  Aiming to simulate the radiation damage effect on a dual alpha+beta phase
Ti-6Al-4V alloy utilized as high-intensity accelerator beam window material, a
series of irradiation experiments were conducted with a 2.8 MeV-Fe^2+ ion beam
in several dpa regions at room temperature. The nano-indentation hardness
increased steeply at 1 dpa and unchanged up to 10 dpa, due to the saturation of
defect clusters and tangled dislocations in the dominant alpha-phase matrix
with a size of 2~3 nm and a density of about 1x10^23 m^-3. In contrast in the
intergranular beta-phase, larger loops of 20~30 nm diameter were observed with
much less density of about 5x10^20 m^-3. The diffraction pattern showed
rectilinear diffuse streaks between the beta-phase reflections, corresponding
to the omega-phase precursor, without dose dependency in its intensity.
FFT/I-FFT analysis of the HREM revealed a sub-nanometer-sized lattice disorder
with local fluctuations, not discrete but continuous, and homogeneously
distributed within the matrix beta-phase stably against the irradiation. The
significantly low dislocation density and the absence of phase transformation
in the beta-phase matrix could be attributed either to the strong sink effect
expected for this distinctive sub-nanometer-sized homogeneous lattice disorder
or to the anomalous point defect recombination induced by the high mobility of
vacancies, both of which are originated from the metastable omega-phase
precursors specifically formed in the beta(BCC) phase of group-4 transition
metals.
