Effect of orientation on plastic deformations of Alloy 617 for VHTR applications

Alloy 617 is considered a primary structure material for building the intermediate heat exchanger (IHX) for the very high temperature reactor (VHTR). In the present work, the effect of orientation of Alloy 617, which may influence the engineering design and performance, was investigated by mechanical tests and associated microstructural analysis. Tensile tests at various temperatures were performed on the specimens at various angles to the rolling plane. The yield strength parabolically dropped once the sampling direction deviated from the rolling plane, whereas the ultimate tensile strength decreased in a linear trend. The mechanical anisotropy reflected by the loss of the transverse strengths exists throughout the range of testing temperatures up to 1000 °C. The microstructural analysis revealed that the intrinsic Bauschinger effect, the loss of transverse yield strength, is caused by mechanical fibering in Alloy 617, which is originally induced during hot rolling processing. The results also indicated that the inclusion particles caused steep strain gradients during plastic deformations in Alloy 617, due to the dislocation pileups on the interphase boundaries. The distribution of strained area is orientation dependent and thus caused localized plastic deformations and early fractures in the transverse direction at low temperatures. In addition, it was found that the strained area could be recovered by forming new grains, leading to an oriented dynamic recrystallization (DRX) at high temperatures.