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In the present work, we study the role of programming strain (50% and 100%), end loads (0, 0.5, 1.0, and 1.5 MPa), and chemical environments (acetone, ethanol, and water) on the exploitable stroke of linear shape memory polymer (SMP) actuators made from ESTANE ETE 75DT3 (SMP-E). Dynamic mechanical thermal analysis (DMTA) shows how the uptake of solvents results in a decrease in the glass temperature of the molecular switch component of SMP-E. A novel in situ technique allows studying chemically triggered shape recovery as a function of time. It is found that the velocity of actuation decreases in the order acetone &gt, ethanol &gt, water, while the exploitable strokes show the inverse tendency and increases in the order water &gt, acetone. The results are interpreted on the basis of the underlying chemical (how solvents affect thermophysical properties) and micromechanical processes (the phenomenological spring dashpot model of Lethersich type rationalizes the behavior). The study provides initial data which can be used for micromechanical modeling of chemically triggered actuation of SMPs. The results are discussed in the light of underlying chemical and mechanical elementary processes, and areas in need of further work are highlighted.

Shear testing can contribute to a better understanding of the plastic deformation of Ni-base superalloy single crystals. In the present study, shear testing is discussed with special emphasis placed on its strengths and weaknesses. Key mechanical and microstructural results which were obtained for the high-temperature (T ≈ 1000 °C) and low-stress (τ ≈ 200 MPa) creep regime are briefly reviewed. New 3D stereo STEM images of dislocation substructures which form during shear creep deformation in this regime are presented. It is then shown which new aspects need to be considered when performing double shear creep testing at lower temperatures (T τ > 600 MPa). In this creep regime, the macroscopic crystallographic [11−2](111) shear system deforms significantly faster than the [01−1](111) system. This represents direct mechanical evidence for a new planar fault nucleation scenario, which was recently suggested (Wu et al. in Acta Mater 144:642–655, 2018). The double shear creep specimen geometry inspired a micro-mechanical in-situ shear test specimen. Moreover, the in-situ SEM shear specimen can be FIB micro-machined from prior dendritic and interdendritic regions. Dendritic regions, which have a lower γ′ volume fraction, show a lower critical resolved shear stress.

The present work introduces a novel circular notched miniature tensile creep (CNMTC) specimen which is used to study the influence of notches on creep and of multiaxial stress states on microstructural evolution in Ni-based single crystal (SX) superalloys. It is briefly discussed how mild circular notches affect the stress state in the notch root of a tensile bar during elastic loading. Then the stress redistribution under creep conditions is calculated using the finite element method (FEM), assuming isotropic material behavior. Two series of interrupted creep experiments with the Ni-based single crystal superalloy ERBO1 (CMSX-4 type) were then performed at 950 °C, with flat uniaxial miniature tensile creep (FUMTC) and CNMTC specimens, respectively. The evolution of cavities and microcracks in both types of specimens was carefully analyzed after 81, 169, and 306 h. In the uniaxial experiments, a growth of cast pores and the formation of new creep cavities were observed. These degradation processes were much less pronounced in the circular notched specimens. The results of the present work are discussed in the light of previous findings on the influence of multiaxial stress states on creep in single crystal superalloys.

The present work studies the diffusion of small molecules (acetone, ethanol, and water) in a shape memory polymer (SMP) of type Estane ETE 75DT3 (SMP-E), which represents a thermoplastic polyurethane. The work aims at providing background information on the chemical reaction between SMPs and small molecules which can limit the service life of SMP actuators operating in harsh chemical environments. Weight gain studies after immersion of plate specimens in liquid acetone, ethanol, and water yield data which can be assessed on the basis of analytical and numerical solutions of Fick’s second law. The diffusion coefficients which are obtained for 21, 30, and 40 °C in the present study scale as D acetone > D ethanol > D water. The diffusion coefficients show Arrhenius types of temperature dependencies with apparent activation energies of 33 (acetone), 59 (ethanol), and 58 (water) kJ mol−1. The diffusion coefficients and the apparent activation energies obtained in the present work are in reasonable agreement with data which were reported for the reaction of the three small molecules with similar polymers in the literature. It is not straightforward to correlate differences in molecular mobility with individual physical properties. The Hansen solubility parameter (originally derived to explain solubility not mobility) qualitatively rationalizes the observed differences.

High-resolution characterization techniques are combined with thermodynamic calculations (CALPHAD) to rationalize microstructural features of single crystal Ni-base superalloys. Considering the chemical compositions of dendritic and interdendritic regions one can explain differences in γ′-volume fractions. Using thermodynamic calculations one can explain, why γ-nanoparticles are observed in the central regions of large cuboidal γ′-particles and why tertiary γ′-nanoparticles form in the γ-channels. The chemical compositions of the γ-channels and of the newly formed γ-particles differ because of the Gibbs–Thomson pressure which acts on the small particles. With increasing size of secondary γ′-particles, their shape changes from spherical to cuboidal. Some general thermodynamic aspects including the temperature dependencies of the Gibbs free energy G, the enthalpy H, and the entropy S and site occupancies in the ordered L12 (γ′) phase are considered. The importance of cooling rate after homogenization is discussed.

In the present work, we use a miniature test procedure to investigate the tensile creep behaviour of the single crystal superalloy ERBO1. We test precisely oriented [0 0 1], [1 1 0] and [1 1 1] creep specimens and determine the stress and the temperature dependence of characteristic creep rates in limited stress and temperature regimes, where the stress and temperature dependence of characteristic creep rates can be well described by power law and Arrhenius type of relations, with stress exponents n and apparent activation energies Qapp. n-values increase with stress and decrease with temperature. Qapp-values, on the other hand, increase with increasing temperature and decrease with increasing stress. Creep curve shapes gradually evolve from the high temperature low stress to the low temperature high stress (LTHS) regime. This implies that there is a gradual change in elementary deformation and softening mechanisms, which is qualitatively confirmed using transmission electron microscopy. While at high tem...

In the present study we investigate the creep behavior of a Ni-base single crystal superalloy. We evaluate the stress and temperature dependence of the minimum creep rate, which shows a power law type of stress dependence (characterized by a stress exponent n ) and an exponential type of temperature dependence (characterized by an apparent activation energy Q app ). Under conditions of high temperature (1323 K) and low stress (160 MPa) creep, n and Q app are determined as 5.3 and 529 kJ/mol, respectively. For lower temperatures (1123 K) and higher stresses (600 MPa) the stress exponent n is higher (8.5) while the apparent activation energy of creep is lower (382 kJ/mol). We show that there is a general trend: stress exponents n increase with increasing stress and decreasing temperature, while higher apparent activation energies are observed for lower stresses and higher temperatures. We use density functional theory (DFT) to calculate the activation energy of diffusion for Re in a binary Ni–Re alloy with low Re-concentrations. The resulting energy is almost a factor 2 smaller than the apparent activation energy of creep. We explain why it is not straightforward to rationalize the temperature dependence of creep merely on the basis of the diffusion of one alloying element. We show that the evolution of the microstructure also must be considered.

Purpose: By adding an osteotomy of the inferior border of the mandibular body to the classic sagittal split osteotomy, the authors expected to prevent unfavorable splits and damage to the inferior alveolar nerve. Materials and Methods: Thirty-five human mandibles were used to perform 70 sagittal split osteotomies as an in vitro study. Conducted as a split-mouth model, each mandible was split at the midline. One side of the mandible was split using the traditional Obwegeser-Dal Pont technique, and the other side was split in the same manner with an additional osteotomy of the inferior mandible border. The torque used to split the mandible was measured, and the fracture line of the mandible was recorded. Results: The average torque associated with the original technique was 1.38 Nm (standard deviation, 0.60 Nm), with a fracture line along the mandibular canal. The average torque required to split the hemimandible with the modified technique was 1.02 Nm (standard deviation, 0.50 Nm), a significant (P

In the present work we investigate three mechanical and microstructural aspects of high temperature and low stress creep of the single crystal superalloy LEK 94. First, we compare the tensile creep behavior of specimens loaded in precise [001] and [110] directions and show that tensile creep specimens with precise [110] directions show significantly lower minimum creep rates. However, small deviations from precise [110] orientations result in a significant increase of creep rate. Second, we use a novel SEM technique to measure dislocation densities. We show that after short periods of creep, dislocation densities in dendritic regions are always higher than in interdendritic regions. This finding is probably associated with wider γ-channels, higher concentrations of W and Re and higher misfit stresses in the γ-channels of dendrites. Finally, we show that internal stresses associated with solidification can drive complex rafting processes during high temperature exposure, which differ between dendrite cores and interdendritic regions.

In the present work, we show how conventional and advanced mechanical, chemical, and microstructural methods can be used to characterize cast single crystal Ni-base superalloy (SX) plates across multiple length scales. Two types of microstructural heterogeneities are important, associated with the cast microstructure (dendrites (D) and interdendritic (ID) regions – large scale heterogeneity) and with the well-known g/g 0 microstructure (small scale heterogeneity). Using electron probe microanalysis(EPMA),wecanshowthatelementssuchasRe,Co,andCrpartitiontothedendriteswhileID regions contain more Al, Ta, and Ti. Analytical transmission electron microscopy and atom probe tomography (APT) show that Al, Ta, and Ti partition to the g 0 cubes while g channels show higher concentrations of Co, Cr, Re, and W. We can combine large scale (EPMA) and small-scale analytical methods (APT) to obtain reasonable estimates for g 0 volume fractions in the dendrites and in the ID regions. The chemical and mechanical properties of the SX plates studied in the present work are homogeneous, when they are determined from volumes with dimensions, which are significantly larger than the dendrite spacing. For the SX plates (140 mm � 100 mm � 20 mm) studied in the present work this holds for the average chemical composition as well as for elastic behavior and local creep properties. We highlight the potential of HRTEM and APT to contribute to a better understanding of the role of dislocations during coarsening of the g 0 phase and the effect of cooling rates after high temperature exposure on the microstructure.

In the present study, the functional fatigue in the commercial SMP Veriflex®, which is associated with repeating up to maximum 40 programming/one-way effect (1WE) cycles, has been examined. The material is characterized by a glass transition temperature ( Tg) of 67°C, above which it looses all its strength. An interesting comparative investigation on thermomechanical cycles, including programming, cooling, unloading, and heating to trigger the 1WE, were carried out for Veriflex at 62°C ( T < Tg, below 5°C but near to Tg) and also at 72°C ( T > Tg, above 5°C but near to Tg) for strains of 140% and for a recovery time of 10 min. Accumulation of strain was estimated during the thermomechanical treatments for using strains of 140% at 62°C ( T < Tg) as well as at 72°C ( T > Tg). Recovery ratios for strains of 140% at 62°C ( T < Tg) as well as at 72°C ( T > Tg) were also estimated. It turns out that programming, cooling, unloading, and heating to trigger the 1WE causes an increase in irreversible strain and is associated with a corresponding decrease in the intensity of the 1WE in particular during the first thermomechanical cycles. Confocal laser scanning microscopic) study shows a very little wavy surface structure evolved during cycling up to strains of 140% at 72°C ( T > Tg). Infrared study features the chemical nature after cycling, processing, and programming of Veriflex.

This paper presents the experimental study of a Thermoplastic Polyurethane (TPU) based Shape Memory Polymer (SMP) produced from granulates of commercially available EstaneTM ETE75DT3 NAT022 (Oevel Westerlo, Belgium). This polymer is characterized by Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Thermal Analysis (DMTA). The experimental procedure was designed to study the functional properties (shape fixity and shape recovery) of this shape memory polymer in multiple programming/shape-recovery cycles at three different programming temperatures. The results are displayed in the temperature-stress-strain space with focus on parameters which characterize the functional fatigue and material stability of the tested polymer during consecutive cycles. These results give a better understanding of this material class that has a potential for actuator applications in engineering.

Dynamic mechanical thermal analysis (DMTA), oftensimply referred to as dynamic mechanical analysis (DMA),is one of the most versatile techniques available for thecharacterization of time-, frequency-, and temperature-dependent mechanical properties of polymers. No othercharacterization method provides more information in singletest.

An interesting comparative case study on thermomechanical cycles including programming, cooling, unloading and heating to trigger the 1WE was done using Veriflex® at 62°C (T Tg, close to and above 5°C of Tg) for slightly low strains (ϵm = 70%) and the recovery time of 10 min. Accumulation of strain was estimated during the thermomechanical treatments for using both 70% strains at 62°C (T Tg). Recovery ratios for 70% strains at 62°C (T Tg) were also estimated. It turns out that programming, cooling, unloading and heating to trigger the 1WE causes an increase of irreversible strain and is associated with a corresponding decrease of the intensity of the 1WE, in particular, during the first thermomechanical cycles. A LSCM (Laser Scanning Confocal Microscopic) study shows very little change in surface structure which evolved during cycling up to 70% strains at 72°C (T > Tg).

Thermomechanical cycles including programming, cooling, unloading, and heating to trigger the 1WE were examined for a shape memory polymer, Tecoflex® (TFX EG − 72D). Cycles were performed at 60°C with 300% strain and a recovery time of 10 min. Strains evolving with time were estimated during the thermomechanical treatments for the total 50 cycles using 300% strain. Recovery ratios for the 300% strain were also estimated. It turns out that programming, cooling, unloading and heating to trigger the 1WE causes an increase of irreversible strain and is associated with a corresponding decrease of the intensity of the 1WE in particular during the first thermomechanical cycles.

Thermomechanical cycles including programming, cooling, unloading and heating to trigger the 1WE were examined for a shape memory polymer (SMP), Tecoflex® (TFX EG-72D). Cycles were performed at 60 °C with 65% strains and the recovery time of 10 min. Strains evolving with time were estimated during the thermomechanical treatments for the total 50 cycles using 65% strains. Recovery ratios for 65% strains were also estimated. It turns out that programming, cooling, unloading and heating to trigger the 1WE causes an increase of irreversible strain and is associated with a corresponding decrease of the intensity of the 1WE in particular during the first thermomechanical cycles. Gold coated scanning electron microscopic study using secondary electron imaging (120 times magnified) shows a very little wavy surface structure evolve during cycling up to 300% strains. IR study accurately features the chemical nature after cycling, processing and programming of TFX EG-72D.

The present investigations were undertaken to find out whether and how often cycling, processing and programming can be repeated, whether repeated programming affects the one way effect and how much irreversible strain the shape memory polymeric material accumulates at a particular temperature. The effect was investigated in dependence of different stress levels, and the effect of both recovery temperature and recovery time was considered. As a model material the commercially and industrially applicable amorphous shape memory polymer Tecoflex® was examined and subjected to 50 programming cycles. Tecoflex® is characterized by a glass transition temperature, Tg, of 74 °C, above which it looses all its strength. During tensile testing at 20 °C (T < Tg), stresses a steady increase to 26 MPa as strains approached the rupture strain of 25%. It is observed that at 60 °C (T < Tg, but near Tg) the material can be strained to more than 2500% before rupture occurs while stresses slowly increase to values less than 0.3 MPa. It turns out that programming, cooling, unloading and heating to trigger the one way effect causes an increase of irreversible strain that is associated with a corresponding decrease of the intensity of the one way effect during the first thermomechanical cycles.

Thermomechanical cycles including programming, cooling, unloading and heating to trigger the 1WE were examined for a shape memory polymer (SMP), Tecoflex® (TFX EG-72D). Cycles were performed at 60°C with 50% and 225% strains and the recovery time of 10 min. Strains evolving with time were estimated during the thermomechanical treatments for the total 44 cycles using 50% strains and the total 50 cycles using 225% strains. Recovery ratios for 50% strains and 225% were also estimated. It turns out that programming, cooling, unloading and heating to trigger the 1WE causes an increase of irreversible strain and is associated with a corresponding decrease of the intensity of the 1WE in particular during the first thermomechanical cycles. In parallel scanning electron microscopic study using secondary electron imaging shows a very slight wavy surface structure evolved during cycling.

NiTiCr is used in medicine for retriever devices, dental- and guide wires [1–3]. Very little has been published in literature about this ternary NiTi-alloy. In the present work, the mechanical properties of NiTiCr shape memory alloy (SMA) wires (0.25 wt.% Cr) in the as-received condition and after a number of different heat treatments were studied. The surface quality of the wires was examined, because it is known that the surface condition has an influence on fatigue life. Mechanical uniaxial loading-unloading tensile and cyclic tests were performed. Bending-rotation fatigue (BRF) tests at higher rpm-levels were conducted in air and in oil. This is important with regard to possible applications, where NiTiCr might be used as a structural component. NiTiCr wird in der Medizin fur Bergeinstrumente, Dental- und Fuhrungsdrahte eingesetzt [1–3]. Uber diese ternare NiTi-Legierung wurde bisher in der Literatur nur sehr wenig berichtet. In der vorliegenden Arbeit wurden die mechanischen Eigenschaften von Drahten aus einer NiTiCr-Formgedachtnislegierung (FGL) (0.25 Gew.% Cr) im Ausgangszustand und nach verschiedenen Warmebehandlungen untersucht. Die Oberflachenqualitat der Drahte wurde getestet da bekannt ist, dass der Oberflachenzustand einen Einfluss auf die Ermudungslebensdauer hat. Es wurden einachsige mechanische Zugversuche und zyklische Versuche durchgefuhrt. Umlaufbiegeermudungs-Versuche (ULB) erfolgten bei hoheren Drehzahlen an Luft und in Ol. Dies ist wichtig mit Hinblick auf mogliche Anwendungen von NiTiCr als Strukturwerkstoff.

In the present study the functional fatigue in the commercial SMP Veriflex® which is associated with repeating up to 20 programming/1-WE cycles has been examined. The material is characterized by a glass temperature Tg of 67 °C, above which it looses all its strength. During tensile testing at 21 °C (T Tg) the material can be strained up to 225 % before rupture occurs while stresses slowly increase to values as low as 0.4 MPa. Thermomechanical cycles including programming, cooling and unloading and heating to trigger the one way effect result in an increase of irreversible strain associated with a corresponding decrease of recovery ratio at the beginning of thermomechanical cycling. In parallel a wavy surface structure evolves during cycling.

The production of high quality powder metallurgical NiTi alloys with elevated phase transformation temperatures is challenging. During processing, an unavoidable pickup of impurity elements (especially oxygen and carbon) results in a decrease of phase transition temperatures and in the formation of brittle secondary phases. We introduce a processing route including melting, gas atomization and hot isostatic pressing for binary NiTi shape memory alloys which minimizes these problems. We demonstrate that the microstructure of the Ti-rich NiTi alloy contains precipitates of Ti2Ni type which can be exploited to dissolve oxygen picked up during later process stages. In this study, three powder fractions with different grain sizes and impurity contents were subjected to hot isostatic pressing. The evolution of microstructures and material properties was studied by chemical analysis, microscopy, differential scanning calorimetry, and mechanical testing. Exploiting the solubility of oxygen in Ti2Ni, the processing route presented in the present paper succeeds in producing powder metallurgical NiTi shape memory alloys with good structural and functional properties.

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. Untersuchungen zur Herstellung und zu den Eigenschaften von NiTi- und NiTiCu-Aktorfedern NiTi-Formgedachtnislegierungen (FGL) zeichnen sich durch eine hohe Attraktivitat fur verschiedene Aktorik-Anwendungen aus. Dabei werden FGL haufig in Form von zylindrischen Federn verwendet, wobei der Einwegeffekt genutzt wird. Die Herstellung von solchen Aktor-Federn ist jedoch keinesfalls trivial. Sowohl die Herstellung von geeignetem Drahtmaterial als auch die Formgebungsbehandlung stellen in gewisser Weise eine Herausforderung dar. Die funktionellen Eigenschaften von NiTi-FGL hangen sehr stark von mikrostrukturellen Randbedingungen ab, und erforderliche thermomechanische Behandlungen sind mit weit reichenden mikrostrukturellen Veranderungen verbunden. Das Ziel dieser Arbeit war, eine Prozesskette zur Herstellung von Ni50Ti50 und Ni40Ti50Cu10-Aktorfedern ausgehend vom Gussmaterial zu erarbeiten und metallkundlich zu charakterisieren. Aus den Gussblocken wurden durch Rundkneten, Walzen, Drahtziehen und speziellen Formgebungsbehandlungen Aktorfedern hergestellt. Dabei wurden die mikrostrukturellen und funktionellen Anderungen der beiden FGL charakterisiert. Zum Schluss wurden die mechanischen und funktionellen Eigenschaften der verschiedenen Draht-Materialien und der fertigen Federn untersucht. Dabei wurde ein besonderes Augenmerk auf die funktionelle Ermudung der Aktorfedern gelegt.

In recent years, pseudoelastic applications of NiTi shape memory alloys have received considerable attention in the medical field due to the development of medical devices and implants. For such applications it can be beneficial to consider hybrid systems like polymer-coated shape memory metals. The objective of the present work is to show that surface treatments can strongly improve adhesion between a pseudoelastic NiTi shape memory ribbon and polyamide (PA6). In our study a pseudoelastic shape memory ribbon was subjected to different kinds of surface treatments, including mechanical, chemical and physical processing steps. We use injection moulding to produce a composite, consisting of a central NiTi ribbon half of which is fully contained in polyamide. Then, pull-out experiments are performed to characterize the adhesion of the NiTi ribbon in the polymer matrix. We investigate if depending on the surface condition, pull-out stresses can vary.

The present work shows how the microstructure and the phase transformation behavior of as-cast Ti-48 at.% Ni-2 at.% Fe shape memory alloys respond to different thermomechanical treatments. The applied procedures involve homogenization, swaging at different temperatures and recrystallization annealing. The as-cast material is characterized by a coarse-grained microstructure. During repeated hot working in combination with recrystallization treatments, a fine-grained microstructure evolves in the surface volume of the swaged Ni–Ti–Fe rod. In contrast, its center remains almost unaffected by recrystallization (because less dislocations are introduced during swaging and recovery processes are more intense). This type of microstructural heterogeneity can be avoided by cold work at the end of the thermomechanical procedure. Swaging at low temperatures results in higher dislocation densities and thus in a more homogeneous microstructural evolution during recrystallization. In the cold-worked state, the phase transition R → B19′ is strongly impeded by the presence of dislocations, whereas the transition characteristics of the recrystallized and of the non-deformed original material state almost coincide.

In the present study we investigate microstructures of NiTi-based shape memory alloys (SMAs) using scanning electron microscopy (SEM) in combination with mechanical polishing and electropolishing. The results show that electropolishing is quite useful for microstructural characterization of NiTi-based SMAs revealing morphologies of second phases, characteristic features of eutectic regions, growth mechanisms of primary carbides and coarsening of second phases. Furthermore, three-dimensional morphologies of martensitic variants, twins, and substructures can be identified in electropolished high temperature SMAs like NiTiHf and NiTiZr.

In the present study, Ni50.7 at.%–Ti shape memory alloys were prepared by vacuum induction melting using graphite crucibles. During melting the carbon concentration of the alloy increased and TiC carbides formed during solidification. The microstructure of the as-cast NiTi alloys was examined using scanning and transmission electron microscopy (SEM, TEM). Only eutectic TiC fibers were found in a low carbon NiTi alloy. For higher carbon concentrations, primary TiC carbides were observed in addition to the fine eutectic TiC phase. TEM results suggest that there is no specific orientation relationship between primary TiC carbides and B2. For the eutectic TiC phase, however, the following orientation relationship could be identified for low and high carbon contents in the NiTi alloy: B2 [0 0 1]||TiC [0 0 1], B2 (2 0 0)||TiC (2 2 0), B2 ( 1 1 ¯ 0 ) ||TiC (2 0 0), and B2 (1 1 0)||TiC (0 2 0). It was also found that the interface between TiC and B2 is semi-coherent. During a subsequent post-cast heat treatment new carbides formed with different morphology (sizes and shapes) but with the same orientation relationship. Experimental evidence including SEM and TEM micrographs and diffraction patterns is presented to document these findings.

In the present study we investigate vacuum induction melting (VIM) of prominent ternary NiTiX (X ¼ Cu, Fe, Hf, Zr) shape memory alloys using graphite crucibles. We apply a melting procedure which was recently developed for binary NiTi alloys and which keeps the carbon pick-up during melting at a minimum. We investigate the microstructures of the as-cast and homogenized alloys using scanning electron microscopy (SEM) in combination with energy dispersive X-ray analysis (EDX). Differential scanning calorimetry (DSC) was performed to study the phase transformation temperatures of the as-cast and homogenized materials. The results show that VIM processing in graphite crucibles provides ternary NiTiX shape memory alloys with good chemical homogeneity and acceptable impurity contents.

The functional and structural properties of NiTi shape memory alloys (SMAs) which are often produced by vacuum induction melting (VIM) using electro graphite crucibles are strongly dependent on the alloy composition. It is known that NiTi melts react with electro graphite crucibles and the associated carbon pick up will affect the alloy properties. So far no effort has been made to study the thermodynamics and kinetics of this reaction. In the present work the reaction between NiTi melts and electro graphite crucibles during VIM processing was investigated. The reaction results in the growth of a TiC layer and a simultaneous increase of carbon concentration in the melt. A physical view of the underlying transport processes and reactions is given and a phenomenological rate equation for the increase of carbon concentration during melting is provided.

Binary NiTi based shape memory alloys can be produced starting from the pure elements (Ni-pellets, Ti-rods) by using vacuum induction melting (VIM). VIM ingot metallurgy is known to produce materials with a good chemical homogeneity; it, moreover, is cheaper than vacuum arc melting (VAM) when small quantities of laboratory materials are needed. In a VIM procedure, graphite crucibles are attractive because they have appropriate electrical properties. For NiTi melting, graphite crucibles are interesting because they are reasonably priced and they show a good resistance against thermal cracking. On the other hand, it is well known that melting of Ti alloys in graphite crucibles is associated with a vigorous interface reaction. And the carbon concentration of NiTi alloys needs to be kept below a certain minimum in order to assure that the functional properties of the alloys meet the required targets. Therefore, it is important to minimize the carbon pick up of the melt. The present work presents experimental results and discusses thermodynamic and kinetic aspects of the reaction of NiTi melts with graphite crucibles; a method is suggested to keep the carbon dissolution into the melt at a minimum.

Off-stoichiometric NiTi sputtering targets were manufactured by cast melting in order to sputter Ni-rich NiTi thin films. Crystalline samples, which reveal a martensitic transformation, were fabricated by magnetron sputtering and subsequent thermal treatment. A systematic investigation to determine the deviation between target and substrate composition was carried out by means of microprobe analysis. Typical loss rates in sputtering targets with Ti content higher than 49.5 at.% were in the order of 4.3 at.%, whereas more Ni-rich targets revealed a Ti loss rate of only 0.5–1.5 at.%. Sacrificial layer technique was applied to obtain free-standing NiTi films. Transformation temperatures and hysteresis properties of the martensitic transformation were characterised by differential scanning calorimetry (DSC) but showed only minor differences concerning composition. Free-standing films obtained from the Ti–53.1 Ni–46.9 at.% sputtering target revealed superelastic properties at 40 °C as demonstrated by tensile testing.

Compression-aging treatments tests were carried out with single crystals in order to nucleate precipitates under a compression stress in the [1 1 1] direction, conditions which favor the formation of a single family of Ni4Ti3 precipitates. The precipitation behavior under this type of stress-assisted aging is investigated. An attempt is made to study the effects of aging on the characteristics of the martensitic transformations. The objective of the present study is to lay the groundwork for investigating the characteristics of martensitic transformations in Ni-rich NiTi alloys by excluding effects associated with heterogeneous nucleation on grain boundaries in polycrystals.

The present work studies the performance of three different kinds of commercial crucible graphite materials used during inductive melting of NiTi-shape memory alloys (SMAs). One low-density extruded graphite and two isostatically pressed graphite-materials with different porosity and purity were used for melting NiTi-SMAs from pure Ni and Ti raw materials. The use of a low grade, lower density graphite crucible led to a high carbon contamination of the alloy. Much better results were obtained using graphite crucibles with higher density and higher purity. But in all cases, the first use of a graphite crucible leads to a higher carbon pickup than the second time of utilization. This can be explained by the formation of a protective TiC rich layer on the surface of the crucible during its first contact with the NiTi melt which acts as a diffusion-barrier later on. Furthermore, attention has to be paid to the thermal conductivity of the graphite. Using a material with a low thermal conductivity may lead to the formation of cracks at the pouring-lip of the crucible during casting. This may lead to spalling off of crucible material and can enhance the risk of non-metallic inclusions in the alloy. Schmelzmetallurgische Herstellung von NiTi Formgedachtnislegierungen – Einfluss des Graphit-Tiegelwerkstoffes auf die Qualitat der Legierung Die vorliegende Arbeit befasst sich mit der Anwendbarkeit von drei unterschiedlichen Graphit-Tiegelwerkstoffen bei der schmelzmetallurgischen Herstellung von NiTi-Formgedachtniswerkstoffen durch Induktions-Schmelzen. Verwendet wurde ein Tiegel aus extrudiertem Graphit mit geringer Dichte und zwei aus isostatisch gepressten Graphit-Werkstoffen mit verschiedener Porositat und Reinheit. Die Verwendung des preiswerten Graphits mit geringer Dichte fuhrte zu einer starken Kohlenstoff-Kontamination der Legierung. Bessere Ergebnisse konnten mit den beiden anderen Tiegelmaterialien erzielt werden. Generell war jedoch auffallig, dass bei der erstmaligen Benutzung eines Graphit-Tiegels unabhangig vom Tiegelwerkstoff eine deutlich hohere Kohlenstoff-Kontamination der Schmelze erfolgte, als bei dem nachfolgenden Schmelzversuch. Dies kann dadurch erklart werden, dass bei der ersten Verwendung eines solchen Tiegels eine TiC-reiche Schicht im Kontaktbereich Tiegel/Schmelze entsteht, welche als Diffusions-Barriere wirkt. Des Weiteren muss der verwendete Graphit eine hohe Warmeleitfahigkeit haben. Sonst konnen sich an der Gies-Kante Risse bilden, welche zur Ablosung von Tiegelmaterial fuhren und somit zu einer Erhohung des Risikos nichtmetallischer Einschlusse im Gussblock beitragen.

In this paper, we present a computationally efficient implementation of a continuum mechanical model for shape memory alloys into a finite element code. The model covers several thermomechanically coupled effects typical for the material behaviour of shape memory alloys, e.g. pseudo-elasticity, the one-way shape memory effect and the two-way shape memory effect due to external loads. Via the use of a finite element formulation based on only one Gauss point, the computational effort is reduced enormously. Eine neue Finite-Elemente-Technologie fur Formgedachtnisverbunde In diesem Paper wird eine numerisch effiziente Implementierung eines kontinuumsmechani-schen Modells fur Formgedachtnislegierungen in ein Finite-Elemente Programm vorgestellt. Das Modell deckt mehrere, fur das Material-verhalten von Formgedachtnislegierungen typische, thermomechanische Effekte ab, z. B. die Pseudoelastizitat, den Einweg-Effekt und den extrinsischen Zweiweg-Effekt. Durch die Verwendung einer Finite-Elemente Formu-lierung basierend auf nur einem Gauspunkt lasst sich der numerische Aufwand zudem erheblich reduzieren.

The usage of shape memory alloys (SMAs) in coupling devices depend considerably on the phase transformation temperatures (PTTs) of the material. It is important that SMAs in such devices feature austenite start temperatures (A s -temperatures) above room temperature for storage purposes as well as low martensite start temperatures (M s -temperatures) in order to guarantee a high level of mechanical resistance service. It is well known that the addition of Niobium to the NiTi-System results in an increase in the width of the PTT-hysteresis, in particularly for the first phase transformation cycle. The present work studies the influence of different amounts of Nb both as a comparison between binary and ternary alloys and as a comparison of diferent levels of Nb [9 and 21 at.%) on. the first cycle shape memory effect. The constrained shape memory effect is simulated using a tensile test machine and the dependence of the recovery stress on the predeformation is determined. The dependence of this functional property on cold working and subsequent annealing is considered; special emphasis is placed on the recovery stress and how it is affected by the thermomechanical treatments. In the present work, solutionannealed material and thermo-mechanically treated material is characterized on the basis of calorimetric (differential scanning calorimetry - DSC) and mechanical experiments.

In the present paper we investigate the shear creep behavior of the single crystal super alloy CMSX-4 at temperatures between 950 and 1100 °C and shear stresses ranging from 80 to 155 MPa. A double shear creep test technique is used to study the shear creep behavior of four specific macroscopic crystallographic shear systems defined by a specific crystallographic shear plane and a specific crystallographic shear direction (systems investigated: {001} , {100} , {011} , and {111} ). The shear creep behavior is analyzed in terms of the shape of individual creep curves and in terms of the stress and the temperature dependence of the secondary shear creep rate. Individual creep curves are generally characterized by a pronounced primary creep range where creep rates decrease by up to three orders of magnitude. A sharp creep rate minimum is not observed. The secondary creep range starts at shear stresses of the order of 0.02 and is followed by a secondary creep range which extends over shear strain ranges of the order of 0.1. No pronounced increase of shear creep rate in the later stages of creep is observed. Norton plots yield power law stress exponents ranging from 5.5 to 9.7. The temperature dependence of the secondary creep rate is of an Arrhenius type and apparent activation energies between 549 and 690 kJ/mol were found. There is a clear influence of crystallography on shear creep rates, which vary between different macroscopic crystallographic shear systems; this effect decreases with increasing temperature. The shear creep results obtained in the present study are discussed in the light of results from uniaxial testing and in the light of underlying microscopic deformation processes. Scherkriechverhalten der einkristallinen Superlegierung CMSX-4 bei hohen Temperaturen und niedrigen Spannungen In der vorliegenden Arbeit geht es um das Scherkriechverhalten der einkristallinen Superlegierung CMSX-4 im Temperaturbereich zwischen 950 und 1100 °C bei Scherspannungen von 80 bis 155 MPa. Mit Hilfe einer Doppelscherkriechprobe werden vier makroskopische kristallographische Schersysteme untersucht, die durch jeweils eine makroskopisch kristallographische Scherebene und eine makroskopisch kristallographische Scherrichtung gekennzeichnet sind: {001} , {100} , {011} , und {111} . Das Scherkriechverhalten wird auf der Grundlage der Form individueller Kriechkurven und der Spannungs- und Temperaturabhangigkeit der sekundaren Kriechrate diskutiert. Alle Kriechkurven weisen einen ausgepragten Primarkriechbereich auf, wo die Kriechrate um bis zu drei Grosenordnungen abnimmt. Es wird kein scharfes Kriechratenminimum beobachtet. Das sekundare Kriechen beginnt nach Scherungen von etwa 2 %; der folgende sekundare Kriechbereich dehnt sich uber einen Scherbereich von 10 % aus. Eine deutliche Zunahme der Schergeschwindigkeit bei grosen Dehnungen konnte nicht festgestellt werden. Beschreibt man die Kriechdaten durch ein Norton-Gesetzt, werden Spannungsexponenten zwischen 5.5 und 9.7 gefunden. Die Temperaturabhangigkeit der sekundaren Kriechrate kann durch ein Arrhenius-Gesetz beschrieben werden, mit scheinbaren Aktivierungsenergien zwischen 549 und 690 kJ/Mol. Es gibt einen deutlichen Kristallographieeinfluss, der mit steigender Temperatur kleiner wird. Die Scherkriechergebnisse der vorliegenden Arbeit werden mit Literaturergebnissen aus einachsigen Zugversuchen verglichen und auf der Grundlage mikrostruktureller Verformungsmechanismen diskutiert.

This study was designed to evaluate the fatigue characteristics and the safety and effectiveness of laparoscopic aortic clamps in a pulsatile circulation model.A heart-lung machine was used to create a pulsatile circulation model with bovine aortas resembling the vessels being cross-clamped. Four different models (A-D) of laparoscopic aortic clamps were investigated, and three identical probes of each model underwent testing. Preliminary examinations were conducted to define the size and thickness of the bovine aortas that would allow effective cross-clamping and to detect gross material or functional deficits of the clamps. Then, the instruments were placed in the circulation model, which was set at a frequency of 82/min and a pressure of 200/120 mm Hg. Each clamp was subjected to these conditions for 120 hours and was opened and closed 40 times to stimulate real-life conditions. Clamping failures and mechanical defects were recorded, and the clamp parts were afterward examined with an electron microscope.Two clamp models had to be eliminated from the study after the preliminary examinations. All three probes of model B displayed mechanical defects after a few applications. All probes of model D were excluded because none effectively occluded the aortas. All probes of model A and one probe of model C provided effective cross-clamping during the 120-hour test phase and showed no signs of mechanical failure. Two probes of model C broke after 51 and 57 hours of testing, respectively. Both times, the defect occurred during application of the clamps. The detailed analysis of all instruments after the testing, including electron microscope imaging, revealed that several construction deficits and weak points were responsible for the mechanical failures.A surprisingly high incidence of clamping failures and mechanical deficits were encountered during the testing. Of the four clamps tested, only one (model A) seemed to be safe and effective enough for routine clinical use. These disappointing results demonstrate the need for further cooperation between vascular surgeons and instrument manufacturers to develop safe and effective laparoscopic vascular clamps.

The present work represents an exploratory study of creep in a Ni-rich NiTi shape memory alloy (50.7 at.% Ni) in the temperature range 470–530°C at stresses betweef 90 and 150 MPa. Creep o...

The present work studies the influence of different thermo mechanical treatments on phase transformation and creep of a Ti-50.7 at% Ni shape memory alloy. Thermo-mechanically processed, solution annealed, tempered and pre-crept material states show different phase transformation behaviour. Precipitation of Ti 3 Ni 4 particles can rationalize a 20°C increase in phase transition temperatures. Changes of dislocation density and subgrain size also have a systematic but small effect on the martensitic transformation. There is a strong influence of small preloads on the shape of individual creep curves in the early stages of creep which is associated with the precipitation of Ti 3 Ni 4 .

The paper reports creep data of a Ni-rich NiTi-alloy at stresses and temperatures in the range of 500 °C and 150 MPa. The apparent activation energy for creep and the creep stress exponent were found to be 421 kJ/mole and 5. Tempering and pre-creep result in a 20°C increase of phase transformation temperatures (PTTs). The effect of an additional plastic deformation (as compared to a stress free aging treatment) is that PTT-peaks broaden and overlap on cooling and broaden on heating. This can only be rationalized on the basis of a difference in microstructures between the tempered and pre-crept materials.

In the present study we characterize the microstructure of pseudoelastic, ultra-fine grained, Ni-rich NiTi-stents using transmission electron microscopy (TEM) and perform structural fatigue experiments. The TEM-samples were taken from small struts, the elementary building units of medical stents. They were prepared using a focused ion beam (FIB) system. Microstents were subjected to approximately 6000 cycles at room temperature in air at a frequency of 1 Hz. They were then integrated into an in situ test rig which allowed to perform cyclic tensile loading in a scanning electron microscope (SEM). The present paper reports the stress–strain response associated with cyclic loading and documents failure mechanisms.

The application of shape memory alloys in coupling devices depends considerably on the phase transition temperatures of the material. On the one hand it is important to achieve a high austenite start temperature. On the other hand the difference between martensitic and austenitic transition start-temperatures should be as large as possible. The present work investigates the influence of thermomechanical treatments on the phase transition temperatures of a NiTiNb alloy. The aim is to optimize the properties of the semiproduct for later applications in coupling devices.

Crack loading and crack extension in pseudoelastic binary NiTi shape memory alloy (SMA) miniature compact tension (CT) specimens with 50.7 at.% Ni (austenitic, pseudoelastic) was investigated using infrared (IR) thermography during in situ loading and unloading. IR thermographic measurements allow for the observation of heat effects associated with the stress-induced transformation of martensite from B2 to BIT during loading and the reverse transformation during unloading. The results are compared with optical images and discussed in terms of the crack growth mechanisms in pseudoelastic NiTi SMAs. Direct experimental evidence is presented which shows that crack growth occurs into a stress-induced martensitic microstructure, which immediately retransforms to austenite in the wake of the crack.

Testing parameters, such as rotational speed and bending radius, have a strong influence on the fatigue life of pseudoelastic NiTi shape-memory alloys during bending rotation fatigue (BRF) experiments [M. F. X. Wagner, Int. J. Mat. Res. 97 (2006), p. 1687-1696. and M. Frotscher, et al., Thermomechanical processing, microstructure and bending rotation fatigue of ultra-fine grained NiTiCr-wires, Proceedings of the International Conference for Shape Memory and Superelastic Technologies (SMST 2007), Tsukuba, Japan, ASM International, (2008), p. 149-158.]. Previous studies showed a decrease in the fatigue life for smaller bending radius (i.e. higher equivalent strain) and larger rotational speed. This observation is associated with an increase of dislocation density, the stabilization of stressinduced martensite during cycling, and an increase of the plateau stresses due to self-heating. In the present study, we examine the influence of these fatigue parameters on the nanohardness and shape recovery of pseudoelastic NiTiCr shape-memory alloy wires by nanoindentation. We show that nanoindentation is a suitable method for the characterization of fatigue-related microstructural changes, which affect the mechanical properties.