There are many possibilities to induce residual stress in Ni-base single crystal superalloy during the manufacture and service procedures. The residual stress combined with the applied stress will affect the microstructure and mechanical performance significantly. Finite Element Method and Convergent Beam Electronic Diffraction have been extensively used to study the stress state in Ni-base single crystal superalloy, but X-ray diffraction (XRD) method and neutron diffraction which have been used wildly for the stress measurement in polycrystal haven’t been applied wildly. The aim of this work is to study the residual stress in Ni-base single-crystal superalloy by XRD and neutron diffraction.
First, XRD method for the stress measurement in single-crystal superalloy was examined by the four-point bending tests. Experiment errors from the repeated measurements for the same stress-free sample was about ±30MPa and this proved XRD method was suitable for the stress measurement in this experiment. Stress states of the superalloy during four-point bending tests were measured by XRD at room temperature. The results from the calculation by separated information of γ/γ' phase showed that the measurement technology and analysis method were correct. The inter-phase stress states of γ/γ' phases during the tests increased with the deformation while the γ (γ') phase was in tension (compression).
Second, samples with different surface direction (A-(100), B-(110), and C-(111)) were shot peened to simulate the residual stress state introduced during the manufacture process and shot peened samples were aged to observe the different microstructures. The experiment results showed that the stress tensors parallel to the sample surface was compressive stress and decreased with the depth; it changed to tensile in interior and balanced with the compressive stress in the sub-surface. The stress tenor along the depth within the deformed layer was tensile. The mechanical performances of the material play an important role in the stress state of shot peened samples. The stress distribution state is while the corresponding yield strength is . Recrystallization and “anomalous” rafting structure were observed in the shot peened sample after ageing heat treatment. Rafting was supposed to be a result of the diffusion process which was consistent of the thermal relaxation of the residual stress. The tensile stress tensor along the depth was supposed to be the critical reason for the rafting structure while the stress tensors parallel to the surface were countervailed.
Third, in-phase thermal-mechanical fatigue (IP TMF) was performed to simulate the service condition. The TMF samples were measured by XRD and neutron diffraction. Experiment results showed that the stress state of γ/γ' phase within the sample was: γ (γ') along the [001] direction was compressive (tensile) and opposite along the [100]/[010] direction. The stress states in the deformed sample were inhomogeneous, as the stress and the changes of the lattice parameter in the center was larger than the other position. The results from the {400} profiles indicated the γ/γ' phases were tetragonal distorted, c-b for γ (γ') phase was negative (positive) and increased with the deformation process; the lattice parameter mismatch in axial (side) case was positive (negative) with the corresponding value of +0.05% (-0.25%).
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