Effect of Nd Addition on the Microstructure and Cyclic Oxidation Behavior of NiAl-Cr(Mo) Eutectic Alloys

The effect of a minor amount of rare-earth element Nd addition (0.1 at.%) on the microstructural properties, microhardness, and cyclic oxidation resistance of NiAl-Cr(Mo) alloy is investigated in detail. The microstructure of investigated alloys is composed of NiAl-based dendrites and a eutectic mixture whose components are NiAl and Cr(Mo) phases. The minor amount of Nd addition re�nes the microstructure and increases the microhardness considerably. The results of the cyclic oxidation tests reveal that the surface scales of both alloys are mainly consisted of α -Al 2 O 3 and little amount of Cr 2 O 3 . A Cr(Mo)-rich layer is observed in the metal/oxide interface. With Nd addition, the oxidation resistance of Ni-33Al-31Cr-3Mo alloy is strongly improved. The Nd-doped alloy exhibits lower oxidation mass gain and increased scale adherence.


Introduction
The B2-type ordered NiAl intermetallic compound exhibits excellent properties including high melting temperature, low density, good thermal conductivity, high Young's modulus, high corrosion, and oxidation resistance [1][2][3][4].Therefore, NiAl is considered a potential material to replace Ni-based superalloys in aerospace turbine blade applications [5][6][7].However, the brittleness and low fracture strength at room temperature and inadequate strength and creep resistance at high-temperature handicap its commercial application [3,5,8,9].Alloying can be considered to be an e cient method to improve the insu cient properties.
On the other hand, oxidation at a high temperature can cause severe degradation and can lead to structural failure [14][15][16][17].Although extensive studies on the oxidation behavior of NiAl have been performed, only limited information is present in the literature on the oxidation behavior of NiAl-Cr(Mo) eutectic alloys containing a minor amount of RE.To our best knowledge, Zhang et al. [7] studied the cyclic oxidation resistance of Dy-doped NiAl-31Cr-3Mo alloy.They showed a two-layer oxide scale formed: a continuous Al 2 O 3 surface layer and a Cr-rich layer between the metallic substrate and Al 2 O 3 scale.They also showed that the oxidation resistance of Dy-doped alloy is much higher than that of the based alloy.In addition, Zheng-sheng et al. [13] investigated the oxidation behavior of NiAl-Cr(Mo)-Hf-Ho alloy in the temperature range of 900-1150°C.They showed that a continuous α-Al 2 O 3 scale was formed in the temperature range of 900-1100°C.Similar to Dy addition, Ho addition also increased the oxidation resistance.
However, no reports have studied the effect of the rare earth element Nd addition on the cyclic oxidation behavior of NiAl-Cr(Mo) eutectic alloys up to now.So the aim of this present study is to investigate the effect of a minor amount (0.1 at.%) of Nd addition on the microstructural properties, microhardness, and cyclic oxidation behavior of Ni-33Al-31Cr-3Mo alloy.
Then, the buttons were cut into slices of 2 mm thickness by electrical discharge machining (EDM) for heat treatment, characterization, and oxidation tests.Then, the specimens were heat-treated: (i) at 1250°C for 48 h /furnace cooled (HT-I) and (ii) at 1250°C for 48 h/water quenched and subsequently aged at 1050°C for 24h/furnace cooled (HT-II).

Characterization
The microstructural examination of as-cast and heat-treated specimens were performed by Zeiss Evo LS-10 model scanning electron microscope (SEM) attached to an energy dispersive x-ray spectrometer (EDS).Prior to the microstructural examination, specimens were mounted in bakelite, ground to 2000 grit using SiC papers, and then polished with 1 µm Al 2 O 3 suspension.Finally, specimens were etched with a % 5 Nital solution.The phase analysis was conducted by X-ray diffraction (XRD) utilizing Cu-Kα radiation with a Bruker D8 model diffractometer.
The Vickers microhardness measurements were carried out with a Microbul 1000D model microhardness tester using a load of 200 g and a dwell time of 10 s.At least eight individual indentations on polished specimens were performed to obtain an average value.

Cyclic Oxidation Tests
For the investigation of the oxidation behavior by cyclic oxidation tests, specimens with the dimensions of 10x10x2 mm 3 were used.To achieve uniform oxidation, all surfaces of the specimens were ground (up to 2000 grit), polished (1 µm Al 2 O 3 ), ultrasonically cleaned in acetone, and dried in air.Cyclic oxidation tests were carried out at 1000°C in a mu e furnace under laboratory air for a maximum time of 25 cycles.One cycle consisted of heating the specimen to 1000°C, holding at 1000°C for 1 h, and air-cooling outside the furnace.After the tests, mass gains/losses of the specimens were recorded using an electronic balance with an accuracy of 0.01 mg.Finally, the characterization of the oxidation products was performed.A detailed description of the characterization could be found elsewhere [18-22].

Microstructural Evolution
The microstructures of as-cast and heat-treated NiAl-Cr(Mo) and NiAl-Cr(Mo)-Nd alloys are shown in Fig. 1.The microstructure of NiAl-Cr(Mo) alloy is comprised of NiAl-based equiaxed dendrites (dark phase) and cellular eutectic mixture, composed of rod-like or lamella Cr(Mo) phase (light phase) and NiAl matrix phase.In the eutectic cell, NiAl and Cr(Mo) phases form a radially emanating pattern from the cell core to the eutectic cell boundary.The NiAl and Cr(Mo) phases are much coarser at the intercellular zone compared to those in the cell interior.The eutectic cell boundaries are the last solidi cation areas and exhibit much coarser morphology [23,24].In addition, ne Cr(Mo) precipitates are present within the NiAl dendrites.The chemical composition of the phases in NiAl-Cr(Mo) alloy is analyzed with EDS and shown in Fig. 2-4.The results of the EDS analysis indicate that the atomic ratio of Ni:Al is close to 1 and Mo tends to dissolve in the Cr phase.
The minor amount of Nd addition considerably alters the microstructure of the alloy: (i) NiAl/Cr(Mo) eutectic lamella is re ned strongly.The interlamellar spacing within the eutectic cell is decreased compared to that of without Nd addition, (ii) the NiAl and Cr(Mo) phases close to the eutectic cell boundary become coarser, (iii) the size of the eutectic cells are decreased markedly, and (iv) small Nd-rich phases start to form within the eutectic cells.
The results of the EDS analysis (Fig. 5-7) reveal that Nd is uniformly distributed in the NiAl and Cr(Mo) phases.Cr(Mo) phase dissolves a slightly higher amount of Nd compared to the NiAl phase.Additionally, the EDS mapping analysis (Fig. 7) shows that Nd is uniformly distributed along the eutectic cell boundaries and cell interior.No Nd-rich regions or phases are observed in the microstructure of NiAl-Cr(Mo)-Nd alloys.
After the rst heat treatment of 1250°C/48h furnace cooling (HT-I) Cr(Mo) lamellas coarsen signi cantly in the microstructures of both Nd-free and a minor amount of Nd-containing alloys.The lamellas spheroidize and transform to particles.Although the coarsening also occurs after the second heattreatment of 1250°C/48h quenching and subsequent aging (HT-II), the coarsening rate is noticeably slower compared to HT-I.A typical microstructure that is composed of alternating lamella or bers of two different phases can be produced after eutectic phase transformation.In this microstructure, the minor phase is embedded in the continuous matrix phase [25,26].At high temperatures, ne eutectic lamella microstructures can coarsen in order to reach thermodynamic equilibrium.In the basic theory of the coarsening, phases tend to reduce the total energy by decreasing the surface energy via minimizing the interfacial area of the relevant phases [11,24,25].In the present study, the coarsening of eutectic after HT-I is relatively less than that of HT-II.Compared to HT-II, relatively slower in HT-I accelerates the diffusion kinetics and coarsens the lamellas much more.
The effect of coarsening is clearly observed in microhardness measurements (Table 1).For both compositions, the as-cast alloys exhibit the highest mean microhardness value.After heat-treatment, the mean microhardness values are decreased as a result of coarsening.The samples heat-treated at 1250°C/48h furnace cooling (HT-I) exhibit the lowest microhardness values since they possess the coarsest microstructure.Moreover, a minor amount of Nd addition increases the mean microhardness values of NiAl-Cr(Mo) alloys because of the re nement effect of Nd.This result indicates that the addition of Nd may impede the growth of Cr(Mo) during the solidi cation process.A similar nding is also observed in the previous research [12,13,27] regarding the Hf and Ho addition to the NiAl-Cr(Mo) eutectic alloys.In addition, the formation of any other phase is not observed in the XRD analysis.

Cyclic Oxidation Test
The weight gain per unit area of the studied alloys as a function of oxidation time is given in Fig. 9.The plots of both alloys show two distinct oxidation periods: the rst period with rapid oxidation (0-12 h) and the nal period with slower oxidation (13-25 h).The slopes of the rst period are clearly sharper than in comparison with the slopes of the second period.A certain amount of scale spallation is observed during the cyclic oxidation tests.Comparing the weight change plots of two alloys, the weight gain of the NiAl-Cr(Mo) alloy is more pronounced.In order to examine the oxidation kinetics, the weight gain plots can be tted to a power law equation [7]; where t is the exposure time, k is the oxidation rate constant, A is the surface area of the sample, ΔW is the weight gain and n is the power law exponent.The inverse value of power law exponents can be calculated from the slope of a linear regression line of the double logarithm (ΔW/A) vs t curves.If the value of n is equal to 2, the oxidation kinetics obey a parabolic rate law.The tting quality standard (R, correlation coe cient) is at least 0.99746.
According to the calculated exponents, only NiAl-Cr(Mo) alloy obeys parabolic kinetics in the initial oxidation period.The calculated power law exponents for NiAl-Cr(Mo)-Nd and NiAl-Cr(Mo) alloy in the second period are different from 2. Therefore, a parabolic behavior may not occur and a parabolic rate constant (k p ) may not be calculated correctly.In several studies regarding the oxidation kinetics of similar alloys [28][29][30][31][32], the apparent parabolic rate constant can be calculated from the parabolic rate law (n = 2).
The calculated k p, app values for both alloys and two oxidation periods are listed in Table 2.
Table 2 Parabolic rate constants for as-cast samples.Alloy (mg 2 cm − 4 sn − 1 ) (mg 2 cm − 4 sn − 1 ) NiAl-Cr(Mo) 4.47x10 − 5 ± 1.54x10 − 6 6.03x10 − 6 ± 1.11x10 − 6 NiAl-Cr(Mo)-Nd 3.42x10 − 5 ± 1.89x10 − 6 6.02x10 − 6 ± 8.81x10 − 7   Compared to the NiAl-Cr(Mo) alloy, the NiAl-Cr(Mo)-Nd alloy has somewhat lower k p,app values which implies a decrease in oxidation rate after Nd addition.For the rst period, the k p values of both alloys are in the same order of magnitude (10 − 5 mg 2 cm − 4 sn − 1 ), which agrees well with the k p values of similar compositions in the literature [33][34][35][36].For longer oxidation times (> 12 h), a considerably lower oxidation rate is observed for both alloys.The k p,app values of both alloys are similar in the second period.For NiAl-Cr(Mo) alloy, the continuous heating-cooling cycles occurring in the surface region generate internal stresses which lead to scale spallation.A substantial Al depletion occurs due to continuous scale spallation and formation of the Al 2 O 3 scale.Thus, the elimination of Al leads to the formation of the Cr(Mo) layer on the alloy surface [7,37].On the other hand, the trace amount of Nd addition results in important improvements in the oxidation resistance of NiAl-Cr(Mo) alloy.With Nd addition, the oxidation mass gain is decreased, scale spallation is extensively reduced and the adhesion of the oxide scale is promoted.It is believed that a trace amount of rare earth Nd addition promotes the inward diffusion of O which results in the removal of interspaces in the oxide scale.Thus, a reduction of mass gain is observed.Moreover, the re nement of microstructure as a result of Nd addition may support the Al 2 O 3 layer to wrap the Cr(Mo) phase quickly.Therefore, the Cr(Mo) layer may be formed more easily between the surface scale and metallic substrate, which has the possibility to reduce the internal stresses and enhance the scale adhesion [37].
The higher concentration of Al in the NiAl matrix phase could promote the development of an Al 2 O 3 scale in the oxidation process.At the same time, the Cr(Mo) phase is also exposed to air and oxidation occurs.
The Mo oxides (i.e., MoO 3 ) are volatile and non-protective, which accelerates the oxidation process and contributes to the reduction of the oxidation mass gain at high temperatures [7,[38][39][40].However, a protective Cr 2 O 3 scale and volatile CrO 3 can be formed as a result of the oxidation of Cr.Protective Cr 2 O 3 forms at relatively lower testing temperatures (T ≤ 1000°C), while non-protective CrO 3 forms at higher temperatures [41].In the present study, we observe the formation of protective Cr 2 O 3 nodules based on the results of the XRD and EDS analysis.
The addition of reactive elements such as Y, Ce, Zr, and Hf to NiAl-based alloys is considered to be bene cial for oxide scale adhesion and have also several positive effects on the microstructure and mechanical properties of NiAl-based alloys [42][43][44][45][46][47][48].However, like other rare-earth elements Dy and Ho, Nd has higher chemical reactivity compared to reactive elements Y, Ce, Hf, and Zr [7,37].In the oxidation process, Nd addition in uences both surface oxide and metallic substrate.For rare-earth element additions, microstructural re nement takes place because of the formation of non-spontaneous cores

Table 1
Mean microhardness values of as-cast and heat-treated NiAl-Cr(Mo) and NiAl-Cr(Mo)of the NiAl-Cr(Mo) and NiAl-Cr(Mo)-Nd alloys are investigated by XRD analysis, as shown in Fig. 8.It is obvious that both alloys are composed of NiAl matrix (JCPDS Card No: 44-1188) and Cr(Mo) (JCPDS Card No: 06-0694) phases.The (110) crystal planes are the strongest diffraction peaks of the NiAl and Cr(Mo) phases.Moreover, the XRD pattern of NiAl-Cr(Mo)-Nd alloy reveals that the intensity of the diffraction peak corresponding to the (211) plane is higher for the NiAl phase than Cr(Mo) phase.

Figure 11 shows
Figure11shows the surface scale morphology of the NiAl-Cr(Mo) alloy after cyclic oxidation at 1000°C.The surface scale is composed of α-Al 2 O 3 as shown in Fig.11 (a).After cyclic oxidation, scale spallation (Fig.11 (b)) and cracks are observed.The spallation occurs due to the thermal stresses originating through from the heating/cooling cycles.In addition, elemental mapping analyses (Fig.11 (c)) show that a Cr(Mo)-rich layer is present between the surface scale and metallic substrate.

p
The surface scale of the Nd-doped specimen is shown in Fig.12.The surface scale is composed of ne and equiaxed α-Al 2 O 3 grains.Moreover, no important scale spallation and cracks are observed.However, investigation of the surface scale morphologies does not indicate the entire oxidation process.Internal oxidation can occur under the surface scale[18,20].Therefore, cross-sectional SEM-EDS analyses (Fig.13) are conducted.The results indicate that the oxide scale of NiAl-Cr(Mo) alloy consists of an α-Al 2 O 3 surface layer (Fig.13(a)) with little Cr 2 O 3 nodules (Fig.14) and a Cr(Mo)-rich layer is formed in the metal/oxide interface.The formation of Cr 2 O 3 particles is con rmed with EDS point analysis in the crosssectional examination.For the Nd-doped alloy, the cross-sectional analyses (Fig.13(b)) reveal that the surface layer is mainly composed of α-Al 2 O 3 and little Cr 2 O 3 protective scale.Similar to base alloy, a Cr(Mo)-rich layer is also formed in the metal/oxide interface.

Figure 9 Mass
Figure 9

Figure 11 Surface
Figure 11

Figure 12 Surface
Figure 12

Figure 13 Cross
Figure 13