Damping of Selective-Laser-Melted NiTi for Medical Implants

NiTi exhibits distinct damping properties associated with the martensite-austenite transformation. We fabricated net-shape NiTi parts layer-by-layer using a laser beam that locally melted the NiTi powder. The damping properties of such NiTi parts were analyzed by the decay of cantilever vibrations in comparison to conventionally prepared NiTi. The dynamic modulus as a function of the temperature was derived from the resonant frequency. We found that the two cantilevers showed a damping ratio of about 0.03 at temperatures below austenite start, maximal values of up to 0.04 in the transformation regions and low values of about 0.005 above austenite finish. The results indicate that selective-laser-melted NiTi qualifies for the fabrication of shock-absorbing medical implants in the same manner than conventionally produced NiTi.     

Wear Properties of Porous NiTi Orthopedic Shape Memory Alloy

Porous NiTi shape memory alloy (SMA) scaffolds have great potential to be used as orthopedic implants because of their porous structure and superior physical properties. Its metallic nature provides it with better mechanical properties and Young??s modulus close to that of natural bones. Besides allowing tissue ingrowth and transfer of nutrients, porous SMA possesses unique pseudoelastic properties compatible to natural hard tissues like bones and tendons, thus expediting in vivo osseointegration. However, the nickel release from debris and the metal surface may cause osteocytic osteolysis at the interface between the artificial implants and bone tissues. Subsequent mobilization may finally lead to implant failure. In this study, the wear properties of porous NiTi with different porosities processed at different treatment temperatures are determined. The results of the study show that the porosity, phase transformation temperature, and annealing temperature are major factors influencing the wear characteristics of porous NiTi SMA.     

Phase Transformation Behavior of Hot Isostatically Pressed NiTi-X (X?=?Ag, Nb, W) Alloys for Functional Engineering Applications

Owing to their unique properties, NiTi-based shape memory alloys (SMAs) are highly attractive candidates for a lot of functional engineering applications like biomedical implants (stents), actuators, or coupling elements. Adding a third element is an effective measure to adjust or stabilize the phase transformation behavior to a certain extent. In this context, addition of alloying elements, which are low soluble or almost insoluble in the NiTi matrix is a promising approach and??with the exception of adding Nb??has rarely been reported in the literature so far, especially if the manufacturing of the net-shaped parts of these alloys is aspired. In the case of addition of elemental Nb, broadening of hysteresis between austenitic and martensitic phase transformation temperatures after plastic deformation of the Nb phase is a well-known effect, which is the key of function of coupling elements already established on the market. In the present study, we replaced Nb with additions of elemental Ag and W, both of which are almost insoluble in the NiTi matrix. Compared with Nb, Ag is characterized by higher ductility in combination with lower melting point, enabling liquid phase sintering already at moderate temperatures. Vice versa, addition of W might act in opposite manner considering its inherent brittleness combined with high melting temperature. In the present study, hot isostatic pressing was used for manufacturing such alloys starting from prealloyed NiTi powder and with the additions of Nb, Ag, and W as elemental powders. Microstructures, interdiffusion phenomena, phase transformation behaviors, and impurity contents were investigated aiming to better understand the influence of insoluble phases on bulk properties of NiTi SMAs.     

Challenges in using waterjet machining of NiTi shape memory alloys: An analysis of controlled-depth milling

The nickel-titanium shape memory alloys (NiTi SMAs) have a very high potential for a wide variety of applications thanks to their unique mechanical properties: shape memory effect and pseudoelasticity. However, they have been proved to be more challenging to cut than other advanced engineering materials because of their high ductility, crystal-oriented and stress-oriented mechanical properties. In stark contrast to the extensive work on the metallurgical/microstructural properties of the SMA, there is limited research regarding non-conventional machining of this group of special alloys.Waterjet technology is well-known for cutting advanced difficult-to-cut materials owing to its benefits of reduced mechanical and thermal damages to workpiece surfaces. This paper reports for the first time the use of waterjet technology to mill the functional shape memory alloys and thus to open new avenues for the utilisation of these alloys for advanced engineering applications (e.g. aerospace, medical fields). However, when it comes to NiTi SMAs (characterised by low temperature phase martensite and parent phase austenite), the insignificant waterjet temperatures become critical to the material behaviour as their crystal structures are sensitive to the variations in both temperature and mechanical compression. This makes the processing (particularly waterjet controlled-depth milling) a real challenging task.By taking into consideration both of the waterjet temperatures at different material removal conditions (i.e. with and without abrasives in the focussing tube) and the transformation temperatures of NiTi, three different working zones (100% martensite; mix of austenite and martensite; 100% austenite) under waterjet process have been proposed. In addition, a combined phase and stress-strain diagram for shape memory effect in martensitic phase and pseudoelasticity in austenite phase of NiTi has been suggested. In this paper, Ni_49.8Ti_50.2 shape memory alloy was considered in which its transition temperature range is overlapped with the waterjet operating temperature; two approaches of waterjet processes (plain and abrasive waterjet milling) were proposed so as to investigate the mechanical and metallurgical effect provoked by the relationship between operating temperatures and transformation temperatures. It was found that abrasive waterjetting is more viable than plain waterjetting for controlled-depth milling of NiTi shape memory alloys.     

孔隙对NiTi形状记忆合金中B2-R相变影响的相场模拟

建立了适用于含孔隙NiTi合金中B2-R相变的相场模型,并用该相场模型研究了多孔NiTi合金中B2-R转变的微观组织演化过程以及孔隙率和孔尺寸对R相变体生长动力学行为的影响.多孔NiTi合金中R相变体以相互协调的方式形成"带状"的三维结构和"鱼骨"状的二维组织,变体之间形成的孪晶面包括{101}B2和{001}B2 2...     

Wide Range Temperature Memory Effect of an In Situ NiTi Alloys Composite

Incomplete martensitic transformation characteristics of NiTi shape memory alloys after cold deformation were studied. By designing the shape of the original surface curve, macroscopic domains with different dislocation density were introduced into the NiTi alloys and materials possessing the characteristics of composites were obtained. Due to the interactions between the dislocation texture and martensite variants, the reverse transformation temperatures were found to expand over a large temperature window, which gives temperature memory effect a higher potential for application.     

Experimental Investigation on the Mechanical Instability of Superelastic NiTi Shape Memory Alloy

In this paper, primary attention is paid to the mechanical instability of superelastic NiTi shape memory alloy (SMA) during localized forward transformation at different temperatures. By inhibiting the localized phase transformation, we can obtain the up-down-up mechanical response of NiTi SMA, which is closely related to the intrinsic material softening during localized martensitic transformation. Furthermore, the material parameters of the up-down-up stress-strain curve are extracted, in such a way that this database can be utilized for simulation and validation of the theoretical analysis. It is found that during forward transformation, the upper yield stress, lower yield stress, Maxwell stress, and nucleation stress of NiTi SMA exhibit linear dependence on temperature. The relation between nucleation stress and temperature can be explained by the famous Clausius-Clapeyron equation, while the relation between upper/lower yield stress and temperature lacks theoretical study, which needs further investigation.     

兰姆波表征形状记忆合金相变试验

通过求解薄板的兰姆波频散方程,绘制了NiTi形状记忆合金薄板的频散曲线,并根据频散曲线选择了对合金相变敏感的兰姆波模式。利用PZT超声探头在合金薄板中激励并接收S1及S3模式的兰姆波,测量了温度变化时兰姆波的群速度。研究结果表明,随着NiTi合金相变过程中微观组织结构的变化,兰姆波群速度明显改变,可以根据兰姆波群速度变化测量合金薄板的相变温度。     

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.     

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.     

CuAlPd alloys for sensor and actuator applications

Currently available shape memory alloys (NiTi, CuAlNi, CuSnAl) lack the high transformation temperatures and long term thermal stability desired in many commercial applications. This paper reports the results of an investigation in which Pd was substituted for Ni to obtain the shape memory ally CuAlPd. The CuAlPd alloys were found to have an austenite transformation temperature range of 115–370°C depending on composition, heat treatment and working process. Optimal shape memory properties were found for a composition of Cu-13.1 wt% A1-2.4 wt% Pd. This alloy has a transformation temperature of 180°C and a recoverable strain of 4.8%. CuAlPd alloys have excellent workability and exhibit fatigue properties comparable to NiTi shape memory alloys. Single crystals of CuAlPd alloys were produced using a modified Bridgeman technique.     

A Study on Micro-Machining Technology for the Machining of NiTi: Five-Axis Micro-Milling and Micro Deep-Hole Drilling

Micro-sized applications are gaining more and more relevance for NiTi-based shape memory alloys (SMA). Different types of micro-machining offer unique possibilities for the manufacturing of NiTi components. The advantage of machining is the low thermal influence on the workpiece. This is important, because the phase transformation temperatures of NiTi SMAs can be changed and the components may need extensive post manufacturing. The article offers a simulation-based approach to optimize five-axis micro-milling processes with respect to the special material properties of NiTi SMA. Especially, the influence of the various tool inclination angles is considered for introducing an intelligent tool inclination optimization algorithm. Furthermore, aspects of micro deep-hole drilling of SMAs are discussed. Tools with diameters as small as 0.5 mm are used. The possible length-to-diameter ratio reaches up to 50. This process offers new possibilities in the manufacturing of microstents. The study concentrates on the influence of the cutting speed, the feed and the tool design on the tool wear and the quality of the drilled holes.     

真空感应重熔NiTi形状记忆合金室温拉伸力学性能

采用真空感应熔炼炉制备NiTi合金,并对重熔后的合金进行不同制度热处理。研究NiTi形状记忆合金边角料重熔合金不同温度时效后的室温拉伸力学性能。结果表明,NiTi形状记忆合金边角料重熔后,其室温屈服强度和抗拉强度均有不同程度的提高,断后延伸率略有增加,应力诱发马氏体相变对应的形状回复率降低;合金塑性变形能力随时效温度的升高而变差;合金中R相重新取向所提供的延伸率从总体上看比由单质原材料一次熔炼而成的合金要高。     

Age-induced multi-stage transformation in a Ni-rich NiTiHf alloy

Abnormal multi-stage transformations have already been studied in binary Ni-rich NiTi alloys. In this research, this kind of transformation was investigated in a low supersaturated Ni-rich ternary NiTiHf high-temperature shape memory alloy by aging at intermediate temperatures for various durations. Meticulous examinations of the results of differential scanning calorimetry tests demonstrated the heterogeneous precipitation of (Ti,Hf)₃Ni₄ particles and the three-stage transformation (one-stage R and two-stage B19′) in the aged alloy. Aging provided a significant rise in transformation temperatures (TTs) until they reached their equilibrium states, corresponding to the equilibrium Ni content at each aging temperature. Equilibrium TTs were higher when aging was performed at a lower temperature. The remarkable increase in TTs was compared with those in aged Ni-rich NiTi alloys, and discussed based on the variation in valence electron concentration. A model was also proposed for the microstructural evolution during aging. Furthermore, aging provided enhanced hardness and strain recovery for the alloy. In particular, aging at a lower temperature resulted in a considerable improvement in hardness and shape recovery, which was discussed based on the microstructural changes in the aged alloy. Equilibrium Ni content at each aging temperature proved to be a crucial parameter in controlling the alloy properties, even in heterogeneous precipitations.     

Rapid fabrication of function-structure-integrated NiTi alloys: Towards a combination of excellent superelasticity and favorable bioactivity

Porous NiTi has brought new expectations to the field of orthopaedic implants due to its excellent mechanical properties such as high strength and superelasticity together with good biocompatibility. In order to facilitate the surrounding bone tissue ingrowth into the implanted porous alloy, reasonably large sized pores and a high amount of porosity are required. There is, however, a major challenge for clinical applications: the higher the porosity, the worse are the mechanical properties and the superelasticity. In this work, therefore, function-structure-integrated NiTi alloys consisting of a central solid and an outer porous layer were fabricated by spark plasma sintering (SPS). When sintered at 750 °C, the NiTi alloy with 14% porosity in the inner part and 49% porosity as well as 350 μm average pore size in the outer layer exhibits an exceptionally high compressive strength (∼1375 MPa), together with an excellent superelastic recovery strain (>4%) and favorable cellular affinity (ROS1728 osteoblasts). Altogether, this work provides a strategy to design materials with function-structure integration and suggests that properly designed function-structure integrated NiTi alloys may be promising as advanced bone implants.     

Achieving Small Structures in Thin NiTi Sheets for Medical Applications with Water Jet and Micro Machining: A Comparison

NiTi shape memory alloys (SMA) are used for a variety of applications including medical implants and tools as well as actuators, making use of their unique properties. However, due to the hardness and strength, in combination with the high elasticity of the material, the machining of components can be challenging. The most common machining techniques used today are laser cutting and electrical discharge machining (EDM). In this study, we report on the machining of small structures into binary NiTi sheets, applying alternative processing methods being well-established for other metallic materials. Our results indicate that water jet machining and micro milling can be used to machine delicate structures, even in very thin NiTi sheets. Further work is required to optimize the cut quality and the machining speed in order to increase the cost-effectiveness and to make both methods more competitive.     

Effect of post-deformation annealing on the R-phase transformation temperatures in NiTi shape memory alloys

In NiTi shape memory alloys, both the annihilation of dislocations and the formation of Ni₄Ti₃ precipitates may occur during post-deformation annealing. Different responses of the R-phase transformation temperatures to the annealing conditions have been reported. In order to find out the main factor(s) affecting the R-phase transformation temperatures during post-deformation annealing, a Ti-49.8 at% Ni and a Ti-50.8 at% Ni alloy were subjected to various post-deformation annealing and thermal cycling treatments. The results show that the R-phase transformation temperatures are very stable in the Ti-49.8 at% Ni alloy, while a significant variation is observed in the Ti-50.8 at% Ni alloy with respect to the annealing and thermal cycling conditions. These findings suggest that the R-phase transformation temperatures are not susceptible to the change of dislocation density and depends mainly on the Ni concentration of the matrix, which can be modified by the formation of Ni₄Ti₃ precipitates.     

冷却速度对Ti-50.9%Ni形状记忆合金相变行为和组织的影响(英文)

为了探索冷却速度对镍钛形状记忆合金相变行为和组织的影响,通过差热扫描、X射线衍射、扫描电镜、X射线荧光分析等方法研究Ti-50.9%Ni(摩尔分数)合金的热处理。结果表明:当冷却速度非常低时(如炉冷)可诱发三级相变的产生;冷却速度对相变温度也有很大的影响,Ms和Mf都随冷却速度的降低而下降,降低冷却速度有利于提高M→A奥氏体相变温度,相变滞后宽度(Af–Mf)随着冷却速度的降低而增大;热处理不能消除镍钛合金热加工时所形成的织构,但能减弱其强度;冷却速度对晶粒大小影响不大。     

Grain size effect on structural, electrical and mechanical properties of NiTi thin films deposited by magnetron co-sputtering

In the present study NiTi films have been deposited on Si (100) substrates by dc magnetron co-sputtering in the temperature range from room temperature to 923 K. The crystallization, surface morphology and structural features were studied using X-ray diffraction (XRD), atomic force microscope (AFM), field emission scanning electron microscope (FESEM) and high resolution transmission electron microscope (HRTEM). In situ hot stage atomic force microscope was used to investigate the micro-structural changes during phase transformation in these films. Substrate temperature was found to have a great impact on the structural features and phase transformation behavior of NiTi films. The grain size and the crystallization extent increase with the increase in substrate temperature. Nanoindentation tests of these films were conducted at room temperature. Low hardness and depth recovery ratio was observed in case of the film deposited at substrate temperature of 923 K that could be due to the dominance of martensite phase at room temperature which results in more plastic deformation. The electrical properties of the films were studied using four probe resistivity method. Electrical resistance versus temperature plots show that grain size of NiTi films plays an important role in their electrical properties. NiTi based shape memory alloys exhibit a very interesting martensite to austenite phase transformation as crystal structure changes from monoclinic to cubic upon heating close to room temperature. The characteristics of this transformation are of immense technological importance due to a variety of MEMS applications.     

Highly Porous NiTi with Isotropic Pore Morphology Fabricated by Self-Propagated High-Temperature Synthesis

Highly porous NiTi with isotropic pore morphology has been successfully produced by self-propagating high-temperature synthesis of elemental Ni/Ti metallic powders. The effects of adding urea and NaCl as temporary pore fillers were investigated on pore morphology, microstructure, chemical composition, and the phase transformation temperatures of specimens. These parameters were studied by optical microscopy, scanning electron microscopy, x-ray diffraction, and differential scanning calorimetry (DSC). Highly porous specimens were obtained with up to 83% total porosity and pore sizes between 300 and 500 μm in diameter. Results show pore characteristics were improved from anisotropic to isotropic and pore morphology was changed from channel-like to irregular by adding pore filler powders. Furthermore, the highly porous specimens produced when using urea as a space holder, were of more uniform composition in comparison to NaCl. DSC results showed that a two-step martensitic phase transformation takes place during the cooling cycles and the austenite finish temperature (A _f) is close to human body temperature. Compression test results reveal that the compressive strength of highly porous NiTi is about 155 MPa and recoverable strain about 6% in superelasticity regime.