In-situ diffraction study on Deformation twinning behaviour in Ni based alloys

Purpose and goal

Twinning induced plasticity (TWIP) is an important mechanism for the material with a combination of high strength and high elongation. As a part of the Ph D work, the aims of this project are to study deformation twinning behaviours in two high Ni alloys using in-situ neutron diffraction experiments under the tensile loading. The TWIP mechanisms in these alloys, especially with the influences of temperature and stacking fault energy, would be investigated.

Expected results and effects

The diffraction spectra and stress/strain profiles obtained show that deformation twinning and TWIP can occur in these two alloys, especially at cryogenic temperatures. The neutron diffraction spectra and lattice strain measured can predict the deformation twinning initiation behaviour. The amounts of twins can be correlated to the Integral intensity ratio of the diffraction patterns. An unexpected new observation is the martensite phase transformation in Alloy 625 during the deformation at the cryogenic temperatures.

Planned approach and implementation

Two high Ni tube materials: Sanicro 28 and Ni based superalloy 625 were used. The Dog Bone type of sample with a diameter of 8 mm was taken in the longitudinal direction of the tubes. The in-situ tensile tests with neutron diffraction were done at 298, 77 and 30K with a strain rate of 8x10-4/s using the ENGIN-X neutron diffractometer at ISIS, UK. The data were collected with the double detector banks, diffraction spectra containing a number of diffraction peaks can be obtained simultaneously along the loading axis and a transverse direction to the loading axis.