706 Alloy

Alloy name: 706
Diagram No.: 1080
Type of diagram: TTP
Chemical composition in weight %: See the table
Alloy group: Nickel-based alloys
Note: TTP diagrams of experimental alloys containing Ti: (a) alloy No.2, (b) No.4, (c) No.5 and (d) Alloy 706.
As described above, four types of precipitates were identitied in the allays containing Ti. gamma' and eta were found in alloys Nos.2 and 4. In addinon to them, gamma” and gamma‘- gamma” co-precipitate were found in alloy No.5 and Alloy 706. The TTP dtagrams of the four alloys are shown in Figure. The regions of gamma', gamma” and gamma‘- gamma” agreed well with the TTH diagram. Gamma” and gamma‘- gamma” are seen only in Nb-containing alloys, suggesting that the gamma” formation requires both Ti and Nb. These precipitates are the cause of the greater hardness of the Nb-bearing alloys, described ealier with respect to their TTH diagrams. That is, the gamma” phase reinforces the matrix more effectively than the gamma' phase.

The region of 7 precipitation grows wider as aging time increases in all the alloys. In fact, the eta phase was found to shoot out from the grain boundary as the aging trme increased at about 800 C. The gamma', gamma“, gamma‘- gamma” all transform eventually to eta when aged for long time at high temperatures in all the alloys tested. However, the region of eta precipitation is much wider in the Nb-free alloys than in the Nb-containing alloys, suggesting that gamma' transforms to eta more readily than gamma” and gamma‘- gamma“. That is, the Nb-containing alloys are thought to be more stable at high temperatures, This is supported by the aging response previously described. Thus, Nb not only reinforces the matrix by the gamma” precipitation, but also enhances the high temperature stability by delaying the transformation from the mtra-granular precipitates to the grain boundary eta phase.

The TTP behavior of alloy No.5 and Alloy 706 is more complicated than those of No.2 and No.4, especially at the temperatures between 700 - 800 C. Figure demonstrates how the precipitate morphlogy develops wtth aging, when Alloy 706 is aged at 730 C. gamma' phase appears faintly at 0.1 h, which is characterized by the ordering spots in the dtftkaction pattern. As the exposure time increases, the gamma‘- gamma” begins to form replacing the gamma' phase, and the gamma” phase becomes predominant. It should be noted that the size of the gamma‘- gamma” co-precipitate in Alloy 706 aged at 730 C for 10 h is much smaller than that of gamma” in alloy No.5 aged at the same condition. This suggests that the stability of the gamma‘- gamma” is greater than that of gamma” at high temperatures, as previously reported for Alloy 718.

The TTP diagrams of alloy No.5 and Alloy 706 are somewhat different, although those of alloys No.2 and No.4 are very similar. The difference between alloy No.5 and Alloy 706 is characterized especially by the region of gamma‘- gamma“. The precipitation occurs in these alloys pass in the same sequence gamma' -> gamma‘- gamma” -> gamma“, but the region of gamma‘-gamma” is fairly wider in Alloy 706 than in alloy No.5. It indicates that the Alloy 706 has better thermal stability than alloy N0.5. Such difference reflects a synergetic effect among Al, Nb and Ti. Therefore, the effect of Al addition is considered to form the stable gamma‘- gamma''.

The solubility for Al in gamma” is extremely low whereas that for Nb in gamma' is very high, hence a low Al content favors the gamma” formation whereas high Al content the gamma' phase. The dominant gamma‘- gamma” co-precipitation should be explained by the same effect of Al addition. Moreover, the same effect is expected in the transformation to eta, since eta has little solubility for Al. Further study is needed to shed more light on the stability of the precipitates as influenced by the chemical composition.
Reference: Takashi Shibata, Yukoh Shudo, and Yuichi Yoshino, EFFECTS OF ALUMINUM, TITANIUM AND NIOBIUM ON THE TIME - TEMPERATURE - PRECIPITATION BEHAVIOR OF ALLOY 706, Superalloys 1996, Edited by R. D. Kissinger, D. J. Deye, D. L. Anton, A. D. Cetel, M. V. Nathal, T. M. l’ollack, and D. A. Woodford, The Minerals, Metals & Materials Society, 1996, pp. 153-162.

Transformation Diagram


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Chemical Composition of Experimental Alloys


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