24 / 70
morphology. However, much more complex microstruc-
tures can be generated in these alloys. Depending on the Nb
content and thermo-mechanical processing condition,
Ti
2
AlNb alloys exhibit different combinations of α
2
, O, and
B2 phases. The B2 phase can be retained on quenching and
subsequently decompose to α
2
and O laths via aging. The α
2
phase can also transform to O phase under certain condi-
tions. A detailed study of the microstructure evolution of
several Ti
2
AlNb alloys can be found in Ref. 6. Figure 3 shows
the microstructure of the Ti-22Al-24Nb base alloy with 0.5%
addition of Mo. The dark, gray, and bright regions
correspond to the α
2
, O, and B2 phases.
In the literature, Gogia summarizes the correlation be-
tween microstructure and mechanical properties of the
ternary Ti
2
AlNb alloy system
[7]
. First, in equiaxed α
2
+B2
or O+B2 microstructure, a higher volume fraction of the
B2 phase results in an increase of strength, ductility, and
fracture toughness. The B2 phase can decompose into α
2
and O by aging, with the higher aging temperature causing
a decrease in strength and increase in ductility. For the B2
phase, higher Al content and larger grain size leads to in-
creased ductility and toughness. Secondly, increasing the
cooling rate in the beta heat-treated microstructure pro-
duces a finer microstructure and hence a higher strength,
which is similar to conventional α/β Ti alloys. Lath
microstructure of the α
2
+O phase is preferred in creep-
driven applications. Quaternary additions such as Mo, V,
Si, Fe, and Zr were also explored. The effect of alloying el-
ements on microstructure and mechanical properties is
summarized in Table 1
[8-17]
.
There is no universal alloy composition suitable for all
applications. Rather, alloy composition must be tailored to-
gether with processing to achieve the required properties
for specific components.
Processing and applications
Wrought Processing:
For relatively low-ductility mate-
rials such as Ti
2
AlNb alloys, wrought processing is partic-
ularly useful to achieve the full potential of materials due to
the need to optimize mechanical properties through mi-
crostructure manipulation. Standard processing practices
such as melting, forging, and hot rolling can be used for
making Ti
2
AlNb products. The effect of alternative pro-
cessing methods such as extrusion and isothermal forging
are also under investigation.
Properties of Ti-base intermetallics are more composi-
tion-sensitive than conventional Ti alloys. Ti
2
AlNb alloys
require additional precautions during processing because
their alloying elements have large differences in melting
points and densities (Ti: 1668
o
C, 4.5 g/cm
3
; Al: 660
o
C, 2.7
g/cm
3
; Nb: 2468
o
C, 8.6 g/cm
3
). Ti
2
AlNb ingot material usu-
ally suffers from pronounced segregation of alloying ele-
ments, shrinkage porosity, coarse microstructures, and
ADVANCED MATERIALS & PROCESSES •
MAY 2014
24
TABLE 1 — EFFECT OF ALLOYING ELEMENTS ON MECHANICAL PROPERTIES
OF Ti
2
AlNb ALLOYS
Element
Type
Effect on Mechanical Property
Reference
Al
a
stabilizer
Oxidation and creep resistance improve, ductility and toughness decrease
10
Nb
b
stabilizer
Density, high temperature strength, elevated temperature oxidation resistance,
10
and room temperature toughness increase
Mo
b
stabilizer
Density, tensile strength, elevated temperature creep resistance, and
11, 12
toughness increase
V
b
stabilizer
Density decreases and room temperature ductility, toughness, elevated
9
temperature strength, and creep properties increase
Zr
—
Creep strength improves, yield stress and room temperature ductility are
17
not effected
Si
b
stabilizer
Creep property and oxidation resistance improve
8
Fe
b
stabilizer
Yield strength, ultimate tensile strength, creep resistance improve
13
W
—
High temperature strength and creep resistance improve
9
Ta
b
stabilizer
Yield strength and B2/
b
transus temperature improve
14
Y
—
Hardness improves
15
B
—
Room temperature ductility, hardness, elevated temperature creep property improve 16
Fig. 3 —
Microstructure of a Ti-22Al-24Nb-0.5Mo alloy
showing
a
2
, O, and B2 phases.
1 µm