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A triple-melt alloy 718 is being used for high strength fasteners on a tokamak fusion reactor that is being constructed at MIT on behalf of U.S. DOE. The fastener is designed for up to 910 MPa tensile stress at 77 K with alternating load during a pulsed magnet operation between 0 and 9 tesla. Manufacturing malpractice for the fastener prompted an interim evaluation which revealed poor fracture toughness (less than 110 MPam) of the direct aged material containing a microstructure of about 10 ym grain size with the preponderance of coarse delta phase, Ni3Nb. An anneal and direct aging was chosen to improve the fracture toughness based on favorable experience with similar high temperature treatments. Due to the large thickness of the fastener and the requirement for high strength a fine adjustment of annealing temperature was necessary. A 1,050 C for one hour anneal followed by a standard direct aging was selected for complete removal of undesirable precipitates while grain size increased to about 70 pm. A fast cooling by forced air convection was used to avoid re-precipitation of Laves or delta phases. Yield strength and ultimate tensile strength of the anneal and direct aged material at room temperature were decreased by about 8 % compared with those of the direct aged condition. Fracture toughness was increased by as much as 100 % and fatigue crack growth rate was reduced by a factor of three. The improvement is attributed to the removal of coarse precipitates which presumably are reused as gamma double prime precipitates in the matrix. The anneal and direct aging is hence recommended for low temperature applications including fusion reactor superstructure. *Visiting student from Baylor College Superalloys 718,625 and Various Derivatives Edited by Edward A. Loria The Minerals, Metals & Materials Society, 1991

Improved Mechanical Properties of Alloy 718 by Anneal and Direct Aging Process for Nuclear Fusion Applications