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Microstructure and defects strongly affect martensitic transformations in metallic alloys. Significant progress has been made in understanding the atomic-level processes that control the role of grain boundaries and precipitates in these solid-to-solid phase transformations. Yet, the role of dislocations and their structures on martensitic transformation temperature and the resulting microstructure remains unclear. Therefore, we used large-scale molecular dynamics simulations to study the forward and reverse transformation of a martensitic material modeled after Ni63Al37 under cyclic thermal loading. The simulations reveal that dislocations in the austenite phase act as one-dimensional seeds for the martensite phase, which is present at temperatures significantly above the martensite start value. We find a reduction in the dislocation density during cyclic thermal loading, which results in the increase in martensite and austenite transition temperatures, in agreement with experiments. Importantly, we extracted a critical martensitic nuclei size for developing stable domains and found that relatively low dislocation densities are needed to grow independent martensitic variants resulting in a multi-domain structure. [ABSTRACT FROM AUTHOR]

To explore a new method to improve the wear resistance of TiNi shape memory alloy (SMA), Ti-50.8Ni alloy was treated by the method of ultrasonic surface shot peening. The microstructure evolution, hardness, and tribological behaviors have been further investigated to evaluate the effect of ultrasonic surface shot peening (USSP). The surface microstructure can be refined to some extent while the basic phase composition has little change. USSP can facilitate the martensitic transformation in the surface layer, which benefits improving the surface hardness. Additionally, the hardness of Ti-50.8Ni alloy increases first and then decreases with the increase of applied load, but the USSP-treated alloy tends to be more sensitive to load. USSP treatment can improve the wear resistance and reduce the coefficient of friction (COF) in case of a low sliding wear speed of 5 mm/s. However, the tribological properties of USSP-treated alloy are reversely worse in the case of 10 mm/s. This is mainly attributed to the combined effect of stress-induced martensite transformation and degeneration resulting from the frictional heating during the dry sliding wear process. [ABSTRACT FROM AUTHOR]

In the present investigation, continuous stiffness measurement (CSM) using nano-indentation technique has been employed to measure the hardness and strain rate sensitivity (SRS) parameter of Ti-6Al-2Mo-4Zr-2Sn (AM-Ti6242) alloy. The measured properties have been subjected to exploratory data analysis and interpreted based on local microstructure, composition and micro-texture. Very interestingly, the chemically homogeneous microstructure showed mechanical heterogeneity. The presence of the martensite phase and formation of the stress-induced nano-twins in the AM Ti6242 have been found to influence its nano-mechanical behaviour in a completely different way compared to the non-additively manufactured Ti6242. [ABSTRACT FROM AUTHOR]

A novel method of preparing continuous NiTif‐reinforced Ti/Al‐laminated composites is proposed via ultrasonic consolidation (UC) and vacuum hot pressing. The microstructure of the composites is characterized by X‐ray diffraction, scanning electron microscopy, and electron backscattering diffraction. The tensile properties of the composites are measured by a universal testing machine. Results indicate that the composite exhibits multilayer structure consisting of residual NiTi fiber, intermetallic compounds (IMCs) layer, reaction layer, Ti layer, and Al layer. The intermetallic layer is composed of Al3Ti and Al3Ni, and some Ni‐rich precipitates are dispersed in the matrix. And with the introduction of UC assisted, grain refinement occurs at the IMC/Al interface, texture variation emerges at the NiTi fiber/IMCs interface, and a weakening of texture strength is observed at both interfaces. Compared to NiTif‐reinforced Ti/Al‐laminated composites without UC assisted, both the tensile strength (from 872.4 to 826.5 MPa) and failure strain (from 17.5% to 20.4%) of NiTif‐reinforced Ti/Al‐laminated composites with UC assisted increase due to the contribution of UC assisted. The reinforcing and fracture mechanisms of the UC assisted are also discussed. [ABSTRACT FROM AUTHOR]

The specific damping capacity variation of heat-treated NiTi was observed during a pseudoelasticity test. The detailed B2 → R-phase transformation process in cold-drawn NiTi wires undergoing middle-temperature aging was studied via X-ray diffraction, transmission electron microscope, and internal friction technique. Results show that, as aging time increased at 450 °C, the dynamic phase transition splitting from B2 → R to B2 → R1 and B2 → R2 became evident. However, such a splitting process was not observed for the sample after aging at 400 °C. The reason for R-phase generation is attributed to non-uniformly distributed stress fields. The splitting of the internal friction peak, in conjunction with high-resolution transmission electron microscope and mechanic results, suggests a substantial occurrence of short-range segregation of Ni atoms in the B2-NiTi matrix. Furthermore, the specific damping capacity (SDC) exhibits a gradual increase with prolonged annealing time. Specifically, the sample with significant dynamic phase transition splitting reaches an SDC value of 0.60. [ABSTRACT FROM AUTHOR]

Nickel‐titanium alloys are produced via laser metal deposition using titanium carbide and titanium diboride as nucleating agents to refine the grain structure. The optimal process parameters are determined by analyzing the quality of the alloy formation. X‐ray diffraction confirms the simultaneous presence of the B19′ martensitic phase and B2 austenitic phase in the nickel‐titanium alloy fabricated by laser metal deposition. Tensile testing is performed on the samples at room temperature. The results show that adding mass fractions 0.9 % titanium carbide and titanium diboride effectively refines the grain size of the nickel‐titanium alloy fabricated by laser metal deposition, leading to improved tensile properties. Specifically, the addition increases the tensile strength by 40 % and 70 %, and the elongation by 50 % and 60 %. [ABSTRACT FROM AUTHOR]

High-pressure torsion (HPT) was applied to the Ni50.6Ti49.4 (at. %) alloy ingot up to 1/4, 2, 16, 32 and 48 turns under a pressure of 6.0 GPa. The samples were examined by X-ray diffraction (XRD), transmission electron microscope (TEM) and microhardness measurement. The results indicate that martensitic transformation and formation of amorphous phase occurred during the HPT process. As the HPT turns increased, more amorphous phase formed. The fraction of amorphization was analyzed based on the X-ray results. The microhardness increased with the HPT turns, which may be related to strain-induced martensite transformation, formation of the amorphous phase, increased dislocation densities and grain refinement. Differential scanning calorimetry (DSC) test revealed that shape memory alloys can be produced by HPT and post-HPT annealing from a NiTi ingot. [ABSTRACT FROM AUTHOR]

The actualization of high strength and ductility in alloys, in addition to providing strong, formable materials, can lead to reduced weights in practical applications. However, increasing strength typically comes at the cost of lowering the ductility and vice-versa, referred to as the strength–ductility trade-off. In this work, we investigate the thermo-mechanical response of a 3-element multifunctional NiTi–Nb nanocomposite material that overcomes this trade-off, as it exhibits a high strength of 980 MPa and an ultrahigh ductility of 58% at fracture. The remarkable properties are attributed to the underlying microstructure of Nb nanofibers dispersed in an NiTi matrix. Deformation is accommodated via the shape memory transformation of the active NiTi matrix in concert with elastoplastic deformation of Nb nanofibers embedded within the matrix. Consequently, the material exhibits multifunctionality and recovers deformation during heating via the reversion of the stress-induced martensitic transformation in the NiTi matrix. The high strength and high ductility of this 3-element nanocomposite material puts it amongst the best performing high-entropy alloys (HEAs) that are typically made up of five or more elements. [ABSTRACT FROM AUTHOR]

Achieving grain refinement in the wire-arc directed energy deposition (DED) process of NiTi alloy is highly challenging. We have successfully obtained NiTi alloy with a fine microstructure and observed an improvement in superelastic recoverability by introducing an ultrasound-assisted field during the deposition process. The results show that the average grain size of the NiTi alloy samples decreased from 147.3 to 72.2 µm. Additionally, the recoverable strain of the samples increased significantly from 3.7% to 4.4%. This is attributed to the ultrasonic cavitation and acoustic streaming phenomena during the ultrasound-assisted process, which promotes the columnar-to-equiaxed transition (CET) during the solidification process. Fundamentally, the key to the enhancement of recoverable strain after ultrasound-assisted is that the formation of equiaxial grains provides more strain compatibility to the grain boundaries, which reduces the formation of dislocations during deformation. [ABSTRACT FROM AUTHOR]

In this work, the interaction of an additively produced Ti-4Al-3V titanium alloy with a nickel superalloy tool and the features of the stir zone formation during friction stir processing have been studied. The stop-action technique was used to produce the samples to be studied using optical and scanning electron microscopy methods, as well as microhardness measurements. As a result, it was revealed that the tool, when moving, forms a pre-deformed area in front of it, which is characterized by a fine-grained structure. The presence of an interface layer between the workpiece material and primary fragmentation by the tool was revealed. It was demonstrated that the transfer of titanium alloy material occurs periodically following the ratio of feeding speed to tool rotation rate. Metal flow around the tool can occur in both laminar and vortex modes, as indicated by the tool material stirred into the transfer layer and used as a marker. [ABSTRACT FROM AUTHOR]

This paper characterises the dissimilar laser welding of 0.5 mm thick plate of shape memory alloy (NiTi) and copper (Cu) at different welding speeds. Properties of laser welds are characterised by microhardness testing, tensile-shear testing, XRD analysis, differential scanning calorimetry (DSC), and optical microscopy. XRD analysis of welded zone demonstrates the presence of NiTi, Cu, and equiatomic NiTi-Cu phases due to diffusion of NiTi and Cu at the interfacial zone of NiTi-Cu joint. DSC analysis shows the start and finish temperature of austenite phase during heating and martensitic phase during cooling for NiTi and laser welded sample. Microhardness studies reveal that weld zone possesses much higher hardness than NiTi and Cu base alloy. Microhardness value at NiTi-Cu joint interface (fusion zone) increases to 280HV and 150HV because of the presence of brittle and hard IMC phases (i.e. NiTiCu). The welded joints deliver higher joint strength than Cu base alloy but lower strength than NiTi SMA. Maximum joint strength of 335MPa (strain of 13.6%) is achieved at 1400W laser power and 1 m/min welding speed due to improved mixing and material flow. Microstructures of the fusion zone have dendrites grains and show the presence of Cu and NiTi constituents in the weld region. [ABSTRACT FROM AUTHOR]

Vacuum brazing of Ti-47Al-2Nb-2Cr-0.15B (at%) alloy with TiH2-Ni-TiB2 powder filler in situ assisted 20 vol% TiB whiskers is performed at 1190-1250 °C for 10 min. The microstructure evolution of the joint at various brazing temperatures and the relationship between microstructure and joint properties are systematically investigated. In the results, it is indicated that the interfacial microstructure of the γ-TiAl joint brazed at 1230 °C is γ-TiAl/Ti3Alþ Al3NiTi2 (layer I)/Al3NiTi2þ Ti3Al (layer II)/Al3NiTi2þ TiBþ Ti3Al (layer III)/Al3NiTi2þ Ti3Al (layer II)/Ti3Alþ Al3NiTi2 (layer I)/γ-TiAl. The layer II containing high-hardness Al3NiTi2 is always the weakest part in the brazed joint. The microstructure refinement and load-bearing behavior of layer III are increased with brazing temperature due to the increasing length of the TiB whisker. Variation of the content of low-hardness Ti3Al in layer II as well as its microstructure refinement and internal bonding strength, the shear strength of γ-TiAl joints is first increased and then decreased with brazing temperature, but that of Al3NiTi2 in layer II is reversed. The joint brazed at 1230 °C possesses the maximum shear strength of 332.87MPa and a quasi-cleavage fracture occurs in the layer II mainly consisted of Al3NiTi2. [ABSTRACT FROM AUTHOR]

The process of argon arc welding of nickel onto titanium with a nonconsumable electrode with feeding of the nickel filler wire into the liquid metal pool is investigated. The influence of the welding modes on the geometrical parameters of the bead and on the susceptibility of the forming surface layer to crack formation is described. The effect of the welding modes on the chemical and phase compositions of the formed surface layer of titanium nickelide is determined. The chemical and phase compositions of the welded-on layer are shown to affect its hardness and wear resistance. [ABSTRACT FROM AUTHOR]

Porous NiTi shape memory alloys (SMAs) possess compatible mechanical properties with human bones and can effectively reduce the risk of stress shielding and stress concentration; therefore, they have been termed promising candidates for orthopedic implants. However, microstructure characteristics of porous NiTi SMAs during plastic deformation have rarely been investigated. The present study aims to specifically investigate microstructure characteristics and the corresponding underlying mechanisms of fabricated porous NiTi SMAs via a conventional sintering (CS) process with NaCl space holder during compressive deformation at room temperature. To realize the aforementioned target, X-ray diffraction (XRD), scanning electron microscope (SEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) are applied in the present study. The results show that the fabricated porous NiTi SMA is 51.8% for porosity, 181.65 μm for the average pore size, and 0.78 μm for the average grain size. Many Ni4Ti3 and NiTi2 phases are formed in the mixed matrix with dominant B2 (NiTi) and some B19′ (NiTi). Severe inhomogeneous deformation happens within compressed specimens, leading to the occurrence of tangled dislocation and shear bands. Microcracks occur within fabricated porous NiTi SMAs at a deformation degree of 9.2%; then, they extend quickly to form macrocracks, which finally results in the failure of regions between pores. The observed nanocrystallization and amorphization around microcrack tips within the 12.5%-deformed sample can be attributed to the relatively small grain size and the grain segmentation effect via statistically stored dislocation (SSD) and geometrically necessary dislocation (GND). [ABSTRACT FROM AUTHOR]

Laser powder bed fusion (LPBF) is an emerging metal additive manufacturing method that can pave a pathway for manufacturing NiTi shape memory alloys (SMAs) with high performance. Considering the unique characterizations of LPBF process, the position and sequence of laser irradiation are different under different laser scanning modes, which will affect the performance of as‐built samples. Herein, four different chessboard sizes are utilized to fabricate NiTi parts. The surface quality and relative density first increase and then decrease with the increasing chessboard size, obtaining the optimal surface roughness of 9.95 μm and relative density of 99.7%, respectively, at a chessboard size of 5 mm. As the chessboard size increases, the more pronounced precipitation of Ni4Ti3 with a higher quantity induces a strengthening effect, leading to a higher microhardness value of ≈290 HV0.2 at a chessboard size of 9 mm. The electrochemical test shows a better corrosion resistance with a corrosion potential of 0.101 V and a corrosion current density of 1.670 × 10−5 A cm−2 at a chessboard size of 5 mm. The corrosion mechanism is further revealed. This work emphasizes the importance of chessboard size as a reference for optimizing the process of additive‐manufactured NiTi SMAs. [ABSTRACT FROM AUTHOR]

In this study, the effect of heat treatment parameters on the optimized performance of Ni-rich nickel–titanium wires (NiTi/Nitinol) were investigated that were intended for application as actuators across various industries. In this instance, the maximum recovery strain and actuation angle achievable by a nitinol wire were employed as indicators of optimal performance. Nitinol wires were heat treated at different temperatures, 400–500 °C, and times, 30–120 min, to study the effects of these heat treatment parameters on the actuation performance and properties of the nitinol wires. Assessment covered changes in density, hardness, phase transition temperatures, microstructure, and alloy composition resulting from these heat treatments. DSC analysis revealed a decrease in the austenite transformation temperature, which transitioned from 42.8 °C to 24.39 °C with an increase in heat treatment temperature from 400 °C to 500 °C and was attributed to the formation of Ni4Ti3 precipitates. Increasing the heat treatment time led to an increase in the austenite transformation temperature. A negative correlation between the hardness of the heat-treated samples and the heat treatment temperature was found. This trend can be attributed to the formation and growth of Ni4Ti3 precipitates, which in turn affect the matrix properties. A novel approach involving image analysis was utilized as a simple yet robust analysis method for measurement of recovery strain for the wires as they underwent actuation. It was found that increasing heat treatment temperature from 400 °C to 500 °C above 30 min raised recovery strain from 0.001 to 0.01, thereby maximizing the shape memory effect. [ABSTRACT FROM AUTHOR]

For NiTi alloys prepared by the Laser Powder Bed Fusion (LPBF), changes in the building directions will directly change the preferred orientation and thus directly affect the smart properties, such as superelasticity, as well as change the distribution state of defects and impurity elements to affect the phase transformation behaviour, which in turn affects the smart properties at different temperatures. In this study, the relationship between impurity elements, the building directions, and functional properties; the effects of building directions on the crystallographic anisotropy; phase composition; superelastic properties; microhardness; geometrically necessary dislocation (GND) density; and impurity element content of NiTi SMAs fabricated by LPBF were systematically studied. Three building directions measured from the substrate, namely, 0°, 45° and 90°, were selected, and three sets of cylindrical samples were fabricated with the same process parameters. Along the building direction, a strong <100>//vertical direction (VD) texture was formed for all the samples. Because of the difference in transformation temperature, when tested at 15 °C, the sample with the 45° orientation possessed the highest strain recovery of 3.2%. When tested at the austenite phase transformation finish temperature (Af)+10 °C, the 90° sample had the highest strain recovery of 5.83% and a strain recovery rate of 83.3%. The sample with the 90° orientation presented the highest microhardness, which was attributed to its high dislocation density. Meanwhile, different building directions had an effect on the contents of O, C, and N impurity elements, which affected the transformation temperature by changing the Ni/Ti ratio. This study innovatively studied the impurity element content and GND densities of compressive samples with three building directions, providing theoretical guidance for LPBFed NiTi SMA structural parts. [ABSTRACT FROM AUTHOR]

Samples prepared using various additive manufacturing methods were compared in terms of structure, texture, transformation temperature and superelastic properties. Samples manufactured using laser engineered net shaping (LE NS) method showed texture several degrees deviated from the <001> build direction, however with composition near to the initial powder composition, enabling superelastic effect. The electron beam additive manufacturing (EBAM) samples showed martensitic structure at room temperature due to a shift of transformation temperatures to the higher range. This shift occurs due to a lower Ni content resulting from different processing conditions. However, EBAM method produced sharper <001> texture in the build direction and made it possible to obtain a good superelastic effect above room temperature. Intermetallic particles of size 0.5-2 mm were identified as Ti2Ni phase using EDS and electron diffraction analyses. This phase was often formed at the grain boundaries. Contrary to the LE NS method, the EBAM prepared samples showed Ni-rich primary particles resulted from different processing conditions that reduce the Ni content in the solid solution thus increase the martensitic transformation temperature. Ageing at 500°C allowed for shifting the martensitic transformation temperatures to the higher range in both, LE NS and EBAM, samples. It resulted from the formation of Ni rich coherent precipitates. In samples prepared by both methods and aged at 500°C, the presence of martensite B19' twins was observed mainly on {011} B19' planes. [ABSTRACT FROM AUTHOR]

In this study, TiB 2 –TiN-based composite ceramic materials modified with NiTi were prepared by a two-step spark plasma sintering process (900 °C/3 min + 1650 °C/5 min). The effects of adding different volume fractions of NiTi (0–12 vol%) on the mechanical properties and microstructure of the ceramic material were investigated. The results showed that the optimal mechanical properties of the composite ceramic were obtained when the NiTi content was 9 vol%. Its flexural strength, fracture toughness and hardness were 942.5 MPa, 5.96 MPa m1/2 and 19.8 GPa, respectively. The addition of NiTi promoted densification during the sintering process, which improved the sinterability and refined the grain size of the materials. The toughening mechanisms were attributed to branching, deflection and bridging of cracks. Furthermore, a new toughening mechanism, phase change toughening was observed due to the addition of NiTi. Its specific manifestation in TiB2–TiN–NiTi cermet materials was the limitation of crack extension, which caused NiTi to transform from austenite to martensite when the crack extended into the region containing NiTi, and this consumed most of the fracture energy. The fracture toughness of the TiB 2 –TiN–NiTi composite ceramic materials was improved by the ability of NiTi to hinder crack growth. [ABSTRACT FROM AUTHOR]

In this study, corrosion and wear tests of NiTi alloy (Ni 55%–Ti 45%) samples, known as shape memory alloy, which offer a shape recovery memory effect between memory temperatures ranging from 25 to 35 °C, have been carried out. The standard metallographically prepared samples' microstructure images were obtained using an optical microscope device and SEM with an EDS analyzer. For the corrosion test, the samples are immersed with a net into the beaker of synthetic body fluid, whose contact with the standard air is cut off. Electrochemical corrosion analyses were performed after potentiodynamic testing in synthetic body fluid and at room temperature. The wear tests of the investigated NiTi superalloy were carried out by performing reciprocal wear tests under 20 N and 40 N loads in a dry environment and body fluid. During wear, a 100CR6-quality steel ball of the counter material was rubbed on the sample surface for a total of 300 m with a unit line length of 13 mm and a sliding speed of 0.04 m/s. As a result of both the potentiodynamic polarization and immersion corrosion tests in the body fluid, an average of 50% thickness reduction in the samples was observed in proportion to the change in the corrosion current values. In addition, the weight loss of the samples in corrosive wear is 20% less than that in dry wear. This can be attributed to the protective effect of the oxide film on the surface at high loads and the effect of reducing the friction coefficient of the body fluid. [ABSTRACT FROM AUTHOR]

In this study, the surface properties of marine structures were improved by applying a twin wire arc spray process to high-strength low alloys. The effect of Al and Mo contents in Ni-Al coatings on their mechanical and corrosion behaviors was analyzed using hardness tests, electrochemical experiments, and immersion tests. The increase in Al content resulted in the formation of oxides and intermetallic compounds, leading to a significant improvement in the mechanical properties by approximately 222 HV. Despite a fine galvanic phenomenon causing a decrease in corrosion resistance by up to 8.91%, a higher Al content demonstrated the highest corrosion resistance after high-temperature exposure, with an enhancement of approximately 20.9%, attributed to the formation of an oxide film generated by intermetallic phase transformation. However, the mechanical properties experienced a reduction of 134.3 HV. This study demonstrated a correlation between the microstructure of the coating layers that form passivation films and their respective mechanical and corrosion properties. It also revealed that the content of Al and Mo significantly affects the mechanical and corrosion behavior of Ni-Al coatings. [ABSTRACT FROM AUTHOR]

In this study, NiTi–x wt.% B4C (x = 0, 2, and 4) composites were consolidated with spark plasma sintering method, and the effects of boron carbide reinforcement addition on the microstructure and wear behavior of samples were investigated. Identification of the constituent phases of samples by the X‐ray diffraction method plus Rietveld analysis revealed that the stability of the martensite phase increased in the composite samples because of mismatch stresses between the NiTi matrix phase and the reinforcing particles, which increases the density of the dislocations and facilitates the diffusion process that subsequently leads to the formation of stable intermetallics. The results of hardness test indicated that the hardness value increased from 3.67 GPa for pure NiTi to 10.99 GPa for NiTi–4 wt.% B4C. Results of wear test revealed that boron carbide reinforced composite specimens had higher wear resistance, whereas wear rate of NiTi sample was 3.6 × 10−3 mm3/N m, and it reached to.21 × 10−3 mm3/N m for NiTi–4 wt.% B4C. Investigation of microstructure by scanning electron microscopy images and EDS analysis revealed that the wear mechanism in NiTi samples was abrasive and the addition of B4C to NiTi changed the wear mechanisms from abrasive to a combination of oxidation, adhesive, and delamination mechanisms. [ABSTRACT FROM AUTHOR]

The martensite phase with low crystal symmetry in NiTi alloy consists of internal twins and nano-layered compounds. The martensite phase has various inelastic deformation modes, such as twinning, reorientation, and nanocrystallization, during the deformation process, which leads to the further understanding of this deformation mechanism is unclear. Therefore, this research performed hot compression tests on the as-cast martensitic NiTi alloy, focusing on the analysis of the hot deformation behavior of the flow curves with abnormally increased stress value under high strain. The results showed that the peak stress, peak strain and softening stage of the flow curve decrease with the increases in deformation temperature, and the stress value of the flow curve increases at the high strain stage. According to the microstructure analysis of the alloy, the Ti2Ni phase is extruded and broken into small particles with the compression progress. Further spheroidization of the Ti2Ni phase with the compression temperature increases. In the process of hot compression, the martensite variants in the B′19 phase reorient, and the deformation twins and nanocrystalline structures are formed. The hindering effect of the broken Ti2Ni particles on dislocation movement and the strengthening role of nanocrystalline grains are the main reasons for the abnormal increase in stress value at high strain. [ABSTRACT FROM AUTHOR]

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Porous NiTi alloys were obtained by self-propagating high-temperature synthesis (SHS) using layer-by-layer combustion in closed and open gas flow reactors under a protective argon atmosphere. The maximum compressive strain of porous NiTi alloys synthesized in the closed reactor was 34% compared to 7% in the open gas flow reactor. X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, energy dispersive spectroscopy, and optical microscopy showed that the reaction products in the two-phase gas zone of peritectic crystallization are in the form of isolated Ti2Ni crystalline clusters in the TiNi matrix. The ductility and strength of the recrystallized Ti2Ni phase hard dendrites increase the effective stiffness of porous NiTi alloys and decrease the maximum compressive strain. A highly porous NiTi alloy with improved mechanical properties was obtained by the SHS method in a closed reactor. [ABSTRACT FROM AUTHOR]

Microstructural parameters of clad metal from Fe – C – Si – Mn – Cr – Ni – Mo – Vpowder wire with an addition containing carbon and fluorine cleaning dust of aluminum production instead of amorphous carbon are determined. The chemical composition and the microstructural parameters including the grain diameter, the interlamellar distance, the type of the structure of the deposited layer and its chemical composition are determined. [ABSTRACT FROM AUTHOR]

NiTi-Fe ternary shape memory alloys were produced by laser powder bed fusion (L-PBF) using the pre-alloyed NiTi and elemental Fe powders. A large process window exists to fabricate dense NiTiFe0.3 and NiTiFe0.5 alloys, while severe cracks occur in the NiTiFe2 alloy. The cracking susceptibility is studied through thermodynamic analysis. Fe is mainly dissolved in the NiTi matrix, which is mainly due to the low Fe addition and rapid cooling associated with L-PBF process. The martensite transformation temperatures (MTTs) decrease monotonously with the increase of scanning speed, the decrease of laser power, or the increase of Fe addition, providing a feasible approach to tailoring the MTTs. Due to the decrease of the MTTs, the NiTiFe0.3 and NiTiFe0.5 alloys show good superelasticity, e.g. the NiTiFe0.3 alloy shows a superelastic recoverable strain of 5.2% (pre-strain of 6%). This work suggests that L-PBF is a potential method to in-situ synthesise ternary NiTiFe alloys. [ABSTRACT FROM AUTHOR]

Ni–Ti alloys are used as functional materials in numerous sectors such as aerospace, automotive engineering, medical technology, and consumer goods. Their properties in terms of shape memory effect and superelasticity offer a great potential for innovative smart products. However, forming and machining of these alloys into concrete products is challenging. Assembling plain structures by laser welding to complex product shapes offers an economical alternative in many cases, but can be associated with negative effects, such as reduction of strength, development of brittle intermetallic compounds, alteration of transformation temperatures, and modification of shape memory effects and superelastic behavior. Against this background, investigations on laser welding of Ni55/Ti45 foil with a thickness of 125 µm by a fiber laser were conducted. Supported by methods of design of experiments, optimal parameters were determined with respect to laser power, welding speed, focus position, and beam oscillation, and the welding results were analyzed concerning the microstructure and mechanical characteristics of the welded joints. The effect of laser beam oscillation was investigated for the first time for the welding of this alloy. Due to the very low thickness, the preparation of the foils for the microstructure characterization is quite demanding. Best results were obtained by ion milling. Fracture surfaces and the influence of the welding were also investigated. [ABSTRACT FROM AUTHOR]

Herein, NiTi alloys are prepared by laser engineered net shaping (LENS) technology. By exploring the mechanical properties and microstructures of NiTi samples under different laser powers, it is concluded that NiTi alloys with a laser power of 350 W have better tensile strength (783 MPa) and strain recovery rate (59.01%). In addition, the mechanical properties and microstructure of NiTi samples with a laser power of 350 W at different aging temperatures (400–800 °C) are also explored. It is found that NiTi alloys aged at 600 °C had better tensile strength (729 MPa) and strain recovery rate (92.85%). In addition, the phase‐transition temperature and endothermic enthalpy and exothermic enthalpy of the NiTi samples after thermal treatment increased, which is attributed to the formation of stable martensite (B19′) inside the NiTi samples after thermal treatment, and the type of internal NiTi phases structure change significantly. Thermal treatment can generate Ni4Ti3, NiTi2 precipitates, and martensite (B19′) with high density of dislocations inside the NiTi samples. Herein, it is showed that LENS can prepare NiTi alloys with excellent mechanical and structural properties by controlling the equipment parameters and aging temperature. [ABSTRACT FROM AUTHOR]

The ability to integrate multiple distinct properties into a single structure is the driving force behind the development of NiTi to Ti6Al4V dissimilar joints. However, differences in thermophysical properties and the subsequent development of brittle Ti2Ni and Ni3Ti intermetallic compounds (IMCs) make fusion welding of these alloys rather challenging. The solidification of brittle IMC causes cracking upon solidification and a significant reduction on the joint mechanical properties. In this study, two strategies were used concurrently to reduce the formation of brittle IMC: zirconium (Zr) interlayer was introduced alongside with the laser beam being offset to the Ti6Al4V side. The influence on microstructural evolution was investigated using various characterization techniques. Scanning electron microscopy aided by energy dispersive spectroscopy and micro-X-ray diffraction supported by Rietveld refinement revealed that the volume fraction of the Ti2Ni brittle phase was drastically reduced from 90 to 9%, leading to improvements on the joint mechanical performance. A joint with a lower hardness of 392 HV was obtained, compared to 515 HV for the conventional NiTi/Ti6Al4V joint. The strength and ductility of the Zr-interlayered joint were improved to 320 MPa and 1.7%, respectively, compared to 148 MPa and 0.8% for the Zr-free joint. [ABSTRACT FROM AUTHOR]

To overcome the disadvantages of comparatively poor surface hardness and deprived resistance against wear of Ti–6Al–4V alloy, the protective NiTi coatings are fabricated at different processing parameters by laser‐directed energy deposition (LDED). This work presents a comprehensive study of deposition morphology, phase constituents, microstructural evolution, composition distribution, and mechanical properties of as‐fabricated NiTi deposition tracks. The relationship among processing parameters and deposition morphology, phase constituents, and mechanical properties is established. The microstructural evolution mechanism and the composition distribution on the cross section of deposition tracks are revealed. Results show that the deposition tracks are successfully formed under all laser processing parameters, and the dilution ratio increases with the increase in laser power and scanning speed. The deposition tracks consist of NiTi2, NiTi, and α–Ti. According to the microstructural analysis, planar crystals are formed between the substrate and the deposition tracks, and columnar and equiaxed dendrites are found in the deposition tracks. The average microhardness increases continuously with the increase in laser power. The deposition track prepared at laser power of 1400 W has the highest average microhardness of 732.45 HV0.2. The deposition track at this laser power exhibits excellent wear properties, with a wear rate of 2.25 × 10−6 mm3 Nm−1. [ABSTRACT FROM AUTHOR]

Nowadays, additive manufacturing (AM) of NiTi shape memory alloy is a challenging topic for the realization of 3D functional parts. Particularly, Laser Powder Bed Fusion (LPBF) of NiTi powder is one of the most challenging processes belonging to AM, thanks to its best performances in terms of productivity and precision of geometrical complexity. The control of the functional performances in NiTi components requires a strong interaction between technological and metallurgical approaches. In fact, a strong correlation among the process conditions, the microstructure, and the final functional performances, beyond the defects associated with the process are needed to be understood and analyzed. In the present work, the correlation between the feasibility map of processability and the obtained microstructure, which can be tailored according to the use of different energy density values, of Ni-rich NiTi powder processed with LPBF is investigated. In detail, discrete energy density values, in the range 60–300 J/mm3, were correlated to microstructure, Ni:Ti ratio, and transformation temperatures of the martensitic transformation, analyzed with SEM, EBSD, EDX, and DSC characterizations, respectively. An increase in laser energy density was found to promote Ni evaporation, which induced a change of the microstructure from austenite to martensite at room temperature. A consequent shift of the transformation temperatures to higher values and a change in microstructural texture was achieved. These achievements can support the identification of the feasibility range for manufacturing functionally graded NiTi SMA, requiring tailored functional properties located in selected positions in the 3D parts. [ABSTRACT FROM AUTHOR]

Joining alloys exhibiting individual superior properties could yield major benefits in terms of design and production flexibility. Nevertheless, differences in thermophysical properties between the materials to be joined complicate the formation of dissimilar combinations, thus limiting the potential applications of multiple engineering alloys. The formation of brittle Ti2Ni intermetallic (IMC) in the fusion zone (FZ) is the main challenge in joining NiTi to Ti6Al4V without an interlayer. Hence, the composition of the FZ needs to be changed to ideally suppress brittle IMC phases to form and decrease the likelihood of crack formation upon solidification. In this study, two strategies were used concurrently to reduce brittle Ti2Ni intermetallic compound: a cobalt interlayer was introduced in a butt joint configuration, and the laser was offset to the Ti6Al4V side. The use of a Co interlayer resulted in a joint free of brittle interaction layer susceptible to microcracks at the NiTi boundary by reducing the amount of brittle Ti2Ni intermetallic compound. A joint with a lower hardness of 438HV was attained, compared to 515HV for the conventional NiTi/Ti6Al4V joint. The maximum strength and fracture strain of the Co-interlayered joint were improved to 285MPa and 1.67%, respectively, compared to 148MPa and 0.8% for the Co-free joint. [ABSTRACT FROM AUTHOR]

Severe plastic deformation via high-ratio differential speed rolling (HRDSR) was applied to the Ni-rich Ti-50.8Ni alloy. Application of HRDSR and a short annealing time of 5 min at 873 K leads to the production of a partially recrystallized microstructure with a small grain size of 5.1 μm. During the aging process for the annealed HRDSR sample at 523 K for 16 h, a high density of Ni3Ti4 particles was uniformly precipitated over the matrix, resulting in the formation of an R phase as the major phase at room temperature. The aged HRDSR sample exhibits excellent superelasticity and superelastic cyclability. This achievement can be attributed to an increase in strength through effective grain refinement and particle strengthening by Ni3Ti4 and a decrease in the critical stress for stress-induced martensite (B19′) due to the presence of the R-phase instead of B2 as a major phase at room temperature. The currently proposed method for using HRDSR and post-deformation heat treatment allows for the production of Ni-rich NiTi alloys with excellent superelasticity in sheet form. [ABSTRACT FROM AUTHOR]

In this study, the thermodynamic and microstructural effects of boron element on a Ni-riched NiTi shape memory alloy (SMA) were investigated and the biocompatibility of the improved alloy was compared with the binary alloy. X-ray diffraction measurements showed that at room temperature the binary NiTi SMA was in the austenite (B2) phase, which was also supported by differential scanning calorimetry (DSC) measurements. When the NiTi alloy was doped with a low concentration of B, new phases appeared, e.g., SEM–EDX analyses of NiTiB alloy reveal the presence of Ni3Ti phase structure and β -Ti and Ni-rich matrix structures. Boron additive decreased the DSC peaks of martensitic phase transformation ( B 2 ↔ B 19 ′ ) of the NiTi alloy; therefore, all thermomechanical parameters investigated in this study were affected. On the other hand, it was determined that although the amount of Ni leakage in both alloys conform to the standards, the biocompatibility of NiTi alloy was improved by adding only 0.8 at% B, which reduced Ni ion release. [ABSTRACT FROM AUTHOR]

Porous NiTi alloys were manufactured by integration of gel-casting and microwave sintering. The effects of sintering temperature on porosity, compressive strength, pore morphology and phase composition of sintered samples were researched. The results show that the porosity and the mean pore diameter of porous NiTi alloys decrease with increasing sintering temperature, whereas the content of the NiTi phase, the elastic modulus and compressive strength of sintered samples increase. When the gel body with the solid loading of 50 vol.% is microwave sintered at 1000 °C for 30 min, porous NiTi alloys are obtained with the porosity of 38.9%, the compressive strength of 254 MPa, elastic modulus of 4 GPa, and predominant phase of NiTi. The results suggest that the method is suitable for rapid preparation of large-size and complex-shape personalized products similar to human bones at a low cost. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this paper is to investigate the effect of remelting each layer on the homogeneity of nickel-titanium (NiTi) parts fabricated from elemental nickel and titanium powders using laser powder bed fusion (LPBF). In addition, the influence of manufacturing parameters and different melting strategies, including multiple cycles of remelting, on printability and macro defects, such as pore and crack formation, have been investigated. Design/methodology/approach: An LPBF process was used to manufacture NiTi alloy from elementally blended powders and was evaluated with the use of a remelting scanning strategy to improve the homogeneity of fabricated specimens. Furthermore, both single melt and up to two remeltings were used. Findings: The results indicate that remelting can be beneficial for density improvement as well as chemical and phase composition homogenization. Backscattered electron mode in scanning electron microscope showed a reduction in the presence of unmixed Ni and Ti elemental powders in response to increasing the number of remelts. The microhardness values of NiTi parts for the different numbers of melts studied were similar and ranged from 487 to 495 HV. Nevertheless, it was observed that measurement error decreases as the number of remelts increases, suggesting an increase in chemical and phase composition homogeneity. However, X-ray diffraction analysis revealed the presence of multiple phases regardless of the number of melt runs. Originality/value: For the first time, to the best of the authors' knowledge, elementally blended NiTi powders were fabricated via LPBF using remelting scanning strategies. [ABSTRACT FROM AUTHOR]

Wire arc additive manufacturing achieves high efficiency and low costs by using a melting wire for directional depositions. Thermal analyses and the finite element method have been applied to predict residual stress and the deformation of fabricated parts. For Ni-based alloy production, a method for predicting solidification microstructure evolution with segregation is needed in order to design precise heat treatment procedures. In this study, a multi-phase field method coupled with a CALPHAD database is developed to simulate the solidification microstructure evolution of a practical Ni-based alloy. Thermal analyses of a wire arc additive manufacturing model were performed by the process modeling of multi-pass depositions with a running cyclic arc. Solidification microstructure evolution was obtained using the temperature profile in each deposited layer by the multi-phase field method. These predicted microstructures are compared with experimental measurements. It is confirmed that the multi-phase field method coupled with the CALPHAD database is effective for predicting solidification microstructure and segregation in the engineering of Ni-based alloys. [ABSTRACT FROM AUTHOR]

For NiTi alloys, different additive manufacturing processes may have different compressive recovery capabilities. In particular, there are relatively few studies on the compressive recovery ability of NiTi alloys by the laser-directed energy deposition (LDED) process. In this paper, the compression recovery properties of NiTi alloys with the LDED process were investigated quasi-in-situ by means of transmission electron microscopy, an electron backscatter diffractometer, and focused ion beam–fixed-point sample preparation. The results showed that the material can be completely recovered under 4% deformation and the B19' martensite phase content and dislocation density are basically unchanged. However, the recovery rate was only 90% and the unrecoverable strain was 0.86% at 8% deformation. Meanwhile, the B19' martensite phase content and dislocation density of the material increased. Furthermore, with the increase in deformation, the relative dislocation pinning effect of the Ti2Ni precipitated phase in the alloy was enhanced, which reduced the compressive strain recovery to a certain extent. [ABSTRACT FROM AUTHOR]

The solid-state refrigeration and heat pumping field is gaining increasing attention as an alternative solution to the conventional vapour-based cooling devices. Among the shape memory alloys (SMAs), besides the reference NiTi system, outstanding elastocaloric performances are exhibited by the NiMnTi alloy. In this work, we produced two cast NiMnTi alloys which were characterized in terms of microstructural, structural, functional, and mechanical properties. Moreover, we assessed the effect of the heat treatments on the microstructure and on the physical properties of the produced alloys by calorimetric, structural and high-temperature microscope analyses. Furthermore, the heat treatments promoted partial mechanical cycling stability and a superelastic behavior without significant residual strains. Finally, the elastocaloric performance of the alloy was evaluated by direct adiabatic ΔT measurements upon elastocaloric cycles in four different loading conditions. The alloys exhibited promising elastocaloric performance for possible solid-state cooling applications. In particular, the best adiabatic ΔT value achieved in this work was 8.7 °C with a strain rate of 400 %/min and up to a strain of 5 % for the NiMnTi heat treated at 900°C. [Display omitted] • All-d metal NiMnTi superelastic alloy exhibits promising elastocaloric effect. • Heat treatments tune NiMnTi microstructure and martensitic transition temperatures. • Heat treated cast NiMnTi shows a significant adiabatic temperature change of 8.7°C. • Cast NiMnTi can be applied in room temperature solid-state heating or cooling field. • Casting can be a low cost and effective method to produce elastocaloric NiMnTi. [ABSTRACT FROM AUTHOR]

Two transmission electron microscopy (TEM) based methods were applied to reconstruct martensite variant microstructures in entire grains of plastically deformed nanocrystalline NiTi wires. First method consists in manual TEM analysis of composite electron diffraction patterns and dark field images of microstructures taken using carefully selected Bragg spots. Second method involves automated orientation/phase mapping of martensite variant microstructures in TEM (ASTAR). Application of these two methods helped us to discover new mechanism of the plastic deformation of monoclinic B19' martensite in NiTi shape memory alloys called kwinking as it combines dislocation slip based kinking and deformation twinning in martensite. In this work, we describe how the martensite variant microstructures in nanograins of plastically deformed NiTi shape memory wire are reconstructed using the TEM based methods and discuss their advantages and disadvantages. The application of the first manual method requires tilting the TEM lamella so that all martensite variants in the analysed grain are oriented in a common low index zone. Such common crystal lattice direction exists in each grain of plastically deformed martensitic NiTi wire because the microstructures were created by the kwinking deformation mechanism. The requirement to orient the selected grain into a common unique direction in monoclinic lattice, on the other hand, is not unique to kwinking and represents a severe limitation of the SAED-DF method due to the constraints on tilting the TEM lamella. Application of the automated crystal orientation mapping ASTAR method enables detailed orientation analysis of nanoscale martensite variant microstructures in generally oriented grains of deformed NiTi wires as well as pointwise ex-situ analysis of interfaces with excellent resolution in real and orientation space. • Nanocrystalline NiTi wire deformed plastically in tension. • Reorientation and plastic deformation of monoclinic B19' martensite by kwinking. • Reconstruction of martensite variant microstructures in grains of deformed wire by TEM. • Manual orientation mapping using electron diffraction and dark field imaging in TEM. • Automated orientation mapping in nanoscale using ASTAR method in TEM. [ABSTRACT FROM AUTHOR]

In this study, (NiTi) 100-x Nb x (x = 5, 10, 15 wt%) ternary alloys with different Nb contents were synthesised in situ by directed energy deposition. Microstructure analysis shows that the (NiTi) 100-x Nb x ternary alloy possess a typical dendritic structure. With the increase of Nb content, eutectic regions were formed between Nb and NiTi alloy, and TiNb intermetallic compounds were segregated at the eutectic. The NiTiNb ternary alloy formed a good metallurgical bond with the substrate at all different Nb contents, and no cracks or unfused areas were observed. (NiTi) 85 Nb 15 demonstrated excellent elasticity at varying loads, especially at a load of 800 mN, with a modulus of elasticity of only 57.37 GPa. Meanwhile, the in situ synthesised (NiTi) 85 Nb 15 demonstrated excellent wear resistance compared to other materials, with a minimum specific wear rate of 0.96 ∗ 10−4 mm3/Nm. The superelasticity of NiTiNb alloys plays a role in alleviating the effect of normal loads during sliding wear, which is the key to improving wear resistance. The wear mechanism of NiTiNb ternary alloys is dominated by furrowing with a small amount of adhesive wear. This work reveals the influence of Nb element content on the microstructure and wear resistance of NiTi alloys, providing insights into in-situ alloying of NiTiNb ternary alloys through laser cladding. In future research, it is recommended to enhance the functional properties of NiTiNb ternary alloys, such as memory properties or superelasticity by heat treatment. • The NiTiNb ternary alloy coatings were synthesised in situ via laser cladding. • The effects of Nb content on the wear resistance of NiTi alloys are revealed. • Provides new insights into the design and development of elastic wear-resistant coatings [ABSTRACT FROM AUTHOR]

[Display omitted] • Deposited HEFs showed a higher capacity for HAp formation and bioactivity. • Low working pressure yielded a dense and compact structure with no cracks. • Ti content disparity occurred due to its lower atomic weight during sputtering. • In the dense microstructure, oxygen diffusion through the depth has been minimized. • HEA-1500-Ar25-P0.5 caused robust protection lowering Ni ion release in AS and SBF. This paper presents the experimental findings on the effect of deposition conditions on the microstructure of the corrosion-resistant TiTaNbZrMo high entropy alloy (HEA) thin films deposited on NiTi substrates with the purpose of enhancing biocompatibility of the NiTi shape memory alloy (SMA). For this purpose, RF magnetron sputtering was employed to fabricate TiTaNbZrMo HEA films with 750 nm and 1500 nm thicknesses. Static immersion experiments were conducted in simulated body fluid (SBF) and artificial saliva (AS) solutions for 1, 14, and 28 days to establish the relationship between Ni ion release and deposition parameters. The results revealed that thin films grown under low working pressure exhibited crystalline body-centered cubic (BCC) microstructure with a highly dense, compact, and crack-free structure, while those deposited under high-pressure conditions exhibited an amorphous structure with inherent cracks. The biocorrosion test results indicated that the dense and compact thin film fulfilled the expected corrosion resistance requirements for prolonged utility in human body. Moreover, the HEA films revealed an outstanding amount of hydroxyapatite (HAp) formation, indicating remarkable bioactivity and favorable bone-bonding capabilities. The findings suggest that the HEA films deposited under low working pressures could constitute promising alternatives to conventional coatings on NiTi SMAs. [ABSTRACT FROM AUTHOR]

Laser alloying has attracted significant attentions due to the advantages of high processing precision, good controllability and low heat effects on the substrate. However, the complexity of laser alloying requires further attentions on its processing parameters. This study aims at improving the wear resistance of the Ti-6Al-4V substrate by means of laser surface alloying with Ni-coated graphite (G@Ni). The effect of laser scanning speed is explored. The result suggests that the coating has a high surface quality and excellent metallurgical bonding with the substrate. NiTi and NiTi2 have a eutectic microstructure as well as in the TiC ceramic-reinforced phase as dendrites distribute in the γ-Ni matrix of the coatings. At higher scanning speeds, the lower energy density and shorter existence time of the molten pool refines the microstructure of the coating, improving its microhardness. At the scanning speed of 15 mm/s, the coating has the lowest wear weight loss due to its high microhardness and dense structure. This paper explores the influence of scanning speed on the microstructure and properties of the coatings, expanding the application of laser alloying on the surface modification of Ti-6Al-4V alloys. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) is an orderly construction process involving track-by-track melt pools, which can modulate three characteristic zones between two adjacent melt pools: the heat-affected zone, transition zone, and cellular grain zone. These three zones constitute a function domain that determines the properties of AM metallic materials. In this study, we constructed a novel ordered function domain by AM of modified Ni50.6Ti49.4 shape memory alloy powder, which was produced by adding uniform Ni nanoparticles into atomised Ni49.6Ti50.4 powder. Interestingly, the three zones of the function domain had the same B2 austenite matrix but inhomogeneous Ni4Ti3 clusters and Ti2Ni nanoprecipitates, which caused different compressive recovery and residual strains in the three zones. The synergetic effect of the three zones generated a stable recovery strain and small hysteresis area in the AMed Ni50.6Ti49.4. Our results provide a novel strategy for modulating the microstructure of NiTi by AM to yield superior properties. [ABSTRACT FROM AUTHOR]

60NiTi alloys have a tremendous potential to be used in aerospace, marine and automotive industries. There is still a need to further improve the deformability due to the high brittleness of the previously prepared 60NiTi. In this work, 200 °C hot silicone oil was selected as the quenching medium for 60NiTi for the first time to overcome its high brittleness. It is found that the unique microstructure of 60NiTi quenched by hot oil has a lamellar structure composed of a channel-like NiTi matrix and lenticular Ni4Ti3 phase containing a nano-lath NiTi phase. The 60NiTi exhibits a high compression fracture strain of 10% and large reversible strain of 7.5%; which originates from the superelastic behavior of the NiTi SMA constituent. Upon loading, the R phase reorientation releases the stress concentration at the initial stage; while the stress-induced martensitic transformation accommodates the large elastic deformation of the Ni4Ti3 phase at the later stage. This synergistic effect of the two promotes the compressive deformability. [ABSTRACT FROM AUTHOR]

Solid-state diffusion bonding of pseudo-α-Ti alloy and Ti-stabilized stainless steel (SS321), with and without Ni interlayer, was investigated in the temperature range of 800-940 °C for different times. Microstructural investigation of the bond interfaces in the directly bonded diffusion couple showed a distinct diffusion zone which was composed of layers of (1) Fe-Ti intermetallic phases (Fe2Ti and FeTi), (2) transformed β-Ti and (3) (α+β) Ti. Shear strength of the joint was seen to be dependent on the thickness of diffusion zone and intermetallic phase layer. For direct diffusion bonding, maximum shear strength of 223.6 ± 17 MPa was observed for the joints processed at a temperature of 920 °C with a holding time of 8 min under a load corresponding to ~0.8 times yield strength (YS) of Ti alloy. Shear strength results showed that the optimum thickness of the Fe-Ti intermetallic phase layer is approximately 2 ± 0.5 µm. For the diffusion bonding joints in the presence of a Ni interlayer, formation of layers of Ni-Ti intermetallic compounds (Ni3Ti, NiTi and NiTi2) was observed at the interface between Ni and Ti alloys. Joint bonded at 940 °C with a holding time of 1 min under a load of 0.8 × YS showed the maximum shear strength of 180 ± 23 MPa. The optimum thickness of the Ni-Ti intermetallic layers for maximum shear strength was found to be approximately 5 ± 2 µm. [ABSTRACT FROM AUTHOR]

NiTi shape memory alloys (Nitinol) are one of the most widely used families of smart materials that are used in smart systems and medical applications. The NiTi alloy with a composition of around 50 at. % Ni is very sensitive and just a small change in the components can make a big difference in the physical properties of the alloy. In this study, five non-equiatomic NiTi alloys with different compositions were fabricated with arc-melting in a controlled atmosphere. The crystal structure and morphology of the alloys were analyzed with x-ray diffraction (XRD) and scanning electron microscope (SEM), also the caloric properties of the alloys were investigated as a function of composition. Additionally, the samples were examined by the Tafel test to determine the corrosivity rate for one year in a simulated body fluid. The XRD and DSC results showed that only Ni44.56Ti55.44 at. % has martensite phase at room temperature, while the other alloys with higher Ni contents did not show martensitic phase transformation for the range of 230 K to more than 400 K. It was found that the alloys have corrosion rate around 10–4 mmpy which has comparably better than CuAlNi-based alloys. On the other hand, it was found that the corrosivity does not depend on the matrix phase of the NiTi alloy, but the composition ratio and other secondary phases play an important role. [ABSTRACT FROM AUTHOR]

NiTi alloys are widely known for their shape memory effect and super-elasticity. In this study, the laser cladding method was applied to prepare Ni-rich NiTi alloy coatings on 316L stainless steel substrate. The microstructure, phase composition, element distribution and phase transformation behavior of the coatings were investigated in as-fabricated and annealing-treated states. The results indicated that the recrystallized microstructure obtained and the content of Ni3Ti and Ti2Ni phases increased significantly with a rising annealing temperature. Annealing treatment also induced a decrease in the phase-transition enthalpy and a rise in the transformation temperature, even though no obvious martensite transformation was observed. This was suppressed due to the Fe element diffused from the substrate and was probably retarded by the mounting metallic compounds formed during annealing as well. The mechanical properties have also improved obviously; coatings annealed under 850 °C exhibited the highest microhardness of 839 HV, and the wear resistance of the coatings after annealing was enhanced with an 11% average wear mass loss reduction. [ABSTRACT FROM AUTHOR]