Structural transformations and wear resistance of titanium nickelide under conditions of sliding friction at a cryogenic (−196°C) temperature

Structural transformations and tribological properties of a Ti_49.4Ni_50.6 alloy have been investigated at the liquid-nitrogen temperature. It has been shown that the alloy under study possesses the resistance to abrasive and adhesive wear smaller by a factor of 1.4–1.7 and the friction coefficient that is (to 1.7 times) higher, as compared to the austenitic steel 12Kh18N9. The only moderate tribological properties of the titanium nickelide are caused by an enhanced brittleness of this material under the conditions of friction-initiated severe plastic deformation. The enhanced low-temperature brittleness of the martensitic structure is seemingly explained by a low symmetry of the crystal lattice of the B19’ martensite, an atomically ordered state of this phase, and the formation of a brittle amorphous phase in the layer several microns thick near the friction surface of the alloy. The appearance of a continuous amorphous layer at the friction surface of the titanium nickelide is favored by the presence of the martensitic structure in the alloy, its stability under the friction conditions with respect to the reverse B19′ → B2 transformation, and a high intensity of the deformation processes occurring in the zone of friction contact. Below the amorphous layer, a mixed amorphous-crystal-line structure is located. The nanocrystallites are textured and range in size from a few to tens of nanometers. The formation of crystallites of the B2 phase in the amorphized layer appears to occur at the stage of warming of the alloy samples to room temperature. A similar amorphous-nanocrystalline structure arises near the abrasive-wear surface of the Ti_49.4Ni_50.6 alloy. It has been shown that the presence of a submicrocrystalline structure in the initial Ti_49.4Ni_50.6 alloy exerts no significant effect on the tribological properties and the character of structural transformations induced in the alloy by the frictional action.     

Application of severe plastic deformation by torsion to form amorphous and nanocrystalline states in large-size TiNi alloy sample

Results of investigations of the initial structure of large-size Ti_49.4Ni_50.6 samples subjected to severe plastic deformation by torsion under a high pressure (HPT) are reported. The study was performed using transmission and scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction, and measurements of mechanical properties. Under an applied pressure of 6 GPa, the alloy was found to undergo a martensitic B2 → B19′ transformation. Even after HPT using a single revolution of anvils, the granular structure of titanium nickelide is refined so that there is formed a nanocrystalline state of B2 austenite (i.e., the reverse martensitic B19′ → B2 transformation occurs) and amorphization of the alloy begins. The HPT with a high number of revolutions leads to the almost complete amorphization of the alloy, which is explained by a high degree of shear deformation. In this case, all nanocrystalline inclusions in the amorphous matrix have an ordered B2 structure.     

Effect of frictional heating on the surface-layer structure and tribological properties of titanium nickelide

The effect of frictional heating (whose intensity was varied at the expense of changes in the sliding velocity from 0.35 to 9.00 m/s) on the rate of wear, friction coefficient, friction thermopower, structure, and microhardness of the Ti_49.4Ni_50.6 alloy in a microcrystalline (MC) state with grains 20–30 μm in size and in a submicrocrystalline (SMC) state with grains 300 nm in size has been investigated. The tribological tests were conducted under the conditions of dry sliding friction in air using the finger-disk (made of steel Kh12M, hardness HRC = 63) scheme at a normal load of 98 N. Due to the frictional heating, the temperature in the surface layer 0.5 mm thick of the samples changed from 150–200 (at a sliding velocity of 0.35 m/s) to 1100°C (at a velocity of 9 m/s). The alloy structure has been studied with the help of metallographic and electronmicroscopic (scanning and transmission microscopy) methods. It has been shown that the rate of wear of the titanium nickelide in the MC and SMC structural states is more than an order of magnitude lower than in the 12Kh18N9 steel and several times less than in the 40Kh13 steel. The fracture of the friction surface of the titanium nickelide occurs predominantly by the fatigue or oxidation-fatigue mechanisms, which are characterized by a relatively low wear rate, whereas the 40Kh13 and 12Kh18N9 steels show a tendency to intense thermal adhesive wear (seizure) at velocities higher than 0.35 m/s. It has been shown by the electron-microscopic investigation that nanocrystalline structures consisting of crystals of the B2 phase, oxides of the TiO₂ type, and some amount of martensite B19′ are formed in the process of friction in the surface layer of the titanium nickelide. It has been concluded that an enhanced wear resistance of the titanium nickelide is caused by the high heat resistance (strength) and high fracture toughness of the nanocrystalline B2 phase and by the presence of high-strength thermostable oxides of the TiO₂ type formed upon friction.     

Structural transformations,strengthening, and wear resistance of titanium nickelide upon abrasive and adhesive wear

Wear resistance and structural transformations upon abrasive and adhesive wear of titanium nickelide Ti49.4Ni50.6 in microcrystalline (MC) and submicrocrystalline (SMC) states have been investigated. It has been shown that the abrasive wear resistance of this alloy exceeds that of the steel 12Kh18N9 by a factor of about 2, that of the steel 110G13 (Hadfield steel), by a factor of 1.3, and is close to that of the steel 95Kh18. Upon adhesive wear in a testing-temperature range from −50 to +300°C, the Ti49.4Ni50.6 alloy, as compared to the steel 12Kh18N9, is characterized by the wear rate that is tens of times smaller and by a reduced (1.5–2.0 times) friction coefficient. The enhanced wear resistance of the Ti49.4Ni50.6 alloy is due to the development of intense strain hardening in it and to a high fracture toughness, which is a consequence of effective relaxation of high contact stresses arising in the surface layer of the alloy. The SMC state produced in the alloy with the help of equal-channel angular pressing (ECAP) has no effect on the abrasive wear resistance of the alloy. The favorable effect of ECAP on the wear resistance of the Ti49.4Ni50.6 alloy takes place under conditions of its adhesive wear at temperatures from −25 to +70°C. The electron-microscopic investigation showed that under conditions of wear at negative and room temperatures in the surface layer (1–5 μm thick) of titanium nickelide there arises a mixed structure consisting of an amorphous phase and nanocrystals of supposedly austenite and martensite. Upon friction at 200–300°C, a nanocrystalline structure of the B2 phase arises near the alloy surface, which, as is the case with the amorphous-nanocrystalline structure, is characterized by significant effective strength and wear resistance.     

Superelastic behavior of nanostructured Ti50Ni48Co2 shape memory alloy with cold rolling processing

Effects of cold rolling followed by annealing on microstructural evolution and superelastic properties of the Ti₅₀Ni₄₈Co₂ shape memory alloy were investigated. Results showed that during cold rolling, the alloy microstructure evolved through six basic stages including stress-induced martensite transformation and plastic deformation of martensite, deformation twinning, accumulation of dislocations along twin and variant boundaries in martensite, nanocrystallization, amorphization and reverse transformation of martensite to austenite. After annealing at 400 °C for 1 h, the amorphous phase formed in the cold-rolled specimens was completely crystallized and an entirely nanocrystalline structure was achieved. The value of stress level of the upper plateau in this nanocrystalline alloy was measured as high as 730 MPa which was significantly higher than that of the coarse-grained Ni₅₀Ti₅₀ and Ti₅₀Ni₄₈Co₂ alloys. Moreover, the nanocrystalline Ti₅₀Ni₄₈Co₂ alloy had a high damping capacity and considerable efficiency for energy storage.     

Effect of heat treatment on the structural and phase transformations and mechanical properties of TiNi alloy subjected to severe plastic deformation by torsion

Effect of annealing on the structural and phase transformations and mechanical properties of large-size samples of the Ti_49.4Ni_50.6 alloy preliminary subjected to severe plastic deformation by torsion under a high pressure (HPT) is studied. The study was performed by transmission and scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction, and measurements of mechanical properties. The annealing was found to result in the nanocrystallization of initial samples amorphized by HPT. In this alloy, the high-strength uniform nanostructured state is formed the size of nanocrystalline grains of which and the mechanical properties depend on the temperature and time of annealing.     

Oxidation Behavior of Ti50Ni40Cu10 Shape-Memory Alloy in 700–1,000 °C Air

The isothermal-oxidation behavior of Ti₅₀Ni₄₀Cu₁₀ shape memory alloy (SMA) in 700–1,000 °C air was investigated by TGA, XRD, SEM and EPMA. Experimental results indicate that a multi-layered oxide scale formed, consisting of an outermost Cu₂O(Ni,Ti) layer, a layer of the mixture of TiO₂, TiNiO₃ and irregular small pores, a layer of the mixture of Ni(Ti,Cu), TiO₂ and irregular large pores, a Ti(Ni,Cu)₃ layer and an innermost Ti₃₀Ni_43–47Cu_27–23 layer. The apparent activation energy for the oxidation reaction of Ti₅₀Ni₄₀Cu₁₀ SMA is determined to be 180 kJ/mol, and the oxidation rate follows a parabolic law. A schematic oxidation mechanism of Ti₅₀Ni₄₀Cu₁₀ SMA is proposed to explain the observed results.     

Influence of Sc addition on microstructure and transformation behaviour of Ni24.7Ti50.3Pd25.0 high temperature shape memory alloy

Vacuum-arc melted Ni_24.7Ti_50.3Pd_25.0 and Ni_24.7Ti_49.3Pd_25.0Sc_1.0 (at.%) alloys were investigated to study effect of Sc micro-addition on microstructure and transformation behaviour of NiTiPd alloy. Study showed that microstructure of homogenized NiTiPd alloy consisted of NiTiPd matrix interspersed with Ti₂(Ni,Pd) precipitates. In contrast, NiTiPdSc alloy showed a single phase NiTiPdSc matrix with a few scandium oxide particles at isolated places. TEM and X-ray diffraction studies confirmed matrix phase of the alloys to be of orthorhombic B19 structure. TEM observations showed that NiTiPdSc alloy had relatively larger martensite plates with a smaller twin ratio compared to that of NiTiPd alloy. Also, APB (anti-phase boundary) like regions with twinless martensites was observed in both the alloys, area fraction of APB-like regions being more in NiTiPdSc alloy. Thermal analysis showed that transformation temperatures (TTs) of NiTiPd alloy decreased significantly with addition of Sc. The martensite finish temperature (M_f) of 181 °C for NiTiPd alloy lowered to 139 °C upon 1.0 at.% Sc addition. The transformation hysteresis of Ni_24.7Ti_49.3Pd_25.0Sc_1.0 (at.%) alloy was measured to be 7 °C, significantly lower than that of 15 °C for Ni_24.5Ti_50.0Pd_25.0Sc_0.5 alloy, reported in literature. Alloy purity, lower volume fraction of second phase and presence of twinless/small twin ratio martensite in microstructure is believed to be the reasons for such low transformation hysteresis. The transformation behaviour of the alloys upon stress-free thermal cycling was found stable, variation in TTs being within 1–2 °C.     

Formation of a wear-resistant nanocrystalline layer strengthened by TiO2 (Rutile) particles on the surface of titanium

The effect of a thermomechanical treatment including severe plastic deformation under dry sliding friction conditions and subsequent heating in air to 350–650°C with further holding for 1 h on the structure and wear resistance of commercial titanium of grade VT1-0 has been studied. It has been shown that the deformation by friction leads to the formation of a nanocrystalline structure with α crystals 20–100 nm in size in a surface layer of titanium of about 10 μm thick. The heating of titanium deformed by friction at temperatures of 450–650°C for 1 h in air leads to the formation in the surface layer of this material ～10 μm thick of nanocrystalline particles of the titanium oxide TiO₂ (rutile), the volume fraction of which reaches tens of percents, while the dimensions are ～10 nm. The presence in the surface layer of titanium of a nanocrystalline two-phase (α-Ti + rutile) structure leads to a significant increase in the wear resistance of the VT1-0 titanium in pair with steel 40Kh13. This is explained by the enhanced strength of the arising nanocrystalline layer and its positive influence (as of a transition layer) on the reduction of the level of internal stresses that exist at the interface between the titanium oxide TiO₂ and the host metal.     

In-situ multi-layer formation in the oxidation of Ti3Al-Nb

The effect of niobium on the oxidation of a Ti₃Al alloy was studied in pure oxygen in the range of 850-1,100°C. The oxidation products for the Ti-30Al-2.7 Nb alloy were mainly TiO₂ (rutile) mixed with A1₂O₃ (alumina) and small amounts of niobium oxide. The oxidation resistance of Ti₃Al was improved by the addition of niobium. An in-situ multiple-layer structure comprising a mixture of rutile and alumina formed on the oxide scale of the alloy at temperatures 1,000°C and above. The number of layers increased as the temperature increased but the individual layer thickness decreased.     

Structure and phase transformations in TiNiFe ternary alloys subjected to plastic deformation by high-pressure torsion and subsequent heat treatment

The results of a complex study of ternary TiNiFe alloys with a low-temperature shape-memory effect subjected to megaplastic deformation by high-pressure torsion (HPT) with subsequent heat treatment are presented. Investigations have been performed using X-ray diffraction, transmission and scanning electron microscopy, and measurements of electrical properties. It has been established that, at moderate degrees of reduction, the plastic deformation in the Ti₅₀Ni₄₉Fe₁ alloy induces a B2 ? B19′ thermoelastic martensitic transformation and the formation of a developed banded dislocation and twin structure in the B19′ martensite; in the Ti₅₀Ni₄₇Fe₃ alloy, a mainly analogous dislocation substructure is formed, but in the B2 austenite. The megaplastic deformation by HPT at room temperature leads to the amorphization of the Ti50Ni49Fe1 alloy and to the high-angle nanofragmentation of the Ti50Ni47Fe3 alloy. Specific features of the evolution of the structure and martensitic transformations in the TiNiFe ternary alloys after plastic deformation and heat treatment have been established. It has been found that the heat treatment of both alloys after HPT at temperatures of 553–773 K results in the formation of a nanocrystalline or mixed nano-submicro-crystalline structure.     

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.     

Surface characterization of NiTi modified by plasma source ion implantation

This paper reports the novel application of an oxygen ion plasma for surface modification of a shape memory alloy. The surface characterization of oxygen ion implanted Ti–50.7 at% Ni alloy samples was performed with the assistance of Auger electron spectroscopy, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy techniques (TEM). TEM identified amorphous TiO₂ and crystalline phases of Ti₁₁Ni₁₄ and Ti₄Ni₂O_x. XPS results reveal that the surfaces of control samples are covered with predominantly TiO₂ and traces of TiO and NiTi, as well as NiO and Ni₂O₃. At the surface of oxygen-implanted samples, however, only TiO₂ and trace amounts of Ni₂O₃, NiO and NiTi were observed. TiO and NiO exist in much larger range than other oxides, and TiO extends to the oxide–metal interface. These results are especially noteworthy because of their implications for interpreting corrosion, wear, and biocompatibility behavior.     

低氧分压下NiTi记忆合金氧化行为的研究

本文研究了NiTi形状记忆合金在H₂-H₂O气氛下400-700 °C的氧化行为。合金的氧化过程遵循立方规律,氧化激活能127.52 kJ/mol。氧化显著降低了试样表面的Ni含量。400 °C氧化的试样,其表面形貌与其他试样不同,并且氧化膜较薄,截面结构无法用SEM分析。500 °C、600 °C、700 °C氧化的试样,表面有两种形貌的氧化物,一种是颗粒状氧化物,另一种是晶须状氧化物。截面分析表明,氧化膜分为两层,上层由TiO₂构成,下层由Ni₃Ti构成,两层界面处有孔洞生成。     

Effect of deviations of composition from the quasi-binary section TiNi-TiCu on structural and phase transformations in rapidly quenched alloys

Methods of X-ray diffraction, transmission and scanning electron microscopy, and selected-area electron diffraction (SAED) have been used to study the phase and elemental composition and structure of alloys close to the stoichiometric Ti₅0Ni₂₅Cu₂₅ alloy. Based on the method of rapid quenching of the melt (free-jet melt spinning), alloys of the quasi-binary TiNi-TiCu section have been prepared, which in the initial as-cast state exhibited the thermoelastic martensitic transformations B ₂ ? B ₁₉ and related shape-memory effects. The chemical composition of the Ti_{50 + x} Ni₂₅Cu_{25 ? x} alloys was varied by changing titanium and copper concentrations within x ≤ ±1 at % (from Ti₄₉Ni₂₅Cu₂₆ to Ti₅₁Ni₂₅Cu₂₄). It has been established that quenching at a cooling rate equal to 10⁶ K/s leads to the amorphization of all the alloys under consideration. Heating to 723 K and higher leads to the devitrification of the alloy with the formation of a nanocrystalline or submicrocrystalline structure of the B2 austenite. The mechanical properties of these alloys have been measured in the initial amorphous state and in the polycrystalline martensitic state. It has been shown that, depending on the extent of the deviations of the alloy composition from the stoichiometry, which cause the decomposition of the alloys in the process of nanocrystallization, regular changes are observed in their mechanical properties and in the shape-memory effects. The kinetics of the processes of the devitrification of the alloys, as well of the forward and reverse martensitic transformations, have been studied, their characteristic temperatures have been determined, and a diagram of the dependence of the characteristic temperatures on the chemical composition of the alloys has been constructed.     

Formation of the nanocrystalline structure in the Ti<Subscript>50</Subscript>Ni<Subscript>25</Subscript>Cu<Subscript>25</Subscript> shape-memory alloy under severe thermomechanical treatment

Results of comparative studies of the structure of the cast martensitic Ti₅₀Ni₂₅Cu₂₅ alloy in the initial state, after severe plastic deformation by high-pressure torsion (HPT), and after subsequent annealing are presented. The studies have been performed by X-ray diffraction, transmission and scanning electron microscopy, and measurements of electrical properties. It has been established that the alloy undergoes almost complete amorphization after torsion using 5 and 10 rev of anvils under a pressure of 7 GPa. This result can be explained by the large value of shear deformation (true strain from 6 to 7 units) and the retention of an extremely large quantity of highly dispersed (less than 3–4 nm in size) nanocrystals with a distorted B2 lattice in the amorphous matrix even at room temperature. Their determining role as nuclei of crystallization ensures the total process of low-temperature nanocrystallization upon subsequent annealing, beginning from 250–300°C. It is shown that the annealing of the alloy amorphized during HPT makes it possible to produce extremely uniform nanocrystalline (NC), submicrocrystalline (SMC), or bimodal (NC + SMC) structures of B2 austenite. For the first time, a complete diagram of thermoelastic martensitic transformations in the field of B2-austenite states, from nanostructured to usual polycrystalline, has been constructed for the Ti₅₀Ni₂₅Cu₂₅ alloy. The size effect of stabilization of the martensite transformation has been found in the nanocrystalline B2 alloy.     

On the nature of anomalously high plasticity of high-strength titanium nickelide alloys with shape-memory effects: I. Initial structure and mechanical properties

This work presents the results of studies of the Ti_49.4Ni_50.6 alloy of enhanced purity with shapememory effects in an ordinary coarse-grained state with an average grain size of 20–30 μm or in a submicrocrystalline state with an average grain size of 0.2–0.3 μm. In this alloy the initial structure, phase composition, martensitic transformations, mechanical properties, and character of fracture have been investigated in a wide temperature range. It has been shown that upon cooling and mechanical tests at room temperature, the alloy exhibits highly reversible thermoelastic martensitic transformations. It has been established that the alloy exhibits high values of the strength and plastic properties and strain-hardening coefficients.     

The Effect of Colored Titanium Oxides on the Color Change on the Surface of Ti-5Al-5Mo-5V-1Cr-1Fe Alloy

In the present investigation, a color change on the surface of Ti-5Al-5Mo-5V-1Cr-1Fe alloy was studied through thermal oxidation experiments in the temperature range of 100-1000 °C with an interval of 50 °C. The phase composition and morphology of oxide layer were characterized by x-ray diffraction and light optical microscopy, respectively. The result shows that the achieved colors after thermal oxidation followed a chromatic scale which went from silver white to light yellow to golden yellow to blue and then to light green and brownish black. The color change on the alloy mainly resulted from the different colored titanium oxides in the oxide layer. The silver white, yellow, and blue on the alloy with the oxidation temperature below 600 °C were the results of TiO₂ white tint, TiO golden tint, and Ti₂O₃ blue color, respectively. The light green was the mixed color of TiO golden tint and Ti₂O₃ blue color in the oxidation temperature range of 600-700 °C. However, at the oxidation temperatures exceeding 750 °C, the color turned to be brownish black. It might be associated with the thick, porous, and multilayered oxide layer. Consequently, it can be suggested that the illustration of the color change is vitally necessary for assessing the quality of the final workpieces according to the color change on titanium alloys.     

Hydrogen storage characteristics of Ti2Ni alloy synthesized by the electro-deoxidation technique

Ti₂Ni alloy was synthesized in the molten CaCl₂ electrolyte by the electro-deoxidation method at 900 °C and the electrochemical hydrogen storage characteristics of the synthesized alloy was observed. The X-ray diffraction peaks indicated that stoichiometric oxides in TiO₂–ZrO₂–NiO mixture reduced to Ti₃O₅, CaTiO₃, CaZrO₃, Ni and Ti₂O₃ within 5 h electro-deoxidation process. Extension of the electro-deoxidation time to 10 h caused formations of TiO and equilibrium Ti₂Ni phase. After 24 h electro-deoxidation the target alloy with the equilibrium Ti₂Ni phase structure and the maximum amount of the dissolved Zr in it was obtained. It was observed that the synthesized alloy had maximum discharge capacity of 200 mA h g⁻¹. Upon increase in the charge/discharge cycles, however, the discharge capacity decayed sharply. According to the gathered EIS data at various DODs, the rapid degradation in the electrode performance of Ti₂Ni alloy was attributed to the developed barrier oxide layer on the electrode surface.     

Vacuum diffusion bonding of Ti2AlNb alloy and TC4 alloy

The Ti₂AlNb alloy was joined with TC4 alloy by vacuum diffusion bonding. The relationship between bonding parameters, and joint microstructure and shear strength was investigated. The results indicated that the diffusion of Al, Ti, Nb and V elements across bonding interface led to the formation of three reaction layers: B2/β layer and α₂ layer on the TC4 side, and α₂+B2/β layer on the Ti₂AlNb side. The bonding temperature determined the atomic activity, thus controlling the growth of reaction layers and influencing the shear strength of the joint. When the Ti₂AlNb alloy and TC4 alloy were bonded at 950 °C for 30 min under 10 MPa, the shear strength of the joint reached the maximum of 467 MPa. The analysis on the fracture morphology showed that the fracture occurred within the B2/β layer and the fracture model was ductile rupture. Meanwhile, the formation mechanism of the Ti₂AlNb/TC4 joint was discussed in depth.