706 Alloy

Alloy name: 706
Diagram No.: 1083
Type of diagram: TTP
Chemical composition in weight %: See the table
Alloy group: Nickel-based alloys
Note: The “on cooling ‘ITP” diagrams of three experimental alloys and Alloy 706 : (a) alloy No.2, (b) No.4, (c) No.5 and (d) Alloy 706.
The “on cooling TTP” diagrams of the four alloys, alloys Nos.2,4,5 and Alloy 706 are shown in Figure. The region of Gamma', Gamma'' and Gamma‘- Gamma” co-precipitates is agreed well with the TTH diagram. 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. The only exception is the presence of the double cuboidal Gamma‘- Gamma” co-precipitate. This co-precipitate appears only in the “on cooling TTP diagram of Alloy 706. The reason can be explained from the view of growth of Gamma’ phase, in the following way.
The Gamma‘- Gamma” co-precipitate is considered to form through initial Gamma’ formation and subsequent Gamma” formation on that Gamma‘. Therefore, in order to form co-precipitate, both Gamma’ and Gamma” are necessary. In fact, as compared alloy No.5 and Alloy 706 with alloys Nos. 2 and 4, this co-precipitate demands niobium that is a required element for the formation of Gamma“, in addition to titanium that can form Gamma‘. However, only the overlaid co-precipitate appears in alloy No.5, while both the overlaid co-orecipitate and the double cuboidal co-orecipitate appear in Alloy 706. This seems to be due to the size of Gamma’ before the formation of Gamma“. When that size is large enough to form Gamma” on its six (100) planes, the double cuboidal co-precipitate can form. That is to say, in order to form double cuboidal coprecipitate, a relatively large Gamma’ prior to the formation of Gamma” is necessary. As compared Alloy 706 with alloy No.5. this co-precipitate demands aluminum that can only substitute in Gamma’ and not in Gamma“. On the contrary, titanium can substitute both Gamma’ and Gamma“. Therefore, aluminum promotes the formation Gamma’ more than titanium does. In other words, this co-precipitate demands the co-existance of niobium for Gamma” and aluminum for Gamma‘. This tendency of the balance among aluminum, titanium and niobium is consisitent with the previous report on Alloy 718.
The reason that this co-precipitate appears only in the “on cooling TTP” diagram is considered to be due to the difference in the rate of Gamma’ formation. As shown in Figure, in case of “on cooling TTP”, the precipitation of Gamma’ occurs in upper side of Gamma” region, whereas, in case of “on heating TTP”, the precipitation of Gamma’ occurs in lower side of Gamma” region. Therefore, the temperature of Gamma’ formation is higher in “on cooling TTP” than in “on heating TTP”, so the rate of Gamma’ formation is greater in “on cooling ‘ITP” than in “on heating TTP”. This high rate in the “on cooling TTP” is consistent with the estimation described in the “on cooling TTH” diagram of titanium-free alloys. As a result, the size of Gamma’ is larger in “on cooling TTP” than in “on heating TTP”.
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