Ti-rich NiTi Alloy Research Articles

Effect of aging time on mechanical and functional properties of graded Ti-Ni alloy joined by laser additive manufacturing

In this work, a functionally graded Ti-Ni alloy was fabricated using directed energy deposition by depositing Ti-rich Ti-Ni alloy section on near-equiatomic TiNi substrate to demonstrate multi-sectional microstructural, thermal, and mechanical behaviors for multifunctionality. The aging effect at 400 °C with various heat treatment time durations on the graded Ti-Ni alloy was applied. Materials characterization indicated that the two sections demonstrate different responses under various aging treatments, which provided more possible combinations of functional properties in a joined component. This work shows a powerful potential application in using directed energy deposition to fabricate multifunctional smart alloys followed by aging treatment.

Investigation of thermodynamic and microstructural characteristics of NiTiCu shape memory alloys produced by arc-melting method

$$\hbox {Ni}_{ 50-x}\hbox {Ti}_{50}\hbox {Cu}_x$$ (at.%) $$x=14, 17, 20)$$ shape memory alloys were produced using the arc-melting technique. The replacing Cu element into Ti-rich NiTi alloy has been investigated through characterization techniques, including differential scanning calorimetry (DSC), X-ray diffraction (XRD), optical and SEM microscopy. Additionally, various phases and precipitations were determined using EDS measurement. The manufactured alloys showed a single-step austenite $$\leftrightarrow $$ martensite phase transformation (B2 $$\leftrightarrow $$$$\hbox {B19}^{\prime }$$ ). The DSC results showed that the samples have comparable high-temperature hysteresis in the range of about (30–50 K). The main detected XRD peaks are the monoclinic martensite phase with cubic austenite phases, which their intensities were decreased by increasing copper addition into the alloy. The microhardness results revealed that the second phase of the alloys has higher hardness compared to the matrix phase. It was concluded that NiTiCu can be used in low-temperature shape memory alloys as they illustrate transformation temperature below $$100\,^{\circ }\hbox {C}$$

Evaluation of palladium diffusion in Ti-Rich TiNi alloys

In order to study the diffusion kinetics of Pd into Ti-rich TiNi alloys, diffusion annealing treatments of palladium-coated Ti53Ni47 and Ti50.2Ni49.8 alloys were carried out at temperatures ranging from 700 to 1000 °C. Linear Arrhenius temperature dependencies with the pre-exponential factors of 2 ± 1.1 × 10−10 and 1.05 ± 1.5 × 10−8 m2 s−1 and the activation energies of −128 ± 4 and −153 ± 10 kJ mol−1 were obtained for Pd diffusivity into Ti53Ni47 and Ti50.2Ni49.8 alloys, respectively. The suggested mechanism for Pd diffusion in Ti53Ni47 was anti-site bridge mechanism. While the rapid diffusion of Pd in Ti50.2Ni49.8 was attributed to the high volume fraction of vacancies in Ni sub-lattice of TiNi lattice and Next Nearest jumps of Pd atoms into abundant Ni vacancies. The lower diffusivity of Pd in Ti53Ni47 was related to the dominant diffusion mechanisms in two systems and low solubility of Pd in Ti2Ni precipitates which located in grain boundaries, as well.The other part of the study was denoted to inter-diffusion studies in Ti53Ni47/Pd and Ti50.2Ni49.8/Pd diffusion couples, which confirmed the higher diffusivity of Pd in Ti50.2Ni49.8.

Investigation on Ce Addition on Microstructure and Martensitic Transformation of a Ti51Ni49 Shape Memory Alloy

The effect of rare earth element Ce addition on the microstructure and martensitic transformation behavior of Ti51Ni49 shape memory alloy was investigated by scanning electronic microscope (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results show that the microstructure of TiNiCe ternary alloy consists of Ti2Ni phase, CeNi phase and the matrix. One-step martensitic transformation is observed in quenched TiNiCe ternary alloys, which is the same as that in quenched TiNi binary alloys. The martensitic transformation temperatures of Ti-rich TiNi alloy hardly increase with Ce addition.

The effect of annealing on the mechanical properties and microstructural evolution of Ti-rich NiTi shape memory alloy

An investigation was carried out into the influence of the annealing temperatures on the thermo-mechanical behavior of Ti-rich NiTi alloy with regard to transformation temperatures, mechanical properties at room temperature and microstructure evolution under deformation. It was found that annealing above the recrystallization temperature (600°C) modulated the mechanical behavior of the alloy significantly. Based on tensile and DSC analysis, it was observed that by increasing the annealing temperature, the shape memory behavior of the alloys improved. Scanning and transmission electron microscopy were used to investigate the fracture surfaces and microstructural evolution of the NiTi samples after failure. Fractography revealed the brittle fracture area produced through the propagation of cleavage cracks; however, ductile fracture via nucleation growth and coalescence of micro-dimples in the martensitic phase at room temperature were also observed. During plastic deformation, the NiTi alloy was also observed to undergo a detwinning process, dislocation slip and the formation of submicrocrystalline grains, nanocrystallization and amorphous bands.

The development of TiNi-based negative Poisson's ratio structure using selective laser melting

There is a growing interest in using additive manufacturing to produce smart structures, which have the capability to respond to thermal and mechanical stimuli. In this report, Selective Laser Melting (SLM) is used to build a Negative Poisson's Ratio (NPR) TiNi-based Shape Memory Alloy (SMA) structure, creating a multi-functional structure that could be used as reusable armour. The study assesses the influence of SLM process parameters (laser power, scan speed, and track spacing) on the microstructural and structural integrity development in a Ti-rich TiNi alloy, as well as the impact of the post-process homogenisation treatment on the microstructure and phase transformations. The builds generally shows stress-induced cracks and residual porosity, which could be minimised through process optimisation. Nonetheless, the homogenisation treatment is essential to reduce the fraction of Ti2Ni intermetallics, which are known to disturb the B19′-chemistry, and hence the required phase transformation temperatures. The optimum process parameters are finally used to fabricate NPR structures, which were mechanically tested to validate the Poisson's ratio predictions. A higher ductility was observed in the structures that have undergone the homogenisation treatment.

Study of the microstructure evolution of heat treated Ti-rich NiTi shape memory alloy

Martensitic evolution in Ti-rich NiTi alloy, Ti50.5Ni49.5, has been investigated as a function of annealing, solution treatment and a combination thereof and a detailed electron microscopic investigation carried out. Self-accommodated martensite plates resulted in all heat treated samples. Martensitic <011> type II twins, which are common in NiTi shape memory alloys, was found in both as-received and heat-treated samples. Solution treated samples, additionally, showed {11-1} type I twinning was also found in samples that have been annealed after solution-treatment. Another common feature of the microstructure in both as-received and heat treated samples is the formation of Ti2Ni precipitates. The size, number and dispersions of these precipitates can be controlled by resorting to a suitable heat treatment e.g. solution treatment.

Shape memory characteristics of highly porous Ti-rich TiNi alloys

Ti50.5Ni49.5 alloy fibers were fabricated using a melt overflow process. The effects of rapid solidification on the microstructure, transformation temperatures, and shape memory characteristics of Ti-rich TiNi alloys were investigated. The addition of 0.5 at% Ni in Ti50Ni50 alloy increased the transformation temperatures. Highly porous billets were produced using a vacuum sintering technology with the fibers. While compression mechanical testing carried out on porous specimens revealed the residual strain of 6.7%, this strain recovered up to 5.4% due to the shape memory effect. The permanent plastic strain resulting from 10% compressive deformation was only 1.3% in the TiNi shape memory alloy with 75% porosity. Because of the high porosity of sintered specimens, an elastic modulus of about 3.03 GPa could also be obtained.

Design, fabrication and characterization of porous Ti-rich Ti-Ni alloy based composites with near-zero thermal expansion behavior

Porous Ti-rich Ti-Ni alloy based composites with near-zero thermal expansion (NZTE) behaviour tailored by micro-sized SiC particles were fabricated using powder metallurgy and a two-step designing strategy. At first, the Ni concentration was tuned to broaden the temperature range of negative thermal expansion (NTE) of porous Ti-Ni alloys; then micro-sized SiC particles of different amounts were introduced and co-sintered with mixed Ti and Ni powders having an optimized atomic ratio so as to obtain SiC/Ti-Ni composites with desirable thermal expansion performance. Results show that there is a clear correlation between the reverse martensitic transformation and NTE temperature range. The porous 12wt.%SiC/Ti-43.8at.%Ni composite exhibits very low coefficient of thermal expansion (CTE) of -0.976×10 −6 K −1 from 114.08 to 131.50 °C. It is proposed that the NTE behavior originates from the volume change accompanying the phase transition of the Ti-Ni matrix, and the NZTE performance of SiC/Ti-Ni composites is attributed to the combination of the NTE produced by the alloy matrix and the positive thermal expansion provided by the SiC phase.

Fabrication of Porous Ti-rich Ti51Ni49 by Evaporating NaCl Space Holder

Net-shaped porous Ti-rich Ti51Ni49 alloy with well-controlled porosity, pore size, and pore shape are fabricated by pressing-and-sintering compacts containing fine Ti and Ni powders and coarse NaCl powders. After sintering at 1323 K (1050 °C) for 30 minutes in a high vacuum, the NaCl space holder is removed by evaporation, and the remaining Ti and Ni powders are sintered with about 2.3 vol pct liquid phase. The sintered Ti51Ni49 compacts have porosities of 26, 64, 70, 78, and 85 pct, and no distortion is observed. DSC tests show that the M S temperature and ΔH are about 347 K (74 °C) and 28 J/g, respectively, and that they are almost independent of the porosity and close to those of wrought Ti-rich TiNi alloys. These porous Ti51Ni49 compacts exhibit a homogeneous microstructure, and the compressive properties and porosity are close to those of human bones.

Enhanced shape recovery in cryogenically treated martensitic Ti–Ni alloys

The present investigation concerns the effect of cryogenic treatment on mechanical properties and shape recovery behavior of the Ti-rich Ti–Ni alloy. The sample quenched in liquid nitrogen (CQ) has resulted in higher volume fraction of martensite (B19′) phases and exhibited higher amount of recoverable strain (∼10%) in the region of iso-stress tensile deformation in comparison to the samples quenched in ice water. Cold rolling of the sample at near liquid nitrogen temperature (CR), with and without subsequent aging treatment, has resulted in the bimodal distribution of martensite plate size and negligible amount of recoverable strain under iso-stress tensile deformation.

Martensitic transformation behaviors of Ti-rich Ti–Ni alloy fibers fabricated by melt overflow

Ti50Ni50, Ti50.5Ni49.5, Ti51Ni49 and Ti51.5Ni48.5 fibers were fabricated by melt overflow process. The rapid solidification can increase the solubility above the equilibrium solubility. The effects of the rapid solidification of Ti-rich Ti–Ni alloys on the microstructure, transformation temperatures and shape memory characteristics are investigated. The addition of 0.5at.% Ti in Ti50Ni50 alloy greatly increases the transformation temperature. However, the transformation temperatures decrease again for Ti content exceeding 51at.%. Results of thermal cycling tests under various constant stress levels reveal that the recoverable elongation associated with B2–B19 martensitic transformation of Ti50.5Ni49.5 fibers is two times larger than that of Ti51.5Ni48.5 alloy fiber.

Mechanical properties of Ti–(∼49 at%) Ni shape memory alloy, part II: Effect of ageing treatment

The present investigation aims to study the effect of ageing treatment on the mechanical properties and shape memory characteristics of the Ti-rich Ti–Ni alloy. The ageing treatment of the homogenized and deformed Ti-rich alloys results in the formation of Ti2Ni precipitate phase. The coarser precipitates occasionally provide the nucleation sites for the martensite plates. In the case of the deformed samples, the austenites to martensite and reverse transformations have been constrained due to the presence of deformation strain as evidenced from the corresponding XRD pattern. In the case of the aged samples, the martensitic transformation evolved through the one-stage (B2→B19′) and two-stage transformation routes via the formation of R phase. The most favorable shape recovery is achieved at the peak aged condition of the samples owing to the formation of optimum particle size conducive to delaying the onset of plastic straining.

Mechanical properties of Ti-(∼49 at%) Ni shape memory alloy: Part I. Effect of cold deformation

The present investigation aims to study the effect of cold deformation on the mechanical properties and shape recovery behavior of the Ti-rich Ti–Ni alloy. The micrographic investigations and XRD analyses have revealed that the microstructure of the homogenized and quenched samples comprises austenite (B2) and martensite (B19′) phases. Significant enhancement of martensite volume fractions in the microstructure has been achieved after 30% cold deformation. The effect of deformation has also been manifested in terms of the sloping tensile curve in the region of shape memory strain. The shape recovery experiment has indicated improved shape recovery ratio in the case of the deformed samples owing to the delay in onset of the plastic straining during the shape deformation.

Effect of warm-working rate on shape memory properties in Ti–49.7at.%Ni alloy

Abstract This study investigated the effect of warm-working rate on the shape memory properties of Ti–49.7 at.%Ni alloy. The ingot was prepared by using an arc melting method and then warm-rolled at 673 K to reduce the thickness by 77%, 85%, and 89%. For comparison, an ingot was cold-rolled at room temperature to reduce the thickness up to 30%. The warm-worked specimens were annealed at 623–723 K for 3.6 ks, and the cold-worked specimens were annealed at 673 K for 3.6 ks. The transformation behavior was analyzed by means of differential scanning calorimeter (DSC) and tensile tests. The critical stress for slip increases with increasing warm-working rate and decreasing annealing temperature. The martensitic transformation temperature ( T R→M ) and the reverse transformation temperature ( T M→B2 ) increase with decreasing warm-working rate and increasing annealing temperature. The critical stress for slip in the warm-worked specimens is higher than that of the specimens annealed at 673 K after cold-working. Therefore, warm-working at 673 K improves the shape memory characteristics of Ti-rich Ti–Ni alloys. This suggests that Ti-rich Ti–Ni alloys are good candidate materials for actuators.

Effects of second phases on mechanical properties and martensitic transformations of ECAPed TiNi and Ti–Mo based shape memory alloys

TiNi and Ti-based shape memory alloys were processed by equal channel angular pressing (ECAP) at 673-773 K along Bc route to obtain ultrafine grains for increasing the strength of parent phase and improving the functional properties. The effects of both thermodynamically stable and metastable second phases on the mechanical properties and martensitic transformations of these alloys were investigated. It is found that thermodynamically stable Ti2Ni phase has no effect on martensitic transformation and superelasticity of Ti-rich TiNi alloy, thermodynamically stable α phase is harmful for ductility of Ti–Mo–Nb–V–Al alloy, but metastable Ti3Ni4 phase is effective for R phase transformation, martensitic transformation and superelasticity of Ni-rich TiNi alloy. The mechanisms of the second phases on the martensitic transformations and mechanical properties were discussed.

Stability of thermal-induced phase transformations in the severely deformed equiatomic Ni–Ti alloys

Nickel–Titanium (Ni–Ti) alloys of two different near-equiatomic chemical compositions (Ni-rich and Ti-rich) are subjected to severe plastic deformation by means of high pressure torsion (HPT) by higher rotation speed and larger total number of rotations. Further, the as-received and severely deformed specimens are subjected to heat treatments at 300 and 350 °C. Phase transformations of the specimens under different conditions are analyzed by employing differential scanning calorimetry and by X-ray diffraction. The results obtained show that in Ti-rich Ni–Ti alloy the sequence of phase transformations is found to be stable against heat treatments and independent of previous HPT process. Also, in Ni-rich Ni–Ti alloy, when it is subjected to HPT, the sequence of phase transformations found to remain unaltered. However, with or without HPT, after the heat treatments at 300 and 350 °C, the sequence of the phase transformation is found to be affected.

Effects of pre-deformation and subsequent low-temperature annealing on transformation, mechanical properties and shape memory behavior of a Ti-rich TiNi alloy

Effects of pre-deformation in the martensitic state and subsequent low-temperature annealing on the phase transformation, tensile properties and shape memory performance of a Ti-49.8 at.% Ni alloy were investigated in order to improve mechanical and memory properties of the alloy. The results show that water quenching from 850 degrees C and subsequent annealing do not induce Ti-rich precipitates in the alloy. Transformation from parent to R-phase appears on cooling for alloys annealed at 350-500 degrees C after pre-deformed to the strain of as high as 19.0%, and the transformation temperature and rate from parent to R-phase are not affected by pre-deformation, but M(s) and M(f) reduce with increasing pre-deformation strain, and the volume fraction of R-phase increases in the microstructure of alloys annealed at low temperature after pre-deformation to higher strain. Tensile and memory properties of the Ti-49.8 at.% Ni alloy can be improved by appropriate pre-deformation (more than 27.8% strain) in the martensitic state and subsequent low-temperature annealing (at 400 degrees C).

Mechanical Behaviour of Ausformed and Marformed Ti-Rich Ni-Ti Shape Memory Alloys

Shape memory effect (SME) in Nickel-Titanium (Ni-Ti) alloys is ascribed to the thermoelastic reversible martensite phase transformation. Phase transformation is established to be affected by the pre- thermal and mechanical history of the alloy. The present work deals with the effect of mechanical working, known as ‘marforming’ and ‘ausforming’, on the phase transformation characteristics and mechanical behaviour of Ti-rich Ni-Ti alloy. Tensile study and measurement of the hardness data were carried out at room temperature. Mere heat treatment or heat treatment at 773 K after the marforming shows similar characteristics, whereas, the as-received and the ausformed samples exhibit different behaviours. Hardness numbers of the heat treated samples are found to be smaller than those of the as-received and mechanically worked samples.

Powder metallurgical processing of NiTi shape memory alloys with elevated transformation temperatures

The production of high quality powder metallurgical NiTi alloys with elevated phase transformation temperatures is challenging. During processing, an unavoidable pickup of impurity elements (especially oxygen and carbon) results in a decrease of phase transition temperatures and in the formation of brittle secondary phases. We introduce a processing route including melting, gas atomization and hot isostatic pressing for binary NiTi shape memory alloys which minimizes these problems. We demonstrate that the microstructure of the Ti-rich NiTi alloy contains precipitates of Ti 2Ni type which can be exploited to dissolve oxygen picked up during later process stages. In this study, three powder fractions with different grain sizes and impurity contents were subjected to hot isostatic pressing. The evolution of microstructures and material properties was studied by chemical analysis, microscopy, differential scanning calorimetry, and mechanical testing. Exploiting the solubility of oxygen in Ti 2Ni, the processing route presented in the present paper succeeds in producing powder metallurgical NiTi shape memory alloys with good structural and functional properties.