The mechanical properties of some ductile niobium and tantalum base alloys prepared by electron beam melting

JOURNAL OF THE LESS-COMMON METALS

69

I. Melting, Sintering and Working

THE MECHANICAL PROPERTIES OF SOME DUCTILE NIOBIUM AND TANTALUM BASE ALLOYS PREPARED BY ELECTRON BEAM MELTING H. R. WITH,

JR., J. Y. I<. HIJ11, A. L)ONLEVY

Temescal Metallurgical

Corporation,

(Received

Richmond.

AND

C. d’A. HIJNT

Calif. (U.S.A.)

June Srd, rg6o)

SUMJiARY This paper presents the results of studies made on a relatively small number of niobium and tantalum base alloys. Using impact transition temperature as a criterion for initial evaluation, a number of alloys have been selected for high temperature tensile and creep rupture tests as well as fabrication studies. These data are being obtained at the present time, and more complex modifications of these alloys are being explored. Tantalum alloys containing more than 20% tungsten are very brittle and this limits the usefulness of the combination. Present investigations in our laboratory, as well as at others, are directed toward the development of more complex alloys to take advantage of the exceptional ductility exhibited by tantalum base alloy systems. Niobium alloy development is still in its infancy. Although the electron beam process restricts the number of combinations of alloying elements that can be used, there is a good possibility that, in addition to tungsten, molybdenum, and zirconium, rhenium and hafnium may materially improve high temperature properties. Economic reasons, however, restrict the amounts of these latter two additions that can be used at present. -4lthough at other laboratories there are higher strength niobium alloys being prepared than the ones discussed here, they are considerably less fabricable and ductile. The efforts of this investigation have been directed toward the development of alloys with good fabricability rather than stronger alloys with severe fabrication problems. IKTHODIJCTION Commercial scale electron beam melting has made possible the routine production of exceptionally high purity refractory metal alloys. This high purity is essential if

ductility and fabricability in such alloys.are to be combined with reasonably good mechanical properties at elevated temperatures. The first alloy development programme at Stauffer-Temescal was conducted on the molybdenum and tungsten systems. Unfortunately, the initial results indicated that electron beam melting produced no significant improvement in the impact transition temperature of even very pure molybdenum and tungsten. Since the magnitude of this particular property is related directly to problems in the fabrication of these metals, particularly in welding, the alloy development effort was shifted to niobium and tantalum base alloys whose impact properties are considerably better. The purity of these alloys is also critical, not only with respect to the interstitial ,J. I.ess-Common

Metals, z (1960) 69-75

70

H. R. SMITH, JR., J. Y. K. HUM, A. DONLEVY,

C. D’A.

HUNT

impurities such as oxygen, nitrogen, carbon, and hydrogen, but also with respect to volatile metallic impurities such as iron and nickel. Electron beam melting provides a means of producing these exceptionally high purity alloys, but there are two main disadvantages: (I) because it produces a metal or alloy of higher purity with accompanying greater ductility, the net result is generally a weaker alloy; and (2) there are certain common elements which cannot be alloyed with tungsten, niobium, tantalum, and molybdenum because they are lost by evaporation during the melting process. These elements, chromium, titanium, vanadium, and, to a lesser degree zirconium, appear to be useful primarily in imparting oxidation resistance to niobium and tantalum base alloys. The first disadvantage can be rectified by the addition of more of the non-volatile alloying elements to replace the effect of the lost interstitials and volatile metals. The second disadvantage appears to be minor in the production of the ductile niobium and tantalum base alloys in view of the evidence that these alloys are ductile ody if the volatile metals are substantially eliminated. The development effort at Stauffer-Temescal was directed toward the production of alloys of tantalum and niobium possessing good high temperature strength coupled with good fabricability. Further, the alloys were developed so that they could be forged or worked at steel forging temperature where, with only ordinary precautions, oxidation losses are small. This is in contrast to the numerous alloys developed in recent years having good high temperature strength, but with very limited fabricability. Since a major objective in all high temperature alloy programmes is to form sheet stock, another criterion for a suitable alloy is that it can be either warm or cold rolled to sheet. M&h of the high loss in the usual fabrication of refractory metal alloys has been due to rolling at high temperature. Low temperature rolling not only reduces the losses, but produces better surfaced sheet. As a matter of general practice in this alloy development work, Charpy V notch impact tests were made on an alloy as soon as practicable. The transition temperature for the brittle to ductile failure was determined for each alloy. At temperatures above the impact transition temperature, fabrication has generally been successful. Below the transition temperature, fabrication has always been unsuccessful. Considerable effort has been expended to develop a multipurpose alloy for high temperature use; one that would have high strength, high oxidation resistance, and good fabricability. This combination of requirements appears to be complicating many alloy development programmes unnecessarily, since the oxidation resistance is incompatible with the other properties. The programme described here has been directed toward attaining high strength wjth good fabricability. It is believed that good oxidation resistance will only come with coatings and that an alloy with a good combination of the three properties is of doubtful feasibility. PROCEDURE

Tantalum- tungsten alloys It has been reported in the literature that tantalum and tungsten form a continuous series of solid solutions with one another. However, until quite recently, the only fabricable alloy of tantalum-tungsten was a 92% tantalum-8% tungsten alloy. J.

Less-Common

Metals,

2

(1960)

69-75

MECHANICALPROPERTIESOF Ni AND Ta BASE ALLOYS With

the advent

tungsten

tungsten

the tungsten

tantalum-15%

alloy presented content

tungsten

talum-10%

tungsten

production The

up to 20%

alloys of tantalum-tungsten

have been prepared.

The 20% tions,

of electron beam melting,

71

fabrication

was reduced.

is marginally

is completely

problems,

It appears

useful

so, for production

at the present

as a production

fabricable

time that

item,

and reproducible

but 90%

opera85% tan-

in its properties

at

levels of several tons per month.

90% tantalum-10% tungsten alloy produced by the electron beam furnace 4-6 in. in diameter and approximately 40 in. long. These are

is in the form of ingots forged

by

2,200”F.

conventional The

forging

2,000-2,200°F

drop

hammer

temperature

since the material

Under these conditions

techniques

must

after

has very high strength,

no difficulty

they

be maintained

have

been heated

in the temperature

to

range

even at these temperatures.

has been experienced

in forging the alloy. It can

be both upset and pierced. For sheet production, removed

the ingots

are forged

by either grinding or machining

for one hour. Rolling

commences

TABLE

forged bar

go% Ta-10% forged bar

W

Ta-10%

2040 3720 3600 4050

5060 (2793) 5060 (2793) 4880 (2693) 5300 5030 4865 4850

W sheet

(2927) (2777) (2685) (2677)

I.920

2.470 3,230 3,230 146,000

as-rolled-go”/OR.A.

20% Elong.

81,400. 75.7oo* 72,4oo* 75.7oo* 59,roo*

1598 ( 8701 1604 ( 873)

0.060 in. thick

to 500°F.

I

5300 (2927)

W

the oxide skin is

annealed at 2,600’F

OF TANTALUM-TUNGSTENALLOYS

R.T.

go%

slabs,

after the annealed slab has been reheated

ULTIMATETENSILESTRENGTH

85% Ta-15%

into suitable

and the slab is vacuum

1795 ( 980)

I795 ( 980) 2008 (1098) 2015 (1102)

61,100*

* All failed outside gauge length. Estimated elongation about ~-IO%. TABLE II ULTIMATETENSILESTRENGTH

2000

2400 2600 2800

3000

0~~90%

0.0055

0.0055 0.0047 0.005 0.0056

Ta-10%

WALLOY

41,600

44,100 37.600 35,400 26,900

J. Less-Common

Metals, 2 (I@)

69-75

H. R. SMITH, JR., J. Y. K. HUM, A. DONLEVY,

72

C. D’A. HUNT

As a general rule, one reannealing operation is sufficient to roll a plate from Q in. down to 0.060 in. thick. After rolling, the sheet is generally annealed at 2,400’F for one hour for maximum fabricability. High temperature tensile tests performed in vacuum have been made on 90% tantalum-10% tungsten and 85% tantalum-q% tungsten alloys. The results are presented in Tables I and II and in Fig. I.

0

600

1600

2400 3200 4000 Tempemture *F

4500

Fig. I. Mechanical properties of electron beam meltedgo% Ta-10% W alloy.

1600

16CCI

20X1

2200 2400 Tem!xrature ‘F

2600

Fig. 2. Annealing of a go% Ta-10%

2800

3000

W alloy.

Tests to determine the recrystallization temperature of 90% tantalum-10% tungsten have been performed on sheet that has been cold rolled to 90% reduction in area. The material is about 50% recrystallized in 15 min at 2,200°F (1,200°C) and is fully recrystallized in 15 min at 2,500’F (1,370%). (see Fig. 2) Niobium alloys In the very high temperature range, tantalum and tungsten are the only readily available metals, but in mid-range the candidates based on melting point multiply rapidly; niobium, tungsten, tantalum, molybdenum, and, possibly hafnium, zirconium, titanium, vanadium and chromium enter into consideration. With electron beam melting, niobium, tungsten, tantalum, molybdenum, and zirconium are about the only possible metals that can be used. The others, with the exception of hafnium, are too volatile. Hence, various combinations of niobium, tungsten, tantalum, molybdenum, and sometimes, small amounts of zirconium have been melted into alloy systems and evaluated. Impact tests at room temperature can eliminate many candidates. A completely J. Less-Common Metals,

2

(1960) 69-75

MECHANICAL

PROPERTIES

OF

Ni AND Ta BASE ALLOYS

73

brittle fracture at room temperature indicates that the alloy will roll only at high temperature. The transition curve is determined for each alloy. An alloy with an impact transition above about 500°F is rejected. As a general rule, it is found that 15% tungsten and 4% molybdenum represent the practical limits for alloying with niobium. If the indicated limits are exceeded, it is found that the ductile brittle transition will be about 500°F. Tantalum additions do not materially affect the impact properties of niobium, although it is reported that additions of up to 30% tantalum materially improve the oxidation resistance of niobium. Tungsten additions are also reported to improve the oxidation resistance of niobium. Therefore, in determining suitable niobium alloys the contents of tungsten, molybdenum, tantalum and small amounts of other alloying elements are balanced so as to obtain an alloy that has the maximum high temperature strength consistent with a high strength-to-weight ratio and ease of fabrication. Oxidation resistance is not considered specifically in the judgment of alloys. Considerable work has been performed on various combinations of niobium, tantalum, molybdenum and tungsten. Many alloys have been eliminated owing to high transition temperatures and it is possible that some alloys worthy of further study have been eliminated by this technique. However, because of the complexity of the systems involved, it is thought that this is a reasonable first screening process (see Fig. 3).

-200

-150

-100

-50

0

50

loo 150 200 Tempwatwe *C

250

300

400

450

500

Fig. 3. Keyhole impact strength of niobium base alloys.

Tables III-VI list a number of niobium alloys that are either under investigation or have been subjected to preliminary screening. Table V gives the results of tests on some modified (grain refined) alloys. Tables VI and VII refer to alloys of the same general composition of 60% niobium, 30% tantalum and 10% other metals. J. Less-Common

Metals, 2 (1960) 69-75

Ta-10%

W

Modified 60% Nb-30% annealed

60% Nb-30% annealed

W

W

* Material was heated to 2,600°F,

Ta-10%

W

60% Nb-30% Ta-10% as cold rolled

Alloy

:::: 67.0 65.0 70.0 60.0

68.5 67.75 62.5

Ta-10%

D3 D4 D3 D3 D3 D3 D3 D3

Nb

Modified 60% Nb-30% as cold rolled

223 199 204 197 zzz

,221

207

206

205 D3

Alloy number

I.5 5.0

-

MO

V

2x.100

22,000 20,100 19.500

0.010

0.0083 0.0083 0.008

2,000 2,000

2,000 2,200 2,200

Air

Air Air Air Air

Q

I4 15 16 17

for 5 set, then dropped to 2,200°F,

22,300 21,900 23,800

held for 15 set and tested.

24,400

0.0046 0.005 o.oog

21,000

I4.900 I4,6oO 13.400

15,000

12.3 13,5 8.4 9.6

9.6 9.7 11.4 I5*400 15,200 I6.000

2,000 2,000

13.8 11.3 8.9 9.8 38,000 32.700 18,600 20,300

0.0056 0.0056 0.0067 0.0067

43,000 36,500

Air

12.7 12.8 11.7 10.3

;

II

18 18 16

Em-“(p.s.i.)

38 18

33

16 18

I7

79 79 36

39,600 40,900 33,800 21,700

2,000 2,200 2,400 *

Elong. %

;;

75

59

86 57

R.A. %

45,800 46,700 39,400 24,200

Helium Helium Helium Helium

0.0060 0.0056 0.0059

o.oogg

IO 13 12

2 3 4

I

39,700 32,600 42,000 4’,000 57,600 43.300

41,5**

39.100

Yield stm@k (p&i.)

Yield stm@k (psi.)

AT Z,OOO*F

Ultimate strength (P&i.)

65,200 79.500 go,600 87,900 65,900 67.000 73,700 96,700 125.6oo

ALLOYS

strain rate par sex

BASE

K

V V V V V

Ultimate strmt$k (psi.)

FORGING

2,000 2,200 2,400

ON NIOBIUM

AFTER

V notch

ALLOYS

2,000

TESTS

TABLE

52

I7I

2

231 267 37

Impact stmgtk at room tmp. (ft. lb.)

III

Helium Helium Helium Helium

MECHANICAL

I.5 0.75 2.5 5.0 3.0 5.0 10.0

W

TABLE OF SOME NIOBIUM

Temp. “F

3.0 3.0 -

10.0

%

PROPERTIES

Atmosphere

Coupolt

:: 30

30 30 30 30 IO 30

TO2

composwJn

MECHANICAL

MECHANICALPROPERTIES

OF

Ni AND Ta BASE ALLOYS

TABLE MECHANICALPROPERTIES

OF SOME

IV

NIOBIUM

ALLOYS

_

tmimate

0

Nb

-

ANNEALED

FOR I

h, 2,ooo”F

~--

composition 0,’

Alloy number

75

MO

TlZ

~~~__

Yield

W

strength (P.S.i.)

strength (p&i.)

R.A.

Eloflg.

%

%

_~

205 D3

68.5

30

-

1.5

49,200

38,800

79

33

D3 207 D4

67.75 62.5

30

1.5

0.75

36,200

80

47

5.0

2.5

77,500

67

23

223 D3

82.5

30 IO

45,500 85,200

3.0

65,900

D3 199 D3

55.0 67.0

30

3.0 10.0

5.0

70 -

38 -

30

3.0

59,300 58,200

57,100 51,800

78

38

,204 D3

65.0

30

56,400

60.0

30

78,900

238 D3

65.0

20

15.0

72,300 -

71 66

25

D3

5.0 10.0

61,900

56

237 D3

60.0

20

27 26.5

239 D3

55.0

20

206

22I

222

-

20.0

97.400 127,800

25.0

130,900

TABLE THE

MECHANICAL

PROPERTIES

temperature

Room

AT ROOM

Forged bar stock stress relieved for

45 IO.2

12.5

VI

TEMPERATURE

OF SOME

DUCTILE

NIOBIUM

BASE

ALLOYS

mechanical properties of some improved niobium base alloys which can be forged to plate at z,ooo”F and cold rolled to sheet. Alloy

Treatment

35.5

Temperature

Ultimate strength

Yield strength

102,000

(P.S.i.)

280 D3

R.T.

112,600

284 D3

R.T.

99,100

(P.S.i.)

Elan& :b

3.5 16.5

89,900

Em-e(p.s.i.)

35.2

I h at 2,200°F Sheet 0.060

280 D3

R.T.

148,600

2.1

284 D3 ST-41

R.T.

5.0

R.T.

145.700 138,100

Sheet 0.060 in.

280 D3

R.T.

100,600

thick - stress

284 D3 ST-41

R.T.

95,800

-

17.0

R.T.

104,500

-

13.0

in.

thick - as-rolled

relieved for I h at

5.0 15.0

2,200°F

TABLE

strainrate

Alloy

Ultimate strength (P.&i.)

Yield strmngth (P.S.i.)

Em-6(p.s.i.)

0.005

23,700 23,800

20,500 22,300

13.7 11.3

0.0054

26,300

22,100

14.7

per set 280

D3

284 D3 ST-41

VII

*Stress relieved As-rolled *Stress relieved

0.0067

* Stress relieved I h at 2,400°F. J. Less-Common

Metals,

2 (1960) 69-75