Biomedical Engineering Reference
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out in a FEI/Philips XL-30 SEM equipped with a fi eld emission gun (FEG) source,
an electron backscatter EBSD detector, as well as an EDS detector. Sample prep-
aration for TEM studies involved drilling of 3 mm diameter cylinders by Electro-
discharge Machining (EDM), followed by sectioning of thin discs from these
cylinders, which were subsequently mechanically thinned and ion-milled to elec-
tron transparency in a Gatan Duo Mill. The TEM specimens were characterized
in a Philips CM200 TEM at an operating voltage of 200 keV.
9.3.3.2 Microstructure and Mechanical Properties.
The microstructure
of the LENS™-deposited TNZT sample is shown in the backscatter SEM image
in Figure 9.4a [28]. It is evident from this image that the as-deposited alloy is pri-
marily single phase with no substantial compositional inhomogeneity. The same
was confi rmed by x-ray diffraction (XRD) studies on the same sample. A repre-
sentative XRD pattern from this sample is shown in Figure 9.4(b). This XRD pat-
tern indicates the presence of a single
phase in the as-deposited TNZT sample.
The composition of the alloy, as determined from EDS studies in the SEM, was
found to be Ti-34%Nb-7%Zr-7%Ta (all in wt%). (The error associated with the
measurement of compositions using EDS in a SEM is typically
β
1 - 2%.) As com-
pared with the target composition, there is a marginal increase in the Zr and Ta
contents in the LENS™ deposited alloy, presumably due to the differences in the
fl ow rate of Zr and Ta powders as compared with Ti and Nb powders.
Nevertheless, considering that this chemically complex alloy was deposited
in situ
from a blend of elemental powders, the composition is within reasonable
limits of the target composition. In addition, the oxygen contents of these samples,
measured using were
∼
0.16 wt%. A bright - fi eld TEM micrograph of the as-
deposited sample is shown in Figure 9.5a. This image shows a grain boundary
between two
∼
β
grains. A selected area diffraction (SAD) pattern recorded along
the [113]
zone axis is shown in Figure 9.5b. In this SAD pattern, in addition to
the primary refl ections arising from the
β
β
matrix, secondary precipitate refl ec-
tions are also visible along the
g
= (21 - 1)
β
vector. The intensity profi le along the
g
= (21 - 1)
vector is shown in Figure 9.5b below the SAD pattern. These second-
ary refl ections are present at the 1/3 and 2/3 (21-1)
β
locations and other equivalent vectors. While the secondary refl ections at the 1/3
and 2/3 (21-1)
β
as well as at the 1/2 (21-1)
β
β
positions can be attributed to precipitates of the
ω
phase in the
β
matrix, the refl ections at 1/2 (21-1)
β
positions can be attributed to nanometer-
scale precipitates of the
matrix [26,29]. Therefore, it can
be concluded that in the as-deposited condition, these alloys consist predomi-
nantly of a
α
phase within the same
β
precipitates.
The engineering stress-strain curve for the as-deposited tensile sample is
shown in Figure 9.6. The corresponding mechanical properties of this sample have
been tabulated as insets in the same fi gure. The alloy exhibits a relatively
low modulus,
β
matrix with nanometer-scale
ω
and
α
∼
55 GPa, but reasonably high yield (
∼
814 MPa) and tensile (UTS
∼
834 MPa) strengths. These strength values are substantially higher than those
reported for alloys of similar composition in the solution-treated condition while
the modulus is comparable. The yield strength and UTS of Ti-35Nb-7Zr-5Ta in
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