Influence of nanocrystalline Ni–Ti reaction agent on self-propagating high-temperature synthesized porous NiTi

Nanocrystalline Ni–Ti was used in self-propagating high-temperature synthesis (SHS) to fabricate porous NiTi. The SHS of porous NiTi using elemental powders was also prepared for comparison. Results showed that the main phase was NiTi with unreacted Ni when using elemental powders, which is detrimental to medical use. A large amount of Ti₂Ni secondary phase was also detected. By employing mechanically alloyed nanocrystalline Ni–Ti as a reaction agent, the secondary intermetallic phase (i.e. Ti₂Ni) was significantly reduced and the unreacted Ni was eliminated. The addition of 25 wt% nanocrystalline Ni–Ti reaction agent produced porous NiTi with an average porosity of 52–55 vol% and a general pore size of 100–600 μm under preheating temperatures of 200 and 300 °C. This general pore size in the range of 100–600 μm is beneficial to biomedical application for osseointegration. By further increase of the reaction agent to 50 wt% in the reactant, a porous NiTi part was produced at ambient temperature (i.e. no preheating was necessary) and a dense part was formed at preheated temperature of 200 °C due to the large amount of energies in the nanocrystalline reaction agent. This revealed that the use of nanocrystalline reaction agent effectively lowered the activation barriers for combustion synthesis reaction.     

The compositional stability of the P-phase in Ni–Ti–Pd shape memory alloys

The precipitation of the P-phase in Ni–Ti–Pd and Ni–Ti–Pt shape memory alloys has been shown to dramatically increase the martensitic transformation temperature and strength in Ni-rich ternary alloys, yet little is known about the phase's compositional stability. Therefore, the compositional limits of the P-phase have been systematically studied by varying the Pd and Ni content while maintaining the general P-phase Ti₁₁(Ni + Pd)₁₃ stoichiometry. Each alloy was solutionized at 1050 °C followed by water quenching, and aging at 400 °C for 100 h. Four distinct phases were identified by electron and x-ray diffraction: Ti₂Pd₃, B2 NiTi, P- and P1-phases. The latter precipitate phases became more stable with increasing Ni at the expense of the Pd content. Atom probe tomography revealed the P-phase composition to be 45.8Ti–29.2Ni–25Pd (at.%) or Ti₁₁(Ni₇Pd₆) as compared to the P1-phase 44.7Ti– 45.8Ni–9.4Pd (at.%) or Ti₅Ni₅Pd.     

Powder metallurgical processing of Ni–Ti–Zr alloys undergoing martensitic transformation: part I

Ni–Ti–Zr materials with Zr 12–25 at.% and Ni 42–50 at.% have been produced by powder metallurgy. Suitable temperatures for sintering in Ar-atmosphere Ni–Ti–Zr compacts are within the range 900–1000 °C. Sintering at temperatures above 1000 °C causes melting of the compacts with high Zr content. The presence of ZrC, ZrO₂, Zr₃O, TiO₂ and TiO of different modifications, complex oxides such as Ni₅TiO₇, Ti_0.5Zr_0.5O_0.2 and equilibrium phases after sintering at temperatures above 1000 °C in alloys with low Zr-content was derived from X-ray diffractometry. During sintering at temperatures below 1000 °C the phases belonging to the binary Ti–Ni and Ti–Zr systems were formed. Long-term sintering and slow furnace cooling allowed the precipitation of Ni₄Ti₃ and Ni₂Ti. The process of sintering is controlled by the diffusion of Ni in Ti and Zr particles during the early stages of sintering. Slow diffusion of Zr atoms in Ti₂Ni, Ti–Ni and diffusion of Ti atoms in Zr₂Ni, Ni–Zr controls the later stages of sintering.     

Microstructure and tensile properties of isothermally forged Ni–43Ti–4Al–2Nb–2Hf alloy

NiTi-Al-based alloys are promising high-tem- perature structural materials for aerospace and astronautics applications. A new NiTi-Al-based alloy Ni--43Ti-4AI- 2Nb-2Hf （at%） was processed via isothermal forging. The microstructure and mechanical properties at room temperature and high temperature were investigated through scanning electron microscope （SEM）, X-ray diffraction （XRD）, and tensile tests. Results show that the micro- structure of as-forged Ni-43Ti--4AI-2Nb-2Hf alloy con- sists of NiTi matrix, Ti2Ni phase, and Hf-rich phase. The simultaneous addition of Nb and Hf, which have strong affinities for Ti sites, promotes the precipitation of Hf-rich phases along the grain boundaries. The tensile strengths of Ni-43Ti-4A1-2Nb-2Hf alloy are dramatically increased compared with the ternary Ni-46Ti-4A1 alloy. At room temperature and 650℃, the yield stress of Ni--43Ti-4Al- 2Nb-2Hf alloy reaches 1,070 and 610 MPa, respectively, which are 30 % and 150 % higher than those of Ni--46Ti- 4Al alloy. The improved tensile property results from the solid solution strengthening by Nb and Hf, as well as the dispersion hardening of the Ti2Ni and Hf-rich phases.     

Vacuum brazing of TiAl-based intermetallics with Ti–Zr–Cu–Ni–Co amorphous alloy as filler metal

An amorphous Ti41.7–Zr26.7–Cu14.7–Ni13.8–Co3.1 (wt%) ribbon fabricated by melt spinning was used as filler to vacuum braze Ti–48Al–2Nb–2Cr (at%) intermetallics. The influences of brazing temperature and time on the microstructure and strength of the joints were investigated. It is found that intermetallic phases of Ti₃Al and γ-Ti₂Cu/Ti₂Ni form in the brazed joints. The tensile strength of the joint first increases and then decreases with the increase of the brazing temperature in the range of 900–1050 °C and the brazing time varying from 3 to 15 min. The maximum tensile strength at room temperature is 316 MPa when the joint is brazed at 950 °C for 5 min. Cleavage facets are widely observed on all of the fracture surfaces of the brazed joints. The fracture path varies with the brazing condition and cracks prefer to initiate at locations with relatively high content of γ-Ti₂Cu/Ti₂Ni phases and propagate through them.     

Vacuum brazing of TZM alloy to ZrC particle reinforced W composite using Ti-28Ni eutectic brazing alloy

Reliable brazing of TZM alloy and ZrC particle reinforced (ZrC_p) W composite was achieved in this study by using Ti-28Ni eutectic brazing alloy. The typical interfacial microstructure of TZM/Ti-28Ni/ZrC_p-W brazed joint consisted of a Ti solid solution (Ti(s, s)) layer, a continuous Ti₂Ni layer and a diffusion layer mainly composed of W particles and (Ti, Zr)C particles. With an increase of brazing temperature, more ZrC particles and W particles entered the molten brazing alloy, which broadened the brazing seam and diminished the Ti₂Ni layer, resulting in the disappearance of the Ti₂Ni layer eventually. Meanwhile, more Ti(s, s) stripes were observed on the TZM side. The presence of continuous Ti₂Ni intermetallic phase and Ti(s, s) stripes structure in joints deteriorated the joining properties, which resulted in the formation of brittle fracture under shear test. In addition, the fracture path was related to the brazing temperature, and cracks initiate and propagate in the continuous Ti₂Ni layer at lower temperatures. However, the fracture path tended to be located at the TZM substrate close to the interface between TZM and the brazing seam when the brazing temperature exceeded 1040 °C. The optimal room temperature shear strength reached 120.5 MPa when brazed at 1040 °C for 10 min and the fracture surface exhibited cleavage fracture characteristics, and the shear strength at high temperature of 800 °C for the specimens with highest shear strength at room temperature reached 77.5 MPa.     

On the constitution of the ternary system Al–Ni–Ti

The constitution of the ternary system Al–Ni–Ti is investigated. Phase equilibria at 1000 °C and 900 °C are clarified confirming the stability of the four ternary phases τ₁ to τ₄ already described in the literature. An additional ternary phase τ₅ occurs as an equilibrium phase in the Al-rich corner. The composition of this phase is Al₆₅Ni₂₀Ti₁₅. The ternary phases τ₃ and τ₄ melt congruently at 1289 °C and ∼1500 °C, respectively. The ternary phases τ₁, τ₂ and τ₅ are found to melt incongruently at 1347 °C, 1225 °C, and 1107 °C, respectively. Two ternary eutectics occur: E₁ (τ₄ + Ni₃Al + TiNi₃) at 1269 °C and E₂ (τ₃ + τ₄ + NiAl) at 1221 °C. The composition reported for E₁ in the literature is corroborated. The composition for the newly discovered E₂ is determined to be 43 at%Al, 31 at% Ni, and 26 at% Ti. The section NiAl–NiTi is pseudobinary, but NiTi melts incongruently into L + τ₄ rather than congruently as binary NiTi. The section Ni₃Al–Ni₃Ti is eutectic, but not pseudobinary. By adding aluminium apparently all invariant equilibria of the binary system Ni–Ti are shifted to higher temperatures.     

Annealing effects on the microstructure and properties of bulk high-entropy CoCrFeNiTi0.5 alloy casting ingot

Most previous researches focused on small casting ingots prepared by arc melting, when studying high-entropy alloys. Large sized ingots were also necessary in exploring the existence of volume effects in the multi-principal element alloys. During the experiments, a large sized CoCrFeNiTi_0.5 alloy casting ingot was prepared by a medium frequency induction melting furnace. A slight volume effect occurred, reflecting mainly in the growth of crystalline grains and the increase of alloy hardness in the ingot. To investigate the effect of annealing temperature on microstructure and properties of CoCrFeNiTi_0.5 alloy, several samples taken from the ingot were annealed at 600 °C, 700 °C, 800 °C and 1000 °C respectively for 6 h. Almost no effects were found to the crystalline structure and elemental distribution when the samples were annealed below 1000 °C. The crystalline structure of CoCrFeNiTi_0.5 alloy was composed of one principal face-centered cubic (FCC) solid-solution matrix and a few intermetallic phases in the form of interdentrite. Dendrite contained approximately equivalent amount of Co, Cr, Fe, Ni and a smaller amount of Ti. When annealed below 1000 °C, the interdendrite stayed in (Ni, Ti)-rich phase, (Fe, Cr)-rich phase and (Co, Ti)-rich phase. After 1000 °C annealing, (Co, Ti)-rich phase disappeared, while (Ni, Ti)-rich phase and (Fe, Cr)-rich phase grew. The microhardness of the as-cast CoCrFeNiTi_0.5 alloy was 616.80 HV and the macrohardness was 52 HRC. The hardness of the samples stayed generally unchanged after annealing. This indicated a high microstructure stability and excellent resistance to temper softening that the CoCrFeNiTi_0.5 alloy exhibited.     

层叠Ni/Ti热扩散形成金属间化合物的规律

选择Ni和Ti粉末及其机械合金化粉末制备Ni/Ti扩散偶,利用扫描电镜和X射线衍射等手段研究了Ni/Ti扩散偶在固相热处理作用下金属间化合物的形成及生长规律.随着热处理温度的提高,Ni3Ti,Ti2Ni和NiTi金属间化合物的数量增加明显;随热处理保温时间的增加,NiTi金属间化合物呈抛物线规律生长,而对Ni3Ti和Ti2Ni的生长影响不大.结果表明,金属间化合物在形成过程中,Ni3Ti和Ti2Ni优先形成,达到一定厚度后,NiTi金属间化合物开始形成并快速增长.     

Mechanical milling of Ti–Ni–Si filler metal for brazing TiAl intermetallics

The Ti–Ni–Si filler metal was manufactured by mechanical milling of TiH₂, Ni and Si powder mixture. The microstructure of the filler metal and TiAl brazed joint was analyzed by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). The effect of milling time on the brazing powder was investigated. It was found that NiSi phase formed when the milling time exceeded 120 min. The typical microstructure of the TiAl brazed joint using Ti–Ni–Si filler metal was TiAl/Ti₃Al/TiAlNi₂/Ti₃Al + Ti₅Si₃/TiAlNi₂/Ti₃Al/TiAl. The effect of Si on the microstructure was investigated and the result suggested that Si addition resulted in the aggregation of Ti and formation of Ti₃Al phase in the middle of joint. The optimal parameters were brazing temperature of 1140 °C and holding time of 30 min. The fracture was brittle and propagated between the TiAlNi₂ layer and Ti₃Al + Ti₅Si₃ layer.     

Micropyretic synthesis of NiTi in propagation mode

The microstructure of NiTi formed by micropyretic synthesis, carried out in the propagation mode, was investigated with preheat and Ti particle size as the two process variables. These variables were found to exert a significant influence on the microstructure. as well as the process. Unstable combustion was observed in specimens combusted without preheat. The microstructure of specimens undergoing unstable combustion showed some features which were similar to those developing during rapid solidification of NiTi in terms of the nature of the phases and their morphology. The microstructure essentially comprised the martensitic phase, the B2 parent phase and the Ti₂Ni intermetallic phase. In some specimens the Ti₁₁Ni₁₄ phase was found to form in an unusual lamellar morphology. The microstructural features of the micropyretically synthesized alloy were compared with those of the conventionally processed alloy. The structures of the B2, Ti₂Ni and Ti₁₁Ni₁₄ phases and of their interfaces with the matrix phase were examined by high resolution electron microscopy (HREM). The mechanism of synthesis of the alloy from elemental powders was studied by stopping the combustion front and carrying out detailed microstructural characterization around the arrested front.     

Ordering process of Al5Ti3, h-Al2Ti and r-Al2Ti with f.c.c.-based long-period superstructures in rapidly solidified Al-rich TiAl alloys

Change in microstructure and stability of superstructural phases in Al-rich TiAl alloys containing 58.0–62.5 at.% Al were investigated using melt-spun ribbons. Ordering processes of long-period ordered phases such as Al₅Ti₃, h-Al₂Ti and r-Al₂Ti in the L1₀ matrix during annealing were examined. The presence of Al₅Ti₃ and h-Al₂Ti phases in the L1₀ matrix was confirmed in melt-spun Ti–60.0 at.% Al and Ti–62.5 at.% Al ribbons by electron diffraction patterns, while diffuse scattering corresponding to the Al₅Ti₃ superstructure appeared in Ti–58.0 at.% Al ribbon. In Ti–58.0 at.% Al ribbon, the Al₅Ti₃ phase developed as an island in the L1₀ matrix having an obscure coherent boundary at and below 800°C, while it dissolved during annealing above 800°C. Although the r-Al₂Ti phase was finally formed as an equilibrium phase, the ordering of Al₅Ti₃ and metastable h-Al₂Ti phases in Ti–60.0 at.% Al and Ti–62.5 at.% Al ribbons occurred prior to the precipitation of the r-Al₂Ti during annealing below 800°C. The priority for the ordering process is discussed on the basis of crystal symmetry and periodicity of Al layers parallel to the (002) plane. The anti-phase boundaries (APBs) based on the Al₅Ti₃-type ordering were observed along {110) planes in Ti–62.5 at.% Al ribbon annealed at 700°C and their energies were calculated using the interaction energy between neighbouring atoms.     

Effect of brazing temperature and brazing time on the microstructure and tensile strength of TiAl-based alloy joints with Ti-Zr-Cu-Ni amorphous alloy as filler metal

An amorphous Ti-37.5Zr-15Cu-15Ni (wt.%) ribbon fabricated by vacuum arc remelting and rapid solidification was used as filler metal to vacuum braze TiAl alloy (Ti-45Al-2Mn-2Nb-1B (at.%)). The effects of brazing temperature and time on the microstructure and strength of the joints were investigated in details. The typical brazed joint major consisted of three zones and the brazed joints mainly consisted of α_2-Ti₃Al phase, α-Ti phase and (Ti, Zr)₂(Cu, Ni) phase. When the brazing temperature varied from 910 °C to 1010 °C for 30 min, the tensile strength of the joint first increased and then decreased. With increasing the brazing time, the tensile strength of the joint increased. The maximum room temperature tensile strength was 468 MPa when the specimen was brazed at 930 °C for 60 min. All the fracture surfaces assumed typical brittle cleavage fracture characteristic. The fracture path varied with the brazing parameter and cracks preferred to initiate at (Ti, Zr)₂(Cu, Ni) phase and propagation path were mainly determined by the content and distribution of α-Ti phase and (Ti, Zr)₂(Cu, Ni) phase.     

Phase equilibria and phase transformations in the Ti-rich corner of the Fe–Ni–Ti system

While the main features of the Fe–Ni–Ti system are well known at low Ti content, literature review of the Ti-rich corner revealed inconsistencies between experimental reports. This investigation presents new experimental results, defined to remove the uncertainties concerning melting behavior and solid-state phase equilibria of the (Ni,Fe)Ti₂ phase with the adjacent (Fe,Ni)Ti (B2, CsCl-type structure) and β-Ti (A2, W-type) phases. Six samples have been prepared and examined by differential thermal analysis performed in yttria and alumina crucibles, and by scanning electron microscopy in the as-cast state as well as equilibrated at 900 °C.     

Microstructural characterization in 7 at% Al-containing NiTi-based alloys

The microstructural evolution of Ni–42Ti–7Al and Ni–41Ti–7Al alloys as a function of solution and aging heat treatment was investigated using transmission electron microscopy（TEM）, electron probe, and X-ray diffraction（XRD）. The results reveal that the volume fraction of Ti2 Ni phase as well as its composition does not change significantly after as-solution treated at 1200 °C and aged at 850 °C. At the early stage of the aging treatment at 850 °C for 1 h, the cuboidal β＇ precipitate keeps coherency with the matrix; further aging, β＇ precipitate coarsens, and the semicoherency between the β/β＇ two phases are observed.The shape of coarsened β＇ precipitates changes to the globule, and the interface dislocations are introduced accompanied by the occurrence of semicoherent precipitates. Under the same heat treatment, compared to the Ni–42Ti–7Al alloy, the lattice misfits of the Ni–41Ti–7Al alloy between the β and β＇ two phases are larger, so the β＇ precipitates in Ni–41Ti–7Al alloy are coarsened severely and easily lose coherency with the matrix. The thermal stability of Ni–41Ti–7Al alloy is much worse when aging at 850 °C.     

Phase diagram of the Sn–Zn–Ti system

Equilibrated Sn–Zn–Ti alloys and (Sn + Zn)/α-Ti diffusion couples have been studied by scanning electron microscopy, metallography, and differential scanning calorimetry. For the first time an isothermal section, at 600 °C, of the ternary Sn–Zn–Ti system has been constructed. A previously unknown ternary phase with approximate formula Ti₃Sn₂Zn₆ (probable homogeneity interval in the range Ti₅Sn₄Zn₁₁ to Ti₅Sn₃Zn₁₂) has been found.The solubility ranges of the titanium based solid solutions and the intermetallic phases have been looked for. As far as we could detect and in agreement with theoretical considerations, zinc dissolves more in Ti–Sn phases than tin into Ti–Zn compounds. Titanium additions of 3 and 4 at.% Ti do not influence significantly the Sn–Zn eutectic temperature. The experimentally determined melting enthalpies of the nearly eutectic alloys have values around 100 J g⁻¹.     

Phase Equilibrium in the Ni-Ti-Zr System at 800 °C

The isothermal Ni-Ti-Zr phase diagram at 800 °C was constructed by means of 50 equilibrated alloys. Electron microprobe analyses were used to determine the phase compositions and phase relationships. There is one ternary phase Ni(Ti,Zr)₂ formed in the central region. Most of the Ni-Ti and Ni-Zr binary intermetallic phases show a large Zr or Ti solubility and extend to the ternary region. According to the results of DTA measurements, there are lower liquidus regions around the ternary phase due to the ternary phase reactions with the binary intermetallic phases. There is a potential region to form the bulk metallic glass.     

Effect of long-term ageing on microstructure stability and lattice parameters of coexisting phases in intermetallic Ti–46Al–8Ta alloy

The lamellar α₂(Ti₃Al) + γ(TiAl) microstructure of intermetallic Ti–46Al–8Ta (at.%) alloy is thermodynamically unstable and transforms to α₂ + γ + τ type during long-term ageing at 750 °C. A new ternary τ-phase with B8₂ type structure (space group P6₃/mmc, Pearson symbol hP6) is identified by XRD analysis and its occurence is included to thermodynamic modelling of Ti–Al–Ta phase diagram by CALPHAD approach. The lattice parameters of the coexisting γ, α₂ and τ phases change during ageing.     

The mechanical properties of intermetallic Ni50−xTi50Alx alloys (x = 6,7,8,9)

High-strength, heat- and oxidation-resistant low density Ti–Ni–Al intermetallic alloys have recently attracted attention competing with some conventional high temperature structural superalloy such as Ni-based superalloy. In the present study, the mechanical properties of Ti-rich Ni_50−xTi₅₀Al_x (x = 6,7,8,9) alloys were examined by compression tests at room temperature and at high temperature from 400 °C to 800 °C. X-ray diffraction, scanning electron microscopy as well as microhardness tester were utilized to characterize the microstructure as well as the structural evolution with the increasing Al additions. The systematic analyses of the mechanical behavior were made according to compression test at different temperatures. A yield stress of 1800 MPa and more than 10% of compression strain were achieved at room temperature; and a yield stress of 400 MPa at 800 °C. It is suggested that controlling the shape, the volume percent and the distribution of second phases in the matrix is most important to obtain good mechanical properties in these alloys. The strengthening mechanism of aluminum addition on the mechanical properties was discussed systemically according to the microstructure evolution and solution hardening and precipitation hardening upon Al addition.     

Oxidation resistance and thermal stability of Ti(C,N) and Ti(C,N,O) coatings deposited by chemical vapor deposition

Ti_0.51C_0.24N_0.25 and Ti_0.51C_0.26N_0.21O_0.02 coatings were deposited by medium temperature chemical vapor deposition, their oxidation resistance and thermal stability at 600 °C were compared. Ti(C,N,O) coating is oxidized faster than Ti(C,N) coating. The oxidation resistance of Ti(C,N,O) coating is impaired not only by the existence of more pores, fissures and cracks in the oxide layer, but also by lower interfacial adhesion. Higher tensile stress has a negative impact on the oxidation resistance of Ti(C,N,O) coating. It promotes the formation and development of more defects and cracks during cyclic oxidation, it is also harmful to the interfacial adhesion. Although the oxidation resistance of Ti(C,N,O) coating is inferior, the thermal stability is improved. The incorporation of oxygen slows down the phase transformation of Ti(C,N,O) at 600 °C. During the annealing, the hardness of Ti(C,N,O) coating is always higher than that of Ti(C,N).