Ni-rich NiTi Shape Memory Alloy Research Articles

Ni 4Ti 3 precipitate structures in Ni-rich NiTi shape memory alloys

Non-uniform distributions of Ni 4Ti 3 precipitate crystallographic variants are investigated in a Ni-rich NiTi shape memory alloy after aging, assisted by external stress. A finite-element method model is presented that considers the elastic anisotropy of the B2 parent phase and also mutual misorientations of grains in a polycrystalline sample. On loading by the external stress, the stress is redistributed in the microstructure and the precipitation of some Ni 4Ti 3 crystallographic variants becomes distinctly favorable in grain boundary regions since these variant configurations minimize the elastic interaction energy. The volume fraction of the affected grain boundary regions is calculated and the numerical results are compared with the data obtained by differential scanning calorimetry and transmission electron microscopy.

Conventional and in-situ transmission electron microscopy investigations into multistage martensitic transformations in Ni-rich NiTi shape memory alloys

Investigations of multi-step martensitic transformations in Ni-rich NiTi single crystals and polycrystals are reported. Attention is mainly given to processes that involve a two-step R → B19′ transition. Single crystals and polycrystals with a Ni content close to 51 at.% were subjected to standard heat-treatments during which microstructures with a homogeneous distribution of Ni 4Ti 3 precipitates form. Results obtained using differential scanning calorimetry and conventional and in-situ transmission electron microscopy clearly show that these microstructures transform from R phase into B19′ martensite in two steps. We suggest that the nucleation conditions for the B19′ phase must be taken into account to rationalize the obtained results.

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.

Effect of Y addition on microstructure and martensitic transformation of a Ni-rich Ti–Ni shape memory alloy

In the present work, 1 at% Y was added to Ti-50.7at%Ni alloy to prepare Ti-50.2Ni-1Y alloy and the effects of rare earth Y addition on the microstructure and martensitic transformation behavior of Ti-50.7at%Ni alloy was investigated by optical microscope, scanning electronic microscope (SEM) X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results show the microstructure of Ti-50.2Ni-1Y alloy is different with that of Ti-50.7at.%Ni alloy, and its microstructure consists of B19′martensite and the Y-rich phase, which may be YNi phase. One-step martensitic transformation occurs in Ti-50.2Ni-1Y ternary alloy. The phase transformation temperatures increase rapidly with Y addition, and the increase of Ms is about 85°C.

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).

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.

Effect of Dy addition on the microstructure and martensitic transformation of a Ni-rich TiNi shape memory alloy

The effect of rare earth element Dy addition on the microstructure and martensitic transformation behavior of Ti 49.3Ni 50.7 shape memory alloy was investigated by optical microscope (OM), scanning electronic microscope (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results show that the microstructure of TiNiDy ternary alloy consists of Ti 2Ni phase, DyNi phase and the matrix. One-step martensitic transformation is observed in quenched TiNiDy ternary alloys, which is the same as that in quenched TiNi binary alloys. The martensitic transformation temperatures increase evidently with Dy addition, and the maximum increase of M s is about 110 °C.

Quantitative phase analysis in microstructures which display multiple step martensitic transformations in Ni-rich NiTi shape memory alloys

In the present study, we investigate a microstructure which shows a three-step martensitic transformation on cooling from the high temperature regime and two DSC peaks on heating. Using in situ neutron diffraction phase analysis based on Rietveld full profile fitting it is shown how the volume fractions of B2, R and B19′ phases evolve during the three step transformation on cooling and the reverse transformation on heating. We used TEM as a complementary method to support our neutron diffraction findings. The first DSC peak on cooling is related to a complete transformation of the B2 matrix to R phase and the second and third peak on cooling are associated with the formation of B19′ in two stages. On heating also, the B19′ → R transformation is accumulating in two stages, with the second stage overlapping with the R → B2 transition.

Influence of thermomechanical processing on the superelastic properties of a Ni-rich Nitinol shape memory alloy

Ni-rich NiTi shape memory alloys usually undergo a series of thermomechanical treatment in their production and component fabrication processes. Comprehension of property changes and microstructural evolutions induced by these treatments is essential for quality control and improvement of these materials. The effect of controlled ageing for prolonged times (of the order of several hours) has been extensively investigated in the literature. This study investigates the effect of actual production processes of NiTi tubing, which often involve thermomechanical treatment for multiple periods of short durations and which are typical of industrial production routines of devices like stents, on the transformation and deformation behaviour of a Ti–50.8 at%Ni alloy. Evolution of the properties is characterized by DSC measurements and by tensile testing at 310 K.

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.

In situ transmission electron microscopy-investigations on the strain-induced B19′-phase in NiTi shape memory alloys structured by focused ion beam

In the present study, we investigate the deformation of a pseudo elastic NiTi shape memory alloy, micro structured with the focused ion beam technique, by in situ transmission electron microscopy (TEM). A polycrystalline Ni-rich NiTi shape memory alloy with a nominal composition of 50.7 at.% Ni was solution heat treated at 850 °C for 15 min. Subsequent water quenching was carried out to stabilize the B2-phase at ambient temperatures. Dog bone shaped TEM tensile specimens were prepared by spark-erosion and ground to a thickness of 120 μm followed by electropolishing in order to obtain thin regions which are transparent for the electron beam. Then, a rectangular hole was sputtered into the specimen using the focused ion beam technique. In situ TEM straining was carried out at ambient temperatures and the stress induced formation of martensite was observed near the rectangular hole. On straining, the sample shows a martensitic transformation from B2 (austenite) to B19′ (martensite). Details of these in situ observations are reported.

Micromemory Effects Involved during Martensitic Transformation in Ni-Rich NiTi Alloy

In the present study, interrupted mechanical cycles are performed to observe ‘micromemory effect’ for the Ni-rich (Ni51.0at%-Ti) Ni-Ti Shape Memory Alloy (SMA) with different strain rates. In addition, the tensile test is coupled with the four-probe electrical resistivity (ER) measurements. Coupled measurements of stress-strain and electrical resistivity for mechanical cycling (with SIM) have not yet been reported in the literature. The behaviour of the alloy during the interrupted cycles is discussed on the basis of the ER profile

Effects of heat treatment on characteristics of porous Ni-rich NiTi SMA prepared by SHS technique

Abstract The effects of heat treatment on the microstructure and compressive properties of porous Ni-rich NiTi shape memory alloy (SMA) fabricated by self-propagating high-temperature synthesis (SHS) were investigated. The solution treatment at 1050°C has little effects on stable Ti 2Ni second phase, however, it decreases the amount of Ni 4Ti 3 phase derived from the SHS process and results in the improvement of the ductility of porous NiTi SMA. The subsequent aging treatment after solution treatment could lead to the precipitation of the discrete Ni 4Ti 3 phase in NiTi matrix grains, which increases the brittleness of porous NiTi SMA. Porous NiTi SMA presents a composite fracture behavior consisting of a ductile fracture of NiTi matrix and a cleavage fracture of second phase particles. Many cracks existing on the interfaces indicate that the bonding of the matrix with second phase particles is weak.

Self-deployable origami stent grafts as a biomedical application of Ni-rich TiNi shape memory alloy foil

This paper describes the design, manufacturing and properties of a new type of stent graft, the origami stent graft. Unlike conventional stent grafts which consist of a wire mesh stent and a covering membrane, the new origami stent graft is made from a single foldable foil with hill and valley folds. The Ni-rich titanium/nickel (TiNi) shape memory alloy (SMA) foil made by the newly developed ultrafine laminates method was used in order to produce the stent graft. The pattern of folds on the foil was produced by negative photochemical etching. The deployment of the stent graft is achieved either by SMA effect at the body temperature or by making use of property of superelasticity. A number of prototypes of the stent graft, which are the same size as standard oesophageal and aortal stent grafts, have been produced successfully. It was demonstrated that the stent graft deploy as expected.

Effects of aging temperature on the martensitic transformation in porous Ni-rich NiTi shape memory alloy fabricated by combustion synthesis

Effects of aging temperature on the martensitic transformation in porous Ni-rich NiTi shape memory alloy fabricated by combustion synthesis

On the effect of aging on martensitic transformations in Ni-rich NiTi shape memoryalloys

We investigate the effect of 450 °C aging on the microstructure and on the martensitic transformations in a Ni-rich (50.8 at.%Ni) NiTi shape memory alloy using transmission electron microscopy (TEM), x-raydiffraction (XRD), neutron diffraction (ND) and differential scanning calorimetry(DSC). On cooling from the high temperature phase, DSC charts show two distinctpeaks after short aging times, three peaks after intermediate aging times and twopeaks again after long aging times (2–3–2 transformation behaviour). After 1.5 h450 °C aging, three DSC peaks were obtained on cooling from the high temperature phase. The first peak oncooling represents the formation of R-phase and the second peak is associated with the formation ofB19′. Due to a macroscopic microstructural heterogeneity a secondB19′ peak (overall third peak) occurs at lower temperatures. We discuss our results in the lightof recent remarks on burst-like events during the growth of thermoelastic martensite andon the effect of oxidation on NiTi microstructures.

Elemental Mapping of NiTi with EFTEM

Martensitic transformations in Ni-rich NiTi shape memory alloys take place as multistage transformations. In Ni-rich alloys with an austenitic B2 matrix, coherent Ni{sub 4}Ti{sub 3} precipitates form from thermo-mechanical processing and affect the sequence of the martensitic transformation. Any composition inhomogenieties that develop during the evolution of the Ni{sub 4}Ti{sub 3} precipitates will have a large influence on the multistage martensitic transformations, since the martensite start temperature, M{sub s}, is strongly dependent on the Ni concentration of the matrix. Since concentration differences on the order of 0.5 at% are sufficient to influence the transformation, providing sufficiently accurate concentration profiles for meaningful structure-property correlations is a challenging experiment. This investigation employs elemental mapping by energy-filtered transmission electron microscopy (EFTEM) to attempt to measure the concentration profiles at these precipitate-matrix interfaces.

Precipitation of Ni4Ti3-variants in a polycrystalline Ni-rich NiTi shape memory alloy

Abstract In the present work we provide a fully quantitative TEM analysis, which shows that different Ni 4 Ti 3 precipitate variants form close to and far from grain boundaries during stress-assisted aging. We discuss how this type of heterogeneity can give rise to multiple step martensitic transformations.

Characterization of transformation behavior in porous Ni-rich NiTi shape memory alloy fabricated by combustion synthesis

Characterization of transformation behavior in porous Ni-rich NiTi shape memory alloy fabricated by combustion synthesis

Phase transformation behaviors in porous Ni-rich NiTi shape memory alloy fabricated by combustion synthesis

The phase transformation behaviors that change with the aging temperature in the SHS-synthesized porous Ni-rich NiTi SMA after solution treated at 1050 °C for 2 h were investigated by differential scanning calorimeter (DSC). The experimental results show that the type of phase transformation changes from one-step (after solution treatment) to two-step (after subsequent aging treatment). With increasing aging temperatures, the transformation temperatures ( R s and A s ) decrease, and the positions of the first peaks on the DSC charts during cooling and the last peaks on the DSC charts during heating shift to lower temperatures. The positions of the last peaks on DSC charts on cooling of the porous Ni-rich NiTi SMA aged at different temperatures between 400 and 550 °C for 1 h correspond to the position of the single DSC peak obtained for the solution treated materials. The interpretation for the features in the phase transformation behaviors of the porous Ni-rich NiTi SMA fabricated by combustion synthesis is proposed.