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JOURNAL OF IRON AND STEEL RESEARCH, INTERNAXIONAL. 2013, 20(6): 88-94

Elemental Partitioning Characteristics of Equilibrium Phases in Inconel 718 Alloy at 600- 1 100 “c

WE1 Xian-ping’,’ , ZHENG Wen-jie’ , SONG Zhi-gang’ , LEI Ting’ , YONG Qi-long‘ , XIE Qing-cheng3

(1. Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, Yunnan, China; 2. Central Iron and Steel Research Institute, Beijing 100081, China: 3. No. 704 Institute of China Shipbuilding Industry Corporation, Shanghai 200031, China)

Abstract: The optimization of heat treatment and chemical composition for Inconel 718 alloy has been investigated un- interruptedly because of its excellent mechanical properties and metallurgical workability. The species, chemical composi- tions and content of equilibrium phases of Inconel 718 alloy in the temperature range of 600- 1100 C were calculated by using thermodynamic software “Thermo-Calc” and the latest relevant datebase of Ni-base superalloys. A concept of elemental partitioning fraction was used to study the partitioning characteristics of alloying elements in each equi- librium phase at different temperatures, such as Ni, Cr, Fe, Nb, Mo, Al, Ti and C, and some calculation results were confirmed under a scanning transmission electron microscope (STEM). The results showed that the elemental partitioning characteristics with the change of temperature revealed the selective partitioning characteristic of alloying elements in equilibrium phases at different temperatures, such as Nb was mainly distributed in 6 and y’ phase, C in carbides, A1 and Ti in y’ phase and Cr, Mo in Laves phase. At the same time, the effect of the change of component and quantity for each precipitated phase on matrix phase can be helpfully understood, which provided a theoretic foundation to optimize the chemical composition and heat treatment in different environments for Inconel 718 alloy. Key words: Inconel 718 alloy: Thermo-Calc; thermodynamic calculation; elemental partitioning fraction

Inconel 718 alloy, a Nb-modified Fe-Cr-Ni base superalloy, has been widely used in gas turbine and related application“-’]. At present, some researches are focusing on the microstructure evolution and high temperature properties of Inconel 718 alloy. The predominant r e sea rche~[~-~] about Inconel 718 alloy can be summarized briefly as follows: the types of stable equilibrium precipitation phases, the pre- cipitation temperature of each precipitation phase, the effect of a change in elemental content on the mi- crostructure and properties, and thermodynamic calcula- tion of a-Cr phase in Inconel 718 alloy. Undoubtedly, these researches are useful for understanding the phase equilibrium and transformation of this alloy at different temperatures. They are also helpful to study the high-temperature properties of Inconel 718 alloy.

At present, “ Thermo-Calc” and relevant date- base of Ni-base superalloy are only used to calculate

the quantity of equilibrium precipitation phase. Also, some experimental results confirmed the validity of these calculated results. But partitioning mechanism especially quantitative interpretation has not been used to explain the calculated and experimental re- sults. Therefore, in the present work, the elemental partitioning fraction concept was introduced to re- search selective partitioning characteristics of the al- loying element in equilibrium phases on the basis of phase composition and phase content, providing the- oretical basis for technology formulation and compo- sition design of Inconel 718 alloy.

1 Experimental Methods

1.1 In order to study the relationship between pre-

cipitation phases and temperature, the composition and content of each equilibrium phase in Inconel 718

Thermodynamic calculation on phase equilibrium

Biography: WE1 Xian-ping(1984-), Male, Doctor Candidate; E-mail: weixp316@163. corn; Received Date: January 11, 2012 Corresponding Author: ZHENG Wen-jie(1964-), Male, Master, Professoriate Senior Engineer; E-mail: sxzwj@sohu. corn

Issue 6 Elemental Partitioning Characteristics of Equilibrium Phases in Inconel 718 Alloy at 600-1 100 C 89

* 8 0.6

0 & 0.4

alloy were calculated by "Thermo-Calc" and relevant latest datebase of Ni-base superalloy at 600-1 100 "C. In this calculation, parameters were set as 1 mol, 298. 15 K and l o 5 Pa, respectively. The chemical composition of the investigated Inconel 718 alloy is given as follows (mass percent, %) : C 0. 03, Cr 19.12, Mo 3.12, A1 0. 60, Ti 0.99, Nb 5.22, Fe 17.61, Mn 0.013, Si 0.080, P 0.0037, S 0.0024, and Ni balance.

-

-

1.2 librium phases

expressed as Ci in equilibrium states.

calculation of elemental partitioning among equi-

The atomic concentration of element i can be

n

ci = z pjcji (1) j = l

where, p j is the phase volume fraction of phase j ; Cji is the atomic concentration of element i in phase j , and j = 1, 2 , . * a , n. Phase j expresses the equi- librium phase which consists of 6, y r , MC, MZ3 C, , M,C, Laves and matrix (y) phase.

n i= Mo zpj=1, z cji=l (2)

i = l i=C,Cr,Fe,-

Then, set the total amount of each element i as 1, and the proportion of element i in equilibrium phase j as a j i , where element i expresses the main alloying element which includes Ni, Cr, Fe, Nb, Mo, Ti , A1 and C.

n c Q,i = 1 j = l

(3)

1.3 Experimental details The calculated chemical composition is consist-

ent with that of the experiment. Three heat treat- ment processes are used in the present study. The

1--6 ' 2--y

3- r' 4- MDCS

6- MC . 6-M&

7--Liquid 8- Laves

~ 2

800 1 000

heat treatment process 1 is ageing at 720 'C for 16 h and then water quenching to room temperature. The process 2 is solution treating at 940 'C for 2 h and water quenching to room temperature. The process 3 is solution treating at 1050 'C for 2 h and water quenching to room terqperature. AccQrding to the distribution of strengthening elements in the matrix phase, some calculation results are confirmed under a scanning transmission electron microscope (STEM).

2 Computation and Experimental Results

2.1 Computation and analysis 2. 1. 1

Fig. 1 shows the phase content in Inconel 718 alloy a t the temperatures ranging from 600 to 1100 'C. The stable phases including 6 , y' , MC, MZ3 C, , C, Laves and matrix y phase are shown in Fig. 1 (a) . Meanwhile, as the main strengthening phase, the metastable strengthening y" phase is also precipita- ted in the investigated alloy. Thus, the 7'' phase cannot be directly represented on the equilibrium calculation. And it can be transformed into 6 phase which has similar chemical composition with y" phase after long time ageing (such as y+y"-+6[*--lo1 1. There- fore, the content of other phases was not changed, when the y" and 6 phase were looked as a ~ h o l e [ ~ * " ~ . The calculated content of 6 represented the sum con- tent of y" phase and 6 phase below 900 "C.

It can be seen from Fig. 1 ( b ) that the content of 6 phase keeps steady from 600 to 750 'C , while it decreases from 750 to 1018 'C. And 914 OC is the in- itial precipitation temperature of y' phase. Both the 6 phase and y r phase had the same equilibrium con- tent of 11% at 600 'C. Fig. 1 (a) shows M,C phase

Content of phase

0.10

0.06

0 600 700 800 900 lo00 110

Temperature/%

(a) Relationship of mass percent of precipitated phase to temperature: (b) Partially enlarged region of (a). Fig. 1 Equilibrium phase diagrams of Inconel 718 alloy

90 Journal of Iron and Steel Research, International VOl. 20

0.25

0.20

OJ5

0.10

0.06

0;

precipitated from 715 to 755 "C , and the initial pre- cipitation temperatures of Mz3C6 and MC were 752 and 1254 'C , respectively. Moreover, MC was more stable in contrast with Mz3 C, and m C . It is clear that some certain transformations exist among Mz3 c6 , M,C and MC. The relationship is shown as follows: MC+ Y+MZ3 c6 (or M6 c) + Y ' ~ ~ ~ - ~ ~ ' , which occurs in Inconel 718 alloy at temperatures from 748 to 755 'C ,

- . - - - -

- A - - - - o t - - 1~ - -

-I

600 700 800 900 lo00 1100

o.80 I (c)

0.75 t \ I I \*"..- I

0.10 0 . 1 5 ~

580600 680 720 760

______Iw - - 6 - I - 1 - I- I

600 620 640 660 680 700 720

and M6C+Mz3 c6 appears at temperatures from 715 to 720 %. But these transformations are limited in non-equilibrium state and even could be neglected. Laves phase begins to precipitate when the tempera- ture fell below 720 "C , and its content increases with decreasing temperature. 2. I. 2 Change of components

Fig. 2 can be obtained from the calculation of

0.40 t I 600 700 900 1000

0.90 1 0.85 1 1

i 0.10

0.05 1

Fig. 2 Component of each phase at different temperatures

Issue 6 Elemental Partitioning Characteristics of Equilibrium Phases in Inconel 718 Alloy at 600-1 100 'C 91

Eqn. (2). Fig. 2 shows the change of element content in each phase varied with temperature. Below 1018 "C , the content of Ni, Nb, and Ti in matrix phase de- creased with decreasing temperature, as shown in Fig. 2 (a). On the contrary, the content of Cr, Fe, and Mo in matrix phase increased with decreasing temperature. The reason can be ascribed to the pre- cipitation of 6 phase when the temperature fell below 1018 'C , as shown in Fig. 1. 6 phase is rich in Ni, Nb, and Ti and poor in Cr, Fe, and Mo. Thus, large amount of Ni, Nb and Ti in matrix can be con- sumed by the precipitation of 6 phase, leading to the depletion of Ni, Nb and Ti in matrix phase. On the other hand, y' phase, Ni3 (Al,Ti,Nb), is rich in Ni (more than 70%), Ti, A1 (more than 7 % ) , Nb (more than 10%) and depleted in Cr, Fe, and Mo. Therefore, the content of Fe in matrix phase in- creased with the precipitation of y' phase. As shown in Fig. 1 (a) , 6 phase and y' phase are the main pre- cipitated phases in Inconel 718 alloy, so the content of relative elements in matrix phase changed obvi- ously, owing to their precipitation.

In respect of carbides, MC phase begins to pre- cipitate at 1254 'C. Due to the forming of 6 and y' phase, the ratio of Ti/Nb in MC particle is changed at 1018 and 914 %. In addition, some fluctuation of the content of Nb and Ti in MC particle can be as- cribed to the element exchange between matrix phase and MC particle. Meanwhile, Cr and Mo are the major alloying elements in M23 cg and M6 c, and the content of Cr and Mo in M23C6 and Cr in M6C in- creased with decreasing temperature, as shown in Fig. 2 (e) and (f ) . The results indicated that the en- richment of Cr and the depletion of Ni in MZ3C6 and M6 C become more remarkable with decreasing tem- perature, and Mz,C6 is more stable than M6C for the higher concentration of Mo.

Fig. 2 (g) illustrates that Laves phase is rich in Cr, Mo, and poor in Ni. Consequently, the content of Mo significantly decreased and Ni steadily increased in matrix phase for the forming of Mz3 c6 , M6C and Laves phase.

But it is impossible to obtain the transfer char- acteristics of each element among phases in Inconel 718 alloy from Fig. 2 , so the conception of distribu- tion content is introduced in this article to illustrate the elemental partitioning characteristics. 2. 1. 3

The content and composition of each phase at the temperatures ranging from 600 to 1100 "C can be obtained by thermodynamic calculation, and Fig. 3

Distribution of alloying element in each phase

can be obtained according to Eqn. (3). The distribu- tion of Cr and Mo in Laves phase is quite obvious, as shown in Fig. 3 ( a ) and ( b). There are about 3. 8% of Cr and 38. 3% of Mo distributing in Laves phase at 620 'C , while their precipitation exerts neg- ative influence on the plasticity and toughness. How- ever, in addition to the precipitation of Laves phase, the decrease of equilibrium distribution amount of Mo in matrix phase is also associated with the precipitation

As the main elements of strengthening phase, the strength of the investigated alloy are dominated by the distribution characteristic of Nb, A1 and Ti among matrix phase, 6 and y' phase. Fig. 3 (c) shows the equilibrium distribution amount of Nb in 6 and 7' phase is 73. 5% and 23.4% at 620 'C , respectively. Thus, the distribution characteristic of Nb is mainly controlled by the precipitated 6 phase. As shown in Fig. 3 (d) and (e) , the equilibrium distribution amount of A1 and Ti in y' phase are 79.2% and 78.2% at 620 'C , respectively, implying that they are controlled by the precipitation of y' phase. Thus, the equilibri- um distribution amounts of Nb, A1 and Ti in matrix phase with equilibrium state are 0. 13%, 0. 03% and 0.12% (by mass percent) at 620 "C , well below their mean content of 5.22%, 0.99% and 0. 60% (by mass percent) , respectively. The results indica- ted that Nb, Ti and A1 are the main constituent ele- ments of precipitated phases, and their solution con- tent in matrix phase is limited.

In addition, Fig. 3 ( f ) shows the element of C is almost concentrated in carbides. The equilibrium content of C in matrix phase is 0.027% at 620 'C only. The equilibrium distribution of C is dominated by the MC-carbide, and the relative quantity in MC-carbide increases with decreasing the temperature above 885 'C , but it has no change in the temperature range of 760- 885 %. On the other hand, with the transformation of C among M6C, Mz3C6 and MC phase, the distri- bution ratio of C varied with temperature. As a result, the C element is mainly distributed in MZ3G at 620 'C. And carbides are not beneficial to improving the strength and corrosion resistance of Inconel 718 alloy.

Similarly, the equilibrium distribution charac- teristics of Ni and Fe are shown in Fig. 3 (g) and (h). Furthermore, as illustrated in Fig. 3, there are about 72.5% Nb, 22.0% Ti distributing in 6 phase, and 20.4% Nb, 68.5% Ti and 62.0% A1 in y' phase, while 7.0% Nb, 9. 5% Ti and 38.0% A1 in matrix (7) phase at 720 'C. The results indicated that the solution content of alloying elerqents in matrix phase

of M23C6.

* 92 * Journal of Iron and Steel Research, International VOl. 20

A Laves

100

80

P g 6 0 -44

Y

9 40 20

0

(a ) Cr; (b ) Mo; ( c ) Nb; (d ) Al; ( e ) Ti; (f) C; (g) Ni; (h ) Fe. Fig. 3 Distribution characteristics of main elements in each phase

at 720 "C are higher than that at 620 'C, which is mainly ascribed to the less precipitation amount of y' phase, as shown in Fig. 1. Additionally, the content of Cr and Mo in matrix phase also increased at 720 "C

for no precipitation of Laves phase. As stated previously, Laves phase is the princi-

pal phase enriched in Mo, Fe and Cr, so it had a sig- nificant effect on the distribution of Mo, Fe and Cr.

Issue 6 Elemental Partitioning Characteristics of Equilibrium Phases in Inconel 718 Alloy at 600-1 100 'C 93

It is well known that when the content of Laves is controlled strictly, the better ductility, plasticity and hot-forming capacity can be obtained. In addi- tion, the precipitation of MC and M2, C, determined the distribution of carbon. Reducing the content of carbon in Inconel 718 alloy is believed to limit the quantity and size of MC, M23CG and M,C particle, and the corrosion resistance also can be increased. I t would be necessary when this alloy is used as a high strength and corrosion resistant alloy at room tem- perature. On the other hand, in order to avoid the transformation from MC into Mz3C6 (or M,C) and the precipitation of M,C, i t is necessary to take ap- propriate heat treatment process.

2.2 Experimental confirmation Fig. 4 (a) shows the morphology of precipitates

when the alloy is solution treated at 940 'C for 2 h , followed by water cooling. It can be seen that a large number of 6 phase are precipitated. Accordingly,

Fig. 4 ( b ) shows the morphology of precipitates when it is aged at 720 "C for 16 h , followed by water cooling, and a large number of dispersion strengthe- ning phases are observed. In addition, Fig. 5 shows the change of strengthening elements in matrix phase, including Nb, A1 and Ti, which is obtained according to the EDS of STEM and Eqn. (3). Com- paring Fig. 5 (a ) with Fig. 5 ( b ) , also as shown in Fig. 3 (c> , (d) and (e) , i t is clear that the curves of experimental results and calculations are similar in trends and shapes, but some misfits also exist, which is basically ascribed to the following two rea- sons. l) The test process is conducted in non-equi- librium state, but the content and composition of precipitates by calculation are obtained in equilibri- um state. 2) The experimental distribution of Nb is obtained only from matrix phase by EDS of STEM, but the calculated distribution is deduced including the matrix phase and precipitates. Thus, the calculation error is inevitable, especially at lower temperature, and

(a) Process 2; (b) Process 1. Fig. 4 STEM images showing morphology of precipitates in investigated alloy treated by different processes

0 . I

1 2 3 Liquid 720 9: 940 9: 10609: Liquid Heat trentment proress Equilibrium temperature

(a) Experimental results; (b) Calculated results.

Fig. 5 Distribution percentage of strengthening elements in matrix phase ( 7 ) vs heat treatment process

94 - Journal of Iron and Steel Research, International VOl. 20

the same problem also exists about A1 and Ti. Therefore, the similar trends between the calculated and experimental results are able to show the validi- ty of the calculated results, although lack of strict correspondence.

It can be also concluded from calculated and ex- perimental results that appropriate heat treatment process to control the types and proportion of pre- cipitation phases is necessary. For avoiding the pre- cipitation of M6C and Laves and minimizing the pre- cipitation of M,, c6 , Inconel 718 alloy was always aged at 720 and 620 'C. Consequently, the corrosion resistance, toughness, ductility and strength of In- cone1 718 alloy can be improved to a certain degree after optimizing heat treatment parameters. And the purpose of this study is to improve the comprehen- sive performance of Inconel 718 alloy.

3 Conclusions

1) It is considered that 6 phase, y' phase, MC, M,,C,, M6C, Laves and y phase are the equilibrium phases in Inconel 718 alloy at temperatures from 600 to 1100 "C. 6 phase is precipitated at temperatures ranging from 750 to 1018 "C , while the precipitation temperature of y' phase, MC, M23G and Laves were 914, 1254, 752, and 720 "C , respectively. Howev- er, the precipitation temperature of M6C was 715- 755 'C.

2 ) The distributions of Nb, Ti and A1 in matrix phase are decreased in the process of the precipitati- on of Ni, Nb and Ni, (A1 ,Ti). In addition, the distri- bution of C is significantly decreased during the for- mation of carbides. Moreover, with the change of types and content of precipitates, the distributions of Cr, Fe, and Mo are also changed accordingly.

3) The elements of Nb, Ti and A1 were mainly consumed during the precipitation of 6 and y' phase. Based on the equilibrium thermodynamic calcula- tions, there are about 73. 5% Nb, 18. 6 % Ti and 13. 5% A1 distributing in 6 phase, and 23.4% Nb, 78.2% Ti and 79.2% A1 in y' phase, while only about 2.5% Nb, 3.2% Ti and 20.8% A1 in matrix ( y ) phase at 620 "C. However, there are about 7.0% Nb, 9. 5% Ti and 38. 0% A1 distributing in matrix

(y) phase at 720 "C.

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