Biomedical Engineering Reference
In-Depth Information
working) and the interstitial (primarily oxygen) content determine the volume
fractions of the a and b phases. The b phase is stable at room temperature only
if the vanadium content is greater than 15%. When slowly cooled, the alloy
may contain up to about 90 vol% of the a phase. This phase precipitates as
plates or needles which have a specific crystallographic orientation within
grains of the b matrix. Depending on the phase morphology, the microstructures
are classified into three categories: lamellar, equiaxed or bimodal (a mixture
of both).
Lamellar structures are obtained by heat treatments. slow cooling at the
furnace from the b phase field allows precipitation of the a phase at the
b-grain boundaries forming plates that grow towards the grain interior, as can
be seen in Fig. 6.4. The resulting lamellar structure is fairly coarse. Higher
cooling rates, for instance in air, give rise to a microstructure consisting of
a fine needle-like a phase referred to as acicular. at intermediate cooling
rates, Widmanstätten structures are obtained. Finer lamellar structures can
be obtained by water quenching the alloy from the b-transus temperature
followed by annealing at intermediate temperatures in the biphasic region.
Quenching from temperatures higher than 900ºC results in an acicular or
sometimes fine-lamellar hcp martensite (a¢), while quenching from the
750-900ºC temperature range produces an orthorhombic martensite (a≤)
that is a rather soft martensite. This phase can also form as a stress-induced
product by straining metastable b.
equiaxed microstructures are obtained by applying a severe deformation
(>75% reduction) to the alloy in the biphasic condition and subsequent
100
m
m
6.4
Optical micrograph showing the microstructure of the Ti-6Al-4V
alloy annealed at 1100°C for 2 hours, then slowly cooled within the
furnace (courtesy of J. Chao, CENIM).
