Influence of carbon on martensitic phase transformations in NiTi shape memory alloys

In the present paper we show how carbon affects martensitic transformations in Ni-rich NiTi shape memory alloys. During vacuum induction melting in graphite crucibles, NiTi melts dissolve carbon and TiC particles form during solidification. Differential scanning calorimetry (DSC) shows that this is associated with a decrease in the phase transition temperatures. We provide new experimental evidence for increasing temperature intervals between the start and the end of the martensitic transformations (from B2 to B19′) with increasing C content in as-cast and solution-annealed (850 °C) microstructures. The nucleation and growth of TiC particles in intercellular/interdendritic regions causes variations in the local Ni/Ti ratios. This results in wider transformation temperature intervals (DSC peak broadening) in as-cast and solution-annealed microstructures. Subsequent intense heat treatments (1000 °C) homogenize the alloy and re-establish sharp DSC peaks during martensitic transformations.     

On the multiplication of dislocations during martensitic transformations in NiTi shape memory alloys

In situ and post-mortem diffraction contrast transmission electron microscopy (TEM) was used to study the multiplication of dislocations during a thermal martensitic forward and reverse transformation in a NiTi shape memory alloy single crystal. An analysis of the elongated dislocation loops which formed during the transformation was performed. It is proposed that the stress field of an approaching martensite needle activates an in-grown dislocation segment and generates characteristic narrow and elongated dislocation loops which expand on {1 1 0}_B2 planes parallel to {0 0 1}_B19′ compound twin planes. The findings are compared with TEM results reported in the literature for NiTi and other shape memory alloys. It is suggested that the type of dislocation multiplication mechanism documented in the present study is generic and that it can account for the increase in dislocation densities during thermal and stress-induced martensitic transformations in other shape memory alloys.     

Energy landscape for martensitic phase transformation in shape memory NiTi

First-principles calculations are presented for parent B2 phase and martensitic B19 and B19′ phases in NiTi. The results indicate that both B19 and B19′ are energetically more stable than the parent B2 phase. By means of ab initio density functional theory, the complete distortion–shuffle energy landscape associated with B2 → B19 transformation in NiTi is then determined. In addition to accounting for the Bain-type deformation through the Cauchy–Born rule, the study explicitly accounts for the shuffle displacements experienced by the internal ions in NiTi. The energy landscape allows the energy barrier associated with the B2 → B19 transformation pathway to be identified. The results indicate that a barrier of 0.48 mRyd atom^?1 (relative to the B2 phase) must be overcome to transform the parent B2 NiTi to orthorhombic B19 martensite.     

Effect of atomic ordering on the phase transformations in Ni–Mn–Ga shape memory alloys

The influence of the L2₁ order degree in single crystalline Ni–Mn–Ga alloys is analysed in the present work. The alloys studied display a sequence of martensitic (MT) and intermartensitic (IMT) transformations on cooling, the reverse transformation to austenite, on heating, taking place in a single step. Quenching from different temperatures produces distinct effects on the MT and IMT: while the MT temperatures are not practically affected by the performed heat treatments, the IMT shifts towards lower temperatures after quenching from increasing temperatures. Such evolution can be related to changes in the L2₁ order degree, in the sense that ordering favours the occurrence of the IMT while it hardly affects the MT temperatures. The closeness of the free energies of the different martensitic structures and the differences between the MT and IMT entropy changes allow this behaviour to be explained. In turn, the Curie temperature increases with the L2₁ ordering for an alloy undergoing magnetic transition in martensite phase, no changes being detected if the Curie temperature is above the martensitic transformation.     

Comparative study on microstructure and martensitic transformation of aged Ni-rich NiTi and NiTiCo shape memory alloys

In this article the influence of aging heat treatment conditions of 250, 350, 450 and 550 °C for 3 h on the microstructure, martensitic transformation temperatures and mechanical properties of Ni₅₁Ti₄₉Co₀ and Ni₄₇ Ti₄₉Co₄ shape memory alloys was investigated. This comparative study was carried out using X-ray diffraction analysis, scanning electron microscope, energy dispersive spectrometer, differential scanning calorimeter and Vickers hardness tester. The results show that the microstructure of both aged alloys contains martensite phase and Ti₂Ni in addition to some other precipitates. The martensitic transformation temperature was increased steadily by increasing the ageing temperature and lowering the value of valence electron number (e_v/a) and concentration. Moreover, the hardness measurements were gradually increased at first by increasing the aging temperature from 250 to 350 °C. Further elevating in aging temperature to 450 and 550 °C decreases the hardness value.     

Age-induced four-stage transformation in Ni-rich NiTi shape memory alloys

A four-stage transformation has been observed in an aged Ti–50.7 at.%Ni shape memory alloy by differential scanning calorimetry (DSC). Such phenomenon is able to occur in the alloys with compositions around 50.7 at.%Ni by aging at 673 K. The multi-stage transformation behavior observed in this study is attributed to the complex microstructural evolution, i.e., formation of large-scale and small-scale heterogeneities which induced various stress fields and affected the phase transformation sequence.     

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.     

Entropy change of martensitic transformation in ferromagnetic shape memory alloys

A consistent analysis of the entropy change that accompanies the martensitic transformation (MT) of ferromagnetic shape memory alloys has been carried out using the Landau theory of phase transitions. The interrelation between the values of entropy change and the widths of the temperature intervals of MTs, observed in experiments with different alloys, was described. General theoretical expressions for the non-magnetic and magnetic parts of the total entropy change were obtained and then applied to the Ni–Mn–Ga and Ni–Fe–Ga shape memory alloys. For Ni–Mn–Ga alloys the theoretical values of the magnetic parts of entropy changes appeared to be close to the measured total entropy changes. This fact demonstrated the crucial role of the interaction between the magnetic and elastic subsystems of these alloys in the thermodynamic characteristics of MTs. A theoretical estimation of the magnetic entropy change has been obtained from the experimental temperature dependence of magnetization of a Ni–Fe–Ga alloy.     

P-phase precipitation and its effect on martensitic transformation in (Ni,Pt)Ti shape memory alloys

A new precipitate phase named P-phase has recently been identified in (Ni,Pt)Ti high temperature shape memory alloys. In order to understand the roles played by the fine coherent P-phase precipitates in determining the martensitic transformation temperature (M_s), strength of the B2 matrix phase, dimensional stability and shape memory effect of the alloys, a phase field model of P-phase precipitation is developed. Model inputs, including lattice parameters, precipitate-matrix orientation relationship, elastic constants and free energy data, are obtained from experimental characterization, ab initio calculations and thermodynamic databases. Through computer simulations, the shape and spatial distribution of the P-phase precipitates, as well as the compositional and stress fields around them, are quantitatively determined. On this basis, the elastic interaction energy between the P-phase precipitates and a martenstic nucleus is calculated. It is found that both the chemical non-uniformity and stress field associated with the P-phase precipitates are in favor of the martensitic transformation. Their relative contributions to the increase in M_s temperature are quantified as a function of aging time and the result seems to agree with the experimental measurements. The shape and spatial distribution of the P-phase precipitates predicted by the simulations also agree well with experimental observations.     

第二相对等径弯角挤压处理TiNi合金和Ti-Mo基合金力学性能及马氏体转变的影响

借助ECAP技术对TiNi合金和Ti-Mo基记忆合金在673~773K进行挤压处理,挤压路径为Bc,以获得超细晶组织,从而增强母相的强度,改进材料的性能。研究热力学稳定相及亚稳相对这两种合金的力学性能和马氏体转变的影响。结果表明,对于富钛TiNi合金,热力学稳定相Ti2Ni对马氏体转变及超弹性没有影响,而稳定相α相则可造成Ti-Mo-Nb-V-Al塑性降低。亚稳Ti3Ni4相对富镍TiNi合金的R相变、马氏体转变及超弹性有很大影响。并对第二相对TiNi合金和Ti-Mo基合金力学性能、马氏体转变的影响进行分析。     

Influence of the post-deformation annealing heat treatment on the low-cycle fatigue of NiTi shape memory alloys

Shape memory alloys (SMA) suffer from the same impairing mechanisms experienced during cycling loading by classic alloys. Moreover, SMA fatigue behavior is greatly influenced by thermomechanical cycling through the zone of thermoelastic phase transformation, which is the basis of shape memory and superelasticity effects. Since the fatigue resistance of any material can be improved by an appropriate thermomechanical treatment, in the present work combined differential scanning calorimetry and microhardness testing were used to determine an optimum annealing temperature for the cold-worked Ni-50.1%Ti alloy. The optimization is based on the assumption that latent heat of transformation is proportional to the mechanical work generated by SMA upon heating, while material hardness is related to the yield stress of the material. It is supposed that an optimum trade-off in these two properties guarantees the best dimensional and functional stability of SMA devices. The level and stability of the mechanical work generated by the material during low-cycle fatigue testing are considered criteria for the material performance and thus of the validity of the proposed optimization procedure.     

Impurity levels and fatigue lives of pseudoelastic NiTi shape memory alloys

In the present work we show how different oxygen (O) and carbon (C) levels affect fatigue lives of pseudoelastic NiTi shape memory alloys. We compare three alloys, one with an ultrahigh purity and two which contain the maximum accepted levels of C and O. We use bending rotation fatigue (up to cycle numbers >10⁸) and scanning electron microscopy (for investigating microstructural details of crack initiation and growth) to study fatigue behavior. High cycle fatigue (HCF) life is governed by the number of cycles required for crack initiation. In the low cycle fatigue (LCF) regime, the high-purity alloy outperforms the materials with higher number densities of carbides and oxides. In the HCF regime, on the other hand, the high-purity and C-containing alloys show higher fatigue lives than the alloy with oxide particles. There is high experimental scatter in the HCF regime where fatigue cracks preferentially nucleate at particle/void assemblies (PVAs) which form during processing. Cyclic crack growth follows the Paris law and does not depend on impurity levels. The results presented in the present work contribute to a better understanding of structural fatigue of pseudoelastic NiTi shape memory alloys.     

Electrochemical behaviour of oxidized NiTi shape memory alloys for biomedical applications

A new oxidation treatment (OT) for NiTi Shape Memory Alloys used in biomedical applications has been developed in a previous work to reduce Ni surface concentration and, consequently, decrease Ni ions release into the exterior medium. This OT treatment is expected to minimize adverse and toxic reactions associated to Ni ions. However, in order to assess the biocompatibility of a metallic material, its corrosion resistance has to be evaluated.In this work, the electrochemical behaviour of NiTi surfaces oxidized by the new OT treatment was compared to untreated NiTi surfaces (NT). For this purpose, tests of open-circuit potential and cyclic voltammetry were performed at 37 °C in a Hanks Balance Salt Solution.A significant increase of the corrosion (E_corr) and breakdown (E_b) potentials was observed for OT surfaces, in comparison with NT surfaces. Moreover, the qualitative potentiodynamic behaviour of OT NiTi and Ti surfaces are similar. This observation suggests that the OT treatment leads to the formation of an oxide on NiTi surface that has similar structure and electrochemical property to native Ti oxide. This new oxidation treatment is efficient to protect NiTi alloys from electrochemical degradation and, therefore, it may be an excellent candidate for biomedical applications.     

Passivation and oxygen ion implantation double surface treatment on porous NiTi shape memory alloys and its Ni suppression performance

The surfaces of porous NiTi shape memory alloys (SMAs) were modified by double treatment of passivation (PA) in HNO₃ solution and oxygen plasma immersion ion implantation (O-PIII) methods. SEM and XPS were used to characterize the modified surface. It has been found that a protective film consists of three layers formed on the surface after modification. From the outmost surface towards inside, they are a 10 nm thick layer consisting of rutile and anatase TiO₂ with pure rutile TiO₂ on the surface and their ratio varying in gradient, a 45 nm thick layer containing anatase TiO₂ and NiTi with constant ratios and a 40 nm thick layer of TiO₂, TiO and NiTi with their ratio varying in gradient. The immersion tests in simulated human body fluid demonstrated that the modified porous NiTi samples exhibit a good Ni suppression performance, approaching to that of the untreated dense NiTi samples. The Ni ion content released from the porous NiTi SMAs double treated by PA and O-PIII is one magnitude lower than that from the untreated porous samples. Moreover, the released Ni ion content after 8 weeks can be reduced into the safe range acceptable to the human body.     

Effect of post-deformation annealing on the R-phase transformation temperatures in NiTi shape memory alloys

In NiTi shape memory alloys, both the annihilation of dislocations and the formation of Ni₄Ti₃ precipitates may occur during post-deformation annealing. Different responses of the R-phase transformation temperatures to the annealing conditions have been reported. In order to find out the main factor(s) affecting the R-phase transformation temperatures during post-deformation annealing, a Ti-49.8 at% Ni and a Ti-50.8 at% Ni alloy were subjected to various post-deformation annealing and thermal cycling treatments. The results show that the R-phase transformation temperatures are very stable in the Ti-49.8 at% Ni alloy, while a significant variation is observed in the Ti-50.8 at% Ni alloy with respect to the annealing and thermal cycling conditions. These findings suggest that the R-phase transformation temperatures are not susceptible to the change of dislocation density and depends mainly on the Ni concentration of the matrix, which can be modified by the formation of Ni₄Ti₃ precipitates.