Determination of the γ solvus surface in NiAlX ternary systems

Materials" Science and Engineering, A 146 ( 1991 ) 123-13(I

123

Determination of the 7 solvus surface in Ni-A1-X ternary systems Yoshinao Mishima Precision and Intelligence Laboratory, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 227 (Japan)

Yong Myong Hong* Department of Materials Science and Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 227 (Japan)

Tomoo Suzukit Department of Metallurgical Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 157 (Japan) (Received March 8, 1991; in revised form April 15, 1991 )

Abstract A systematic investigation was carried out to determine the ~, solvus in N i - A I - X ternary systems with X being a transition metal or subgroup B-element. Differential thermal analysis (DTA) was employed as a major experimental technique for this purpose, by which the y solvus can be obtained as a surface with respect to the composition and temperature. Chemical analyses using energy-dispersive X-ray spectroscopy not only proved that the results obtained by DTA are highly accurate but also provided additional information on the phase relations such as the constitution of the three-phase triangle neighbouring the y - y' two-phase field.

1. Introduction Although chemical compositions of commercial nickel-based superalloys are very complex, their microstructures consist mainly of two phases: nickel solid solution (y phase) and intermetallic compounds (~' phase) based on Ni3A1. Consequently the high temperature mechanical properties of the precipitation-strengthened alloys are basically controlled by the dispersion characteristics of the y' phase in the ~, matrix, such as volume fraction and interparticle spacing. One of the most fundamental databases for the alloy design of the superalloys should therefore be the ternary phase diagrams of the Ni-AI-X with an emphasis on ~-y' equilibria. It seems, however, that such information is not sufficient in both quality and quantity probably because tedious experimental work is in most cases required to establish the phase reltions for wide compositional and temperature ranges.

*Present address: Korean Academy of Industrial Technology, Inchon, Korea. 0921-5093/91/$3.50

The objective of the present work is to furnish accurate and systematic data on the phase relations in the nickel corner of the ternary Ni-AI-X alloy phase diagram. The focus is on the 7 solvus, here being defined as the solubility limit of 7' in 7 solid solution. As a ternary element X, transition metal elements of groups IVA, VA and VIA and elements of groups IIIB and IVB are chosen. Differential thermal analysis (DTA) is employed as a major experimental tool. The method is relatively uncommon for such purposes but is found to be superior over conventional methods such as optical metallography and X-ray analysis because the solvus is readily obtained as a surface with respect to composition and temperature. In other words, the present results provide the y solvus at any temperature of a ternary system investigated and they can be compared with any isothermal sections which have been reported fragmentally.

2. Experimental details Ni-AI-X ternary alloys were prepared with raw materials of the highest purity readily available by arc melting under an argon atmosphere. Nominal compositions were accepted because © Elsevier Sequoia/Printed in The Netherlands

124

the weight loss after melting was less than 0.05%. All the alloys were homogenized for 504 ks between 1300 and 1523 K, which is in a single7-phase region. At least 20 alloys were prepared for each alloy system to be examined by DTA. DTA was carried out using a Rigaku Thermoflex with a cylindrical specimen o f approximate diameter 3 mm and 4 mm long. The specimen was placed in an alumina crucible and the measurement was made at a heating and cooling rate of 0.25 K s-~ using platinum as the reference. For selected alloys, chemical compositions of the constituent phases were examined by scanning electron microscopy (SEM)-energy-dispersive X-ray spectroscopy (EDXS) using a Tracor-Northern TN-4500 instrument in combination with a JEOL-455 scanning electron microscope. A conventional ZAF calibration was applied to the spectroscopy taking into account the effects of atomic number Z, absorption A and fluorescence F. 3. Results and discussion

on Tf and Ts was examined for the range from 0.08 to 0.33 K s- 1 and it was found that the average of the two temperatures was not sensitive to the rate, although each of them certainly was. An example of the composition depenence of the 7 solvus temperature is shown in Fig. 2 for the Ni-AI-Cr alloys: in Fig. 2(a) the variation is shown with aluminium concentration and in Fig. 2(b) the variation with chromium concentration [1, 2]. It should be noted that (1) the 7 solvus can be expressed by smooth curves and (2) another set of curves can be drawn at high aluminium and chromium concentrations. The former enables us to evaluate the solvus at any temperature or for any composition by interpolation, whereas the latter tells us that there is a solvus involving a situation where a phase other than y' is neighbouring to the 7-7' equilibria. These by themselves represent the advantage of the present method of DTA well because it is able not only to provide the solvus surface with respect to composition and temperature but also to present additional and yet systematic information on the phase relations nearby.

3.1. Determination of 7 solvus by differential thermal analysis Figure 1 shows a schematic DTA curve to determine the 7 solvus. The solvus is determined by averaging the temperature Tf corresponding to the completion of 7' dissolution upon heating and the temperature T~ corresponding to the onset of 7' precipitation upon cooling, both being defined as in Fig. 1. The effect of heating and cooling rate

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Fig. 1. Schematic DTA curve to determine the 7 solvus.

Fig. 2. Dependence of the ? solvus temperature on (a) aluminium concentration and (b) chromium concentration.

125

3.2. Ternary systems with a transition metal element Results on the determination of the 7 solvus in the Ni-AI-X ternary systems with X being transition metal elements are presented in the form of solvus isotherms. Figures 3-5 are those for X~-Ti, Zr and Hf (group IVA) [3], Figs. 6-8 for X - V , Nb and Ta (group VA) [2, 4], Figs. 9-11 for X ~- Cr, Mo and W (group VIA) [1, 2]. Supplemental results are for X - M n (group VIIA) in Fig. 12 [3]. In each figure, except for Fig. 6 and Fig. 12, the 7 solvus is expressed by full curves

connected to broken curves at clearly defined inflections. The boundary between them is drawn not by speculation but by consideration of such findings as described in Fig. 2 or as to be described in Section 3.4. For the Ni-AI-V system in Fig. 6, systematic information is obtained through analyses on results similar to Fig. 2 regarding the phase relations with respect to Ni3V. Besides the data shown in the figures the pseudobinary constitution has been revealed between Ni3AI and Ni3V, for which the details have been described elsewhere [2, 3]. The results

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Fig. 6. Results for Ni-AI-V.

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126

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Fig. ]0. Results for Ni-AI-Mo.

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Fig. 8. Results for Ni-AI-Ta.

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Fig. 12. Results for Ni-AI-Mn.

I0

Ni

127

for the Ni-AI-Mn system in Fig. 12 are not complete in the sense that the terminating compositions for the solvus isotherm cannot be determined through the present investigation. When a comparison is made between the results for elements of the same group, the role that the periodic number of an element plays in the direction and extent of the solvus at 1300 K can be demonstrated as shown in Figs. 13-15 for groups IVA, VA and VIA respectively. It is generally stated that on increasing the periodic number the tangent of solvus evaluated at the Ni-A1 edge becomes steeper and the extent of the solvus becomes less. Also by interrelating the figures, the same tendency is found with increasing group number for elements of the same period number. Such systematic information has never been available previously.

Ge in Figs. 16-18 because in these systems a continuous solid solution is formed between Ni3A1 and Ni3X [5]. Figures 19-21 are the results for X - In, Sn and Sb, where the limited solubility of these elements to both Y' and y is characteristic. A similar type of comparison to that in Figs. 13-15 is given in Figs. 22 and 23 for the 7 solvus at 1100 K for all the subgroup B elements investigated and it is found that there is no clear tendency as was found for the ternary systems with a transition metal element in the direction and extent of the 7 solvus as a function of group or periodic number.

3.4. Accuracy of the evaluation Chemical analyses of the constituent phases are carried out using SEM-EDXS on selected alloys in order to check the validity of the present method of utilizing DTA to determine the 7

3.3. Ternary systems with a subgroup B element The same experimental strategy has been used to determine the 7 solvus in the ternary systems with a subgroup B element as X [3]. The solvus isotherms are presented first for X - S i , Ga and Ni-AI-X at 13001(

0

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Ni-AI-X

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at 1300K

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\ 10

20

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X I at%

Fig. 15. Role that the periodic number of an element plays in the extent and direction of the solvus at 1300 K for group VIA.

Ni

3o X / at.%

Fig. 13. Role that the periodic number of an element plays in the extent and direction of the solvus at 1300 K for group IVA.

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Fig. 16. Results for Ni-A1-Si.

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Fig. 17. Results for Ni-AI-Ga.

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Fig. 20. Results for Ni-AI-Sn.

solvus. Such an examination is carried out for most of the ternary systems investigated; however, because of the limited amount of space available here, only an example is shown. References already published or to be published should be referred to for the details of the results in each alloy system [1-4]. Figure 24 shows the results for the Ni-AI-Cr system where an alloy with a composition shown by the full circle is chosen for chemical analyses of the constituent phases [1]. The alloy is equili-

brated at 1400 K and is found to be in the threephase triangle next to the 7-7' equilibrium. Chemical analyses reveal that the ternary phase is NiAI (/7) and then the compositions of the vertices for the triangle are determined as shown by the full squares. The composition of the 7 phase is of importance because it is a unique value giving the terminal composition of the 7 solvus at a specific temperature. The composition is found to be in almost perfect agreement with what is found in Fig. 9 based on the results shown

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20

-

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•

Alloy Composition

•

Phase Composition

Sb / at%

-

at 1400K

Fig. 21. Results for N i - A I - S b .

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,

20

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Fig. 22. Role that the periodic number of an element plays in the 7 solvus at 1100 K for X ~- Si, Ga and Ge.

Fig. 24. Results for N i - A I - C r .

in Fig. 2 by which the accuracy of the present method is proved. Such agreement is observed in all the ternary systems investigated. It should be noted that the chemical analyses using S E M - E D X S are useful for obtaining quantitative information on the neighbouring phases to 7 and 7'. Also by the chemical analysis on the alloys with 7-7' two-phase alloys, the phase boundary of 7' facing the 7 phase can be determined and consequently the tie line can be drawn. The end of the tie line in the 7-7' two-phase region is, of course, the edge of the three-phase triangle if it is known as in Fig. 24. As is mentioned above, by the present method the 7 solvus is obtained as a surface with respect to temperature and composition in the ternary system. It then enables us to compare the present results with any fragmental information in the literature. For each alloy system, such a compari-

son has been made with as many available data as possible [1-4] and in general it has been pointed out that in many cases the data are inaccurate and sometimes inconsistent. It is felt that the data tend to be more accurate in newer publications in which advanced analytical techniques are utilized. 4. Conclusions

Systematic investigations on the determination of the 7 solvus have been carried out using DTA as a key experimental technique. The solvus isotherms are obtained in the Ni-AI-X ternary systems with X being a transition metal or subgroup B element belonging to many groups and periods. The following are the conclusions drawn. (1) DTA is found to be a useful experimental tool for the purpose of the present work. The 7 solvus is obtained as a surface with respect to

130

composition and temperature in each ternary system. (2) The 7 solvus obtained is found to be highly accurate by the chemical analyses of the constituent phases using SEM-EDXS on selected alloys. Moreover the results provide much additional information on the phase relations in the vicinity of the 7-7' equilibria. (3) The quality and quantity of the data presented here are sufficient to create a fundamental database for alloy design of nickel-based superalloys.

References 1 Y. M. Hong, H. Nakajima, Y. Mishima and T. Suzuki, Iron Steel lnst. Jpan. Int., 29 (1989) 78. 2 Y.M. Hong, Y. Mishima and T. Suzuki, in C. C. Koch, C. T. Liu and N. S. Stoloff (eds.), High Temperature Ordered Intermetallic Alloys I11, Materials Research Society Syrup. Proc., Vol. 133, Materials Research Society, Pittsburgh, PA, 1989, p. 429. 3 Y.M. Hong, Y. Mishima and T. Suzuki, Iron Steel Inst. Jpn. Int., to be published. 4 Y. Mishima, Y. M. Hong and T. Suzuki, Iron Steel Inst. Jpn. Int., to be published. 5 S. Ochiai, Y. Oya and T. Suzuki, Acta Metall., 32 (1984) 289.