Biomedical Engineering Reference
In-Depth Information
titanium boride precipitates within the soft beta titanium matrix and substantially
enhances its wear resistance.
Based on the above discussion, it is evident that basically two types of materi-
als will need to be processed using LENS™ for these functionally-graded implants.
The individual materials that will eventually be integrated into the femoral hip
implant are listed below:
1. Beta titanium based Ti - Nb - Zr - Ta alloys deposited using LENS ™ from a
blend of elemental powders.
2. Metal - matrix composites based on titanium boride reinforced Ti - Nb - Zr -
Ta alloys deposited using LENS™ and compositionally graded compos-
ites with gradations from Ti - Nb - Zr - Ta to Ti - Nb - Zr - Ta + TiB (borides)
deposited using LENS ™ .
9.3.3 Laser Processed Ti-Nb-Zr-Ta Alloys
9.3.3.1 Details of Processing and Characterization Methods Used.
The
specifi c alloy composition chosen was Ti - 35Nb - 7Zr - 5Ta (wt%) (henceforth
referred to as TNZT), based on the good balance of properties reported for the
conventionally-processed material of this composition [27]. All the LENS™
depositions were carried out on a commercially procured Ti-6Al-4V alloy
substrate. The LENS™ unit used for these depositions was manufactured by
Optomec Inc., Albuquerque, New Mexico. The
in situ
alloys were deposited from
a blend of pure elemental Ti, Nb, Zr, and, Ta powders mixed in the ratio of 53 wt%
Ti + 35 wt% Nb + 7 wt% Zr + 5 wt% Ta. The elemental powder feedstock used for
these depositions are as follows:
Ti: 99.9% pure,
−
150 mesh (from Alfa Aesar)
Nb: 99.9% pure,
−
200
×
325 mesh (from Reading alloys)
Zr: 99.8% pure,
−
140/+325 mesh (from CERAC)
Ta: 99.98% pure,
−
100 mesh (from Alfa Aesar)
The laser power used in these depositions was
350 W. The hatch width (gap
between two successive passes of the laser beam), layer spacing (the gap between
two successive layers deposited), and travel speed (the speed at which the sub-
strate is moved with respect to the laser beam), used for all depositions were
0.38 mm, 0.25 mm, and, 635 mm/min, respectively. The LENS ™ deposited alloys
had a cylindrical geometry with a diameter of 12 mm and a height of 30 mm.
Tensile specimens were machined from the as-deposited cylinders and tested
in tension to failure by fracture. The tensile specimens were machined as per
ASTM standards, E-8(00) with the dimensions of the gage being 3.6 mm diame-
ter
∼
20.6 mm length. These specimens were subsequently characterized in detail
by scanning electron microscopy (SEM), orientation imaging microscopy (OIM),
and transmission electron microscopy (TEM). The SEM/OM studies were carried
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