Study of mechanical state of Nb3Sn conductors during heat treatment

by Dr Mahmoud Abdel Hafiz (CEA Saclay)

Monday 31 Oct 2022, 16:00 → 17:00 US/Pacific

0006-CR (47)

Speaker: Mahmoud Abdel, CEA Saclay, France

Host: Jean-Francois Croteau

Zoom: https://lbnl.zoom.us/j/5104867866

Title: Study of mechanical state of Nb3Sn conductors during heat treatment

Abstract: The development of future particle colliders such as the HiLumi - Large Hadron Collider (HL-LHC) and the Future Circular Collider (FCC) relies on the ability to generate high magnetic fields allowed by improved superconducting Nb3Sn magnets. The coil fabrication starts with the winding of cables followed by a precipitation of the superconducting brittle Nb3Sn phase during a long 650°C heat treatment.

This study focuses on Rod Restacked Process (RRP) conductors which heat treatment is composed of several dwells, activating different diffusion mechanisms and phase transformations. These mechanisms, located into the filament area of each strand, naturally lead to variations of the crystalline structure hence of the mechanical state.
As it is well known that Nb3Sn superconducting properties are stresses (or strain) sensitive, estimating the mechanical state of Nb3Sn conductors after the heat treatment might have a considerable implication concerning the tooling and enhancing its properties.
Thus, a multiphysic model of the strand is proposed, taking into account the stresses created by differential thermal expansion and phase transformations. Diffusion kinetic are here determined experimentally using Energy Dispersive X-ray Spectroscopy and microscope analysis on strand cross sections.
In the other hand, an experimental device, using high temperature in situ Digital Image Correlation (DIC), has been designed. This experiment allows the determination of a 2D displacement field at the surface of Nb3Sn conductors during heat treatment.

DIC measurements are consistent with literature. The experiment provided in particular the first observations of Rutherford cable width changes as a function of temperature. In the other hand, the multiphysic model achieves to reproduce part of the strains measured.

Biography: Mahmoud obtained his bachelor in fundamental physics at Sorbonne Université, Paris VI and his master’s in mechanical engineering at the Ecole Normale Supérieure (ENS). To improve his knowledge in material science and multiphysics phenomena, he spent a year in his degree at the ENS Paris-Saclay. As a graduate student in different research laboratories, i.e. Institut Langevin, Jean le rond ∂’Alembert, and the Commissariat à l’Energie Atomique et aux Energies Alternatives (CEA), he performed a variety of experiments, e.g. SEM analyses and strain measurements with DIC. Lastly, during his PhD thesis at CEA Paris-Saclay and the Laboratoire de Mécanique et Technologie (LMT), now the Laboratoire de Mécanique de Paris-Saclay (LMPS) of ENS, he studied the mechanical state of Nb3Sn conductors during heat treatment, with a focus on theoretical thermodynamics behavior of materials and modeling of multiphysics phenomena.