UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

160 IMMC 2018 | 19th International Metallurgy & Materials Congress

Eff ect of Alloying Elements on Microstructure of Ni-15Al Superalloys

Mehmet Şahin Ataş¹,², Mehmet Yıldırım¹,²

¹Selçuk University, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Konya, Turkey²Konya Technical University, Faculty of Engineering and Natural Sciences, Department of Metallurgical and Materials Engineering, Konya, Turkey

Abstract

Ni-base superalloys can be considered as promising materials at high temperature structural applications due to their excellent corrosion and oxidation resistance, high creep strength and high strength at elevated temperatures (above 500 oC). These unique physical and mechanical properties are directly attributed their microstructure composed of L12 ordered strenthening precipitates dispersed in the disordered matrix. The life time of these alloys strongly depends on the microstructure and its temporal evolution. The microstructural properties are directly effected by alloy composition and proper heat-treatment. In this study, it is aimed to investigate the effect of alloying additions on the microstructural properties such as size, distribution, morphology and volume fraction of precipitates in as-cast Ni-15Al composition.

1. Introduction

Ni-base superalloys can be widely used in commercial and military jet engines, land-based gas turbine blades, aerospace and marine propulsion engines due to their high strength at elevated temperatures, considerable fatique resistance and excellent corrosion resistance [1-6]. The superior high temperature mechanical and physical properties of Ni-base superalloys can be attributed to their unique microstructure consisting of L12 type ordered, (Ni3Al), precipitates distributed coherently within a FCC (Ni) matrix phase [7]. In Ni-base superalloys, alloying element additions have generally resulted in either solid-solution strengthening for phase stabilizers such as W, Mo, Cr and Co or precipitation hardening with optimization of volume fraction of the phase for forming elements such as Ti, Nb and Ta [6].

Moreover, the levels of alloying additions have also an important effect on the microstructure and the resultant mechanical and physical properties of polycrystalline Ni-base superalloys. When designing a new Ni-base superalloy containing phase forming alloying elements such as Ti, Nb Ta, Hf or Y, the amount of these alloying elements have to be optimized [6]. For relatively low alloying amounts, these alloying elements preferentially partition the phase

and improve the strength of the alloy [8]. However, when the concentration of these alloying additions exceed the solubility limit, unwanted secondary phases such as (Ni3Nb), (Ni6AlNb), Ni5Hf, Ni5Y, Ni17Y2 and etc. can be formed within the microstructure [9-11]. These secondary phases may have detrimental effects on mechanical properties, temperature capability and some other physical properties of the alloy. In this study, it is aimed to investigate the effect of alloying additions of Nb and Y on microstructure, phase relationships and microhardness of Ni-15Al alloy.

2. Experimental Procedure

2.1. Production of the samples

The Ni85Al15 nXn, where X= Nb and Y, alloys (n=2 and 4 at.%) were prepared by arc melting using a non-consumable tungsten electrode and a water-cooled copper tray in a high-purity argon atmosphere. Samples of 10–15 g were produced using high-purity constituent elements (Ni=99.95 wt. %, Al=99.9 wt. %, Nb=99.9 wt. %, and Y=99.9 wt. % purity). Prior to arc melting the furnace chamber was evacuated to 10 5 mbar. Arc melting was then performed under an argon inert gas atmosphere. Each alloy was remelted several times to achieve greater composition homogeneity. The weight loss during the arc-melting process was less than 0.5 wt. %.

2.2. Characterization

X-ray diffraction (XRD) analyses were conducted using a Rigaku D/Max-2200 PC diffractometer with Cu-Kradiation ( = 1.54056 Å) and an X-ray source operating voltage of 40 kV to identify the phases present in the samples. XRD scans were performed in the 2 range of 20°–130° using a scanning rate of 2°/min. Themicrostructures were examined using scanning electron microscopy with SM Zeiss LS-10 model and FEI Nova Nano 430 field-emission gun (FEG) coupled with energy-dispersive X-ray spectrometry (EDS). The SEM samples were prepared using standard metallographic techniques and etched with a solution containing 100 ml HCl/100 ml deionized H2O/1 g K2S2O5 mixture. Vickers microhardness

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ı Eğ i t im MerkeziBildir i ler Kitab ı

16119. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2018

was measured using a Microbul-1000D instrument on samples polished to 1 μm, with an applied load of 200 g sustained for 10 s, using the average value of 10 independent measurements made on several grains. The standard errors for all quantities are calculated based on counting statistics and reconstruction scaling errors using standard error propagation methods [12], and represent two standard deviations from the mean. Individual precipitates within the images were traced by hand using commercial software (Adobe Photoshop 6.1) and the dimensions of the traced areas were determined using NIH Image 1.62, a freeware image-processing program which fits the precipitate cross-sectional areas to a rectangular box or to a best-fit ellipse.

3. Results and Discussion

Before going through a detailed analysis of the effect of Nb and Y on the structural properties of Ni85Al15 alloy, microstructural examination and phase analysis of Ni85Al15alloy have been performed. The optical microscopy image for binary Ni85Al15 alloy is shown in Figure 1. The solidification microstructure of the binary Ni85Al15 alloy consists of ( + ) dendrites and interdendritic regions. The size of the precipitates are relatively larger because interdendritic regions are last solidifying regions.

Figure1. Optical micrograph of binary Ni85Al15 alloy.

Figure 2. XRD patterns of binary Ni85Al15 alloy. Phase analysis of binary Ni85Al15 alloy have been conducted by XRD technique (Figure 2). The presence of the (100), (110), (210) and (211) super-structure peaks confirms the existence of L12-type ordered phase. However, the fundamental peaks referring as (111), (200), (220), (311) and (400) are composed of two individual peaks corresponding to and phases. The lattice parameters of and phases are very close to each other and their diffraction peaks overlap. In addition, the lattice parameter of the phase have been calculated by applying well-known Bragg’s Law. Calculated lattice parameter for binary Ni85Al15 alloy is 3.569 Å and it coincides well with the literature data (3.560 Å) [13] and standard data (3.572 Å) [JCPDS 09-00097].

Figure 3. SEM micrographs of Ni85Al15 nYn alloys,n= (a) 2 and (b) 4.

The microstructures of ternary Ni85Al15 nYn alloys are given in Figure 3. The microstructures of Ni85Al15 nYn alloys consisted of primary crystalized dendrites, eutectic mixture and Ni5Y phase enclosing the dendrites. The constituents of the eutectic phase mixture were phase (dark) and Ni17Y2 (light) intermetallic compound. The constituent phases we shown in Figure 5. The volume fraction of Ni5Y and Ni17Y2 phases increased with increasing Y content. Ni5Y and Ni17Y2 phases formed due

UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

162 IMMC 2018 | 19th International Metallurgy & Materials Congress

to the limited solid solubility of Y in (Ni) phase. In ternary Ni-Al-Y ternary phase diagram, investigated compositions lie between the (Ni), Ni17Y2 and Ni5Yregions. In addition, following invariant reaction occurs at 1333 °C for Ni-18.3Al-3.7 composition [Ni-Al-Y phase diagram]:

Liquid Ni5Y + Ni17Y2 + (Ni) (1)

However, (Ni3Al) phase has not been observed in the microstructures of investigated Ni85Al15 nYn alloys.

Figure 4. XRD patterns of Ni85Al15 nYn alloys.

Formation of Ni5Y and Ni17Y2 phases were also confirmed with XRD (Figure 4) and EDS analyses. The presence of diffraction lines corresponding to Ni5Y and Ni17Y2 revealed the formation of these phases for both compositions. Based on the results of EDS analyses, the compositions of the phases were as following; (i) phase: Ni-11.7Al-0.2Y, (ii) Ni5Y: Ni-5.5Al-14.1Y and (iii) Ni17Y2: Ni-3.1Al-11.9Y.

Figure 6. Detailed SEM micrographs of Ni85Al13Y2 alloyshowing the constituent phases.

Figure 6 shows the microstructures of ternary Ni85Al15 nNbn alloys. The microstructure of Ni85Al13Nb2alloy consists of (Ni) phase, formation of phase was

only observed for Ni85Al11Nb4 alloy. Cuboidal precipitates have average particle size of 221.6 nm and they are uniformly distributed along the matrix. The exact composition of the precipitates were determined as Ni-14.4Al-8.5Nb by EDS analysis.

Figure 6. FESEM micrographs of Ni85Al15 nNbn alloys,n= (a) 2 and (b) 4.

For the two-dimensional (2D) precipitate size distributions (PSD), the numbers of -precipitates were plotted in a histogram (Figure 7). A narrow PSD yielding a tail for large values of RPS/<RPS>.

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ı Eğ i t im MerkeziBildir i ler Kitab ı

16319. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2018

Figure 7. -precipitate size distribution forNi85Al11Nb4 alloy.

The results of the microhardness measurements are listed in Table 1. Microhardness value strongly increased with addition of Nb and Y. Moreover, hardness also increased with increasing alloying element content. In Ni-Al-Y alloys, the hardening mechanism is precipitation strengthening arises from Ni5Y phase. In Ni-Al-Nb system, hardening mechanism is composition dependent; for (i) Ni85Al13Nb2 alloy solid solution strengthening is only hardening mechanism, whereas for (ii) Ni85Al11Nb4 alloysolid solution strengthening and precipitation strengthening due to phase are the hardening mechanisms.

Table 1. Vickers microhardness measurements for Ni85Al15 nXn alloys.

Alloys Micro-Hardness (Vickers)

Ni85Al15 194±6

Ni85Al13Y2 302±13

Ni85Al11Y4 335±19

Ni85Al13Nb2 344±19

Ni85Al11Nb4 363±6

4. Conclusion

The microstructures, phase relationships and microhardness of Ni85Al15 nNbn and Ni85Al15 nYn, where n = 2 and 4 at.%, were investigated, and the following conclusions can be drawn:

• phase precipitation was only observed for Ni85Al11Nb4 alloy and precipitates have average particle size of 221.6 nm and cuboidal morphology.

• For Ni85Al15 nYn alloys, Ni5Y and Ni17Y2 phases were formed as a secondary phase due to the limited solid solubility of Y in (Ni) phase.

• Volume fraction of the Ni5Y and Ni17Y2 phases increased with increasing Y content.

• Addition of both Nb and Y to Ni-15Al alloy increased the microhardness.

References

[1] C.K. Sudbrack, T.D. Ziebell, R.D. Noebe, D.N. Seidman, Acta Materialia 56 (3) 448-463. [2] A.J. Ardell and R.B. Nicholson, Acta Metall, 14 (1966), pp. 1295-1309. [3] A.J. Ardell and R.B. Nicholson J Phys Chem Solids, 27 (1966) 1793-1804. [4] E. Hornbogen and M. Roth, Z Metallk, 58 (1967) 842-855.[5] D.J. Chellman and A.J. Ardell, Acta Metall, 22 (1974) 577-588.[6] S. Antonov, M. Detrois, D. Isheim, D. Seidman, R.C. Helmink, R.L. Goetz, E. Sun and S. Tin, Materials & Design 86 (2015) 649-655. [7] C.K. Sudbrack, Decomposition Behavior in Model Ni-Al-Cr-X Superalloys: Temporal Evolution and Compositional Pathways on a Nanoscale, Ph.D. Thesis, Northwesten University, 2004, Evanston, Illinois, U.S.A. [8] T.M. Pollock and R.D. Field, Dislocat. Solids, 11 (2002) 547-618. [9] P.M. Mignanelli, N.G. Jones, K.M. Perkins, M.C. Hardy and H.J. Stone, Mater. Sci. Eng. A, 621 (2015) 265-271.[10] M. Detrois, R.C. Helmink and S. Tin, Mater. Sci. Eng. A, 586 (2013) 236-244. [11] M. Detrois, R.C. Helmink and S. Tin, Metall. Mater. Trans. A, 45 (2014) 5332-5343. [12] L.G. Parratt, Probability and experimental errors in science, John Wiley, 1966, New York. [13] N.I. Kourov, S.Z. Nazarova, A.V. Korolev, Y.A. Dorofeev, N.V. Volkova, and E.V. Belozerov, Physics of Metals and Metallography, 110 (2010) 1-4.