Low temperature mechanical behavior of Ni-15Cr-Ai-Ti-Mo alloys

A series of Ni-15 Cr-Al-Ti-Mo alloys with varying γy-γ’ mismatch, Ti/Al atomic ratio, and weight fraction of γ’ were tension tested at ambient temperature after aging to peak yield strength at 760°C. After subtracting the alloy yield stress in the solution treated condition, σss, the increment in yield stress due to precipitation of γ’, Δσy, was found to be principally influenced by the weight fraction of γ’ and the measured γ-γ’ mismatch. Only a small contribution of APBE to alloy strength was observed between two compositions of similar mismatch and differing measured APBE. The work hardening behavior of the alloys was similarly influenced by weight fraction of γ’ and γy-γ’ mismatch. Alloys with low mismatch exhibited sheared γ’ precipitates following tensile deformation. A model for shear of ordered γ’ precipitates by residual dislocation loops in low γ-γ’ mismatch alloys is proposed to account for the low work hardening rates observed.     

Effect of gamma-gamma prime mismatch,volume fraction gamma prime,and gamma prime morphology on elevated temperature properties of Ni, 20 Cr, 5.5 Mo,Ti, Al alloys

This study was designed to provide a critical test for the postulate that the mismatch between the lattices of austenite (γ) and the age-hardening gamma-prime (γ′) precipitates and the resultant coherency strains have a significant influence on the elevated temperature, particularly stress rupture, properties of a nickel-base superalloy. Two experimental alloys with a base analysis of Ni, 20 Cr, 5.5 Mo were designed with variable titanium and aluminum additions. To discern the effect of mismatch, an alloy without molybdenum was also experimented with. By manipulating the mismatch and volume fraction γ′ by heat treatment and chemistry, it was shown that a lower γ-γ′ mismatch indeed is beneficial to stress rupture life. Importance of volume fraction γ′ on this elevated temperature was also established.     

The influence of cobalt,tantalum, and tungsten on the microstructure of single crystal nickel-base superalloys

The influence of composition on the microstructure of single crystal nickel-base superalloys was investigated. Co was replaced by Ni, and Ta was replaced by either Ni or W, according to a matrix of compositions based on MAR-M247. Substitution of Ni for Co caused an increase inγ′ solvus temperature, an increase inγ-γ′ lattice mismatch, and the precipitation of W-rich phases in the alloys with high refractory metal levels. Substitution of Ni for Ta caused large decreases inγ′ solvus temperature,γ′ volume fraction, andγ-γ′ lattice mismatch, whereas substitution of W for Ta resulted in smaller decreases in these features. For the alloys withγ′ particles that remained coherent, substitution of Ni for Co caused an increase inγ′ coarsening rate. The two alloys with the largest magnitude of lattice mismatch possessedγ′ particles which lost coherency during unstressed aging and exhibited anomalously low coarsening rates. Creep exposure at 1000 °C resulted in the formation ofγ′ lamellae oriented perpendicular to the applied stress axis in all alloys.     

Rhenium additions to a Ni-base superalloy: Effects on microstructure

Refractory metal alloying to superalloy single crystals has resulted in alloys possessing higher temperature capabilities than have previously been known. In this study, additions of rhenium to a modified MAR-M200* alloy were made at the expense of tungsten. A study was carried out on the γ′ precipitate growth characteristics, morphology, and lattice misfit as a function of rhenium content and aging time and temperature. Rhenium additions substantially lowered the γ′ coarsening kinetics and resulted in negative γ-γ′ misfit alloys. A qualitative model is developed which explains these results. Particle growth was shown to bevia the widely accepted diffusion controlled coarsening mechanism. Precipitates were also shown to grow by particle coalescence into irregular blocky shapes at high aging temperatures while remaining cuboidal at lower temperatures, and the necessity of using large precipitate volume fraction modifications to theoretical coarsening models is illustrated.     

Nickel-rich portion of the Ni-Al-Nb phase diagram

The solubility of Nb + Al in the γ solid solution decreases markedly with decreasing temperature; thus alloys can be prepared that are γ at 1200°C and yet contain 50 pct γ’ precipitate after aging at 800°C. Thermal stability of the γ’ precipitate is related to the lattice mismatch between the γ and γ’ phases; the smaller the mismatch the lower is the interfacial elastic energy and the more stable is the γ’. Upon aging certain alloys at 800°C a γ’ growth interface other than the normal (100)_γll(100)_γ’ is observed. The maximum solubility of the niobium in γ’ is ∼7 at. pct; the width of the γ’ field increases with increasing niobium content but it is essentially independent of temperature. Replacing aluminum by niobium in γ’ gives hardnesses of up to 400 Dpn.     

Effect of initial gamma prime size on the elevated temperature creep properties of single crystal nickel base superalloys

The influence of initial γ′ size and shape on the high temperature creep properties of two single crystal nickel-base superalloys was investigated. The two alloys were chosen to represent different magnitudes of γ-γ′ lattice mismatch. A range of initial microstructures was produced by various quenching and aging treatments. Creep-rupture testing at 1000 °C was performed under stresses where γ′ directionally coarsens to form γ-γ′ lamellae in the early portion of the creep life. Both alloys exhibited a peak in creep resistance as a function of initial γ′ size. The peak corresponded to an initial microstructure consisting of cuboidal precipitates aligned along [001] directions. These aligned cuboidal γ′ particles directionally coarsened into a relatively perfect lamellar γ-γ′ structure in the early stages of creep, whereas the more irregularly shaped and distributed γ′ particles in both under and overaged material formed more irregular lamellae with more imperfections. The alloy with a lower magnitude of mismatch was less sensitive to initial γ′size and shape.     

Effect of filler alloys on heat-affected zone cracking in preweld heat-treated IN-738 LC gas-tungsten-arc welds

The effect of filler alloys C-263, RENé-41, IN-718, and FM-92 on heat-affected zone (HAZ) cracking susceptibility of cast IN-738 LC, which is a high-temperature Ni-based superalloy used at temperatures up to 980 °C and is precipitation hardened by the γ′ (Ni₃Al,Ti) phase, by gas-tungsten-arc (GTA) welding was studied. In addition, autogenous welds were also made on the IN-738 parent material. The preweld treatments consisted of the standard solution treatment at 1120 °C for 2 hours followed by air cooling, and a new heat treatment, which was developed to improve the HAZ cracking resistance of IN-738 LC. This heat treatment consisted of solution treating at 1120 °C followed by air cooling then aging at 1025 °C for 16 hours followed by water quenching. Welds were observed to suffer intergranular HAZ cracking, regardless of the filler alloy; however, the autogenous welds were most susceptible to HAZ cracking. In general, the cracking tendency for both heat treatments was maximum for C-263 and RENE-41 fillers and decreased with the use of FM-92 and IN-718 filler alloys. The HAZ cracking was associated mainly with constitutional liquation of γ′ and MC carbides. On some cracks, liquated low melting point containing Zr-carbosulfide and Cr-Mo borides were also observed to be present. The cooling portion of the weld thermal cycle induced precipitation hardening via γ′ phase in the γ matrix of the weld metal. The HAZ cracking increased as the weld metal lattice mismatch between γ′ precipitates and γ matrix of the weld and its hardness (Ti + Al) increased. However, the weld-metal solidus and solidification temperature range, determined by high-temperature differential scanning calorimetry, did not correlate with the HAZ cracking susceptibility. It is suggested that the use of filler alloys with small γ′-γ lattice mismatch and slow age-hardening response would reduce the HAZ cracking in IN-738 LC superalloy welds.     

Effects of carbon content and ausaging on γ ↔ α′ transformation behavior and reverse-transformed structure in Fe-Ni-Co-Al-C alloys

The effects of carbon content and ausaging on austenite γ ↔ martensite (α′) transformation behavior and reverse-transformed structure were investigated in Fe-32Ni-12Co-4Al and Fe-(26,28)Ni-12Co-4Al-0.4C (wt pct) alloys. TheM _s temperature, the hardness of γ phase, and the tetragonality of α′ increase with increasing ausaging time, and these values are higher in the carbon-bearing alloys in most cases. The γ → α′ transformation behavior is similar to that of thermoelastic martensite; that is, the width of α′ plate increases with decreasing temperature in all alloys. The αt’ → γ reverse transformation temperature is lower in the carbon-bearing alloys, which means that the shape memory effect is improved by the addition of carbon. The maximum shape recovery of 84 pct is obtained in Fe-28Ni-12Co-4Al-0.4C alloy when the ausaged specimen is deformed at theM _s temperature and heated to 1120 K. There are two types of reverse-transformed austenites in the carbon-bearing alloy. One type is the reversed y containing many dislocations which were formed when the γ/α′ interface moved reversibly. The plane on which dislocations lie is (01 l)_γ if the twin plane is (112)_α′. The other type of reverse-transformed austenite exhibits γ islands nucleated within the α′ plates.     

High refractory, low misfit ru-containing single-crystal superalloys

Single-crystal Ru-containing nickel-base superalloys with spherical γ′ precipitates have been observed in alloys with substantial amounts of Re and W and high levels of Ru. The γ′ precipitates did not experience stress-induced shape changes (rafting) during creep deformation at 950 °C and 290 MPa, indicative of a γ-γ′ lattice misfit very near zero. Furthermore, interfacial dislocation networks were not formed during creep deformation in the low misfit alloys. The alloys containing spherical precipitates had lower creep strengths than the alloys containing cuboidal precipitates at 950 °C and 290 MPa. Element partitioning between the phases was investigated in order to determine the origin of the unusual microstructural features. Transmission electron microscopy (TEM)-based energy-dispersive spectroscopy (EDS) analysis of the γ and γ′ phases indicates that Ru affects the partitioning of Re, which partitions much less strongly to the matrix than previously observed in Re-containing superalloys, consistent with a lattice misfit very near zero. With high levels of Ru, the addition of Cr also has a strong influence on partitioning. These investigations demonstrate that Ru and Cr control the lattice misfit, precipitate shape, and creep behavior, through the associated changes in the γ-γ′ phase equilibrium.     

Creep deformation of Ni3Al-Mo in situ composites

High temperature γ′(Ni₃A1)-α(Mo) in situ composites solidified with growth rates ranging from 0.12 cm/h to 2.4 cm/h, were creep tested at temperatures 710 °C, 830 °C, and 950 °C under a tensile stress of 300 MPa. Creep rupture lives of rapidly grown alloys were comparable to those of γ′/γ-δ and Nitac eutectics whereas strain to rupture was up to three times greater. In comparison, creep rupture lives of slowly grown alloys were more than ten times and strain to rupture about two times smaller than in rapidly grown alloys. In slowly grown alloys, failure occurred by formation of shear bands caused by cooperative shear of matrix and fibers whereas no shear bands were seen in rapidly grown samples. Shear band formation was due to pile-up stresses and parallel orientation of matrix and fiber slip systems, the latter resulting from a change in the crystallographic phase relationship as growth rate decreased. There was evidence that shear band formation depended on a threshold stress. The creep behavior of rapidly grown alloys was in qualitative agreement with predictions obtained from a linear visco-elastic model composite strengthened by the “mean matrix stress”.     

Alloy design of FeMnSiCrNi shape-memory alloys related to stacking-fault energy

The stacking-fault energy (γ _sf ) of iron-based shape-memory alloys was calculated by the extended dislocation-node method. The results show that Ni and Mn increase the γ _sf of the alloys with an austenite structure, while Cr and Si decrease it. An expression relating the alloying elements of Ni, Cr, Mn, and Si to the γ _sf of the alloys is established. Moreover, in terms of the γ _sf values of the alloys and the Schaeffler diagram, the alloy design of iron-based shape-memory alloys is carried out. It is found that the alloy having the lowest γ _sf has the best shape-memory effect (SME), with a martensite transition temperature (M _s ) being a little lower than ambient temperature. The corresponding composition of the alloy is located in the γ-phase zone near the phase line between the γ and γ+ε phase zone and the triple point of α+γ+ε in the Schaeffler diagram.     

Morphology of y’ and y” precipitates and thermal stability of inconel 718 type alloys

The precipitation of the γ^’ (Ll₂) and γ^" (DO₂₂) phases has been studied in four alloys Fe-Ni-Cr-Ti-Al-Nb containing a higher Ti + Al/Nb ratio than that of the INCONEL 718 alloy. For these alloys, the precipitation microstructure varies rapidly with aging temperature and composition. Bct γ^"particles have always been found to precipitate on γ^’ phase. Moreover, by aging three alloys above a critical temperature, a “compact ntorphology” has been observed: cube-shaped γ^’ particles coated on their six faces with a shell of γ^" precipitate. This microstructure has proved to be very stable on prolonged aging. A thermal stability better than that encountered in nominal INCONEL 718 alloy can thus be achieved. The influence of composition and aging temperature on the conditions that bring about this “compact morphology” has been investigated. A minimal Ti + Al/Nb ratio between 0.9 and 1 has been determined, allowing the “compact morphology” to be obtained. This paper is based upon a thesis submitted by R. COZAR in partial fulfillment of the requirements of the degree of Doctor of Philosophy at the University of Nancy.     

The role of twinning in the cyclic stress-strain behavior of directionally solidified γ/γ′-δ

Under cyclic straining of the γ/γ’-δ directionally solidified alloy a special type of hysteresis loop with an inflection in the compressive loading region was obtained. Observations gave direct evidence that the inflection was due to a process in which the 5 twins formed during tension untwinned under the subsequent compression. Thin foil electron microscopy established the 5 twins to be two specific variants of the four {211} variants, the selectivity being imposed by the crystallographic relation of the 5 phase with the γ/γ’ matrix. Characteristic distribution of (010) 5 faults was also noted in the foils and they were found associated with the {211} untwinning events. The compressive {011} 5 twinning mode reported in literature was not observed; the forward plastic strain in tension was reversed by {211} untwinning mode at a compressive stress level much lower than that required for the onset of the {011} twins.     

Low-cycle fatigue behavior of INCONEL 718 superalloy with different concentrations of boron at room temperature

Symmetrical push-pull low-cycle fatigue (LCF) tests were performed on INCONEL 718 superalloy containing 12, 29, 60, and 100 ppm boron (B) at room temperature (RT). The results showed that all four of these alloys experienced a relatively short period of initial cyclic hardening, followed by a regime of softening to fracture at higher cyclic strain amplitudes (Δɛ _t /2≥0.8 pct). As the cyclic strain amplitude decreased to Δɛ _t /2≤0.6 pct, a continuous cyclic softening occurred without the initial cyclic hardening, and a nearly stable cyclic stress amplitude was observed at Δɛ _t /2=0.4 pct. At the same total cyclic strain amplitude, the cyclic saturation stress amplitude among the four alloys was highest in the alloy with 60 ppm B and lowest in the alloy with 29 ppm B. The fatigue lifetime of the alloy at RT was found to be enhanced by an increase in B concentration from 12 to 29 ppm. However, the improvement in fatigue lifetime was moderate when the B concentration exceeded 29 ppm B. A linear relationship between the fatigue life and cyclic total strain amplitude was observed, while a “two-slope” relationship between the fatigue life and cyclic plastic strain amplitude was observed with an inflection point at about Δɛ _p /2=0.40 pct. The fractographic analyses suggested that fatigue cracks initiated from specimen surfaces, and transgranular fracture, with well-developed fatigue striations, was the predominant fracture mode. The number of secondary cracks was higher in the alloys with 12 and 100 ppm B than in the alloys with 29 and 60 ppm B. Transmission electron microscopy (TEM) examination revealed that typical deformation microstructures consisted of a regularly spaced array of planar deformation bands on {111} slip planes in all four alloys. Plastic deformation was observed to be concentrated in localized regions in the fatigued alloy with 12 ppm B. In all of the alloys, γ″ precipitate particles were observed to be sheared, and continued cyclic deformation reduced their size. The observed cyclic deformation softening was associated with the reduction in the size of γ″ precipitate particles. The effect of B concentration on the cyclic deformation mechanism and fatigue lifetime of IN 718 was discussed.     

Carbide reinforcement in two directionally solidified alloyed nickel eutectic alloys

Two directionally solidified carbide-reinforced alloyed nicel eutectics were evaluated; an alloy consisting of monocarbide fibers in a single phase matrix and one containing monocarbide fibers in a two-phase γ-γ′ matrix. The mechanical properties and microstructures of these alloys are compared to those of two directionally solified alloys having the same nominal matrix compositions, but not containing carbide fibers. The calculated strengths of the monocarbide fibers in the γ′-containing eutectic alloy are 1,400,000 psi (9650 mn/m²) and 243,000 psi (1680 mn/m²) at room temperature and 1000°C, respectively, while those in the single phase γ matrix eutectic at the same temperatures are 590,000 psi (4060 mn/m²) and 298,000 psi (2050 mn/m²). At room temperature, the lower strength of fibers from the γ matrix alloy is believed to result from stress concentrations induced by the presence of growth facets on the fibers. The lower apparent strength at 1000°C of fibers from the γ′-containing eutectic alloy is related to nucleation of needles believed to be M₂₃C₆ on the monocarbide fibers during deformation. These needles appear to act as stress raisers and cause early failure of fibers.     

Activities of oxygen in liquid Cu-Sb and Cu-Ge alloys

In order to determine the activity coefficients of oxygen, γΩ in liquid Cu-Sb and Cu-Ge alloys at 1373 K as a function of alloy composition, the modified coulometric titrations, described previously, have been performed by using the galvanic cell: O in liquid Cu-Sb or Cu-Ge alloys/ZrO₂ (+CaO)/Air, Pt. A pronounced point of inflection in the In γΩvs alloy composition curve has been observed both for Cu-Sb and Cu-Ge alloys, as predicted by Jacob and Alcock’s quasichemical equation. The measured data itself, however, are significantly different from those predicted by their equation. The validity of Wagner’s solution model with one or two energy parameters has been also tested. YOSHIHIRO MATSUMURA, formerly a Graduate Student, Osaka University.     

Effect of melt spinning on the microstructure and mechanical properties of three Ni-base superalloys

A microstructural and mechanical analysis was performed on three rapidly solidified nickel base superalloys. Transmission and scanning electron microscopy along with tensile tests were performed on the ribbons in the as-cast and aged condition. This investigation permitted a correlation to be made between cooling rate, microstructure, and mechanical properties. It was found that melt spinning significantly altered the physical characteristics of the alloys studied. The rapid cooling rates (∼10⁷ K/s) produced ribbons with a low dislocation density and small (∼1 μm) low angle cells. The precipitation of γ′ was suppressed, producing alloys with a smaller volume fraction of precipitates and lower APBE than in the conventionally cast condition. Also, the matrix/precipitate lattice mismatch was higher in the melt spun foils. Tensile strengths were similar to those in the conventional form; however, no measurable ductility was present. Aging the ribbons resulted in increases in γ′ volume fraction, tensile strengths, APBE, and elongations compared to the as-cast ribbons. The results of this work suggest that many of the microstructural refinements produced by melt spinning are lost after a short aging time at moderate temperatures.     

The substitution of nickel for cobalt in hot isostatically pressed powder metallurgy UDIMET 700 alloys

Nickel was substituted in various proportions for cobalt in a series of five hot-isostatically-pressed powder metallurgy alloys based on the UDIMET 700 composition. These alloys were given 5-step heat treatments appropriate for use in turbine engine disks. The resultant microstructures displayed three distinct sizes of γ′ particles in a γ matrix. The higher cobalt-content alloys contained larger amounts of the finest γ′ particles, and had the lowest γ-γ′ lattice mismatch. While all alloys had approximately the same tensile properties at 25 and 650°C, the rupture lives at 650 and 760°C peaked in the alloys with cobalt contents between 12.7 and 4.3 pct. Minimum creep rates increased as cobalt contents were lowered, suggesting their correlation with the γ′ particle size distribution and the γ-γ′ mismatch. It was also found that, on overaging at temperatures higher than suitable for turbine disk use, the high cobalt-content alloys were prone to sigma phase formation.