitic transformation (MMT) together with the two-stage one. B2–B19′ martensitic transformation occurs in the NiTi matrix containing Ni4Ti3 precipitates with relatively large inter-particle space, while B2–R–B19′ transformation takes place in the NiTi matrix with Ni4Ti3 precipitates having relatively small inter-particle space.

Orientation dependence of elastocaloric effect in an aged Ni-rich Ti-Ni alloy

Elastocaloric effects caused by the stress-induced B2-R and the reverse martensitic transformations (B2: the parent phase, R: the martensite phase) were studied by using an aged single crystal of Ti-50.8Ni shape memory alloy including coherent precipitates of Ti3Ni4. Inverse elastocaloric effect was observed when tensile stress is applied in the [100]B2 direction, while conventional one was observed in the [111]B2 direction. This result implies that transformation strain in the [100]B2 direction is negative for any variants.

Coexistence of martensite and strain glass phases in homogenized Ni-rich TiNi shape memory alloys

The transport and thermal properties of homogenized Ti50-xNi50+x shape memory alloys (SMAs) in the range of 1.0 ≤ x ≤ 1.3 (in at. %) by means of electrical resistivity, Seebeck coefficient, thermal conductivity, and heat capacity measurements are presented. From the electrical resistivity measurement, we have observed thermal hysteresis in cooling and subsequent warming curves for x = 1.1 to 1.2 suggesting the occurrence of a martensitic transition. The martensitic transition is also detected in the Seebeck coefficient, thermal conductivity, and heat capacity measurements in these alloys. Despite this, the x = 1.2 alloy shows a negative temperature coefficient of resistivity over the entire temperature range under investigation, indicating the formation of strain glass phase at low temperatures. The existence of the strain glass state is confirmed by the frequency dispersion of the storage modulus measurement. These observations suggest a coexistence of martensite and strain glass phases in a well-controlled composition of the Ti50-xNi50+x system with 1.0 ≤ x ≤ 1.3. It is revealed that the crossover from martensite to strain glass in Ti50-xNi50+x SMAs is mediated via a coexisting of these two phases, which are triggered by a competing interaction between the thermodynamic and kinetic processes at low temperatures.