Structural transformation and residual stresses in surface layers of α + β titanium alloys nanotextured by femtosecond laser pulses

Surface topography, phase composition and compressive residual stresses were characterized in surface layers of α + β titanium alloys VT6 (Ti–6Al–4V) and VT16 (Ti–3Al–4.5V–5Mo) nanotextured by 744-nm, 120-fs-laser pulses. The relative content of β-phase, along with the residual compressive stresses, increases significantly in both these materials over the depth ~0.2 μ in the nanotextured surface layer comprised by nanograins and remains almost unchanged at higher depths of 1–2 μ in bulk. The laser-driven α → β phase transformation is related to the combined effect of ultrafast heating/cooling, strong (multi-GPa) shock waves, chemical segregation and nanograin formation induced during the ablative surface nanotexturing.