Creep and long-term strength of titanium reinforced with molybdenum fibers

C R E E P AND L O N G - T E R M S T R E N G T H OF T I T A N I U M

R E I N F O R C E D W I T H M O L Y B D E N U M F I B E R S

E. S. U m a n s k i i , L. I . T u c h i n s k i i , V. V. K r i v e n y u k , a n d V. Ya . F e f e r

UDC 53 9.4

The resul ts of many recent investigations show that the re inforcement of metals is one of the most important ways of improving their s t ructura l strength. It is, therefore, a mat ter of considerable interest to investigate the regular i t ies observed in the c reep and long- te rm strength of such composi te mater ia ls . For this reason an investigation was car r ied out at this Institute on molybdenum-fiber reinforced titanium orientated along the tensile axis.

The creep and long- te rm strength charac te r i s t i c s obtained under uniaxial loading conditions were de- termined using a vacuum device described in great detail in [1, 2]. The specimens were heated by nickel hea ters in a vacuum of about 1-5 �9 10 -5 mm Hg. Their c ross - sec t iona l area was 2 • 4 mm 2 and working length was 20 ram. The tests were ca r r i ed out at a tempera ture of 550~ Strain was measured using a KM-6 eathetometer with an e r r o r of less than 0.02%. C h r o m e l - A l u m e l the rmocoupleswere usedfor meas - uring specimen temperature .

The experimental procedure was as follows: heating up to a specified tempera ture in 3 h, holding at this t empera ture for 10 min, loading at a rate of 0.2%/mtn.

Compositions containing Vf = 8, 22, and 38 vol. % of Mo f ibers and pure titanium (VT-10) were p re - pared.

The specimens were made by dynamic compacting of a packet of 0.08 mm thick foils onto which M4 molybdenum wire with a thickness of 0.08 mm was wound. The specimens were compressed in vacuum of 10-2-10 -3 mm Hg.

The resul ts obtained in testing titanium foil reinforced with molybdenum wire for long- te rm strength are given in Fig. 1. This shows that the long- te rm strength of titanium with molybdenum wire is sa t i s fac- tori ly described by the following power equation

t = B~, (1)

a,60gf/mrn2k -' , ..

O01 o

'0 I 10

I 0 if} ~ !.n I I 0 e r , h

Fig. 1. Long-term strength curves for titanium foil, molybdenumw~re, and the composition obtained at 500~

Institute of Strength Problems at the Academy of Sciences of the Ukrainian SSR, Kiev. Translated f rom Problemy Prochnosti , No. 1, pp. 24-27, January, 1973. Original ar t icle submitted Februa ry 15, 1972.

�9 1974 Consultants Bureau, a division o f Plenum Publishing Corporation, 227 g/est 17th Street, New York, N. Y. 1001t. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, withol, t written permission of the publisher. A copy o f this article is available from the publisher for $15.00.

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e,~

4<0

aO

26

I0 5

O 15 . 24 ,Y2 ~0 #fl r, h 0 a

6=/6,Skgf/mm'/ I ]1

10 20 50 /70 50 50 78 O0 ,90 r, h b

1 5 - -

]O

5 �84

'----Vr~$Mo -25,2 I

-2~,1 I

20

T i m 2

zd I

z/o tTO ' r h e

! ;F / 2 ~ __._m 0

6 : 13,,4 ~ m z'~-"

I

I 200 0o0 6o0 ~o0

d

V.f -= 22 vol. %

t000 1200 tdOO r . h

E,~i-17 r I ~-5 :JS,/kgf/mm z

I I[--aO,5 1 0 - - ~ - -

j f l

O 5 I0

7

Vf = 38 vol. % Mc

/

15 20 r, (~ e

Fig. 2.

E,;'o l~/sn kgf/~m 2

5

O 200

Vf = 38 vol. %Mc I

y 408 500 r, h

f

Creep curves for t i t an ium-molybdenum composition.

where t is the time to failure; o- is the s t ress ; and B, /3 are mater ia l and testing condition coefficients. Fo r the composition with Vf = 8 vol. % Mo the experimental resul ts are also in good agreement with Eq. (1).

A further increase in fiber content resu l t s in an increased sca t te r of data. Nevertheless the approxi- mation of experimental resul ts shown [n Fig. 1 by a broken line is in good agreement with Eq. (1)

To find the reasons for this scat ter , consider creep curves plotted for investigated specimens.

The creep curves for pure titanium obtained at ~ = 5.0, 6.4, and 9.1 kg/mm 2 {Fig. 2a) produced for investigated t ime ranges, have steady sections, sections for accelera ted creep, and a section of rapid failure, The size of plast ici ty r e se rved e r [3] is 8-10%. The percentage elongation after f rac ture ep is relat ively high (40-50%).

According to creep curves the composit ions with Vf = 8% of Mo (Fig. 2b) differ only in that within the same time intervals they show a small strengthening section which makes them s imi lar to the c lass ica l creep curves. In this case e r and ep values are much lower, with the e r values being 2-5% and ep values 15-23%.

The creep curves for composit ions with 22 and 38% of f ibers (Fig. 2c-h) show a more pronounced strengthening section. This is because in this mater ia l the major par t of the load is a lready car r ied by the molybdenum f ibers for which such sections are typical.

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o t

JO

20

10

r/mm 2

9 % JO Vf, vol.

~=lOOOh

20 0 10 20 JOVp vol. ~o Fig. 3 Fig. 4

Fig. 3. Effect of the content of paral lel f ibers on long- te rm strength of the composition.

Fig. 4. Dependence of the coefficient of strengthening of the com- position titanium-molybdenum on the volume content of fibers.

For the composit ion containing 22% fibers e r var ies within the range 1-4% and ep changes f rom 9 to 16%. A reduction of s t r e s s with corresponding increase in fatigue life to 400 h had no effect on the shape of creep curves . The initial section of the curve for ff = 13.1 kg/mm 2 and a fatigue life of 1550 h shows that this time is not sufficient to make any physical changes which would cause a reduction in e r o r cause distort ions of creep regular i t ies observed in tests with much shor te r fatigue life. Hence the er values can be used for a rapid determination of the time to failure [3] using the equation

t = ~ , (2)

where t is the time to failure; e r is the plast ici ty reserve ; and k is the rate of plastic s t rain during the steady creep.

An examination of creep curves shows that the composi t ions with Vf = 22% Mo and more have p r a c - tically identical plast ici ty charac te r i s t i c s . Apparently these cha rac te r i s t i c s are determined mainly by the plast ici ty proper t ies of molybdenum f ibers at a given tempera ture .

A sca t te r of creep and long- te rm strength cha rac te r i s t i c s could be caused by a number of fac tors such as the changing ratio of load car r ied by fibers to load ca r r i ed by the matr ix during the deformation process , etc.

The p rocess ing of experimental data on s h o r t - t e r m behavior of strength cha rac t e r i s t i c s observed by severa l workers [4-6 and others] conf i rms the following relation:

% -- afVf -{- (sMV M, (3)

where (7 c is the ultimate tensile strength of the composition with a single direction of fibers; o-f is the ul- t imate tensile strength of fibers; (7 M is the s t r e ss in the matr ix at the moment of fai lure of fibers; and Vf and VM are the fiber and matr ix contents in the composition.

Figure 3 shows the long- te rm strength at constant fatigue life o- t as a function of f iber content. It may be seen that the crt-V f function differs little f rom a l inear function represent ing the additivity law [4] and those obtained by p rocess ing experimental data by the l eas t - square method. The d iagram also shows that the composit ion is much s t ronger than pure titanium even if it contains only a relat ively smal l amotmt of f ibers (e. g . , for a fatigue life of t = 1 h it increases f rom 10 kg/mm 2 for pure titanium to 15.5 kg/mm 2 for a composit ion containing Vf = 8% Mo and to 34 kg/mm z for a composit ion containing Vf = 38% Mo).

This strengthening may be seen in Fig. 4, where k = (fft)K/((Tt)Ti ((o't)lr and (r are the long- te rm strength of composition and titanium at equal fatigue life values).

C O N C L U S I O N S

1. Under the conditions of the descr ibed investigation the relationship between the load and fatigue life for the composit ion t i t an ium-molybdenum and its components is descr ibed by a power function.

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2. In assessing the strength of t i tanium-molybdenum compositions use can be made of the law of additivity.

3. The reinforcement of titanium by molybdenum fibers substantially improves its strength.

1~ 2.

3.

4. 5. 6.

L I T E R A T U R E C I T E D

G. S. Pisarenko, V. V. Krtvenyuk, and V. P. Dubinin, Zavod. Lab., No. 9 (1966). G. S. Pisarenko, V. N. Rudenko, et al. , High-Temperature Strength of Materials [in Russian], Naukova Dumka (1966). N. A. Oding, V. S. Ivanova, et al. , Theory of Creep and Long-Term Strength of Metals [in Russian], Metallurgtzdat, Moscow (1959). D. M. Karpinos, t~. S. Umanskii, et al . , Probl. Prochnosti, No. 1 (1969). D. M. Karpinos, 1~. S. Umanskii, et al . , Probl. Proehnosti, No. 5 (1970). G. S. Holtster and C. Thomas, Fiber Reinforced Materials [Russian translation], Metallurgiya, Moscow (1969).

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