Journal of Engineering and Technology

ARTICLE
Year
: 2014  |  Volume : 4  |  Issue : 2  |  Page : 119--122

Characterization of Gamma Prime Phase in Hafnium Doped IN-738LC Nickel Base Superalloy


Ali Akbar Saghafi1, Jamshid Aghazade Mohandesi2, Hamid Reza Alizade Attar1,  
1 Mapna Turbine Blade Engineering and Manufacturing, Karaj, Iran
2 Department of Mining and Metallurgical Engineering, Amirkabir University of Technology, Tehran, Iran

Correspondence Address:
Ali Akbar Saghafi
Mapna Turbine Blade Engineering and Manufacturing, Karaj
Iran

Abstract

In this article, effect of hafnium (Hf) addition to IN-738LC nickel-base superalloy, on the morphology, size distribution and content of gamma prime (γ′) precipitate has been studied. Scanning electron microscopy employed for observation of γ′ precipitates in the matrix. Image analysis process has been performed by Clemex Image Analysis software (Clemex Technologies, Inc., 800 Guimond Blvd, Longueuil, Quebec, Canada). The results show that Hf addition increases the amount of γ′ phase in the alloy, change it morphology from cubical to irregular and spherical shape and forms more stable and finer γ′ particles. Furthermore, selection of a suitable heat treatment is very important for achievement to the obtained results.



How to cite this article:
Saghafi AA, Mohandesi JA, Attar HA. Characterization of Gamma Prime Phase in Hafnium Doped IN-738LC Nickel Base Superalloy .J Eng Technol 2014;4:119-122


How to cite this URL:
Saghafi AA, Mohandesi JA, Attar HA. Characterization of Gamma Prime Phase in Hafnium Doped IN-738LC Nickel Base Superalloy . J Eng Technol [serial online] 2014 [cited 2019 Nov 12 ];4:119-122
Available from: http://www.onlinejet.net/text.asp?2014/4/2/119/141200


Full Text

 1. Introduction



The beneficial application of nickel-base superalloys at high temperature depends on their microstructure. A complex combination of different phases, created because of the vast amount of alloying elements, determines the reliability of alloy at high temperature and stress. Addition of hafnium (Hf) was found to be very effective in improvement of microstructure and mechanical properties of nickel-base superalloys [1],[2],[3],[4],[5] . Hf has high solubility in gamma prime (γ′) relative to γ [6] . Further during the solidification strong partitioning of Hf to γ′ results in convoluted γ′-γ structures in the grain boundary area. This inhibits rapid crack propagation and retard grain boundary slidings and separations during the creep and gives in higher stress-rupture life for Hf-modified alloys [3] . Also, Hf has an added advantage of increasing the oxidation resistance of the base alloy. However, its high reactivity causes an added "adventure," creating difficult (but controllable) problems during ingot melting and component processing. Beside Hf is a strong oxygen, nitrogen and sulfur scavenger [7] .

While most minor elements are added up to the amounts much less than 0.1%, Hf is found at levels from about 0.5% to nearly 2.0%. Customary levels might be 0.5% for polycrystalline cast nickel-base superalloys to 1.5% for columnar grain directionally solidified cast nickel-base superalloys [1] . In this article, effect of different amounts of Hf addition on the characteristic of γ′ phase IN-738LC (Low Carbon) have been studied.

 2. Experimental Procedure



Vacuum induction melted IN-738 samples have been sand blasted and then grinded. After grinding, vacuum arc remelting has been used in order to add excess high purity Hf to the base alloy. The amount of superimposed Hf in the different samples is stated in [Table 1]. The purity of added Hf was 99.99%.

After value added reseller processing, three solution treatments has been used in order to homogenize the microstructure. [Table 2] shows these treatments.

Metallographic investigations on the heat treated samples have been performed using the scanning electron microscope. In order to observe γ′ phase by scanning electron microscope (SEM), electro etching in 10% oxalic acid reagent was employed. Electro etching carried out at 5 V for 4 s.

Quantitative analysis of γ′ phase was performed by the Clemex Image Analysis software (Clemex Technologies, Inc., 800 Guimond Blvd, Longueuil, Quebec, Canada). In the quantitative analysis procedure, the image was loaded and calibrated using available scale line. A strong delineate function was applied to remove intermediate gray levels. The binarization was performed using gray thresholding. Artifacts were removed and particles were separated. An example of image before and after preparing for quantitative analysis is shown in [Figure 1].{Table 1}{Table 2}{Figure 1}

 3. Results and Discussion



[Figure 2] shows the γ′ phases in all alloys after solution treatment at 1120°C for 2 h. It is obvious that γ′ phase have the blocky shape in Hf free alloy, but in Hf bearing alloys, its shape changes to irregular and spherical shape. [Figure 3] shows the γ′ phases in all alloys after solution treatment at 1220°C for 2 h. It could be seen that in Hf free alloy the size of γ′ phase greatly reduced after applying the second type solution treatment; however, in Hf bearing alloys, there is a distribution of large and small particles of γ′ phases. The very small size of γ′ phase in Hf free alloy is the result of nearly completely dissolution of primary γ′ and nucleation of secondary γ′ phases. These fine particles called as cooling γ′ because slow cooling rate from solution temperature causes the formation of these phases. The dissolution temperature of γ′ phase increases by the addition of Hf. Therefore, incomplete dissolution of primary γ′ causes the formation of bimodal γ′ in the matrix. It is expected that by enhancing soaking time at solution temperature, completely dissolution of primary γ′ occurs. So, a third solution treatment employed for solution treatment of Hf bearing alloy at 1220°C for 4 h. The results [Figure 4] show a uniform distribution of fine γ′ particles by extending soaking time at solution temperature.{Figure 2}{Figure 3}{Figure 4}

The results of quantitative analysis [Figure 5] and [Figure 6] performed on each sample confirmed last results; [Figure 7] and [Figure 8] show the variation of aspect ratio of γ′ phase versus Hf content and solution temperature. Aspect ratio is the proportion of longest to shortest length of γ′. The aspect ratio is always more than one. The lower aspect ratio shows the tendency toward the circular γ′ morphology. The results of quantitative analysis of aspect ratio confirm the obtained results from microscopic investigation. It indicates that Hf addition causes more circular γ′ phases. So does the increasing solution temperature and time, causes more circular γ′ in the alloy. Also, quantitative analysis has been used in order to calculate average fraction area of γ′ phases under different conditions. It is clear from [Figure 9] that Hf addition and solution treatment condition affect average fraction area of γ′ phase. Higher the Hf content higher is the average fraction area of γ′ phase. Due to partitioning of Hf in γ′ phase, the amount of γ′ forming elements increases and causes increment of γ′ phase. Also, [Figure 9] shows that increasing solution temperature (i.e., to above dissolution temperature of γ′ phase ≈1136°C [8] ), decreases γ′ phase percentage in all three alloys, but its decrement is severe in Hf free alloy, because of enhancement dissolution temperature of γ′ phase by the addition of Hf. Subsequently, the reduction in γ′ phase percentage could be related to the difference between the dissolution rates of γ′ phase during soaking at solution temperature and the rates of nucleation and growth of γ′ phase during cooling from solution temperature. The rates of dissolution are more than the rates of nucleation and growth of γ′ phases.

It could be implicated from [Figure 9] and [Figure 10] that Hf addition retards dissolution of γ′ phase in IN-738LC. It's of prime concern for precipitate hardening alloys to form stable precipitates at high temperatures. Therefore, Hf seems to be a suitable choice for the formation of more stable γ′ precipitates in IN-738LC.{Figure 5}{Figure 6}{Figure 7}{Figure 8}{Figure 9}{Figure 10}

 4. Conclusion



The effect of Hf addition to the IN-738LC nickel-base superalloy on the morphology, shape, size distribution and stability of γ′ phases was investigated by SEM and Clemex Image Analysis software. The following conclusions were obtained:

Hf addition to the IN-738LC nickel-base superalloy changes the morphology of γ′ phase in as-cast condition from cubical shape in Hf free alloy to irregular and spherical shapes in Hf bearing alloys.Hf addition to the IN-738LC increases the amount of γ′ phase after accomplished solution treatments.Hf addition to the IN-738LC causes the formation of more stable γ′ phase at high temperature because partitioning of Hf to the γ′ phase increases the dissolution temperature of this phase.Selection of a good solution treatment condition causes the formation of uniform distribution of finer γ′ phases in Hf bearing alloy than Hf free alloy.

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