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     

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.     

On the development and investigation of quaternary Pt-based superalloys with Ni additions

The objective of this work is to mimic the microstructure and strengthening mechanisms of Ni-based superalloys in a new group of high-temperature alloys based on the system Pt-Al. The elements Cr and Ni were chosen as further alloying components. Having a face-centered cubic (fcc) crystal structure with an Ll₂-ordered and coherently embedded phase, these new alloys should increase creep and corrosion resistance beyond Ni-based superalloys. After arc melting and heat treatment, the alloys were investigated by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). In the aged condition, the alloy composition 13 at. pct Al, 3 at. pct Cr, 7 at. pct Ni, and balance Pt showed the most promising microstructure with cubical precipitates, 30 pct precipitate volume fraction, and a lattice misfit of about −0.1 pct at room temperature. 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.     

Embrittlement of ferritic stainless steels

The mechanical properties and microstructures of commercial 11 to 29 pct Cr ferritic steels were examined as functions of aging times to 1000 h at 371, 482, and 593°C. Of the properties evaluated, changes in impact transition temperatures were the best measure of embrittlement. Embrittlement at 482°C occurs most rapidly in the 29 pct Cr alloy and somewhat more slowly in the stabilized 26 pct Cr alloy. The stabilized 18 pct Cr alloy embrittles much more slowly while little, if any, embrittlement was detected in a stabilizedll pct Cr alloy. Embrittlement at 482°C was characterized by a rapid change in properties followed by a plateau region and then further property changes. The early property change is attributed to precipitation of interstitial compounds and the later change to classic 475°C embrittlement. The onset of 475°C embrittlement in the two highest Cr alloys was accompanied by clustering of Cr atoms along {100} planes indicative of spinodal decomposition. Concurrent with clustering there was also a change from turbulent slip to a more planar slip along {110} planes. Some embrittlement was observed after longer exposures at 371°C which was attributed to a combination of 475°C embrittlement and the precipitation of interstitial compounds. Two of the alloys also embrittled at 593°C, accompanied by optically observable precipitates. The precipitate in the stabilized 18 pct Cr alloy was identified as Laves (Fe₂Ti) phase. One of the precipitates in the 29 pct Cr alloy was identified as sigma phase. Formerly with Allegheny Ludlum Steel Corporation.     

High Cycle Fatigue of (Fe,Ni, Co)3V type alloys

High cycle fatigue tests in vacuum have been performed on ordered (Fe, Co, Ni)₃V alloys between 25 °C and 850 °C. Heat-to-heat variations in fatigue properties of a Co-16.5 wtpct Fe-25 pct alloy, LRO-1, appeared to be due to differing quantities of grain boundary precipitates. Modification of this alloy with 0.4 pct Ti, to produce an alloy designated LRO-23, reduced the density of grain boundary precipitates and increased ductility, resulting in superior fatigue strength at high temperatures. The fatigue lives of LRO-1 and LRO-23 decreased rapidly above 650 °C, and increased intergranular failure was noted. The fatigue resistance of a cobalt-free alloy, Fe-29 pct Ni-22 pct V-0.4 pct Ti (LRO-37), was examined at 25 °C, 400 °C, and 600 °C; there was little evidence for intergranular fracture at any of these temperatures. Fatigue behavior of the LRO alloys is compared to that of conventional high temperature alloys.     

An investigation of the high-temperature and solidification microstructures of PH 13-8 Mo stainless steel

Differential thermal analysis (DTA), high-temperature water-quench (WQ) experiments, and optical and electron microscopy were used to establish the near-solidus and solidification microstructures in PH 13-8 Mo. On heating at a rate of 0. 33 °C/s, this alloy begins to transform from austenite to δ-ferrite at ≈1350 °C. Transformation is complete by ≈1435 °C. The solidus is reached at ≈1447 °C, and the liquidus is ≈1493 °C. On cooling from the liquid state at a rate of 0. 33 °C/s, solidification is completed as δ-ferrite with subsequent transformation to austenite beginning in the solid state at ≈1364 °C. Insufficient time at temperature is available for complete transformation and the resulting room-temperature microstructure consists of matrix martensite (derived from the shear decomposition of the austenite) and residual δ-ferrite. The residual δ-ferrite in the DTA sample is enriched in Cr (≈16 wt pct), Mo (≈4 wt pct), and Al (≈1. 5 wt pct) and depleted in Ni (≈4 wt pct) relative to the martensite (≈12. 5 wt pct Cr, ≈2 wt pct Mo, ≈1 wt pct Al, ≈9 wt pct Ni). Solid-state transformation of δσ γ was found to be quench-rate sensitive with large grain, fully ferritic microstructures undergoing a massive transformation as a result of water quenching, while a diffusionally controlled Widmanstätten structure was produced in air-cooled samples.     

Transformation squence in a Cu-Al-Ni shape memory alloy at elevated temperatures

Precipitation sequences in a Cu-14 pct Al-4 pct Ni (wt pct) shape memory alloy were studied by means of transmission electron diffraction and microscopy as well as X-ray microanalysis techniques. On aging thin foil specimens up to 550 °C in the electron microscope, an as-quenched sample having a mixture of 2H-type and D0₃-type metastable structures transformed to the stable simple cubic γ₂ phase at or above 450 °C. The remaining matrix either showed precipitates of the fcc α-phase on prolonged annealing at 500 to 550 °C for a longer period, or transformed to martensite on cooling below theM _s temperature (~150 °C).     

Microstructure and high-temperature tensile deformation of tiai(si) alloys made from elemental powders

Two ternary TiAl-based alloys with chemical compositions of Ti-46.4 at. pct Al-1.4 at. pct Si (Si poor) and Ti-45 at. pct Al-2.7 at. pct Si (Si rich), which were prepared by reaction powder processing, have been investigated. Both alloys consist of the intermetallic compounds y-TiAl, α₂-Ti₃Al, and ξ-Ti₅(Si, Al)₃. The microstructure can be described as a duplex structure(i.e., lamellar γ/α₂ regions distributed in γ matrix) containing ξ precipitates. The higher Si content leads to a larger amount of ξ precipitates and a finer y grain size in the Si-rich alloy. The tensile properties of both alloys depend on test temperature. At room temperature and 700 °C, the tensile properties of the Si-poor alloy are better than those of the Si-rich alloy. At 900 °C, the opposite is true. Examinations of tensile deformed specimens reveal ξ-Ti₅(Si, Al)₃ particle debonding and particle cracking at lower test temperatures. At 900 °C, nucleation of voids and microcracks along lamellar grain boundaries and evidence for recovery and dynamic recrystallization were observed. Due to these processes, the alloys can tolerate ξ-Ti₅(Si, Al)₃ particles at high temperature, where the positive effect of grain refinement on both strength and ductility can be utilized.     

Decomposition of Fe-Ni martensite: Implications for the low-temperature (≤500 °C) Fe-Ni phase diagram

The low-temperature (<500 °C) decomposition of Fe-Ni martensite was studied by aging martensitic Fe-Ni alloys at temperatures between 300 °C and 450 °C and by measuring the composition of the matrix and precipitate phases using the analytical electron microscope (AEM). For aging treatments between 300 °C and 450 °C, lath martensite in 15 and 25 wt pct Ni alloys decomposed with γ [face-centered cubic (fcc)] precipitates forming intergranularly, and plate martensite in 30 wt pct Ni alloys decomposed with γ (fcc) precipitates forming intragranularly. The habit plane for the intragranular precipitates is {111}_fcc parallel to one of the {110}_bcc planes in the martensite. The compositions of the γ intergranular and intragranular precipitates lie between 48 and 58 wt pct Ni and generally increase in Ni content with decreasing aging temperature. Diffusion gradients are observed in the matrix α [body-centered cubic (bcc)] with decreasing Ni contents close to the martensite grain boundaries and matrix/precipitate boundaries. The Ni composition of the matrix α phase in decomposed martensite is significantly higher than the equilibrium value of 4 to 5 wt pct Ni, suggesting that precipitate growth in Fe-Ni martensite is partially interface reaction controlled at low temperatures (<500 °C). The results of the experimental studies modify the γ/α + γ phase boundary in the present low-temperature Fe-Ni phase diagram and establish the eutectoid reaction in the temperature range between 400 °C and 450 °C. Formerly Research Assistant, Department of Materials Science and Engineering, Lehigh University     

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.     

The structure of rapidly solidified Al- Fe- Cr alloys

Four aluminum alloys, designed for use at elevated temperatures, were studied. The alloys were supersaturated with iron and chromium, and one of them contained small amounts of Ti, V, and Zr. The starting materials were alloy powders made by the RSR (Rapid Solidification Rate) centrifugal atomization process. Extrusion bars were made from the four powders. The as-extruded microstructure and the microstructure of the alloys after annealing at 482 °C were investigated by optical and transmission electron microscopy and by X-ray diffraction. The microstructure consists of equiaxed grains of aluminum matrix and two types of precipitates, namely, Al₃(Fe ,Cr) and a metastable phase, Al6(Fe,Cr). The precipitates were different in their shape, size, distribution, and location within the grains.     

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.     

Effect of Aging on Microstructure and Shape Memory Properties of a Ni-48Ti-25Pd (At. Pct) Alloy

The microstructure and properties of a precipitation-hardenable Ni-48Ti-25Pd (at. pct) shape memory alloy have been investigated as a function of various aging conditions. Both the hardness and martensitic transformation temperatures increased with increasing aging time up to 100 hours at 673 K (400 °C), while no discernable differences were observed after heat treatment at 823 K (550 °C), except for a slight decrease in hardness. For aging at 673 K (400 °C), these effects were attributed to the formation of nano-scale precipitates, while precipitation was absent in the 823 K (550 °C) heat-treated specimens. The precipitation-strengthened alloy exhibited stable pseudoelastic behavior and load-biased-shape memory response with little or no residual strains. The precipitates had a monoclinic base-centered structure, which is the same structure as the P-phase recently reported in Ni(Pt)-rich NiTiPt alloys. 3D atom probe analysis revealed that the precipitates were slightly enriched in Ni and deficient in Pd and Ti as compared with the bulk alloy. The increase in martensitic transformation temperatures and the superior dimensional stability during shape memory and pseudoelastic testing are attributed to the fine precipitate phase and its effect on matrix chemistry, local stress state because of the coherent interface, and the ability to effectively strengthen the alloy against slip.     

Effect of methane on sulfur dioxide and sodium sulfate-lnduced corrosion of nickel-base alloys

The corrosion of nickel-base binary alloys, with up to 20 wt pct Cr or Al, by sulfur dioxide contained in a nitrogen gas stream, has been found to be considerably enhanced between 800 and 1000°C in the presence of low concentrations of gaseous methane. The magnitude of the effect, which is related to the ability of the alloy surface to catalyze the decomposition of the hydrocarbon to solid carbon, decreases with increasing Cr and Al content of the alloy. Ni-Al alloys containing less than 10 wt pct Al are more readily attacked by SO₂ alone than are the corresponding Ni-Cr alloys, although the deleterious effects of added methane are more marked in the Ni-Cr series. The extent of high temperature corrosion of these alloys by deposited sodium sulfate at 900 to 1000°C is also generally increased when small amounts of methane are introduced, although in this case the addition of Cr to the alloy confers greater corrosion resistance than does the addition of an equivalent concentration of Al. In the presence of either sodium sulfate or SO₂, the addition of methane to the ambient nitrogen atmosphere results in localized reducing conditions and increased sulfur potentials near the alloy surface.     

Microstructure and ordering of L12 titanium trialuminides

The present article reports and discusses the results of the microstructural characterization of various modifications of Ll₂ trialuminides containing various titanium contents, including the first ever report on their degree of ordering. The Ll₂ trialuminide alloys Al₃Ti + X, where X = Cu, Fe, Cr, and Mn were studied. The as-cast structure contains a very low level of porosity, and the amount of second phase depends on the particular alloy. After homogenization, the second phase is reduced in almost all the alloys to the level less than 0.5 pct, except for the Mn-high Ti alloy in which it remains at about 20 pct and its composition is 67.9 ± 0.6 at. pct Al, 2.2 ± 0.6 at. pct Mn, and 29.9 ± 0.3 at. pct Ti. In almost all the alloys, porosity after homogenization increases about twofold, except in the Al₃Ti + Cr alloy in which it remains at almost the as-cast level. Limited transmission electron microscopic observations have revealed the existence of very fine (≈10 nm) unidentified precipitates in the homogenized Al₃Ti + Cu alloy. The homogenized Al₃Ti + Cr and Mn alloys have greater lattice parameters than the Al₃Ti + Fe and Cu alloys. It is also found that the long-range order parameterS of the ho- mogenized Ll₂ Al₃Ti + X alloys dramatically decreases with increasing titanium content.     

Effects of chromium and copper additions on precipitation in Al−Zn−Mg alloys

The effects of chromium and copper additions on precipitation in several Al?Zn?Mg alloys have been investigated. Results show that chromium additions heterogenize precipitation in aged Al?Zn?Mg alloys by creating special nucleation sites. Multirowed bands of incoherent precipitates appeared in the grain boundaries and subboundaries in an Al-5 pct Zn-2 pct Mg-0.1 pct Cr alloy. It is believed that fine nuclei associated with the existence of chromium-rich regions are formed during solidification and are retained after solution heat treatment. These nuclei would lead to the formation of incoherent precipitates during quenching and aging. Chromium is, therefore, considered to causehigh temperature nucleation. Copper additions to Al?Zn?Mg alloys accelerate precipitation at lower aging temperatures and increase the density of G. P. zones nucleated at relatively lower temperatures (20 to 90°C). In this way copper considerably strengthens Al?Zn?Mg alloys. Copper, in contrast to chromium, causes increased low-temperature nucleation of G. P. zones.     

Magnesium silicide intermetallic alloys

Methods of induction melting an ultra-low-density magnesium silicide (Mg₂Si) intermetallic and its alloys and the resulting microstructure and microhardness were studied. The highest quality ingots of Mg₂Si alloys were obtained by triple melting in a graphite crucible coated with boron nitride to eliminate reactivity, under overpressure of high-purity argon (1.3 X 10⁵ Pa), at a temperature close to but not exceeding 1105 °C ± 5 °C to avoid excessive evaporation of Mg. After establishing the proper induction-melting conditions, the Mg-Si binary alloys and several Mg₂Si alloys macroalloyed with 1 at. pct of Al, Ni, Co, Cu, Ag, Zn, Mn, Cr, and Fe were induction melted and, after solidification, investigated by optical microscopy and quantitative X-ray energy dispersive spectroscopy (EDS). Both the Mg-rich and Si-rich eutectic in the binary alloys exhibited a small but systematic increase in the Si content as the overall composition of the binary alloy moved closer toward the Mg₂Si line compound. The Vickers microhardness (VHN) of the as-solidified Mg-rich and Si-rich eutectics in the Mg-Si binary alloys decreased with increasing Mg (decreasing Si) content in the eutectic. This behavior persisted even after annealing for 75 hours at 0.89 pct of the respective eutectic temperature. The Mg-rich eutectic in the Mg₂Si + Al, Ni, Co, Cu, Ag, and Zn alloys contained sections exhibiting a different optical contrast and chemical composition than the rest of the eutectic. Some particles dispersed in the Mg₂Si matrix were found in the Mg₂Si + Cr, Mn, and Fe alloys. The EDS results are presented and discussed and compared with the VHN data. Formerly Formerly     

Preparation and properties of some ODS Fe-Cr-Al alloys

Several alloys based on Fe-25Cr-6Al and Fe-25Cr-11Al (wt pct) with additions of yttrium, Al₂O₃, and Y₂O₃ have been prepared by mechanical alloying of elemental, master alloy and oxide powders. The powders were consolidated by extrusion at 1000°C with a reduction ratio of 36:1. The resulting oxide contents were all approximately either 3 vol pct or 8 vol pct of mixed Al₂O₃-Y₂O₃ oxides or of Al₂O₃. The alloys exhibited substantial ductility at 600°C: an alloy containing 3 vol pct oxide could be readily warm worked to sheet without intermediate annealing; an 8 vol pct alloy required intermediate annealing at 1100°C. The 3 vol pct alloys could be recrystallized to produce large elongated grains by isothermal annealing of as-extruded material at 1450°C, but the high temperature strength properties were not improved. However, these alloys, together with some of the 8 vol pct materials, could be more readily recrystallized after rod (or sheet) rolling; sub-stantially improved tensile and stress rupture properties were obtained following 9 pct rod rolling at 620°C and isothermal annealing for 2 h at 1350°C. In this condition, the rup-ture strengths of selected alloys at 1000 and 1100°C were superior to those of competitive nickel-and cobalt-base superalloys. The oxidation resistance of all the alloys was ex-cellent. F. G. WILSON and C. D. DESFORGES, formerly with Fulmer Re-search Institute     

Role of Mo and W during sensitization of superaustenitic stainless steel—crystallography and composition of precipitates

This study concerns the crystallographic identification and compositions of precipitates formed in superaustenitic stainless steel. Three experimental alloys, all containing 24 wt pct Cr, 22 wt pct Ni, and 0.5 wt pct N but with varying amounts of Mo and W, were investigated after sensitization heat treatment (aging) at 900 °C. The contents of Mo and W in the three alloys were 7 wt pct Mo, (6 wt pct Mo + 2 wt pct W) and (5 wt pct Mo + 5 wt pct W), respectively. While σ and x were the main secondary phases found in the W-free alloy, replacement of Mo by W was found to promote the formation of Laves-related phases with high Mo + W content. The complex crystallographic nature of Laves-related precipitates was exemplified through the formation of intergrowing C14 Laves, μ, and C phases, all with closely related crystal structures. There was no difference in chemical composition between the three phases. Prolonged aging resulted in intragranular precipitation of different intermetallic phases, as well as formation of nitrogen bearing phases, π and Cr₂N, adjacent to previously formed intermetallic precipitates. The content of Mo + W was found to decrease with increasing aging time for all secondary phases.