IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
alloy undergoes a martensitic
transformation when quenched
from the α phase field (1143K)
forming a hexagonal
martensite. However, β phase if
sufficiently enriched with
stabilizers results a transformed
β microstructure (Fig. 1(a)) on
water quenching, when
isothermally treated in the α+β
phase field at 973K, which is
below martensite start (Ms)
temperature. Analysis of SAD
patterns (inset of Fig. 1(a)) from
retained β showed the presence
of phase. The orientation
relationship between α and β,
ω and β phases were found to
be obeyed, as reported for Ti
and its alloys. Dark-field
microscopy of ω reflections,
showed a dense distribution of
fine, ellipsoidal particles of
sizes ranging from 4 to 10 nm
(Fig. 1(b)). The ellipsoidal
morphology of the particles is
attributed to the low misfit
between α and β phases, since
it can be ellipsoidal or cuboidal
depending on the degree of
misfit. The lower misfit in this
alloy is attributed to the similar
atomic size of Ti, Ta and Nb.
The hardness of the β annealed
alloy, the structure prior to the
isothermal treatment showed no
significant increase after
isothermal treatments, which is
also attributed to the poor solid
solution strengthening effect of
Ta and Nb. It was also observed
that aging this alloy under
similar conditions showed no
evidence for decomposition of
β phase. These observations
therefore confirm that athermal
phase forms in this alloy.
IV.C.2. Effect of Deformation Mode on Texture
Development in Ti-5Ta-2Nb Alloy
Ti-5Ta-2Nb alloy has been
developed as a structural
material for fuel reprocessing
applications due to its excellent
oxidation and corrosion
resistance. The alloy undergoes
a variety of phase
transformations depending on
the temperature and cooling
rate. The products show distinct
microstructural features and
texture. Further, the texture is
influenced by the prior
deformation.
This alloy has α (hcp) phase
as the predominant phase at
room temperature, while β
(bcc) phase gets stabilized at
higher temperatures. The alloy
Fig.1 XRD of wire and sheet sample before and after heat treatment,
compared with powder sample. Surface of sheet and wire cross-section
have been studied.
FUEL CYCLE 105