Scripta METALLURGICA
et MATERIALIA
Vol. 26, pp. 35-40, 1992
Printed in the U.S.A.
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Pergamon Press plc
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T1N THE Ti3~+IL~
L.M. Hsiung and H.N.G. Wadle~
Department of Materials Science and Engineering
University of Virginia
Charlottesville, VA 22903 USA
(Received October 3, 1991)
(Revised October 15, 1991)
Introduction
Ordering of the ~ phase to a B2 structure in the Ti3AI+Nb alloy system has been four~ and
confirmed by many investigators[l-4].
It is known that in additi~ to the B2 phase, a DO~-type
ordered ~ phase also exists in many alloy systems, for instance noble metal based alloys[8],'shape
memory alloys[5,6], and other ~B-type aluminide systen~ such as Fe3AI[7 ] . The formatic~ of th~_e
two ordered ~ phases, for instance in the Fe~Al aluminide[7] and the CuZnAI shape memory alloy[8],
is sensitive to the cooling rate. The ~ -> B2 ordering transition is more likely to form during
a rapid cooling, and the ~ -> D ~ transition is favored during a slower cooling or by reheating the
B2 phase below the B2 -> D~ ~ransitic~ temperature.
No report so far has been made an the
existence of a phase similar to the D~-type in the Ti3AI+Nb system. Since the nature of Ti3AI+Nb
alloys is similar to that of the above-menti~ed alloy "systems[l,2], it is of interest to determine
if the B2 -> ~ ordering transition also occurs in the TilAl+Nb system. From a preliminary study
of phase stability in a plasma-sprayed Ti3Al+Nb alloy usihg transmission electron microscopy and
electron diffraction techniques, evidence has indeed been fom~d for ordering of the B2 phase to a
structure similar to the D~-type phase, but with a tetragcsml distortion. The result is reported
here.
Exper/mental
A plasma-sprayed Ti3AI+Nb alloy with a nominal compositi~ of Ti-14wt%AI-21wt%Nb (Ti-24at%Alllat%Rb) was chosen for ~ i s study. The alloy was produced from Ti3Al+Nb powder via an inductively
coupled plamga depositic~ (ICPD) process[9] by GE Aircraft ~/ines, Lynn, MA. During the ICPD
p r o c e s s , aluminide powder was passed through a plasma a r c t o cause m a l t i n g . The molten d r o p l e t s
were imnwdiately d e p o s i t e d c ~ t o a ~andrel i n s i d e a vacuum chamber where they were r a p i d l y quenched
to a solid state. In this process, as depositicm continued, the molten alloy was first deposited
at a temperature of -8000 C, held for varying periods, and then subeequently cooled to room
temperature. The microstructure of the alloy was exauined in its as-sprayed and aged status using
tran.~ssion electron microscopy (TEM), microdiffracton (MD) and convergent beam electrc~
diffractic~ (CB~3) in a Philips-4OOT transmission electron microscope (operated at 120 keV).
Before aging, specime~ were wrapped with tantalum foils and sealed in cleaned and evacuated quartz
a~i~ules. Aging was Performed for different times at 650o C.
R e s u l t s end Di.-~'ussien
Mi crostructure
Figure l(a) is dark-field TEM image observed from an as-sprayed alloy showing a network of
thermal anti-phase bom~daries (APEs) formed in a B2 grain. A tweed-like or modulated structure,
Fig. l(b), accumpar~ed with diffuse streaking and local diffuse maxima at various 1/2<i10> and
i/2<i12> positicrm in diffraction patterns, was frequently observed in the B2 phase; an example is
35
0036-9748/92 $5.00 + .00
Copyright (c) 1991 Pergamon Press plc
36
ORDERED TETRAGONAL PHASE
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1
shown in Fig. l(c). According to Strychor et al.[2], these anomalies may be attributed to the
man/festation of a (110}<i10> type lattice displacement wave (LDW) similar to prmrertensitic
phenomena found in many noble metal-based alloys and shapememory alloys[5]~ They postulated that
the cubic symmetry of the B2 phase canbe altered as a result of the {ll0}<il0> type shear strain.
To examine the s~m~try of the B2 phase, convergent-beamelectrondiffraction(CBED) was applied.
The CSED patterns of <001>, <iii> and <ii0> zones generated from the B2 phase are shown in Figs.
2(a) - 2(c). These patterns reveal the expected cubic slawnetry of the B2 phase. Strychor et al.
also reported that the diffuse maxima at various half-integer positions subsequently intensified
and sharpened to reflections during aging. A high density of fine particles or domains was
associated with those reflectiorm.
Yet, no satisfactory expla~ati~ was provided.
Similar
phenomena also were found by us. As is shown in Fig. 3, fine particles or domains were imaged from
a local area of an as-sprayed alloy using the sharpened 1/21i12] reflection. These domains can
further grow upon aging. Detailed examination of the domain using the electron diffraction method,
as will be shown later, reveals that its structure is no longer cubic. An alternation of the
symmetry froma cubic to tetragonal has been identified. Formation of a new ordered phasebecause
of further chemical ordering in the B2 lattice is considered to be responsible for th/s
alternation.
A typical microstructure ohserved inanalloy after aging for 1 0 m in at 650° C is shown in Fig.
4(a). Coarsened domains associatedwith thermal anti-phasebotmdaries (indicated by arrows) were
imaged using the 1/21i10] (superlattice) reflection under two-besm conditions. Microdiffraction
(MD) patterns of tbe<100>, <iii> and<ll0> zones (generated from different domains) are shown in
Figs. 4(b) - 4(d). Notice that besides the 1/2<i10> and 1/2<1i2> positions, the (superlattice)
reflections also appear at various i/4<i12> and3/4<li2>positions in the MD patterns of the <111>
and <ii0> zones, Figs. 4(c) and 4(d). More importantly, a tetragonal distortion was found by a
careful exmninationof the MDpatterns. To show the existence of this distortion, a comparison of
geametric parameters determined from the diffraction patterns of the B2 and the distorted
(tetragcmal) phases is shown in Fig. 5. Symmetry of the new phase also was examined. The CBED
whole patterns of the <100>, <001>, <ii0>, and <Iii> zones of the new phase are shown in Figs.
6(a) - 6(d). Since the intensity of these patterns is blurred due to the existence of distortion,
interpretation has to be made with caution. By comparing these patterns with the CB~)patterns of
the B2 phase shown in Fig. 2, we find that while the symmetry of the <001>and <ii0> zone patterns
remains nearly four and two-fold, respectively, the symmetry of the <i00> zone pattern reduces to
two-fold, and the <111> zone pattern no longer has s three-fold symmetry. These are cansistent
with the sla~metry of a tetragonal system according to the international
tables for
crystallography[10].
Apparently, an ordered tetragcrml phase (designated as T) was formed. The
lattice constant a 0 is determined to be 0.65 ± 0.01 ran and the tetragonality ~ / ~ % 1.02.
Crystallography of the new phase
We view the T phase as a D~-like ordered phase with a tetragonal distortion. We believe the
tetragcnal distortion arises due to an ordering of Nb atoms in the lattice. An ordered tetragcmal
superlattice based on the composition of TisAI~NB is proposed and illustrated in Fig. 7. The Nb
atoms occupy the (1/2,0,0) and (0,1/2,0) positions.
A tetragcnal distortion along the [001]
direction is generated due to an atomic size effect.
The structure factor for superlattice
reflections from the superlattice can be expressed as
F % Fa {I + exp[2riC2h + 2k + 21)/2]}
where Fa is a term including the atomic scattering factors, and h, k and I are allowed to be halfintegers since the Miller indices (h,k,l) are referred to the basic b.c.t, cell in the
superlattice. When h + k + I = half-integer, F = 0, i.e reflectiems are forbidden. Reflections
occur as the condition h + k + I = integer is satisfied. That is, reflectioms can be observed at
positions where each indice is an integer, or one is an integer and other two are half-integers.
The superlattice reflecticrm observed in Figs. 4(b) - 4(d) are consistent with the reflectioms
allowed for the ordered tetragmnal superlattice. The reflections at various 1/4<112> and 3/4<112>
positions, see Figs. 4(c) and 4(d), also are allowed since the h + k * I = integer for those
reflections. The T phase is a transition phase, and it transforms to an ordered orthorhumbic (O)
phase when the aging proceeds. Details of the T phase stability and its relaticrmhips with the O
and ~ phases will be published elsewhere.
1. A change of the cubic slnmmetry of the B2 phase upon aging has been identified using a
Vol.
26, No.
ORDERED TETRAGONAI, PHASE
I
37
convergent-bean electron diffraction method. This change is attributed to a further chemical
ordering of the B2 phase.
2. An ordered tetragonal phase (T) is proposed to account for our observatiorm, and existence of
the B2 ~>Jorderingtransformation
in the Ti3AI+Nb system is suggested.
3. The T prmse canoe regarded as a D~-like p~iase with a tetragonal distortion. The tetragonaliy
c o / a ~ ~ 1.02, and a o : 0.65 ± 0.01 rcn.
Acknowle~emm~ts
This work was c o - s p o o r e d by the DefenseAdvancedResearch Projects Agency, and the National
Aeronautics and Space Administration through Contract Nunber NAGW-1692, and GE Aircraft Engines
through Contract Ntm~berMS-GE-5222-92.
Reference
i. R. Strychor and J.C. Williams, in Solid-Solid Phase Transformations, edited by H.I. Aaronson et
al., p. 249, TMS-AIME, New York (1982).
2. D. Banerjee, T.K. Nandi and A.K. Gogia, Scripta Met., 21, 597 (1987).
3. R. Strychor, J.C. Williams and W.A. Soffa, Met. Trans. A, 19A, 225 (1988).
4. L.A. Bendersky and W. J. Boettinger, Mat. Res. Soc. Syrup. Proc., edited by C.T. Liu et al., 133,
p. 45, Materials Research Society (1989).
5. H. Warlinx~nt and L. Delaey, Procjress in Materials Science, 18, 1 (1974).
6. K. Shimizu and T. Tadaki, in Shape Memory Alloys, edited by H. Funakubo, p. I, Gordon a~.d Breach
Science Pttblishers, (1984).
7. M.J. Marcinkowski and N. Brown, Acta Met., 9, 764, (1961).
8. J. Dutkiewicz and J. Morgiel, Journal of Materials Science, 21, 429, (1986).
9. D.G. Backrnan, E.S. Russel, D.Y. Wei and Y. Pang, Proc. o£ the 1989 Syrup. in Intelligent
Processing of Materials, edited by H.N.G. Wadley and W.E. Eckhart, Jr., p. 17, The Minerals, Metals
& Materials Society (1990).
i0. "International Tables for X-ray Crystallography", Vol. i, 3rd ed. Kynoch
press, Birmingham,
England, (1969).
J
•
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o I
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i
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0
(c)
Fig. i. Dark-field (DF) T~M image observed from an as-sprayed alloy showing
formation of (a) thermal anti-phase domain boundaries, Z (zone axis) -- [i00],
g = 001); (b) tweed-like structure, Z ~ [i00], g = 002 in the B2 phase;
(c) microdiffraction (MD) patterns of the <O01>B2, <Ill>B2 and <II0>B2 zones.
38
ORDERED TETRAGONAL PHASE
Vol.
26,
No.
Fig. 2. Ccmvergent-besm electr~ diffracticm (CBED) whole patterns generated
from the B2 phase, (a) the <001> z ~ e showing 4ram symmetry, (b) the <iii> zone
showing 3m slmmetry, and (c) the <ii0> zane showing 2am symmetry.
e
I
0
i
lid
II
Fig. 3. Dark-field TSM image showing high density of fine particles or dc.ains
in a B2 phase observed fram a local ares of the as-sprayed alloy, Z -" [ll0],
g = ½ i12.
1
Vol.
26,
No.
1
ORDERED TETRAGONAL PHASE
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,~ '~ ~ m ~ ,
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Fig. 4. (a) Dark-field TSM image showing coarsemed dm~ins
associated with
thermal anti-phase bom~daries (indicated by arrows) observed frun an alloy after
aging for i0 rain at 6500 C, Z ~ [ii0], g = ½ ii0; the MD patterns of (b) the
<100> zone, (c) the <111> zone, and (d) the <110> zone.
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Fig. 5. A comparison of geometric parameters determined frum the diffraction
patterns of the B2 and tetragonal phases. The crosses in one quadrant of the
diagrmn indicate the positicr~ of the superlattice reflections for the B2 phase.
40
ORDERED
TETRAGONAL
PHASE
Vol.
26, No.
Fig. 6. CBED whole patterns generated fram the new phase; (a) the <001> zone
showing two-fold symmetry, (b) the <100> zone showing four-fold s~an~etry, (c) the
<ii0> zone showing two-fold symmetry, (d) the <111> zone.
Fig. 7. A proposed ordered tetragonal superlattice. The atomic radii (nm)
of Ti, A1 and Nb are 0.147, 0.143 and 0.143 respectively.
1