Dilatometric study of martensitic transformation in NiTiCu and NiTi shape memory alloys

Dilatometric measurements have been carried out for the study of nature of martensitic transformation in the NiTiCu and NiTi shape memory alloy wire samples. Investigation has been done in the heat-treat temperature range 300–800°C. NiTiCu exhibits only single stage A M martensitic transformation in the entire heat-treat temperature range indicating the suppression of R-phase by Cu substitution. NiTi shows the two-stage A R M martensitic transformation in the heat-treat temperature range 340–410°C and the single-stage A M martensitic transformation above heat-treat temperature 410°C. The extent of dilation during phase transformation decreases with increasing heat-treat temperature in both the alloys. Effect of first 15 thermal cycles on transformation temperatures in both the alloys has been studied. It is found that transformation temperatures are unaffected with thermal cycles in NiTiCu whereas considerable decrease in transformation temperatures has been observed in the case of NiTi. The stability of transformation temperatures in NiTiCu during M A transformation against thermal cycling may be attributed to the associated smaller thermal hysteresis compared to NiTi.     

Microstructural,mechanical and citotoxicity evaluation of different NiTi and NiTiCu shape memory alloys

Transformation temperatures and mechanical properties such as transformation stresses at different temperatures and the superelasticity have been investigated in NiTiCu alloys with various Copper concentrations. The results have been compared with the conventional NiTi alloys. The addition of copper was effective to narrow the stress hysteresis and to stabilize the superelasticity characteristics. Moreover, it produced greater stability on both the transformation temperatures and the forces applied to the different tissues. However, the studies of cell cultured with human fibroblasts showed certain toxicity.     

Densification and microstructural evolution of spark plasma sintered NiTi shape memory alloy

The effect of particle size and sintering temperature on the densification and microstructural characteristics of nickel-titanium shape memory alloy (NiTi-SMA) has been investigated using spark plasma sintering (SPS) process. The Ni and Ti elements in different particle sizes were alloyed in the composition of Ni_50.6Ti_49.4. The milled NiTi powders were consolidated using SPS process in a temperature range of 700–900?°C. The densification was characterized by plotting temperature, current and relative displacement of punch as a function of holding time. The results showed that a maximum relative density of ～98% can be achieved for NiTi-SMA with an average particle size of 10?µm at a sintering temperature of 900?°C. The microstructure of the sintered NiTi-SMA was examined using scanning electron microscope (SEM) and composition of NiTi alloy was analyzed using energy dispersive spectroscopy (EDS) analysis. The effect of sintering temperature on the microstructural evolution and transformation was also studied.     

Processing and property assessment of NiTi and NiTiCu shape memory actuator springs

Among the multifarious engineering applications of NiTi shape memory alloys (SMAs), their use in actuator applications stands out. In actuator applications, where the one‐way effect (1WE) of NiTi SMAs is exploited, SM components are often applied as helical coil springs. Ingots are generally used as starting materials for the production of springs. But before SM actuator springs can be manufactured, the processing of appropriate wires from NiTi ingots poses a challenge because cold and hot working of NiTi SMAs strongly affect microstructure, and it is well known that the functional properties of NiTi SMAs are strongly dependent on their microstructure. The objective of the present paper is therefore to produce binary Ni₅₀Ti₅₀ and ternary Ni₄₀Ti₅₀Cu₁₀ SMA actuator springs, starting from ingots produced by vacuum induction melting. From these ingots springs are produced using swaging, rolling, wire drawing and a shape‐constraining procedure in combination with appropriate heat treatments. The evolution of microstructure during processing is characterized and the mechanical properties of the wires prior to spring‐making are documented. The mechanical and functional characteristics of the wires are investigated in the stress‐strain‐temperature space. Finally, functional fatigue testing of actuator springs is briefly described and preliminary results for NiTi and NiTiCu actuator springs are reported.     

Phase transformation and microstructure and mechanical properties of as cast NiTiRe shape memory alloys

Abstract

The microstructure, martensitic transformation and mechanical properties of as cast Ni₅₂Ti_48?xRe_x shape memory alloys (SMAs) were investigated. The microstructure of these alloys consists of B19′ martensite phase as a matrix and B2 austenite in small percentages in addition to some precipitations of NiTi intermetallic compounds. There are two types of NiTi precipitates: the first one is Ti₂Ni, which can be seen in the all microstructures of the three alloys, and the other is Ni₂Ti, which is found only in the microstructure of Ni₅₂Ti_47·7Re_0·3 alloy. According to differential scanning calorimetry measurements, one stage of transformation reaction B2 to B19′ accompanied with forward and backward martensitic transformation was observed. The martensitic peak and the austenitic peak were increased with the addition of rhenium. Both are increased as the number of valence electron per atom increase and the valence electron concentration decrease. Hardness measurements of Ni₅₂Ti_48?xRe_x SMAs are improved by the Re additions.     

Fracture of precipitated NiTi shape memory alloys

The fracture mechanisms in single crystal and polycrystalline Ti-50.8at%Ni shape memory alloys containing Ti₃Ni₄ precipitates are studied using the scanning electron microscope (SEM). Aged materials with three different precipitate sizes (50 nm, 150 nm, and 400 nm), which have interfaces ranging from semi-coherent to incoherent, are considered. The mechanisms of material fracture identified in the single crystal NiTi are: 1. Nucleation, growth, and coalescence of voids from the Ti₃Ni₄ precipitates, 2. Cleavage fracture on {100} and {110} crystallographic planes, 3. Nucleation, growth, and coalescence of voids from fractured Ti-C inclusions. Cleavage and ductile tearing mechanisms also operate in polycrystalline NiTi, however, since the Ti-C inclusions are an artifact of single crystal growth processes, mechanism 3 was not discovered in the polycrystalline materials. Cleavage fracture and ductile tearing are found to act in conjunction, with the relative dominance of one over the other depending on the local precipitate size and concentration. As the Ti₃Ni₄ precipitate size increases to about 400 nm, the overall fracture is dominated by failure mechanism 1, and the cleavage markings become diffuse. Finally, we assert that the high tensile ductility of drawn NiTi polycrystals is due partially to the fact that drawn bar and wire stock usually have a strong {111} fiber texture. Such a texture promotes the initiation of the transformation at low stresses and concurrently prevents primary cleavage on the {100} or {110} planes.     

高温热变形条件下Ni-Ti形状记忆合金的微结构演变

采用真空感应法炼制了不同氧、碳含量的NiTi和NiTiNb形状记忆合金 .分析研究了合金的微观组织、相结构以及高温热变形对合金微观组织的影响 ,重点研究和探讨了合金微观结构在高温热变形过程中的演变规律 .确定了合金组织中TiNi基体相、Nb相及在晶界形成的氧化物和碳化物的形变特点 .结果表明 ,合金中C、O含量决定其组织中碳化物和氧化物的形态和含量 ,高温热变形时 ,TiNi基体相和Nb相粒子均产生较大塑性变形 .当合金中氧和碳含量较高时 ,氧化物和碳化物在变形时碎裂 ,在组织中的分布比铸态更均匀 .在本文研究的氧、碳含量的范围内 ,氧化物和碳化物的存在对合金的力学性能 ,如拉伸强度、延伸率等影响不大 .     

Optimization of spark plasma sintering parameters for NiTiCu shape memory alloys

Nanostructured nickel titanium copper-shape memory alloys (NiTiCu-SMAs) were fabricated using spark plasma sintering (SPS) by varying the significant process parameters. The NiTiCu elements with different particle size were consolidated in a temperature range of 700–900°C and pressure from 20 to 40 MPa with 5 min of soaking time. The sintered products were subjected to mechanical analysis such as density and microhardness. Genetic algorithm (GA) and particle swarm optimization (PSO) techniques were used with integrated artificial neural network (ANN) to optimize the SPS process parameters to obtain better mechanical characteristics. The results indicate that the density and microhardness can be enhanced by the reduction of particle size and increase in pressure and temperature. A maximum density of 6.21 g/cc and Vickers hardness of 766 Hv were obtained the optimal for process parameters of temperature, pressure, and particle size of ~ 800°C, ~ 26 MPa and ~ 6 µm, respectively, in case of NiTiCu nanostructured SMAs.     

NiTi形状记忆合金的加工方法

对NiTi形状记忆合金的多种加工方法如锻造、热挤压、轧制和拉拔、冷加工、粉末成形、包套碎片挤压成形、溅射沉积加工等进行了详细介绍.     

Si content dependence on shape memory and tensile properties in Fe-Mn-Si-C alloys

Fe-17Mn-xSi-0.3C alloys (x = 0, 2, 4, 6 mass%) were used to investigate the influence of Si on the tensile properties and the shape recovery strain. We considered three kinds of tensile properties: critical stress for ?-martensitic transformation, critical stress for dislocation gliding, and work hardening rate. A significant increase in the shape recovery strain was obtained in the 6%Si added alloy, when the alloys were heated to 873 K after a pre-straining of 8% in tension. The critical stresses for both the ?-martensitic transformation and the dislocation gliding increased with an increase in Si content from 0 to 4% but were similar in the 4%Si and 6%Si added alloys. However, the work hardening rate between the 4%Si and 6%Si added alloys was significantly different and was much smaller in the 6%Si added alloy. Hence, a 6%Si addition suppresses the plastic deformation due to the dislocation gliding through the decrease in the work hardening rate along with the solution hardening. As a result, ?-martensitic transformation occurs as the predominant deformation mode at smaller strains and improves the shape recovery strain.     

Surface structure and biomedical properties of chemically polished and electropolished NiTi shape memory alloys

The surface structure and biomedical properties of NiTi shape memory alloy (SMA) samples after undergoing electropolishing and chemical polishing are determined and compared employing scanning electron microscopy, X-ray photoelectron spectroscopy, inductively-coupled plasma mass spectrometry, hemolysis analysis, blood platelet adhesion test, and MTT test. The results indicate that after chemical polishing, there is still a high Ni concentration on the surface of the NiTi SMA. On the other hand, electropolishing can form a thin surface titanium oxide film (about 10 nm thickness) with depleted Ni. In addition to the TiO₂ phase, some titanium suboxides (TiO and Ti₂O₃) are found in the surface film. Compared to chemical polishing, electropolishing can more effectively mitigate out-diffusion of Ni ions and the wettability, blood compatibility, and thromboresistance are also better. However, no difference on the cytocompatibility can be observed from samples that have been chemically polished or electropolished.     

Micromechanics of precipitated near-equiatomic Ni-rich NiTi shape memory alloys

The specific thermo-mechanical behavior of precipitated, near-equiatomic Ni-rich NiTi shape memory alloys, i.e., thermal actuation under stress and pseudoelasticity, are investigated via the finite element method. The deformation response of the material-at-large is simulated using a representative volume element, taking into account the structural effect of the precipitates, as well as the effect of the Ni-concentration gradient in the matrix. An existing rate-independent constitutive model, similar to the one employed to describe the matrix behavior, is calibrated based on the deformation response of the representative volume elements. The actuation and pseudoelastic response of the homogenized material are found to be very close to those of the representative volume elements. The obtained results reproduce and provide important insight into several of the experimentally observed precipitation-induced changes on the transformation characteristics of these materials.     

Effects of microstructural mechanisms on the localized oxidation behavior of NiTi shape memory alloys in simulated body fluid

Localized oxidation and corrosion behavior of a nickel–titanium (NiTi) shape memory alloy (SMA) was investigated via static immersion experiments in a simulated body fluid solution. Detailed electron microscopy examinations on the sample surfaces revealed preferential formation of local oxide particles around dislocation networks, which constitute high-energy zones. Moreover, various intermediate phases were detected in addition to the parent NiTi phase around dislocation networks. These are also areas with enhanced diffusion, which promotes Ni release. These findings emphasize the significant role of fine microstructural features, such as dislocation networks, on the oxidation and Ni release, and thus, the biocompatibility of the NiTi SMAs.     

Detecting NiTi two-way shape memory effect based on microstructural and macromechanical parameters

ABSTRACT

The two-way shape memory effect in NiTi shape memory alloys is identified according to the evolution of the apparent modulus of the martensite during mechanical cycling. The microstrain and texture index of the NiTi samples are evaluated with synchrotron data to relate the evolution to the changes in the NiTi microstructure caused by mechanical cycling. The results show that a progressive decrease in the apparent modulus of the martensite during load, together with an increase in the apparent modulus of the reoriented martensite, are a sign that the NiTi sample is developing the two-way memory effect by mechanical cycling. When the two moduli show the same value, the two-way shape memory effect is fully developed in the NiTi alloy.

This paper is part of a thematic issue on Titanium.     

Investigating microstructural evolution during homogenization of the equiatomic NiTi shape memory alloy produced by vacuum arc remelting

The vacuum arc remelting process is widely used for the commercial production of NiTi shape memory alloys. Due to the absence of electromagnetic forces in this method, several remelting and long-time homogenizing are required. In this work, a Ti-50 at.% Ni alloy was prepared using the non-consumable vacuum arc melting technique in a water-cooled copper crucible. After four times of remelting process, specimens were subjected to homogenization at 1000 °C. Micro/macrostructural changes during homogenization were investigated by optical microscope and SEM equipped with EDS analyzer. The results showed that the as-cast specimen consisted of mostly Ni₃Ti, Ti₂Ni and monoclinic (B1^9′) phases with high segregation. By increasing the holding time during the homogenization process at 1000 °C, the amount of austenite (B₂) phase was increased, while segregation and unfavorable phases, and accordingly, hardness were decreased. After 4 h of homogenization, austenite (B₂) was the only phase maintained in the microstructure of Ti-50 at.% Ni. In addition, macrostructure of the alloy was turned into polygonal structure after such a homogenization treatment.     

Surface form memory in NiTi shape memory alloys by laser shock indentation

An indentation-planarization method for NiTi shape memory alloys has been developed that produces a robust surface topographical memory effect that we call "surface form memory", or SFM. Surface form memory entails reversible transitions between one surface form (flat) and another (say, wavy) that occur on changing temperature. These transitions are cyclically stable and exhibit very high mechanical energy density. Our previous study has demonstrated SFM transitions in NiTi alloys derived from quasistatic (i.e., low strain rate) spherical indents, as well as other geometries. Here, we report on experiments using confined laser ablation to indent a similar martensitic NiTi substrate, but in the dynamical regime (very high strain rate). As in the quasistatic case, subsurface plastic strain gradients are created via martensite twinning reactions, and later by dislocation-mediated slip. The resulting defects and stress fields support the two-way shape memory effect underlying SFM. In the dynamical case however, relative cyclic two-way displacements are found to be significantly larger, when normalized to the initial indent depth, than is the case with quasistatic indentation. This confers certain processing and boundary condition advantages. Analysis of the shock dynamics is found to be consistent with the observed surface displacements.     

Microstructure and martensitic transformation and mechanical properties of cast Ni rich NiTiCo shape memory alloys

Abstract

The influence of Co additions on the microstructure, second phase precipitates, phase transformation and mechanical properties of cast Ni_51?xTi₄₉Co_x (x?=?0, 0·5, 1·5 and 4 at-%) shape memory alloys was investigated. At the expense of Ni, Co added to NiTi alloy significantly increases the martensitic transformation temperature. The matrix phase in the microstructure of Ni₅₁Ti₄₉Co₀ alloy is the austenite phase (B2) in addition to martensite phase (B19′) and precipitates of NiTi intermetallic compounds. However, the parent phase in the other three alloys, Ni_50·5Ti₄₉Co_0·5, Ni_49·5Ti₄₉Co_1·5 and Ni₄₇Ti₄₉Co₄, is martensite. Ti₂Ni phase was found in the microstructures of the all investigated alloys; however, Ni₃Ti₂ phase precipitated only in the NiTi alloy with 0 at-%Co. The volume fraction of Ti₂Ni phase decreased by the additions of 0·5 and 1·5 at-%Co, while it is slightly increased with 4 at-%Co. The hardness value of NiTi alloy is affected by Co additions.     

Relevant aspects in the clinical applications of NiTi shape memory alloys

NiTi shape memory alloys showing pseudoelastic behaviour have great potential in dental and orthopaedic applications where constant correcting loads may be required. In most of the clinical applications the device may have been heat treated and during its life in service it will be cyclically deformed. It is therefore important to investigate the effect of cyclic straining and heat treatments upon the transformation stresses and temperatures of the material. The aim of this work is to study the thermal and mechanical ageing of a pseudoelastic NiTi shape memory alloy, as well as the environmental in vitro degradation of the alloy due to the effect of artificial saliva.This paper was accepted for publication after the 1995 Conference of the European Society of Biomaterials, Oporto, Portugal, 10–13 September.