The effect of grain size on the deformation dynamics of commercial purity α-titanium

Scripta METALLURGICA

Vol. 2, pp. 239-242, 1968 Printed in the United States

Pergumon P r e s s , Inc.

THE EFFECT OF GRAIN SIZE ON THE DEFORMATION DYNAMICS OF COMMERCIAL PL~AITY G-TITANIUM

R. L. Jones

and R. Conrad t

Franklin Institute, Philadelphia, Pennsylvania 'University of Kentucky, Lexington, Kentucky (Received February

15, 1968)

In the course of a continuing investigation of the deformation and strengthening mechanisms of the solid solution phases of titanium, the effect of grain size on the deformation dynamics of A-70 con~nercial purity a-titanium has been studied.

Tensile and deformation

dynamics data for the identical material recrystallized to a grain size of 2.6 microns and tested in the temperature range 4.2°K to 800°K have already been reported (1,2).

These

results, together with others from the literature for materials of different grain sizes and purities (3-9) have recently been reviewed (10-12).

Using the approach of thermally

activated deformation, it was concluded that the rate controlling dislocation mechanism during low temperature deformation is thermally activated dislocation glide on the first order prism planes over barriers to motion association with individual interstitial impurity atoms.

The thermal activation analysis (10-12) suggests that the rate controlling process is

independent of grain size but it is not possible to definitely conclude this because of the ~ c k of sufficiently detailed experimental data for materials of identical impurity content but different recrystallized grain sizes.

The present work is intended to demonstrate that

for A-70 a-tltanium, the rate controlling process is indeed independent of grain size. Small tensile specimens recrystallized to grain sizes of 0.8 microns and 18-19 microns have been subjected to continuous,change-of-temperature and change-of-strain rate tensile testing in the temperature range 77°K to 723°K.

The techniques used for specimen

preparation and testing were similar to those described earlier (1,2), except that continuous and change-of-temperature tests were run at a tensile strain rate of about 3.3 x 10 -4 -i sec Identical conditions to DhQse previously described were used for change-of-strain rate tests with 4:1 changes between about 3.3 x 10-4sec -I and 0.83 x 10-~sec -I

Grain sizes

were determined by mean linear intercept counts, using transmission electron metallography for the fine grain size specimens and conventional optical metallography for the coarse grain size specimens. The stress-strain behavior observed in continuous tests was very similar to that reported for the same material recrystallized to an intermediate grain size (1,2). from initial yield point effects, the relation between true stress o and true strain

239

Apart c at

240

GRAIN SIZE AND DISLOCATION DYNAMICS IN ~ - T t

Vol. 2, No. 4

all testing temperatures could be written as = a(O) + hE I/2

(i)

The extrapolated yield stress a(0) decreased with increasing grain size while the hardening coefficient h increased with increasing grain size.

Small yield drops were observed for the

0.8 micron specimens at all testing temperatures whereas yield drops for the 18-19 micron specimens were restricted to the temperature range 300°K to 523°K. Values obtained for ~(0) are plotted against testing temperature in Fig. i.

140

120 I00

~I n\

I I A-?O Q°TI

~\

~ a 3 , Io"4 s~c "1 \

I

I

I

\

The

I -

a a s / , G.s.

|. b

ZO

]

I00

]

200

I

I

I,,

300 400 500 TEMP~RATURIr {'K)

I

600

I

700

800

FIG. I Extrapolated yield stresses for commercial purity u-titanium specimens of two grain sizes tested at various temperatures. curves for the t~vo grain sizes are essentially parallel showing that @he effect of decreased grain size is an approximately constant increase in yield stress, independent of testing temperature, within the temperature range 77°K to 723°K.

Following the earlier approach

(10-12), the yleld stress values have been subdivided into two components, a temperature insensitive or athermal component a , and a temperature sensitive or thermal component u . At high temperatures ~(0) equals a .

and at lower temperatures u(0) equals the sum of ~

and

For consistency with the previous work (i, 2, i0, ii, 12) ~(0) has been put equal to

at 650°K.

Values of u

at other temperatures were calculated from a knowledge of the tempera, ture sensitivity of the tensile modulus (13), and values of e deduced by subtraction. The

thermal component o£ the shear stress T of the absolute temperature in Fig. 2.

(taken as 1/2 ~*) is plotted against the square-root The linear relation plotted is that obtained by Jones

and Conrad (1) for 2.6~ grain size A-70 a-tltanlum, which also agrees with the data of Orava et al (2).

The agreement with the present results is quite good, with small deviations at

high temperatures which can be attributed to the higher strain rate used in the present work. One can conclude that the effect of grain size on the yield stress of A-70 a-tltanlum is completely athermal in nature.

Vol. 9, No. 4

G R A I N SIZE A N D D I S L O C A T I O N D Y N A M I C S IN m-Ti 8o i I I i i

241

A-?O cr-Ti ~'~ 6x I0 "4 SEC'I

7o!

A ,8-19 F G.S.

~ z

a

o.wa.

G.S. -

30-

zo

~, u

,.5•

see"

~E

I0

o 0

I

I

i

5

I0

15

I~ 20

25

~0

T~'2 (,K'J2) FIG. 2 The variation of the thermal component of the shear stress with the square root of the absolute temperature for commercial purity o-titanium of two grain sizes. Additional evidence of the athermal nature of the effect of grain size is obtained from the results of the change-of-temperature and change-of-strain rate tests.

The activa-

tion enthalpy for a thermally activated deformation process can be deduced from the experimental data using the equation developed by Conrad and Wiedersich (14),

H = kT 2

~ £

. a~/_.

\ a~

/T \ ~ /

(2)

In agreement with the conclusions of Orava et al (2) the present values of ~ aT "~

)

are independent of strain, evidence that the rate oontrolling deformation

process is independent of the work hardening process. testing temperature. comparison.

and T

Fig. 3 shows the variation of H with

Values deduced from the data of Orava e t

al (2, 12) are included for

To a good first approximation a linear relation is observed, independent of

grain size, showing that the activation process is unaffected by grain size within the accuracy of the experimental data. From the present results it is concluded that while the yield stress is grain size dependent, the contribution due to the grain size is completely contained in the athermal stress component o u , In addition, the data establish that the thermally activated rate controlling process for low temperature deformation of A-70 u-titanium is independent of grain size, which is in agreement with the earlier view (10-12) of the nature of the process, The authors are indebted to the Systems Engineering Group, WPAFB, U.S. Air Force for financial support under Contract No. AF33(615)-3864 and for permission to publish.

242

GRAIN SIZE AND DISLOCATION DYNAMICS IN c~-Ti f.S 1,4--

1.2 - -

r

J

J

r

Vol. 2, No. 4

i

A-70~-r~ 0 1 8 - J 9 ~ GS. / ~ 0 2.6F¢GS.(ORAVAetoJ) O O.Sg. G.S. o

I.O--

/

>

~0.8

0,6--

0

0,4 _ _ ~ ~ ~

0.2

--

,,

00

I00

~

f

200

300

I

400

,,,

TEMPERATURE (°K)

I

500

600

FIG. 3 The temperature dependence of the activation enthalpy for commercial purity u-tltanium of three grain sizes. References i.

R. L. Jones and H. Conrad, Acta Met 15, 649 (1967)

2.

R. N. Orava, G. Stone and H. Conrad, Trans. ASM, 58, 171 (1966)

3.

F. D. Rosl and F. C. Perkins, Trans. ASM, 45, 972 (1953)

4.

G. W. Geil and N. L. Carwile, J. Res. N.B.S. 54, 91 (1955)

5.

G. W. Gell and N. L. Carwile, J. Res. N.B.S. 59, 215 (1957)

6.

E. D. Levlne, Kennecott Copper Corp., TR-67 (Sept. 1965)

7.

G. Spanglec and M. Herman, Franklin Institute Research Lab., AFOSR-TN-60-695 I-A1878-7 (July 1960)

8.

H. I. Burrier, Jr., M. F. Amateau and E. A. Steigerwald, AFML-TR-65-239 (July 1965)

9.

R. J. Wasilewski,

Trans ASM 56, 221 (1963)

I0.

H. Conrad, 'High Strength Materials,' Wiley, N.Y., 436 (1965)

ii.

H. Conrad, Acta Met 14, 1631 (1966)

12.

H. Conrad, Canadian Jnl. Phys. 45, 581 (1967)

13.

P. E. Armstrong and H. L. Brown, Trans. AIME 230, 962 (1964)

14.

H. Conrad and H. Wiedersich, Acta Met ~, 128 (1960)