MATEC Web of Conferences 14, 01005 (2014)DOI: 10.1051/matecconf/20141401005c© Owned by the authors, published by EDP Sciences, 2014

Intermetallic eutectic alloys in the Ni-Al-Zr system with attractive hightemperature properties

Chandrasekhar Tiwary, Vilas V. Gunjal, Dipankar Banerjee, and Kamanio Chattopadhyaya

Department of Materials Engineering Indian Institute of Science, Bangalore 560012, India

Abstract. We describe a group of alloys with ultrahigh strength of about 2 GPa at 700◦C and exceptionaloxidation resistance to 1100◦C. These alloys exploit intermetallic phases with stable oxide forming elementsthat combine to form fine nanometric scale structures through eutectic transformations in ternary systems. Thealloys offer engineering tensile plasticity of about 4% at room temperature though both conventional dislocationmechanisms and twinning in the more complex intermetallic constituent, along with slip lengths that are restrictedby the interphase boundaries in the eutectics.

1. IntroductionIntermetallics such as Ni3Al and NiAl have beenextensively evaluated for high temperature alloy systemsand indeed have been used for several decades in theclassical Ni base superalloys which derive their hightemperature strength from nanoscale dispersions of thealuminide, Ni3Al(γ ’) in a matrix of disordered fcc Ni(γ ), alloyed with expensive, high density, refractoryelements such as Re and Ru. We describe a newalloy system based on intermetallic phases realisedfrom solidification of compositions in the Ni-Al-Zrsystem. Our choice of alloy compositions was basedon an earlier assessment of equilibria and liquidusprojections [1–3]. We have identified in this systemthree binary eutectics, NiAl(β) + Ni7Zr2(τ ), Ni3Al(γ ’) +Ni7Zr2(τ ), Ni3Al(γ ’)+Ni5Zr(ε) and a ternary eutecticNiAl+Ni3Al+Ni7Zr2, We have evaluated the mechanicalbehaviour of these in compression as a function oftemperature, and in some cases in tension at roomtemperature. We have also assessed the oxidationbehaviour of these alloys. The alloys show ultrahighstrength to temperatures of 700◦C and attractive oxidationresistance and high temperature structural microstructuralstability. The high strength is accompanied by tensileplasticity.

2. ExperimentalAlloys were non-consumable vacuum remelted severaltimes into 50 gm buttons and suction cast into 3 mmdiameter rods. The composition of the alloys that wereexamined in this study are shown in Fig. 1 as well as inTable 1.

The as-melted alloys were characterised by opticalmicroscopy, scanning electron microscopy (SEM), and

a Corresponding author: karthik@materials.iisc.ernet.in

electron probe microanalysis (EPMA). Samples fortransmission electron microscopy were prepared by eitherelectropolishing in a solution of picric acid, glycerol andethanol at a temperature of 5 ◦C, or by ion milling.

Both compression (3 mm diameter and 4.5 mm height)and flat microtensile samples (thickness 2 mm, gaugelength 10 mm) were prepared from the suction cast rods.These samples were polished prior to testing to permitthe examination of slip lines after tests. Fracture surfacesof the tensile samples were also examined to elucidatefracture modes at room temperature.

Oxidation experiments were carried out in air withcontinous weight measurements using a micro balancewith a sensitivity of about 100 µg. Samples with flatpolished surfaces were used in the oxidation tests.

3. Results3.1. Microstructure

Microstructures of fully eutectic alloys are shown inFig. 2. These alloys lie essentially along the 11% Zrsection. We have identified in this section a varietyof eutectic combinations that form under the coolingconditions that are realised experimentally. These areNi3Al+Ni5Zr in alloy E1, Ni3Al+Ni7Zr2 in alloy E2NiAl+Ni7Zr2 in alloy E3 and the ternary eutecticNiAl+Ni3Al+Ni7Zr2 in alloy TE. The eutectic colonies inalloy E3 show a central lamellar structure that degeneratesto a more irregular appearance at the boundaries. Theeutectic in alloy E2 consists of an extremely fine fibrousstructure, while the eutectic in alloy E1 again has acentral fine lamellar structure surrounded by a coarserirregular lamellar morphology The ternary eutectic showsthe phases in dark (NiAl), light grey (Ni3Al), and lightcontrast (Ni7Zr2). Combinations of the binary eutecticsNi3Al+Ni7Zr2 and NiAl+Ni7Zr2 are realised in alloysME2/3 and ME3/2, with the latter containing greater

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Figure 1. Alloy compositions are designated to show binaryeutectic compositions (E and ME), a ternary eutectic (TE) andcompositions with primary intermetallic solidification phases (P).

Table 1. Alloy designations indicate binary eutectics (E),mixtures of binary eutectics (ME), a ternary eutectic (TE) andcompositions with primary phase compositions (P).

Designation Al Zr NiE1 12 11 77E2 13.5 11 75.5E3 19 12 69TE 18.4 11.6 70ME2/3 15 11 74ME3/2 18 11 71Pε 9 11 80Pγ ′1 15 8 77Pγ ′2 18 8 74Pγ ′3 14 6 80Pγ ′4 15 4 81Pτ1 15 14 71Pτ2 12 14 74Pβ, γ ′1 20 11 69Pβ,γ ′2 20 6 74

amounts of the eutectic E3. Other alloys show the presenceof primary solidification phases along with a eutectic(Fig. 3). Thus alloy Pγ ′1 contains the primary Ni3Alphase while alloys Pβ,γ ′1 and Pβ,γ ′2 show the primaryNiAl and Ni3Al phases. Alloy Pτ1 and Pτ2 has a primaryNi7Zr2 phase while alloy Pε contains the primary Ni5Zrphase.

3.2. Mechanical behaviour

Figure 4 shows the hardness and modulus of Ni3Al, Ni5Zrand Ni7Zr2 determined by picohardness measurements onthe primary solidification phases. We note that there is noprior report of the mechanical behaviour of the Ni5Zr andNi7Zr2 phases available in the literature. The properties ofthe NiAl and Ni3Al phases as available in the literature areincluded in the Figure. The Ni7Zr2 has the highest hardnessand modulus followed by the Ni5Zr phase.

These data were obtained with low load (10 µN)picoindentation consisting of Berkovich indenter with aSiC tip. The low load indenter (PI 85 SEM PicoIndenterfrom Hysitron) is used in conjunctionwith a scanningelectron microscope (SEM).

a d

b e

c f

NiAl+Ni7Zr

2

NiAl+Ni7Zr

2

Ni3Al+Ni

7Zr

2

Ni3Al+Ni

7Zr

2

Figure 2. Eutectic structures: (a) alloy E3 (b) alloy (E2) (c) alloyE1 (d) alloy TE (e) alloy ME2/3 (d) alloy ME3/2.

a

d

b

e

c

f

Figure 3. Alloys containing primary intermetallic phases:(a) alloy Pγ ′1 (b) Pε (c) alloy Pβ,γ ′1 (d) alloy Pβ,γ ′2 (e) alloyPτ1 (f) alloy Pτ2.

Figure 5 shows data from compression tests as afunction of temperature for the eutectic alloys. The eutecticNi3Al+Ni5Zr has the highest strength of all the eutectics.The strength levels observed are significantly above thoseof typical Ni base superalloys. The strength levels persistto 700C and then drop significantly.

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Figure 4. The hardness and modulus of intermetallic phasesdetermined by picoindentation in alloys containing these phasesas primary solidification phases.

Figure 5. Yield strength in compression as function oftemperature of the eutectic alloys.

Figure 6. Tensile stress-strain curves of some eutectic alloys.

Tensile data at room temperature are shown inFig. 6 for three of the alloys, one containing theeutectic Ni3Al+Ni5Zr (alloy E1) and the others containingdifferent proportions of the eutectics Ni3Al+Ni7Zr2 andNiAl+Ni7Zr2. Significant tensile plasticity of 3–4% isobserved at these extremely high strength levels.

In Fig. 7, we show an example of failure initiationand fracture in tension from alloy E1 containing theNi3Al+Ni5Zr eutectic. Figure 2 shows that the eutecticexits in two length scales. Failure initiation occurs throughvoid nucleation at the Ni3Al/Ni5Zr interfaces in the coarser

a

b

Figure 7. (a) Void nucleation and growth within the Ni3Al phasein alloy E1. (b) The fracture surface in tension.

segments of the microstructure and subsequently growththrough the Ni3Al phase around Ni5Zr. The resultingfracture surface has a macroscopically ductile appearance,with evidence for void formation in Ni3Al in both the fine,lamellar regions and the coarse irregular morphology ofthe eutectic.

The scale of fracture features is related to the finesubmicron scale of the eutectic and the Ni5Zr lamellaeprobably fail by tearing of the lamellar ligaments aftervoids have formed within Ni3Al. Thus deformationappears to occur by extensive plasticity within Ni3Albefore it spreads to the stronger Ni5Zr phase. We haveexamined deformation structure within these intermetallicphases by transmission electron microscopy. Conventionaldeformation by dislocation-based plasticity is observed inNi3Al, NiAl and Ni5Zr (Fig. 8), while Ni7Zr2 deforms byan unusual microtwnning mechanism in the (Fig. 9).

We have also evaluated the effect of the presence of theprimary intermetallic phases in the microstructure on roomtemperature strength in compression. Figure 10 shows thisfor two different base alloys, the E1 eutectic and the ME2/3eutectic combination. In all cases the presence of primaryphases lowers the strength

3.3. Oxidation resistance

Figure 11 shows oxidation data for alloys E1(Ni3Al+Ni5Zr) and ME3/2 (Ni3Al+Ni7Zr2 and NiAl+Ni7Zr2).These suggest that oxidation occurs in 3 distinct stagesassociated with different reaction kinetics.

An initial stage of very rapid oxidation is seen,followed by a transition stage to a steady state associatedwith progressively decreasing rate constants. In some

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Figure 8. Dislocation structures in various intermetallic phasesin the eutectics.

Figure 9. Microtwinning in Ni7Zr2.

Figure 10. The effect of primary intermetallic phases in themicrostructure on the yield strength at room temperature.

cases, particularly in alloy C, there is evidence ofinstability in the oxidation curves that suggests spalling ofthe oxide layer in early stages before a final steady stateregime is reached.

The rate constants associated with the steady stateregime correspond to the rate constants that are associatedwith the oxidation of Ni3Al and NiAl and superalloys withAl203 or spinel formation rather than the formation ofthe NiO or Zr2O3 phases which are associated with rateconstants that are orders of magnitude higher (Fig. 12).The significantly larger rate constant associated with theinitial rapid oxidation phase is more likely to be associatedwith the initial formation of the latter oxides.

a

b

Figure 11. Plots of the square of weight gain against time in(a) alloy E1 and (b) alloy ME3/2.

Figure 12. Activation plots for Ni3Al, NiAl, Ni7Zr2 and alloysE1 (A in figure) and ME2/3 (C in figure). Data for Ni5Zr from [8]and NiAl and Ni3Al from [9].

However the activation energies in the steady stateregime are significantly lower than those associated withthe oxidation of Ni3Al or NiAl which correspond moregenerally to Al2O3 formation or to the formation of mixedoxides of NiO, Al2O3 and spinel. In fact these activationenergies correspond to no known oxidation or diffusionprocesses. As we shall see below the microstructure ofthe oxygen affected layers in these alloys are considerablymore complex and indicative of several processes thatoperate simultaneously. It is thus not possible to correlatethese apparent activation energies with any one simpleprocess.

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Figure 13. Microstructure of the oxidized layer of alloy ME2/3after 16 hr at 900C and a schematic description of the structuremagnified at the scale of the eutectic.

Figure 13 shows the crossection of the oxidized layerin alloy ME3/2 after 16 hours at 900C. An outermost layer,Zone 5, is composed of NiO and ZrO2 (no Al is present inthis layer). Zone 4 consists of Zr rich and Ni rich regions.The intermediate layers between zone 1 and zone 4 consistof a thin Ni depleted Al, Zr and O rich layer (zone 3). Alayer, zone 2, containing relatively lower amounts of Al, Zrand O is present between zone 3 and zone 1. Zone 3 and 2do not contain oxides of Al or Zr The composition range

Figure 14. Weight gain in alloys containing primary intermetallicphases in comparison to fully eutectic structures.

of zone 3 suggests that this region might be composedprimarily of the intermetallic NiAlZr. Zone 1 is marginallydepleted in Zr and Al and therefore enriched in Ni but notsubstantively altered in microstructure. A detailed analysisof the EPMA images leads to the schematic descriptionof the oxygen affected crosssection shown in Fig. 13. Thefollowing steps may be envisaged for the evolution of theoxygen-affected microstructure

(i) O ingress into the Ni5Zr intermetallic in solidsolution.

(ii) The formation of NiO by the outward diffusionof Ni from Ni5Zr and Ni3Al.

(iii) Interaction of Al and Zr in the base metal withoxygen at the NiO scale /metal interface toform Ni-Al-Zr-O complexes in the intermediatelayers.

The presence of Ni3Al as a primary solidification phaselowers the oxidation resistance, while the presence ofthe Zr rich intermetallics does not affect the oxidationresistance compared with the baseline eutectic alloys asshown by the weight gain data in Fig. 14.

4. SummaryIn summary, we find that submicron scale eutecticstructures in the Ni-Al-Zr systems comprised of theNi3Al, NiAl, Ni5Zr and Ni7Zr2 intermetallics offer veryhigh strength combined with reasonable tensile plasticityat room temperature under the processing conditionsthat have been explored in this study. Certainly themore complex intermetallic phases Ni5Zr and Ni7Zr2 are

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expected to be brittle in room temperature. Nevertheless,under the fine scale, constrained deformation conditionsthat have been engineered in these microstructures, wesee no evidence of brittle behaviour. This exceptionalmechanical behaviour is complemented with extremelygood high temperature structural stability and oxidationresistance. It is tempting to speculate that stability of theoxide scales is also related to the scale of the underlyingmicrostructure and distribution of alloying elements thatreact form submicron or nano-grained dispersions ofmixed oxides. Considering in addition the low densitiesof these alloys, ranging from 7.35–7.95 gm/cc, andthe relatively low cost of the constituent elements, thissystem is clearly of great potential for high temperatureapplications.

Two of the authors KC and DB are grateful for support fromthe J.C. Bose Fellowship of the Department of Science andTechnology, India. The use of facilities at AFMM, IndianInstitute of Science is gratefully acknowledged.

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