G. Eggeler

Ni4Ti3-precipitation during aging of NiTi shape memory alloys and its influence on martensitic phase transformations

The present work studies the microstructure of a Ni-rich NiTi shape memory alloy and its influence on the thermal characteristics of martensitic transformations. The solution annealed material state is subjected to various isothermal aging treatments at 773 K; this results in the nucleation and growth of lenticular coherent Ni4Ti3-precipitates, which were quantitatively characterized using transmission electron microscopy (TEM). Stress free aging for 36 ks results in a heterogeneous microstructure with precipitates near grain boundaries and precipitate free regions in grain interiors; this microstructure shows a three step (’multiple step’) transformation behavior in a differential scanning calorimetry (DSC) experiment on cooling from the B2 regime, which can neither be rationalized on the basis of a coherency stress argument (Bataillard et al., 1997) nor on the basis of varying Ni-concentrations between growing precipitates (Khalil-Allafi et al., 2002). A new interpretation of evolving DSC chart features is proposed which takes the evolution of microstructures during stress free and stress-assisted aging into account. Most importantly it is shown that stresses as small as 2 MPa strongly affect the precipitation process.

Neutron diffraction phase analysis during thermal cycling of a Ni-rich NiTi shape memory alloy using the Rietveld method

Abstract Martensitic transformations in an aged Ni-rich NiTi shape memory alloy are analysed using neutron diffraction and electron microscopy. The material shows a two step transformation on cooling from B2. After the first release of latent heat on cooling two martensitic phases (R-phase and B19 ′ ) are present. This is due to a heterogeneous microstructure after solution annealing and ageing.

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

We investigate the effect of 450u2009°C aging on the microstructure and on the martensitic transformations in a Ni-rich (50.8xa0at.% Ni) NiTi shape memory alloy using transmission electron microscopy (TEM), x-ray diffraction (XRD), neutron diffraction (ND) and differential scanning calorimetry (DSC). On cooling from the high temperature phase, DSC charts show two distinct peaks after short aging times, three peaks after intermediate aging times and two peaks again after long aging times (2–3–2 transformation behaviour). After 1.5xa0h 450u2009°C aging, three DSC peaks were obtained on cooling from the high temperature phase. The first peak on cooling represents the formation of R-phase and the second peak is associated with the formation of B19. Due to a macroscopic microstructural heterogeneity a second B19 peak (overall third peak) occurs at lower temperatures. We discuss our results in the light of recent remarks on burst-like events during the growth of thermoelastic martensite and on the effect of oxidation on NiTi microstructures.

Creep of binary Ni-rich NiTi shape memory alloys and the influence of pre-creep on martensitic transformations

Abstract 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...

New aspects of bending rotation fatigue in ultra-fine-grained pseudo-elastic NiTi wires

Abstract Bending rotation fatigue experiments are a standard evaluation procedure for pseudo-elastic NiTi wires used in the medical device industry. However, the non-linear mechanical behavior and the possible occurrence of localized deformation during bending complicate the physical interpretation of fatigue life data. In the present study, some related experimental and theoretical aspects are addressed: Optical measurements are used to characterize the local deformation of the wire specimens in a typical experimental setup, and a simple mechanical model of stresses and strains associated with bending rotation is employed to rationalize the impact of several experimental parameters on fatigue life data. Dissipated energies per cycle and unit length are calculated. How dissipation leads to an increase of both specimen temperature and maximum stresses and, therefore, considerably decreases fatigue lives is demonstrated.

On the determination of the volume fraction of Ni4Ti3 precipitates in binary Ni-rich NiTi shape memory alloys

Abstract The size distributions and volume fractions of Ni4Ti3 precipitates in Ni-rich Ni–Ti shape memory alloys affect their mechanical and functional properties. In the present work precipitate sizes and volume fractions were measured after various thermo-mechanical treatments using transmission electron microscopy (TEM) in combination with quantitative metallography. TEM tilting experiments revealed that the Ni4Ti3 particles have a disk-like shape with a disk diameter D and a disk thickness t. These two parameters together with the number density of particles per unit volume were determined after stress-free and stress-assisted aging at 400, 500 and 530 °C. Volume fraction results are presented after different thermo-mechanical treatments and our quantitative metallographic procedure is documented.

SEM micrographs from NiTi-based shape memory alloys after mechanical polishing and electropolishing

Abstract 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.

Comparison of laparoscopic aortic clamps in a pulsatile circulation model.

PURPOSEnThis study was designed to evaluate the fatigue characteristics and the safety and effectiveness of laparoscopic aortic clamps in a pulsatile circulation model.nnnMETHODSnA 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.nnnRESULTSnTwo 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.nnnCONCLUSIONnA 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.

Experimental investigation of the interaction of a dendritic solidification front with foreign particles

Metal matrix composites (MMC) have many technical applications, e. g. in automotive industry, and have still a great potential. Usually they are produced by expensive powder metallurgy. Cost effective solidification of the material directly from a particle containing melt requires an understanding of the interaction of the ceramic particles with the advancing solidification front. The knowledge is limited about dendritic solidification with particles. The experimental idea in this work is to deeply undercool the melt below its melting point, resulting in a fast propagating dendritic solid/liquid interface. Due to the rapid solidification, microstructure is frozen and the spatial distribution of particles in the microstructure can be analyzed postmortem. Experimental techniques are processing in a terrestrial electromagnetic levitation facility (EML) and, under low gravity conditions during parabolic flight, in the TEMPUS facility. Convection has a strong influence on the interaction between particles and dendritic solidification front. The experiments under low gravity show the importance of the reduction of melt convection on particle distribution within the rapidly solidified sample.

Phase evolution in aged Ni-rich NiTi shape memory alloys - A neutron diffraction and TEM study

After solution annealing and subsequently aging at temperatures between300 and 550 degrees C lenticular coherent Ni4Ti3-particIes precipitate.The presence of these precipitates strongly affects the martensitictransformations in Ni-rich NiTi alloys. In this work aging treatmentswere performed at 400 degrees C from 1 to 640 hours. Subsequently DSC,TEM, and neutron diffraction (ND) analysis were performed. TEM resultsshow that short and intermediate aging times result in a heterogeneousmicrostructure with Ni4Ti3 precipitates on grain boundaries andprecipitate-free regions in grain interiors. Correspondingly, theneutron diffraction indicates the presence of all four phases, 132, R,B19’, and Ni4Ti3 in the same sample after the first peak on cooling.Heterogeneous microstructures lead to complex transformation behaviourwhere regions with precipitates do not transform in the same way asprecipitate free regions.