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.     

The mechanism of multistage martensitic transformations in aged Ni-rich NiTi shape memory alloys

Usually aged Ni-rich NiTi alloys undergo martensitic transformations on cooling from high temperatures in two steps: B2 to R and then R to B19′ (normal behaviour). But under certain ageing conditions, the transformation can also occur in three or more steps (unusual multiple step behaviour). In the present study we use differential scanning calorimetry (DSC) for a systematic investigation of the evolution of transformation behaviour with ageing temperature and time. We demonstrate that during ageing of Ni-rich NiTi alloys, DSC curves exhibit two transformation peaks on cooling after short ageing times, three after intermediate ageing times and finally again two peaks after long ageing times (2–3–2 transformation behaviour). In the present study we propose a new explanation for the 2–3–2 transformation behaviour which consists of two basic elements: (1) The composition inhomogeneity which evolves during ageing as Ni₄Ti₃ precipitates grow. (2) The difference between nucleation barriers for R-phase (small) and B19′ (large). These two elements explain all features of the evolution of DSC charts during ageing including the number of distinct DSC peaks and their positions.     

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.     

Modeling and simulation of irradiation effects on martensitic transformations in shape memory alloys

A model is proposed for the calculation of the irradiation induced changes of the martensitic transformation temperatures of shape memory alloys. It considers the transition temperatures being determined by a chemical and a non-chemical term in the Gibbs free energy that can be affected by the irradiation induced changes of the vacancy density and the chemical ordering of the crystal structures. Numerical simulations for TiNi SMAs have shown that upon irradiation the austenite finish temperature A_f can be raised slightly at the beginning stage. After that, both A_f and the martensite start temperature M_s will decrease strongly and reach some stable values after extensive irradiations. Both the dose rate and the temperature of irradiations will affect the changes of M_s and A_f. According to the simulation, the increase of A_f at the beginning of irradiations is resulted from the irradiation induced point defect production that increases mainly the non-chemical term in the Gibbs free energy change of the martensitic transformation. The irradiation induced chemical disordering reduces both the chemical and non-chemical terms in the Gibbs free energy change and leads to the strong decreases of M_s and A_f.     

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.     

Influence of compositional ratio on microstructure and martensitic transformation of CuZr shape memory alloys

In the present work the effect of Cu/Zr atomic ratio on structural and calorimetric properties of this high temperature shape memory alloy (HTSMA) is studied. It has been discussed the changes induced by Cu/Zr ratio on the martensitic transformation temperatures and the corresponding transformation heats coupled with the phases microstructure. The modification of the Cu content in the range ±2% at, around the equiatomic composition, does not drastically change the thermal properties of the alloys. Moreover, the Cu/Zr ratio strongly influences the microstructure in terms of the presence and amount of the other characteristic phases, Cu₁₀Zr₇ and CuZr₂, in the place of the CuZr phase. The understanding of the basic properties of the binary system can be of great help for further investigations on CuZr based systems with other alloying elements.     

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.     

Cyclic deformation mechanisms in precipitated NiTi shape memory alloys

Results are presented on the cyclic deformation of single crystal NiTi containing Ti₃Ni₄ precipitates of various sizes. Mechanical cycling experiments reveal that the cyclic degradation resistance of NiTi is strongly dependent on crystallographic orientation. Under compression, orientations approaching the [100] pole of the stereographic triangle possess the highest fatigue resistance. Orientations approaching the [111] pole of the stereographic triangle demonstrate the lowest fatigue resistance. Aging to produce small coherent Ti₃Ni₄ precipitates (10 nm) improves the fatigue resistance of NiTi compared to the other heat treatments (solutionized or overaged) for nearly all orientations. NiTi with 10 nm Ti₃Ni₄ precipitates consistently showed stabilized martensite due to mechanical cycling, and an absence of dislocation activity. Samples with large incoherent Ti₃Ni₄ precipitates (500 nm) consistently showed significant dislocation activity due to mechanical cycling in addition to stabilized martensite colonies. The first cycle stress–strain hysteresis was found to correlate to the fatigue resistance of the material. Samples demonstrating large inherent hysteresis, with different heat treatments and orientations, showed poor fatigue performance. Rational for the observed behaviors is discussed in terms of operant deformation mechanisms and ramifications on modeling the cyclic deformation of NiTi are presented.     

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.     

Comparative study of R-phase martensitic transformations in TiNi-based shape memory alloys induced by point defects and precipitates

We studied the influence of point defects (Fe) and precipitates (Ti₃Ni₄) on the characteristics of R-phase martensitic transformation by comparing the transport and thermal properties of as-quenched Ti₅₀Ni₄₆Fe₄ and annealed Ti_48.7Ni_51.3 shape memory alloys. Both alloys undergo a weak first-order R-phase transformation with a small thermal hysteresis (less than 7 K) and non-zero transformation strain, suggesting the introduction of point defects and precipitates lead to a stable R-phase in these alloys due to the defects induced local lattice deformations. Furthermore, our study revealed that the transition temperature, transformation width, and transformation strain of the investigated R-phase TiNi-based alloys are strongly affected by the induced defects. As a result, the annealed Ti_48.7Ni_51.3 has a higher transition temperature than that of Ti₅₀Ni₄₆Fe₄, as expected.     

Microstructure and martensitic transformation of TiNiNbB shape memory alloys

In present work, microstructure, martensitic transformation and mechanical properties of Ti₄₄Ni_47−xNb₉B_x (x = 0, 0.5, 1, 5 at.%) alloys were investigated as a function of B content. The results show that the addition of B significantly influences the microstructure of the alloys. The microstructure of Ti₄₄Ni₄₇Nb₉ alloy consists of B2 parent phase matrix and β-Nb phase. When the B content is 0.5 at.%, Nb₃B₂ phase presents. With further increasing B content to above 1 at.%, TiB and NbB phases present instead of Nb₃B₂ phase. With increasing B content, the transformation temperatures increase due to the reduced Ni/Ti ratio and Nb content in the matrix. The mechanical properties can be optimized by the addition of 1 at.% B.     

Biocorrosion and cytocompatibility assessment of NiTi shape memory alloys

Ti-based alloys are characterised by an excellent corrosion resistance, high mechanical performances, an increased biocompatibility, and in particular by the deforming force of NiTi. Five alloys were examined by electrochemical assays and cell culture tests with different cell types. All tests show the high biological and electrochemical performances of Ti6Al4V and NiTi, and in particular a significant influence of living cells on corrosion.     

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.