Study On The Textured Ni8W Alloy And Composite Substrates Used For Coated Conductors

Coated conductor, so called the second generation high temperaturesuperconductor has the advantages to be low AC loses, high in-field performace andlow processing costs. It is of immense potential and promising for electric and energyapplication in the coming future. The RABiTS (rolling assisted bi-axially texturedsubstrate) process is well known to be one of the promising ways that can fabricatethe biaxially textured Ni based alloy substrates for YBCO coated conductors withhigh critical current density (Jc). This method consists of the fabrication of metal oralloy substrates by cold rolling and recrystallization heat treatment suitable for YBCOcoated conductors. Compared to the IBAD (ion beam assised deposition) method tofabricate coated conductor, RABiTS process is more cost-effective and simple devicesdepended. Whereas in recent years, there are still several primary challenges for thecommercialized nickel based substrate tapes such as the poor mechanical andmagnetic properties for commercial use in coated conductor applications. What ismore, in order to obtain optimization of cube texture quality in recrystallized NiWalloy tapes with high tungsten content (W>9at.%), the process may become muchmore complicated than others. In this thesis, applying the RABiTS technic route,sharp cube textured Ni8W and Ni8W/Ni12W/Ni8W composite alloy tapes withreinforced strength and reduced magnetism were fabricated by spark plasma sintering(SPS) technic, while the evolution mechanism of rolling texture and recrystallizationtexure in the Ni8W tapes is clarified. Furthermore, the influence of element diffusionto the properties of composite tapes was systematically studied. The primary noveltiesof his thesis are described in details as follows.We study on the influence of high energy ball milling process on the morphologyand component of green mixed powders, as well as the influence of homogenizationtreatment on the initial grain size and homogeneity of Ni8W alloy ingots. It is reliableroute for the fabrication of reinforced textured Ni8W alloy tapes involving powdermetallurgical techniques, as well as optimized ball milling and homogenizationtreatment processes. According to typical cold rolling and two step annealingprocesses, textured Ni8W alloy tapes with high quality of cube texture which reach tofraction of90.2%(<10o) are obtained from the ingots prepared by SPS technic, beinghighly promising for them and composite taps in coated conductor applications. Deformation and recrystallization texture in Ni8W alloy tapes were studied byboth electron backscattering diffraction (EBSD) and X-ray diffraction (XRD) analysis.It is recognized that in Ni8W alloy substrates, such low stacking fault energy FCCalloy, Cube oriented grains have advantage neither on so called Oriented Nucleation,nor40o<111> boundaries relationship in Oriented Growth theory. This theory ismuch more different from those of high stacking fault energy FCC alloys. Due to thegrain size advantage of Cube oriented grains during recrystallization and grain growth,Ni8W alloy finally obtain strong cube texture.Powder-type Ni8W/Ni12W/Ni8W composite ingots were prepared by SPSpowder metallurgy route. After a systematic investigation of the fabrication processesfor composite ingots with different layer thickness ratios and the heat treatments forcomposite tape, a composite tape with sharp cube texture at the surface, high strengthand reduced magnetism in the whole layers was successfully obtained. The propertiesof strength, magnetism and cube texture could be balanced in the composite tapes.The optimized Ni8W/Ni12W/Ni8W triple-layer structure was found to performexcellent properties. It indicate that the fraction of cube texture on the compositesurpace reaches to96.5%(<10o). Moreover, the yield strength (ζ0.2) of the compositetape reaches to339MPa, a factor more than two times higher than that of Ni5W alloytape, while its saturation magnetization is only0.39emu/g at77K, i.e.98.4%lowerthan for competing Ni5W alloy tape. The combination property of this composite tapehas reached the leading level around the world.It is found that both of the gradient distributions of rolling textures and theelement diffusion in the Ni8W/Ni12W/Ni8W composite substrate dominate thetexture development of the outer layer in composite tapes. The components of rollingtextures on the surface of composite tape are much stronger than the inner’s.Moreover, elements diffution may affect stacking fault energy and have enfluence onthe grain growth during annealing. They have further influence on the formation ofcube texture in some extent. This explanation is useful in view of discussing therelationships governing the texture formation of each layer in composite tapes.In summary, this thesis mainly focused on the recent highlights of coatedconductor materials and some ideas and routes to solve the problems existing in thetextured tapes, which were proposed and studied systematically. According to SPStechnic and two step annealing processes, optimized high energy ball milling processand homogenization treatment method have been studied, and the cube texture component of Ni8W tapes is dramatically improved. This is significant for the futherfabrication of high performanced composite tapes. Furthermore, the study on theevolution of rolling texture and recrystallization texture in Ni8W is of greattheoretical significance in guiding the subsequent research on hign tungsten contentNi-W alloy substrates. Both of the gradient distributions of rolling textures and theelement diffusion in the Ni8W/Ni12W/Ni8W composite substrate have strong effecton the texture development on the surface of composite tape. The proposeddevelopment mechanism of cube texture in NiW composite alloy tapes is helpful forfully understanding the recyrstallization behavior of composite materials. Finally, highperformance Ni8W/Ni12W/Ni8W composite substrate is obtained, thus offering apromising base for the coated conductors with high quality.