Fabrication of shape memory TiNi foils via Ti/Ni ultrafine laminates

Thin foils of TiNi shape memory alloy were produced by a diffusion treatment of ultrafine laminates with 50 μm thick composed of pure Ti and Ni layers. The M_s and transformation heat of the TiNi foil were similar to those of the bulk alloys. The foil exhibited the shape memory effect with the recovery strain of about 8.6×10⁻³.     

Microstructure and shape memory behavior of Ti55.5Ni44.5−xCux (x = 11.8–23.5) thin films

The microstructure and shape memory behavior of Ti_55.5Ni_45.5−xCu_x (x = 11.8–23.5) thin films annealed at 773, 873, and 973 K for 1 h were investigated. None of the films except the Ti_55.4Ni_32.8Cu_11.8 film annealed at 773 K for 1 h had any precipitates in the B2 grain interiors and their grain sizes were small (less than 1 μm). Increasing the annealing temperature caused grain growth and thus a decrease in the critical stress for slip and an increase in the martensitic transformation start temperature (Ms). The grain size was also controlled by the growth of a second phase. In the three-phase equilibrium region of Ti₂Ni, Ti₂Cu and TiNi, Ti₂Cu grains grew faster than Ti₂Ni grains, leading to a decrease in the critical stress for slip and an increase in the Ms temperature with increasing Cu content.     

Formation of the Nanocrystalline Structure in an Equiatomic NiTi Shape-Memory Alloy by Thermomechanical Processing

The microstructural evolution during cold rolling followed by annealing of an equiatomic NiTi shape-memory alloy was investigated. The high purity Ni₅₀Ti₅₀ alloy was cast by a copper boat vacuum induction-melting technique. The as-cast ingots were then homogenized, hot rolled, and annealed to prepare the suitable initial microstructure. Thereafter, annealed specimens were cold rolled up to 70 % thickness reduction at room temperature. Post-deformation annealing was conducted at 400 °C for 1 h. The microstructure was characterized using scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and differential scanning calorimetry techniques. The initial microstructure was free from segregation and Ti- or Ni-rich precipitates and was composed of coarse grains with an average size of 50 μm. The cold rolling of NiTi alloy resulted in a partial amorphization and the deformation-induced grain refinement. A nanocrystalline structure with the grain size of about 20-70 nm was formed during the post-deformation annealing.     

Correlation between microstructure and tensile behavior of metal–intermetallic laminate compound with different initial Ni foil thickness

Ni-base metal–intermetallic laminate composites were obtained from in situ reaction synthesis between Ni and Al foils by utilizing plasma activated sintering. The effects of Ni foil thickness on the microstructure and tensile properties of the composites were investigated. The results show that the phases forming during reaction synthesis are independent of the starting thickness of the Ni foils. However, thicker reacted layers are obtained in the samples fabricated from 100 lm Ni foils(Ni100) than those obtained in the samples from 50 lm Ni foils(Ni50)when treated at the same process. The tensile strength of Ni100 samples increases with the temperature increasing at the expense of ductility. Dissimilarly, Ni50 composites treated at higher temperatures exhibit enhanced strength and ductility. Both Ni50 and Ni100 laminate fracture in a similar mechanism. Cracking first occurs in the brittle intermetallic layers. These original cracks result in shear bands in Ni layers emitted from the crack tips, and thus producing local stress concentration, which initiates new cracks in adjacent intermetallic layers. The multiplication of cracks and shear bands leads to the failure of the laminates.     

Vacuum Brazing TC4 Titanium Alloy to 304 Stainless Steel with Cu-Ti-Ni-Zr-V Amorphous Alloy Foil

Dissimilar metal vacuum brazing between TC4 titanium alloy and 304 stainless steel was conducted with newly designed Cu-Ti-Ni-Zr-V amorphous alloy foils as filler metals. Solid joints were obtained due to excellent compatibility between the filler metal and stainless steel substrate. Partial dissolution of stainless steel substrate occurred during brazing. The shear strength of the joint brazed with Cu_43.75Ti_37.5Ni_6.25Zr_6.25V_6.25 foil was 105 MPa and that with Cu_37.5Ti₂₅Ni_12.5Zr_12.5V_12.5 was 116 MPa. All the joints fractured through the gray layer in the brazed seam, revealing brittle fracture features. Cr₄Ti, Cu_0.8FeTi, Fe₈TiZr₃ and Al₂NiTi₃C compounds were found in the fractured joint brazed with Cu_43.75Ti_37.5Ni_6.25Zr_6.25V_6.25 foil, and Fe₂Ti, TiCu, Fe₈TiZr₃ and NiTi_0.8Zr_0.3 compounds were detected in the joint brazed with Cu_37.5Ti₂₅Ni_12.5Zr_12.5V_12.5 foil. The existence of Cr-Ti, Fe-Ti, Cu-Fe-Ti, and Fe-Ti-V intermetallic compounds in the brazed seam caused fracture of the resultant joints.     

Microstructure and hydrogen permeation of cold rolled and annealed Nb40Ti30Ni30 alloy

The ultimate rolling reduction rate (r_u) of the as-cast Nb–TiNi alloys was evaluated by measuring the thickness changes in cold rolled samples. Furthermore, the effects of cold rolling and subsequent anneal on hardness, microstructure, hydrogen permeability (Φ) and hydrogen flux J of the as-cast Nb₄₀Ti₃₀Ni₃₀ alloy (mol%) were examined by a Vickers hardness tester, a scanning electron microscope (SEM), and a mass flow meter, respectively. The value of Φ for the Nb₄₀Ti₃₀Ni₃₀ alloy was reduced with increasing rolling reduction rate and attained to one third of the original one by 50% rolling reduction, but recovered to the original one by anneal at 1373 K for 605 ks. Hydrogen flux J varied inversely proportional to the membrane thickness at 673 K. J of 12 ccH₂/cm²/min was attained for the sample with the thickness of 120 μm. The present work has demonstrated that rolling and the subsequent anneal are effective and useful for the preparation of the hydrogen permeation Nb–TiNi alloy membrane.     

Microstructures of crystallized Ti51.5Ni48.5−xCux (x = 23.4–37.3) thin films

The microstructures of Ti_51.4Ni_25.2Cu_23.4, Ti_51.3Ni_21.1Cu_27.6, Ti_51.2Ni_15.7Cu_33.1, and Ti_51.4Ni_11.3Cu_37.3 thin films annealed at 773, 873 and 973 K for 1 h were investigated. The Ti_51.2Ni_15.7Cu_33.1 and Ti_51.4Ni_11.3Cu_37.3 films showed very small grain sizes (120 and 50 nm, respectively) compared with the Ti_51.4Ni_25.2Cu_23.4 and Ti_51.3Ni_21.1Cu_27.6 films (1 and 0.3 μm, respectively). They had no precipitates within the B2 grains. On the other hand, the Ti_51.4Ni_25.2Cu_23.4 films annealed at 773 and 873 K showed GP zones and Ti₂Cu precipitates, respectively, and the Ti_51.3Ni_21.1Cu_27.6 film annealed at 773 K showed TiCu precipitates in the grain interiors. The formation of precipitates in the grain interior was discussed in terms of the lattice mismatch between the precipitates and the matrix. The difference in grain size was attributed to different crystallization processes.     

Work Production and Variation in Shape Memory Effects during Thermal Cycling of Equiatomic TiNi Alloy

The influence of thermal cycling under a symmetric scheme for work performance on the functional properties and work output of equiatomic TiNi shape memory alloys was studied. It was found that the Ti₅₀Ni₅₀ alloy produced an effective work output of 9.7 MJ/m³ and an efficiency of 1.47% in a symmetrical scheme for work production where the stress acting on cooling was 50 MPa, and the stress acting on heating was 200 MPa. It was observed that the Ti₅₀Ni₅₀ alloy demonstrated the monotonic dependence of plastic strain on the number cycle, and 3.1% of the residual strain was accumulated over 30 cycles. The data have shown that using a ‘symmetric’ scheme for work production allows one to reduce a plastic strain 13-fold in comparison with an ‘asymmetric’ scheme. Thus, the symmetric scheme for work production provides for the high stability of the functional properties and work output of TiNi-based shape memory alloys.     

Powder metallurgical production of TiNiNb and TiNiCu shape memory alloys by combination of pre-alloyed and elemental powders

The common Ti₄₄Ni₄₇Nb₉ and Ti₅₀Ni₄₀Cu₁₀ ternary shape memory alloys were produced by sintering techniques and the microstructure, phase structure and phase transformation behaviour were investigated. A combination of pre-alloyed binary TiNi powder and elemental Nb, Ni and Cu, Ti powders, respectively, were used. In contrast to the use of pre-alloyed ternary powders, which have to be produced in each new composition, a higher flexibility in the alloy composition becomes possible. In case of the Ti₄₄Ni₄₇Nb₉ alloy, liquid phase sintering was done to obtain the eutectic phase structure known from cast material. In case of the Ti₅₀Ni₄₀Cu₁₀ alloy, the pore size and porosity can be improved by choosing a two-step sintering process, as a eutectic melt between Ti and Cu is formed at low temperatures which influences the sintering behaviour. Controlling the impurity contents and the resulting secondary phases is necessary for both alloys in the same way as for binary TiNi alloys.     

Effects of equal channel angular extrusion and aging treatment on R phase transformation behaviors and Ti3Ni4 precipitates of Ni-rich TiNi alloys

Ni-rich TiNi alloys were subjected to the effect of multiple equal channel angular extrusion (ECAE) treatments by B_C path at 500 °C. The characteristics of R phase transformation in aging treatment were dissimilar in the appearance and the temperature range to those counterparts induced by ECAE treatments. The fine lens-like shape Ti₃Ni₄ particles precipitated mainly in the regions of near grain boundaries and on the tangled grain boundaries after ECAE treatments. The effects and mechanisms of aging treatments and ECAE treatments on R phase transformation behaviors and Ti₃Ni₄ precipitates were investigated and discussed.     

Superelastic properties of nanocrystalline NiTi shape memory alloy produced by thermomechanical processing

Effects of thermomechanical treatment of cold rolling followed by annealing on microstructure and superelastic behavior of the Ni₅₀Ti₅₀ shape memory alloy were studied. Several specimens were produced by copper boat vacuum induction melting. The homogenized specimens were hot rolled and annealed at 900 °C. Thereafter, annealed specimens were subjected to cold rolling with different thickness reductions up to 70%. Transmission electron microscopy revealed that the severe cold rolling led to the formation of a mixed microstructure consisting of nanocrystalline and amorphous phases in Ni₅₀Ti₅₀ alloy. After annealing at 400 °C for 1 h, the amorphous phase formed in the cold-rolled specimens was crystallized and a nanocrystalline structure formed. Results showed that with increasing thickness reduction during cold rolling, the recoverable strain of Ni₅₀Ti₅₀ alloy was increased during superelastic experiments such that the 70% cold rolled–annealed specimen exhibited about 12% of recoverable strain. Moreover, with increasing thickness reduction, the critical stress for stress-induced martensitic transformation was increased. It is noteworthy that in the 70% cold rolled–annealed specimen, the damping capacity was measured to be 28 J/cm³ that is significantly higher than that of commercial NiTi alloys.     

Microstructures evolution and phase transformation behaviors of Ni-rich TiNi shape memory alloys after equal channel angular extrusion

Ni-rich TiNi shape memory alloys were subjected to the effect of equal channel angular extrusion (ECAE) processes at 773 K by Bc path. The effects of ECAE processes on microstructures evolution and phase transformation behaviors were investigated. The initial 60-80 μm equiaxed coarse grains of samples were elongated along the shearing force direction of ECAE and refined to 300-400 nm after eight passes ECAE. The R phase transformation of Ni-rich TiNi shape memory alloys was stimulated by ECAE processes within a larger temperature range. The martensite transformation peak temperature (Mp) dropped in previous 1-3 ECAE treatments, but the dropped Mp increased gradually with the increase of ECAE processes. Ti₃Ni₄ phase was observed in the regions with high density of dislocations after ECAE treatment. Reasons for microstructures evolution and phase transformation changes were also discussed.     

Damage-free highly efficient polishing of single-crystal diamond wafer by plasma-assisted polishing

Single-crystal diamond (SCD) is considered to be an ideal material for next-generation power devices. Plasma-assisted polishing (PAP) without using an abrasive was applied to polish SCD fabricated by chemical vapor deposition. Argon-based plasma containing water vapour was used in the PAP to modify the surface of polishing plate and SCD (100), and SCD was polished under a polishing pressure ranging from 10 to 52.6 kPa. Raman spectroscopy measurement showed that there was no residual stress on the polished SCD surface, and a polishing rate of 2.1 μm/h and a surface roughness of 0.13 nm Sq were obtained.     

Phase transformation and damping behavior of lightweight porous TiNiCu alloys fabricated by powder metallurgy process

Porous TiNiCu ternary shape memory alloys (SMAs) were successfully fabricated by powder metallurgy method. The microstructure, martensitic transformation behavior, damping performance and mechanical properties of the fabricated alloys were intensively studied. It is found that the apparent density of alloys decreases with increasing the Cu content, the porous Ti₅₀Ni₄₀Cu₁₀ alloy exhibits wide endothermic and exothermic peaks arisen from the hysteresis of martensitic transformations, while the porous Ti₅₀Ni₃₀Cu₂₀ alloy shows much stronger and narrower endothermic and exothermic peaks owing to the B2-B19 transformation taking place easily. Moreover, the porous Ti₅₀Ni₄₀Cu₁₀ alloy shows a lower shape recovery rate than the porous Ti₅₀Ni₅₀ alloy, while the porous Ti₅₀Ni₃₀Cu₂₀ alloy behaves reversely. In addition, the damping capacity (or internal friction, IF) of the porous TiNiCu alloys increases with increasing the Cu content. The porous Ti₅₀Ni₃₀Cu₂₀ alloy has very high equivalent internal friction, with the maximum equivalent internal friction value five times higher than that of the porous Ti₅₀Ni₅₀ alloy.     

Evolution of the bulk microstructure in 1100 aluminum builds fabricated by ultrasonic metal welding

The annealed 1100 aluminum foils were welded at room temperature with an ultrasonic metal welding (UMW) method. Effects of two key parameters (the oscillation amplitude and the deformation reduction of the welded foils) on the microstructural evolution were investigated. With the increase of oscillation amplitude, the deformation and the grain refinement of the foils in the welded specimen were homogeneous, but the grain size was not less than 25 μm. With the increase of deformation reduction, the microstructures were inhomogenously changed from the initial coarse grains (45 μm) into the dynamically recrystallized fine grains (2 μm) in the upper foil, but they changed little in the lower foil. For both cases, the microstructural evolutions attributed to the grains and/or sub-grains rotation. The dynamic recovery and the followed continuous dynamic recrystallization were the active deformation mechanism during UMW according to the observation of the thermal and the deformation textures. The effects of both ultrasonic amplitude and deformation reduction on the hardness of the builds were measured.     

Thermal and transport properties of as-grown Ni-rich TiNi shape memory alloys

Electrical resistivity, Seebeck coefficient, specific heat and thermal conductivity measurements on the Ti_50−xNi_50+x (x = 0.0–1.6 at.%) shape memory alloys are performed to investigate their thermal and transport properties. In this study, anomalous features are observed in both cooling and heating cycles in all measured physical properties of the slightly Ni-rich TiNi alloys (x ≤ 1.0), corresponds to the transformation between the B19′ martensite and B2 austenite phases. Besides, the transition temperature is found to decrease gradually with increasing Ni content, and the driving force for the transition is also found to diminish slowly with the addition of excess Ni, as revealed by specific heat measurements. While the signature of martensitic transformation vanishes for the Ni-rich TiNi alloys with x ≥ 1.3, the characteristics of strain glass transition start to appear. The Seebeck coefficients of these TiNi alloys were found to be positive, suggesting the hole-type carriers dominate the thermoelectric transport. From the high-temperature Seebeck coefficients, the estimated value of Fermi energy ranges from ∼1.5 eV (Ti_48.4Ni_51.6) to ∼2.1 eV (Ti₅₀Ni₅₀), indicating the metallic nature of these alloys. In addition, the thermal conductivity of the slightly Ni-rich TiNi alloys with x ≤ 1.0 shows a distinct anomalous feature at the B19′ → B2 transition, likely due to the variation in lattice thermal conductivity.     

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

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.     

Hot rolling of gamma titanium aluminide foil

Metal flow and microstructure evolution during the thermomechanical processing of thin-gage foil of a near-gamma titanium aluminide alloy, Ti–45.5Al–2Cr–2Nb, with an equiaxed-gamma microstructure was investigated experimentally and theoretically. Foils of thickness of 200–250 μm were fabricated via hot rolling of sheet in a can of proprietary design. The variation in gage of the rolled foils was ±15 μm except in very sporadic (local) areas, with variations of approximately 60 μm relative to the mean. Metallography revealed that the larger thickness variations were associated with large remnant colonies lying in a hard orientation for deformation. To rationalize these observations, a self-consistent model was used to estimate the strain partitioning between the softer (equiaxed-gamma) matrix and the remnant colonies. Furthermore, the efficacy of pre- or post-rolling heat treatment in eliminating remnant colonies was demonstrated and quantified using a static-spheroidization model. The elimination of remnant colonies via spheroidization prior to foil rolling gave rise to improved gage control.     

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.     

Solid state reactions in Al/Ni alternate foils induced by cold rolling and annealing

Al/Ni composites made of alternate foils having overall composition Al₅₀Ni₅₀ and Al₆₆Ni₃₄ were rolled up to 75 times folding them after every rolling pass to restore approximately the original thickness. It was found that the deformation of the composite is sustained by the Ni with Al acting as transmitting medium. The logarithmic reduction of foil thickness scales with the number of rolling passes. A nanocrystalline state of the elements, particularly Ni, is progressively reached. No detectable reaction is caused by repeated co-deformation. Reactions in the composites occur on annealing. The sequence of phases obtained at Al/Ni interfaces via nucleation and growth, and identified by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, reproduces that found on annealing deposited multilayers and ball-milled powders: Al₃Ni, Al₃Ni₂, NiAl. All reactions are strongly activated by deformation, i.e. they occur at lower temperature as revealed by continuous heating experiments in a differential scanning calorimeter. The overall set of experimental results is consistent with reaction mechanisms of nucleation and growth with grain-boundary interdiffusion as the rate-determining step. This view is supported by comparison with a collection of data for the activation energy of diffusion, grain growth, and ordering in Al–Ni phases.