The superconducting transition behavior of ternary alloys of titanium

Scripta

METALLURGICA

V o l . 8, pp. 3 2 9 - 3 3 6 , 1974 Printed in t h e U n i t e d States

THE SUPERCONDUCTING

Pergamon

Press,

Inc.

TRANSITION BEHAVIOR OF TERNARY

ALLOYS OF TITANIUM

by W. L. Cotton,

R. Taggart,

College of Engineering,

Seattle, WA

(Received

and D. H. Polonis

University

of Washington

98195

February

i,

1974)

Introduction It has been shown that ternary additions

of the alpha stabilizing

elements

inhibit the formation of the omega phase and promote a phase separation beta stabilized Ti base alloys retained

in quenched alloys,

of the electron:atom

(i).

The omega phase can be formed or the beta phase completely

if the addition of high valency

ratio in excess of four (2).

values of the superconducting

transition

ternary elements

It is generally recognized

temperature

leads to values that high

depend upon an electron:atom

between 4.6 and 4.8 (3), which would require the addition of high concentrations or group 6 elements

to either Ti or Zr base alloys.

nominal composition

the phase separation

employed zones;

to increase

In beta stabilized

ratio

(e/a)

of group 5

alloys of low

reaction or the omega reversion process can be

the value of e/a in the beta matrix due to the formation of solute lean

this approach is expected

These fundamental designed

Sn, AI, and O

reaction in certain

concepts

to test the feasibility

to enhance the superconducting

form the basis for a series of experiments of increasing

to alloys of the type Tiy(NbaXl_a)l_y. fraction and the conposition

properties

the value of T

of such alloys. that have been

by adding a ternary element X

c Thermal treatments have been used to vary the volume

of the beta phase and the microstructural

of the beta and alpha phases, while at the same time maintaining

distribution

of mixtures

the value of e/a in the beta

matrix in the range 4.6 - 4.8. Alloys of the titanium-niobium

system containing

ternary additions

of either molybdenum

or aluminum were selected on the basis of the following considerations: i.

The Ti-Nb binary system exhibits Type II superconductivity

the presence of non-superconducting length for the alloy

precipitates

that can be enhanced by

having a spacing of the order of the coherence

(4), e.g., a few hundred Angstroms

(5). o

2.

The atomic radii of Ti, Nb, Mo, and A1 are 1.467, 1.456, 1.386 and 1.429 A respectively

These values provide the suitable matching of the atomic size that is necessary solubility during solution treatment at elevated 3.

In alloys based on titanium and zirconium, molybdenum

it possible

to achieve e/a > 4.5 at relatively

is a beta stabilizer

low solute concentrations.

329

for beta

temperatures. that makes

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TRANSITION

OF

TERNARY

ALLOYS

OF

Ti

Aluminum is a Group III element which stabilizes the alpha phase

Vol.

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4

and inhibits the

formation of the omega phase in alloys based on Ti and Zr. 5.

By combining Ti-Nb alloys with ternary additions of AI or Mo the value of e/a can be

varied readily over the range 3.8 to 5.0 in alloys of the type Tiy(NbaXl_a)l_y where X is either Mo or AI.

These alloys provide an opportunity to evaluate the suggestion by Matthias

(6), that the value of T

c

should be highest when the electron:atom ratio is between 4.6 and 4.8. Experimental Procedure

The ternary alloys were prepared by levitation melting in an atmosphere of purified helium.

The Ti-Nb-Mo alloys contained from 2 to 62 at.% Nb and from 3 to 39 at.% Mo.

Ti-Nb-AI alloys contained from 5 to 45 at.% Nb and from 5 to 25 at.% AI.

The

Binary Ti-Nb and

Ti-Mo alloys were prepared for reference purposes, and contained from 2 to i00 at.% Nb and 3 to 39 at.% Mo, respectively. Each alloy charge was levitated until the component elements were molten and thoroughly mixed; the melts were then chill cast to produce ingots 0.5 cm dia. and 5 cm in length.

The

ingots were individually wrapped in tantalum foil, to getter any residual oxygen, and sealed in Vycor capsules at a vacuum of 10 -6 tort for solution treatment at I050°C, followed by either quenching or slow cooling. The specimens were quenched to room temperature by breaking the capsules in a water bath; the subsequent omega precipitation and reversion heat treatments were performed in neutral salt baths at 350°C and 500°C respectively.

The specimens that were transformed isothermally

in the ~ + 8 region were cooled from the homogenization temperature at a rate of i00 - 150°C per hour and subsequently aged for 40 hours at 750, 650, or 550°C. The superconducting transition temperature was measured using a germanium thermometer and an induction coil (7).

The transition temperature T c is defined by the value correspond-

ing with 50% of the inductance change. Needle specimens for x-ray studies were prepared by rolling portions of ductile samples to a thickness of 0.2 - 0.5 mm, followed by encapsulation, heat treatment, fine grinding, and etching in a solution containing 14 parts HNO 3 to one part HF.

Debye-Scherrer x-ray patterns

were used to identify the constitution of the heat treated alloys. Experimental Results and Discussion The values of T c were measured for alloys of the type Tiy(NbaXl_a)l_y where X was /either AI or Mo.

The thermal treatments include the quenched, slow-cooled, and isothermally tr~6s-

formed conditions.

Reversion treatments were employed to produce solute fluctuations in the

beta phase in those alloys where it was possible to form the omega phase during low temperature aging. a.

Ti-Nb-Mo Alloys Figure i shows the transition temperatures of Ti-Nb-Mo alloys in the as-quenched condition

plotted as a function of the nominal e/a ratio.

This family of curves provides a means for

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TERNARY

ALLOYS

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Ti

531

I0

r,-n,-

6

~E Z O_ ~-- 4 u3 Z h.I.-

2

42

4.0

4.4

4.6

4.8

5.0

E L E C T R O N : A T O M RATIO

FIG. i.

The transition temperatures of Ti-Nb-Mo alloys in the as-quenched as a function of the nominal e/a ratio.

evaluating

condition plotted

the combined contribution

of all three elements on the value of T . Since Ti, Nb c and Mo have 4, 5, and 6 valence electrons, respectively, the e/a ratio of an alloy of the type Tiy(NbaMOl_a)l_y

is related to the alloy composition

as follows:

e/a = 4y + 5a(l-y) + 6(l-a)(l-y) A ternary alloy with a fixed Nb content of 12 at.% and an e/a of 4.8 should contain 54 at.% Ti, 12 at.% Nb, and 34 at.% Mo. The transition

temperature values for the binary Ti-Nb and Ti-Mo alloys are in agreement

with previously published values not show the beneficial

(8,9).

The addition of the ternary element molybdenum did

effect predicted on the basis of the model proposed by Matthias

and tended to depress the value of T

c

(6),

well below that of the binary alloys of equivalent

niobium content. The reduction

in the value of T

c

due to the addition of molybdenum is demonstrated

Figure 2 and Table I which represent many measurements the Ti-Nb-Mo

ternary system.

and thermal treatments

in

for alloys of

The addition of 9 at.% Mo to a Ti-Nb binary alloy containing

approximately condition,

52 at.% Nb reduces the value of T from 9.83°K to 6.98°K in the quenched c whereas a value of 7°K is exhibited by a binary Ti-Nb alloy containing only 22 at.%

Nb. The aging of the as-quenched

specimens at 350°C for periods up to 40 hours had a signifi-

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SUPERCONDUCTING

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OF TERNARY

ALLOYS

OF

Ti

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Nb

/ I+A "7/f/ / / I t l : JS/ S // i

(046°)-~6

o

~1 o 5 . , 5 0 - 2 . 6 °

o=o,

2.1 °

( 0 , 9 6 °)

o

\_2o TRANSITION TEMPERATURE (°K) COMPOSITION (at%) O EXPERIMENTAL ALLOY COMPOSITIONS v COMPOSITION MARKED IN INCREMENTS OF IO at % ALONG AXES. NUMBERS INDICATE Tc VALUES FOR CORRESPONDING COMPOSITION POINTS ON DIAGRAM

FIG. 2.

Contours of constant values of T c for Ti-Nb-Mo ternary alloys in the beta stabilized condition.

TABLE I Transition Temperature Values For Heat Treated Ti-Nb-Mo Alloys

e/a

% Nb

% A1

Tc Furnace Cool

4.22

12.90

4.50

4.87

4.40

12.00

4.50

12.00

4.60

12.00

23.90

12.00

4.80

Tc Furnace Cool + 40 hrs. @ 550

Tc Water quenched

5.95

5.46

14.00

5.15

5.42

19.00

4.65

4.87

4.18

4.31

34.00

2.95

3.34

4.17

3.84

6.80

6.77

7.20

7.21

4.20

2.00

9100

4.50

32.00

9.00 9.00

4.15 3.60 7.01

4.60

42.00

4.70

52.00

9.00

7.20

6.98

4.50

20.00

15.00

5.45

5.62

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cant effect only on those alloys having a value of e/a in the vicinity of 4.2. process broadened

the superconducting

a double transition

transition

in the aged Ti-12Nb-4Mo

by as much as 1.2°K with the progressive aging.

When the temperature

The aging

zone in the impedance curve and resulted

alloy.

The corresponding

value of T

in

was lowered

c of the isothermal omega phase during

development

of the aged alloy was raised to 55°C for two minutes

the omega phase the value of T

333

OF TERNARY ALLOYS OF Ti

to revert

such c behavior is consistent with that reported for quenched and aged Zr-Nb alloys in which the omega phase was reverted

became equal to that of the original quenched condition;

(i0).

For alloys in which alpha precipitation

did not occur, furnace cooling resulted in a

decrease of 0.2 to 0.8 ° in the value of T . An increase in the value of T of i - 5 ° occurred c c after furnace cooling the alloys containing Mo as a ternary element and having a value of e/a less than 4.13. 4.22.

Similar results were obtained for Ti-Nb alloys with a value of e/a less than

These observations

solute enrichment

can be explained on the basis of alpha precipitation which promotes

in the beta phase,

thereby increasing

the bulk value of T c for the phase

mixture. The as-quenched

Ti-Nb binary alloys remained ductile over the entire compositon range.

Binary Ti-Mo alloys became brittle at concentrations

over 15 at.% and in Ti-Nb-Mo alloys

brittleness was encountered when the value of e/a exceeded 4.6. b.

Ti-Nb-AI Alloys The transition

function of e/a.

temperatures

of as-quenched

Ternary alloys containing

Ti-Nb-AI alloys are shown in Figure 3 as a

from 5 to i0 at.% AI showed sharp transitions

io

8

J r~ t'~

bJ I-Z _0 F-

4

z oc I.-

2i

$.8

FIG. 3.

The transition ratio.

4.0

temperatures

4.2 4.4 ELECTRON'ATOM

of as-quenched

4.6

I 4.8

I

2.0

RATIO

Ti-Nb-AI alloys as a function of the e/a

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ALLOYS

O F Ti

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from the normal to the superconducting state and a relationship between T that exhibited by the dotted Curve f o r t h e Ti-Nb binary alloys.

and e/a similar to c The ternary alloys containing

from 20 to 25 at.% AI showed a temdency for broad and double transitions.

There was no apparent

relationship between the values of T and the nominal e/a ratio for these alloys, thereby c suggesting that aluminum contents exceeding 15% alter the constitution of the alloy so that the influence of the beta phase is overshadowed.

When a mixture of the alpha and beta phases is

produced in Ti-Nb-AI alloys the aluminum separates to the alpha phase and the beta phase becomes enriched in niobium, leading to an improvement in the value of T original as-quenched condition.

compared with the c This is illustrated by the results for an alloy containing

45% Nb and 10% AI, for which T increases from 5°K in the quenched condition to 9.1°K after the c alloy was furnace cooled to Promote the formation of an alpha plus beta mixture. Thermal treatments designed to enrich the Nb content of the beta phase make it possible to approach the maximum T

c

value of 9.9°K in the ternary Ti-Nb-AI alloys.

The T

values increased by less than 1 ° from the as-quenched condition following excessive c aging and reversion treatments at 350°C and 550°C respectively. This stability of T is c attributed to the suppression of omega formation by the alpha stabilizing influence of aluminum. Aging for 40 hour periods at 550°C after furnace cooling generally increased the value of T for pseudo-binary alloys containing up to 25% Nb as shown in Table II.

c

The decrease in the

TABLE II Transition Temperature Values For Heat Treated Ti-Nb-AI Alloys

e/a

% Nb

% AI

TC Furnace Cool

TC Furnace Cool + 40 hrs. @ 550

Tc Water quenched

3.97

6.57

9.36

6.65

7.17

2.98

4.01

10.23

9.65

6.55

7.10

3.51

4.05

14.98

9.99

6,73

7.17

3.81

4. I0

19.55

9.75

6.24

7.65

4.70

4.11

18.02

6.07

6.98

7.15

4.80

4.15

24.97

10.03

6.36

7.89

5.00

4.25

34.94

i0.00

8.82

7.90

6.20

4.35

45.00

10.03

9.10

7.89

4.60

value of T

after aging the higher Nb alloys for 40 hours at 550°C reflects the dissolution of c the alpha phase at the aging temperature. The subsequent redistribution of solute leads to a

decrease in the average niobium content of the beta phase.

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335

Conclusions The studies conducted sufficient alloys.

criterion

so far have made it clear that the electron:atom

to use as a basis for predicting

This fact becomes particularly

having a non-uniform the electron:atom

distribution

concentration

that of the alloy. electron:atom

ratio is not a

the optimum value of T in multi-component c evident in alloys containing more than one phase and

of the solute elements.

of the superconducting

Furthermore,

In such cases it is apparent

that

phase itself may be quite different

the addition of a third element

than

that tends to raise the

ratio can reduce the value of Tc, as revealed in the present work for Ti-Nb-Mo

alloys. Niobium additions

are especially

effective in enhancing

for the beta c factor is the distribution of the niobium

phase of titanium alloys, but an equally important within the alloy. increase

the value of T

This is evidenced by the fact that appropriate

the value of T

for a beta stabilized

thermal treatments

can

alloy if solute enrichment of the beta matrix is

c promoted by invoking phase separation processes. creased by ternary additions

or treatments

Alternatively, the value of T can be inc that promote alpha phase formation accompanied by

beta phase enrichment. This work is part of a program sponsored by the Division of Research,

U.S. Atomic Energy

Commission under Contract No. AT(45-1)-2225-TI3. References

i.

J. C. Williams,

2.

C. A. Luke, R. Taggart and D. H. Polonis,

3.

B. T. Matthias,

4.

J. D. Livingston and H. W. Schadler,

5.

C. Baker and J. Sutton, Phil. Mag., Vol. 19, p. 190, 1969.

6.

B. T. Matthias, Prog. Co., Vol. 2, 1957.

7.

T. S. Luhman,

8.

J. K. Hulm and R. D. Blaugher,

9.

J. C. Ho and E. W. Collings,

i0.

B. S. Hickman,

and D. H. Leslie, Met. Trans.,Vol.

2, p. 477, 1971.

Jnl. of Nucl. Mtls., Vol. 16, p. 7, 1965.

Physics Today, p. 23, August 1971. in Mater.

in Low Temp. Physics,

Ph.D. dissertation,

S. L. Narasimhan,

Prog.

University

Sc., Vol. 12, p. 249, 1964.

edited by J. C. Goster, North Holland Publishing

of Washington,

1970.

Phys. Rev., Vol. 123, p. 1569, 1961.

Phys. Letts., Vol. 29A, p. 206, 1969.

R. Taggart and D. H. Polonis,

Jnl. Nucl. Mtls., Vol. 43, p. 258, 1972.