Comparison of γ'/γ" Precipitates and Mechanical Properties in Modified 718 Alloys

Microstructural instability and loss in strength of Alloy 718 at and above 65O’C is caused by the dissolution of the precipitation strengthening y ” phase and the formation of the brittle and crack-like 6 phase. Two improved compositional modifications of Alloy 718 were given heat treatments that could change the morphology of the coexisting y ‘/y ” precipitates. A conventional non-compact y ‘/y ” structure was obtained in one alloy and a compact y ‘/y ” structure in the other alloy. A third alloy specifically designed to produce the compact y ‘/y ” cuboidal precipitate was included for comparison purposes. Higher tensile strength averaging 105 MPa (2.3 ksi) between 400 and 700°C (750 and 1300’F) and almost three times longer rupture life at 650°C (1200’F) under a stress of 686 MPa (99.5 ksi) were obtained with the non-compact y ‘/y ” precipitate compared to the compact y ‘/y ” precipitates and the non compact y ‘/y ” structure of conventional heat treated 718. Also obtained were significant improvements in tensile yield strength and stress rupture life at 483 to 724 MPa (70 to 105 ksi) in the 650 to 730°C (1350’F) temperature regime corresponding to y ” instability in conventional 718. For this particular high stress range, a 100 hr rupture life at 25 to 40°F higher temperature was calculated for the modified alloy with the non-compact y ‘/y ” structure compared to the trendline established for today”s high-quality 718. For this particular composition, the mechanical properties were apparently optimized by the size and spacing of the non-compact y ‘/y ” precipitates being more effective in impeding the motion of dislocations through the lattice. There was an increase in the primary strengthening y ” phase, and the matrix and y ’ + y ” phases were strengthened by a tungsten addition to this alloy. Also, the grain boundary was strengthened by a small chain of MgC phase which inhibited long range grain boundary sliding during stress rupture testing. Superalloys 718,625 and Various Derivatives Edited by Edward A. Lxia The Minerals, Metals & Materials Society, 1991 397