Comparison of deformation induced precipitation behaviour in Alloy 718 under two microstructural conditions

Alloy 718 samples under two initial microstructural conditions, viz., solution annealed to form only gamma phase (ST) and aged to precipitate only delta particles (DELTA), were deformed in tension till fracture in the temperatures range from 200 degrees C - 700 degrees C. From the comparison of the evolved microstructure of deformed and undeformed specimens that have been subjected to similar thermal history, deformation induced precipitation could be identified. Deformation in the range of 550 degrees C to 650 degrees C promoted the precipitation of gamma' and gamma '' phases in both structures. In case of DELTA alloy, the gamma' precipitation was found to precede gamma '' the phase precipitation while no such preference for precipitation could be identified in ST specimens. This difference in the precipitation behaviour and the sequence of precipitation has been explained on the basis of the relative concentration of solutes in the matrix of the starting microstructures of ST and DELTA specimens.