LASER PROCESSING OF ORTHOPEDIC BIOMATERIALS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 286

indicated the preferential texture

has been previously reported for LENS™ deposited Ti-Cr alloys [33] and is likely
a result of the preferential texture resulting from the alloy solidifi cation. Further-
more, this

< 001 > orientation of the β grains. Such a < 001 >

texture might infl uence the deformation mechanisms in these
LENS™ deposited samples and could lead to the enhanced yield and tensile
strengths observed experimentally. However, more detailed studies are required
in order to investigate these aspects of the deformation behavior.
A bright - fi eld TEM image of the tensile tested TNZT sample is shown in
Figure 9.8a. A large number of distinct shear bands are clearly visible in this TEM
image. The presence of such shear bands indicates deformation dominated by a
slip mechanism and also slip localization in shear bands. A SAD pattern recorded
along the [113]

< 001 >

zone axis from the same sample is shown in Figure 9.8b. In this
diffraction pattern, in addition to the primary refl ections arising from the

β

β matrix,

additional refl ections are also visible along the g = (21 - 1) β vector and vectors of

the same type. A intensity profi le along the g = (21 - 1) vector is also shown in

Figure 9.8b. From this intensity profi le, it is evident that while the secondary
refl ections at the 1/2 (21-1)

β

β locations are quite distinct, those at the 1/3 and 2/3

(21 - 1) locations are much lower in intensity and not as distinct as in case of the

as-deposited LENS™ sample prior to tensile testing (refer to the SAD pattern
and the intensity profi le in Figure 9.5b). Therefore, it appears that while well-
defi ned

β

α precipitates, giving rise to the secondary refl ections at the 1/2 (21-1) β

locations, are present after the tensile testing, the ω precipitates, giving rise to the

secondary refl ections at the 1/3 and 2/3 (21-1) locations, are not as well-defi ned.

The intensity at these locations appears to be streaked along the g = (21 - 1)

β

β

vector. Furthermore, comparing the relative intensity of the ω refl ections before

and after tensile testing, it appears that the precipitates are likely to be of

smaller size in the tensile tested samples as compared to the LENS™ as-
deposited sample. The reduction in the average size of the

ω

precipitates is likely
to be a consequence of shearing of these precipitates by the dislocations during
plastic fl ow. Such precipitate shearing at the nanoscale would consequently result
in slip localization and the formation of shear bands that is experimentally
observed in the tensile-tested samples.
The dislocations within the shear band have been imaged under two-beam
imaging conditions in the TEM and these results have been shown in Figure 9.9.
A two - beam bright - fi eld TEM image recorded using g = (011)

ω

β near the [1 - 11] β

zone axis is shown in Figure 9.9a. The shear bands and the dislocations within the
shear bands are clearly visible in this diffraction contrast image. Another set of
two images from the exact same location, recorded using two different two-beam
imaging conditions are shown in Figures 9.9b and 9.9c. Thus, while Figure 9.9b
shows the two - beam bright - fi eld image recorded using g = (011)

β near the [3 - 11] β

zone axis, Figure 9.9c has been recorded using g = ( - 1 - 21) zone

axis. The dislocation contrast within the shear band, running almost parallel to the
edge of the TEM foil, is visible in Figure 9.9b but is invisible in the same region in
Figure 9.9c. The invisibility criterion for g = ( - 1 - 21)

β near the [3 - 11] β

suggests that most of these
dislocations possibly have a burgers vector of the type 1/2

β

< 111 > β as would be