BETA-PHASE INSTABILITY AND MARTENSITIC TRANSFORMATION IN Ti-22 a/o Nb ALLOY

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BETA-PHASE INSTABILITY AND MARTENSITIC TRANSFORMATION IN Ti-22 a/o Nb ALLOY

P. Hochstuhl, B. Obst

To cite this version:

P. Hochstuhl, B. Obst. BETA-PHASE INSTABILITY AND MARTENSITIC TRANSFORMA- TION IN Ti-22 a/o Nb ALLOY. Journal de Physique Colloques, 1982, 43 (C4), pp.C4-133-C4-138.

�10.1051/jphyscol:1982413�. �jpa-00222114�

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JOURNAL DE PHYSIQUE

Co2Zoqv.e C.?, suppZQmenL au n o 12, 'l'ome 45, dEcembre 7982 puge C4-133

BETA-PHASE

INSTABILITY

AND

MARTENSITIC TRANSFORMATION

I N

Ti-22 a/o

N b

ALLOY

P. ~ o c h s ~ u h l + and B. ~ b s t ++

' r ~ n i v e r s i t i i t ~ a r l s r u h e , i- n c t i t u t j G r FkperimenteZZe Kemphysik, P.R.C.

++

Kerni"orschungszenLivmochungsen L i:ur%smche, S-nstitut j'iir. Techn1:sche Physik, 1.'. R. C . (Revised text accepted 28 September 1982)

Abstract.- The variation of the electrical resistivity with temperature (4 to 300 K) and strain has been measured in solid solution Ti-22 a/o Nb. The experiments have been conducted on a tensile machine equipped with a temperature- variable flow cryostat. Depending on the sample history, the value of the temperature-coefficient of resistivity appears to be abnormal. In the as-quenched condition, the alloy shows a negative dp/dT in a certain range of temperatures.

The resistivity-temperature relation is found to be reversible (no hysteresis);

variations with ageing time have been observed. The resistance anomaly appears to be closely related to the incipient instability of the 3 phase as the system tends towards thermodynamic equilibrium. Polymorphic transformations will produce a kinetic barrier for further 6 instability.

1ntroducrion.- Alloys of the Ti-Nb system are commonly used commercial superconduc- tors. For a long time, it was a mystery that for a given field, coils of Ti-Nb wire had a much lower critical current than a short straight sample of the same wire. Re- cently, this phenomenon of current degradation has been attributed to stress induced microstructural instabilities which occur in the material itself ( 1 , Z ) . In limiting

the performance of superconducting magnets, these instabilities are of technological, as well as scientific, importance.

In this paper, the electrical resistivity of solid solution Ti-22 a/o Nb has been measured to study the resistivity-temperature-strain relationship, particularly at low temperatures. From the temperature variation of the resistivity near structural phase transitions,a simple relationship between the static aspects of a transformation and the occurrence of temperature-dependent (i.e. 'soft') modes resulting in dynamical fluctuations can be deduced. Based on this knowledge, the change of the temperature-coefficient of resistivity from negative to positive values due to a strain transition may give insight into the nature of the dynamic mechanism of a phase transformation.

Experimental and Results.- The Ti-22 a/o Nb wire samples employed in this study were prepared from a commercial superconductor. The diameter of the TiNb single core was 0.4 mm and the over-all diameter including the copper coating was 0.56 mm. After removing the copper in a solution of nitric acid, the specimens were cleaned and sealed (under argon atmosphere, together with Ti-getter) in silica tubes. Subsequent- ly,a heat treatment at 1123 K was carried out for 2 hrs and finally the tubes were water quenched to room temperature. With this procedure, the high temperature single- phase 6 (cf. Fig. 1 ) could be retained to room temperature. According to X-ray dif- fraction studies ( 3 ) , no transformationoecurred on simply cooling. Cooling to cryoge- nic temperatures after prior deformation, however, resulted in a two-phase structure.

The electrical resistivity was determined by a conventional 4-probe method using a lock-in technique. The current and potential leads were soldered to the sample after copperplating the ends again in an electrolytic bath; the distance between the potential connections was about 40 mm. The measurements have been conducted on a tcnsile machine equipped with a continuous flow cryostat. The temperature was

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982413

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C4-134 JOURNAL DE PHYSIQUE

varied in the range from 300 to 4 K at a rate of about 1 K/min. Simultaneously with the resistance, stress-strain curves could be measured, the maximum strain (force) being about 6 % (500 N). Details of the experimental set-up are described elsewhere

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In the as-quenched samples, the resistivity at 300 K is anomalous high com- pared to that of 'normal' metals and further, a negative d p l d ~ is observed down to about 80 K (Fig. 2a). For temperatures below 8 0 K, dp/dT becomes positive but the resistivity at 10K (which is slightly above the superconducting transition temperature) is still higher than that at 300 K (RRR = 0.96). On heating again to temperatures of about 240 K (which indicates a marked departure from the "linear" (, vs. T relation in Fig. 2a), no effect of cycling is observed. In the experiment shown in Fig. 2 b , the specimen has been cooled down again to 180 K (which lies roughly half- way between the two temperatures limiting the linear part of curve (a)). At 180 K, the wire was deformed in tension to about 2.4 %. After stress release, a residual strain of about I % is retained and the resistivity has decreased by about 0.5 % (as indicated with the arrow at 180 K). In addition, d p / d ~ is now found to be positive on further cooling down to 10 K and during the subsequent heating process to 300 K.

It is worth noting that deformation of as-quenched samples at room temperature or an ageing treatment may also result in a positive temperature coefficient in the whole temperature range from 300 to 10K.

As-quenched samples of the bcc alloy Ti-22 a/o Nb are easily deformed even at low temperatures. A typical stress-strain curve of a tensile experiment at 77 K is shown in Fig. 3 a . The strain obtained during loading is recovered on unloading for more than would correspond to the elastic strain. This deformation behaviour is characteristic of a "pseudoelastic" material. Provided the extent of deformation was not too large, the original undeformed state could be recovered by heating the sample to temperatures well above room temperature (500 K). This behaviour is characteristic of a "shape memory" material.

As to the electrical resistance as a function of strain, an anomaly was observed on stressing beyond an apparent yield point. The resistance vs. strain dependence was found to be non-linear and showed a hysteresis. The absolute values of the resistance were markedly below the values resulting simply from shape changes of the specimen during the tensile experiment. The discrepancy between the measured resistance values and those calculated from the specimen dimensions at any given strain level is due to a decrease of resistivity with strain. Qualitatively, this resistivity- strain dependence resulting from the resistance-strain measurements is plotted in Fig 3 b (A p in arbitrary units).

Discussion.- A characteristic feature of the Ti-Nb phase diagram (Fig. 1 ) is the body-centered cubic (6) solid solution with a complete range of solubility above

1155 K and the close-packed hexagonal (u) solid solution restricted to temperatures below 1155 K and to Ti-rich alloys (6). By its very nature, the diagram considers only thermodynamic equilibrium conditions, it yields no information on the phase di- stribution morphologically and ignores surface energy effects at phase boundaries, and strain energy effects in transformations as well. External and internal stresses, however, may change the relative stability of the phases, leading to various sequen- ces of phase transitions. Thus, the constitutional diagram is affected in a very pro- found way by stress ( 7 ) .

Ti-22 a/o Nb can easily be quenched to room temperature without precipitation of CY occurring. By this way, limitations of the phase diagram are circumvented and the alloy is in a state of thermodynamic non-equilibrium (1,2,5). As will be evi- denced in the following, this state of the matrix has a fundamental bearing in deter- mining the physical properties of Ti-Nb systems in a certain composition range.

Structural instabilities are a natural consequence of this state and various inter- mediate (metastable) phases may arise during quenching from the high temperature

B-

field to the low temperature (cc+O)-composition of thermodynamic stability.

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A producL of the lattice instability, w (hex.), may precipitate from bcc phase on rapid cooling (8-11). This w phase has been variously interpreted in terms of a transition structure between Lhe high temperature B phase and the low temperature u (hcp); L) will be metastable relative to a . The structure of w is closely related to that of fi from which it forms (12). Depending o n the nucleation kinetics, w formation may be suppressed in as-quenched samples because of another competing martensitic transformation to a", an orthorhombic metastable structure, which is a heavily distorted form of the hcp structure,

a

(4,13,14). In the Ti-22 a/o Nb alloy which forms as-quenched w phase, transition to a different kind of martensite (te-

tragonal (3,14)) can form as deformation product. (In the following, am denotes both kinds of martensite).

In Fig. 2a, one consequence of retaining the high temperature bcc phase in a metastable state at low temperatures shows itself in the abnormal variation of the electrical resistivitywith temperature, and further, evidence that the quenched B phase in such a condition is unstable with respect to mechanical stresses is given in Figs. 2b and 3.

The anomalous high resistivity at room temperature and the negative dp/dT within the temperature interval 300 K to about 80 K is thought of as manifesting an incipient instability of the bcc lattice - dynamical h1 fluctuations, i.e. short-range correlations of the ions between parent and product states (9,10,13,15) - as the alloy tends towards thermodynamic equilibrium. This lattice instability can be para- phrased in terms of anomalous decreases of phonon frequency with wave vector and with temperature, so-called phonon softening, a process which may ultimately lead to, and is interrupted by a structural instability. In this picture, the excess resistivity simply arises from additional electron scattering due to the displacement fluctuations, or their quantized equivalent, the instability-related phonons which develop with cooling over a limited temperature-range. The decrease of electrical resistivity below about 8 0 K is interpreted to indicate a change in the long-range correlations of the ions, i.e. correlated displacements over large spacetime volumes.

This leads to structural precipitates known as the w transformation of the

B

phase (15 - 17).

w forms coherently with the matrix. The preservation of coherence results in the development of elastic fields with the formation of a regular space phase distri- bution of these "elastic-concentration domains" (1,7,16). With its small lattice distortion in addition to shuffle displacements, the phonon-induced structural change 3 ^-+w serves to stabilize the bcc alloy outside its compositional range of stability and may produce a kinetic barrier against further B instability (9 - 11). As a result, with the w phase precipitate, dp/dT of Ti-22 a/o Nb becomes positive at low temperatures (Fig. 2a, T < 80 K) like in 'normal' metals. The instantaneous reversi- bility of p vs. T strongly indicates that the w transformation is diffusionless and that both the nucleation and the kinetic barrier to the athermal w reaction are extremely small (12). The transformation proceeds with the velocity of elastic distur- bances in the crystal.

The nonmartensitic quenched Ti-22 a/o Nb samples used in these experiments do not transform to a'' simply on cooling to LRe-temperature. This result which is quite surprising in view of the phase diagram (Fig. I) has been well documented in a series of experiments (3). The tendency towards a martensitic reaction was shown to be extremely sensitive to cold-working rather than to the conditions of quenching. This instability under deformation suggests the existence of a "strain spinodal" (Figs.

2b, 3 1 , i.e. the phase diagram of the Ti-Nb system (Fig. I) is severely affected by

stress.

While in Fig. 2a the B + w precipitation causes dp/dT to become positive, it is the highlystrained am marten:;ite in Fig. 2b forming in the quenched alloy during deformation that changes the temperature-coefficient of resist-ivil-yfrom negative to positive values. Again, the polymorphic tr'arlsformatim in evaluating a strain field, results in stabilization of the parent phase retained. For the displacive transformation + om to trigger, the presence of lattice defects such as dislocations, stacking faults, and boundaries are a means to overcome the nucleation difficulty.

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The lattice defects may also act as sites of a strain spinodal lattice decomposi- tion.

The shift of thermoelastic equilibrium on application of static stress not only influences the temperature-coefficient of resistivity but also gives rise to the thermo-mechanical anomalies known as "pseudoelasticity" and the "shape memory effect" (I

-

5). Fig. 3a shows a typical U

-

^Ecurve at Td = 77 K of the shape memory alloy Ti-22 a/o Nb. When the material is deformed beyond an apparent yield point, a rapid decrease in the electrical resistivity is observed (Fig. 3 b) which parti- ally recovers as the strain recovers on stress release. Again, this decrease of p with E at isothermal conditions is the result of decomposing the single-phase metastable 6 into a two-phase mixture without further structural fluctuations. Because of the essential equivalence of temperature and stress for thermoelastic martensite, the 0-value will change to the original one upon thermal activation of the alloy above the level of am stability.

Summary and Conclusion.- In 6-stabilized Ti-Nb alloys which are known to favour the precipitation of the w phase, the resistivity-temperature relation is found to be abnormal in a range of temperatures below 300 K.. This anomaly is intimately linked with the stare of the matrix which, in such a condition, is in a highly non-equilibri- umstate: The 'virtual-8' pllase will produce changes in all thermodynamic and trans- port properties. (For an excellent review of the interrelationships between lattice stability, mechanical, normal-state physical, and superconducting properties of Ti- alloys we refer the reader to Ref. (13)).

In Ti-22 a/o Nb, polymorphic transformations to w or am may eventually occur on further decrease in hcc stability with cooling. They appear to be the systems of elastic-concentration domains in minimizing the free energy (7), and they should be regarded as 'competitive' phases. Which one of the transformations will develop de- pends on the nucleation kinetics, however, both structural precipitates, w and am

,

are 6-stabilizing. The transformation from single-phase metastable 6 to a hetero- phase structure produces a kinetic barrier (stress field) for further instability and renders the value of d;/dT increasingly positive. Hence, in the "soft-mode" ter- minology, the negative dp/dT arises from electron scattering due to soft mode phonon density fluctuations in the matrix.

Theseconc~usionsconfirm earlier arguments which were inferred froma series of experiments with the B-isom~rphous system ~ i - C r (18).

Finally, we want to mention that the 6-instability of TiNb alloys does not only manifest itself in anomalies of the e l e c t r i c a l r e s i s t i v i t y , a s d i s c u s s e d in this paper, but also in their mechanical properties. Inview of that,we studied low tempera-

ture pecularities of plastic flow and its associati.on with microstructural instabilities which occur in the macerial. With measurements of elastic constants and internal friction, we try to locate transition points in the Ti-Nb phase diagram and to get information on the mechanism of structural changes.

References

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( 5 1 6 ) (1980) 595.

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HOCHSTUHL P., KfK 3141-Bericht (1981).

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⁽¹⁾ (1981) 55.

ROITBURD A.L. and KURDJUNOV G.V., Mat. Science and Eng. 39 (1979) 141.

COLLINGS E.W. and GECEL H.L. in: "physics of solid solutzn strengthening", (ed. by Collings E.W. and Gegel H.L.), Plenum Press (1975) 147.

DE FONTAINE D., PATON N.E., and WILLIAMS J.C., Acta Met.

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DE FONTAINE D. and KIKUCFII R., Acta Xet., (1974) 1139.

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12 COMETTO D . J . , HOUZE G.L.JR., and HEHEMANN R.F., T r a n s . o f M e t a l l . S o c . o f AIME, 2 3 3 ( 1 9 6 5 ) 3 0 .

13

LINGS

E.W. i n ' T i t a n i u m ' 8 0 S c i e n c e and T e c h n o l o g y ' ( e d s . Kimura H . and I z u m i O.), P r o c . 4 t h I n t . Conf. o n T i t a n i u m , Kyoto, J a p a n May 19-22 (1980)

14 HATT B.A. and RIVLIN V.G. B r i t . J . A p p l . P h y s . ( J . P h y s . D)

1

( 2 ) ( 1 9 6 8 ) 1145.

15 PERKINS J . , Met. T r a n s . 4 (1973) 2709.

16 COOK H.E., A c t a Met. 23 7 1 9 7 5 ) 1027, 1041.

17 S I K K A S.K., VOHRA Y . K ~ and CHIDAMBARAM R . , P r o g r

.

M a t e r . S c i .

27

( 3 1 4 ) ( 1 9 8 2 ) 245.

18 CHANDRASEKARAN V., TAGGART R., and POLONIS D.H., J. Mat. S c i e n c e

9

(1974) 961.

T i N b -

-

F i g . I : P a r t of t h e p h a s e d i a g r a m f o r Ti-Nb

--

a f t e r r e f e r e n c e s ( 6 , 1 3 )

The s o l i d l i n e s i n d i c a t e b o u n d a r i e s b e t w e e n e q u i l i b r i u m p h a s e s . R e g i o n s w h e r e t h e m e t a s t a b l e p h a s e s a'' and w may o c c u r a r e l i m i t e d by t h e d a s h e d and d o t t e d l i n e s , r e s p e c t i v e l y . The e x p e r i m e n t a l ?Is-curve t e r m i n a t i n g a t a b o u t 400 K i s b a s e d o n da- t a o f r e f s . ( 4 , 6 ) , t h e e x t r a p o l a t i o n i n t o t h e c r y o g e n i c t e m p e r a t u r e r a n g e may r e - p r e s e n t a Md ( d e f o r m a t i o n - i n d u c e d m a r t e n s i t i c t r a n s f o r m a t i o n ) a c c o r d i n g t o r e f . (I 3 ) . The r e p r e s e n t a t i o n of t h e B+w r e g i o n i s s c h e m a t i c and c o n s i s t e n t w i t h o b s e r v a t i o n s g i v e n i n t h e l i t e r a t u r e . The v e r t i c a l l i n e i n d i c a t e s t h e p o s i t i o n o f t h e T i - 2 2 a / o Nb s p e c i m e n s w i t h r e s p e c t t o c o m p o s i t i o n . T h i s a l l o y i s w e l l l o c a t e d i n a r e g i o n w h e r e , d e p e n d i n g o n t h e q u e n c h i n g c o n d i t i o n s , q u e n c h i n g f r o m t h e 6 - f i e l d ( I ) t o room tem- p e r a t u r e ( 2 ) may r e s u l t i n t h r e e d i f f e r e n t p r o d u c t s t a t e s : H e t e r o p h a s e B+aW o r R+w, o r s i n g l e - p h a s e " v i r t u a l - B " ( 6 , 1 3 ) .

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JOURNAL DE PHYSIQUE

99 I I

96

-

Fig. 2: Electrical resistivityvs.

temperature for bcc Ti-22 a / o ~ b .

2hrsl1123K-water-quenched The sample in the as-quenched condition shows a negative temperature-coefficient of resisti-

Td = 180K-Elrr = 1% vity in a certain range of tern-

peratures (a) while dp/dT becomes positive after deforming

t s c . , I 1

the sample at 180 K (b).

0 100 200 300

T

[Kl-

100 2hrs/1123 K

-

water-quenched

Fig. 3: Effect of isothermal tensile deformation in as- quenched Ti-22 a/o Nb.

(The specimen was wire shaped with a diameter of 0.4 nun and an active length of 4 0 mm).

The stress-strain curve shows pseudoelastic recovery upon stress-release, the origina1,undeformed state recovering during a heating process to 500 K. Upon stressing beyond an apparent yield point, the resistivity decreases with strain (for details see text).