Ni-rich NiTi Alloys Research Articles

Interrelation of grain boundary microstructure and texture in a hot rolled Ni-rich NiTi alloy

This investigation deals with the evolution of grain boundary microstructure and crystallographic texture during hot rolling of a Ni-rich NiTi alloy. Electron backscattered diffraction studies revealed the occurrence of several coincidence site lattice (CSL) boundaries. The origin of these boundaries has been identified. The crystallographic texture of the deformed sample consists mainly of (1 1 1)‖ normal direction fiber. The texture components on the fiber exhibit some correlation with the type of CSL boundary.

Effects of equal channel angular extrusion and aging treatment on R phase transformation behaviors and Ti 3Ni 4 precipitates of Ni-rich TiNi alloys

Ni-rich TiNi alloys were subjected to the effect of multiple equal channel angular extrusion (ECAE) treatments by B C path at 500 °C. The characteristics of R phase transformation in aging treatment were dissimilar in the appearance and the temperature range to those counterparts induced by ECAE treatments. The fine lens-like shape Ti 3Ni 4 particles precipitated mainly in the regions of near grain boundaries and on the tangled grain boundaries after ECAE treatments. The effects and mechanisms of aging treatments and ECAE treatments on R phase transformation behaviors and Ti 3Ni 4 precipitates were investigated and discussed.

Influence of applying external stress during aging on martensitic transformation and the superelastic behavior of a Ni-rich NiTi alloy

Superelastic properties and martensitic transition temperatures of Ni 50.9Ti shape memory wires were studied after aging under different applied stresses at 450 °C for 45 min. The results indicate that applying external stresses during aging of Ni 50.9Ti shape memory wires changes the strain value and stress level of plateaus related to stress induced martensitic transformation. Furthermore, martensitic transformation sequences change after applying external stresses but transformation temperatures measured with DSC remain constant in samples aged under stresses up to 120 MPa. Applying stress of 60 MPa during aging enhances the superelastic properties by increasing the strain of plateaus during loading and unloading cycles. Applying stresses more than 60 MPa, decreases the strain of the plateau during unloading.

Effects of ECAE and Aging on Phase Transformations and Superelasticity of a Ni-Rich TiNi SMA

Effects of equal-channel angular extrusion (ECAE) process at 773K and heat treatment on phase transformations and superelasticity of a Ni-rich TiNi shape memory alloy were investigated by differential scanning calorimeter (DSC) measurement and cyclic tensile test. The R phase transformation of Ni-rich TiNi alloy was stimulated after ECAE processes and within a larger temperature range. The martensitic transformation start temperature Ms of Ni-rich TiNi alloy decreased sharply after ECAE processes at 773 K, then rapidly rose back after the specimen aging at 773K for 30min. Reasons for the changes of phase transformation behaviors have been discussed. ECAE processed TiNi samples exhibit better super-elasticity than solution treated one. With the increase passes of ECAE, the superelasticity becomes more stable, and a completely recoverable strain of 6% is obtained for TiNi sample after 8 passes ECAE.

Characterization of porous, net-shaped NiTi alloy regarding its damping and energy-absorbing capacity

Porous NiTi alloys are highly attractive for energy absorbers, damping devices and biomedical implants. In the present work, metal injection moulding (MIM) in combination with the application of a suitable space holder material was used for the production of NiTi parts with well defined pore sizes and porosities in the range of 30–70vol.%. For comparing the properties, porous titanium and Ti–6Al–4V samples were prepared in the same manner.Focus of the present work was a detailed investigation of the mechanical properties of porous NiTi to estimate its potential regarding the abovementioned applications. For a Ni-rich NiTi alloy with a porosity of 50vol.%, fully pronounced pseudoelasticity after 6% compression was demonstrated. An energy dissipation of 1.5MJ/m3 was measured, which could be directly related to the reversible austenite–martensite phase transformation. At higher deformations, pseudoelasticity becomes more and more superposed by the onset of plastic deformation. Nevertheless, even at deformations of up to 50%, a clearly pronounced amount of pseudoelastic shape recovery still remained. Fatigue of pseudoelasticity was investigated by conducting of up to 230,000 load cycles to 4% compression at a frequency of 1Hz.

An Investigation on Phase Formations and Microstructures of Ni-rich Ni-Ti Shape Memory Alloy Thin Films

Ni-rich Ni-Ti alloy thin films were fabricated by radio frequency (RF)-direct-current (DC) magnetron sputtering using elemental Ni and Ti as sputter targets. Si (100) was chosen as a substrate that was either held at room temperature or at 573 K (300 °C) during the depositions. The 380-nm-thick films were characterized by field emission scanning electron microscopy, energy dispersive spectroscopy, grazing incidence X-ray diffraction, atomic force microscopy, high-resolution transmission electron microscopy, and Vickers microhardness tester. The results suggests that because of the lack of surface mobility of the adatoms at the room temperature, the deposited films were smooth and amorphous, with a crystallite size of 15 nm accompanied with a porous morphology. At higher substrate temperatures, an increase in the surface diffusion leads to the formation of partially crystalline, rougher films with a denser, compact, fibrous grain microstructure. The formation of Ni-rich precipitates such as Ni4Ti3, Ni2Ti, and Ni3Ti along with small amount of the NiTi phase were attributed to the localized heating and cooling within the grains. Few grains exhibited band structures that are believed to be a result of type II twins.

Phase diagram ofTi50−xNi50+x: Crossover from martensite to strain glass

We systematically investigated the variation in transition behavior and physical properties over a wide excess Ni (acting as defect) concentration range $(x=0--2.5)$ in ${\text{Ti}}_{50\ensuremath{-}x}{\text{Ni}}_{50+x}$ alloys. This enables the establishment of an updated quantitative phase diagram for this important system. The phase diagram shows not only the well-known parent phase and martensite phase but also a premartensitic state and a strain glass state. Our experiments were able to determine quantitatively the borders of these states, the latter two having been unclear so far. The new phase diagram shows that a crossover from martensite to strain glass occurs at $x=1.3$, and the appearance of a ``premartensitic phase'' below a critical temperature ${T}_{\text{nd}}$ for defect-containing compositions $(x>0)$. We propose that point defects (excess Ni here) play two roles in a ferroelastic/martensitic system: (i) changing the thermodynamic driving force for the formation of long-range strain order (martensite) and (ii) creating random local stress that favors a premartensitic nanostructure and strain glass. Our work enables a simple explanation for several long-standing puzzles, such as the appearance of premartensitic nanostructure, the vanishing of transition latent heat with increasing Ni content and the anomalous negative temperature coefficient of electrical resistivity in Ni-rich Ti-Ni alloys.

Study on Homogenization Time and Cooling Rate on Microstructure and Hardness of Ni-42.5wt%Ti-3wt%Cu Alloy

Over the last decades, numerous investigations have been conducted on Nitinol properties. However, the effects of alloying elements on Ni-rich NiTi alloys have been considered less. In this research, different effects of homogenization time and cooling rate on the behaviors of Ni-42.5wt%Ti-3wt%Cu alloy were evaluated. The mentioned alloy was fabricated by vacuum arc melting method. Three different homogenization times (half, one and two hours) and three cooling media (water, air and furnace) were selected. The microstructure and hardness were examined by means of optical and scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) analysis and hardness test, respectively. According to the microscopic investigations, no significant changes were observed after half an hour. However, results indicate that increasing time of homogenization leads to finer precipitations and a uniform distribution of them. The various cooling environments result in the formation of two types of precipitation phases. It was seen that in the high cooling rate, the majority of precipitations consisted of Ti2(Ni,Cu) while by decreasing cooling rate NiTiCu precipitates appeared too, which affect the hardness.

Effect of compressive stress aging on transformation strain and microstructure of Ni-rich TiNi alloy

The Ti–50.7%Ni (atom fraction) alloy rods were compressive stress aged at 400 °C, 450 °C and 500 °C for different time, their strain behaviors accompanied by temperature elevation were investigated, and their microstructures were observed. It is found that the compressive stress aged TiNi alloy rod displays an obvious contractive strain behavior in the stress direction as the temperature is elevated from approximately 55–75 °C. Compressive stress causes the parallel alignment of the aging precipitate Ti 3Ni 4 in the TiNi alloy, which controls the martensitic transformation (B19′ transformation) and its reverse transformation, leading to its contractive strain behavior accompanied by temperature elevation. The contractive strain of the TiNi alloy compressive stress aged at 400 °C for 100 h is increased with increasing compressive stress up to 140 MPa. Higher aging temperature and longer aging time lead to the coarsening of the precipitates and the enlarging of the inter-precipitate spacing, and therefore result in a decrease in the contractive strain.

Multi-stage martensitic transformations induced by repeated thermal cycling of equiatomic TiNi alloy

Usually a multi-stage martensitic transformation is observed in Ni-rich TiNi alloys after heat treatment at 350–500 °C. It is due to the internal stresses created by the Ni 4Ti 3 participate. In the present work it was found that the multi-stage martensitic transformation appeared in Ti–50.0 at.% Ni alloy after thermal cycles through the temperature range of the phase transitions. Annealed sample undergoing one-stage phase transition was subjected to 32 thermal cycles in the DSC apparatus. The results had shown that three-stage forward martensitic transformation observed after 32 thermal cycle was due to the B2 → R, B2 → B19′ and R → B19′ phase transitions. It was found that the B19′ phase obtained from the B2 phase underwent the reverse transformation at higher temperatures than the B19′ phase obtained from the R phase. After annealing the cycled sample at 400 °C the transformation behavior was similar to the non-thermal cycled alloy. It was concluded that the main reason for the multi-stage phase transition induced by the thermal cycles was the phase hardening.

The influence of Ni 4Ti 3 precipitates orientation on two-way shape memory effect in a Ni-rich NiTi alloy

In this present work two-way shape memory effect (TWSME) induced by thermomechanical cycling treatments in a Ni-rich NiTi alloy was investigated.Effect of orientation of metastable and lenticular Ni 4Ti 3 precipitates as a governing parameter in different aging temperatures and time as well as applied strain on TWSME were studied. The results indicate that 450 °C is the optimum aging temperature to achieve highest recovery rate. The increasing of applied strain up to 3.38% and aging time up to 2.5 h at 450 °C improves the recovery rate. It has been concluded that formation of especial variants of Ni 4Ti 3 precipitates during aging and increasing of applied strain on thermomechanical cycling treatments could enhance the TWSME.

Dependence on composition of the internal friction of Ni 100− xTi x alloys containing hydrogen

Low frequency (0.1–50 Hz) internal friction (IF) measurements have been made as a function of temperature (100–300 K) in the H-free and H-doped Ni 50Ti 50 and Ni 52Ti 48 alloys. In the H-free equiatomic alloy no IF peak occurs in the martensitic state, while hydrogen doping gives rise to a well developed Snoek-type relaxation within the martensite (B19′). In the Ni-rich alloy not undergoing martensitic transition, a well evident peak ( P TWM) appears at around 150 K. H-doping cancels this peak and introduces an H Snoek relaxation in the B2 austenite. A recently reported peak associated with a sort of strain–glass transition occurring in Ni-rich NiTi alloys is shown to coincide with P TWM, while the “200 K peak”, earlier reported by many authors with non-intentionally H loaded NiTi alloys is confirmed to be a H-twin boundary interaction effect ( P TWH).

Structure and thermomechanical behavior of NiTiPt shape memory alloy wires

The objective of this work is to understand the structure–property relationships in polycrystalline NiTiPt (Ti 42.7 at.% Ni 7.5 at %Pt) with a composition showing pseudoelasticity at ambient temperatures. Structural characterization of the alloy includes grain size determination and texture analysis while the thermomechanical properties are explored using tensile testing. Variation in heat treatment is used as a vehicle to modify microstructure. The results are compared to experiments on Ni-rich NiTi alloy wires (Ti–51.0 at.% Ni), which are in commercial use in various biomedical applications. With regards to microstructure, both alloys exhibit a <1 1 1> fiber texture along the wire drawing axis; however, the NiTiPt alloy grain size is smaller than that of the Ni-rich NiTi wires, while the latter materials contain second-phase precipitates. Given the nanometer-scale grain size in NiTiPt and the dispersed, nanometer-scale precipitate size in NiTi, the overall strength and ductility of the alloys are essentially identical when given appropriate heat treatments. Property differences include a much smaller stress hysteresis and smaller temperature dependence of the transformation stress for NiTiPt alloys compared to NiTi alloys. Potential benefits and implications for use in vascular stent applications are discussed.

3D Reconstruction of Ni&lt;sub&gt;4&lt;/sub&gt;Ti&lt;sub&gt;3&lt;/sub&gt; Precipitates in a Ni&lt;sub&gt;51&lt;/sub&gt;Ti&lt;sub&gt;49&lt;/sub&gt; Alloy in a FIB/SEM Dual-Beam System

Ni4Ti3 precipitates play an important role in the shape memory and superelastic behaviour of thermo-mechanically treated Ni-Ti material. The 3D morphology and distribution of such precipitates with lenticular shape and rhombohedral atomic structure in the austenitic B2 matrix of a binary Ni-rich Ni-Ti alloy has been elucidated via a slice &amp; view procedure in a Dual-Beam FIB/SEM system. With the sequence of cross-section SE images obtained from the SEM, a 3D reconstruction has been achieved after proper alignment and image processing, from which both qualitative and quantitative analysis can be performed. Careful imaging is needed to ensure that all variants of the precipitates are observed with equal probability, regardless sample orientation. Moreover, due to the weak contrast of the precipitates, proper imaging conditions need to be selected to allow for semi-automated image treatment. Finally, a volume ratio of 10.2% for the Ni4Ti3 precipitates could be calculated, summed over all variants, which yields a net composition of Ni50.36Ti49.64 for the matrix, leading to an increase of 113 degrees for the martensitic start temperature Ms. Also, the expected relative orientation of the different variants of the precipitates could be confirmed. In the near future, other quantitative measures on the distribution of the precipitates can be expected.

Effect of cooling rate on the phase transformation behavior and mechanical properties of Ni-rich NiTi shape memory alloy

TiNi alloy is a well-known shape memory alloy and has been widely used for bio-medical, mechanical and electrical applications. In this study, a Ni-rich NiTi alloy was prepared by vacuum arc melting in a water-cooled copper crucible. Three samples of this alloy were heated to 1000 °C and cooled in three media: furnace, water, and dry-ice bath. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), hardness measurement and tensile test were carried out to investigate the effect of cooling rate on transformation temperature and mechanical properties. The results show that Ni3Ti intermetallic compounds have a great influence on martensitic phase transformation temperature. These tests clearly showed the correlation between cooling rate and properties of the alloy.

Effect of ageing on transformation kinetics and internal friction of Ni-rich Ni–Ti alloys

In Ni-rich Ni–Ti alloys the ageing treatments create finely dispersed Ti 3Ni 4 precipitates in the B2-based matrix. Formed precipitates are known to influence the phase transformation temperatures and to facilitate the R-phase transformation. In addition to the change in the phase transformation behaviour, the ageing treatments are also reported to affect internal friction and mechanical properties of Ni–Ti alloys. In the present work, by means of systematic DSC and DMA studies the effects of the ageing treatments on the phase transformation as well as on the internal friction and mechanical properties of Ni-rich Ni–Ti alloy were studied. As an extension to earlier studies, the present study concentrates both on the effects of the ageing time and the ageing temperature systematically. By combining the results of the DSC measurements of phase transformation behaviour to the DMA measurements of internal friction and mechanical behaviour of the Ni-rich Ni–Ti alloy, the present study gives yet unpublished information about the comprehensive effects of the ageing treatments on these alloys. These results enable to control the phase transformation temperatures, internal friction and mechanical properties of the Ni-rich Ni–Ti alloy by selecting the suitable ageing treatment. Results showed that the noticed effects of the ageing treatments on mechanical properties could be explained with the changes in the size, distribution, density and coherency of the formed Ti 3Ni 4 precipitates. The high internal friction values can be explained as the contribution of the internal stress fields and increased amount of mobile interfaces as the result of the formed Ti 3Ni 4 precipitates. The amount of mobile interfaces seems to be more dominant factor for the increased internal friction value than the effect of the internal stress fields. Therefore, the optimal internal friction values can be obtained with a proper aging treatment which will yield a high density of small Ti 3Ni 4 precipitates.

Superelasticity in a New BioImplant Material: Ni-rich 55NiTi Alloy

With the drive towards minimally invasive procedures, the medical industry is looking towards ‘avant-garde’ materials, with 50NiTi currently being the prime choice for many critical components/applications. This paper examines a new Ni-rich NiTi alloy that exhibits superelasticity (SE) and shape memory (SM) properties. Superelastic (SE) properties of 55NiTi* [all compositions are quoted in atomic% throughout the paper. The reader should note the following conversions: 50NiTi (at.%)≈55NiTi (wt.%) and 55NiTi (at.%)≈60NiTi (wt.%)] are studied here as a function of heat-treatment between 400–800°C, and compared with the corresponding response of 50NiTi*, with an aim to develop and optimize thermal treatment procedures to maximize recoverable elastic strains. While optimal tuning of the SE properties in 50NiTi necessitates cold working in conjunction with specific heat treatment/aging, 55NiTi does not require cold work to achieve its optimal SE behavior. Moreover, it can be heat treated to produce strong, stable SE and SM response from the same ingot, with transformation temperatures being a strong function of heat treatment. The main difference between the two alloys is that Ni–Ti alloys with Ni content greater than 50.6 at.% are sensitive to heat treatment; aging in these materials leads to precipitation of several metastable phases. The initial work focuses on SE properties relevant to biomedical use, such as: plateau stress, recoverable strains and strength, as a function of heat treatment and microstructure.

Effect of thermal cycling on the shape memory transformation behavior of NiTi alloy: Powder X-ray diffraction study

The results of studies carried out on the effect of thermal cycling on the phase transformation behavior of Ni-rich NiTi shape memory alloy (SMA), using powder X-ray diffraction (XRD) technique, are presented in this article. In Ni-rich NiTi alloy heat-treated at 660 °C, the integrated peak area (integrated intensity) corresponding to (1 1 0) A plane of the austenitic phase increases and the area corresponding to martensitic phase decreases with increasing temperature. Austenitic start and finish temperatures change during the first few thermal cycles. The former decreases by ∼15 °C and the latter decreases by ∼35 °C during the first cycle, whereas martensitic start and finish temperatures do not get affected by thermal cycling. The transformation characteristics determined through XRD are compared with those obtained from differential scanning calorimetry (DSC).

Understanding of multi-stage R-phase transformation in aged Ni-rich Ti–Ni shape memory alloys

An abnormal three-stage martensitic transformation behavior (one-stage R and two-stage B19′) has been found in Ni-rich Ti–Ni alloys when aged at intermediate temperatures (350–500 °C). This strange behavior was successfully explained by considering the kinetics of Ti 3Ni 4 precipitation in polycrystalline samples. However, it was found that R-phase transformation occurs in two stages, when aging is done at low temperatures (250–300 °C). In order to find the origin of this abnormal phenomenon, we made a comparative study with Ni–49.4 at.% Ti single crystals and polycrystals, which were aged at 250 °C for different time. Differential scanning calorimetry showed that all single crystals undergo normal one-stage R-phase transformation, while polycrystals exhibit the abnormal two-stage R-phase transformation after short time aging. These different transformation behaviors can be understood by considering the kinetics of precipitation in supersaturated solid solution. The abnormal two-stage R-phase transformation is attributed to a large-scale heterogeneity of Ni-content between grain boundary region and grain interior, resulted from the preferential precipitation in grain boundary regions. Therefore, it is suggested that there is a unified scenario for seemingly different transformation behaviors of both low-temperature-aged and intermediate-temperature-aged Ti–Ni alloys.

Effect of ageing on the transformation behaviour of Ti–49.5 at.% Ni

It is known that ageing treatment causes complex multiple stage transformation behaviour in Ni-rich Ni–Ti alloys. This has been attributed to the formation of coherent Ti 3Ni 4 precipitates. There have been different views in the literature on the actual roles of the precipitates on the transformation behaviour, including the creation of microscopic variations in local composition and stress field and spatial inhomogeneous distribution of the precipitates. This study investigated the response to ageing treatment of a Ti–49.5 at.% Ni alloy, which does not develop Ti 3Ni 4 precipitates. It was found that multiple-stage transformation behaviour was also induced by ageing treatment, involving three stages in the forward transformation on cooling and two stages in the reverse transformations on heating. This is attributed to the formation of coherent Ti-rich precipitates in the matrix.