Effects of Aluminum, Titanium and NioBium on the Time-Temperature-Precipitation Behavior of Alloy 706

Abstract

Ni-Fe-base superalloy 706 has been used for high temperature services.The time -temperature -precipitation (TTP) diagram is essential in the design of heat treatments for any precipitation strengthened superalloy.The TTP diagrams have been already presented for Alloy 706. However, effects of aluminum, titanium and niobium, important substitutional elements of y ' and y " precipitates, on the TTP behavior are not clear in the literature.In this study, the TTP and the time -temperature -hardness (TTH) diagrams are presented for experimental alloys containing only one or two of Ti, Nb and Al, in a temperature range of 600 -900°C. The observation by optical microscopy, scanning electron microscopy and transmission electron microscopy revealed that y ' , y ", y '-y " co-precipitates and 17 form in alloys containing Ti.Among the three elements, Ti plays the most important role in the precipitation strengthening behavior of Alloy 706. Furthermore, neither Al nor Nb can demonstrate their effects without Ti addition. Nb promotes y " formation and prevents 7 formation. Al enhances the formation of stable y '-y " co-precipitates, more effectively in the co-existence of Ti and Nb.

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

“…The selected area diffraction pattern indicates that the 77 phase has a specific orientation relationship with the y matrix, as [Oil J, // [211011 and (111)9 // (OOOl)y. This is consistent with our previous work (11,13). The 7 phase appears parallel to each other in order to meet this orientation relationship.…”

“…On the contrary, alloys Nos. 3 and 6 were not hardened in case of usual "on heating TTH" (13). It suggests that the rate of formation of strengthening precipitate is higher in "on cooling TTP" than in "on heating TTP".…”

“…The "on cooling TTP" diagrams are consistent with the estimation from the "on heating TTP" diagrams. For the role of strengthening elements, which are titanium, niobium and aluminum, in TTP behavior, there is no substantial difference between the "on cooling TTP" behavior and the "on heating TTP" behavior (13). The only exception is the presence of the double cuboidal y '-y " co-precipitate.…”

“…Especially for Alloy 706, 'ITP behavior is very important because many kinds of precipitates exist in this alloy, and its mechanical properties are greatly affected by the precipitation at the heat treatment (6-l 1). From this point of view, the TIP diagrams of Alloy 706 have already been presented (2,3,12), and recently we presented more updated one (13). That TIP diagram indicates that four types of precipitates, that is y ', Y '0 overlaid y '-y " co-precipitate (so called, "non-compact morphology") and 7 , appear in Alloy 706.…”

“…All of the samples were solution-treated at 980°C for 3h. This condition was determined according to a previous report (13) rate to the solution treatment temperature was SO"C/h, simulating a practical large size forging. After the solution treatment, the samples were cooled down to various temperatures between 900 and 600°C at SO"C/h and held isothermally up to 1OOh as shown in Figure 1 (a).…”