Ni4Ti3 Precipitate Research Articles

Fiber Laser Surface Melting of a NiTi Superelastic Alloy: Influence on Structural and Mechanical Properties

The surface melting of a NiTi superelastic alloy using a high-power laser Yb:Fiber was investigated. The influence of this process on the microstructural and mechanical properties was also examined. The reference material was a 3 mm nitinol strip with a homogeneous austenitic B2 phase. For the laser surface melting process, input fluences were applied from 17.5 to 45 J/mm2. The morphology of the structure and the chemical composition of several regions were determined by optical microscopy, scanning electron microscopy, dispersive energy spectra, and X-ray diffraction techniques. The mechanical properties, such as modulus of elasticity and hardness, were determined using nanoindentation and microindentation techniques. The greatest surface finishing of the fusion zone was observed for the condition 35 J/mm2. Three well-defined regions (fusion zone (FZ), heat-affected zone (HAZ), base metal (BM)) could be observed and dimensions of grain size, width, and depth of the melted pool were directly affected by the laser fluence. The geometry of the molten pool could be controlled by the optimization of the laser parameters. High laser fluence caused preferential volatilization of nickel, dynamic precipitation of intermetallic phases, including Ti2Ni, Ni3Ti, and Ni4Ti3, as well as solubilization of TiC in the matrix, which led to grain refinement. Thus, high laser fluence is a suitable technique to enhance mechanical properties such as hardness and Young’s modulus.

Evolution of thermal and mechanical properties of Nitinol wire as a function of ageing treatment conditions

As-received and cold-worked 55Ni–45Ti wt% Nitinol wire samples were subjected to various ageing treatments. Experiments show that the properties of as-received Nitinol, including microstructure, critical temperatures in thermally induced phase transformation, and critical stresses/strains in mechanically induced phase transformation, change significantly after ageing process at 773 K for different durations ranging from 10 min to 5 h. BSE and EBSD/EDS analysis shows that the predominant microstructural evolution is a large increase in Ni4Ti3 precipitate volume fraction, while grain growth is negligible. DSC results show that as-received material exhibits little to no phase transformation during heating-cooling cycling at 5K/min, while various ageing durations lead to the emergence of phase transformation that occurs at different critical temperatures. Tensile loading experiments conducted using a test rig equipped with a heater revealed the evolution of mechanically induced phase transformation behaviour. Mathematical models are proposed to predict the dependence of critical temperatures and critical stresses/strains on the ageing treatment conditions and alloy composition.

Effects of thermal aging on phase transformation and microstructural characteristics of NiTi shape memory alloy

In this study, the effects of aging temperature and time on microstructure and phase transformation behaviors of the NiTi shape memory alloy were investigated with the effects of precipitates. NiTi SMA was synthesized using powder metallurgy process followed by the solution annealing at 850 °C for 1 h. The thermal aging process has been executed on NiTi alloy in the temperature range of 450 °C–750 °C for 1 h, 3 h and 5 h respectively. It was found that the enhancement of transformation temperatures due to the formation of secondary phases during the transformation of cubic to monoclinic. The lenticular structured Ni4Ti3 precipitate was mainly influenced the phase transformation by changing the Ni composition in the NiTi matrix. The significant changes were observed in the austenite and martensite finish temperatures with increasing the aging temperature and time. This study suggests that the NiTi SMA can be implemented in biomedical applications when heat treated at 650 °C for 5 h.

Influence of precipitation on phase transformation and mechanical properties of Ni-rich NiTiNb alloys

Ni-rich NixTiyNbz (x/y = 1.0645/x-y ≈ 3, z = 0, 2, 4, 6, 9 at.%) alloys were designed and prepared in this work. Based on these alloys, the precipitation evolution of Ni-rich NiTiNb alloys and its influence on phase transformation and mechanical properties were investigated. Ni4Ti3 precipitates are observed in Nb0 and Nb2 alloys after aging and furnace cooling. Ni4Ti3 precipitation sharply increases Ms in Nb0 alloy, however, causes no obvious change of Ms in Nb2 alloy. With increasing Nb addition, large numbers of β-Nb precipitates densely distribute among eutectic regions of Nb6 and Nb9 alloys. β-Nb precipitates and eutectic β-Nb particles decrease Ms. The critical yield stresses of all Ni-rich NiTiNb alloys are determined by Ms, and present same minus linear relationship with Ms. Precipitation and element variation change Ms and then influence mechanical properties. Meanwhile, this result is demonstrated by thermodynamic analysis.

Selective growth of Ni4Ti3 precipitate variants induced by complicated cyclic stress during laser additive manufacturing of NiTi-based composites

NiTi-based composites were in-situ synthesized by selective laser melting (SLM) additive manufacturing technology with formation of a great many Ni4Ti3 precipitates. The differential scanning calorimetry (DSC) were carried out to exhibit the similar phase transformation temperature (As = 51.82 °C, Af = 94.02 °C, Ms = 55.87 °C and Mf = 18.93 °C) by comparing with that of SLM-fabricated equi-atomic NiTi alloy, which could be attributed to the compensation of Ni4Ti3 precipitates for the effect of an addition of TiC particles. Three different Ni4Ti3 precipitate variants ((111)R//11¯1¯B2, (111)R//1¯1¯1B2, and (111)R//1¯11¯B2) with various precipitate diameters were observed within SLM fabricated part, showing apparently selective growth behavior. By high-resolution transmission electron microscopy, severe lattice defects formed surrounding the smaller variant, while relatively intact lattice structure was found around the larger one, which indicated that high strain energy was produced in the interface between the matrix and the smaller variant. Furthermore, finite element simulation method was applied to disclose the evolution features of complicated cyclic stress during SLM processing of TiC/NiTi composites. By predicting, the main constraint mode within as-fabricated track was tensile stress effect. Based on the redistribution behavior of thermal stress and the coupling effect between the thermal stress and strain field induced by Ni4Ti3 precipitate, the selective growth mechanism of Ni4Ti3 variant was revealed.

In-situ formation of Ni4Ti3 precipitate and its effect on pseudoelasticity in selective laser melting additive manufactured NiTi-based composites

Selective laser melting (SLM) additive manufacturing technology was applied to synthesize NiTi-based composites via using ball-milled Ti, Ni, and TiC mixed powder. By transmission electron microscope (TEM) characterization, it indicated that the B2 (NiTi) matrix was obtained during SLM processing. In spite of more Ti content (the Ti/Ni ratio >1), a mass of Ni-rich intermetallic compounds containing Ni4Ti3 with nanostructure features and eutectic Ni3Ti around in-situ Ti6C3.75 dendrites were precipitated. Influence of the applied laser volume energy density (VED) on the morphology and content of Ni4Ti3 precipitate was investigated. Besides, nanoindentation test of the matrix was performed in order to assess pseudoelastic recovery behavior of SLM processed NiTi-based composites. At a relatively high VED of 533 J/mm3, the maximum pseudoelastic recovery was obtained due to the lowest content of Ni4Ti3 precipitates. Furthermore, the precipitation mechanism of in-situ Ni4Ti3 was present based on the redistribution of titanium element and thermodynamics analysis, and then the relationship of Ni4Ti3 precipitate, VED and pseudoelastic recovery behavior was also revealed.

Correlating microstructure and superelasticity of directed energy deposition additive manufactured Ni-rich NiTi alloys

Laser-based directed energy deposition (LDED) additive manufacturing of Ni-rich NiTi shape memory alloys was shown to produce inhomogeneous precipitate morphologies and characteristic grain structures consisting of columnar grains coexisting with equiaxed and subgrain structures. Post-processing solutionizing and aging heat treatments impacted microstructure and martensitic phase transformation (MT) responses underpinning superelastic shape memory responses. A solution treatment of 950 °C for 24 h was found to produce a uniform composition of the B2 austenite parent phase without affecting the coexistence of columnar and equiaxed substructures. Aging the solution treated material brought about a spatially uniform Ni4Ti3 precipitate morphology. Due to the uniform morphology, an underlying austenite-martensite interface motion accompanies the compressive stress-induced MT (SIMT). Reversible interface motion underpinned the compressive superelastic response for the solutionized and aged condition. On the other hand, strain concentrations existed at different spatial locations in the as built condition as well as when the as built material was aged. The stark contrasts in the SIMT exposed precipitate morphology as a controlling factor in tailoring the superelastic response of Ni-rich NiTi SMAs fabricated by LDED.

Dependence of processing window and microstructural evolution on initial material state in direct electric resistance heat treatment of NiTi alloy

Direct electric resistance heat treatment (DERHT) is especially suitable for the rapid functional property tailoring of NiTi alloys. However, the dependence of processing window and microstructural evolution of DERHT on the initial material state remains unclear so far. To clarify this issue, NiTi wires of 1mm in diameter with aged, non-aged, nanocrystalline and coarse-grained material states were respectively undergone DERHT with electric currents of 12 and 8 A within the heating time from 0 to 600s. The variations of phase transformation and superelasticity were studied and the corresponding DERHT processing windows were obtained. The underlying microstructural evolutions were identified and the variations of grain size and Ni4Ti3 precipitates were revealed. The results show that the nanograins grew rapidly from 34nm to 2.1μm within 12 A/0–25s and the Ni4Ti3 precipitates of 15–25nm were formed after 8 A/600s in coarse-grained samples, which respectively affected the functional properties of nanocrystalline and coarse-grained samples. In addition, 12 A/10s dissolved the preformed Ni4Ti3 nanoprecipitates in the aged samples, which also influenced their functional properties. Furthermore, DERHT induced precipitation and growth of Ni4Ti3 were suppressed in nanograins, leading to the less conspicuousness of aging effect in nanocrystalline samples.

Thermokinetic Simulation of Precipitation in NiTi Shape Memory Alloys

Considering classical nucleation theory and evolution equations for the growth and composition change of precipitates, we simulate the evolution of the precipitates structure in the classical stages of nucleation, growth and coarsening using the solid-state transformation Matcalc software. The formation of Ni3Ti, Ni4Ti3 or Ni3Ti2 precipitate is the key to hardening phenomenon of the alloys, which depends on the nickel solubility in the bulk alloys. The microstructural evolution of metastable Ni4Ti3 and Ni3Ti2 precipitates in Ni-rich TiNi alloys is simulated by computational thermokinetics, based on thermodynamic and diffusion databases. The simulated precipitate phase fractions are compared with experimental data.

Role of Severe Plastic Deformation in Suppressing Formation of R Phase and Ni4Ti3 Precipitate of NiTi Shape Memory Alloy

Microstructural evolution of NiTi shape memory alloy (SMA) with a nominal composition of Ni50.9Ti49.1 (at %) is investigated on the basis of heat treatment and severe plastic deformation (SPD). As for as-rolled NiTi SMA samples subjected to aging, plenty of R phases appear in the austenite matrix. In terms of as-rolled NiTi SMA samples undergoing solution treatment and aging, Ni4Ti3 precipitates arise in the austenite matrix. In the case of as-rolled NiTi SMA samples subjected to SPD and aging, martensitic twins are observed in the matrix of NiTi SMA. With respect to as-rolled NiTi SMA samples subjected to solution treatment, SPD, and aging, neither R phases nor Ni4Ti3 precipitates are observed in the matrix of NiTi SMA. The dislocation networks play an important role in the formation of the R phase. SPD leads to amorphization of NiTi SMA, and in the case of annealing, amorphous NiTi SMA samples are subjected to crystallization. This contributes to suppressing the occurrence of R phase and Ni4Ti3 precipitate in NiTi SMA.

Optimization of Automated Crystal Orientation Mapping in a TEM for Ni4Ti3 Precipitation in All-Round SMA

Automated crystal orientation and phase mapping in TEM are applied to the quantification of Ni4Ti3 precipitates in Ni–Ti shape memory alloys which will be used for the implantation of artificial sphincters operating using the all-round shape memory effect. This paper focuses on the optimization process of the technique to obtain best values for all major parameters in the acquisition of electron diffraction patterns as well as template generation. With the obtained settings, vast statistical data on nano- and microstructures essential to the operation of these shape memory devices become available.

Obtaining and characterization of Ni-Ti/Ti-Mo joints welded by TIG process

The assessment of heat treatment on dissimilar Ni-Ti and Ti-Mo welding has been evaluated in this work. An experimental design 32 was used as optimization tool to evaluate the effect of time and temperature after the welding process on the welding quality. Welded joints without significant difference in the thickness of the welded wires were obtained successfully. After the heat treatment welded joints with shape memory were obtained. After the welding process a change in the chemical composition of the melting zone has been observed. There was a decrease of nickel, molybdenum and zirconium content and an increase of the titanium content. The temperature and time of heat treatment affected the corrosion resistance performance. It was observed that the short time and lower heat treatment temperature favors the formation of Ni4Ti3 precipitate, which is responsible for the formation of stable passivation film. The longer the time and temperature of heat treatment more the decrease in the corrosion resistance and increase in the fragility of the obtained joints, and consequently the amount of the precipitated Ni4Ti3.

Studies on the surface morphology and hydrophobic property of NiTi thin films under in situ and post annealing various temperatures

Surface morphology and hydrophobic property of NiTi thin films are investigated under various in situ and post annealing temperatures. The XRD and AFM results reveal that in situ annealed NiTi thin films crystallize at lower temperature and have larger surface roughness than post annealed samples. Heating during sputtering provides energy for atoms to move to position of equilibrium, thus more regular crystal structure is observed in in situ annealed NiTi thin films. Another reason for the smoother surface observed in post annealed thin films is that large amount of Ni4Ti3 precipitation in hinders the formation of B19′ phase. The contact angle becomes larger with the increase of surface roughness and the maximum contact angle of 123.5° is achieved in 600°C in situ annealed thin film with 41.7nm surface roughness.

Anisotropic microstructure and superelasticity of additive manufactured NiTi alloy bulk builds using laser directed energy deposition

The microstructure and superelasticity in additive manufactured NiTi shape memory alloys (SMAs) were investigated. Using elementally blended Ni and Ti powder feedstock, Ni-rich build coupons were fabricated via the laser-based directed energy deposition (LDED) technique. The build volumes were large enough to extract tensile and compressive test specimens from selected locations for spatially resolving microconstituents and the underlying stress-induced martensitic phase transformation (SIMT) morphology. In the as-deposited condition, X-ray diffraction identified the B2 atomic crystal structure of the austenitic parent phase in NiTi SMAs, and Ni4Ti3 precipitates were the predominant microconstituent identified through scanning electron microscopy. The microstructure exhibited anisotropy, which was characterized by the Ni4Ti3 precipitate morphology being coarsest nearest the substrate, while a finer morphology was observed farthest from the substrate. In-situ full-field deformation measurements calculated using digital image correlation confirmed that the SIMT predominately occurred in the finer precipitate morphology. Heat treatment reduced the degree of anisotropy, and DIC analysis revealed localized SIMT strains increased compared to the as-deposited condition.

Molecular dynamics modeling of NiTi superelasticity in presence of nanoprecipitates

The presence of nano-sized coherent precipitates is well known to have crucial impact on the mechanical behaviors of a broad class of superelastic alloys. As a representative material, the pseudoelasticity of austenitic NiTi alloy in presence of a lenticular coherent Ni4Ti3 precipitate is investigated using atomic scale simulations. We predict the local stress gradient at the matrix-precipitate interface induced by inter-lattice atomic disregistry. The calculated stress distribution conforms to the latest high resolution electron microscopy measurements in the literature. Due to the presence of the local disturbance fields, the preference for activating different martensitic variants, given the uni-directionality thereof, is influenced substantially. The resultant constitutive attributes are thus observed to undergo adjustments in terms of reduced transformation stress, strain and hysteresis in general agreement with experimental literature.

Positive and negative two-way shape memory effect in [111]-oriented Ni51Ti49 single crystals

This study investigates the effect of precipitates and compressive loading at selected temperatures on the two-way shape memory effect (TWSME) properties of a [111]-oriented Ni51Ti49 single crystal shape memory alloy. The single crystals were either solution-treated or aged at 500°C for 1.5h to form Ni4Ti3 precipitates, and then deformed to obtain TWSME. When the solution-treated or aged specimens were loaded at low temperature in martensitic phase, positive (compressive) TWSME was observed. After compressive loading at high temperature in austenitic phase, the solution-treated specimen did not show TWSME while the aged specimen revealed negative (tensile) TWSME. TEM investigation revealed that the negative TWSME strain is produced by the selection of martensite variants due to the back stress created by the dislocation formation around Ni4Ti3 precipitates that affects the R-phase formation. A maximum negative TWSME strain of 1.9% was obtained in the aged specimen after compressive deformation of 6% at 200°C.

DFT-supported phase-field study on the effect of mechanically driven fluxes in Ni4Ti3 precipitation

Formation of the Ni4Ti3 precipitate has a strong effect on the shape memory properties of NiTi alloys. In this work, growth of this precipitate is studied using phase-field modelling and density functional theory (DFT) calculations. Using first-principles calculations, the composition-dependent stability and elastic properties of the B2 phase are obtained. Composition-dependent elastic constants are incorporated into our phase-field model to investigate the interplay between stress and concentration fields around the precipitate. The model introduces a source of diffusion due to mechanical relaxation which is accompanied by local softening/hardening of the B2 phase. The results are discussed in light of previous experimental and simulation studies.

Structural and mechanical stability of the nano-crystalline Ni–Ti (50.9 at.% Ni) shape memory alloy during short-term heat treatments

In this study, the nano-crystalline Nitinol (50.9 at.% Ni) was prepared by 40% cold deformation. Subsequently it was subjected to 15-min-heat treatments at 300–550 °C. Changes of the structure and mechanical properties were studied by transmission electron microscopy, micro X-ray diffraction, differential scanning calorimetry, Vickers hardness measurements, tensile and bend-type fatigue testing. It was shown that the cold drawn material contains textured nano-crystalline B2 grains of 50 nm in thickness and a high concentration of lattice defects. Its tensile strength, hardness and fatigue life were 1521 MPa, 421 HV0.05 and 2435 bending cycles to fracture, respectively. After heat-treatment up to 450 °C/15 min the material underwent Ni4Ti3 precipitation and partial recovery processes. Heat-treatments at above 450 °C induced recrystallization, grain and precipitate growth. Hardness and fatigue lives showed maxima of 692 HV0.05 and 5883 cycles, respectively, after heat-treatments at 450 °C/15 min. In contrast, both tensile strength and B2 → B19′ transformation stress decreased with increasing heat-treatment temperature, but a decrease of the tensile strength after heat-treatments at 300–450 °C was slow (tensile strength after heat-treatment at 450 °C/15 min was 1486 MPa). The observed variations of mechanical characteristics were discussed in relation to structural changes observed.

PHASE FIELD SIMULATION OF AUTO-CATALYTIC GROWTH EFFECT OF COHERENT Ni4Ti3 PRECIPITATE IN Ni 4 Ti 3 SHAPE MEMORY ALLOY

The precipitation behavior and distribution of Ni4Ti3 particles in NiTi alloys have a significant influence on the subsequent martensitic transformation, which may consequently affect shape memory effect and superelasticity of NiTi alloys. The latest experimental studies confirmed that in single crystal NiTi alloy, the Ni4Ti3 particles nucleate and grow as an autocatalytic way leading to a step–like configuration. The autocatalytic nucleation known as collective manner has been widely studied in martensitic transformation while causing relatively less attention in diffusion transformation such as precipitation. Previous studies have been launched to investigate the nucleation and orientation issues of Ni4Ti3 precipitate mainly by analyzing the stress or concentration field around the Ni4Ti3 particle. However, the coupling between stress and concentration field is necessary in * 50871039, 10151064101000017, 20110490881 ! 2012ZZ0014 : 2012–05–10, ! : 2012–07–22 $%! : , , 1981 , # &, DOI: 10.3724/SP.J.1037.2012.00264

Modeling of Ni4Ti3 precipitation during stress-free and stress-assisted aging of bi-crystalline NiTi shape memory alloys

The phase field method was applied to study the microstructure evolution of Ni4Ti3 precipitates during stress-free and stress-assisted aging of bi-crystalline NiTi shape memory alloys (SAMs) with two different initial Ni-contents of 51.5% and 52.5% (mole fraction), respectively. The simulation results show that, during stress-free aging of the NiTi alloy with a low supersaturation of Ni (i.e., Ti-51.5%Ni), the Ni4Ti3 precipitates exhibit a heterogeneous distribution with a high number density of particles at the grain boundary, leaving most of the grain interiors free of precipitates; while for the NiTi alloy with a high supersaturation of Ni (i.e., Ti-52.5%Ni), the Ni4Ti3 precipitates show a homogeneous distribution across the entire simulation system. The stress-assisted aging can give rise to homogeneous distribution of the precipitates, regardless of the initial Ni-content; however, the distribution of variant type within the two grains is heterogeneous.