Effect of heat treatment on structural and phase transformations in the Ti49.5Ni50.5 alloy amorphized by high-pressure torsion

Results are presented for a study of the structural and phase transformations that occur in the titanium-nickelide shape-memory alloy Ti_49.5Ni_50.5 subjected to heat treatment after deformation-induced amorphization by megaplastic high-pressure torsion (HPT) using five or ten revolutions of Bridgman anvils. The investigations were performed using transmission and scanning electron microscopy, X-ray diffraction, and measurements of the temperature dependences of electrical resistivity and magnetic susceptibility. It has been established that the crystallization of the alloy already occurs upon low-temperature treatment, beginning with ～500 K. The evolution of the structure and the stage character of the development of crystallization and recrystallization processes depending on temperature have been determined. It has been shown that the annealing of the amorphized alloy makes it possible to obtain highly homogeneous nanostructured, submicrocrystalline, or bimodal states in the B2 austenite. A complete diagram of thermoelastic martensitic transformations of the B2 austenite has been constructed in the region from a nanostructured to a conventional polycrystalline state (with a grain size of 20–50 μm). The effect of size on the stabilization of austenite has been revealed and its specific features have been studied for the B2 → R and B2(R) → B19′ martensitic transformations depending on the structural state of the alloy.     

Phase transformation behavior and mechanical properties of Ti50Ni49−x Fe1Co x shape memory alloys

In this article, the influence of Co addition on phase transformation behavior and mechanical properties of TiNiFe shape memory alloy was investigated extensively. Differential scanning calorimetry (DSC) measurements shows that martensitic start transformation temperatures (Ms ) decrease drastically with increasing Co content, while the R phase transformation start temperatures (Rs ) vary slightly. Nevertheless, the substitution of Ni with Co does not exert substantial influence on the two-stage transformation behavior of the TiNiFe alloy. The results from stress-strain curves indicate that higher critical stress for stress-induced martensitic transformation (rSIM ) has been obtained because of Co addition. In such cases, the Ti50Ni48Fe1 Co1.0 alloy maintains a good shape memory effect, and a maximum recoverable strain of 7.5 % can be obtained.     

Microstructures and Mechanical Properties of NiTiFeAlCu High-Entropy Alloys with Exceptional Nano-precipitates

Three novel NiTiFeAlCu high-entropy alloys, which consist of nano-precipitates with face-centered cubic structure and matrix with body-centered cubic structure, were fabricated to investigate microstructures and mechanical properties. With the increase in Ni and Ti contents, the strength of NiTiFeAlCu alloy is enhanced, while the plasticity of NiTiFeAlCu alloy is lowered. Plenty of dislocations can be observed in the Ni₃₂Ti₃₂Fe₁₂Al₁₂Cu₁₂ high-entropy alloy. The size of nano-precipitates decreases with the increase in Ni and Ti contents, while lattice distortion becomes more and more severe with the increase in Ni and Ti contents. The existence of nano-precipitates, dislocations and lattice distortion is responsible for the increase in the strength of NiTiFeAlCu alloy, but it has an adverse influence on the plasticity of NiTiFeAlCu alloy. Ni₂₀Ti₂₀Fe₂₀Al₂₀Cu₂₀ alloy exhibits the substantial ability of plastic deformation and a characteristic of steady flow at 850 and 1000 °C. This phenomenon is attributed to a competition between the increase in the dislocation density induced by plastic strain and the decrease in the dislocation density due to the dynamic recrystallization.     

The microstructure and properties of rapidly quenched Ti-rich Ti-Ni binary alloys with shape memory effects

The structure, phase composition, and martensitic transformations in binary titanium-rich Ti-Ni alloys with shape memory effects, produced by ultrarapid quenching using melt jet spinning, have been studied using electron microscopy, X-ray diffraction, and measurements of some physicomechanical properties in a wide temperature range. The alloys with a Ti content that exceeded the stoichiometric composition by 5% and more can be produced in an amorphous state. The alloys with a smaller deviation from the stoichiometry, as well as the Ti₅₀Ni₅₀ alloy, are crystallized in a submicrocrystalline state and undergo a B2 → B19’ martensitic transformation at temperatures above room temperature. They have high strength and plastic properties and demonstrate narrow-hysteresis shape-memory effects.     

Influence of Ti additions on martensitic transformation and magnetic properties of cast Ni51Fe22−xGa27Tix shape memory alloys

The effect of Ti addition on the microstructure, martensitic transformation, magnetic and mechanical properties of polycrystalline Ni₅₁Fe_22?x Ga₂₇Ti _x (x=0, 2 and 4) ferromagnetic shape memory alloy was investigated by scanning electron microscope, differential scanning calorimetry and X-ray diffraction. The results showed that the martensitic transformation temperature increases monotonously with the increase of fraction of Ti substitution for Fe. The increase in the martensite transformation temperatures should be related to the change of the electron concentration after the addition of Ti to Ni₅₁Fe_22?x Ga₂₇Ti _x alloys. According to the results of X-ray diffraction and magnetic properties, Ti has significant effect the structure of Ni₅₁Fe_22-x Ga₂₇Ti _x . Adding of 4 at% Ti altered the structure of the matrix from five-layered tetragonal martensite of Ni₅₁Fe₂₂Ga₂₇ and Ni₅₁Fe₂₀Ga₂₇Ti₂ alloys to non-modulated tetragonal martensite. Magnetic properties proved that the alloy transits from ferromagnetic, five-layered tetragonal martensite, to paramagnetic, non-modulated martensite structure, with increasing Ti content to 4 at.%. Saturation magnetization, remnant magnetization and coercivity of the alloy were significantly influenced by Ti additions. Hardness values of Ni₅₁Fe₂₂Ga₂₇ increased by the addition of Ti.     

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.     

Microstructure and superelastic properties of free forged Ti–Ni shape-memory alloy

Elemental titanium (Ti) and nickel (Ni) powders were consolidated by spark plasma sintering (SPS) to fabricate Ti–51%Ni (mole fraction) shape-memory alloys (SMAs). The objective of this study is to enhance the superelasticity of SPS produced Ti–Ni alloy using free forging as a secondary process. Products from two processes (with and without free forging) were compared in terms of microstructure, transformation temperature and superelasticity. The results showed that, free forging effectively improved the tensile and shape-memory properties. Ductility increased from 6.8% to 9.2% after forging. The maximum strain during superelasticity increased from 5% to 7.5% and the strain recovery rate increased from 72% to 92%. The microstructure of produced Ti–51%Ni SMA consists of the cubic austenite (B2) matrix, monoclinic martensite (B19′), secondary phases (Ti₃Ni₄, Ti₂Ni and TiNi₃) and oxides (Ti₄Ni₂O and Ti₃O₅). There was a shift towards higher temperatures in the martensitic transformation of free forged specimen (aged at 500 °C) due to the decrease in Ni content of B2 matrix. This is related to the presence of Ti₃Ni₄ precipitates, which were observed using transmission electron microscope (TEM). In conclusion, free forging could improve superelasticity and mechanical properties of Ti–51%Ni SMA.     

Structural and phase transformations,mechanical properties,and shape-memory effects in quasibinary Ni<Subscript>50</Subscript>Ti<Subscript>38</Subscript>Hf<Subscript>12</Subscript> alloy obtained by quenching from the melt

Methods of transmission and scanning electron microscopy and chemical microanalysis, electron diffraction, and X-ray diffraction were used to systematically study the structure and the chemical and phase composition of the Ni₅₀Ti₃₈Hf₁₂ alloy synthesized by rapid quenching from the melt and subjected to various heat treatments. The critical temperatures of the devitrification of the initially amorphous rapidly quenched alloy and the B2 ? B19′ thermoelastic martensitic transformations have been determined. The lattice parameters of the B2 austenite and thermoelastic B19′ martensite have been measured. The main features of the formation of an ultrafine-grained structure in the alloy and the subsequent phase transformations (martensitic transformation and the decomposition with the formation of an intermetallic phase of the (Ti,Hf)₂Ni type) have been studied depending on the regimes of heat treatment. Based on the results of measurements of mechanical properties upon tension (σ_M, σ_u, and δ) and the shape-memory effects (degree of shape recovery depending on the deformation by bending; and magnitude of the reversible strain ε_rev), regimes for obtaining high-strength and plastic states of the alloy with a shape-memory effect have been established.     

Glass-forming ability and stability of ternary Ni-early transition metal (Ti/Zr/Hf) alloys

Four Ni-bearing Ti, Zr and Hf ternary alloys of nominal composition Zr_41.5Ti_41.5Ni₁₇, Zr₂₅Ti₂₅Ni₅₀, Zr_41.5Hf_41.5Ni₁₇ and Ti_41.5Hf_41.5Ni₁₇ were rapidly solidified in order to produce ribbons. The Zr–Ti–Ni and Ti–Hf–Ni alloys become amorphous, whereas the Zr–Hf–Ni alloy shows precipitation of a cubic phase. The devitrification of all three alloys was followed and the relative tendency to form nanoquasicrystals and cF96 phases analysed. The relative glass-forming ability of the alloys can be explained by taking into account their atomic size difference. Addition of Ni often leads to quasicrystallisation or quasicrystal-related phases. This can be explained by the atomic radius and heat of mixing of the constituent elements. The phases precipitated at the initial stages of crystallisation indicate the possible presence of Frank–Kasper polyhedral structure in the amorphous alloys. Structural analysis reveals that the Laves and the anti-Laves phases have the same polyhedral structural unit, which is similar to the structural characteristics of glass.     

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.     

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.     

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.     

Structure and phase transformations in copper-alloyed rapidly melt-quenched Ni<Subscript>50</Subscript>Ti<Subscript>32</Subscript>Hf<Subscript>18</Subscript>-based alloys with high-temperature shape memory effect

Methods of transmission and scanning electron microscopy, chemical microanalysis, electron diffraction, and X-ray diffraction have been used to carry out the comparative study of the structure and chemical and phase composition of thin ribbons of four quasi-binary alloys (Ni₅₀Ti₃₂Hf₁₈, Ni₄₅Ti₃₂Hf₁₈Cu₅, Ni₃₅Ti₃₂Hf₁₈Cu₁₅, and Ni25Ti32Hf18Cu25) obtained in the amorphous state by rapid quenching from the melt by jet spinning. The critical temperatures of the devitrification and B2 ? B19′ martensitic transformation of the alloys have been determined based on the data of temperature dependences of the electrical resistivity. The specific features of the formation of the ultrafine-grained structure upon the devitrification and of the phase transformations have been studied depending on the heat-treatment regimes and chemical composition of the alloys (concentration of copper atoms).     

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.     

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.     

Microstructure,martensitic transformation and mechanical properties of Ni–Mn–Sn alloys by substituting Fe for Ni

The effects of partial substitution of Fe element for Ni element on the structure, martensitic transformation and mechanical properties of Ni_50–xFe_xMn₃₈Sn₁₂ (x=0 and 3%, molar fraction) ferromagnetic shape memory alloys were investigated. Experimental results indicate that by substitution of Fe for Ni, the microstructure and crystal structure of the alloys change at room temperature. Compared with Ni₅₀Mn₃₈Sn₁₂ alloy, the martensitic transformation starting temperature of Ni₄₇Fe₃Mn₃₈Sn₁₂ alloy is decreased by 32.5 K. It is also found that martensitic transformation occurs over a broad temperature window from 288.9 to 352.2 K. It is found that the mechanical properties of Ni–Mn–Sn alloy can be significantly improved by Fe addition. The Ni₄₇Fe₃Mn₃₈Sn₁₂ alloy achieves a maximum compressive strength of 855 MPa with a fracture strain of 11%. Moreover, the mechanism of the mechanical property improvement is clarified. Fe doping changes the fracture type from intergranular fracture of Ni₅₀Mn₃₈Sn₁₂ alloy to transgranular cleavage fracture of Ni₄₇Fe₃Mn₃₈Sn₁₂ alloys.     

Phase and structural transformations in the Ti<Subscript>49.5</Subscript>Ni<Subscript>50.5</Subscript> alloy with a shape-memory effect during torsion under high pressure

Results of investigations of structural and phase transformations that occur in the titanium-nickelide-based alloy Ti_49.5Ni_50.5 with a shape memory effect during severe plastic deformation by torsion under high pressure (HPT) are reported. The studies were performed using transmission and scanning electron microscopy, neutron and X-ray diffraction, and measurements of temperature dependences of electrical resistivity. The martensitic B2 → B19′ transformation was found to be induced in the alloy when applying a high pressure. After unloading, the martensitic B19′ phase is retained in the alloy. The fine structure of the B19′ martensite and its evolution into nanocrystalline and, subsequently, amorphous state during HPT with 1/4, 1/2, 1, 5, and 10 rev have been studied. It was shown that, after HPT, all nanosized crystallites whose sizes are less than 30–50 nm have a B2-type structure and, therefore, the reverse martensitic B19′ → B2 transformation is realized in the alloy at room temperature after unloading.     

Effect of the deviation of the chemical composition from the stoichiometric composition on the structural and phase transformations and properties of rapidly quenched Ti50 + xNi25 ? xCu25 alloys

Methods of X-ray diffraction, transmission and scanning electron microscopy, and electron diffraction have been used to study phase composition and structure of an almost stoichiometric alloy Ti₅₀Ni₂₅Cu₂₅. The alloys of the quasi-binary section TiNi-TiCu to be studied, which exhibit in the initial ascast state thermoelastic martensitic transformations B2 ↔ B19 and related shape-memory effects, have been produced by rapid quenching of the melt (melt spinning technique). The chemical composition of the Ti_{50 + x} Ni_{25 − x} Cu₂₅ alloys was varied with respect to titanium and nickel within x ≤ ±1% (from Ti₄₉Ni₂₆Cu₂₅ to Ti₅₁Ni₂₄Cu₂₅). It has been shown that the rapid quenching from the melt at a cooling rate of 106 K/s provides amorphization for all the alloys under consideration. Heating to 723 K or higher temperatures leads to the devitrification of the amorphous alloys with the formation of a polycrystalline structure of the B2 austenite. The mechanical properties of the alloys have been measured in the initial amorphous state and after subsequent heat treatment. It has been established that, depending on the degree of deviation of the alloy from the stoichiometric composition, which leads to solid solution decomposition in the process of nanocrystallization upon heat treatment, there occur regular changes in the mechanical properties and shape-memory effects of the alloys. The characteristic temperatures of the onset and finish of the process of crystallization from the amorphous and amorphous-crystalline states and the critical temperatures of the onset and finish of the forward and reverse thermoelastic martensitic transitions have been determined by measuring temperature dependences of the electrical resistivity of the alloys. The diagram of the dependence of the critical temperatures on the chemical composition of the alloy has been constructed.     

Influence of Fe addition on phase transformation behavior of NiTi shape memory alloy

Three different NiTi-based alloys, whose nominal compositions were Ni₅₀Ti₅₀, Ni₄₉Ti₄₉Fe₂, Ni₄₅Ti_51.8Fe_3.2 (mole fraction, %), respectively, were used in the current research to understand the influence of Fe addition on phase transformation behavior in NiTi shape memory alloy (SMA). The microstructure and phase transformation behavior of the alloys were investigated by optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analysis. The results show that the matrix of the Ni₅₀Ti₅₀ alloy consists of both B19′ (martensite) phase and B2 (austenite) phase. Moreover, the substructures of twins could be observed in the B19′ phase. However, the ternary alloys of NiTiFe exhibit B2 phase in the microstructures. Such microstructures were also characterized by large presence of Ti₂Ni precipitates dispersed homogenously in the matrix of the two kinds of alloys. The addition of Fe to the NiTi SMA results in the decrease in phase transformation temperatures in the ternary alloys. Based on mechanism analysis, it can be concluded that this phenomenon is primarily attributed to atom relaxation of the distorted lattice induced by Ni-antisite defects and Fe substitutions during phase transformation, which enables stabilization of B2 phase during phase transformation.     

Influence of aging on microstructure,martensitic transformation and mechanical properties of NiTiRe shape memory alloy

Aging is an effective way to adapt the microstructure, phase transformation and consequently the mechanical properties of NiTi shape memory alloys. In the present study, Ni₅₂Ti_47.7Re_0.3 shape memory alloy was solution treated at 1000 °C for 24 h then aged at various temperatures of 300, 400, 500 and 600 °C for 3 h. The influence of aging treatment on microstructure, martensitic transformation and mechanical properties of Ni₅₂Ti_47.7Re_0.3 was investigated. The microstructure of the solution treated alloy was martensite as a matrix phase and precipitates of Ti₂Ni phase. The aged alloys had a microstructure as same as that of solution treated alloy in addition to the existence of other types of precipitates like Ni₄Ti₃ and Ni₃Ti. The martensitic — austenitic transformation during heating and cooling was going through one stage of transformation. The martensitic phase transformation temperature increased by the increase of aging temperature but still lower than that of solution treated alloy.