Study On Deformation And Recrystallization Texture Of Ni5W Substrates

For the long length of coated superconducting wires with excellent performance,it is important to dispose of a textured, high quality and flexible metal templatesubstrates. The Ni-5at.%W (Ni5W) alloy substrate is considered as the first choice forlarge-scale production of YBCO coated superconductor, due to its sharp cube texture,excellent mechanical property, low magnetism and oxidation resistance. Presently, thetraditional Ni5W alloy substrate has been scaled up to a long length of industrialproduction. However, the formation mechanism of recrystallization cube texture inNi5W substrate is not clear and has not yet been reproved broadly worldwide. In thepresent thesis，the cube textured Ni5W substrates with heavy cold-rolled, startingfrom advanced melting casting ingots followed by Rolling Assisted Biaxial TexturedSubstrate (RABiTS) process, were prepared. The evolution of deformation andrecrystallization textures in the Ni5W substrate was investigated by using EBSD andX-ray diffraction. The formation process and development mechanism of cube texturein the as obtained Ni5W substrate was investigated systematically. The primaryconclusions of this thesis are described in details as follows:The deformation texture for the cold rolled Ni5W alloy substrate is a typicalCu-type rolling texture which is often observed in FCC metal with high stacking faultenergy. The volume fraction of Copper-type rolling texture and S-type rolling textureare reached18.38%and38.31%, respectively. EBSD results indicate that the cubiccrystal nucleus form and grow at first on the TD-RD surface due to the existence ofCopper and S-types rolling textures. The rolling oriented regions are observed as longthin bands along the thickness direction of deformed substartes. The volume fractionof high angle grain boundary is60.2%. Cube bands are characterized by a highlyoriented gradient in which the point-to-origin misorientation accumulated over6μmdistance is reached to a high value, being close to about13°. The observed results arealso corresponded well with the fact that the cube grain is formed in those regionswith high orientation gradient.After a systematic investigation of the recrystallization annealing temperature,the sharp cube texture on the surface of the Ni5W alloy substrate with small grain sizewas successfully obtained. The results indicate that, with increasing the annealingtemperature, the higher the content of cube texture and low angle of grain boundary is,the lower the content of high angle grain and twin boundaries, When annealing at 1350℃for60min, the abnormal grains were appeared in which the high angle ofgrain boundary and twin boundary have become dominant. Finally when annealing at1100℃for60min, the cube texture content in Ni5W alloy substrate is99%(＜10°),and the low grain boundary content increases83.5%. Meanwhile the twin boundarycontent is only2.3%and the average grain size being51.8204μm. It is determinedthat the optimal annealing process is1100℃for60min. According to the EBSDanalyses of the fully recrystallized Ni5W substrate, the uniform grain size and highquality of cube texture were obtained using the iso-thermal heat treatment. In a word,the recrystallization annealing technique of Ni5W alloy substrate was successfullydeveloped, which has an important significance for promoting the practicalapplication of the coated conductors.A formation mechanism of cube texture in Ni5W alloy substrates along NDdirection was described for the first time. Due to the nucleation advantage, cube gainsare easily produced on the surface of the Ni5W substrates at the early stages ofrecrystallization and there is a nucleation gradient along the thickness of the substrates.The formation of cube texture in Ni5W alloy is appeared primarily during the sizeadvantage and preferential growth of cube grains. A high orientation gradient in thecube deformation bands is favored to the cube sub-grain coalescence with a sizeadvantage. In a word, the formation of a sharp cube texture was found to be due to theco-function of both the size advantage and the orientation growth in the Ni5Wsubstrate.In summary, due to its broad application in coated conductors, the preparationof highly textured Ni5W alloy substrate becomes one of the key points worldwide.However, the formation mechanism of cube texture in the Ni5W substrate is still notclear. The present thesis was focusing on the main problems of the Ni5W alloysubstrates used for coated conductors, and exploring the formation mechanism ofcube texture in the as fabricated substrate materials. The deformation texture in coldrolled Ni5W substrate prepared by a vacuum melting route is typical Cu-type texture,and the cube texture (<10°) content was reached to99%through the optimized heattreatment process. Moreover, the isothermal heat treatment was applied to furtherimprove the quality of the cube texture and grain boundary in the as obtainedsubstrate, which is highly promising to use this optimized annealing process for theproduction of long-length Ni5W substrate with homogeneous cube texture in thefuture. To explore the texture development, the Ni5W alloy system is more complex compared to that of the pure Ni material. The present thesis demonstrates theevolution and distribution of the cube texture along the thickness direction of theNi5W substrate for the first time. It was found that the formation of the cube texturewas mainly due to the size advantage and growth advantage of cubic grains. Theobserved formation mechanism of cube texture in the Ni5W substrate would enrichthe theories of the deformation and recrystallization in the Ni5W alloy, demonstratingthe possibility of large scale application of the as obtained substrates.