Contrasting irradiation behavior of dual phases in Ti-6Al-4V alloy at low-temperature due to co-phase precursors in β-phase matrix

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 co -phase precursor, without dose dependency in its intensity. FFT/I-FFT analysis of the HREM revealed a sub-nanometersized 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 co -phase precursors specifically formed in the beta (BCC) phase of group -4 transition metals.