Evaluation of macro- and microscopic residual stresses in laser shock-peened titanium alloy by FIB-DIC ring-core milling with different core diameters

The residual stresses in a laser shock-peened (ISP) plate of titanium alloy were measured by means of the ring core Focused Ion Beam Digital Image Correlation (FIB-DIC) technique using pillar diameters of 10 pm and 5 pm. A cross-section of the Ti-6Al-4V laser shock peened plate was electrochemically polished to avoid induced residual stress modification due to grinding. FIB-DIC measurements with pillar diameters of 10 pm show that a near-surface compressive residual stress of approximately 350 MPa is present in the direction parallel to the peened surface. This changes to a tensile residual stress of approximately 100 MPa at the depth of 2 mm, and then changes back to a small compressive residual stress on the opposite face. These results are in good agreement with the previously obtained set of measurements using high-energy synchrotron X-ray diffraction. However, FIB-DIC measurements with pillar diameters of 5 pm show strong deviations from this general macroscopic trend that corresponds to the macroscopic average (Type I) residual stresses. The reason for the apparent discrepancy lies in the smaller gauge volume used in the second set of measurements, which causes the evaluated residual stress values to be dominated by the microscopic (Type II + III) residual stress components. These Type II + III residual stresses are the result of the local microstructure and intra- and intergranular interactions. The results demonstrate that the FIB-DIC ring-core technique can be tailored to determine either macro- or microscopic residual stress, by tuning the core diameter with respect to the grain size. Type II + III stresses have significant magnitude sufficient to produce local tensile stress in the region that is subjected to macroscopic compression due to ISP treatment, and vice versa. The implications of this finding are discussed.