6.Role of mechanical deformation in the texturing of coated conductor composites

I NSTITUTE OF P HYSICS P UBLISHING S UPERCONDUCTOR S CIENCE AND T ECHNOLOGY Supercond.Sci.Technol.16(2003)1077–1081PII:S0953-2048(03)62226-8Role of mechanical deformation in the texturing of coated conductor composites Y X Zhou,S Bhuiyan,S Scruggs,H Fang and K SalamaTexas Center for Superconductivity and Advanced Materials,University of Houston,Houston,TX77204,USAReceived8April2003,infinal form14July2003Published8August2003Online at /SUST/16/1077AbstractNi and Ni-based alloys have received considerable attention as the substratesfor HTS coated conductors.A critical step in the successful fabrication ofcoated conductor composites on metal substrates is to produce cube textureby mechanical deformation using the rolling assisted biaxial texturedsubstrate(RABiTS)approach.In this paper,we investigated the effects ofmechanical deformation and heat treatment on the growth of texture innickel and nickel–tungsten alloys with the objective of understandingmechanisms of mechanical texturing in coated conductor substrates.Theresults show that the sharpest cube texture is obtained by cold rolling nickeland nickel tungsten alloys in passes of less than5%deformation followedby annealing at high temperature.(Somefigures in this article are in colour only in the electronic version)1.IntroductionIt is well known that biaxially textured ReBCO is necessary to obtain high transport critical current densities.Therefore, practical applications using HTS have been hampered by difficulties in fabricating these brittle oxide materials into long biaxially texturedflexible wires capable of carrying large critical currents(J c).Thus,significant efforts have focused on studying crystallographic alignment in superconducting wires.In order to obtain a high transport J c,superconducting tape should ideally consist of a network of low angle grain boundaries[1].This has prompted extensive investigations into the development of the rolling assisted biaxially textured substrate(RABiTS)[2]process at Oak Ridge National Laboratory,IBAD(ion-beam assisted deposition)[3–5],and ISD(inclined substrate deposition)[6,7].Among them, RABiTS is a relatively simpler and more cost effective technique for the production of long lengths necessary for many applications.The aim of the RABiTS process is to fabricate biaxially textured metal substrates,which involves a number of steps, namely,preparation of the polycrystalline metal,mechanical deformation of the metal and recrystallization of the deformed metal[8].When a polycrystalline metal is plastically deformed,the lattice orientation of the individual grain is altered towards a preferred orientation in which certain lattice directions are aligned with the principal directions of plastic flow in the metal.The actual orientation distribution of grains is termed deformation texture.When such a material is recrystallized it acquires a texture,which may be identical to the previous deformation texture but more often is quite different.Nickel and Ni-based alloys have become the substrate of choice for coated conductors because they form a very strong cube texture after mechanical deformation texturing and recrystallization.In addition,they have comparatively better resistance to oxidation,better chemical compatibility and small lattice and thermal mismatch with diffusion barriers such as yttria stabilized zirconia(YSZ)or cerium oxide which allows epitaxial growth of the buffer and HTSfilms[9–12].The potential for further improvement in the performance of such substrates has been increased by the production of even higher quality textured Ni and Ni alloy substrates;for example,groups have produced Ni substrates with FWHM φ=10◦–15◦and ω=7◦–9◦(Gladstone et al[13]), φ=8◦–10◦, ω=8◦–10◦(Truchan et al[14]) and φ=6◦, ω=7◦(Goyal et al[2]).In addition,Goyal et al[8]have studied Ni–W,Ni–Cr,Ni–Al,Ni–Cu and Ni–W–Al alloy substrates.These alloy substrates have reduced magnetism and have significantly increased yield strength and ultimate tensile strength,with φ=7◦–10◦, ω=6◦–8◦.In this paper,we have developed a mechanical deformation texturing method to fabricate biaxially textured sheets of Ni and Ni-based alloy substrates with desirable compositions.We also investigated the effects of mechanical0953-2048/03/091077+05$30.00©2003IOP Publishing Ltd Printed in the UK1077Y X Zhou et aldeformation and heat treatment on the growth of texture in nickel and nickel tungsten alloys with the objective of understanding mechanisms of mechanical texturing in coated conductor substrates.2.Experimental detailsFor Ni substrates,the starting material was very pure nickel 99.995%obtained from Alfa Aesar.The powder-in-tube method was used to fabricate the Ni3at%W alloy,using 99.99%purity starting Ni and W powders from Alfa Aesar. First,97at%Ni and3at%W powders were milled withagate pestle and mortar for90min,then packed in a rubber tube,CIPed and sintered to fabricate the Ni–W alloy.The Ni strips were mechanically deformed to obtain various thicknesses of(0.24,0.18and0.1mm)tape.Three values of average deformation reduction per pass,2%,5%and 10%,were used in this work.After the rolling procedure, each of the deformed tapes is cut into small pieces for heat treatment at1000◦C for90min(details in[15]),which was performed using a tube furnace in reducing atmosphere consisting of a mixture of5%H2in argon.The Ni–W alloy substrate was fabricated using almost the same conditions which produced optimal texturing of pure Ni except the higher annealing temperature to form the uniform Ni–W alloy.Deformation texturing properties were characterized by a Siemens D5000series x-ray diffractometer for the purpose of measuring the Bragg diffraction angle2θdetermined by θ–2θscans for the textured samples.The sharpness of the texture was measured using x-ray rocking curves.A Siemens general area detector diffraction system(GADDS)with a polefigure goniometer was used to obtain polefigure measurements.The polefigure is a stereographic projection, which shows the distribution of a particular crystallographic direction in the assembly of grains that constitutes the specimen.Finally,JOEL JSM6330-F,an enhanced SEM-based technique system,was also used to perform the texture analysis on the samples.3.Results and discussions3.1.Effects of mechanical deformationFigure1shows the XRDθ–2θscan for the as-received nickel samples,after mechanical deformation and annealing.After mechanical deformation,it can be seen that deformation texture has developed.During the mechanical deformation, plasticflow causes a reorientation of the lattice of individual grains and the development of a texture or preferred orientation of the lattice in the grains.The progress of reorientation is gradual;orientation change proceeds as plasticflow continues, until a texture is reached against indefinitely continuedflow of given type.The nature of stable deformation texture and the manner in which it is approached are characteristic of the material and the nature of theflow throughout the deformation process(the magnitude of the three principal strains at all points within the specimen and at successive times in the process).The significant point from this20406080(220)(200)(111)Before rolling2 Theta angle (degrees)(220)(200)(111)After rolling(200)After annealingIntensity(counts)Figure1.XRD scans for the as-received undeformed,after mechanical deformation and after annealing,nickel samples.figure is the abnormally high peak{002}and the almost complete suppression of other peaks in the annealed sample. This implies that the nickel has been preferentially textured. Also,from thisfigure it can be deduced that nickel with a sharp in-plane cube texture can be processed using this thermomechanical method.3.2.Effects of deformation per passTo study the effect of the reduction per pass on pure nickel, samples were mechanically deformed using three values of deformation per pass,namely,2%,5%and10%.Thefinal thickness reached in each sample was about0.1mm giving a total reduction of95%.The samples were then annealed at 1000◦C for90min.Figure2shows(111)polefigures for these Ni samples.For higher values of deformation per pass, significant amount of retained texture is present.Even with5% deformation per pass,the substrate still presents a large amount of retained texture.Small components of untextured materials can result in a significant fraction of undesirable high-angle grain boundaries as has been determined by electron back scatter Kikuchi diffraction[16].From the above study, it is determined that lower reductions per pass are more conducive to developing higher degrees of cube texture in the Ni tapes.In general,longitudinal as well as lateral spread was observed during mechanical deformation.In order to minimize lateral spread,the tape was held in tension while mechanically deformed;however,a substantial amount of lateral spread was still observed and was found to increase considerably with larger values of reduction per pass.One of the implications of lateral spread is that the direction of materialflow during mechanical deformation is no longer unidirectional.To obtain pure cube textured substrate after recrystallization,it is essential to have a uniaxial deformation during mechanical deformation treatment.Thus the amount of lateral spread,which depends on the deformation per pass used during mechanical deformation strongly influences the quality of the cube texture obtained after recrystallization.1078Role of mechanical deformation in the texturing of coated conductorcomposites(a) (c)(b)Figure2.(111)polefigures of mechanically deformed Ni substrate with(a)2%,(b)5%and(c)10%deformation per pass annealed at 1000◦C/90min.Figure3.(111)polefigures of mechanically deformed Ni substrate with(a)95%,(b)90%and(c)88%total deformation annealed at 1000◦C/90min.3.3.Effects of total deformationFor this study,samples with an initial gauge thickness of 2mm were cold rolled to thicknesses of0.24mm,0.18mm and0.1mm successively.The total reduction attained in each case was88%,91%and95%.A reduction rate of2% per pass was used to achieve the desired thickness in each sample.The samples were then annealed at1000◦C for about90min.Figure3shows(111)polefigures for these Ni samples.For a lower percentage of total deformation, a significant amount of retained texture was present.Even with90%total deformation,the substrate still presents a large amount of retained texture.For the sample with95%total deformation,only the peaks corresponding to the{100} 100 cube orientations can be seen in the polefigures indicating a sharp textured material.3.4.Mechanical deformation of Ni substrateAccording to the above studies,RABiTS requires careful optimization of mechanical deformation to achieve a sharp cube texture with few misaligned grains.Our work indicates that,with a deformation rate of2%per pass and total deformation of95%,one can obtain sharp cube textured substrates.Figure4shows x-rayφ-scans andω-scans for mechanically deformed textured Ni substrates with2% deformation per pass and total deformation larger than95% annealed at1000◦C for90min.The values of full width at half maximum for these substrates were6.9◦and6.1◦for in-plane and out-of-plane respectively.For these samples,the surface roughness and the microstructure were studied. The quality of the substrate surface is primarily dependent on the quality of the rolls used.For the scale-up to continuous production of coated conductors,it would be ideal if no polishing steps were needed.If polishing is to be avoided, grain boundary grooving during annealing must be minimized. Figure5(a)shows a SEM image where the grains are distinctly separated by grain-boundary grooves;however,both the depth and width of the grooves are very small,indicating the annealing temperature and holding time are reasonable. Figure5(b)shows AFM image of the Ni substrate.Roughness value(R rms)is about0.343nm calculated over an area of 1µm2which was selected randomly from the sample surface. The rms(root mean square)roughness is a very simple measure of the height profile roughness.The rms roughness describes thefluctuations of surface heights around an average surface height.This roughness indicates the surfaces of the mechanically deformed textured Ni substrates are suitable enough for further deposition of buffer layers.3.5.Mechanical deformation textured Ni–W alloy substrate The Ni3at%W alloy strip is mechanically deformed by rolling to a degree greater than95%total deformation using about2% reduction per pass and reversing the rolling direction during each subsequent pass.It is annealed at about1200◦C for about90min to produce a sharp biaxial texture.Annealing is performed inflowing4%H2in Ar.Figure6shows a (111)x-ray diffraction polefigure of the biaxially textured1079Y X Zhou et al-50 0 50 100 150 200 250 300 35050100150200c o u n tφNi (111)FWHM=6.9oθ10 1520 25 303540C o u n t s(b )(a )Figure 4.X-ray (a )φ-scans and (b )ω-scan for mechanical deformation textured Ni substrate.R rms =0.343nm(b )(a )500nmFigure 5.(a )SEM and (b )AFM micrograph of mechanical deformation textured Ni substrate.alloy substrate.As can be seen,only four peaks are evident.Each peak refers to one of four crystallographically similarFigure 6.X-ray (111)pole figure of mechanical deformation textured Ni–W alloy substrate.0500100015002000250030003500-300-200-1000100Phi ScansC o u n t sFigure 7.X-ray φ-scans for mechanical deformation textured Ni–W alloy substrate.Rocking Curve of Ni-W Substrate 02000040000600008000010000010203040The taC o u n t sFigure 8.X-ray ω-scans for mechanical deformation textured Ni–W alloy substrate.orientations corresponding to {100} 100 ,such that the (100)plane is parallel to the surface of the tape and 100 direction is aligned along the length of the tape.Figure 7shows a φ-scan of the [111]reflection showing the degree of in-plane texture.The FWHM of the tape is about 7.5◦determined by fitting a Gaussian curve to the data.Figure 8shows the rocking curve or the out-of-plane texture as measured by scanning the [200]reflection of the substrate.It is a rocking curve with the sample being rocked in the rolling direction and shows a FWHM of 4.5◦.This is truly a single orientation texture with all crystallographic axes being aligned in all directions within 8◦.Together,these results indicate that using our thermomechanical processing,we can obtain sharp texturing in Ni–W alloy substrates.1080Role of mechanical deformation in the texturing of coated conductor composites4.SummaryA study regarding the role of mechanical deformation in texturing of coated conductor composites has been carried out.Mechanical deformation per pass and total deformation have determinant roles on texturing of Ni substrate.The degree of cube texture increases with increasing percentage of total mechanical deformation and using a smaller percentage reduction per pass.To obtain pure cube textured substrate after recrystallization,it is essential to have uniaxial deformation during the mechanical deformation treatment.Thus lateral spread during mechanical deformation should be avoided because the amount of lateral spread,which depends on the deformation per pass,strongly influences the quality of the cube texture obtained after recrystallization.Our best pure Ni substrate has an in-plane alignment of6.9◦and out-of-plane alignment of6.1◦.We have also fabricated the Ni3at%W alloy substrates with sharp cube texture in-plane alignment of7.5◦and out-of-plane alignment of4.5◦.In addition,these texture characteristics are found to be reproducible. 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