Albert C. Kneissl

Influence of the microstructure on the stability of the intrinsic two-way shape memory effect

Abstract This work presents a study of the development of the two-way shape memory effect (TWSME) and its degradation due to working cycles. An intrinsic TWSME was induced in wire specimens of two near equiatomic NiTi alloys by thermomechanical training. The development of the effect was analyzed and discussed with respect to the different microstructures obtained by preliminary heat-treatment of the samples. The investigations show that training of cold worked specimens results in a smaller extension of the TWSME, but the effect remains more stable during service cycles compared to annealed samples. Precipitates have a strong negative effect on the trainability of the alloy. They considerably impede the evolution of the recoverable strain, thus limiting the obtainable magnitude of the two-way strain.

Ätztechniken für die Phasencharakterisierung von niedriglegierten Dual-Phasen- und TRIP-Stählen

Kurzfassung Dual-Phasen- und TRIP-Stähle bilden eine neue Generation von hochfesten Stählen mit einer überlegenen Kombination von Festigkeit und Duktilität. Damit kann Stahl vor allem seine Position in der Automobilindustrie als Konstruktionswerkstoff für den Leichtbau weiter ausbauen. Ähnlich dem Prinzip von Verbundwerkstoffen bestimmen Menge und Anordnung von harten Zweitphasen wie Martensit, Bainit und Restaustenit das Eigenschaftsprofil dieser Mehrphasenstähle. Über Modellüberlegungen zum selektiven Ätzabtrag und zur Kontrastierung durch Farbniederschlagsätzungen, sowie über die Überprüfung dieser Modelle anhand der Anwendung unterschiedlicher Ätzmittel wird ein Beitrag zum Verständnis der Ätztechnik und damit zur korrekten Interpretation des Gefügeaufbaues von DP- und TRIP-Stählen geleistet.

Physics of hydroxyapatite plasma coatings on TiNi shape memory materials

Abstract The microstructure of hydroxyapatite (HAP) ceramics coatings on TiNi shape memory alloy substrates was investigated. From the point of view of adherence, these coatings posses higher strength than approx. 30 MPa. The three levels of power input parameters were applied during the spraying process (55, 50 and 45 V). At lower voltage, the prepared HAP coatings contained pure Ca 10 (PO 4 ) 6 (OH) 2 with a Ca/P ratio of 1.65. At the highest voltage (55 V), the transition phase Ca 10 (PO 4 ) 6 (OH) 0.5 O 0.75 was detected. The observed good metal/ceramic interface strength is given by the formation of chemical bonding and by the energy dissipation due to stress induced martensite formation (SIM) and/or martensite reorientation (RE) during stressing the investigated composite.

Dispersion strengthening of copper by internal oxidation of rapidly solidified Cu–RE alloys: Part I: The microstructure and stability of rapidly solidified ribbons

Abstract Rapidly solidified ribbons of Cu-0.5-at.% Er and Cu-0.5 at.% Yb alloys have been prepared by melt spinning. An overall assessment of the ribbons has shown that up to three microstructural regions are distinguished: fine equiaxed grains, zone with a columnar structure and coarse equiaxed grains. Their number, type and height are controlled by wheel velocity. The growth behavior of dispersed intermetallic particles depends on the obtained microstructure, i. e., on the ribbon thickness. The large particles in thin ribbons coarsened by bulk diffusion, controlled by the diffusivity of the rare earth (RE) elements, and grow with the cube root of time at all temperatures. In thick ribbons with a high fraction of the coarse equiaxed zone, the coarsening takes place much more rapidly along the grain boundaries, which became a dominant transport mechanism.

Investigation of Bending Trainings, Transformation Temperatures, and Stability of Two-Way Shape Memory Effect in NiTi-Based Ribbons

This paper discusses the application of rapid solidification by the melt-spinning method for the preparation of thin NiTi-based ribbons. Generally, the application of rapid solidification via melt-spinning can change the microstructure, improving the ductility and shape memory characteristics and lead to small-dimensioned samples. Several thousand thermal cycles were performed on the trained ribbons using bending deformation procedure, continuously observing the changes in the shape memory and transformation behaviors. These changes are due to the appearance of an intermediate phase which was stabilized probably by the accumulation of defects introduced by thermomechanical training. The influence of training and thermal cycling on characteristics of ribbons was studied by x-ray diffraction and transmission electron microscopy and differential scanning calorimetry. The results displayed that bending training methods were useful in developing a two-way shape memory effect (TWSME). All samples show a shape memory effect immediately after processing without further heat treatment. The addition of copper in NiTi alloys was effective to narrow the transformation hysteresis. The W addition has improved the stability of the TWSME and mechanical properties. The TWSME of ribbons and its stability are well suited for important applications such as microsensors and microactuators.

In Situ Monitoring of Vacuum Carburizing

The present article describes development and testing of a new measurement method that enables nondestructive and in situ monitoring of a vacuum carburizing process. The principle of the method is based on monitoring the carbon diffusion during vacuum carburizing by the in-situ measurement of electrical resistance changes in the carburizing sample. Using this method the electrical resistance changes during vacuum carburizing of pure iron were acquired and analyzed. The experiments were performed in a laboratory vacuum furnace under a low pressure acetylene atmosphere (5 mbar) at a temperature of 950°C. The results of monitoring the kinetics of carburized zones obtained by the novel measurement method were compared with kinetics obtained by metallographic analysis of carburized samples.

The characterisation of microstructural changes in rapidly solidified Al-Fe alloys through measurement of their electrical resistance

Abstract Microstructural changes of rapidly solidified Al alloy ribbons of various thicknesses and containing different concentrations of iron were analysed. The kinetics and sequence of microstructural changes occurring at a constant heating rate were analysed through the measurement of electrical resistance. This method allowed detection of the transition between metastable and stable phases of rapidly solidified Al – Fe alloy ribbons with various concentrations of iron and various thicknesses. After determining the temperature regions throughout the heating cycle, the quenched microstructure was analysed at transition points using both optical and transmission electron microscopy. Precipitation is the most common reaction for these types of alloys, and the most important alloy formed is Al13Fe4.

Microstructural Changes During Internal Oxidation of a Ag-Sn Alloy

Abstract Electrical contact materials are expected to combine improved mechanical and physical properties with good corrosion resistance at increased temperatures. One of the most widely used groups of contact materials are internally oxidized dispersion hardened silver alloys. The principle of their production involves selective oxidation of less noble solute elements (usually Mg, Al, Sn, Cd and Zn) forming the fine dispersion of oxide particles in the solvent silver matrix. In this study the internal oxidation of dilute silver alloy containing 2 at. % of tin was extensively investigated. The morphological evolution of the alloy and the internal oxidation kinetics were determined by several metallographic examinations and in-situ electrical resistance measurements in the air atmosphere and in the temperature range from 600°C to 800°C. The results of experiments and their analyses show that the microstructural changes during internal oxidation of Ag-Sn alloy are strongly dependent on the annealing temperature. At the highest temperatures the perfectly dispersive distribution of the oxide particles SnO2 in the silver matrix was found, while at lower temperatures the formation of inner oxide bands and distribution of oxide particles along the grain boundaries is predominate.

Charakterisierung von Dual-Phasen- und TRIP-Stählen mittels Nanohärte und Röntgendiffraktometrie

Abstract For DP and TRIP steels, the nanohardness of ferrite, martensite, tempered martensite and bainite was measured. The lowest values showed ferrite, the differences detected in the hardness of martensite depend on the carbon content. Bainite in TRIP steels is slightly harder than tempered martensite in DP steel. The amount of retained austenite was measured by XRD methods. Values between 4 Vol.-% up to 14 Vol.-% were shown. According to lattice extension of austenite, the interstitial carbon content was calculated. Amounts between 0.9 and 1.1 mass percent carbon were detected. The influence of the substitutional elements like Al and Si has to be taken into account to get correct carbon contents.

Characterization and properties of niTi(W) and cuAlNi shape memory alloys

Abstract Shape memory wires were trained under constant stress in order to introduce a uniaxial shape memory effect by a thermomechanical treatment. These investigations were carried out on three different alloy systems (NiTi, NiTiW and CuAlNi) with different microstructures (cold-worked and annealed condition, with and without particles, large and small grain size). Several thousand thermal cycles were performed on the trained shape memory elements, continuously observing the changes in the deformation behaviour. The influence of the microstructure on development and stability of the intrinsic two-way shape memory effect has been discussed. Furthermore, this work deals with the production of thin specimens of shape memory alloys by melt-spinning and splat-cooling, the training procedure and their characterization with respect to microstructure and functional properties.