Titanium alloys: modelling of microstructure18
through refined fully lamellar (RFL) heat treatment is as follows: the alloy
should be heat-treated in the single-phase α field range from T
α
, or even
better from T
α+β
to 1450 °C within the α + β phase field. In this latter
regime, α grains are formed but are prevented from growing due to the
presence of β-phase particles, so the competition of nuclei formation limits
the growth of α grains. Higher temperatures in the range generally result in
finer grain sizes. The annealing time is typically 10–20 min. The cooling rate
is important for controlling the grain size and the lamellar spacing. In order
to minimise coarsening of lamellae, once they are formed the cooling rate
should be made as fast as possible (Cao et al., 2000; Wang and Xie, 2000;
Wen et al., 2000).
Among the results reported on the effect of the heat treatment on
microstructure, there is controversy at present. For example, some believe
that microstructure can widely be changed solely by heat treatment, and the
nature of phase transformation to FL microstructure depends strongly on
both cooling rate and composition of the alloy. This contradicts the opinion
of other authors that the effect of cooling rate is not that important for
microstructure and that the chemistry (complex alloying effects) controls the
grain growth mainly (Hu and Botten, 2002; Zhang et al., 2000).
Controversy concentrates on the influence of heat treatment on two main
microstructure parameters, grain size and lamellar spacing. The two parameters
are usually related to each other, and the earlier experimental results do not
separate the two parameters properly (Cao et al., 2000). One school of
opinion suggests that refining lamellar spacing is more important than grain
size refining to enhance the mechanical properties. Many papers are devoted
to studying only the effect of heat treatment (cooling rate, in particular) on
lamellar spacing, although some papers indirectly support that the grain size
is determined by how the sample is cooled (Yamaguchi et al., 2000).
The authors who studied the effect of heat treatment on grain size gave
varying results on the effect of annealing temperatures, holding time (Cao et
al., 2000) and cooling rate (Qin et al., 2000; Wen et al., 2000).
The titanium aluminide alloy described extensively in this book, Ti-46Al-
1.9Cr-3Nb (all titanium aluminide compositions are quoted in atomic percent,
at.%), is representative of a group of TiAl-based alloys.
2.2.2 Effects of heat treatment on microstructure and
grain size
This section will show the heat treatment effect on the grain size of fully
lamellar microstructure in Ti-46Al-1.9Cr-3Nb (at.%) at high temperatures
(>1400 °C) and clarify the controversy on this subject existing in the literature.
Focus is on the current understanding of the phase transformation leading to
the fully lamellar microstructure.