Measurement of elastic characteristics of single-crystals of a nickel-base superalloy by speckle interferometry

The elastic properties of single crystals of a nickel-base superalloy VGM7 have been investigated by speckle interferometry. Plate-shaped specimens of different crystallographic orientations were loaded under pure shear conditions and speckle interference patterns were imaged. Numerical processing of the interference patterns allowed us to determine the values of Young’s modulus in directions [001] and [011], $E_{001}$= 138 GPa and $E_{001}$= 241 GPa, the basic value of Poisson’s ratio ${\nu }_0$= 0.39 in the coordinate system ⟨001⟩, as well as its minimum and maximum values ${\nu }_{\text{min}}$= −0.10 and ${\nu }_{\text{max}}$= 0.69 under longitudinal loading along [101] and transverse deformation along [10$\overline{1}$] and [010], respectively. Using the measured values $E_{001}$, $E_{011}$, ${\nu }_0$, ${\nu }_{\text{min}}$ and ${\nu }_{\text{max}}$ the single-crystal elastic stiffnesses  $C_{11}$= 264 GPa, $C_{12}$= 166 GPa and  $C_{44}$= 133 GPa, and elastic compliances  $S_{11}$= 7.35 TPa^–1, $S_{12}$= –2.84 TPa^–1 and $S_{44}$= 7.52 TPa^–1 we calculated. The applied method allows one to unambiguously determine the sign of Poisson’s ratio and, therefore, it should be recommended for studying the elastic properties of auxetic materials, for which determination of the sign of Poisson’s ratio is of great importance.