Materials Science Forum Vols. 738-739

Abstract: The short-time thermal treatment strategy has been proved to be very efficient in improving the mechanical properties of various titanium based alloys. The mechanical properties of alloys such as Ti-Nb, Ti-Nb-Zr and Ti-Nb-Zr-Sn based alloys, are extremely sensitive to the β phase stability, microstructure and phase constitution. The concept of the short-time treatment is designed to control precisely the material structure (phase precipitation, etc…) without extensive modification of the distribution of alloying elements. This results in reliable optimizations regarding the balance between elastic modulus, pseudo- (super-) elasticity and strength. Currently, the structural evolution mechanisms involved in the STAT are under systematic investigations in the aim of achieving accurate control of the microstructures and optimized balance of mechanical properties.

Abstract: Phase stresses that arise from volume mismatch between austenite and martensite crystal lattices were studied at different stages of direct and reverse martensitic transformations in NiTi alloy for medical application. The absolute stress values of tensile and compressive stresses that operated in austenite and martensite respectively were shown to be Gaussian functions of phase composition: they increased from zero in the initial single-phase state, achieved maximum at the equilibrium between the initial and new phases and decreased to zero in the ultimate phase. Phase stress distribution on the surface of simulated two-phase sample and related distinctions in osteoblast-like cells’ behavior were analyzed. Negative effect of compressive stress on cell viability and probable mechanism of cell apoptosis are discussed. Recommendations concerning the choice of permissible size discrepancy between organ and implant as well as NiTi alloy composition to avoid/minimize negative consequences of two-phase state after implant installation are suggested.

Abstract: This paper focuses on study of the kinetics and the mechanism of passive film formation on novel superelastic biomedical Ti-22Nb-6Ta and Ti-21.8Nb-6Zr (at.%) alloys in simulated physiological solutions using XPS and electrochemical techniques in comparison with commercially pure titanium and Nitinol. In vitro biocompatibility study of Ti-22Nb-6Ta alloy was carried out as well. Ti-22Nb-6Ta alloy manifests a strong trend to protective passive film formation during the first stage of oxidation in simulated physiological solutions. It is shown that Ti-22Nb-6Ta and Ti-21.8Nb-6Zr electrochemical characteristics are comparable to those of titanium and Nitinol, and the alloy can be recommended for use in implantology.

Abstract: Mechanical behavior of structural nitrogen-containing steels with various structures and compositions, including the same steels with different summary C+N content and C/N ratio were studied using pressing tests in a wide temperature range, tensile tests, impact bending tests, hardness measurements and shock-wave loading resistance. The tempering and aging under load processes after quenching or thermomechanical treatment with various regimes have been investigated using optical and electron microscopies, X-ray diffraction analysis, calorimetric and dilatometric analyses. Hot strain resistance of the austenite is determined essentially by the steel composition, while the final structure and mechanical properties of hot-deformed austenite are determine mainly by hot deformation conditions. The higher the nitrogen content and C/N ratio, the higher hot strain resistance was and earlier the softening processes start, especially recrystallization process. The nitrogen microalloying of low-alloyed structural steels changes kinetics of the martensite tempering. Application of the high temperature thermomechanical treatment or combined thermomechanical strengthening with following tempering under load allows the use of these steels in a high-strength state after low-temperature tempering.

Abstract: Titanium-platinum (Ti50Pt50) alloy has been identified as a potential candidate for high temperature shape memory alloy (HTSMA) applications such as actuators in jet engines. This work evaluates powder metallurgy as an alternative processing route with the added benefit that near net shaped components can be formed. Spark plasma sintering (SPS) of blended elemental titanium and platinum powders was carried out. The sintering conditions used were: temperatures ranging from 1200 to 1400 °C and sintering times ranging from 10 to 30 minutes. SEM with EDS, XRD and DSC were carried out to study the results. Results showed that SPS only achieved high density, but not homogenisation of the alloys. All sintering conditions resulted in formation of the martensitic TiPt phase of interest, together with some or all of the following phases: Ti, Ti3Pt, Ti4Pt3, Ti3Pt5 and Pt. Post sintering annealing treatments were carried out to homoginise the alloys at 1300 °C for times ranging from 5 to 15 hours. With sufficient annealing times a two phase microstructure was produced with TiPt as the majority matrix phase and Ti3Pt5 as the precipitate phase.

Abstract: The rhombohedral R-phase is an intermediate phase during the transformation from cubic austenite (B2) to monoclinic martensite (B19’). Compared to the martensitic transformation (B2 → B19’), the R-phase transformation (B2 → R) in NiTi based shape memory alloys (SMA) has a lot of advantages. The main advantages of the R-phase transformation are the very small temperature hysteresis (2-5K) and the high durability (> 100.000 cycles). The main disadvantage of actuators based on R-phase transformation is the small SMA effect (approx. 0,8%). Due to the small temperature hysteresis, the R-phase actuators are usable for temperature sensitive applications in heating technology or in air conditioning. The R-phase transformation doesn’t appear in each SMA alloy and needs a special thermo-mechanical treatment to be induced. Fuzzy logic is an approach to computing, based on degrees of truth, rather than the usual true or false (1 or 0) Boolean logic. It includes 0 and 1 as extreme cases of truth but also the various states of truth. The objective of this paper is to show the applicability of the approach of fuzzy logic in the field of SMA. To gain basic knowledge about influences, different binary NiTi alloys were investigated. The parameters annealing temperature, annealing time and Ni-concentration were varied and their influence on the transformation behavior of the R-phase was noted and added to the fuzzy logic knowledge base.

Abstract: The present authors have invented a new method of changing coil orientation of a coil spring. The major merit of coil orientation change for SMA are reduction of space and control of two way shape memory effect. For instance, when a coil is heated above Af temperature with a load, the coil contracts. There is unrecovered strain which is attributed to elastic deformation by the load. The elastic strain can be eliminated by changing coil orientation. By removing elastic strain, the necessary space can be reduced. Besides generation and control of two way shape memory effect is presented using the reverse of coil orientation.

Abstract: Some technical applications of superficial modifying by materials with SME in mechanical engineering and aspects of practical realisation are considered. Economic feasibility of use of superficial modifying in thermomechanical operated connections of "shaft-plug" type and in bolted connection working in the conditions of vibrations is proved. The necessary thickness of blankets providing reliability of connections are defined. On the basis of the analysis of the stress-strain state of carving connection it is shown, that a material with SME carries out some functions: prevents self-unscrewing, stabilizes force of an inhaling, protects from corrosion, influences friction factor.

Abstract: Keywords: Lithium-ion storage battery (LISB), bypass device, shape memory effect (SME) thermal treatment, reactivity, reliability Abstract. Deformation and strength characteristics of a thermal drive spring made of alloy with shape memory effect (SME) оn the base of TiNi in bypass device (BD) of Lithium-ion storage battery (LISB) for space application were investigated. A technique was developed for this spring thermal preparation for a certain magnitude of shape memory deformation, due to which during reverse martensite transformation a spring generates reactive force, sufficient for BD operation. It allowed to develop small-sized multiple action BDs for LISBs intended for “Glonass-K” spacecrafts. Study of technological and operational factors’ impact proved high level of BD failure-free operation.