Hall coefficient measurement for residual stress assessment in precipitation hardened IN718 nickel-base superalloy

We investigated the feasibility of residual stress assessment based on Hall coefficient measurements in precipitation hardened IN718 nickel-base superalloy. As a first step, we studied the influence of microstructural variations on the galvanomagnetic properties of IN718 nickel-base superalloy. We found that the Hall coefficient of IN718 increases from ≈ 8.0×10⁻¹¹ m³/C in its fully annealed state of 15 HRC Rockwell hardness to ≈ 9.4×10⁻¹¹ m³/C in its fully hardened state of 45 HRC. We also studied the influence of cold work, i.e., plastic deformation, at room temperature and found that cold work had negligible effect on the Hall coefficient of fully annealed IN718, but significantly reduced it in hardened states of the material. For example, measurements conducted on fully hardened IN718 specimens showed that the Hall coefficient decreased more or less linearly with cold work from its peak value of ≈ 9.4×10⁻¹¹ m³/C in its intact state to ≈ 9.0×10⁻¹¹ m³/C in its most deformed state of 22% plastic strain. We also studied the influence of applied stress and found that elastic strain significantly increases the Hall coefficient of IN718 regardless of the state of hardening. The relative sensitivity of the Hall coefficient to elastic strain was measured as a unitless gauge factor K that is defined as the ratio of the relative change of the Hall coefficient ΔR_H/R_H divided by the axial strain ε = σ/E, where σ is the applied uniaxial stress and E is the Young’s modulus of the material. We determined that the galvanomagnetic gauge factor of IN718 is κ ≈ 2.6 − 2.9 depending on the hardness level. Besides the fairly high value of the gauge factor, it is important that it is positive, which means that compressive stress in surface-treated components decreases the Hall coefficient in a similar way as plastic deformation does, therefore the unfortunate cancellation that occurs in fully hardened IN718 in the case of electric conductivity measurements will not happen in this case. Additionally, the temperature dependence of the Hall coefficient was measured at three different hardness levels and the influence of thermal exposure was studied in fully hardened IN718 up to 700 °C.