Papers by Keyword: Diffusion Annealing

Abstract: In the present study, the Nb₃Sn-based multifilamentary wires with coupled Nb filaments have been investigated by SEM and TEM after various regimes of intermediate annealing including short high-temperature heat treatments and after two-staged diffusion annealing. The formation of some amount of pre-reacted Nb₃Sn layers has been revealed in all the wires studied, and their amount depends on the wire diameter, temperature and duration of the intermediate heat treatment. The structure of final diffusion layers is also affected by the regimes of these preliminary treatments. This research enables the revealing of the optimal heat treatment schedules for the formation of most perfect nanocrystalline structure of superconducting layers ensuring the highest critical current densities.

Abstract: The evolution of structure and morphology of superconducting Nb₃Sn layers forming under various regimes of the diffusion annealing in multifilamentary Nb/Cu–Sn superconductors of different design is briefly reviewed based on the authors’ (with their coauthors) original studies and the available publications. Different modes of the solid-state diffusion formation of the superconducting phase are compared. Possible mechanisms of the Nb₃Sn nucleation upon the diffusion reaction of the Nb filaments and bronze matrixes are discussed. The effects of different doping elements, especially Ti, on the formation of the superconducting phase, its structure and morphology, and, consequently, on the current-carrying capacities of multifilamentary Nb₃Sn-based wires are considered. Special attention is paid to the effect of the mode of Ti doping, the content of this alloying element and the regimes of the diffusion annealing on the structure, morphology and resulting performance of the bronze-processed wires with coupled Nb filaments worked out at Bochvar Institute of Inorganic Materials for the creation of International Thermonuclear Experimental Reactor.

Abstract: The structure and morphology of Nb₃Sn layers in superconducting Nb/Cu-Sn composites with ring (tubular) Nb filaments have been studied by transmission (TEM) and scanning (SEM) electron microscopy after various regimes of diffusion annealing. It is demonstrated that the tubular geometry of Nb filaments, in which Sn diffuses from the bronze matrix both from inside and outside, ensures practically complete transformation of Nb into the superconducting Nb₃Sn phase. Besides, at certain regimes of the diffusion annealing this geometry enables the improvement of the superconducting layers morphology compared to that of wires with continuous filaments, namely, to obtain wide Nb₃Sn layers with fine equiaxed grains and to avoid the formation of columnar grains, which promotes enhanced current-carrying capacities of the wires.

Abstract: Although 6.5wt% high silicon electrical steel has pretty excellent soft magnetic properties, it is hard to be obtained by cold rolling at room temperature, by virtue of appearance of ordered phases in this alloy. In this research we report that very thin 6.5wt%Si composite sheet with 0.05-0.08mm thickness was fabricated by proper heat treatment and cold rolling after hot deformation. The 6.5wt%Si composite plate exhibits some extent of ductility at room temperature when the stage of hot deformation was finished. The effect of heat treatment on mechanical properties of this composite plate was investigated by optical microscope. After heat treatment, ductility of this composite plate is increased due to the ordered DO₃ phase transformation inhibited by cooling fast. It is provided a referable method to produce 6.5wt% high silicon steel with conventional forming process.

Abstract: The 6.5wt%Si composite plate, which contains three layers and the middle one is about 10wt% ferrosilicon alloy, is fabricated by clad casting and conventional thermomechanical processes. Experimental results reveal that the high silicon composite plate could get lager plastic deformation on the traditional rolling mill. Microstructure of different stages such as hot rolling, warm rolling and heat treatment are observed by optical microscope. Defects appeared in core layer of 6.5wt% high silicon composite plate could be eliminated in the diffusion annealing process. The values of iron loss are investigated at different frequencies which thickness of these homogenous thin sheets are near 0.3-0.5mm. Measurement results of 6.5wt%Si alloy samples produced by chemical vapor deposition (CVD) and powder rolling processing (DPR), and 3wt%Si grain oriented steel are compared with samples prepared by laminar composite technique.

Abstract: 6.5wt% high silicon electrical sheet is hard to be obtained through conventional different rolling processes due to its remarkable brittleness at room temperature and appearance of ordered phases in the alloy substrates, especially it has very excellent soft magnetic properties. In this study, 6.5wt%Si three-layer composite panels were prepared by advanced technology of laminar composite. The defects, bonding strength, and distribution of elements of the coating and core layers are analyzed by the photographs of metallographic phase and SEM. Experimental results show that the high silicon composite plate could carry out lager plastic deformation on the traditional hot mill after the inner layer which is10%Si ferrosilicon alloy was coated. The defects generated during hot deformation were reduced and uniform distribution of elements were achieved through proper diffusion annealing process. This article provides a short process method to produce 0.3-0.5mm 6.5wt%Si thin sheet.

Abstract: The structure of high-purity copper and stabilizing copper of Cr-plated Nb₃Sn-strands with different RRR (residual resistance ratio) values has been studied. Cr diffusion into peripheral layers of stabilizing Cu has been revealed. The variations of RRR and Cr content in Cu at annealing have been estimated. It is demonstrated that an additional drop of RRR may be caused by oxygen diffusion from Cr coating obtained by electroplating. Optimal regimes of the diffusion annealing of Cr-plated Nb₃Sn-strands for ITER (International Thermonuclear Experimental Reactor) have been determined.

Abstract: Diffusion annealing experiments in Al-Ni cold rolled strip was conducted in nitrogen protection. The characterization of diffusion layers between pure Al and pure Ni metals has been analyzed. The analysis results show that the diffusion layers would be composed of solid-solution of Al in Ni marked as Ni (Al), Ni3Al and Ni5Al3 compounds. During diffusion processing, the diffusion speed of Al to Ni was faster than that of Ni to Al. This implied a possibility to form chemical connection of Al-Ni by solid-solution without intermetallic compounds formation to obtain good mechanical and electric properties through optimizing annealing temperature and holding time.

Abstract: The structure of Nb3Sn-based, bronze-processed Ti-doped multifilamentary superconducting wires has been studied by TEM and SEM after the first (5750C,100 h) and the second (6500C,100 h) stage of the diffusion annealing. The Nb3Sn layers formation in all the composites proceeds by one and the same mechanism and starts with nucleation of particles and very fine grains of this phase in Nb filaments where Sn diffuses from the bronze matrix. Ti, inserted both in the bronze matrix, or Nb filaments, diffuses into the growing superconducting layer and promotes its more active formation. At the first stage of annealing (5750C, 100 h) Nb3Sn grains have an average size of 40 nm, and at the second stage (6500C, 100 h) they increase by a factor of 1.5 and the grain size distribution gets wider. After the two-stage annealing the amount of the residual niobium is small, and some Nb filaments, especially in doped composites, almost completely transform into Nb3Sn. In the Nb3Sn layers of a zone of columnar grains is adjacent to the residual Nb.

Abstract: A homogenous intensity distribution along the cube texture fibre is important to achieve an easy magnetization in non-oriented electrical steels. Several alternatives have been discussed in literature to achieve this goal namely, tertiary recrystallization (surface energy controlled), decarburization annealing, two step cold rolling (strain induced boundary migration), twin-roll thin strip casting (directional solidification), phase transformation (surface energy anisotropy) and columnar grains formation (selective grain growth). In the present study, a hypoeutectic Al-Si alloy was deposited on the surface of cold rolled Fe-Si steels with a hot dipping simulator and subsequently annealed at 1000°C for different times. This procedure was developed previously in order to enrich the substrate with Al and/or Si and consequently improve their resistivity. Of specific interest was the formation of columnar grains in the low Fe-Si steel after annealing. These columnar grains were found to grow from the surface towards the centre of the substrate. The microstructure and texture in the columnar grains were significantly different than those in the middle of the material. Therefore, the evolution of these features during processing was studied in detail in this work.