Ni-rich NiTi Shape Memory Alloy Research Articles

Effect of Ni4Ti3 precipitates on the functional properties of NiTi shape memory alloys: A phase field study

Ni4Ti3 precipitates, which generally exist in aged Ni-rich NiTi shape memory alloys (SMAs), can have a profound effect on material properties. However, the fundamental insights into the effects of Ni4Ti3 precipitates on the functional properties, including the superelasticity (SE), elastocaloric effect (eCE), and shape memory effects (SMEs), are not well understood yet, especially those originating from the B2-B19′ martensite transformation (MT). In this work, a phase field model coupling the precipitation of Ni4Ti3 and the B2-B19′ MT is proposed, where the thermo-mechanical coupling effect and grain size effect are considered. The precipitate-dependent SE, eCE, one-way SME (OWSME), and stress-assisted two-way SME (SATWSME) of single-crystal, polycrystalline, and gradient-nanograined NiTi SMAs are simulated. The effects of the precipitate density, grain orientation range (texture), and gradient-distributed precipitate are examined, and the underlying microscopic mechanisms are revealed. The simulation results and new findings not only contribute to a more comprehensive insight into the effect of Ni4Ti3 precipitates on the MT, martensite reorientation, and functional properties of NiTi SMAs but also provide a reference for the development of excellent SMA-based solid refrigerants or SMA smart materials with designable functional properties.

Effect of crystallographic anisotropy on phase transformation and tribological properties of Ni-rich NiTi shape memory alloy fabricated by LPBF

This article investigated the effects of crystallographic anisotropy on the microstructure, phase transformation and tribological properties of NiTi Shape memory alloy (SMA) fabricated by laser powder bed fusion (LPBF). The maximum tensile recovery strain of 4.13 ± 0.16% was obtained at room temperature by adjusting the hatch spacing. The contributions of metallurgical factors under different crystal orientations, such as active twinned correspondence variant pairs (CVP), Schmid factor, critical stress for the stress-induced martensitic transformation (SIMT), Yong's modulus and nanohardness to superelastic response and tribological properties was clarified through various building orientations (0°, 45°, 90°). The results indicated that the superelasticity of LPBF-fabricated NiTi alloy was crystallographic orientation-dependent. Sample 0° with a strong (001) texture possessed the highest recoverable superelastic strain of 7.91 ± 0.14%. We describe, in detail, how the sliding friction mechanisms such as delamination, adhesive and formation of iron base oxide layers were attributed to superelasticity and the friction pairs under different loading conditions and building orientations. This new understanding reveals how different LPBF-induced microstructures affect mechanical properties and wear, providing a powerful guide for the tribological design or application and tailoring the functional behavior in LPBF-fabricated Ni-rich NiTi alloy.

Correlations between processing technology, microstructure and properties of a Ni-rich Ti–Ni shape memory alloy

A Ti-50.8Ni (at.%)/Ti-55.9Ni (wt.%) VIM processed ingot was rotary swaged and rolled in parallel, obtaining bars with a total Area Reduction above 90% (from forged/rolled raw state one). Throughout the successive stages, the microstructural evolution, thermal and mechanical properties were compared. Deformation bands, grain morphology, precipitates and oxidation were evaluated (by OM and SEM/EDS). An essentially (∼100%) austenitic phase was detected at room temperature (via XRD/Rietveld method), while the Martensitic Transformation temperatures occurred at negative temperatures (via DSC). The rolled bar got through the process more regularly, with homogeneous and refined structure. Typical defects such as superficial microcracks and creases were relevant in the last stages of the two-dies rotary swaging with inductive heating. The work hardening level (via Hardness Test) was similar in both metal forming processes, being 5–7% more pronounced at the edge area of the bars due to redundant work.

Effects of Mechanical Alloying on Ni-Rich NiTi Shape Memory Alloys Produced by Hot Isostatic Pressing

Effects of Mechanical Alloying on Ni-Rich NiTi Shape Memory Alloys Produced by Hot Isostatic Pressing

Effect of Crystallographic Anisotropy on Phase Transformation and Tribological Properties of Ni-Rich Niti Shape Memory Alloy Fabricated by Lpbf

Effect of Crystallographic Anisotropy on Phase Transformation and Tribological Properties of Ni-Rich Niti Shape Memory Alloy Fabricated by Lpbf

Experimental study on performance characteristics of Ni-rich NiTi shape memory alloy in wire electric discharge machining (WEDM)

Ni-rich NiTi shape memory alloys (SMAs) are gaining more prominence compared to near equiatomic NiTi SMAs due to their excellent superelasticity and shape memory properties. The low density and high work output compared to steels make them an excellent choice for automotive and aerospace industries. The study explores the effect of machining parameters, namely, pulse off time, pulse on time, spark gap voltage, wire tension and wire feed rate on material removal rate (MRR), surface roughness (Ra), and surface morphology of Ni-rich NiTi SMA. The experimental results reveal that MRR & Ra increase with the increase in pulse on time while decrease with the increase in pulse off time and spark gap voltage. Wire feed rate and wire tension have negligible influence on MRR and SR. Surface defects, namely, recast layer, micro-cracks & voids were examined through scanning electron microscope (SEM). Energy dispersive X-ray (EDS) and X-ray diffraction (XRD) analysis results reveal the material transfer from wire electrode and the dielectric fluid on the machined surface. Copyright © 2018 VBRI Press.

Solidification characterization and its correlation with the mechanical properties and functional response of NiTi shape memory alloy manufactured by electron beam freeform fabrication

Compared with the common additive manufacturing techniques, the vacuum operating environment and high deposition efficiency of wire-based vacuum additive manufacturing are more expective in the preparation of NiTi shape memory alloys. In the present work, the strategy of interlaminar residence was adopted to deposit the dense wall-structure NiTi shape memory alloy on the commercially pure titanium substrate using the electron beam freeform fabrication (EBF 3 ) technique. The principle of phase constitution, microstructure evolution, martensitic transformation behavior, mechanical properties, and the functional response was studied in detail. The stable regions of as-deposited NiTi alloy were composed of B2 austenite at ambient temperature, and a small amount of B19′ martensite also was observed. Meanwhile, the nano-scaled Ti 4 Ni 2 O x precipitate existed in the interdendritic and grain interior of the EBF 3 -fabricated NiTi alloy. The stable regions of EBF 3 -fabricated NiTi alloy presented a single reversible martensitic transformation (B2 ↔ B19′) during the cooling and heating process. In addition, the solidification characterization, such as microstructure evolution and crystallographic orientation in the building direction and horizon direction, brought about certain differences in the mechanical properties and functional response, such as tensile strength, elongation, superelasticity, and damping capacity. The corresponding intrinsic mechanism has been discussed. Our work also provides a promising method for the flexible manufacture of dense shape memory alloys with large scale and complex geometry, which is beneficial to expanding their application fields. • The dense wall-structure slightly Ni-rich NiTi shape memory alloy was successfully deposited on the commercially pure titanium substrate using the electron beam freeform fabrication (EBF 3 ) additive manufacturing technique. • The stable regions of EBF 3 -fabricated NiTi alloy presented a single reversible martensitic transformation (B2 ↔ B19′) during the cooling and heating process. • The solidification characterization and its correlation with the mechanical properties and the functional response have been discussed.

Study on shape memory effect of hot deep-drawn NI-TI shape memory alloy sheet part

This study investigates the properties of Ni-rich Ni-Ti shape memory alloy sheet parts during and after deep drawing process at an elevated temperature, which is under a plane tensile-compressive stress state. According to the results, the sheet metal shows superelasticity and ductility at 500°C, 550°C, and 600°C. In the hot deep drawing process, the working parameters affect the result. The higher the forming temperature and the longer the holding time at the bottom dead center of the punch or the longer the ageing shape memory treatment, the greater the height of the drawn part and the lesser the springback. The results of the subsequent free recovery experiment show that the deep drawn sheet part without ageing treatment still possesses the shape memory effect. It also shows in the further constraint recovery experiment that in a wide temperature span the deep drawn sheet part possesses a different recovery load when reacting to the phase transformation of the shape memory alloy. Furthermore, the higher the forming temperature and the shorter the ageing treatment, the greater the part recovery load.

Controlling martensitic transformation characteristics in defect-free NiTi shape memory alloys fabricated using laser powder bed fusion and a process optimization framework

Laser Powder Bed Fusion (L-PBF) was utilized to fabricate fully dense, near-equiatomic (Ni50.1Ti49.9) and Ni-rich NiTi (Ni50.8Ti49.2) shape memory alloy (SMA) parts which exhibited tensile ductility up to 16%, shape memory strain of 6%, and tensile superelasticity up to 4%. Annealing heat treatments marginally improved the superelasticity of the as-fabricated Ni-rich NiTi parts due to the formation of a small volume fraction of Ni4Ti3 precipitates. The selection of optimum processing parameters that yielded fully dense parts was guided by a process optimization framework based on a computationally inexpensive analytical model used to predict the melt pool dimensions. The framework also included single-track experiments to validate the model predictions and a criterion for the maximum allowable hatch spacing to prevent the formation of lack of fusion porosity. This framework allowed for constructing L-PBF processing maps for the present NiTi SMAs and revealed that fully dense parts could be printed over a wide range of process parameters. By controlling the L-PBF process parameters, in particular laser power, laser scan speed, and volumetric energy density, in the processing space that result in fully dense parts, it was demonstrated systematically that the composition of the printed parts could be precisely changed by controlling the evaporation of Ni. The flexibility of parameter selection to print defect-free NiTi SMAs and composition control by preferential evaporation of Ni opens the possibility to print functional NiTi SMA parts or devices without post-processing.

Origin of the inverse elastocaloric effect in a Ni-rich Ti-Ni shape memory alloy induced by oriented nanoprecipitates

In the region of a martensitic transformation, conventional elastocaloric effect causes cooling when the forward parent to martensite transition is induced by loading under adiabatic conditions and by heating when the load is removed and the reverse transition occurs. An inverse elastocaloric effect might be expected if the reverse transition could be induced upon loading. However, this behavior is quite uncommon and inverse elastocaloric effect rarely occurs. Herein, we have successfully manufactured a Ti-50.8 at.% Ni crystal in a high proportion of favorable $R$ phase (martensite) variant containing single-variant ${\mathrm{Ti}}_{3}{\mathrm{Ni}}_{4}$ nanoprecipitates aligned along the ${\ensuremath{\langle}111\ensuremath{\rangle}}_{B2}$ direction of the $B2$ parent phase, by means of a compressive stress associated aging along the ${\ensuremath{\langle}111\ensuremath{\rangle}}_{B2}$ direction. The material displays inverse elastocaloric effect induced by compressive stress along the ${[111]}_{B2}$ direction in the temperature range of stability of the $R$ phase. This inverse elastocaloric effect is caused by the stress-induced reverse transformation of the high proportion of favorable $R$-phase variant that occurs when the external compressive stress is applied along the direction of the internal tensile stress induced by the aligned ${\mathrm{Ti}}_{3}{\mathrm{Ni}}_{4}$ nanoprecipitates. The mechanism giving rise to this inverse elastocaloric effect is explained by the transformation strain in the ${[111]}_{B2}$ and ${[1\overline{1}0]}_{B2}$ directions for different $R$-phase variants based on crystallography. Results indicate that the stress-associated aging process is an effective way of patterning materials to display inverse elastocaloric effect through controlling the alignment of the ${\mathrm{Ti}}_{3}{\mathrm{Ni}}_{4}$ nanoprecipitate in the Ni-rich Ti-Ni shape memory alloy.

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.

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 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.

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.

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.

Parametric study on the recast layer during EDWC of a Ni-rich NiTi shape memory alloy

Shape memory alloys (SMAs) have exceptional properties, namely, shape memory effect, superelasticity and other enriched physical and mechanical properties. The conventional machining of these alloys is associated with several severe difficulties. Electric discharge wire cutting (EDWC), an advanced machining process, is utilized to machine Ni50.89Ti49.11 SMA under this investigation. The aim of this investigation was to reveal the effect of EDWC variable parameters such as pulse on time ( TON), wire feed (WF) rate, spark gap voltage (SV), wire tension (WT) and pulse off time ( TOFF) on recast-layer thickness (RLT) and a foreign element atomic content (FEAC) using the one-factor-at-a-time approach. Scanning electron microscope equipped with energy dispersive X-ray spectroscopy facility was utilized to analyse RLT and FEAC. RLT and FEAC are varied linearly and nonlinearly respectively with TON, TOFF and SV. A thick recast layer with a high percentage of FEAC was formed on the machined surface of Ni50.89Ti49.11 SMA at a higher TON and a lower TOFF and SV. However, RLT and FEAC are slightly varied with WF and WT.

A Study on the Nanoparticles Evolution in Isothermally Aged Strain Glass of Ti48.7Ni51.3 Shape Memory Alloy by In Situ Small-Angle X-ray Scattering

Ni-rich nanodomains and Ti3Ni4 nanoprecipitates in the strain glass of Ni-rich Ti-Ni shape memory alloys (SMAs) play an important role in the origin and characteristic control of the strain glass transition. Ni-rich nanodomains are formed in the quenching process and dissolve during isothermal aging. Ti3Ni4 nanoprecipitates nucleate, grow and coarsen during aging. This study performed in situ small-angle X-ray scattering (SAXS) and ex situ X-ray diffraction to investigate the structural evolutions with aging time of nanoparticles in Ti48.7Ni51.3 SMA aged isothermally at 250 °C in terms of relative volume fraction, radius, thickness, orientation, lattice ordering, and morphology. The mechanism and kinetics of the concurrent phase evolutions of Ni-rich nanodomains and Ti3Ni4 nanoprecipitates are revealed and discussed in detail. Frequency-dependent storage modulus vs. temperature at different frequencies for different aging times was measured to characterize the strain glass transition. Quantitative correlation of all structural evolutions and kinetics of Ni-rich and Ti3Ni4 nanoparticles with respect to the variation and loss of the strain glass transition during aging times of 0 to 20 h is presented. The roles of different nanoparticles in causing or suppressing the strain glass behavior are also experimentally revealed.

The machining characteristics and surface integrity of Ni-rich NiTi shape memory alloy using wire electric discharge machining

Owing to the increasing demand for Ni-rich shape memory alloys in various sectors such as biomedical, aerospace, and robotics, the efficient machining of shape memory alloys is vital for their productive exploitation. The aim of this experimental investigation is to explore the influence of wire electric discharge machining process parameters such as spark gap voltage, wire tension, spark off time, wire speed, and spark on time, on the cutting efficiency and surface roughness of Ni50.89Ti49.11 SMA using one factor at a time approach. The results reveal that cutting efficiency and surface roughness are strongly influenced by spark off time, spark on time, and spark gap voltage, whereas wire speed and wire tension have the inconsequential effect. The presence of many microcracks, craters, voids, bulges of debris, and the re-solidified layer of molten material on the machined surface have been detected in scanning electron micrographs. The results of phase analysis using energy-dispersive X-ray spectroscopy and X-ray diffraction divulge the migration of foreign elements from the brass wire and dielectric to the machined surface. Due to the formation of recast layer and various oxides, the hardening effect near the machined surface was also observed. The hardness near the machined surface has been increased several times in comparison to bulk hardness.