Study of Metallic Carbide (MC) in a Ni-Co-Cr-Based Powder Metallurgy Superalloy

The evolution of carbides in a Ni-Cr-Co-based powder metallurgy (PM) superalloy in the as-atomized, as-atomized + annealed, hot isostatic pressed (HIPed) and HIPed + annealed conditions were systematically analyzed to understand the formation of blocky metallic carbide (MC) along the previous particle boundary (PPB). The results show that the carbides both on the powder surfaces and in the bulk of the powder particles are mainly fan-shaped MC whose decomposition temperatures are in the range of 1473 K to 1493 K (1200 °C to 1220 °C). PPB carbides in the HIPed alloy are mainly block-shaped MC, and the fan-shaped MC densely distributed in the area that have not been consumed by the recrystallized grains. The formation mechanism of PPB carbides can be described as follows: When the powders are HIPed at 1453 K (1180 °C), the fan-shaped carbides are decomposed at the migrating boundaries of recrystallized grains, and the preferential precipitation of block-shaped MC at PPB is promoted by the carbide-forming elements released by the fan-shaped carbides. When the HIPed alloy is annealed at 1453 K (1180 °C), the area fraction of PPB carbides increases with an increase in annealing time but that of the fan-shaped carbides exhibits opposite behavior. This proves the above formation mechanism of PPB carbides.     

Preservation of Geometrical Integrity of Supersolidus-Liquid-Phase-Sintered SKD11 Tool Steels Prepared with Powder Injection Molding

The powder injection molded SKD11 tool steels often manifest shape retention problems during supersolidus liquid phase sintering due to the difficulties in controlling the amount of liquid phase. The typical temperature range for the sintering of SKD11 is only 10 K, between 1503 and 1513 K (1230 and 1240 °C), and this narrow sintering range demands a special furnace with very uniform temperature distribution. Through the addition of carbides, in particular TiC, this problem is resolved by enlarging the liquid + γ + carbide region in the phase diagram and by impeding the grain growth with the carbides. The resulting sintering window is broadened to 40 K, between 1513 and 1553 K (1240 and 1280 °C). The relevant mechanisms on the improvement of shape retention are discussed with a focus on the effect of carbide addition on the changes in the phase diagram and the microstructure. A guideline for the selection of effective carbides is also proposed based on the experimental results and the phase diagram analyses.     

Application of Computational Thermodynamics to the Design of a Co-Ni-Based γ′-Strengthened Superalloy

A currently available commercial Calphad thermodynamic database was utilized to investigate its applicability to alloy design in the new class of Co-Ni-based γ′-strengthened high-temperature alloys. A simple primary design criterion was chosen: maximize the γ′ solvus temperature in the six-component Co-Ni-Al-Ti-W-Ta system while ensuring no formation of secondary, potentially deleterious phases. Secondary design considerations included the effects of alloying elements on equilibrium γ′ volume fraction and on solidus and liquidus temperatures. The identified composition, Co-30Ni-9Al-3Ti-7W-2Ta-0.1B (expressed in mole percent), representing a conservative estimate of the maximum allowable concentrations of alloying additions Al, Ti, W, and Ta, was subsequently produced and characterized. The experimentally measured γ′ solvus temperature of the new alloy was 1491 ± 3 K (1218 ± 3 °C), about 35 K (35 °C) above any previously reported two-phase γ−γ′ Co-(Ni)-based alloy. No secondary phases were observed in the alloy after annealing at temperatures between 1173 K and 1473 K (900 °C and 1200 °C). Additional alloy compositions with experimentally measured γ′ solvus temperatures in excess of 1533 K (1260 °C) were also identified employing the same basic approach. The efficacy of currently available thermodynamic databases in their application to Co-based γ′-strengthened superalloy development is discussed, including expanding design efforts to include additional alloying elements, as well as specific areas for improvement of future databases.

     

Processing and Characterization of Cu-Al-Ni Shape Memory Alloy Strips Prepared from Prealloyed Powder by Hot Densification Rolling of Powder Preforms

The present work deals with the preparation of near-full density Cu-Al-Ni shape memory alloy (SMA) strips from argon-atomized prealloyed powder via a powder metallurgy (PM) route comprising cold die compaction to prepare powder preforms, sintering, and hot densification rolling of unsheathed sintered powder preforms under protective atmosphere at 1273 K (1000 °C). It has been shown that argon-atomized spherical Cu-Al-Ni SMA powder consisted of very fine equiaxed grains and no appreciable grain growth occurred during sintering at 1273 K (1000 °C). It also has been shown that no appreciable densification occurred during sintering, and densification was primarily achieved by hot rolling. The densification behavior of the sintered powder preforms during hot rolling was discussed. The hot-rolled Cu-Al-Ni strips were heat-treated at 1223 K (950 °C) for 60 minutes and water quenched. The heat-treated strips consisted of equiaxed grains with average size approximately 90 μm. The heat-treated Cu-Al-Ni SMA strips consisted of self-accommodated b₁^￠ \beta_{1}^{'} martensite primarily, and showed smooth b₁ T b₁^￠ \beta_{1} \Rightarrow \beta_{1}^{'} transformation behavior coupled with a very low hysteresis (≈25 K (25 °C)). The heat-treated strips exhibited an extremely good combination of mechanical properties with fracture strength of 530 MPa and 12.3 pct fracture strain. The mode of fracture in the finished strip was primarily void-coalescence-type ductile together with some brittle transgranular type. The shape memory tests showed almost 100 pct one-way shape recovery after 100 bending-unconstrained heating cycles at 4 pct applied prestrain, exhibiting good stability of Cu-Al-Ni strips under thermomechanical actuation cycling. The two-way shape memory strain was found approximately 0.45 pct after 15 training cycles at 4 pct training strain.     

粉末粒度和氧含量对HIP态FGH96合金组织的影响

对氩气雾化法制备的高温合金FGH96粉末进行了热等静压(HIP)处理,分析了粉末粒度和氧含量对HIP态合金组织的影响,研究了FGH96合金组织中PPB的类型、相结构和形成机制。结果表明,氩气雾化FGH96粉末的氧含量较低,平均氧含量约为50×10-6,随着粉末粒度降低,颗粒比表面积增大,促进了粉末氧含量的升高;粉末经HIP处理后氧含量具有遗传特征,原始粉末氧含量越高,HIP态合金氧含量也越高,且平均氧含量增至83×10-6;粉末尺寸和氧含量对合金致密化行为无明显影响,HIP态合金密度约为8.33 g·cm-3。小尺寸粉末制备的HIP态合金原始颗粒边界主要析出ZrO2和MC碳化物,而大尺寸粉末制备的HIP态合金原始颗粒边界主要析出大尺寸花瓣状γ’相和少量MC碳化物。粉末粒度和氧含量影响PPB析出,小尺寸粉末因氧含量高经HIP处理时颗粒边界处存在更多、尺寸更大稳定的ZrO2,ZrO2成为MC碳化物析出形核的核心,促进了大量MC碳化物的析出。     

Stress Rupture Fracture Model and Microstructure Evolution for Waspaloy

Stress rupture behavior and microstructure evolution of nickel-based superalloy Waspaloy specimens from tenon teeth of an as-received 60,000-hour service-exposed gas turbine disk were studied between 923 K and 1088 K (650 °C and 815 °C) under initial applied stresses varying from 150 to 840 MPa. Good microstructure stability and performance were verified for this turbine disk prior to stress rupture testing. Microstructure instability, such as the coarsening and dissolution of γ′ precipitates at the varying test conditions, was observed to be increased with temperature and reduced stress. Little microstructure variation was observed at 923 K (650 °C). Only secondary γ′ instability occurred at 973 K (700 °C). Four fracture mechanisms were obtained. Transgranular creep fracture was exhibited up to 923 K (650 °C) and at high stress. A mixed mode of transgranular and intergranular creep fracture occurred with reduced stress as a transition to intergranular creep fracture (ICF) at low stress. ICF was dominated by grain boundary sliding at low temperature and by the nucleation and growth of grain boundary cavities due to microstructure instability at high temperature. The fracture mechanism map and microstructure-related fracture model were constructed. Residual lifetime was also evaluated by the Larson–Miller parameter method.     

Influence of hot isostatic pressing on microstructure and mechanical behaviour of nanostructured Al alloy

Abstract

Aluminium alloy AA 5083 [Al–4·4Mg–0·7Mn–0·15Cr (wt-%)], powder was ball milled in liquid nitrogen via the cryomilling method to obtain a nanocrystalline (NC) structure. Samples of the powder were hot vacuum degassed to remove interstitial contaminants, then consolidated by hot isostatic pressing (HIPing) at six temperatures (from 0·46T_m to 0·89T_m), before being high strain rate forged (HSRF) to produce plate material. The microstructure was analysed at the different processing stages. The compressive properties of the as HIPed material, plus tensile properties of the final product were studied. Despite grain growth during HIPing, an ultrafine grain (UFG) structure was retained in the consolidated material, which consequently had increased strength over conventionally processed AA 5083. As the HIP temperature was increased, the density increased. Strength changes were minimal in compression and tension with varying HIP temperature, once near full density was attained at 275°C (～0·64T_M). Yield strength data indicate negligible variation in the grain size of the materials.     

Influence of Thermal Aging on the Microstructure and Mechanical Behavior of Dual-Phase, Precipitation-Hardened, Powder Metallurgy Stainless Steels

The effects of thermal aging on the microstructure and mechanical behavior of dual-phase, precipitation-hardened, powder metallurgy (PM) stainless steels of varying ferrite–martensite content were examined. Quantitative analyses of the inherent porosity and phase fractions were conducted on the steels, and no significant differences were noted with respect to aging temperature. Tensile strength, yield strength, and elongation to fracture all increased with increasing aging temperature reaching maxima at 811 K (538 °C) in most cases. Increased strength and decreased ductility were observed in steels of higher martensite content. Nanoindentation of the individual microconstituents was employed to obtain a fundamental understanding of the strengthening contributions. Both the ferrite and martensite nanohardness values increased with aging temperature and exhibited similar maxima to the bulk tensile properties.     

Tensile Properties and Deformation Characteristics of a Ni-Fe-Base Superalloy for Steam Boiler Applications

Ni-Fe-base superalloys due to their good manufacturability and low cost are the proper candidates for boiler materials in advanced power plants. The major concerns with Ni-Fe-base superalloys are the insufficient mechanical properties at elevated temperatures. In this paper, tensile properties, deformation, and fracture characteristics of a Ni-Fe-base superalloy primarily strengthened by γ′ precipitates have been investigated from room temperature to 1073 K (800 °C). The results showed a gradual decrease in the strength up to about 973 K (700 °C) followed by a rapid drop above this temperature and a ductility minimum at around 973 K (700 °C). The fracture surfaces were studied using scanning electron microscopy and the deformation mechanisms were determined by the observation of deformed microstructures using transmission electron microscopy. An attempt has been made to correlate the tensile properties and fracture characteristics at different temperatures with the observed deformation mechanisms.     

High-Temperature Creep Deformation and Fracture Behavior of a Directionally Solidified Ni-Base Superalloy DZ951

The high-temperature creep deformation and fracture behavior of a directionally solidified Ni-base superalloy DZ951 have been investigated over a wide stress range of 110 to 880 MPa at high temperatures (700 °C to 1000 °C). In this article, the detailed creep deformation and fracture mechanism have been studied. The results show that the creep curves exhibit strong temperature dependence. From transmission election microscopy (TEM) observations, it is suggested that the deformation mechanism is temperature dependent and mainly consists of three dislocation-controlling mechanisms: stacking faults and dislocation-pair shearing, dislocation bowing, and dislocation climbing. It is found that the fracture mode of DZ951 alloy changes from cleavagelike fracture at low temperature to ductile fracture at high temperature. At 700 °C, the creep cracks mainly initiate at the surface and propagate along the cleavagelike facets. With increasing temperature, cracks can initiate at the surface, carbide/matrix interface, and cast pore. The growth of microcrack has a direction perpendicular to the stress direction. The creep-rupture data follow the Monkman–Grant relationship in different temperature regions.     

High temperature-high strain rate fracture of inconel 600

The hot fracture of Inconel 600 has been studied over the temperature range from 800° to 2000°F using a hot torsion tester that is capable of superimposing either axial tensile or compressive stresses on the torsional shearing stresses. Microscopic studies of fracture initiation have been made over the entire temperature region. From 800° to 1200°F fracture initiates at inclusions and propagates by transgranular shear. In the temperature region of minimum ductility, 1300° to 1500°F, fracture initiates at grain boundaries and propagates readily in an intergranular manner. At 1600°F and above, fracture initiates easily at grain boundaries, but because recrys-tallization intervenes crack propagation is difficult and strain to fracture is high. Microcracks initiate at the peak in the torque-twist curve. The higher the temperature the smaller is the strain at which fracture initiates. Correlations have been found between the stress state and the shearing strain at crack initiation and total fracture strain. These correlations show the strong influence of a compressive normal stress on retarding crack initiation and resisting crack propagation.     

The Fracture of Ordered (Fe,Co)3V

Observations have been made of the fracture surfaces of ordered (Fe₂₂Co₇₈)₃V deformed at low strain rates (3.3 and 42 x 10^-3 s^-1) in tension at temperatures from 20 to 1000 °C, that is, above and below the order/disorder transformation temperature of 950 ± 10 °C. From 20 to 700 °C, transgranular fracture occursvia the nucleation, growth, and coalescence of micro voids; correspondingly, high ductilities are obtained (34 to 44 pct). From 700 to 960 °C, the fracture mode changes from transgranular to intergranular and the ductility falls to a minimum of 4 pct. At 1000 °C a third mode operates, consisting of localized dynamic recrystallization followed by boundary sliding of recrystallized grains; ductility now rises sharply. The underlying mechanisms are considered and are supported through the results of additional experiments performed at distinctly higher strain rates.     

Comparison of fatigue deformation and fracture in a dispersion-strengthened and a conventional nickel-base superalloy

INCONEL alloy MA 753 is a dispersion strengthened nickel-base superalloy made by mechanical alloying which combines γ’ precipitation hardening and yttria dispersion strengthening with good oxidation and sulfidation resistance. At temperatures up to 1227 K (1750°F), the fatigue strength of MA 753 is greater than that of a conventional wrought superalloy which has a composition close to that of the MA 753 matrix. Fatigue strength at elevated temperatures is strongly dependent on testing frequency. This behavior is correlated with the strain rate dependence of tensile strength. Fatigue crack initiation sites and propagation modes in MA 753 are discussed as a function of temperature and microstructure and compared to those in the conventional superalloy. The transition from transgranular to intergranular fracture mode in MA 753 occurs at a higher temperature than found in conventional nickel-base superalloys. While the γ’ precipitate controls the fatigue strength at low and intermediate temperatures, the oxide dispersoid and carbides also affect deformation in this temperature range. At elevated temperatures, fatigue deformation is controlled by the dispersoid and carbides. Trademark of The International Nickel Company, Inc.     

Comparison of fatigue deformation and fracture in a dispersion-strengthened and a conventional nickel-base superalloy

INCONEL alloy MA 753 is a dispersion strengthened nickel-base superalloy made by mechanical alloying which combines γ’ precipitation hardening and yttria dispersion strengthening with good oxidation and sulfidation resistance. At temperatures up to 1227 K (1750°F), the fatigue strength of MA 753 is greater than that of a conventional wrought superalloy which has a composition close to that of the MA 753 matrix. Fatigue strength at elevated temperatures is strongly dependent on testing frequency. This behavior is correlated with the strain rate dependence of tensile strength. Fatigue crack initiation sites and propagation modes in MA 753 are discussed as a function of temperature and microstructure and compared to those in the conventional superalloy. The transition from transgranular to intergranular fracture mode in MA 753 occurs at a higher temperature than found in conventional nickel-base superalloys. While the γ’ precipitate controls the fatigue strength at low and intermediate temperatures, the oxide dispersoid and carbides also affect deformation in this temperature range. At elevated temperatures, fatigue deformation is controlled by the dispersoid and carbides.     

Contributors

Abstract

The chemistry of a high performance cast superalloy, ZhS6–K (Ni–10Cr–5Co–5W–5Al–3·5Mo–3Ti–0·2C–0·02B), was modified by slight reductions in carbon, titanium, and aluminium content and minor additions of niobium and hafnium. Two variants of the modified alloy chemistry with different boron contents (0·02 and 0·08 wt–%) were prepared by vacuum induction melting, argon atomization, and consolidation by hot isostatic pressing at three temperatures. It was observed that, unlike carbon, an increase in boron content did not promote the formation of continuous precipitates at the prior powder particle boundaries. Increased boron content narrowed down the consolidation temperature range and changed the morphology of γ′ particles from cuboidal to dendritic. Precipitation of an eutectic γ + γ′ structure and formation of continuous boride films at the grain boundaries severely degraded the mechanical properties of the high boron PM superalloy that was consolidated at a temperature marginally above the γ′ solvus. An optimum consolidation schedule was determined for the high boron alloy, which after a suitable heat treatment produced significant property improvement in stress rupture and tensile properties. PM/0416     

Thermal and Irradiation Creep Behavior of a Titanium Aluminide in Advanced Nuclear Plant Environments

Titanium aluminides are well-accepted elevated temperature materials. In conventional applications, their poor oxidation resistance limits the maximum operating temperature. Advanced reactors operate in nonoxidizing environments. This could enlarge the applicability of these materials to higher temperatures. The behavior of a cast gamma-alpha-2 TiAl was investigated under thermal and irradiation conditions. Irradiation creep was studied in beam using helium implantation. Dog-bone samples of dimensions 10 × 2 × 0.2 mm³ were investigated in a temperature range of 300 °C to 500 °C under irradiation, and significant creep strains were detected. At temperatures above 500 °C, thermal creep becomes the predominant mechanism. Thermal creep was investigated at temperatures up to 900 °C without irradiation with samples of the same geometry. The results are compared with other materials considered for advanced fission applications. These are a ferritic oxide-dispersion-strengthened material (PM2000) and the nickel-base superalloy IN617. A better thermal creep behavior than IN617 was found in the entire temperature range. Up to 900 °C, the expected 10⁴ hour stress rupture properties exceeded even those of the ODS alloy. The irradiation creep performance of the titanium aluminide was comparable with the ODS steels. For IN617, no irradiation creep experiments were performed due to the expected low irradiation resistance (swelling, helium embrittlement) of nickel-base alloys.     

Notched Tensile and Impact Fracture of Ti-15-3 Laser Welds

The notched tensile strength (NTS) and impact toughness of Ti-15V-3Cr-3Sn-3Al (β-type titanium alloy Ti-15-3) laser welds aged at temperatures ranging from 590 K to 866 K (317 °C to 593 °C) were determined, and the results were compared to those of unwelded Ti-15-3 plates aged at the same temperature. At a given aging temperature, α precipitates in welded specimens were finer and exhibited higher hardness than those in unwelded specimens. Among the tested specimens, the weld aged at 644 K (371 °C) was most susceptible to notch sensitivity. In those welds aged at or above 755 K (482 °C), the coarse columnar structure was prone to interdendritic fracture during notched tensile tests, which reduced the NTS of the weld relative to that of the unwelded plate aged at an equivalent temperature. Of the tested specimens, the weld that was not subjected to the postweld aging treatment possessed the highest impact toughness among the specimens.     

Effect of Boron Precipitation Behavior on the Hot Ductility of Boron Containing Steel

The effect of boron (B) precipitation behavior on the hot ductility of B containing steel was investigated. Hot ductility of B containing steel was sensitive to the cooling rate (CR) in the range of 1 to 20 K/s (1 to 20 °C/s), whereas that of B-free steel showed little change with CR. Increased CR causes deepening and widening of the ductility trough in B containing steel. Particle tracking autoradiography (PTA) analysis and transmission electron microscope (TEM) image of the samples show that boron nitride (BN) particles form along prior austenite grain boundaries, and that as CR increases, these particles become smaller and more numerous. This increase in the number of small BN precipitates may promote intergranular fracture, leading to a decrease in hot ductility in the lower austenite temperature region (1173 to 1273 K (900 to 1000 °C)). Furthermore, the formation of filmlike ferrite at ~1123 K (850 °C) causes a decrease in the hot ductility of this steel regardless of B addition and CR.     

FGH4102粉末高温合金等温热模拟压缩试验及动态再结晶组织研究

本文研究了新型第四代粉末高温合金FGH4102在等温热模拟压缩过程中的组织演变,对γ′相在动态再结晶过程中的作用进行了探讨。结果表明,热等静压态合金在1 060～1 120℃温度范围变形时,热加工性能较好。1 140℃变形后试样容易发生开裂,合金热加工性能较差。合金在γ+γ′两相区变形时均发生了不同程度的动态再结晶,再结晶晶粒尺寸远小于热等静压态的晶粒尺寸。变形过程中,尺寸较大的γ′相起到促进动态再结晶的作用。变形参数对动态再结晶的影响非常显著。低温高应变速率变形时,γ′相促进动态再结晶形核占主导地位,再结晶晶粒比较细小;高温低应变速率变形时,晶粒长大逐渐占据主导地位,再结晶晶粒尺寸较大。     

The Origin of Microstructural Diversity,Texture, and Mechanical Properties in Electron Beam Melted Ti-6Al-4V

An additive layer manufacture (ALM) technique, electron beam melting, has been used for the production of simple geometries, from prealloyed Ti-6Al-4V powder. Microstructure, texture, and mechanical properties achieved under standard operating conditions have been investigated. Three transitional regions are observed with a change in microstructural formation dependent on the thermal mass of deposited material. Prior β-phase reconstruction, from room temperature α-phase electron backscatter diffraction (EBSD) data, reveals a strong texture perpendicular to the build axis. Variation of build temperature within the processing window of 898 K to 973 K (625 °C to 700 °C) is seen to have a significant effect on the properties and microstructure of both as-deposited and hot isostatically pressed (HIP) samples.