Investigation of crystal structure formation by electropulse treatment of amorphous Ti50Ni25Cu25 alloy ribbons

The study of the effect of electropulse treatment with a variable duration on the crystallization processes and the structure of a amorphous TiNiCu alloy with 25 at.% Cu in comparison with isothermal annealing and heating at a constant speed was carried out. The alloy was fabricated by rapid-quenching from the liquid state (melt spinning technique) at the cooling rate of the melt of about 106 °C/s in the form of a ribbon with a thickness of 28 μm with a surface crystal layer with a thickness of about 2-3 μm. To remove the crystal layer, the method of double-sided electrochemical polishing was used. The studies were carried out by methods of differential scanning calorimetry, metallography and scanning electron microscopy. It was established that the formation of the crystalline phase in the electropulse treatment of the amorphous ribbon occurs from the surface to the inner part due to the predominant formation and growth of columnar crystals with subsequent nucleation and growth of crystals in the rest of the ribbon.

Resistance spot welding of a thin 0.7 mm EN10130: DC04 material onto a thicker 2.4 mm 817M40 engineering steel

The effects of welding current, electrode force, and welding time in a resistance spot weld were studied to investigate the effectiveness of welded joints between a thin EN10130: DC04 material and a thicker 817M40 part, through analysis of the microstructural and mechanical properties. All welded specimens were subjected to tensile testing at room temperature (25°C) and sub-zero temperature (-46°C) to test the strength of the welded joints. No full button failure was observed at either room temperature or sub-zero temperature after optimization of the weldng parameters. The fusion zone was observed to consist mainly of martensitic phase, due to rapid quenching, while the HAZ was composed of clusters of martensite in a ferrite and bainite matrix. The base 817M40 metal remained fully ferritic after welding. The hardness was found to increase with increasing welding current. An increase in nugget size, indicating good fusion of the weld, was observed with an increase in the welding current.

Effect of aluminum alloying on the structure and properties of rapidly quenched TiNiCu alloy

The efficiency of shape memory alloys for the MEMS technology has been recently demonstrated. Quasibinary intermetallic TiNi-TiCu alloys produced by rapid quenching from liquid phase in the form of thin (about 40 um) ribbons are an attractive material for the fabrication of micro-actuators due to their narrow temperature hysteresis of the shape memory effect (SME) and relatively large recoverable strain. In order to broaden the functionality of SME microdevices, in this work we have alloyed TiNiCu containing 25 at.% copper with aluminum. The results have shown that alloying with 0.6 at.% Al increases the cast characteristics of the composition and favors its amorphization. Upon crystallization by isothermal annealing or electropulse treatment the resultant microstructure and SME properties of the Al containing alloy change but slightly in comparison with the original alloy however there is a significant shift (by more than 15°C) of the SME temperature range toward lower temperatures.

Effect of External Impacts on the Structure and Martensitic Transformation of Rapidly Quenched TiNiCu Alloys

TiNi-TiCu quasibinary system alloys with a high Cu content produced by rapid quenching from liquid state in the form of thin amorphous ribbons exhibit pronounced shape memory effect after crystallization and are promising materials for miniaturized and fast operating devices. There is currently no complete clarity of the mechanisms of structure formation during crystallization from the amorphous state that determine the structure-sensitive properties of these alloys. This work deals with the effect of the initial amorphous state structure and crystallization method of the alloys on their structure and phase transformations. To this end the alloy containing 30 at.% Cu was subjected to thermal and mechanical impact in the amorphous state and crystallized using isothermal or electropulse treatment. We show that after all types of treatment in the amorphous state the structure of the alloy remains almost completely amorphous but the characteristic temperatures and enthalpy of crystallization become slightly lower. Isothermal crystallization of alloy specimens produces a submicrocrystalline structure with an average grain size in the 0.4–1.0 μm range whereas electropulse crystallization generates a bimorphic structure consisting of large 4–6 μm grains and 2–3 μm high columnar crystals in the vicinity of the surface. The grains have nanosized plate-like and subgrain structures. The largest grains are observed in thermally activated samples, meanwhile, mechanical impact in the amorphous state leads to the formation of equiaxed finer grains with a less defective subgrain structure and to the shift of the temperature range of the martensitic transformation toward lower temperatures.

A Computational Study of Structure Development and Texture Formation in Carbonaceous Mesophase Fibers

In this thesis, thermal relaxation phenomena after the melt-extrusion of a rigid discotic uniaxial nematic mesophase pitch were studied using mathematical modeling and computer simulation. The Eriksen and Landau-de Gennes continuum theories were used to investigate the structure development and texture formation across mesophase pitch based carbon fibers. It is found that during the thermal relaxation, discotic nematic molecules stored elastic free energy decays. The distorted nematic molecular profile reoriented to release the stored elastic free energy. The difference in time scales for molecular reorientation and thermal relaxation resulted in different transverse textures. The rate at which the fibers are cooled is the main factor in controlling the structure development. A slow cooling rate would permit nemiatic discotic molecules to reorient to a well developed (radial or onion) texture. The random texture is a result of rapid quenching. The numerical results are consistent with published experimental observations.

A Computational Study of Structure Development and Texture Formation in Carbonaceous Mesophase Fibers

In this thesis, thermal relaxation phenomena after the melt-extrusion of a rigid discotic uniaxial nematic mesophase pitch were studied using mathematical modeling and computer simulation. The Eriksen and Landau-de Gennes continuum theories were used to investigate the structure development and texture formation across mesophase pitch based carbon fibers. It is found that during the thermal relaxation, discotic nematic molecules stored elastic free energy decays. The distorted nematic molecular profile reoriented to release the stored elastic free energy. The difference in time scales for molecular reorientation and thermal relaxation resulted in different transverse textures. The rate at which the fibers are cooled is the main factor in controlling the structure development. A slow cooling rate would permit nemiatic discotic molecules to reorient to a well developed (radial or onion) texture. The random texture is a result of rapid quenching. The numerical results are consistent with published experimental observations.

Intrinsic hydroquinone-functionalized aggregation-induced emission core shows redox and pH sensitivity

Aggregation-induced emission (AIE) fluorophores exhibit strong fluorescence in an aggregated state but emit no or weak fluorescence in dilute solutions. This emerging class of AIE optical materials comprise a variety of functionalities. Here an AIE luminescence core, 1-hydroquinol-1,2,2-triphenylethene (HQTPE), has been designed and synthesized. This AIE core is simple but is fundamentally important to chemistry because of its intrinsic redox and pH activities. The incorporation of hydroquinone (HQ) moiety into a common AIE core tetraphenylethene (TPE) yields HQTPE with unique fluorescent properties like nonlinear self-quenching over most other AIE-active fluorophores (AIEgens) so far reported. There are differences of photochemical properties between HQTPE, 1-benzoquinol-1,2,2-triphenylethene (QTPE, the oxidized counterpart) and its anions. Interestingly, as the solution concentration is increased, AIEgen HQTPE shows stronger fluorescence but QTPE exhibits rapid quenching of fluorescence in a nonlinear fashion, which are in agreement with theoretical studies. The fluorescence of HQTPE is also highly dependent on the pH value of media. We have further explored HQTPE as an ultrasensitive redox probe and efficient deoxidizer, which could lead to potential applications in health care, food security, environmental monitoring, optic and electronic devices.