Ti-rich NiTi Alloy Research Articles

Two-way shape memory effect of a Ti rich NiTi alloy: experimental measurements andnumerical simulations

In this paper the two-way shape memory effect (TWSME) of a Ni–51 at.% Tialloy is investigated and a numerical model is developed, which allows real timesimulations of its hysteretic behaviour strain versus temperature. The two-wayshape memory effect (TWSME) was induced through a proper thermo-mechanicaltraining, carried out at an increasing number of training cycles and for two values oftraining deformation. The TWSME was measured under different appliedstresses and the hysteretic behaviour in the strain–temperature response wasrecorded. In order to evaluate the thermal stability of the hysteresis loops thematerial was subjected to many cycles, by repeated heating and cooling, betweenAf (austenite finishtemperature) and Mf (martensite finish temperature). The numerical method isbased on a phenomenological approach and was developed in aMatlab® function, which calculates the model parameters from a set of experimental data, and aSimulink® model, which is efficient enough for use in real time applications. The accuracy of theproposed model was analysed through systematic comparisons between experimentalmeasurements and numerical predictions.

Shape memory effect and superelastic behavior of TiNi shape memory alloy processed by vacuum plasma spray method

Machining of TiNi shape memory alloy into a complicated three-dimensional (3D) shape is quite difficult, thus a near-net shape forming of 3D shaped TiNi alloy is attractive and cost-effective. Vacuum plasma spray (VPS) process is one of such near-net shape forming processes. In this paper, two kinds of thick TiNi layer, Ni-rich and Ti-rich compositions, were fabricated by VPS process and their shape memory effect (SME) and superelastic (SE) behaviors were characterized. As-VPS processed Ni-rich TiNi which was subjected to homogenization at 1163 K for 7.2 ks and subsequent aging at 773 K for 18 ks exhibited good martensitic transformation behavior. The recoverable strain of the TiNi alloys due to SME and SE behavior were measured to be 2.4% and 5.0%, respectively. The Ti-rich TiNi alloy also exhibited good martensitic transformation behavior and SME in the as-homogenized state.

Martensitic transformation and shape memory effect of Ti–49 at.%Ni alloys

For binary TiNi shape memory alloys, most research work has been devoted to the equiatomic or Ni-rich TiNi alloys, while very few systematical researches are reported on Ti-rich TiNi alloys. In this paper, the martensitic transformation and shape memory effect of Ti-rich Ti–49 at.%Ni alloy were investigated by differential scanning calorimetry, optical microscopy, X-ray diffraction and testing of shape memory in a self-fabricated device. Ti–49 at.%Ni alloy specimens were annealed at 850 °C for 2 h and then cooled in a furnace or in water. The results show that the microstructure of Ti–49 at.%Ni alloys at room temperature consists of three phases, martensite phase B19′ and precipitate phase Ti 2Ni, as well as a small amount of retained parent phase B2. The difference of cooling conditions from annealing temperature 850 °C does not affect the martensitic transformation temperatures and microstructure of Ti–49 at.%Ni alloy. However, furnace cooling contributes to better shape recovery of Ti–49 at.%Ni alloy compared with water quenching. With increasing pre-strain, both one-way and two-way shape recovery of Ti–49 at.%Ni alloy decreases gradually. After 0.38% pre-strain deformation at −30 °C, the Ti–49 at.%Ni specimen exhibits 100% one-way shape recovery upon heating to 180 °C.

Structural evolution on thermal cycling in Ti-rich NiTi SMA

Shape-memory alloys (SMA) represent a class of metallic materials that has the capability of recovering a previously defined initial shape when subjected to an adequate thermomechanical treatment. Annealing of a Ti-rich Ni–Ti alloy has been followed by in situ high temperature diffraction in order to register the texture as well as the microstrain/microstress evolution. This type of study is relevant to the envisaged applications, because the type of preferential orientation and the corresponding anisotropic response of the material conditions contributes to the success of the SMA application. In the present study we have tested the feasibility of high temperature pole figures determination at ROBL (BM20) at the ESRF, and we have shown that there is a relation between the preferential orientation changing and the structural evolution taking place during annealing.

Phase transformation behavior of Ti-rich NiTi alloy by a calorimetric method

Nowadays a lot of shape memory alloys are known, and of these the NiTi alloy has excellent properties not only in shape memory but also in corrosion and wear resistance, so that it is more commonly used in various engineering fields in comparison with any other shape memory alloys [1–3]. A near equiatomic NiTi alloy shows a thermoelastic martensitic transformation, and it is known that the shape memory and transformation pseudoelasticity are caused by the reverse transformation from the phase with B19’ structure to the parent phase with B2 structure [4–6]. These phenomena are sensitive to the crystal structure, inner stress and defects. Therefore, factors such as Ni concentration, thermal treatment, mechanical working and addition of a third element play important roles in controlling the behavior of the shape memory in NiTi [7–10]. It is known that the transformation behavior of near equiatomic NiTi is sensitive to the composition and thermal cycles [11–15]. However, the investigation on the effect of thermal cycles has been scarcely carried out for Ti-rich NiTi alloys. The purpose of this study is to reveal the phase transformation behavior of a Ti-rich NiTi alloy. Details of the experiment are as follows. Ti-rich NiTi alloys were prepared using an arc melting furnace. The prepared samples were Ni48Ti52, Ni49Ti51, and Ni50Ti50 in at%. Several remelts were carried out for homogenization and then the samples were annealed at 1000 ◦C for an hour for homogenization. After cutting, the obtained samples were reannealed at 1000 ◦C. The transformation behavior was measured by using a differential scanning calorimeter with a liquid nitrogen cooling accessory. Fig. 1 shows exothermic behavior as a function of temperature during cooling. The onset point of a sharp peak in the high-temperature side corresponds to the start temperature of the transformation to the low temperature phase (Ms). The Ms of Ni48Ti50 alloy, Ni49Ti51, Ni50Ti50 is 65.95, 75.09, and 45.81 ◦C, respectively. Because the Ms of a Ni-rich NiTi alloys is lower in comparison with that of Ni50Ti50, the Ms reaches a maximum temperature in the vicinity of 51 at.%Ti [16]. Moreover, small variations in the Ms are observed for Ti-rich NiTi alloys in contrast to Ni-rich NiTi. Fig. 2 shows the shift of the Ms against the number of thermal cycles. It can be seen that the Ms is gradually decreased with increasing number of thermal cycles. The shift of the Ms between the 2nd and the 10th thermal cycle for Ni49Ti51, Ni48Ti52, and Ni50Ti50 is about Figure 1 The exothermic behavior during cooling for 2nd cycle.

The effect of severe marforming on shape memory characteristics of a Ti-rich NiTi alloy processed using equal channel angular extrusion

A Ti-49.8 at. pct Ni alloy was severely deformed at three different temperatures using equal-channel angular extrusion (ECAE). Three deformation temperatures—room temperature (below the martensite finish temperature), 50 °C (below the austenite start temperature), and 150 °C (above the austenite finish temperature)—were selected such that the initial deforming phase (B2 austenite or B19’ martensite) and the initial governing deformation mechanism (martensite reorientation, stress-induced martensitic transformation, or dislocation slip in martensite) would be different. The X-ray analysis results revealed that all processed samples mostly contained a deformed martensitic phase, regardless of the initial deforming phase and the deformation mechanism. Although the martensite start temperature did not change, the austenite start temperature decreased significantly in all deformation conditions, probably because of the effect of the internal stress field caused by the deformed microstructure. All deformation conditions led to an increase in the strength levels and some deterioration of shape-memory characteristics. However, a subsequent low-temperature annealing treatment significantly improved pseudoelastic strain levels while preserving the ultrahigh strength levels. The sample deformed at room temperature followed by the low-temperature annealing resulted in the most promising strength and shape-memory characteristics under compression, such that a 5.3 pct shape-memory strain at a 2200 MPa strength level and a 3.3 pct pseudoelastic strain at a 1900 MPa strength level were achieved. The differences between the strength levels and the shape-memory characteristics after severe deformation at different temperatures were attributed to the different amounts of plastic deformation and the resulting deformation textures, since at each deformation temperature the deformation mechanism was different. It is concluded that the severe marforming using ECAE could easily improve strength levels of NiTi alloys while preserving the shape-memory and pseudoelasticity (PE) characteristics and, thus, improve the thermomechanical fatigue behavior. However, lower deformation temperatures are necessary to hinder formation of macroshear bands, and ECAE angles larger than 90 deg should be used to reduce the amount of strain applied in one pass.

Ti過剰Ti-Ni合金スパッタ蒸着薄膜の結晶化に伴う析出物の生成とマルテンサイト変態への影響

The crystallization temperature and composition dependece of morphology of precipitates and precipitates effect on martensitic transformation of sputter-deposited Ti-rich Ti-Ni alloy films were investigated.For the investigation by the electron microscopy on precipitates morphology behavior, the deposited amorphous films were crystallized and concurrently aged by holding at various temperatures between 693 K and 823 K. The following schemes were observed depending on Ti-rich Ti-Ni alloy films composition and heat-treatment temperature: (a) Precipitate-free; (b) Plate-like precipitates exist in the B2 phase; (c) Plate-like precipitates and spherical Ti2Ni precipitates coexist in the B2 matrix; (d) Ti2Ni precipitates exist in the B2 matrix; (e) Ti2Ni precipitates with and without orientation relationship to the matrix.Then, for investigating composition and crystallization temperature dependence of B2⇔R transformation temperatures the amorphous films were crystallized by holding at various temperatures, 723 K, 773 K and 973 K, for 3.6 ks. For investigation of precipitates effect on the martensitic transformation behavior, the sputter-deposited amorphous films were crystallized and concurrently aged by holding at various temperatures, 695 K and 723 K. The specimens were analyzed with differential scanning calorimetry (DSC) and the following was found. The martensitic transformation behavior changed depending on crystallization temperature and composition. The B2⇔R transformation temperatures of the thin films crystallized at temperatures between 723 K and 773 K were the lowest at Ti-48.5 at%Ni and plate-like precipitates lowered the B2⇔R transformation temperatures.The R⇒B19′ transformation temperatures were lowered by distribution of fine Ti2Ni precipitates of Ti-47.0 at%Ni∼Ti-48.5 at%Ni sample crystallized at low temperatures.

Two-way shape memory effect of sputter-deposited Ti-rich Ti–Ni alloy films

Two-way shape memory effect of sputter-deposited Ti-rich Ti–Ni alloy films was investigated. Amorphous films of Ti–48.6at.% Ni alloy obtained by sputtering were crystallized by holding at 743 K for 0.3 ks and then aged under constraint (circular shape) at the same temperature for 1.8 ks. On these specimens, observation of spontaneous shape change during cooling and heating, and also differential scanning calorimetry (DSC) were done. The aged films showed two-way shape memory effect in association with the B2⇔R⇔B19′ transformation.

Internal structures and shape memory properties of sputter-deposited thin films of a Ti–Ni–Cu alloy

Low-temperature heat treatments of the sputter-deposited amorphous films, which were previously proved to be a new method to produce very good shape memory properties for Ti-rich Ti–Ni alloys, have been applied to a ternary Ti–43.0Ni–6.2Cu alloy (at.%). The basically same nanometric structures as in the binary alloy are formed, i.e. the nanometric structures consist of extremely thin plate precipitates of bct structure, which are formed on {100} planes of the parent B2 structure and have the c-axis normal to the habit planes. High-shape recovery stresses of about 500 MPa with recoverable shape strains of 5% are obtained without accompanying any permanent strains. A shape recovery stress of more than 870 MPa is attained if it is allowed to involve about 1% permanent strain. Although these bct precipitates have large tetragonalities, they are perfectly coherent with the parent bcc lattice. The maximum shape recovery stress is nearly twice that of the Ti-rich Ti–Ni binary alloy having a similar nanometric structure. It is suggested that this remarkable increase in recovery stress may be attributed to the change in Burgers vector of dislocations caused by partial disordering in Ti–Ni–Cu alloys. It is emphasized that the shape recovery stress in this ternary alloy is four times that of the Ti 2Ni containing samples and 10 times that of a bulk Ti–45Ni–5Cu alloy.

Structure of Sputter-Deposited Ti-Rich Ti-Ni Alloy Films

Composition and heat-treatment temperature dependence of the micro-structure of sputter-deposited (amorphous in the as-deposited state) and crystallized Ti-rich Ti-Ni alloy films were investigated by transmission electron microscopy. The following schemes were observed depending on alloy composition and heat-treatment temperature: (a) No precipitates ; (b) plate-shape precipitates; (c) coexisting plate-shape precipitates and spherical Ti 2 Ni precipitates with orientation relationship to the matrix; (d) only spherical Ti 2 Ni precipitates with the orientation relationship; (e) spherical Ti 2 Ni precipitates with and without orientation relationship.

Ti過剰Ti-Niスパッタ薄膜のマルテンサイト変態と二方向形状記憶効果

The martensitic transformation and associated two-way shape memory effect of sputter-deposited Ti-rich Ti-Ni alloy films were investigated.For investigation of aging (without constraint) on the martensitic transformation behavior, the deposited amorphous films were crystallized and concurrently aged by holding at various temperatures between 693 and 1123 K.The specimens were analyzed with differential scanning calorimetry (DSC) and the following was found: The martensitic transformation behavior changes depending on crystallization temperatures and composition. The B2 ⇔ R transformation temperatures of the thin films crystallized at temperatures between 693 and 773 K were the lowest at Ti-48.5 at%Ni.Then, for investigating the aging effect on the spontaneous shape change associated with the transformations, the amorphous films were crystallized by holding at various temperatures between 693 and 743 K followed by aging under constraint (circular shapes) at several different temperatures between 693 and 763 K. On these specimens, observation of spontaneous shape change during cooling and heating was made. The obtained results are the following: The specimens aged under constraint show the two-way shape memory in association with the B2 ⇒ R ⇒ B19′ and the reverse transformations. While the hysteresis of the B2 ⇔ R transformation is less than 2 K, that of the R ⇒ B19′ is about 60 K. Aging at 743 K under constraint gives the largest spontaneous shape change both for B2 ⇒ R and B2 ⇒ R ⇒ B19′ transformations.

Effect of thermal cycling on the R-phase and martensitic transformations in a Ti-rich NiTi alloy

The effect of thermal cycling on transformation temperature was studied on a Ti-rich NiTi alloy. The study was carried out by determining the electrical resistance, the internal friction, and the elastic modulus vs temperature. This study shows that the martensite microstructure is modified by the successive cycling transformation. In addition, we established that both the martensite internal friction and the transition peak are sensitive to the transient effect (the vibration frequency lies around 300 Hz). But the major results concern the behavior associated with the R phase occurrence and its evolution. We have stated that the premartensitic phase becomes stable following the diminishment of the beginning of the martensite formation (M s ). Interrupted cooling has also shown that, contrary to the martensite, the R phase exhibits no hysteretic behavior.

Thermal Cycling Effect on the Premartensitic and Martensitic Transition in a Ti Rich NiTi Alloy

The study was carried on a Ti-rich NiTi shape memory alloy. The electrical resistance and the elastic modulus were determined versus temperature. The aim of this work is to study the thermal cycling effect on the transformation features as the temperatures or the phase succession as well as the structural modifications. The major results of this work concern the R-phase occurrence and its evolution. We have shown that the premartensitic phase could exist in a narrow range of temperature on cooling and is revealed only after a few cycles. In fact, its presence and stability is directly linked to the diminishing of Ms. The main condition for the austenite to be transform in a rhombohedral structure is that Ms becomes low enough to be below Tr. Interrupted cooling alms also show that contrarily to the austenite-martensite transition, the austenite-R-phase transition doesn't exhibit any hysteretic behaviour.

Microstructural aspects of precipitation and martensitic transformation in a Ti-rich Ni-Ti alloy

Gradient materials can achieve both external strengthening and internal toughening, which has great potential to break the strong plastic tradeoff. Titanium carbide (TiC) particle is conducive to form a wearable surface in biomedical Titanium (Ti) alloy. In this work, gradient TiC/Ti6Al4V composites were successfully prepared by friction stir processing (FSP). Microstructure, texture and mechanical properties of the material were systematically characterized. Scanning electron microscopy (SEM) showed that the equiaxed grains near the machined surface were transformed into elongated grains inside. The results of energy spectrum analysis (EDS) showed that there was element aggregation near TiC particles. X-ray diffraction (XRD) indicated martensitic transformation in the stir zone and in the transition zone. The results of nano indentation showed that the hardness of stir zone was higher. In order to promote recrystallization and improve the hardness of the transition zone, high temperature rapid heat treatment was carried out after FSP. When the sample was heated at 700 °C and cooled in the furnace, the hardness increased to the maximum. In the process of rapid heating, the dissolution of elements near TiC particles also led to the increase of hardness. This study provides a new idea for surface modification of gradient titanium alloy by using FSP method to improve its mechanical properties.

The martensitic transformation in Ti-rich TiNi shape memory alloys

The martensitic ( Ms) transformation temperatures and their ΔH values of Ti 51Ni 49 and Ti 50.5Ni 49.5 alloys are higher than those of equiatomic or Ni-rich TiNi alloys. The Ti-rich TiNi alloys exhibit good shape recovery in spite of a great deal of second phase Ti 2Ni or Ti 4Ni 2O existing around B2 grain boundaries. The nearly identical transformation temperatures indicate that the absorbed oxygen in Ti-rich TiNi alloys may react with Ti 2Ni particles, instead of the TiNi matrix, to form Ti 4Ni 2O. Martensite stabilization can be induced by cold rolling at room temperature. Thermal cycling can depress the transformation temperatures significantly, especially in the initial 20 cycles. The R-phase transformation can be promoted by both cold rolling and thermal cycling in Ti-rich TiNi alloys due to introduced dislocations depressing the Ms temperature. The strengthening effects of cold rolling and thermal cycling on the Ms temperature of Ti-rich TiNi alloys are found to follow the expression Ms = To — KΔσ y . The K values are affected by different strengthening processes and related to the as-annealed transformation temperatures. The higher the as-annealed Ms (or As), the larger the K value.

Texture monitoring in aluminium alloys: a comparison of ultrasonic and neutron diffraction measurement: Clark, A.V. Jr.; Reno, R.C.; Thompson, R.B.; Smith, J.F.; Blessing, G.V.; Fields, R.J.; Delsanto, P.P.; Mignogna, R.B. Ultrasonics, vol. 26, no. 4, pp. 189–197 (Jul. 1988)

This study investigated the effect of warm-working rate on the shape memory properties of Ti–49.7 at.%Ni alloy. The ingot was prepared by using an arc melting method and then warm-rolled at 673 K to reduce the thickness by 77%, 85%, and 89%. For comparison, an ingot was cold-rolled at room temperature to reduce the thickness up to 30%. The warm-worked specimens were annealed at 623–723 K for 3.6 ks, and the cold-worked specimens were annealed at 673 K for 3.6 ks. The transformation behavior was analyzed by means of differential scanning calorimeter (DSC) and tensile tests. The critical stress for slip increases with increasing warm-working rate and decreasing annealing temperature. The martensitic transformation temperature (TR→M) and the reverse transformation temperature (TM→B2) increase with decreasing warm-working rate and increasing annealing temperature. The critical stress for slip in the warm-worked specimens is higher than that of the specimens annealed at 673 K after cold-working. Therefore, warm-working at 673 K improves the shape memory characteristics of Ti-rich Ti–Ni alloys. This suggests that Ti-rich Ti–Ni alloys are good candidate materials for actuators.