Large stress concentrations around micropore near a crack-tip induced deformation twinning in Ni-based single crystal superalloy

Thermo-mechanical fatigue (TMF) test was performed in a [0 0 1] oriented single crystal superalloy TMS-75. After TMF tests, detailed microstructural evolution were observed from the interior and outer surfaces of the specimens. The path of the crack initiation and propagation on the failure surface of the ruptured specimen has a concrete representation. It was found that the distribution of deformation twins occurs around the micropores near the crack-tip. Considering the twinning dislocation slip mechanism, the critical tensile stress value 1.34 GPa for twinning in single crystal superalloy was obtained by means of theoretical calculations. Associated with crack propagation in varying degrees, finite element method was performed in an effort to clarify the stress fields around the micropore near the crack-tip region. The finite element method analysis results reveal that the stress in the region between the micropore and crack-tip is larger than the critical tensile stress value to promote the nucleation and propagation of deformation twins. Both theoretical calculations and finite element method analysis results are well consistent with the experimental results. The stress concentration around the micropore near the crack-tip region results in a high density of deformation twins.