Ordering and tensile ductility in a TiAlGa alloy

Vol. 9, pp. 75-78, 1975 Printed in the United States

ScriptaMETALLURGICA

ORDERING AND TENSILE DUCTILITY W. N. Roberts*

Pergamon

Press,Inc.

IN A Ti-AI-Ga ALLOY%

and M. J. Godden**

*Head, Metal Physics Section and **Research Scientist, Engineering Physics and Refractory Metals Section, Physical Metallurgy Division, Mines Branch, Department of Energy, Mines and Resources, Ottawa, Canada. (Received November In titanium-aluminum forms by a first-order

18, 1974)

binary alloys,

transformation.

the ordered phase Ti3AI

The single-phase

from 21 at.%Al to 37 at.%Al at 700°C(I).

(~2 type)

field of Ti3AI extends

In the ~ - T i + ~

phase field, ordering

to produce ~2 can be suppressed by rapid quenching when the aluminum content is low.

However,

a long-range

above about 18 at.%Al,

order parameter

ordering cannot be suppressed

approaching

unity at room temperature

Fully-ordered

Ti3AI has limited ductility,

cleavage-type

failure mode.

fracture

surfaces

The lack of significant

Ti3AI , DO19, exhibits

Several workers(4-7)

at.%Ai-7.41

of slip in directions

However Mg3Cd , which is the same structure good ductility

in the fully-ordered

have modified

alloys with ternary element additions characteristics.

the composition

~2 alloys to be brittle, in compression

at.%Ga was ductile

however the alloy

because a second phase

the intense planarity of slip and increased the occurrence In earlier work(6),

sition 12 at.%Al-12 quenching

12.19

Williams et al(5) have re-

12.5 at.%Al-12.5 slip.

of the binary Ti-AI

Hoch et al(4) have reported an alloy of the composition

ported several ternary single-phase

~

type as

condition(3).

in an attempt to modify the deformation

at.%Ga to be single phase and ductile.

Ti2Ga decreased

showing a largely

ductility has been attrib-

uted to the severe planarity of slip and the difficulty other than (2).

and a2 with is found.

we have reported an alloy having the compo-

at.%Ga to be ductile

from 1300°C.

of

in tension at room temperature

after

This work reports the result of aging heat treatments

on this alloy composition. The alloy was melted and worked in a similar manner to that previously reported(6).

Specimen blanks were solution treated

in the B~hase

field at

1300°C in a vacuum of less than 1.3 x 10-4pa for 4 hr and quenched by dropping out of the hot zone into a flowing helium gas stream. rate was 60°C/sec.

After machining

to avoid any possibility

of contamination,

vacuum of 6.5 x 10-4pa for 5 hr at various # Crown Copyright

The approximate

0.5 mm from the surface of the

Reserved. 75

quench

gage length

specimens were aged in a dynamic temperatures

and oil-quenched.

76

ORDERING IN A TI-AI-Ga ALLOY

they were subsequently the results

Vol.

9, No. I

tested in tension at a strain rate of 2 x 10-4sec-l;

are presented

in Table i. TABLE1

Tensile Behavior of Alloy Aged at Various Temperatures Aging Temperature,

0.2% Yield Stress, MPa

°C

As quenched

U.T.S.,

Elongation, %

MPa

648

759

21

200

596

688

21

300

869

893

4

400

807

827

5

500

761

786

5

600

662

666

4

It can be seen that the ductility

shown by this alloy after quench-

ing from the 8-phase field is largely lost when it is aged for 5 hr above 200°C, although the alloy is not completely brittle An examination by transmission the as-quenched

condition,

~ith weak antiphase

Fig.

after quenching long-range

showed no "anomalous

film of the e solid solution(l).

diffraction

spots was also detected,

contrast" The presence

indicating

that even

some level of

order.

Gehlen(8)

any estimation

of the long-range

order parameter

shows that the atoms are slightly displaced

sites in Ti3AI and a similar effect would be expected displacements

would radically change the relative

it is believed that in the as-quenched

condition

is diffi-

from the lattice

in ternary e2"

intensities

spots, making an estimate of the degree of long-range ordered,

e2

such as Ti2Ga, was

from the 8-phase field, the alloy has developed

However, cult.

to be single-phase

No second phase,

domain boundaries

associated with a thin, trapped of weak superlattice

electron microscopy of the alloy in

I, shows the structure

domain boundaries.

observed and the antiphase

in this condition.

These

of the observed

order difficult.

However,

this alloy is only partly

the gallium tending to retard the kinetics of the ordering process.

Shamblen and Rosa(9)

have shown that in s-titanium,

the diffusion

coefficient

for gallium is lower than for aluminum and the activation energy for diffusion is higher for gallium than for aluminum. fusion-controlled,

retention of some disorder ternary additions

Since the ordering process

is dif-

the reduced rate of gallium diffusion could permit the on quenching.

Also, Crossley(7)

tend to slow the formation of ~2"

the lowering of the degree of long-range

has found that some

It would be expected that

order would reduce the antiphase boun-

dary energy which would permit easier cross-slip.

The significant

ductility

shown by specimens aged below 300°C is attributed to enhanced cross-slip.

Vol.

9, No. I

ORDERING IN Ti-AI-Ga ALLOY

77

When aging below T c (estimated to be somewhat below 640°C for this alloy(10)), diffusion permits the degree of long-range order to increase,

causing

the dramatic decrease in ductility for aging temperatures at 300°C and above. Further diffusion causes the growth of domains by the annihilation of antiphase domain boundaries

in a manner analogous to grain growth.

decrease in yield strength as the aging temperature

This results in a

is increased,

a result

similar to that obtained by Blackburn(2). A similar alloy prepared using titanium sponge stead of crystallette titanium

(oxygen <0.01 wt%)

as a base material,

ductility after quenching from the 8-phase field. that oxygen tends to stabilise the ~2 phase.

(oxygen 0.053 wt%)

in-

showed no

This observation suggests

Namboodhiri e~ al(ll)

found that

oxygen stabilised Ti3AI to higher temperatures in two-phase alloys, which would support this view. REFERENCES i.

M. J. Blackburn,

2.

M. J. Blackburn and J. C. Williams,

3.

N. S. Stoloff and R. G. Davies,

4.

M. Hoch, J. V. Hackworth, R. J. Usell and H. L. Gegel, The Science, Technology and Application of Titanium, Eds. R. I. Jaffee and N. E. Promisel 359, Pergamon Press (1970).

5.

J. C. Williams and M. J. Blackburn, Ordered Alloys, Eds. B. H. Kear, C. T. Sims, N. S. Stoloff, and J. H. Westbrook, 425, Claitor's Publishing (1970).

6.

M. J. Godden and W. N. Roberts, Titanium Science and Technology, R. I. Jaffee and H. M. Burte, 2207, Plenum Press (1973).

7.

F. A. Crossley,

8.

P. C. Gehlen, The Science, Technology and Application of Titanium, R. I. Jaffee and N. E. Promisel 349, Pergamon Press (1970).

9.

C. E. Shamblen and C. J. Rosa, Met. Trans.

i0. E. W. Collings, and Technology,

Trans. AIME 239, 1200

Met. Trans.

(1967).

Trans. ASM 62, 398

Trans. ASM 57, 247

i, 1921

(1969).

(1964).

Eds.

(1970).

2, 1925

Eds.

(1971).

J. E. Enderby, H. L. Gegel and J. C. Ho, Titanium Science Eds. R. I. Jaffee and H. M. Burte, 801, Plenum Press (1973).

ii. T.K.G. Namboodhiri,

C. J. McMahon and H. Herman, Met. Trans.

4, 1323

(1973).

78

ORDERING IN A Ti-AI-Ga ALLOY

FIG.

Vol.

1

Antiphase domains in a Ti-12 at.% AI-12 at.% Ga alloy. Dark field taken with a (2110)~ 2 reflection. Zone axis

[Olil] ~2"

9, No.

i