C. Lal'balestier, X. FidCh, 0.6. rlawksworth, ?roceroln?s of 6th lnternatilnal. Conference on iiaqaec ircilno:oyy (;.;7-6), Bra~islava, Czechoslovakia, Yul. ;, pp.
IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-15, NO. 1, JANUARY 1979
400
GENERALIZED CRITICAL AND Nb53w/o
CURRENT DENSITY OF COMMERCIAL Nb46.5, Nb50 T i MULTIFILAMENTARY SUPERCONDUCTORS
K. F. Hwang* and0.
ABSTRACT We have measured the c r i t i c a l c u r r e n t d e n s i t y J co ff o u rd i f f e r e n t commercial compositions of NbTi over the range of f i e l d s up t o H and a t reduced temperatures. We have presentedcfheresultsin scaledform so t h a t a t t a i n a b l e v a l u e s of J c i n NbTi a t a r b i t r a r y f i e l d s and temperatures may e a s i l y b e derived. We have a1 so measured the e f f e c t ofpost manufacturethemaltreatments on J c and shown t h a t t h e change in Jc scales with a temperaturecompensated timefactor. INTRODUCTION NbTi a l l o y s a r e s t i l l t h e most widely used superconducting a1 loys and h i g h c r i t i c a l c u r r e n t d e n s i t i e s can be obtainedover a range of a l l o y contents. To a c e r t a i ne x t e n td i f f e r e n ta l l o y compositions have become standardindifferent T i i n B r i t a i n , Nb46.5 w/o c o u n t r i e s , e.g. Nb44w/o w/o Ti i n Germany. All Ti i nt h e U.S.A, and Nb50 ofthesealloycompositionsarecapable of giving J > 1700 A/mm2 a t 5 T and 4.2 K. However, i t has aCso been reported that values twice as large as t h e s e can e obtained from Nb50 Ti when properly optimized.' F o r superconductingmagneticenergy storage (SMES) a p p l i c a t i o n s NbTi i s on$ of the two and we majordeterminants of the s t o r a g ec o s t have t h e r e f o r e been s t u d y i n g t h e p o s s i b i l i t i e s and c h a r a c t e r i s t i c s of highJc NbTi a l l o y s of d i f f e r e n t composition.withparticularinterest. In t h i s paper we report on t h e c r i t i c a l c u r r e n t p r o p e r t i e s of four d i f f e r e n t commercial a l l o y s , of nominal compositions Nb46.5, Nb50 (2) and Nb53w/o Ti a t f i e l d s from 0 t o Hc2, both a t 4.2 K and a t reducedtemperatures. A common m i c r o s t r u c t u r a l f e a t u r e of each of thesecompositions i st h at h e y a 1 possess a high cm/cm ) and t h a t t h e s e d i s l o c a t i o nd e n s i t y ( Z 10" dislocationarrangementsare of decisiveimportance A t some s t a g e s ,t y p i c a l l yw i t h i n i nd e t e m i n i n gJ c . 4-6 timesthefinalwirediameter and a t t h e f i n a l wire size, the manufacturer gives controlled heat treatmentstothewire,theeffect of which a r e t o c o n s t r u c t i v e l y modify t h e d i s l o c a t i o n d i s t r i b u t i o n and possiblyencourage a p r e c i p i t a t i o nr e a c t i o n . The high d i s l o c a t i o n d e n s i t i e s a r e , however, thermallyunstable and t h e i r s e n s i t i v i t y t o t h e exactheatinput of the thermaltreatment is probablythecause of t h e g r e a t e r p a r t of t h e batch t o b a t c hs c a t t e ri n J seeninproducing 3 nominallyidenticalbillets'ofsuperconductor. In l a r g e magnet systemsthere may, i n a d d i t i o n , be postdeliveryheatinputsreaching the NbTi and we have followedtheresponse of t h e s e a l l o y s t o post production heat tredtments partly to establish s a f e l i m i t s f o r such inputs and p a r t l y t o g i v e us
i
28, 1978. ManuscriptreceivedSeptember *EngineeringExperimentStation, Univ. ofWisconsin, 1500 JohnSon Drive, Madison, Wisconsin 53706.
C. L a r b a l e s t i e r
the r o l e o f microA preliminaryreport has a1 readyappeared.
fundamentalinformationabout s t r u c t u r e i n determining Jc. o f t h i s work f o r Nb46.5w/oTi
EXPERIMENTAL PROCEUURE Four wires were studied of nominal composi t i o n two Nb46.5 Ti (MCA) , Nb53 T i (Airco)togetherwith samples of Nb50 Ti (Vacuumschmel z e ) Thisrange covers a compositionin which few a-Ti p r e c i p i t a t e s a r e found (Nb46.5 T i ) , a composition, lNb50 T i , i n which t h e an be finelydispersed and extremely e f f e c t i v 3 " and a composition, Nb53 T i i n which Jc theycan form rather easily, permitting high with less cold work than i s needed for the lower Ti alloys.Otherdetails of thecompositesare 1 . Alloycompositionsofthe Nb 50 giveninTable and Nb53 Ti a1 loys were determined by e l e c t r o n probemicroanal{sisagainstcalibratedstandards. Ti i s s t a t e d t o The t o l e r a n c e o t h e Nb46.5w/o be c l . 5 w/o Ti.
.
rl
TABLE 1 Sample S p e c i f i c a t i o n
