In Situ Deformation of Nb₃Sn Cables During Heat Treatment Using Digital Image Correlation

Nb₃Sn magnets are primary candidates for future particle colliders such as the High-Luminosity Large Hadron Collider (HL-LHC) and the Future Circular Collider (FCC). In particular, the Rod Restack Process (RRP®) requires a heat treatment typically to 650-680 °C to form the Nb₃Sn superconducting phase. This heat treatment activates several diffusion and phase-transformation mechanisms at the sub-element scale. At the conductor scale, strain measurements highlight a nonlinear behavior with respect to temperature. It is assumed that part of these deformations arises from phase transformations and mechanical stress relaxation within the conductors. This paper investigates the thermomechanical response measured at the conductor scale. Macroscopic deformation measurements of RRP®Rutherford cables were obtained during heat treatment using in situ Digital Image Correlation. This method allowed, for the first time, a quantitative evaluation of the Rutherford cable behavior in both the transverse (width) and longitudinal (length) directions.