Tailoring the tensile deformation modes in the Ni-Cr-Fe-base superalloy GH4070T at 750 ℃ by adjusting Fe content

A novel Ni-Cr-Fe-base superalloy GH4070T containing around 25wt.% of iron has recently been developed and evaluated as a candidate material of superheater/reheater tubes in the boiler of 700 °C-class advanced ultra-supercritical power plants. In order to determine the optimum addition of iron, the impact of Fe element on the tensile properties and deformation modes of the alloy at 750 °C is investigated by adjusting the Fe content from 15wt.% to around 35wt.%. We found that the yield strength and ultimate tensile strength decrease monotonically with increasing the addition of iron, but the tensile plasticity has an optimum value when the concentration of iron is at 25wt.%. Microstructural observations reveal that the γ′ volume fraction decreases with the Fe content, but without obvious changes in the grain sizes and particle sizes of γ′ precipitates. Meanwhile, it is also found that the primary deformation mode transits from dislocations cutting through γ′ precipitates to dislocations striding over them via Orowan looping and cross-slip/climb with enlarging the concentration of iron, but all the experimental alloys display the mixed-mode fracture. Simulation calculations disclose that the stacking fault energy of the matrix increases with the Fe content, whereas the anti-phase boundary energy of the γ′ phase shows an opposite changing tendency. Given these results, it is deemed that the changes in the microstructural features with the Fe content are responsible for variations in deformation modes and fracture mechanisms as well as tensile properties for GH4070T.