Mechanical properties of Ir-Nb alloys containing Ni and Al

Two alloys made by adding 5 or 10 at. pct, respectively, of Ni-18.9 at. pct Al to an Ir-15 at. pct Nb alloy were investigated. The microstructure and compressive strength at temperatures between room temperature and 1800 °C were investigated to evaluate the potential of these alloys for ultra-high-temperature use. Their microstructural evolution indicated that the two alloys formed fcc and L1₂-Ir₃Nb two-phase structures. The fcc and L1₂ two-phase structures were examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The 0.2 pct flow stresses were above 1000 MPa at temperatures up to 1200 °C, about 150 MPa at 1500 °C, and over 100 MPa at 1800 °C. The strength of the quaternary Ir-base alloys at 1200 °C was even higher than that of Ir-base binary and ternary alloys. And the strength of quaternary Ir-Nb-Ni-Al was equivalent to that of the Ir-15 at. pct Nb binary alloy at 1800 °C. The compressive ductility of quaternary (around 20 pct) was improved drastically compared with that of the Ir-base binary alloy (lower than 10 pct) and the ternary Ir-base alloys (about 11 pct). An excellent balance of high-temperature strength and ductility was obtained in the alloy with 10 at. pct Ni-18.9 at. pct Al. The effect of Ni and Al on the strength of the Ir-Nb binary alloy is discussed.     

Ir-Nb-Si ternary refractory superalloys with a three-phase Fcc/L12/silicide structure for high-temperature applications: Phase and microstructural evolution

To find a new phase with the potential to improve the high-temperature strength of Ir-based superalloys, the novel idea of introducing silicides into the Ir-Nb binary was implemented. Hypoeutectic Ir-10Nb, eutectic Ir-16Nb, and hypereutectic Ir-25Nb alloys were used as bases, and 5 mol pct Si was added through the removal of Ir. XRD (XRD), scanning electron microscopy (SEM), and electron-probe microanalysis (EPMA) revealed the formation of a three-phase fcc/L1₂/silicide microstructure in the Ir-Nb-Si ternary after Si addition. The type of silicide formed was dependent on heat-treated temperatures and Nb content. After heat treatment at 1750 °C and 1600 °C, a tie-triangle composed of fcc/L1₂/silicide (Ir₂Si) appeared in the Ir-10Nb-5Si and Ir-16Nb-5Si alloys; in the Ir-25Nb-5Si alloy, an L1₂ and silicide (Ir,Nb)₂Si tie-line was observed. In the as-cast and 1300 °C heat-treated samples, the Ir-10Nb-5Si microstructure changed to a two-phase fcc/silicide structure, while the Ir-16Nb-5Si alloy maintained a three-phase fcc/L1₂/silicide structure. The Ir-25Nb-5Si alloy, however, had the same phases as that at 1600 °C. Silicides typically continuously or discontinuously distribute along the interdendritic regions or grain boundaries of the fcc or the L1₂ phase. With the addition of Si, it was found that both the eutectic point and solid solubility of Nb in Ir would shift toward Ir.     

Properties of the Ir85Nb15 two-phase refractory superalloys with nickel additions

The effects of nickel content on the properties of the polycrystalline Ir₈₅Nb₁₅ refractory superalloy were studied. To examine the possibility of replacing Ir with Ni in Ir_85-X Nb₁₅Ni _X , we varied the nickel content from 0 to 50 at. pct. The yield strength of the Ir₇₅Nb₁₅Ni₁₀ alloy was 2150 MPa at room temperature, much greater than that of the binary Ir₈₅Nb₁₅ alloy, and 728 MPa at 1473 K, similar to that of the binary alloy. The fracture mode of an Ir_85−X Nb₁₅Ni _X alloy changed from predominantly intergranular in fcc and L1₂ two-phase Ir₈₅Nb₁₅ to transgranular in the two-phase Ir₇₅Nb₁₅Ni₁₀ alloy and a mixture of intergranular and transgranular in the other tested alloys when the Ni content exceeded 20 pct. The Ni addition also increased the compression ductility for Ni content up to 50 at. pct. The maximum compression ductility was approximately 13 pct, obtained from Ir₆₅Nb₁₅Ni₂₀. Ir_85-X Nb₁₅Ni _X two-phase refractory superalloys with X ≤ 10 were superior to the binary Ir₈₅Nb₁₅ alloy as an ultra-high-temperature structural material in terms of strength, fracture behavior, and density.     

Ir-base refractory superalloys for ultra-high temperatures

The microstructure and compression strengths of Ir-15 at. pct X (X=Ti, Ta, Nb, Hf, Zr, or V) binary alloys at temperatures between room temperature and 1800 °C were investigated to evaluate the potential of these alloys for ultra-high-temperature use. The fcc and L1₂ two-phase structures of these alloys were examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The strengths of the Ir-Ta, -Nb, -Hf, and -Zr alloys were above 800 MPa at temperatures up to 1200 °C and about 200 MPa at 1800 °C. The strengths of these alloys under 1000 °C are equivalent to or higher than those of the commercially used Ni-base superalloys, MAR-M247 and CMSX-10. The Nb concentration dependence of strength was investigated using a series of Ir-Nb alloys with Nb concentrations from 0 to 25 at. pct. It was found that the Ir-base alloys were strengthened by L1₂ precipitation hardening. The potential of the Ir-base alloys for ultra-high temperature use is discussed.     

Toughness and strength characteristics of Nb-W-Si ternary alloys prepared by Arc melting

This article describes the room-temperature and high-temperature mechanical properties and failure modes of series Nb-W-Si alloys—Nb-10W, Nb-10Si, Nb-10Si-5W, Nb-10W-5Si, and Nb-10W-10Si—prepared by arc melting. For the Nb-10W alloy, the microstructure was a monolithic Nb solid solution (Nb _ss ) with a grain size up to a few hundred microns, while the other four alloys consisted of primary Nb _ss and a eutectic of Nb _ss /Nb₅Si₃ (5-3 silicide) as a result of replacing Nb with Si. Among all alloys, the Nb-10W showed the highest fracture toughness of about 15.3 MPa√m^1/2 and the lowest 0.2 pct yield compressive strength of 90 MPa at 1670 K. Conversely, the Nb-10Si-10W had the highest 0.2 pct yield strength of about 330 MPa at 1670 K and the lowest fracture toughness of 8.2 MPa√m^1/2. It is suggested that toughness is supplied by the metallic Nb _ss phase, while high-temperature strength is mainly provided by the brittle silicide phase. For the Nb-10W alloy with the monolithic Nb _ss , intergranular cleavagelike crack propagation is the fracture mode at room temperature, and dislocation movement within the grains and grain-boundary sliding are the dominant modes of high-temperature failure. With two-phase Nb _ss /Nb₅Si₃ microstructures, the compressive damage of all four alloys at high temperature was dominated by debonding of the interfaces between the Nb _ss and the silicide; however, the fracture mode at room temperature is transgranular, controlled by the primary Nb _ss cleavage.     

Phase equilibria in prototype Nb-Pd-Hf-Al alloys

The phase equilibria in two prototype alloys with nominal compositions 60Nb-20Pd-10Hf-10Al and 40Nb-30Pd-15Hf-15Al (in at. pct) are investigated using scanning electron microscopy and X-ray diffraction. The alloys were heat treated at 1200 °C and 1500 °C for 200 hours each. The phase analysis revealed that the alloys were, for the most part, in the three-phase equilibrium between (Nb), Pd₂HfAl, and Pd₃Hf. The compositions of these three phases along with other observed phases such as PdAl and (α-Hf) provide important data for establishing the Nb-Pd-Hf-Al quaternary phase diagram. A preliminary Nb-Pd-Hf-Al phase diagram, with pertinent tie-tetrahedra, was constructed based on the available composition data. The lattice parameters of (Nb), Pd₂HfAl, Pd₃Hf, and the coefficient of thermal expansion of Pd₂HfAl were measured, and models were developed to predict the composition dependence of the mean atomic volumes/lattice parameters of (Nb) and Pd₂HfAl and the temperature dependence of the lattice parameter of the (Nb) phase. The validity of the models was confirmed by good agreement between predicted and experimental values.     

Partial phase relationships in Ir-Nb-Ni-Al and Ir-Nb-Pt-Al quaternary systems and mechanical properties of their alloys

Two quanternary systems, Ir-Nb-Ni-Al and Ir-Nb-Pt-Al, were successively investigated to assess their possible use in ultra-high-temperature applications. The phase relationships concentrated on the fcc/L1₂ two-phase region were primarily established, and the mechanical properties were studied. Ir-Nb-Ni-Al quaternary alloys around the Ir-rich or Ni-rich corners of the Ir-Nb-Ni-Al tetrahedron showed a coherent fcc/L1₂ two-phase structure, analogous to that of Ni-base superalloys; however, most of the alloys presented three or four phases with two types of L1₂ phases. Although these alloys showed a high compressive strength at high temperature, they exhibited a higher creep rate than Ir-base binary and ternary alloys. Another quanternary system, Ir-Nb-Pt-Al, showed promising results. Only an fcc/L1₂ two-phase structure was found in all the alloys investigated with compositions ranging from the Ir-rich side to the Pt-rich side, and the lattice misfit between the fcc and L1₂ phases was small. The high-temperature strength at 1200 °C of Ir-Nb-Pt-Al alloys was higher than that of Ir-Nb-Ni-Al alloys with the same Ir content (at. pct). Moreover, Ir-Nb-Pt-Al alloys exhibited excellent creep resistance at 1400 °C and 100 MPa. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Creep behaviors of an Ir-23Nb alloy at ultra-high temperatures

The ultra-high-temperature creep behaviors of an Ir-base, Ir-23Nb (in at. pct), two-phase refractory superalloy have been investigated. The compression creep experiments were performed at temperatures from 1650 °C to 1800 °C at initial applied stresses from 49 to 200 MPa. The results show that Ir-23Nb alloy has higher creep resistance and longer creep life in comparison to Ir-17Nb alloy under the same experimental conditions. The steady-state creep behavior can be described in terms of power-law creep with the apparent stress exponent of 4.5 and apparent activation energy of 653 kJ/mol. Basing on the investigation, the possible reasons for the great improvement on the creep resistance and creep life in Ir-23Nb alloy are discussed. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

The growth and morphology of directionally solidified nickel base γ/γ′- σ superalloys

A series of Ni-Nb-Al-Cr(γ/γ′- σ) alloys in the composition ranges Nb 19.3 to 23.2 wt pct, Al 2.5 to 5.2 wt pct and Cr 0 to 7.05 wt pct have been directionally solidified under high thermal gradient (G) at both steady state and under conditions of abruptly or gradually changing growth rate(ft). The critical ratio of G andR, (g/r)*, to achieve two-phase plane frontin- situ composite growth increases as chromium and niobium (Cb) concentration deviates from the trough or surface of two-fold saturation. Interlamellar spacing of composites tend to decrease with increasing chromium content. Structures produced at steady state growth in whichG/R < (G/R)* are consistent with previous work and can be related to the location of the alloy composition with respect to the line of two-fold saturation. For alloys, which at lowG/R exhibited σ dendrites, any perturbation in growth velocity (atG/R > (G/R)*) precipitated a single phase σ (Ni₃Nb) band. For alloys which at lowG/R exhibited γ dendrites a similar effect was achieved only when growth rate was reduced abruptly by more than an order of magnitude. Interlamellar spacing of two alloys (approximately Ni-20 wt pct Nb-2.5 wt pct Al-6 wt pct Cr) was studied and for abrupt reductions in growth rate in which bands were not produced, it was observed to decay slowly to the new steady state value over distances which are inconsistent with the assumption of simple niobium diffusion control. A gradual increase in growth velocity for one of these alloys resulted in extremely slow adjustment of interlamellar spacing occurring over a period greater than one hour. An abrupt increase in growth velocity for all alloys caused immediate adjustment of interlamellar spacing to the new steady state value.     

An investigation of the fracture and fatigue crack growth behavior of forged damage-tolerant niobium aluminide intermetallics

The results of a recent study of the effects of ternary alloying with Ti on the fatigue and fracture behavior of a new class of forged damage-tolerant niobium aluminide (Nb₃Al-xTi) intermetallics are presented in this article. The alloys studied have the following nominal compositions: Nb-15Al-10Ti (10Ti alloy), Nb-15Al-25Ti (25Ti alloy), and Nb-15Al-40Ti (40Ti alloy). All compositions are quoted in atomic percentages unless stated otherwise. The 10Ti and 25Ti alloys exhibit fracture toughness levels between 10 and 20 MPa√m at room temperature. Fracture in these alloys occurs by brittle cleavage fracture modes. In contrast, a ductile dimpled fracture mode is observed at room-temperature for the alloy containing 40 at. pct Ti. The 40Ti alloy also exhibits exceptional combinations of room-temperature strength (695 to 904 MPa), ductility (4 to 30 pct), fracture toughness (40 to 100 MPa√m), and fatigue crack growth resistance (comparable to Ti-6Al-4V, monolithic Nb, and inconnel 718). The implications of the results are discussed for potential structural applications of the 40Ti alloy in the intermediate-temperature (∼700 °C to 750 °C) regime.     

Atomic diffusion and phase equilibria at the interfaces of the CoAl/Ir multilayer on Nb<Subscript>5</Subscript>Si<Subscript>3</Subscript>-base alloys

Atomic diffusion and phase equilibria have been investigated at the interfaces of Ir/CoAl and Ir/Nb₅Si₃ to evaluate the suitability of a diffusion-barrier layer of Ir between an oxidation-resistant layer of B2-CoAl and a base material Nb₅Si₃. Diffusion couples were prepared by hot pressing and annealed at 1573 K for up to 178 hours. Diffusion layers of (Ir, Co) solid solution and B2-(Ir, Co)Al were formed at the Ir/CoAl interface. The concentration of Al dramatically dropped at the interface, which indicates that the Ir layer effectively works as the diffusion barrier against the inward diffusion of Al. To quantitatively evaluate the potential of Ir as a diffusion barrier, the Boltzmann-Matano analysis was employed to determine the diffusion coefficient of Al using Ir-8 at. pct Al/Ir diffusion couples annealed at temperatures of 1573, 1673, and 1773 K. For instance, an extremely low value of 7.0×10⁻¹⁹ m²/s is evaluated for Ir-4 at. pct Al at 1573 K. At the Ir/Nb₅Si₃ interface, the intermetallic phases Ir₃Si and Ir₃Nb are formed on the Ir side and the Nb₅Si₃ side, respectively. The formation of Ir₃Si is controlled by the diffusion of Si through Ir₃Nb in which the solubility of Si is limited quite small. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Effects of temperature and environment on the tensile and fatigue crack growth behavior of a Ni3AI-base alloy

The effects of temperature, frequency, and environment on the tensile and cyclic deformation behavior of a nickel aluminide alloy, Ni-9.0 wt pct Al-7.97 pct Cr-1.77 pct Zr (IC-221), have been determined. The tensile properties were obtained in vacuum at elevated temperatures and in air at room temperature. The alloy was not notch sensitive at room temperature or at 600 °C, unlike Cr-free Ni₃Al + B alloys. In general, crack growth rates of IC-221 increased with increasing temperature, decreasing frequency, exposure to air, or testing at higherR ratios. At 25 °C, crack growth rates were slightly higher than for a previously investigated Cr-free Ni₃Al alloy. However, at 600 °C, the crack growth rates for IC-221 were lower than for the Cr-free alloy. Substantial frequency effects were noted on crack growth of IC-221 at both 600 °C and 800 °C in both air and vacuum, especially at highK. The relative contributions of creep and environmental interactions to fatigue crack growth are discussed.     

Deformation behavior of a Ni3Al(B,Zr) alloy during cold rolling: Part I. changes in order and structure

A hypostoichiometric Ni₃Al(B,Zr) alloy was homogenized and cold rolled by amounts ranging from 25 to 73 pct. The alloy consisted of two phases—a partially ordered γ′ phase (L1₂) and a Ni-rich fcc solid solution (γ). On deforming the alloy by rolling at room temperature, the order parameter showed a gradual change. In fact, between 35 and 45 pct deformation, the order characteristic of the L1₂ structure changed into that of a DO₂₂ structure. The possibility of transition from L1₂ to DO₂₂ structure is also corroborated from strain parameter, microhardness, and detailed x-ray diffraction (XRD) measurements. This structural transformation is accompanied by a change in the deformation mode (from slip to twinning), as is evident from the relevant microstructures.     

Aging behavior of Fe-30 Ni alloys containing niobium

A study has been made of the precipitation reactions in Fe-30 wt pct alloys containing up to 5 wt pct Nb. The as-quenched structures of these alloys consist, of austenite, martensite in twinned as well as in massive form, and Ni₃Nb and Fe₂Nb precipitates. On aging at 700° and 800°C the main precipitation reaction results in the formation of hexagonal Laves phase Fe₂Nb, but Ni₃Nb in both bct and orthorhombic structures also precipitates. The precipitation of Fe₂Nb is a heterogeneous process and results in a considerable increase in the hardness of the alloy.     

Effects of ternary additions on the thermoelastic martensitic transformation of NiAl

Nickel-rich β-NiAl alloys, which are potential materials for high-temperature shape-memory alloys, show a thermoelastic martensitic transformation, which produces their shape memory effect. However, the transformation to Ni₅Al₃ phase during heating of NiAl martensite can interrupt the reversible martensitic transformation; consequently, the shape memory effect in NiAl martensite might not appear after heating. The phase transformation process in binary Ni-(34 to 37)Al martensite was investigated by differential thermal analysis (DTA) method, and we found that the condition of reversible martensitic transformation was not the β → Ni₅Al₃ transformation, but rather the M → Ni₅Al₃ transformation occurring at 250 °C to 300 °C. Therefore, the transformation temperature of M → Ni₅Al₃ determined the highest operating temperature for the shape memory effect. For verifying the critical temperature, the phase transformation process was investigated for eight ternary Ni-33Al-X alloys (X=Cu, Co, Fe, Mn, Cr, Ti, Si, and Nb). Only Ti, Si, and Nb additions were found to be effective in dropping the M _s temperature, and they facilitated the shape memory effect in Ni-33Al-X alloys. In particular, the addition of Si and Nb raised the transformation temperature of M → Ni₅Al₃, a potentially beneficial effect for shape memory at higher temperatures. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

Strength of Initially Virgin Martensites at — 196 °C after Aging and Tempering

The compressive strength at —196°C of martensites in Fe-0.26 pct C-24 pct Ni, Fe-0.4 pct C-21 pct Ni, and Fe-0.4 pct C-18 pct Ni-3 pct Mo alloys, all with subzero M_s temperatures, has been determined in the virgin condition and after one hour at temperatures from —80 to +400 °C. The effects of ausforming (20 pct reduction in area of the austenite by swaging at room temperature prior to the martensitic transformation) were also investigated. For the unausformed martensites, aging at temperatures up to 0 °C results in relatively small increases in strength. Above 0 °C, the age hardening increment increases rapidly, reaching a maximum at 100 °C. Above 100 °C, the strength decreases continuously with increasing tempering temperature except for the molybdenum-containing alloy, which exhibits secondary hardening on tempering at 400 °C. For the ausformed martensites, the response to aging at subzero temperatures is greater than for unausformed material. Strength again passes through a maximum on aging at 100 °C. However, on tempering just above 100 °C, the ausformed materials show a slower rate of softening than the unausformed martensites. The strengthening produced by the ausforming treatment is largest for the Fe-0.4 pct C-18 pct Ni-3 pct Mo alloy, but there is no evidence of carbide precipitation in the deformed austenite to account for this effect of molybdenum. This paper is based on a presentation made at the “Peter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.