The correlation between alloy softening and the strain-rate sensitivity in VTi alloys

Scripta METALLURGICA

Vol. 8, pp. 733-738, 1974 Printed in the United States

Pergamon

Press,

THE CORRELATION BETWEEN ALLOY SOFTENING AND THE STRAIN-RATE

SENSITIVITY

IN V-Ti ALLOYS

E. Pink and H.P. Erich-Schmid-Institut der Osterreichischen

StGwe

fur FestkSrperphysik

Akademie

Leoben,

der Wissenschaften

Austria

(Received April

One conclusion in b.c.c,

25, 1974)

from many publications

metals

is the direct

(AS) and the strain-rate we shall demonstrate the deformation

[1-5]

correlation

on the alloying between

sensitivity k = ~ T / ~ l n

alloy softening

(~/~o).

In this note

that this is true only in the special

mechanism

is not altered

effects

case where

due to alloying.

AS can be defined by the condition (1)

AT

(the

indices

metal",

= TA -

A and B here

respectively).

athermal

~

decreases

with

AS m u s t b e r e s t r i c t e d whether

a test

condition sions. zing T

is

i n T~ i s

(1). shear

A full various the

condition

alone,

T can be

and

o f AS w h i c h

is

therefore

understanding

T

base an

. T

above a critical r~,

becomes a mere matter decrease

into

stress

increase

a temperature

"pure

separated

and vanishes generally

by the

must

and more suitable stress

at

outweighed

Any c o n s i d e r a t i o n

it

"alloy"

"effective"

will

t o T . Thus

theoretical

only

stress

temperature

additions

stresses

on d e n o t e

activatable

can be performed

increase

of total

The s h e a r

increasing

TO . A l l o y

0

and later

and a thermally

temperature

the

~B <

so that of chance

low enough so that in r based

fulfill

on a c o m p a r i s o n

to

wrong conclu-

c a n come o n l y

from recogni-

experimental

lead

to

aspects.

component which exhibits

Thus,

AS, a b e t t e r

and since basic

is

(2)

~ * =T~-r B<

Thfs would,

for instance,

o.

be accomplished

733

if T o were lower in alloys,

Inc

734

ALLOY SOFTENING AND STRAIN-RATE SENSITIVITY IN V-Ti ALLOYS

Vol. 8, No. 6

with but this has never been observed. However, the increase of decreasing temperature can be diminished by alloying while T o remains constant, so that another condition for AS is (3)

Z~. _--- J =

-

<

I~-~ IA ~ B 0 I'~TI When the absolute values of the slopes (not taking into account their negative signs) are used. T

obeys the rate equation of thermally activated deformation,

and

thus (4)

- - =

c --

T where I~T*/~T I again represents the absolute value of the slope, and C equals & H / k T = In(~o/i)~.in(0.?Yn/~).v Provided that the deformation characteristics ~ H and Yo are not altered, another condition for AS follows from equations

(5)

(3) and (4) as

A~ =~A -~B < 0

In several alloy systems, such as Fe-N Ill, Fe-Mo/Si/P [2,3,4] and various niobium alloys [5], the conditions (2), (3) and (5) are fulfilled simultaneously with the exception that 2~l~./~Tll changes over to positive values at lowest temperatures, since ~ * -- still negative - - h a s passed its maximum and approaches zero. This is apparently caused by a gradually vanishing solution effect while the modified double-kink mechanism gains importance [6 ]. Apart from this behaviour in a limited temperature range, the correlation between AS and the three negative values A t * , Ai~r*/~T I and A X is straight forward. It is generally agreed that essentially one single deformation mechanism-though m o d i f i e d - - is rate-controlling in these alloys. For V-Ti alloys, however, such statements do not hold. The criteria ~ n e g a t i v e values of ~ or A I ~ r * / ~ T I.are not always consistent with the occurence of ASo The relation is reversed at certain intermediate temperatures, where negative A ~ and A II~ T'* /"~ T are measured while the effective shear stress of the alloy is in fact larger than that of the unalloyed metal. To understand this unexpected behaviour, we have to reconsider the deformation characteristics of the V-Ti alloys. The original results, from which Figs. 1 abc are compiled, have been published previously [7 ]. Several indications for a change or a severe modification of the deformation mechanism were reported:

Vol. 8, No. 6

ALLOY SOFTENING AND STRAIN-RATE SENSITIVITY IN V-Ti ALLOYS

+5

i

,

,

V-~3 at'/, Ti

a ,

,

,

I

I

I

+0.2

!io.i "~ -0.',

N -0.2 o

Test rerr~r~ure ( K ) Fig. 1.

(a) The change of k - T and v

(activation volume)

- T

dependences

due to alloying (cf. Fig. lb). (b) The change in the A H - r effective stress

relation:

the enthalpy ~ H o at zero

(or temperature of maximum activation To) of

alloys is about 1.5 eV and thus three to four times larger than that of the base metal. (c) Where the new mechanism is operative the strength values increase linearly with the square of the concentration,

c t/z.

Implicit in the published results is an increase of the apparent frequency factor %

for alloys.

This can be demonstrated by means of

equation (4), plotting the relationship between k/T and the slope of the T -T curve. The results

(Fig. 2) differ from previous ones for

iron [8] and Fe-Ni alloys [9] in several respects. First, the slope C of pure vanadium is not uniform. While increasing linearly at the beginning,

the curve assumes a higher slope below lO0°K +). Second,

the slopes for the various alloys m among themselves m the knowledge

while not significantly different

are higher than the slope for pure vanadium. From

of slope C follow the values of the frequency factors:

for vanadium, % =

1.4 x 107; for the alloys close to the origin of

+) Such behaviour has not been reported before. While it lacks interpretation, its understanding is not of importance for the problem of ASo

735

736

ALLOY SOFTENING AND STRAIN-RATE SENSITIVITY IN V-Ti ALLOYS

Vol.

8, No. 6

0.4

. . . • Vanadium o =V'O'8at'% Ti I. 7 at.% u 4.3 at~% lO-~/sec

.

i~50 K ' ///K 30 K . 1 7 ~" I00 K o

-~-02

\ % ('~

125

100 K ~

/

/

5OK 77 Y ~a~'~" a ~ 125 K

--0.1

; % 5o7

. K ;so K

2

0

~

I

6

10~I0-~ Fig. 2.

Fig. 2 , 7 o = 1.4 x 1013sec. -1. At low temperatures resume the behaviour of pure vanadium;

the low-Ti alloys

only the 4.3 at.% alloy exhi-

bits a distinctly different behaviour down to 30°K. Since no twinning was detected,

this appears to be a genuine effect.

One consequence

of higher activation enthalpies and pre-exponential

factors is an increased temperature T o . Using the new values of L~Ho and

%,

we obtain in fact T o ~ 450°K for the alloys

(compared with

T o ~ 300°K for vanadium) which is consistent with the experiment [7 ]. Now it is immediately obvious why tures:

~T*

is positive at high tempera-

since T o of the alloys is increased,

even a small I~T*/aTI

raises the curve above the effective stress of pure vanadium, though ~ X

soon

and al-

is negative below 240°K, still no softening is observed.

Note that in Fig. 1 the temperature for ~ X not agree with the temperature for ~I~T*/~TI

= 0 (i.e. X A = XB) does = 0 as is usually does.

It can be readily explained considering equation (4), and taking into account the appropriate values of % , the proportionality

which enter the equation through

factor Co

All this indicates that a new mechanism is operative in the alloys. It has been argued on grounds of the c ~/~ dependence of the strength that it is related to Fleischer's mechanism of tetragonal distortions [7]. Indeed this process could provide an explanation for the reduced temperature dependences of ~ and T*. Even the true softening in 4°3 ato%-Ti alloys below 100°K (see Fig. 1), previously interpreted as the consequence of facilitated nucleation of double kinks [7 ], may in fact occur due to this still to come extent remnant impurity process. Although,

around 50°K, activation parameters had

indicated the return to the double-kink mechanism,

the stresses were

Vol.

8, No. 6

ALLOY SOFTENING AND STRAIN-RATE

still found to be low.

Transition

SENSITIVITY

proporties

bited.

If tests at still lower temperatures

values

of ~ r *

(i.e. a total return

have been encountered The findings paper

[lO],

stress

are apparently

exhi-

had been possible,

to the double-kink

zero

process)

might

as in the low-Ti alloys.

of R. Gibala and co-workers,

associations

IN V-Ti ALLOYS

summarized

in a recent

that interstitial/substitutional-interstitial

are a necessary

prerequisite

for AS in certain alloys.

The fact that the V-Ti alloys with a total of 300 to 500 at.ppm interstitials theory.

[7] exhibit AS, seem to comply at first with Gibala's

However,

dictions.

a quantitative

consideration

It has been found in damping

of interstitials

experiments

occurs at very low titanium

plain the general

softening

phenomenon

contents

temperature gingo

where

found higher, dependence dominance quence

atoms:

by the larger

due to a pure double-kink

of scavenging.

high-temperature

peaks

AS and the occurence

between

[15 ]. The fact that there is no evidence iour in any of the investigated

of such anelastic

nium atoms

the rest of the titanium

alloy)

strengthening

appears

mechanism, dencies

to be obscured

parameters,

is present as solid solu-

(still visible in favour

titanium atoms.

different

have been measured

to AS,

are rather as follows.

is tied up by part of the tita-

scavenging

due to the excess

that activation

sub-

as the cause for AS. The com-

in 1.7 and 4.3 at.%-Ti alloys

at.%-Ti

between

behav-

[16,17] excludes

of interstitials

only,

additional

in the Ta-Re-N system

The total amount

At room temperature,

in

and interstitial

A clear correlation

plete mechanisms

tion.

dependence

with AS. There is another

V-Ti alloys

interactions

Its pre-

(defined here as the

apart from the Snoek peak,

in damping tests.

temperature

[13].

[14] may be a conse-

substitutional

of such a peak exists

stitutional-interstitial

substaninter-

the room-

theoretical

mechanism

temperature

is incompatible

those which produce,

reduced

contents

But the increased

of interactions

similar

at 160 and 200°K were

alloy shows that scavenging

of interstitials)

is further

This agrees with the idea of scaven-

in metals with low titanium

possibility

This

that the shear stresses

may be explained

of •

this 0.25 at.%-Ti removal

[12].

that scavenging

with perhaps

only 0°25 at.% titanium

shear stress

The observation

some contra-

levels and cannot ex-

Ill].

tiated by recent work on Fe-Ti alloys stitial

reveals

in the Fe-0.25

of solid-solution They are the reason

from those for the double-kink

for all alloys.

They also produce

or actual AS (see Fig. 1), depending

ten-

on the tempera-

737

738

ALLOY SOFTENING AND STRAIN-RATE

SENSITIVITY

IN V - T i

ALLOYS

Vol,

8,

ture and on the amout of the alloying agent present. Let us conclude: the statement that reductions of the temperature dependence of r and of the strain-rate sensitivity in alloys are simultaneous aspects of AS is not always true. In b.c.c, alloy systems, where the basic double-kink mechanism is replaced by another mechanism, the temperature range of thermal activation can be extended. Although the strain-rate sensitivity, or the temperature dependence of the stress may be smaller in alloys, softening must not necessarily be exhibited. In V-Ti alloys it is not possible to establish general correlations between ~ , X and ~T*/~T which are valid over the entire temperature range. These results support the idea that AS in one type of alloy has to be attributed to a change of the deformation mechanism, and not to substitutional-interstitial interactions.

References 1. 2. 3. 4. 5.

6. 7. 8. 9. lO. ll. 12. 13. 14. 15. 16.

17.

Y. Nakada and A. S. Keh, Acta Met. 16, 903 (1968) T. Sakuma and S. Karashima, Trans. ISIJ ll, 240 (1971) W. A. Spitzig and W. C. Leslie, Acta Met. 19. 1143 (1971) W. A. Spitzig, Metall Trans. 3. 1183 (1972) M. G. Ulitchny, A. K. Vasudevan and R. Gibala, Proc. 3rd Intern. Conf. Strength of Metals and Alloys, Vol. l, Insto of Metals, London 1973, p. 505 E. Pink, Z. Metallkd. 64, 871 (1973) E. Pink and R. J. Arsenault, Metal Sci. Jo 6, 1 (1972) H. Conrad and S. Frederick, Acta Met. lO, lO13 (1962) U. Hildebrandt and W. Dickenscheid, Scripta Met° 6, 465 (1972) R. Gibala and T. E. Mitchell, Scripa Met. 7, 1143 (1973) D. F. Hasson and R. J. Arsenault, 2nd Intern° Conf. Strength of Metals and Alloys, Vol.1, ASM, Metals Park 1970, p. 267 D. Leemans and M. E. Fine, Proc. 3rd Intern. Conf. Strength of Metals and Alloys, Vol.1, Inst. of Metals, London 1973, po 510 E. Pink, phys. stat. sol. (a) ll, 87 (1972) W. A. Spitzig, Mater. Sci. Eng. 12, 191 (1973) A. A. Sagues and R. Gibala, Scripta Met. 5, 689 (1971) D. F. Hasson and R. J. Arsenault, Treatise on Materials Science and Technology, Edt. H. Hermann, Vol.1, Academic Press, New York 1972, p. 179 D.F. Hasson, R.J. Arsenault, J. Less-Common Met. 27, 417 (1972)

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