Scripta METALLURGICA
et MATERIALIA
Vol. 27, pp. 605-610, 1992
Printed in the U.S.A.
Pergamon Press Ltd.
All rights reserved
STABILITY OF THE ORDERED ORTHORHOMBIC PHASE IN Ti-24AI-11N-b
L.M. Hsiung and H.N.G. Wadley
Department of Materials Science and Engineering
University of Virginia
Charlottesville, Va 22903-2442 U.S.A.
(Received June 26, 1992)
The existence of an ordered orthorhombic (O) phase in the Ti-A1-Nb ordered alloy system was first reported by
Banerjee et al.[1], and has subsequently been confirmed by several investigators[2-4]. The O phase has been found in
Ti-AI-Nb alloys either slowly cooled from the temperature regime above 1200°C (1~ stable regime) or after quenching
from that temperature regime followed by isothermal aging. Recently, we have also reported the formation of the O
phase through a sequential transition (1~ - B2 ~ T - O) in an aged Ti-24Al-llNb alloy[5,6]. This study showed that
the O phase was metastable; an O ~ a 2 transition took place when the O phase was further aged. Detailed studies of
the B2 - T and T - O transitions have been reported elsewhere[7,8]. Here, the structural relationships between the O
and a 2 phases have been investigated in order to elucidate the mechanism of the O - a 2 transition.
A plasma-sprayed TijAI+Nb alloy with a nominal composition of Ti-24at%Al-1 lat%Nb similar to that studied
earlier[7,8] was used for this study. The sample was produced from powder through an inductively coupled plasma
deposition (ICPD) process at GE Aircraft Engines, Lynn, MA. During the ICPD process, titanium aluminide powder
was melted by passing through a plasma. The molten droplets were immediately deposited onto a mandrel inside a
vacuum chamber where they were rapidly quenched to a solid state forming a 25 mm thick foil. Specimens of this foil
were wrapped with tantalum foils and sealed in cleaned and evacuated quartz ampoules. Isothermal aging was
subsequently performed at 650 ° and 800°C. The microstructure of the alloy was examined using transmission electron
microscopy (TEM), selected area diffraction (SAD) and convergent beam electron diffraction (CBED) methods in a
Philips-400T transmission electron microscope.
3. Results and Di~ussion
The 0 ~ ~ 2 transition
A typical microstructure obtained from a sample aged for 4 h at 650°C is shown in Fig. l(a). A plate-like a 2 phase
was observed to form within an equiaxed O grain. Notice that two different orientation variants of the a 2 plate can
be found. The O/-, 2 interface (habit) planes were determined to be (10i'0)a211(110)O and (1010)a2](l10)o. We note
that the angle between the two O/a 2 interface planes is equal to (ll0)oA(110)o, i.e. 63.4 ° (Fig. l(b)). A selected area
diffraction (SAD) pattern generated from the (O + "2) two-phase region is shown in Fig. 2. The orientation
605
0956-716X/92 $5.00 + .00
Copyright (c) 1992 Pergamon Press Ltd.
606
ORDERED PHASE IN T i - A 1 - N b
Vol.
27,
relationshipsbetween the O and a 2 phases can be derived from the SAD pattern: (001)ol(0001)~ 2, [100]oA[1120]ct2
= [110]0^[1010]~ 2 = 1.5°.
The "2 plates eventually grew and coaiescenced to an equiaxed te2 grain as the aging was further extended to 24 h
at 650°C (Fig. 3(a)). Notice that the plate-like structure disappeared and only a small number of dislocations were left
within the t¢ 2 g r a i n . CBED patterns generated from the [2110] and [0001] zones of such an equiaxed ~2 grain are shown
in Fig. 3(b) and 3(c), respectively. A 2ram symmetry is displayed in the [2110]~2 zone pattern and a 6mm symmetry
can be found in the [0001]~2 zone pattern.
Structure relationships between the 0 and ~2 Phases
The unit cell of the O phase can be derived from a T phase through a shape deformation mechanism[7] as shown in
Fig. 4(a). The O phase may also be considered as a pseudo-hexagonal phase with a lattice distortion resulting from a
supersaturation of the Nb in the "2 lattice. Note that the Nb saturation concentration in a 2 below 900°C is -9.2 at%
according to Weykamp et al.[9], i.e. -1.5 Nb atoms per "2 unit cell (16 atoms/unit cell). To accommodate the excess
Nb content (- 1.8 at%), the supersaturated "2 lattice is no longer able to maintain its hexagonal symmetry and becomes
an O lattice. Notice that the Nb atoms occupy the (0,1/4,1/2) and (0,3/4,1/2) sites of the O lattice. The dimensions of
the O lattice are a = 0.605 nm, b = 0.98 nm and c = 0.47 nm. The perfect hexagonal "2 lattice can be restored by
diffusion of the excess Nb away from the parent O lattice together with a homogeneous lattice distortion and an atomic
shuffling on every third (100)o. That is, the ct2 phase may be formed through a decomposition reaction: O (parent)
"2 0Wb-lean) + O (Nb-rich)[6].
Similar to the T -, O transition[7], a shape deformation mechanism may also be applied to predict the structural
relationships between theO and "2 phases. A s illustrated in Fig. 4(a), the lattice correspondence between the O and
a 2 phases is [110]0 - [1010]a2, [110]O ~ [0110]s2 and [001]O - [0001]a2. The "~2lattice was drawn here by assuming
that Nb does not occupy a specific lattice site. By redefinition of the "2 lattice on the basis of an orthorhombic cell
(shaded area in Fig. 4(b)), the dimensions of the a 2 lattice becomes a ,, 0.58 nm, b = 1.0 nm and c = 0.465 rim. Thus,
the "2 lattice can be obtained by contracting the O lattice by 4.1% along [I00]O to create [100]ffi2,expanding [010]O by
2% to create [010]ffi2,and contracting [001]O by 1.1% to create [001]a2. Referring to the directions [100]O, .[010]O and
[001]O as x-, y-, z- axes, these can be expressed as follows:
l! :1 l°i :1
e~,
0
ffi
0.020
¢33
0
(1)
0.011
The principal strains, denoted by vlij, of the homogeneous distortion B for the O -. a 2 transformation are given by
0
Vls3
0
1+e33
0
1.011
The condition for there to be an invariant strain (undistorted) plane is that one of the principal strains be unity, and
the other two be greater or less than unity. Since q33 = 1.011 is close to unity, the amount of slip or twinning
(corresponding to shear deformation S) is quite small. Alternatively, the invariant plain strain can be obtained by
combining the lattice distortion B with a uniaxiai strain (tension e) along the z-axis, This gives an invariant plain strain
B':
No.
5
Vol.
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5
ORDERED
PHASE
IN T i - A I - N b
607
(3)
where u is Poisson ratio (-0.3).
A theoretical prediction of the habit plane and orientation relationships between the O and a 2 phases can be made
in the manner shown in Fig. 5. The effect of the invariant plain strain B' on a spherical O crystal, viewed along the
z-axis(ll[001]o), is illustrated. The spherical crystal was deformed into an ellipsoid due to the strain B'. However, the
planes OQ' and OP' are not distorted by the strain, but were rotated from their initial positions OQ and OP. To
produce a unrotated as well as a undistorted habit plane, a rotation about the z-axis has to be added to B' to return
one of these planes to the initial position (OQ' to OQ for instance). Let Q be the point (x,y). The coordinates of
Q'(x',y') can be determined by:
y/ --/
0
1.017
y
0
0
0
(4)
That is, x' = 0.956x, y' = 1.017y. Since OQ = OQ', x2 + y2 = (0.956x)2 + (1.017)2. Thus, ZQOY = tan -1 (x/y) = 32.2 °.
The O/n 2 interface (habit) plane therefore makes an angle 32.2 ° with the (100) O plane; this is only 0.5 ° away from the
(110) O plane ((110)_OA(100)O = 31.7°). zQ'OY = tan "1 (x'/y') = 30.6°; thus the rotation angle 0 = 1.6°. This rotation
makes (110)o~(1010)a2. Note that the alternative (110)o habit plane through OP (Fig. 5) is crystaUographically
equivalent to the (110)o habit plane through OQ. The predicted orientation relationships between the O and ~2 phases
are (001)ol[(0001)a 2, and [100]oA[1120]a 2 = [110]OA[0110]a2 = 1.6°. These are in agreement with the experimental
results shown in Figs. 1 and 2.
t._Sammar~
A study of the stability of the O phase in Ti-24AI-11Nb alloy has led to the following conclusions:
1. The O ~ "2 transition occurs with a plate-like a 2 phase formed within the O matrix.
2. The O - a 2 transition may be explained by a shape deformation mechanism accompanied by diffusion of the excess
Nb away from the O/¢t 2 interface.
3. T h e 0 / , , 2 interface (habit) plane is {110}ol{ 1010} a2"
4. The orientation relationships between the O and tt 2 phase are (001)Ol(0001)a2, and [100]oA[l120]a2 =
[110]oA[01 i0]a2 = 1.6°.
~ ] ~ r ~ e . d e . m r ,m
This work was co-sponsored by the Defense Advanced Research Projects Agency (DARPA), the National
Aeronautics and Space Administration (NASA) through Contract Number NAGW-1692, and GE Aircraft Engines
through Contract Number MS-GE-5222-92. The authors thank E.S. Russell and D. Backman of GE Aircraft Engines,
Lynn, MA for providing the material used in this investigation.
608
ORDERED
PHASE
IN Ti-AI-Nb
Vol.
27, No.
1. D. Banerjee, A.K. Gogia, T.K. Nandi and V.A. Joshi, Acta Metall. Mater., 36, 1988, p. 871.
2. H.T. Weykamp, D.R. Baker, D.M. Paxton, and M.J. Kaufman, Scripta Metall. Mater., 24, 1990, p. 445.
3. J.A. Peters and C. Bassi, Scripta Metall. Mater., 24, 1990, p. 915.
4. L.A. Bendersky, W.J. Boettinger and A. Roytburd, Acta Metall. Mater., 39, 1991, p. 1959.
5. L.M. Hsiung, W. Cai and H.N.G. Wadley, presented in Proc. of the 1991 International Conference on High
temperature Aluminides and Intermetallics, to be published in Mater. Sci. and Eng., A152, in press.
6. L.M. Hsiung, W. Cai and H.N.G. Wadley, Acta Metall. Mater., in press.
7. L.M. Hsiung and H.N.G. Wadley, Scripta Metall. Mater., 26, 1992, p. 35.
8. L.M. Hsiung and H.N.G. Wadley, Scripta Metall. Mater., 26, 1992, p. 1071.
9. H.T. Kestner-Weykamp, C.H. Ward, T.F. Broderick and M.J. Kaufman, Scripta Metall. Mater., 23, 1989, p. 1697.
O
~
~ ~
°o,.o.,..
"'°,,O,o
Fig. 1. (a) Dark-field (DF) TEM image showing formation of the plate-like a 2 phase within an O grain
(650°C, 4h), Z -- [001]oll[0001]a2,(b) a [001]O stereographic projection showing the orientation of the
interface.
O/a2
5
Vol.
27, No. 5
ORDERED PHASE
IN T i - A I - N b
0 = 1.5"
609
~I
• .............. 0
0 ......... c
Fig. 2. A selected area diffraction (SAD) pattern generated from the (O + a2)
two-phase region in Fig. l(a), Z = [001]ol[0001]a 2.
(b)
(a)
(c)
Fig. 3. (a) Dark-field_=TEM image showing a coalescenced a 2 grain (650°C, 24h), CBED patterns
of (b) the [2110].,2 zone: 2ram symmetry and (c) the [0001]=2 zone: 6ram symmetry.
610
ORDERED PHASE IN Ti-AI-Nb
[001] 0
0 Ti
[COOl]%
•
( ~ Ti (Nb)
AI
•
Vol.
[0001]a2
A=
Nb
[010]0
•
1
[11010
01 a 2
[100] 0
(a)
,12o]%
(b)
,1~o]%
Fig. 4. Schematic illustrations of(a) lattice correspondence between the O and ¢2 lattice, and
(b) unit cell of the ~2 phase.
[0101o
Y
[1001o
Fig. 5. An ellipsoid developed from a sphere of the O crystal by the invariant plane strain B'.
27, No.
5