Effect of thermomechanical treatments on the room-temperature mechanical behavior of iron aluminide Fe3AI

The room-temperature hydrogen embrittlement (HE) problem in iron aluminides has restricted their use as high-temperature structural materials. The role of thermomechanical treatments (TMT),i.e., rolling at 500 °C, 800 °C, and 1000 °C, and post-TMT heat treatments,i.e., recrystallization at 750 °C and ordering at 500 °C, in affecting the room-temperature mechanical properties of Fe-25A1 intermetallic alloy has been studied from a processing-structure-properties correlation viewpoint. It was found that when this alloy is rolled at higher temperature, it exhibits a higher fracture strength. This has been attributed to fine subgrain size (28/μ) due to dynamic recrystallization occurring at the higher rolling temperature of 1000 °C. However, when this alloy is rolled at 1000 °C and then recrystallized, it shows the highest ductility but poor fracture strength. This behavior has been ascribed to the partially recrystallized microstructure, which prevents hydrogen ingress through grain boundaries and minimizes hydrogen embrittlement. When the alloy is rolled at 1000 °C and then ordered at 500 °C for 100 hours, it shows the highest fracture strength, due to its finer grain size. The alloy rolled at 500 °C and then ordered undergoes grain growth. Hence, it exhibits a lower fracture strength of 360 MPa. Fracture morphologies of the alloy were found to be typical of brittle fracture,i.e., cleavage-type fracture in all the cases.     

Structure and high temperature mechanical behavior of Ni-Ni3Nb unidirectional eutectic

The structure and mechanical behavior of directionally solidified Ni-Ni₃Nb eutectic have been investigated. The lamellar eutectic grows with the Ni-Ni₃Nb interface semicoherent and, within experimental error, parallel to close-packed planes in both phases. When the temperature is lowered, supersaturation in the nickel-rich phase is relieved mostly by precipitation. Mechanical twinning is the typical deformation mode of the Ni₃Nb phase and is also observed in the nickel phase between 700∮ and 1000°. The unusually high ductility of the regular structure below 600° is related to the high strain hardening rate due to mechanical twinning of the intermetallic and the difficulty of crack propagation. Formerly Graduate Student, Ecole des Mines de Paris, Paris, France     

Effect of niobium on hot ductility of low C-Mn-steel under continuous casting simulation conditions

The effect of niobium on hot ductility of low C-Mn steel under continuous casting simulation conditions in the range from 700°C to 1 000°C was investigated on a Gleeble-1500 tester by using in-situ melting and solidification method. It was found that the ductility trough becomes deeper and wider as Nb content increases. The steel has more refined structure and is more sensitive to interdendritic fracture as Nb content gets higher. The segregation of impurities or alloy elements and formation of sulphides and Nb-compounds in the interdendritic regions promote the interdendritic fracture.     

Precipitation of an intermetallic phase with Pt2Mo-type structure in alloy 625

The microstructure of Alloy 625, which has undergone prolonged (∼70,000 hours) service at temperatures close to but less than 600 °C, has been characterized by transmission electron microscopy. The precipitation of an intermetallic phase Ni₂(Cr, Mo) with Pt₂Mo-type structure has been observed in addition to that of the γ″ phase. Six variants of Ni₂(Cr, Mo) precipitates have been found to occur in the austenite grains. These particles exhibit a snowflake-like morphology and are uniformly distributed in the matrix. They have been found to dissolve when the alloy is subjected to short heat treatments at 700 °C. The occurrence of the Ni₂(Cr, Mo) phase has been discussed by taking the alloy chemistry into consideration. Apart from the intermetallic phases, the precipitation of a M₆C-type carbide phase within the matrix and the formation of near continuous films, comprising discrete M₆C/M₂₃C₆ carbide particles, at the austenite grain boundaries have been noticed in the alloy after prolonged service.     

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.     

Grain growth behavior of cryomilled INCONEL 625 powder during isothermal heat treatment

Nanocrystalline INCONEL 625 powders were fabricated via cryomilling (mechanical alloying under a liquid nitrogen environment), and their grain growth behavior during isothermal heat treatment was investigated in detail. The grain size after milling for 8 hours was approximately 22 nm, measured by transmission electron microscopy (TEM) observations and X-ray diffraction (XRD). Along with this refined structure, the NiO and Cr₂O₃ oxide particles were distributed in the cryomilled material with average size of 3 nm. Following heat treatment at 800 °C, correspond to T/T _m = 0.65, for 4 hours, the grain size was approximately 240 nm, which represents an improved grain stability compared to that of conventional INCONEL 625 and cryomilled pure Ni. The improved grain stability of cryomilled INCONEL 625 is originated from a particle pinning effect by the oxide particles in addition to solute drag. The grain stability of the cryomilled powders at 900 °C was better than that at lower temperatures. This behavior was attributed to the formation of two types of secondary particles that precipitated at this temperature, which were identified as spherical NbC carbides and cylindrical-shaped Ni₃Nb intermetallic precipitates. These precipitates promote grain growth resistance at this particular temperature via a grain-boundary pinning effect. Contribution of 30 pct Nb solute atoms in alloy on the forming precipitates on grain boundary, the grain growth will be restricted to approximately 200 nm, on the basis of a Zener mechanism. This calculation is in qualitative agreement with the experimental results. The observation that precipitation kinetics were accelerated over those of conventional INCONEL 625 was rationalized on the basis of the shortened diffusion paths and more nucleation sites available in the nanocrystalline materials.     

Dissolution of iron intermetallics in Al-Si Alloys through nonequilibrium heat treatment

Conventional heat treatment techniques in Al-Si alloys to achieve optimum mechanical properties are limited to precipitation strengthening processes due to the presence of second-phase particles and spheroidization of silicon particles. The iron intermetallic compounds present in the microstructure of these alloys are reported to be stable, and they do not dissolve during conventional (equilibrium) heat treatments. The dissolution behavior of iron intermetallics on nonequilibrium heat treatment has been investigated by means of microstructure and mechanical property studies. The dissolution of iron intermetallics improves with increasing solution temperature. The addition of manganese to the alloy hinders the dissolution of iron intermetallics. Nonequilibrium heat treatment increases the strength properties of high iron alloys until a critical solution temperature is exceeded. Above this temperature, a large amount of liquid phase is formed as a result of interdendritic and grain boundary melting. The optimum solution treatment temperature for Al-6Si-3.5Cu-0.3Mg-lFe alloys is found to be between 515 °C and 520 °C.     

Strength and ductile-phase toughening in the two-phase Nb/Nb5Si3 alloys

The effect of heat treatment on the mechanical properties of Nb-Nb₅-Si₃ two-phase alloys having compositions Nb-10 and 16 pct Si (compositions quoted in atomic percent) has been investigated. This includes an evaluation of the strength, ductility, and toughness of as-cast and hot-extruded product forms. The two phases are thermochemically stable up to ∼1670 °C, exhibit little coarsening up to 1500 °C, and are amenable to microstructural variations, which include changes in morphology and size. The measured mechanical properties and fractographic analysis indicate that in the extruded condition, the terminal Nb phase can provide significant toughening of the intermetallic Nb₅Si₃ matrix by plastic-stretching, interface-debonding, and crack-bridging mechanisms. It has been further shown that in these alloys, a high level of strength is retained up to 1400 °C.     

Effect of the method of producing Ni3Al-based alloy single crystals on the macro- and microhomogeneity of component distribution,structure, and properties

The mechanisms of hardening heterophase Ni₃Al-based cast alloys, which are thermally stable natural eutectic composites, are studied in the operating temperature range. The distribution of basic and alloying elements and impurities in macrovolumes along the height of a charge billet prepared in a vacuum induction furnace is analyzed. The effect of the deviation of the axis of intermetallic alloy single crystals from the 〈111〉 orientation on their mechanical properties is considered. It is shown that the deviation from this orientation within 2.5°–5.4° does not affect the short-term strength characteristics and substantially affects the ductility characteristics of the single crystals. The effect of the method of introducing basic components and refractory reaction- and surface-active alloying elements in the alloys on the structure-phase state of Ni₃Al-based alloys and their service life is investigated.     

The effects of solution and intermediate heat treatments on the notch-rupture behavior of Inconel 718

The effects of solution and intermediate heat treatments on the notch stress-rupture properties of Inconel 718 were investigated using commercial quality bar stock. It was determined that a solution heat treatment of 1038°C for as short a time as 20 min could render this alloy notch-sensitive by solutioning extensive quantities of the orthorhombic Ni₃Cb delta (δ) phase. Restoration of notch ductility subsequent to a 1038°C (20 min) solution heat treatment was accomplished by the insertion of a 917°C (10 h) intermediate heat treatment, prior to aging, during which significant quantities of the δ phase were precipitated. This intermediate heat treatment was not successful in restoring notch ductility to specimens which had been given longer, 1 h, solution heat treatments at 1038°C. A 954°C (3 h) intermediate heat treatment was also not successful in restoring notch ductility to specimens solution heat treated for 20 min at 1038°C. It was confirmed that the most important factors controlling the notch-rupture ductility in Inconel 718 are the size, quantity and distribution of the δ phase. The grain size of the material did not directly control the notch properties. A portion of this work was performed under the auspices of the U.S. Atomic Energy Commission.     

Effect of Ti Addition and Microstructural Evolution on Toughening and Strengthening Behavior of as Cast or Annealed Nb–Si–Mo Based Hypoeutectic and Hypereutectic Alloys

Room temperature fracture toughness along with compressive deformation behavior at both room and high temperatures (900 °C, 1000 °C and 1100 °C) has been evaluated for ternary or quaternary hypoeutectic (Nb–12Si–5Mo and Nb–12Si–5Mo–20Ti) and hypereutectic (Nb–19Si–5Mo and Nb–19Si–5Mo–20Ti) Nb-silicide based intermetallic alloys to examine the effects of composition, microstructure, and annealing (100 hours at 1500 °C). On Ti-addition and annealing, the fracture toughness has increased by up to ~ 75 and ~ 63 pct, respectively with ~ 14 MPa√m being recorded for the annealed Nb–12Si–5Mo–20Ti alloy. Toughening is ascribed to formation of non-lamellar eutectic with coarse Nb_ss, which contributes to crack path tortuosity by bridging, arrest, branching and deflection of cracks. The room temperature compressive strengths are found as ~ 2200 to 2400 MPa for as-cast alloys, and ~ 1700 to 2000 MPa after annealing with the strength reduction being higher for the hypoeutectic compositions due to larger Nb_ss content. Further, the compressive ductility has varied from 5.7 to 6.5 pct. The fracture surfaces obtained from room temperature compression tests have revealed evidence of brittle failure with cleavage facets and river patterns in Nb_ss along with its decohesion at non-lamellar eutectic. The compressive yield stress decreases with increase in test temperature, with the hypoeutectic alloys exhibiting higher strength retention indicating the predominant role of solid solution strengthening of Nb_ss. The flow curves obtained from high temperature compression tests show initial work hardening, followed by a steady state regime indicating dynamic recovery involving the formation of low angle grain boundaries in the Nb_ss, as confirmed by electron backscattered diffraction of the annealed Nb–12Si–5Mo alloy compression tested at 1100 °C.

     

Microstructure and mechanical behavior of Cr-Cr2Hfin situ intermetallic composites

A detailed investigation of the effects of microstructural changes on the mechanical behavior of twoin situ intermetallic composites with Cr and Cr₂Hf phases in the Cr-Hf system was performed. The nominal compositions (at. pct) of the alloys were Cr-5.6Hf (hypoeutectic) and Cr-13Hf (eutectic). The study included evaluations of strength, ductility, and fracture toughness as a function of temperature and creep behavior. Two microstructures in each alloy were obtained by heat treatments at 1250 ‡C (fine microstructure) and 1500 ‡C (coarse microstructure). A decrease in elastic strength (stress at the onset of inelastic response in the load-deflection curve) with the coarsening of the microstructures was noted for both alloys below 1000 ‡C. The Cr-13Hf alloy retained strength to a higher test temperature, relative to Cr-5.6Hf alloy, under both microstructural conditions. The alloys showed no evidence of ductility at room temperature. However, in the coarse microstructure of the Cr-5.6Hf alloy, the primary Cr exhibited ductility at and above 200 ‡C; ductility in primary Cr could be seen only at and above 1000 ‡C for the fine microstructure. In other words, the temperature at which ductility was first observed decreased from about 1000 ‡C to about 200 ‡C due to high-temperature heat treatment in this alloy. Both microstructures of Cr-5.6Hf alloy showed a significant increase in fracture toughness with increasing test temperature. However, the increases in fracture toughness with temperature for the Cr-13Hf alloy microstructures were relatively small. Both alloys showed about four orders of magnitude reduction in steady-state creep rates relative to pure Cr at 1200 ‡C. The results are analyzed in the light of deformation characteristics and fracture micromechanisms. The effects of microstructural factors, such as the size and continuity of phases, solubility levels of Hf as well as interstitial elements in Cr, on the observed mechanical behavior are discussed. Formerly Research Scientist, Materials and Processes, UES, Inc.     

Effect of alloy preheating on the mechanical properties of as-cast Co-Cr-Mo-C alloys

The effect of various alloy preheatings followed by full solid solution treatments on the resultant strength and ductility of as-cast Co-Cr-Mo-C alloys was investigated. Three preheating temperatures were evaluated: 815 °C, 950 °C, and 1100 °C for 4 hours and then solid solution treated at 1225 °C for 4 hours. Tensile and compressive tests were carried out on the heat-treated alloys. It was found that the strength and ductility of the heat-treated alloys exhibited significant improvements over the as-cast condition. In particular, optimum ductility of the heat-treated alloys and alloy strength were promoted by preheating at 815 °C. A relatively fine grained structure coupled with a uniform distribution of second-phase particles promoted homogeneous plastic deformation in the bulk. Fractographic observations indicated that the exhibited ductility was associated with the development of numerous plastic bands combined with band interlockings. Alloy preheats at 950 °C and 1100 °C prior to solutionizing lead to inferior strength and ductility. Although preheating at 1100 °C led to slight improvements, in both cases, the fracture path was dominated by the presence of continuous carbide films surrounding the dendritic grains. Hence, less than optimum combinations of strength and ductility were achieved by the heat treatments at the higher temperatures.     

Effect of rare-earth element additions on high-temperature mechanical properties of AZ91 magnesium alloy

The present article focuses on the high-temperature mechanical properties of the magnesium alloy AZ91. The addition of rare-earth (RE) elements up to 2 wt pct improves both yield and tensile strengths at 140 °C by replacing the Mg₁₇Al₁₂ phase with RE-containing intermetallic compounds. This intermetallic phase is thermally and metallurgically stable and is expected to boost the grain-boundary strengthening. It also increases the resistance of grain boundaries to flow at high temperatures. Further increases of RE additions reduce strength and ductility due to growth of the Al₁₁RE₃ brittle phase, which has sharp edges. Still, at a 3 wt pct RE addition, the strength of the alloy at high temperatures is more than that of AZ91.     

Mechanical properties of a 29 Pct Cr-4 Pct Mo-2 Pct Ni ferritic stainless steel

The mechanical properties of a new ferritic stainless steel consisting essentially of 29 pct Cr, 4 pct Mo, 2 pct Ni (29-4-2) have been evaluated. The mechanical properties of the alloy are dependent on the thermomechanical processing and the final heat treatment conditionsi.e., both annealing temperature and cooling rate from the anneal. The alloy has excellent toughness, ductility and strength at room temperature when fast cooled from elevated temperatures. Slow cooling from elevated temperatures results in a degradation of impact resistance and an increase in strength. The alloy is subject to two major forms of embrittlement. One form results from the precipitation of intermetallic compounds in the temperature range 704°C (1300°F) to 954°C (1750°F) while the other results from the classical phenomenon called 475°C (885°F) embrittlement in the temperature range 399°C (750°F) to 510°C (950°F). Degradation of room temperature impact resistance occurs faster after the high temperature type of embrittlement and failure is characterized by an intergranular fracture mode. Embrittlement after exposure to 475°C (885°F) results in a slower degradation in toughness and results in failure by a transgranular cleavage mode. Impact resistance and tensile ductility are also decreased by exposure to 593°C (1100°F); however, to a lesser degree than 475°C (885°F) or 760°C (1400°F) exposure. The alloy deforms by slip or twinning depending on the metallurgical condition of the material. Deformation by twinning rather than slip is not manifested by a reduction in either toughness or ductility. Exposure to 482°C (900°F) promotes deformation by twinning whereas exposure to 760°C (1400°F) does not.     

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.     

Development of thermo-mechanical treatments of a maraging steel for yield strengths above 3 GPa

Hot-rolling of austenite and cold-rolling of martensite were combined with aging treatments to obtain new microstructures in a maraging steel with a high Mo-content. Purpose of the investigation is to acquire an optimum combination of ultra high yield stress (σ_r>3000 MPa) and ductility (or toughness). The best results were obtained by treatments consisting of high amounts of plastic deformation between 1000°C and 600°C (ausforming) and aging of the martensite which had formed from the deformed austenite. Ausaging around 550°C induced pronounced intercrystalline embrittlement. Intercritical heat treatment at 700°C provide considerable ductility (uniform elongation) however, insufficient strength (<2000 MPa). The most favourable thermo-mechanical treatments are discussed in context with fabrication methods for the investigated steel.     

Tensile properties and microstructure of haynes 25 alloy after aging at elevated temperatures for extended times

The tensile properties of Haynes 25 alloy have been measured after various aging treatments, time, and temperature: as received; and aged at 600 °C for three months; 800 °C for 6 and 12 months; and 1000 °C for 3 and 6 months. Contour plots in temperature-ln (time) space were constructed based on the literature and our own data, detailing changes in yield strength, ultimate tensile strength, and tensile elongation. Scanning electron microscopy and transmission electron microscopy observations of the Haynes 25 alloy microstructure provided an explanation of why the properties changed with aging. Intense lattice distortions after aging at 600 °C, the presence of an α-Co₃W, a L1₂-ordered, fcc phase, a=0.357 nm, after aging at 800 °C, and the nucleation and growth of W₃Co₃C carbides from aging at 800 °C and 1000 °C produced the changes in tensile properties. We did not observe either the Co₂W Laves phase or Co₇W₆ γ phase in any of the material conditions we examined, using TEM of thin foils: as received and aged at 600 °C, 800 °C, and 1000 °C. Other researchers believe these phases cause a loss of ductility in the Haynes 25 alloy with prolonged high-temperature exposure.     

The mechanical response of the Ni−Ni3Nb eutectic composite: Part I. Monotonic behavior

To understand the mechanical behavior of the Ni?Ni₃Nb eutectic composite, it was necessary to determine the operative deformation and fracture mechanisms in the Ni₃Nb intermetallic phase. It was found that Ni₃Nb deforms primarily by twinning along {112} planes and {011} planes when tension and compression, respectively, are applied parallel to the [100] growth direction. The {112} twins were observed to serve as crack nucleation sites with cracks forming along the twin boundaries. The monotonic response of the Ni?Ni₃Nb eutectic composite was investigated with tension and compression tests, metallography, and electron fractography. Room temperature tensile testing of the Ni?Ni₃Nb composite revealed this material to be capable of sustaining tensile strains in excess of 11 pct. This large composite ductility was associated with extensive {112} twinning of the Ni₃Nb lamellae and subsequent twin boundary cracking. When amassed in sufficient numbers in a given cross-section, these {112} twin boundary fissures initiated composite rupture. The room temperature ultimate tensile and compressive strengths of the alloy were found to be 109 and 235 ksi, respectively.     

The balance of mechanical and environmental properties of a multielement niobium-niobium silicide-basedIn Situ composite

This article describes room-temperature and high-temperature mechanical properties, as well as oxidation behavior, of a niobium-niobium silicide basedin situ composite directionally solidified from a Nb-Ti-Hf-Cr-Al-Si alloy. Room-temperature fracture toughness, high-temperature tensile strength (up to 1200 °C), and tensile creep rupture (1100 °C) data are described. The composite shows an excellent balance of high- and low-temperature mechanical properties with promising environmental resistance at temperatures above 1000 °C. The composite microstructures and phase chemistries are also described. Samples were prepared using directional solidification in order to generate an aligned composite of a Nb-based solid solution with Nb₃Si- and Nb₅Si₃-type silicides. The high-temperature mechanical properties and oxidation behavior are also compared with the most recent Ni-based superalloys. This composite represents an excellent basis for the development of advanced Nb-based intermetallic matrix composites that offer improved properties over Ni-based superalloys at temperatures in excess of 1000 °C.