Intermetallic eutectic alloys in the NiAl-Zr system with attractive high temperature properties

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


Introduction
Intermetallics such as Ni 3 Al and NiAl have been extensively evaluated for high temperature alloy systems and indeed have been used for several decades in the classical Ni base superalloys which derive their high temperature strength from nanoscale dispersions of the aluminide, Ni 3 Al(γ ') in a matrix of disordered fcc Ni (γ ), alloyed with expensive, high density, refractory elements such as Re and Ru.We describe a new alloy system based on intermetallic phases realised from solidification of compositions in the Ni-Al-Zr system.Our choice of alloy compositions was based on an earlier assessment of equilibria and liquidus projections [1][2][3].We have identified in this system three binary eutectics, NiAl(β) + Ni 7 Zr 2 (τ ), Ni 3 Al(γ ') + Ni 7 Zr 2 (τ ) , Ni 3 Al(γ ')+Ni 5 Zr(ε) and a ternary eutectic NiAl+Ni 3 Al+Ni 7 Zr 2 , We have evaluated the mechanical behaviour of these in compression as a function of temperature, and in some cases in tension at room temperature.We have also assessed the oxidation behaviour of these alloys.The alloys show ultrahigh strength to temperatures of 700 • C and attractive oxidation resistance and high temperature structural microstructural stability.The high strength is accompanied by tensile plasticity.

Experimental
Alloys were non-consumable vacuum remelted several times into 50 gm buttons and suction cast into 3 mm diameter rods.The composition of the alloys that were examined in this study are shown in Fig. 1 as well as in Table 1.
The as-melted alloys were characterised by optical microscopy, scanning electron microscopy (SEM), and a Corresponding author: karthik@materials.iisc.ernet.inelectron probe microanalysis (EPMA).Samples for transmission electron microscopy were prepared by either electropolishing in a solution of picric acid, glycerol and ethanol 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, gauge length 10 mm) were prepared from the suction cast rods.These samples were polished prior to testing to permit the examination of slip lines after tests.Fracture surfaces of the tensile samples were also examined to elucidate fracture modes at room temperature.
Oxidation experiments were carried out in air with continous weight measurements using a micro balance with a sensitivity of about 100 µg.Samples with flat polished surfaces were used in the oxidation tests.

Microstructure
Microstructures of fully eutectic alloys are shown in Fig. 2.These alloys lie essentially along the 11% Zr section.We have identified in this section a variety of eutectic combinations that form under the cooling conditions that are realised experimentally.amounts of the eutectic E3.Other alloys show the presence of primary solidification phases along with a eutectic (Fig. 3).Thus alloy Pγ 1 contains the primary Ni 3 Al phase while alloys Pβ,γ 1 and Pβ,γ 2 show the primary NiAl and Ni 3 Al phases.Alloy Pτ 1 and Pτ 2 has a primary Ni 7 Zr 2 phase while alloy Pε contains the primary Ni 5 Zr phase.8), while Ni 7 Zr 2 deforms by an unusual microtwnning mechanism in the (Fig. 9).

Mechanical behaviour
We have also evaluated the effect of the presence of the primary intermetallic phases in the microstructure on room temperature strength in compression.Figure 10 shows this for two different base alloys, the E1 eutectic and the ME2/3 eutectic combination.In all cases the presence of primary phases lowers the strength An initial stage of very rapid oxidation is seen, followed by a transition stage to a steady state associated with progressively decreasing rate constants.In some 01005-p.3cases, particularly in alloy C, there is evidence of instability in the oxidation curves that suggests spalling of the oxide layer in early stages before a final steady state regime is reached.

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The rate constants associated with the steady state regime correspond to the rate constants that are associated with the oxidation of Ni 3 Al and NiAl and superalloys with Al 2 0 3 or spinel formation rather than the formation of the NiO or Zr 2 O 3 phases which are associated with rate constants that are orders of magnitude higher (Fig. 12).The significantly larger rate constant associated with the initial rapid oxidation phase is more likely to be associated with the initial formation of the latter oxides.However the activation energies in the steady state regime are significantly lower than those associated with the oxidation of Ni 3 Al or NiAl which correspond more generally to Al 2 O 3 formation or to the formation of mixed oxides of NiO, Al 2 O 3 and spinel.In fact these activation energies correspond to no known oxidation or diffusion processes.As we shall see below the microstructure of the oxygen affected layers in these alloys are considerably more complex and indicative of several processes that operate simultaneously.It is thus not possible to correlate these apparent activation energies with any one simple process.

Summary
In MATEC Web of Conferences expected to be brittle in room temperature.Nevertheless, under the fine scale, constrained deformation conditions that have been engineered in these microstructures, we see no evidence of brittle behaviour.This exceptional mechanical behaviour is complemented with extremely good high temperature structural stability and oxidation resistance.It is tempting to speculate that stability of the oxide scales is also related to the scale of the underlying microstructure and distribution of alloying elements that react form submicron or nano-grained dispersions of mixed oxides.Considering in addition the low densities of these alloys, ranging from 7.35-7.95gm/cc, and the relatively low cost of the constituent elements, this system is clearly of great potential for high temperature applications.
Two of the authors KC and DB are grateful for support from the J.C. Bose Fellowship of the Department of Science and Technology, India.The use of facilities at AFMM, Indian Institute of Science is gratefully acknowledged.

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

Figure 4
Figure 4 shows the hardness and modulus of Ni 3 Al, Ni 5 Zr and Ni 7 Zr 2 determined by picohardness measurements on the primary solidification phases.We note that there is no prior report of the mechanical behaviour of the Ni 5 Zr and Ni 7 Zr 2 phases available in the literature.The properties of the NiAl and Ni 3 Al phases as available in the literature are included in the Figure.The Ni 7 Zr 2 has the highest hardness and modulus followed by the Ni 5 Zr phase.These data were obtained with low load (10 µN) picoindentation consisting of Berkovich indenter with a SiC tip.The low load indenter (PI 85 SEM PicoIndenter from Hysitron) is used in conjunctionwith a scanning electron microscope (SEM).

Figure 5 2 EUROSUPERALLOYS 2014 Figure 4 .
Figure 5 shows data from compression tests as a function of temperature for the eutectic alloys.The eutectic Ni 3 Al+Ni 5 Zr has the highest strength of all the eutectics.The strength levels observed are significantly above those of typical Ni base superalloys.The strength levels persist to 700C and then drop significantly.

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

Figure 11
Figure 11 shows oxidation data for alloys E1(Ni 3 Al +Ni 5 Zr) and ME3/2 (Ni 3 Al+Ni 7 Zr 2 and NiAl+Ni 7 Zr 2 ).These suggest that oxidation occurs in 3 distinct stages associated with different reaction kinetics.An initial stage of very rapid oxidation is seen, followed by a transition stage to a steady state associated with progressively decreasing rate constants.In some

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

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

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

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

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

Figure 14 .
Figure 14.Weight gain in alloys containing primary intermetallic phases in comparison to fully eutectic structures.
summary, we find that submicron scale eutectic structures in the Ni-Al-Zr systems comprised of the Ni 3 Al, NiAl, Ni 5 Zr and Ni 7 Zr 2 intermetallics offer very high strength combined with reasonable tensile plasticity at room temperature under the processing conditions that have been explored in this study.Certainly the more complex intermetallic phases Ni 5 Zr and Ni 7 Zr 2 are 01005-p.5 Zr 2 ).Combinations of the binary eutectics Ni 3 Al+Ni 7 Zr 2 and NiAl+Ni 7 Zr 2 are realised in alloys ME2/3 and ME3/2, with the latter containing greater MATEC Web of Conferences