Effect of Cooling Rate from Solution Treatment on Precipitation Behavior and Mechanical Properties of Alloy 706

Abstract

The time temperature precipitation behavior of N&Fe-base superalloy 706 is very complicated because many kinds of precipitates exist in this alloy. Therefore, TTP behavior of this alloy vary with the heat treatment program and condition, and so do mechanical properties accordingly. In this study, TTP and TTH diagrams on cooling from the solution treatment of three experimental alloys and Alloy 706 are presented in a temperature range from 600 to 900°C for up to 100h. The observations by optical microscopy, scanning electron microscopy and transmission electron microscopy revealed that y ‘, y “, double cuboidal y ‘y ” co-precipitate (compact morphology), overlaid y ‘y ” co-precipitate (non-compact morphology) and 7 appeared in this alloy despite that the double cuboidal co-precipitate does not exist in the usual TTP diagram which is composed on re-heating from room temperature after the solution treatment. The dependence of microstructure and mechanical properties of aged Alloy 706 on the cooling rate after the solution treatment was also examined. The solution treatment was conducted at 980°C for 3h, and the cooling rate was varied from 0.5 to 8.5 “C/min. Tensile strength and 0.2% yield strength were maximized at 35 “C/min, while elongation and reduction of area suddenly decreased below 3-5 “C/min. Creep rupture life was also maximized at 3-5 “C/min, but creep rupture ductility suddenly increased below 3-5 “C/min. These changes can be explained in terms of the precipitation behavior such as the formation of 7 at grain boundaries and the double cuboidal y ‘y ” co-precipitate inside grains in the case of low cooling rate. Superalloys 718,625,706 and Various Derivatives Edited by E.A. Loria The Minerals, Metals &Materials Society, 1997