Superplastic behavior of a powder metallurgy TiAl alloy with a metastable microstructure

Superplasticity in a powder metallurgy (P/M) TiAl alloy (Ti–47Al–2Cr–1Nb–1Ta) with a metastable B2 phase coexisted with a fine-grained γ+α₂ duplex structure has been studied. Alloy samples were tested at temperatures ranging from 650 to 1100°C, and at strain rates ranging from 10⁻⁶ to 10⁻⁴ s⁻¹. An elongation of over 300% was obtained at a strain rate of 2×10⁻⁵ s⁻¹ and at a temperature of 800°C, which is close to the ductile-to-brittle transition temperature of the alloy. This is in contrast to the prior observations of superplastic behavior of TiAl alloys in which a typical temperature of 1000°C is usually required for achieving superplasticity. It is suggested that the occurrence of low-temperature (800°C) superplasticity in the present alloy is primarily due to the presence of a metastable B2 phase in addition to a fine-grained (α₂+γ) duplex microstructure. The metastable B2 phase continues to decompose into fine-grained α₂ and γ phases, which promotes grain boundary sliding during superplastic deformation. The retained fine B2 grains accommodate the sliding strains to reduce the propensity of cavitation at grain triple junctions and thus delay the cavitation and fracture process.     

Microstructural evolution and mechanical properties of as-cast and directionally-solidified Nb-15Si-22Ti-2Al-2Hf-2V-(2, 14) Cr alloys at room and high temperatures

Arc-melting (AC) and directional solidification (DS) techniques were used to prepare Nb-15Si-22Ti-2Al-2Hf-2V-(2, 14) Cr alloys (hereafter referred as to 2Cr and 14Cr alloys, respectively), and the microstructural evolution and mechanical properties, including Vickers hardness, room temperature fracture toughness and high temperature strength, of the two AC and DS alloys were compared. The results showed that with heat-treatment at 1350 °C for 50 h, the AC-2Cr alloy composed of Nb solid solution (Nb_SS) and α-Nb₅Si₃ silicide, while Laves C15-Cr₂Nb phase arose in the 14Cr alloy. With two-phase Nb_SS/α-Nb₅Si₃ microstructure, the AC-2Cr alloy showed excellent room-temperature fracture toughness (K_Q: 14.2 MPa m^1/2) and 0.2% yield strength at 1250 °C (σ_0.2: 315 MPa) and 1350 °C (σ_0.2: 294 MPa), better than the AC-14Cr alloy with tri-phase Nb_SS/α-Nb₅Si₃/C15-Cr₂Nb microstructure (K_Q: 9.4 MPa m^1/2, σ_0.2: 189 MPa at 1250 °C and 87 MPa at 1350 °C). The DS technique was found not to change the phase constituent of each alloy, but it made the microstructure slightly orient to the growth direction, resulting in a significant improvement in room-temperature fracture toughness (by ∼43%) and high-temperature yield strength σ_0.2 (by ∼55%), as compared with the AC samples.     

The oxidation behavior of high Cr and Al containing Nb-Si-Ti-Hf-Al-Cr alloys at 1200 and 1250 °C

The oxidation behavior of two alloys containing different content of Al and Cr from the Nb-Si-Ti-Hf-Al-Cr system has been evaluated at 1200 and 1250 °C. The alloy compositions in atomic percent are Nb-24Ti-16Si-2Hf-2Al-10Cr (B1), and Nb-24Ti-16Si-2Hf-6Al-17Cr (B2). The oxidation kinetic of B1 alloy at 1200 and 1250 °C followed a mixed parabolic-linear law, while the oxidation kinetic of B2 alloy at 1200 and 1250 °C followed a parabolic law. The weight gain of B2 alloy was 18.9 mg/cm² after oxidation at 1200 °C for 100 h, which was a seventh of the value of that of B1 alloy. Besides, oxidation became more severe as temperature increased to 1250 °C. The oxide scales of B2 alloy consisted of CrNbO₄, TiNb₂O₇ and SiO₂, which were relatively compact and protective. In addition, the oxidation mechanism of Nb-Si based alloys were also discussed.     

Beneficial Effects of Ce Implantation into Preformed Cr2O3 Scales on the Subsequent Oxidation of Ni–20Cr Alloy

The influence of Ce implantation into preformed Cr₂O₃ scales with a dose of 1 × 10¹⁷ ions/cm² on the subsequent oxidation behavior of Ni–20Cr alloy at 1050°C in air has been investigated. The pre-oxidation was carried out at 1050°C in air for 0.5 and 1 hr respectively Cr₂O₃ and NiCr₂O₄ formed on Ni–20Cr alloy. The oxidation rate was decreased remarkably due to Ce implantation regardless of whether it was implanted into the alloy or into the pre-formed oxide scales, and the beneficial effect decreased with increasing pre-oxidation time, the alloy implanted directly with Ce had the lowest oxidation rate constant. During cyclic oxidation (350 cycles) Ce implantation played a similar benefical effect on the oxide-spallation resistance for blank and pretreated alloys. The result indicates that Ce incorporated into the oxide scale affected the diffusion of the reaction species and also the spallation resistance of the oxide scales. The change of the oxidation process is attributed to the segregation of Ce at the oxide grain boundaries     

High temperature oxidation characteristics of Nb–10W–XCr alloys

The effect of Cr content on the microstructure and cyclic oxidation behavior of Nb–10W–XCr alloys with four different compositions has been investigated. Experiments were conducted in air at 900 °C and 1300 °C; the oxidation kinetics have been evaluated in terms of weight change per unit area with respect to exposure time. Alloy's microstructure consists of Nb solid solution phase regions surrounded by a network of NbCr₂ Laves phase. A trend of improvement in oxidation resistance with increase of the intermetallic phase is observed at 1300 °C and oxidation kinetics follow a parabolic behavior. At 900 °C, alloys with higher Cr content exhibit higher oxidation rates than alloys with lower Cr content. The oxidation products are a mixture of CrNbO₄, and Nb₂O₅ and the amount of each oxide present in the mixture is related to the intermetallic phase content. Results delineate the influence of microstructure and composition on the oxidation mechanisms of these alloys that represent a promising base for high-temperature intermetallic alloy development.     

Evolution of microstructure,hardness, and corrosion properties of high-entropy Al0.5CoCrFeNi alloy

In this study, we investigate the microstructure, hardness, and corrosion properties of as-cast Al_0.5CoCrFeNi alloy as well as Al_0.5CoCrFeNi alloys aged at temperatures of 350 °C, 500 °C, 650 °C, 800 °C, and 950 °C for 24 h. The microstructures of the various specimens are investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe X-ray microanalysis (EPMA). The results show that the microstructure of as-cast Al_0.5CoCrFeNi comprises an FCC solid solution matrix and droplet-shaped phases (Al–Ni rich phases). At aging temperatures of between 350 and 950 °C, the alloy microstructure comprises an FCC + BCC solid solution with a matrix, droplet-shaped phases (Al–Ni rich phase), wall-shaped phases, and needle-shaped phases (Al–(Ni, Co, Cr, Fe) phase). The aging process induces a spinodal decomposition reaction which reduces the amount of the Al–Ni rich phase in the aged microstructure and increases the amount of the Al–(Ni, Co, Cr, Fe) phase. The hardness of the Al_0.5CoCrFeNi alloy increases after aging. The optimal hardness is obtained at aging temperatures in the range 350–800 °C, and the hardening effect decreases at higher temperatures. Both the as-cast and aged specimens are considerably corroded when immersed in a 3.5% NaCl solution because of the segregation of the Al–Ni rich phase precipitate formed in the FCC matrix. Cl^? ions preferentially attack the Al–Ni rich phase, which is a sensitive zone exhibiting an appreciable potential difference, with consequent galvanic action.     

粉末冶金态与铸态Ti-5553合金高温不连续屈服行为和绝热温升效应对比研究

本文系统地研究了粉末冶金态与铸态Ti-5553合金在温度为700 ℃~1100 ℃、应变速率为0.001 s_^-1~10 s_^-1条件下的高温不连续屈服行为和绝热温升效应,并对这两种同名义成分不同制备工艺的钛合金进行了对比研究。结果表明：两种合金不连续屈服的幅度均与应变速率呈正相关关系,并与温度呈近似负相关关系, 两种合金中出现的不连续屈服现象符合动态理论。在相同变形条件下,铸态合金中不连续屈服的幅度更大,其原因是相对于粉末冶金态合金,铸态合金中的起始位错密度低,这更有利于晶界处可动位错的突然增殖与扩展。两种合金在热变形中绝热温升的大小均随应变速率的升高而逐渐增大,并随着变形温度的升高而逐渐降低。在相同变形条件下,粉末冶金态合金的绝热温升效应相比与铸态合金较弱,这是因为粉末冶金态合金具有较低的变形抗力和较高的协调变形能力。     

Tensile properties of hot rolled Mg_(97)Zn_1Y_2 alloy sheets at elevated temperatures

The elevated temperature tensile properties of Mg97Zn1Y2 magnesium alloy sheets, hot rolled at 390, 420 and 450 ℃ respectively, were tested in a temperature range from room temperature to 250 ℃ with a strain rate of 1.0×10-3 s-1. The results show that the variations in yield strength for Mg97Zn1Y2 magnesium alloy sheets hot rolled at 390 ℃ and 420 ℃ with temperature resemble each other due to their similar morphology of the chain-shaped strengthening phase. The yield strength maintains at a high level of 283 MPa before 200 ℃ and decreases significantly at 250 ℃. Despite of the fine lamellar structure of Mg97Zn1Y2 magnesium alloy sheet hot rolled at 450 ℃, its yield strength decreases linearly owing to occurrence of the coarse grain, and drops to 239 MPa at 250 ℃. The elongation for all hot rolled Mg97Zn1Y2 magnesium alloy sheets increases slightly with increasing testing temperature.     

Microstructure control in two-phase Al–21Ti–23Cr alloy

To improve the mechanical properties of the Al–21Ti–23Cr two-phase alloy consisting of L1₂ matrix and 20 vol% Cr₂Al as a second phase, microstructure control was conducted through the aging treatment as a thermal process and the addition of V and Zr as conventional alloying. It was found that TiAlCr was precipitated as a third phase in the L1₂ matrix by the aging treatment at 800 and 1000°C, and its size was smaller at 800°C than at 1000°C. The yield strength of the aged alloy increased rapidly only at 800°C although the third phase was precipitated at both 800 and 1000°C. In the V-added two-phase alloys, the yield strength and the strain increased simultaneously when V was added up to 3 at%, which is attributable to the improvement in the ductility of Cr₂Al. Microstructure control conducted in this study suggests the possibility of improving the mechanical properties of L1₂ (Al,Cr)₃Ti-based two-phase alloy by precipitating the fine third phase in the L1₂ matrix and enhancing the ductility of the second phase.     

Microstructure and properties of Cu−2Cr−1Nb alloy fabricated by spark plasma sintering

Cu?2Cr?1Nb alloy was fabricated by spark plasma sintering (SPS) using close coupled argon-atomized alloy powder as the raw material. The optimal SPS parameters obtained using the L₉(3⁴) orthogonal test were 950 °C, 50 MPa and 15 min, and the relative density of the as-sintered alloy was 99.8%. The rapid densification of SPS effectively inhibited the growth of the Cr₂Nb phase, and the atomized powder microstructure was maintained in the grains of the alloy matrix. Uniformly distributed multi-scale Cr₂Nb phases with grain sizes of 0.10?0.40 μm and 20?100 nm and fine grains of alloy matrix with an average size of 3.79 μm were obtained. After heat treatment at 500 °C for 2 h, the room temperature tensile strength, electrical conductivity, and thermal conductivity of the sintered Cu?2Cr?1Nb alloy were 332 MPa, 86.7% (IACS), and 323.1 W/(m·K), respectively, and the high temperature tensile strength (700 °C) was 76 MPa.     

Annealing effects on the microstructure and properties of bulk high-entropy CoCrFeNiTi0.5 alloy casting ingot

Most previous researches focused on small casting ingots prepared by arc melting, when studying high-entropy alloys. Large sized ingots were also necessary in exploring the existence of volume effects in the multi-principal element alloys. During the experiments, a large sized CoCrFeNiTi_0.5 alloy casting ingot was prepared by a medium frequency induction melting furnace. A slight volume effect occurred, reflecting mainly in the growth of crystalline grains and the increase of alloy hardness in the ingot. To investigate the effect of annealing temperature on microstructure and properties of CoCrFeNiTi_0.5 alloy, several samples taken from the ingot were annealed at 600 °C, 700 °C, 800 °C and 1000 °C respectively for 6 h. Almost no effects were found to the crystalline structure and elemental distribution when the samples were annealed below 1000 °C. The crystalline structure of CoCrFeNiTi_0.5 alloy was composed of one principal face-centered cubic (FCC) solid-solution matrix and a few intermetallic phases in the form of interdentrite. Dendrite contained approximately equivalent amount of Co, Cr, Fe, Ni and a smaller amount of Ti. When annealed below 1000 °C, the interdendrite stayed in (Ni, Ti)-rich phase, (Fe, Cr)-rich phase and (Co, Ti)-rich phase. After 1000 °C annealing, (Co, Ti)-rich phase disappeared, while (Ni, Ti)-rich phase and (Fe, Cr)-rich phase grew. The microhardness of the as-cast CoCrFeNiTi_0.5 alloy was 616.80 HV and the macrohardness was 52 HRC. The hardness of the samples stayed generally unchanged after annealing. This indicated a high microstructure stability and excellent resistance to temper softening that the CoCrFeNiTi_0.5 alloy exhibited.     

Phase transformation in the β phase of super α2 Ti3Al base alloys during static annealing and superplastic deformation at 700–1000 °C

Ti₃Al based alloys have admirable superplasticity with elongation greater than 1000% at temperatures between 950 and 1000 °C. In this study, the phase evolution and mechanical behavior of a super α₂ alloy Ti–25%Al–10%Nb–2%V–1%Mo (in at.%) at lower temperatures of 700–900 °C are further examined, and the reasons for the reduced elongations are examined. The initial grain size was around 2 μm. Although the super α₂ alloy exhibited superior superplastic elongations of 1500% at 960 °C, the elongation dropped to 600% at 900 °C, 330% at 850 °C and 140% at 750 °C. The β→α₂^′ transformation occurred in accordance with the equilibrium phase diagram during static annealing, but the transformation was enhanced during dynamic straining at temperatures from 750 to 900 °C. The β→α₂^′ transformation in the two-phase region is shown to be diffusion controlled, and not a massive or martensitic transformation. With the fine acicular α₂^′ plates inside the β grains, the grain boundary sliding accommodation process across the BCC β grains was impeded, leading to lower tensile elongations.     

Effect of neutron irradiation on the properties of the FeSe compound in the superconducting and normal states

The influence of atomic disorder induced by irradiation with fast neutrons on the properties of normal and superconducting states of polycrystalline samples of FeSe has been studied. The irradiation with fluences of fast neutrons up to 1.25 × 10²⁰ cm^?2 at the irradiation temperature T _irr ?? 50°C leads to relatively small changes in the temperature T _c of the superconducting transition and in the electrical resistivity ??₂₅ at 25 K. This behavior is related to the relatively low concentration of radiation defects arising at a given irradiation temperature, which is a consequence of a specific crystal structure of FeSe, which is more simple as compared to other layered compounds of this class.     

Precipitation Behavior of Pt2Mo-Type Superlattices in Hastelloy C-2000 Superalloy with Low Mo/Cr Ratio

This paper focuses on the precipitation behavior of superlattices phases in new Hastelloy C-2000 alloy with low Mo/Cr ratio owing to their detrimental effects on both mechanical and corrosion-resistance properties of the alloys. The precipitation behavior of superlattices phases in the C-2000 alloy was investigated at 600 °C in the aging time range of 100-500 h. The results revealed that Pt₂Mo-type superlattices phases have been precipitated after aging at 600 °C for 100 h. Typically, the Pt₂Mo-type precipitated phases meet to a stoichiometric ratio of Ni₂(Cr, Mo) in this alloy. As increasing aging time from 100 to 500 h, size of the phase increases from around 13 to 55 nm. Besides, morphology of the Ni₂(Cr, Mo) precipitated phases changes from a lean to a fat ellipse with increasing aging time due to the effect of the Mo/Cr atomic ratio and alloying elements on transformation paths from disorder to order. In addition, solution temperature of the Pt₂Mo-type superlattices is around 725 °C determined by differential scanning calorimetry method, which was significantly dependent on the heating rate.     

Oxidation behavior of atomized Fe40Al intermetallics doped with boron and reinforced with alumina fibers

Isothermal oxidation resistance of Fe40 (at.%) Al-based atomized and deposited intermetallic alloys has been evaluated. The alloys included Fe40Al, Fe40Al + 0.1B, and Fe40Al + 0.1B + 10Al₂O₃ at 800, 900, 1000, and 1100 °C. The tests lasted approximately 100 h, although in most cases there was scale spalling. At 800 and 900 °C, the Fe40Al + 0.1B alloy had the lowest weight gain, whereas the Fe40Al alloy had the highest weight gain at 800 °C (0.10 mg/cm²) and the Fe40Al + 0.1B + 10Al₂O₃ alloy was the least oxidation resistant at 900 °C with 0.20 mg/cm². At 1000 °C, the Fe40Al + 0.1B alloy showed the highest weight gain with 0.12 mg/cm² and the Fe40Al alloy the lowest. At 1100 °C, again, as at 900 °C, the Fe40Al alloy was the least resistant, whereas the Fe40Al + 0.1B alloy performed the best, but the three alloys exhibited a paralinear bahavior on the weight-gain curves, indicating the spalling, breaking down, and rehealing of the oxides. This spalling was related to voids formed at the metal-oxide interface.     

Physical metallurgy and mechanical properties of transition-metal Laves phase alloys

This paper provides a comprehensive review of the recent research on the phase stability, point defects, and fracture toughness of AB₂ Laves phases, and on the alloy design of dual-phase alloys based on a soft Cr solid solution reinforced with hard XCr₂ second phases (where X=Nb, Ta and Zr). Anti-site defects were detected on both sides of the stoichiometric composition of NbCr₂, NbCo₂, and NbFe₂, while they were observed only on the Co-rich side of ZrCo₂. Only thermal vacancies were detected in the Laves phase alloys quenched from high temperatures. The room-temperature fracture toughness cannot be effectively improved by increasing thermal vacancy or reducing stacking fault energy through control of phase stability. Microstructures, mechanical properties, and oxidation resistance of dual-phase alloys based on Cr–NbCr₂, Cr–TaCr₂, and Cr–ZrCr₂ were studied as functions of heat treatment and test temperature at temperatures to 1200°C. Among the three alloy systems, Cr–TaCr₂ alloys possess the best combination of mechanical and metallurgical properties for structural use at elevated temperatures.     

Transient state scale formation on Fe 32 Ni 20 Cr and Fe 20 Cr in H2-H2O-H2S atmospheres

The transient state of simultaneous oxidation and sulfidation of Fe-32 Ni-20 Cr and Fe-20 Cr was studied at 700°C for short time exposures in H₂-H₂O-H₂S. After heating the specimens in pure, dry hydrogen they were corroded by introduction of the oxidizing and sulfidizing atmosphere for 2, 4 or 15 min. After quenching the layer was investigated by SEM, AES, X-ray and electron diffraction. Four different gas compositions were applied: pS₂ = 10^?12 bar and pO₂ = 10^?25, 10^?26, 10^?27, 10^?28 bar, all within the thermodynamic stability range of Cr₂O₃. After the short time exposures oxides and sulfides were present on the surface, Cr₂O₃ and Cr₃S₄ had grown side by side and in case of the alloy Fe-32 Ni-20 Cr Fe- and Ni-containing sulfides formed patches on top of the scale. The amount of sulfides was higher for the lower oxygen pressures. After a longer time exposure, 120 min, all sulfides had vanished. Simultaneous formation of oxides and sulfides occurs in the transient state during phase boundary reaction or transport control. Upon transition to diffusion control the sulfides vanish by dissolution into the alloy and reaction with the gas atmosphere. This is valid for low pS₂ where no iron and nickel sulfides are stable.     

Formation of an <Emphasis Type="Italic">L</Emphasis>1<Subscript>0</Subscript> superstructure in austenite upon the α → γ transformation in the invar alloy Fe-32% Ni

Structure of a metastable austenitic invar alloy Fe-32% Ni preliminarily quenched for martensite and subjected to α → γ transformation using slow heating to various temperatures (430–500°C) with the formation of variously oriented nanocrystalline lamellar austenite, which was subjected to an additional annealing at 280°C (below the calculated temperature of ordering of the γ phase), has been studied electron-microscopically. An electron diffraction analysis revealed the presence of an L1₀ superstructure in the disperse nickel-enriched nanocrystalline γ phase both after annealing at 280°C and in the unannealed alloy immediately after α → γ transformation upon slow heating to 430°C.     

The effect of internal stable nitride and oxide dispersion on the high temperature oxidation of FeCr alloys

The behaviour of FeCr alloys containing either a dispersed nitride or oxide phase is examined. Fe-12, 14 or 18% Cr containing 0.5 or 1 wt% Ti were used. The oxide dispersion was introduced by an internal oxidation treatment using a 50/50 Cr/Cr₂O₃ mixture in a sealed quartz capsule at 1100 or 1200°C. Internal nitrides were produced by nitridation in either an 80/20 or a 90/10 H₂/N₂ mixture at 1150°C. Subsequent oxidation tests of the treated alloys were carried out isothermally at 900, 1000 and 1100°C, or by thermal cycling (3 h cycles) at 1100°C.The dispersed nitride phase is not as effective in improving the oxidation resistance as corresponding dispersed oxides. Considerable particle coarsening is observed and the nitrides tend to dissociate and are converted to oxide near to the alloy/scale interface.     

机械合金化及热压烧结制备Fe-Cr-Mn基超细晶奥氏体不锈钢研究

采用高能研磨诱导的机械合金化方法制备了Fe-Cr-Mn基不锈钢合金粉末;对机械合金化粉末分别进行了退火和热压烧结,分析了退火过程中的相变规律,并对热压烧结获得的奥氏体不锈钢进行了组织和耐蚀性能研究.结果表明:机械合金化获得的不锈钢合金粉由亚稳态的纳米晶铁素体构成;退火/热压烧结处理后,铁素体逐渐转变为热力学上更加稳定的...