Optimization of composition and heat treatment of age-hardened Pt-Al-Cr-Ni alloys

The objective of this work was to produce an alloy showing a microstructure similar to Ni-base superalloys, but with Pt as base metal. The Pt-base alloys with various contents of Al, Cr, and Ni were arc melted. Solution heat treatments at 1450 °C followed by water quenching lead to single-phase alloys. Ageing at 1000 °C resulted in the precipitation of Ll₂ ordered particles. An alloy with 11 at. pct Al, 3 at. pct Cr, 6 at. pct Ni, and Pt balance shows cuboidal precipitates with edge lengths of 200 to 500 nm along with a volume fraction of 23 pct and a lattice misfit of −0.1 pct. Aging at 1100 °C leads to coarsening of precipitates. Volume fraction and morphology of the precipitates were investigated by scanning electron microscopy and optical microscopy. X-ray diffraction as well as transmission electron microscopy (TEM) were applied to verify the crystal structure.     

Optimization of composition and heat treatment of age-hardened Pt−Al−Cr−Ni alloys

The objective of this work was to produce an alloy showing a microstructure similar to Ni-base superalloys, but with Pt as base metal. The Pt-base alloys with various contents of Al, Cr, and Ni were are melted. Solution heat treatments at 1450 °C followed by water quenching lead to single-phase alloys. Ageing at 1000 °C resulted in the precipitation of L1₂ ordered particles. An alloy with 11 at. pct Al, 3 at. pct Cr, 6 at. pct Ni, and Pt balance shows cuboidal precipitates with edge lengths of 200 to 500 nm along with a volume fraction of 23 pct and a lattice misfit of −0.1 pct. Aging at 1100 °C leads to coarsening of precipitates. Volume fraction and morphology of the precipitates were investigated by scanning electron microscopy and optical microscopy. X-ray diffraction as well as transmission electron microscopy (TEM) were applied to verity the crystal structure. M. Huller, formerly with Metallic Materials, University Bayreuth, D-95440 Bayreuth, Germany     

Design of quaternary Ir-Nb-Ni-Al refractory superalloys

We propose a method for developing new quaternary Ir-Nb-Ni-Al refractory superalloys for ultra-high-temperature uses, by mixing two types of binary alloys, Ir-20 at. pct Nb and Ni-16.8 at. pct Al, which contain fcc/L1₂ two-phase coherent structures. For alloys of various Ir-Nb/Ni-Al compositions, we analyzed the microstructure and measured the compressive strengths. Phase analysis indicated that three-phase equilibria—fcc, Ir₃Nb-L1₂, and Ni₃Al-L1₂—existed in Ir-5Nb-62.4Ni-12.6Al (at. pct) (alloy A), Ir-10Nb-41.6Ni-8.4Al (at. pct) (alloy B), and Ir-15Nb-20.8Ni-4.2Al (at. pct) (alloy C) at 1400 °C; at 1300 °C, three phase equilibria—fcc, Ir₃Nb, and Ni₃Al—existed in alloys A and C and four-phase equilibria—fcc, Ir₃Nb, Ni₃Al, and IrAl-B2—existed in alloy B. The fcc/L1₂ coherent structure was examined by using transmission electron microscopy (TEM). At a temperature of 1200 °C, the compressive strength of these quaternary alloys was between 130 and 350 MPa, which was higher than that of commercial Ni-based superalloys, such as MarM247 (50 MPa), and lower than that of Ir-based binary alloys (500 MPa). Compared to Ir-based alloys, the compressive strain of these quaternary alloys was greatly improved. The potential of the quaternary alloys for ultra-high-temperature use is also discussed.     

Microstructure and phase identification in type 304 stainless steel-zirconium alloys

Stainless steel-zirconium alloys have been developed at Argonne National Laboratory to contain radioactive metal isotopes isolated from spent nuclear fuel. This article discusses the various phases that are formed in as-cast alloys of type 304 stainless steel and zirconium that contain up to 92 wt pct Zr. Microstructural characterization was performed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), and crystal structure information was obtained by X-ray diffraction. Type 304SS-Zr alloys with 5 and 10 wt pct Zr have a three-phase microstructure—austenite, ferrite, and the Laves intermetallic, Zr(Fe,Cr,Ni)_2+x. whereas alloys with 15, 20, and 30 wt pct Zr contain only two phases—ferrite and Zr(Fe,Cr,Ni)_2+x. Alloys with 45 to 67 wt pct Zr contain a mixture of Zr(Fe,Cr,Ni)_2+x and Zr₂(Ni,Fe), whereas alloys with 83 and 92 wt pct Zr contain three phases—α-Zr, Zr₂(Ni,Fe), and Zr(Fe,Cr,Ni)_2+x. Fe₃Zr-type and Zr₃Fe-type phases were not observed in the type 304SS-Zr alloys. The changes in alloy microstructure with zirconium content have been correlated to the Fe-Zr binary phase diagram.     

Analysis of the composition of α plates isothermally formed in titanium binary alloys

The solute distribution within and near the proeutectoid α plates which were isothermally formed from the β solid solution in Ti-V, Cr, and Fe alloys was measured in a scanning transmission electron microscope (STEM) equipped with an energy dispersive X-ray (EDX) analyzer. In the Ti-4.9 at. pct Cr and 5.1 at. pct Fe alloys in which all measurements were made above the calculated T_o temperature (the maximum temperature at which the diffusionless β to α transformation can occur), the solute concentration in the α plates was close to final equilibrium as long as the analyzed plates had grown to the sizes comparable to the STEM spatial resolution (∼50 nm). In the Ti-5.4 at. pct V and 2.6 at. pct Fe alloys, the solute concentration in the α plates was already close to equilibrium at very short reaction times at temperatures more than 100 °C below T_o. The ledgewise diffusion growth mechanism can roughly account for the thickening behavior of these plates, consistent with the results of the composition analysis. In the Cr alloy, a microstructure resembling the lower bainite in steels, i.e., nonlamellar distribution of TiCr₂ particles in thick α plates, was formed by coalescence of solute-depleted thin α plates which were formed in aggregates (in sheaves) at an earlier stage of growth. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

The effect of γ′ content on the densities of Ni-based superalloys

The γ′ phase increases the mechanical strength of Ni-based superalloys. Equations have been derived to calculate the atomic percent of γ′ phase from the Al content of the alloy. It is shown that increased γ′ phase content results in an increase in density (of the solid) and a decrease in the thermal expansion coefficient. Equations based on the Al content of the alloy have been derived to calculate the magnitude of these effects. The coarsening of the γ′ phase from 800 °C to 1000 °C was found to be accompanied by an increase in the temperature dependence of the thermal expansion coefficient. Densities for the entire temperature range of the solid alloys calculated with the equations are within 2 pct of measured values. The strong bonding between the Ni and Al was maintained in the liquid and resulted in increased densities. Equations were derived to quantify the effect on density (of the liquid) and the calculated density values for Ni-based superalloys were within 2 pct of the measured values. These uncertainties are commensurate with the experimental uncertainty in the measurements.     

Microstructural evolution and mechanical properties of Cr-Ru alloys

In an effort to enhance ductility and strength of Cr-base alloys, a series of Cr-Ru alloys with Ru contents ranging from 3 to 30 at. pct were made to study their microstructure evolution and mechanical properties. The microstructure of the alloys with 6 to 20 at. pct Ru showed signs of a eutectic structure. However, no corresponding eutectic reaction is indicated in the published Cr-Ru phase diagram. The yield strength of the Cr-Ru alloys increased with increasing Ru content at both room temperature and 1200 °C. The tensile ductility of Cr-3 at. pct Ru is about 1.5 pct at room temperature, while the alloys containing 6 at. pct or more Ru showed zero tensile elongation. The deformation mechanisms of the Cr-Ru alloys are discussed in terms of the microstructure and fracture behavior. 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.     

Phase stability and yield stress of Ni-base superalloys containing high Co and Ti

Ni-base superalloys containing high Co (>20 wt pct) and Ti (>5.5 wt pct) were designed in order to study the effects of Co16.9 wt pct Ti addition on phase stability and mechanical property. These new alloys, though they contained high Ti, mainly consisted of γ and γ′ phases. Ni₃Ti (η) phase was observed along the grain boundaries in some of the alloys. The formation of η phase was mainly related to the Ti/Al ratio, Ti content, and alloy composition. Tensile and compression tests showed that these new alloys exhibited higher yield stress than that of the baseline alloy, TMW-1(U720LI). The possible strengthening mechanisms were discussed in terms of solid-solution and precipitation strengthening effects by the Co16.9 wt pct Ti additions. Preliminary results show promising trends for the development of new superalloys for turbine disc applications.     

Martensitic transformations induced by plastic deformation in the Fe-Ni-Cr-C system

Martensitic transformations induced by plastic deformation are studied comparatively in various alloys of three types: Fe-30 pct Ni, Fe-20 pct Ni-7 pct Cr, and Fe-16 pet Cr-13 pct Ni, with carbon content up to 0.3 pct. For all these alloys the tensile properties vary rapidly with temperature, but there are large differences in the value of the temperature rangeM _s toM _d, which strongly increases with substitution of chromium for nickel or with carbon addition. Using the node method, it is found that the intrinsic stacking fault energy in the austenite drastically increases with temperature in all the chromium-bearing alloys investigated. This variation is consistent with the observed influence of temperature on the appearance of twinning or ε martensite during plastic deformation. Very different α’ martensite morphologies can result from spontaneous and plastic deformation induced transformations, especially in Fe-20 pct Ni-7 pct Cr-type alloys where platelike and lath martensites are respectively observed. As in the case of ε martensite, the nucleation process is analyzed as a deformation mode of the material, using a dislocation model. It is then possible to account for the morphology of plastic deformation induced α’ martensite in both Fe-20 pct Ni-7 pct Cr and Fe-16 pct Cr-13 pct Ni types alloys and for the largeM _s toM _d range in these alloys. This paper is based upon a thesis submitted by F. LECROISEY in partial fulfillment of the degree of Doctor of Philosophy at the University of Nancy.     

The wetting characteristics and surface tension of some ni-based alloys on yttria,hafnia, alumina,and zirconia substrates

Sessile drop experiments were carried out in order to measure surface tensions and to investigate wetting characteristics of some Ni-based alloys on various ceramic substrates. The liquid-vapor surface tension (γ_LV) was found to be 1.764 N/m for pure Ni, 1.45 ± 0.11 N/m for Ni-20 pct Cr, 1.29 ± 0.06 N/m for Ni-20 pct Cr-1 pct Al, and 1.31 ± 0.09 N/m for Ni-20 pct Cr-4 pct Al. The commercial alloys UD520, UD718, UD720, and WASPALOY* showed non-wetting behavior on zirconia but wetting tendency on alumina substrates. Ni-20 pct Cr-1 pct Al showed non-wetting behavior on alumina, hafnia, and yttria substrates whereas Ni-20 pct Cr and Ni-20 pct Cr-4 pct Al were observed to be non-wetting on hafnia but wetting on yttria and alumina substrates. All the systems that exhibited wetting behavior were found to be non-wetting in the beginning; however, wet-ting improved with time. The wetting characteristics were apparently related to impurification of droplets during measurements, which is reflected in the solidification structure, rather than to the presence of oxides on the surface.     

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.     

Microstructure and its development in Cu-Al-Ni alloys

The microstructure of as-cast Cu-AI-Ni alloys, based on copper containing 9 to 10 wt pct Al and up to 5 wt pct Ni, has been examined. The development of the microstructure on continuous cooling has also been investigated. For alloys with 9.2 to 9.3 wt pct Al, and less than 1 wt pct Ni, the as-cast microstructure consists of proeutectoid α solid solution, α + γ₂ eutectoid, and martensitic β. If the nickel content is more than 2.5 wt pct, the α + γ₂ eutectoid is replaced by α + β ₂ ^′ eutectoid, and no martensitic β is observed in the as-cast alloys. The morphologies of the β ₂ ^′ and γ₂ eutectoid phases are similar; both have the Kurdjumov-Sachs (K-S) orientation relationship with the a phase. Two eutectoid reactions, involving β to α + γ₂ and β to α + β′₂, have been observed in an alloy containing 9.7 wt pct Al and 2.7 wt pct Ni. When both eutectoid reactions occur, the Nishiyama-Wassermann (N-W) orientation relationship exists between γ₂ or β ₂ ^′ and the α phase. During continuous cooling, proeutectoid α solid solution is the first phase to precipitate from the high-temperature β phase. The β to α + β ₂ ^′ eutectoid reaction starts at higher temperatures than the β to α + γ₂ reaction. Tempering of the as-cast alloys results in the elimination of the martensitic β. Y.S. SUN formerly Research Associate with the Manchester Materials Science Centre.     

Effects of silicon on the oxidation,hot-corrosion,and mechanical behavior of two cast nickel-base superalloys

Cast specimens of nickel-base superalloys 713C and Mar-M200 with nominal additions of 0, 0.5, and 1 wt pct Si were evaluated for oxidation and corrosion resistance, tensile and stress-rupture properties, microstructure, and phase relations. Results are com-pared with those of an earlier study of the effects of Si in B-1900. Si had similar effects on all three superalloys. It improves oxidation resistance but the improvement in 713C and Mar-M200 was considerably less than in B-1900. Hot-corrosion resistance is also improved somewhat. Si is, however, detrimental to mechanical properties, in particular, rupture strength and tensile ductility. Si has two obvious microstructural effects. It in-creases the amount of γ precipitated in eutectie nodules and promotes a Mo(Ni,Si)₂ Laves phase in the alloys containing Mo. These microstructural effects do not appear responsible for the degradation of mechanical properties, however.     

Microstructure of Precipitation Hardenable Powder Metallurgical Ni Alloys Containing 35 to 45 pct Cr and 3.5 to 6 pct Nb

Ni-based alloys with high Cr contents are not only known for their excellent high temperature and hot corrosion resistance, but are also known for poor mechanical properties and difficult workability. Powder metallurgical (PM) manufacturing of alloys may overcome several of the shortcomings encountered in materials manufacturing involving solidification. In the present work, six PM Ni-based alloys containing 35 to 45 wt pct Cr and 3.5 to 6 wt pct Nb were produced and compacted via hot isostatic pressing. Samples were heat treated for up to 1656 hours at either 923 K or 973 K (650 °C or 700 °C), and the microstructures and mechanical properties were quantified and compared to thermodynamic calculations. For the majority of the investigated alloys, the high Cr and Nb contents caused development of primary populations of globular α-Cr and δ (Ni₃Nb). Transmission electron microscopy of selected alloys confirmed the additional presence of metastable γ″ (Ni₃Nb). A co-dependent growth morphology was found, where the preferred growth direction of γ″, the {001} planes of γ-Ni, caused precipitates of both α-Cr and δ to appear in the form of mutually perpendicular oriented disks or plates. Solution heat treatment at 1373 K (1100 °C) followed by aging at 973 K (700 °C) produced a significant strength increase for all alloys, and an aged yield strength of 990 MPa combined with an elongation of 21 pct is documented for Ni 40 wt pct Cr 3.5 wt pct Nb.     

The effects of composition and γ′/γ lattice parameter mismatch on the critical resolved shear stresses for octahedral and cube slip in NiAlCrX alloys

Prototypical single-crystal NiAlCrX superalloys were studied to examine the effects of the common major alloying elements, Co, Mo, Nb, Ta, Ti, and W, on yielding behavior. The alloys contained about 10 at. pct Cr, 60 vol pct of the γ′ phase, and about 3 at. pct of X in the γ′. The critical resolved shear stresses (CRSSs) for octahedral and primary cube slip were measured at 760 °C, which is about the peak strength temperature. The CRSS_Oct and CRSS_cube are discussed in relation to those of Ni₃ (Al, X) γ′ alloys taken from the literature and the γ′/γ lattice mismatch. The CRSS_Oct of the γ+γ′ alloys reflected a similar compositional dependence to that of both the CRSS_Oct of the γ′ phase and the γ′/γ lattice parameter mismatch. The CRSS_cube of the γ+γ′ alloys also reflected the compositional dependence of the γ′/γ mismatch, but bore no similarity to that of CRSS_cube for γ′ alloys since it is controlled by the γ matrix. The ratio of CRSS_cube/CRSS_Oct was decreased by all alloying elements except Co, which increased the ratio. The decrease in CRSS_cube/CRSS_Oct was related to the degree in which elements partition to the γ′ rather than the γ phase.     

Hardenability in high carbon steels

Multiplying factors (MF) for the hardenability effects of Mn, Si, Cr, Ni, Mo, Al, and B at high carbon levels were successfully derived to a pure iron or alloy free base for austenitizing conditions ranging from 1475°F (800°C) to 1700°F (927°C). Base factors were also determined for carbon in the range of 0.60 to 1.10 pct. These data supersede a similar set of MF’s determined previously by the author to a reference or base composition of 1.0 pct C and 0.25 pct of each of Mn, Si, Cr, and Ni. The new MF’s are presented in both tabular and graphical form and can be used to predict hardenability from composition for homogeneous high carbon steels as well as the so-called “case” hardenability of high carbon regions in carburizing grades. Case hardenability can be calculated for both the single quench practice wherein the steel is hardened by direct or delay quenching from carburizing, and for the double quench practice wherein the steel is reheated for hardening to some lower temperature after a prior air cooling (normalizing) or quenching from the carburizing treatment. The accuracy of hardenability prediction using these new factors has been found to be within ±10 pct of the measured hardenability atD _I’s as high as 26 in. (660 mm). This paper is based on a presentation made at a symposium on “Hardenability” held at the Cleveland Meeting of The Metallurgical Society of AIME, October 17, 1972, under the sponsorship of the IMD Heat Treatment Committee.     

A review of very-high-temperature Nb-silicide-based composites

The temperatures of airfoil surfaces in advanced turbine engines are approaching the limits of nickel-based superalloys. Innovations in refractory metal-intermetallic composites (RMICs) are being pursued, with particular emphasis on systems based on Nb-Si and Mo-Si-B alloys. These systems have the potential for service at surface temperatures >1350 °C. The present article will review the most recent progress in the development of Nb-silicide-based in-situ composites for very-high-temperature applications. Nb-silicide-based composites contain high-strength silicides that are toughened by a ductile Nb-based solid solution. Simple composites are based on binary Nb-Si alloys; more complex systems are alloyed with Ti, Hf, Cr, and Al. In higher-order silicide-based systems, alloying elements have been added to stabilize intermetallics, such as Laves phases, for additional oxidation resistance. Alloying schemes have been developed to achieve an excellent balance of room-temperature toughness, high-temperature creep performance, and oxidation resistance. Recent progress in the development of composite processing-structure-property relationships in Nb-silicide-based in-situ composites will be described, with emphasis on rupture resistance and oxidation performance. The Nb-silicide composite properties will be compared with those of advanced Ni-based superalloys. 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.     

Microstructure and yield strength of UDIMET 720LI alloyed with Co-16.9 Wt Pct Ti

We proposed a new method for developing Ni-base turbine disc alloy for application at temperatures above 700 °C by mixing a Ni-base superalloy U720LI with a two-phase alloy Co-16.9 wt pct Ti in various contents. The microstructure and phase stability of the alloys were analyzed using an optical microscope, a scanning electron microscope, energy-dispersive spectroscopy, and an X-ray diffractometer. The yield strength was studied by compression tests at temperatures ranging from 25 °C to 1200 °C. The results show that all the alloys had a dendritic structure. Ni₃Ti (η) phase was formed in the interdendritic region in the alloys with the addition of Co-16.9 wt pct Ti, and its volume fraction increased with the increase in the addition of Co-16.9 wt pct Ti. The results of exposure at 750 °C show that the addition of Co-16.9 wt pct Ti to U720LI had a great effect on suppressing the formation of σ phase due to the reduced Cr content in the γ matrix. Compared to U720LI, the alloys with the addition of Co-16.9 wt pct Ti possessed higher yield strength. The solid-solution strengthening of γ and γ′ and higher volume fraction of γ′ were assumed to cause this strength increase.     

Work strengthening by a deformation-induced phase transformation in “MP alloys”

Work strengthening and microstructure were investigated for a class of alloys, designated “MP Alloys”, containing 20 pct Cr, 10 pct Mo, and the remainder cobalt and nickel in proportions ranging from 60Co∶10Ni to 30Co∶40Ni. These alloys, in the fully annealed, homogenized condition, have a fcc structure with yield strengths ranging from about 45 to 60 ksi. Deformation at room temperature rapidly increases the yield strength of the alloys to about 250 ksi. Structural analyses by X-ray and electron diffraction techniques indicate that this marked increase in strength is associated with a deformation-induced martensitic transformation forming a network of extremely thin hcp platelets within the fcc grains. The nature of this martensitic transformation was studied as a function of alloy composition, deformation temperature, and structural variables, such as the platelet size, thec/a ratio of the hcp phase, and twinning.     

Thermal stability of the nickel-base superalloy B-1900 + Hf with tantalum variations

The microstructure of the solutionized and aged nickel-base superalloy B-1900 + Hf was examined after additional aging at 982 °C for 72, 250, and 1000 hours. Alloy compositions that were examined contained the normal 1.34 at. pct (4.3 wt pct). Ta as well as 0.67 at. pct and zero Ta levels. The γ phase agglomerated, became plate-like in morphology, and decreased in volume fraction for all three alloys throughout the aging treatments. Changes which occurred in the γ and γ' phase compositions were nearly complete after 72 hours of aging while changes in the MC carbide composition continued throughout the aging. Blocky M₆C carbides precipitated along the grain boundaries of all three alloys in the first 72 hours of aging. In addition, an acicular form of this Mo/Cr/Ni-rich carbide developed in the intragranular regions of the Ta-containing alloys. Formerly an Undergraduate Student, Department of Metallurgical Engineering, Michigan Technological University.