B2-TiNi Phases Research Articles

Study on microstructure and mechanical properties of Ti–Co–Ni–Nb multi-principal component alloys designed by proportionally mixing intermetallics and eutectic units

The compositions of Ti–Co–Ni–Nb multi-principal component alloys (MCAs) were designed by proportionally mixing the Ti2Co intermetallics and Ni60Nb40 eutectic unit. The Ti30Co15Ni33Nb22 alloy mainly consists of body-centered cubic (BCC) TiNi matrix phase, along with small quantities of NiNb and Ti2Ni phases. The Ti32Co16Ni31.2Nb20.8 and Ti34Co17Ni29.4Nb19.6 alloys are primarily composed of a B2 TiNi matrix phase, with small amounts of NiNb and Ti2Ni phases. The Ti30Co15Ni33Nb22 alloy exhibits high yield strength and specific yield strength, while the Ti32Co16Ni31.2Nb20.8 alloy demonstrates high fracture strength (2802 MPa) and specific fracture strength. The plastic deformation behavior of these alloys is related to the structure of the TiNi phase. The limited plastic deformation observed in the Ti30Co15Ni33Nb22 alloy is attributed to the deformation of the brittle BCC TiNi phase, whereas the significant plastic deformation observed in the Ti32Co16Ni31.2Nb20.8 and Ti34Co17Ni29.4Nb19.6 alloys is a result of the deformation of the more ductile B2 TiNi phase. This study reaffirms that the proportional mixing of intermetallics and eutectic units is an important method for designing MCAs with high strength.

Porosity and phase composition effects on SHS-NiTi structure and mechanical properties

NiTi samples with 59–70% porosity were obtained by the self-propagating high-temperature synthesis (SHS) using Ti, Ni, and NiTi powders as inert. The maximum and average pore sizes increase, while the maximum and average dimensions of the bridges decrease with porosity, controlled by adjusting the amount of inert additives, based on the surface and cross-sectional microscopy studies. The phase composition of porous NiTi was determined by X-ray diffraction. The alloy contains an austenitic B2 TiNi phase, a martensitic B19′ TiNi phase, and a secondary Ti2Ni phase. SHS-NiTi samples with different porosity were tested by uniaxial compression, revealing mechanical properties’ dependence on porosity. It is shown that with an 11% increase in the NiTi alloy porosity, the yield and compressive strength are reduced by almost a factor of 2.

Design and coherent strengthening of ultra-high strength refractory high entropy alloys based on laser additive manufacturing

The WNbMoTa-based refractory high entropy alloy (RHEA) is expected to become a new generation of high-temperature materials due to its high thermal stability, which can be used in aerospace, nuclear energy and other fields. In this paper, TN10 (TN is the abbreviation of Ti and Ni elements), TN15, TN20 and TN25 non-equimolar WNbMoTa-based RHEAs were designed based on phase engineering and formed by laser powder bed fusion (LPBF) technology. The solidification thermodynamics, microstructure and mechanical properties (compressive strength at 25 °C) of the four alloys were analyzed, and the strengthening mechanism of TN20 alloy was studied. The compressive yield strength of the four alloys exceeds 2050 MPa, the ultimate compressive strength exceeds 2550 MPa, and the elongation exceeds 9%. The compressive yield strength of TN20 alloy is 2513 MPa, the ultimate strength is 3127 MPa, and the elongation is 11.38%. The room temperature (25 °C) compressive yield strength of TN20 alloy is the highest among the high entropy alloys studied so far. TN20 alloy is composed of BCC matrix phase and grain boundary phase containing three different structures, which are B2 TiNi phase, Ni55Nb25Ti15Ta5 phase generating stacking faults and Ni70Ti20Nb10 secondary precipitated phase. The micrometer matrix phase and nanometer grain boundary phase of TN20 alloy show a network distribution in fine crystal region. After the matrix phase cracks due to high thermal stress, the flow TiNi-rich grain boundary phases precipitate to fill the cracks. The grain boundary phase with high proportion in this region is enhanced by the secondary phase and its nanoprecipitated phase. In other regions, stacking faults are generated by the Ni55Nb25Ti15Ta5 phases, which convert the thermal stress that induces crack into lattice defects of the crystal. On the other hand, the content of B2 phase is the highest in grain boundary phase, and the grain boundary between B2 phase and matrix phase is a coherent grain boundary, which realizes the high quality combination of grain boundary phase and matrix phase. The design and strengthening of WNbMoTa-based non-equimolar RHEAs are studied in this paper, which is expected to provide a reference for the design of a new generation of high entropy alloy.

Microstructures, Martensitic Transformation and Mechanical Properties of TiNi-Based Amorphous-Crystalline Composites

By successively introducing minor Al and Sn into the (Ti46.5Ni46.5Hf2Si5)98Nb2 alloy, the amorphous-crystalline composites were fabricated by rapid solidification. The main microstructures were identified as B2 TiNi dendrites together with a few TiNi martensitic crystals. The intercrystalline regions were composed of amorphous phase, B2 TiNi phase, and Ni3.02Ti2Si0.98 intermetallic compounds, whose morphology gradually transformed into striped microstructures. The segregation of Si within intercrystalline regions leads to the formation of the amorphous phase. During deformation, the amorphous-crystalline composites exhibit a double yielding behavior. The yield strength and fracture strength increase to 1114 ± 10 MPa and 2470 ± 15 MPa, respectively, resulting from the reduction of grain size, the existence of the amorphous phase, Ni3.02Ti2Si0.98 intermetallic compounds, and martensitic transformation. All the samples exhibited plastic strains larger than 10% due to the interaction between martensitic transformation and the shear banding process.

Effect of Heat Treatment Temperature on Martensitic Transformation and Superelasticity of the Ti49Ni51 Shape Memory Alloy

The martensitic transformation and superelasticity of Ti49Ni51 shape memory alloy heat-treatment at different temperatures were investigated. The experimental results show that the microstructures of as-cast and heat-treated (723 K) Ni-rich Ti49Ni51 samples prepared by rapidly-solidified technology are composed of B2 TiNi phase, and Ti3Ni4 and Ti2Ni phases; the microstructures of heat-treated Ti49Ni51 samples at 773 and 823 K are composed of B2 TiNi phase, and of B2 TiNi and Ti2Ni phases, respectively. The martensitic transformation of as-cast Ti49Ni51 alloy is three-stage, A→R→M1 and R→M2 transformation during cooling, and two-stage, M→R→A transformation during heating. The transformations of the heat-treated Ti49Ni51 samples at 723 and 823 K are the A↔R↔M/A↔M transformation during cooling/heating, respectively. For the heat-treated alloy at 773 K, the transformations are the A→R/M→R→A during cooling/heating, respectively. For the heat-treated alloy at 773 K, only a small thermal hysteresis is suitable for sensor devices. The stable σmax values of 723 and 773 K heat-treated samples with a large Wd value exhibit high safety in application. The 773 and 823 K heat-treated samples have large stable strain–energy densities, and are a good superelastic alloy. The experimental data obtained provide a valuable reference for the industrial application of rapidly-solidified casting and heat-treated Ti49Ni51 alloy.

Effects of rolling and annealing on hydrogen permeability and crystal orientation in Nb-TiNi two-phase alloys

The Nb19Ti40Ni41 alloy has a lamellar structure with bcc-Nb and B2-TiNi phases. It is known that a granule Nb phase forms in the TiNi matrix after thermal annealing and that the hydrogen permeability of unrolled annealed alloy is higher than that of rolled annealed alloy, even though the two alloys have granule Nb phase. Although the “cube-on-cube” relationship of crystal orientation for these phases has been observed in unrolled annealed alloy, no specific crystal orientation relationship has been seen in rolled annealed alloy. This indicates that the crystal orientation between the two phases strongly affects hydrogen permeability in Nb-TiNi two-phase alloys.

Friction-Induced Hardening Behaviors and Tribological Properties of 60NiTi Alloy Lubricated by Lithium Grease Containing Nano-BN and MoS2

Hardened 60NiTi alloy, which possesses a unique set of desirable properties, has been considered as an attractive candidate for the bearing materials used in space mechanisms. However, the typical hardening process (quenching from high temperatures with a high cooling rate) may result in quench cracking, especially in casting parts. With this in mind, the feasibility of friction-induced surface hardening of 60NiTi under the lubrication of lithium-based greases containing nanoparticles was explored in our research. In addition, the tribological properties of 60NiTi alloy lubricated with lithium-based greases containing different proportions of nanoparticles were investigated. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive spectroscopy (EDS) were used to characterize the surface of worn 60NiTi. An obvious friction-induced hardening effect on the 60NiTi alloy disc surface was identified, which was due to the formation of a hard metastable Ti3Ni4 phase and B2TiNi phase during the friction process. The lubrication effects of all of the modified grease were superior to those of the base grease, and the antiwear properties were closely related to the types of single nano-additives and the proportion of composite nano-additives.

Shape memory TiNi powders produced by plasma rotating electrode process for additive manufacturing

Abstract This study aimed to produce spherical TiNi powders suitable for additive manufacturing by plasma rotating electrode process (PREP). Scanning electron microscopy, X-ray diffractometry and differential scanning calorimetry were used to investigate the surface and inner micro-morphology, phase constituent and martensitic transformation temperature of the surface and inner of the atomized TiNi powders with different particle sizes. The results show that the powder surface becomes smoother and the grain becomes finer gradually with decreasing particle size. All the powders exhibit a main B2-TiNi phase, while large powders with the particle size ≥178 μm contain additional minor Ti2Ni and Ni3Ti secondary phases. These secondary phases are a result of the eutectoid decomposition during cooling. Particles with different particle sizes have experienced different cooling rates during atomization. Various cooling rates cause different martensitic transformation temperatures and routes of the TiNi powders; in particular, the transformation temperature decreases with decreasing particle size.

Relationship between hydrogen permeation and microstructure in Nb–TiNi two-phase alloys

The relationship between the microstructure and hydrogen permeability of the as-cast two-phase Nb–TiNi alloys is investigated and discussed on the basis of the mixing rule. The alloy compositions consisted of the bcc-(Nb, Ti) and B2–TiNi phases are expressed as (Nb4Ti46Ni50)1−x(Nb85Ti13Ni2)x. The alloy for x = 0.19 has a fully eutectic structure of the (Nb, Ti) and TiNi phases in the as-cast state. The hydrogen permeability of this alloy corresponds to that of a model alloy in which these two phases are distributed randomly. The primary TiNi and (Nb, Ti) phases are formed in the alloys for x < 0.19 and x > 0.19, respectively. Their volume fractions decrease and increase with x, respectively. According to the mixing rule, the hydrogen permeability of alloys having a primary (Nb, Ti) phase can be expressed as the model alloy in which the primary phase is isolated in the eutectic structure. However, the hydrogen permeability of alloys having a primary TiNi phase is higher than that expected for the above model alloy.

On the mechanism that leads to vanishing thermal hysteresis of the B2-R phase transformation in multilayered (TiNi)/(W) shape memory alloy thin films

The film stresses in two-phase (TiNi)/(W) shape memory alloy (SMA) multilayer thin films were evaluated using synchrotron diffraction analysis. The phase transforming B2-TiNi phase is under tensile stress due to the mismatch of the coefficient-of-thermal-expansion (αB2-TiNi>αW>αSi-substrate) and the elastic modulus (EW>ESi>EB2-TiNi) with respect to the bcc-W layers and the Si-substrate. The amount of stress on the B2-TiNi phase increases with increasing W amount in the film, which is proportional to the W layer thickness. This led to important changes in the behavior of the B2-R transformation. On cooling, a B2-R transformation proceeds under increasing tensile stress which increases the transformation start temperature (Rs). Upon transformation to the R phase, the TiNi layers undergo stress-relaxation by reorientation of R phase variants to accommodate the mismatch. During heating the film always starts from a relaxed stress-state, so the reverse transformation proceeds without adversely affecting the reverse transformation temperature (Af). With increasing amount of W in the film Rs increases more on cooling, while Af is not significantly affected on heating, and this leads to vanishing thermal hysteresis (ΔTB2-R=Af−Rs).

Production of Nb–Ti–Ni alloy in molten CaCl2

A Nb–Ti–Ni hydrogen permeable alloy was synthesized as a powder using the OS process, which involves the simultaneous reduction of a mixture of Nb, Ti, and Ni oxides by Ca that is generated from molten CaCl2 via electrolysis. The mechanism confirmed that a BCC-(Nb, Ti) phase easily formed at an early stage of the reaction followed by the generation of a B2-TiNi phase from the lower oxides such as NbO and Ti2O. The oxygen content of the synthesized alloy decreased significantly to 0.27mass% with increased electric charge. The three elements were uniformly distributed at the macroscopic level.

Hydrogen permeation in rapidly quenched amorphous and crystallized Nb 20Ti 40Ni 40 alloy ribbons

Formation of an amorphous ( a-) Nb 20Ti 40Ni 40 alloy by rapid quenching and its crystallization behavior were investigated by X-ray diffraction, differential scanning calorimetry and scanning electron microscopy. The B2-TiNi phase was firstly crystallized and the remaining amorphous ( a-) phase subsequently decomposed into the bcc-(Nb, Ti) + B2–TiNi phases. The crystallized ( c-) Nb 20Ti 40Ni 40 samples obtained by annealing above 1173 K were ductile and insusceptible to hydrogen embrittlement between 598 and 673 K. Hydrogen permeability at 673 K ( Φ 673 K ) for the a- and c-Nb 20Ti 40Ni 40 alloys were 8.1 × 10 −9 and (3.0–4.8) × 10 −9 mol H 2/m/s/Pa 0.5, respectively. Φ 673 K increased with the increase in the diameter of the bcc-(Nb, Ti) phase, although this led to poor resistance to hydrogen embrittlement. The lower hydrogen permeability of the c-Nb 20Ti 40Ni 40 alloy was due to its lower hydrogen diffusivity. The present work demonstrated that rapid quenching and subsequent annealing are useful to prepare thermally stable Nb–TiNi hydrogen permeation alloy membranes.

パラジウム代替の水素透過合金の開発戦略

Palladium (Pd) and its alloys are industrially used for separation and purification of hydrogen gas. However, Pd is too expensive and a rare natural resource, so that the development of non-Pd based hydrogen permeation alloys is strongly desired. The present authors have found out that high hydrogen permeability (Φ) is compatible with resistance against the hydrogen embrittlement in the Nb-TiNi alloys consisting of the bcc-(Nb, Ti) and the B2-TiNi phases. The value of Φ for Nb40Ti30Ni30 alloy, for example, is slightly higher than that of pure Pd. Then, in this paper, the possibility of substitution of these alloys for the Pd based hydrogen permeation alloys is discussed based on the experimental data, the prices of the metals and so on.