Microstructure of high-temperature aged alloy 625 before and after tensile deformation

Alloy 625 is a nickel-chromium alloy with excellent resistance to pitting and crevice corrosion. It also has good resistance to radiation embrittlement and is widely used in nuclear, aerospace and marine environments. The alloy exhibits good weldability and can be used in the as-welded condition. Aging the alloy 625 at 540 deg C results in the appearance of micro-twins along with precipitation of ordered phases, Ni2(Cr,Mo) and γ' (Ni3Nb). However, at higher temperatures of aging, (700 deg C and 850 deg C), along with precipitation of γ' at 700 deg C and orthorhombic δ (Ni3Nb) at 850 deg C, secondary recrystallization sets in leading to disappearance of annealing twins. It also appears from the selected area diffraction patterns obtained from samples deformed after aging at 540 deg C that nano-scale ordered phases, namely, Ni2 (Cr,Mo) and γ' dissolves, at least partially, due to the shearing of the coherent precipitates by the dislocations. However, micro-twin as well as macro-twin remains unaffected by the deformation. (author)