Diffusion creep of intermetallic TiAl alloys

The creep behaviour of γ-TiAl with L1₀ structure without second phases, γ-TiAl with precipitated particles of α₂-Ti₃Al with D0₁₉ structure, and γ-TiAl with the H-phase Ti₂AlC has been studied at low stresses in the temperature range 900–1200°C. The obtained data allow the construction of creep deformation mechanism maps for the studied alloys which may be used for an extrapolation of the observed creep behaviour. At higher stresses dislocation creep occurs in all alloys, which is well described by the Dorn equation with stress exponents in the range 3–5. Extended Coble creep with threshold stress was observed only for the studied two-phase alloys. A strong temperature dependence of the threshold stress for Coble creep was found for the TiAl alloy with carbide particles.     

Thermal-cycling creep of γ-TiAl-based alloys

In two-phase TiAl-based alloys, the coexisting α₂ and γ phases exhibit a thermal expansion mismatch, so that increased creep rates during thermal cycling may be expected. Creep deformation of two γ-TiAl-based alloys was investigated during thermal cycles between 900 and 300 or 350°C with applied tensile stresses of 32.5 or 37.0 MPa. Measured thermal cycling creep rates were compared with isothermal creep rates calculated at the effective average temperature. No creep enhancement was measured upon thermal cycling within uncertainties of 1.6×10⁻³% strain per cycle.     

Improved High-Temperature Microstructural Stability and Creep Property of Novel Co-Base Single-Crystal Alloys Containing Ta and Ti

The influence of Ta and Ti additions on microstructural stability and creep behavior in novel Co-Al-W base single-crystal alloys has been investigated. Compared to the ternary alloy, the γ′ solvus temperature and γ′ volume fraction were raised by individual additions of Ta and Ti, and increased further in the quinary alloy containing both alloying additions. In contrast to ternary and quaternary alloys, an improved microstructural stability with the stable γ–γ′ two-phase microstructure and more than 60% γ′ volume fraction existed in the quinary alloy after prolonged aging treatment at 1050°C for 1000 h. The creep behavior at 900°C revealed lower creep rates and longer rupture lives in the quaternary alloys compared to the ternary alloy, whereas the quinary alloy exhibited even better creep resistance. When the creep temperature was elevated to about 1000°C, the creep resistance of the quinary alloy exceeded the previously reported Co-Al-W-base alloys and first-generation Ni-base single-crystal superalloys. The improved creep resistance at approximately 1000°C was considered to be associated with high γ′ volume fraction, γ′ directional coarsening, and dislocation substructure, which included γ–γ′ interfacial dislocation networks and the sheared γ′ precipitates containing stacking faults and anti-phase boundaries.     

Microstructures,mechanical properties and oxidation behaviour of single-phase Fe3Al (D03) and two-phase α-Fe,Al (A2) + Fe3Al (D03) FeAlV alloys

Microstructures, mechanical properties and oxidation behaviour of single-phase Fe₃Al (D0₃) and two-phase α-Fe,Al (A2) + Fe₃Al (D0₃) FeAlV alloys have been studied. Alloying with vanadium substantially increases the strength of the single-phase alloys through solid solution hardening. In addition, vanadium markedly increases the D0₃/B2 transition temperature. The change from B2 to D0₃ at a given temperature yields an additional increase in strength. The two-phase alloys have a fine-scaled coherent microstructure with D0₃ precipitates ranging between 10 and 30 nm in the as-cast state. However, marked coarsening of D0₃ is observed after annealing at 700 °C for 1000 h. Up to this temperature the alloys show an appreciable flow stress and creep strength, comparable to those of other iron aluminide based alloys with coherent microstructures. Oxidation tests at 700–900 °C revealed formation of thick and porous Fe₂O₃ + Al₂O₃ scales above 700 °C. Therefore, above 700 °C the oxidation behaviour of these alloys is inferior compared to that of other iron aluminide alloys with about the same Al content. This poor oxidation behaviour does not improve when 4–8 at.% Cr are added.     

Creep behavior of Ni-added Ir85Nb15 two-phase refractory superalloys at 1800 °C

To provide information relevant to Ir-based two-phase alloys for future ultra-high-temperature applications, the compression creep properties for the Ni-added Ir₈₅Nb₁₅ alloys were investigated at 1800 °C under 137 MPa. The results show that Ni addition has a significant effect on the creep resistance of the Ir₈₅Nb₁₅ two-phase refractory superalloy.     

Effect of Thermal Annealing on Machining-Induced Residual Stresses in Inconel 718

Nickel-based alloys are widely employed in the manufacturing of aero-engines. These alloys are difficult to machine, and tensile residual stresses are generated during machining. These tensile residual stresses can negatively affect the performance of aero-engine components. Nevertheless, residual stresses can vary due to thermal or mechanical loading. These variations must be considered to evaluate the real influence of residual stresses on component behavior. This paper studies the effect of thermal loads on machining-induced residual stresses in the alloy Inconel 718. A ring-shaped Inconel 718 part was face-turned, and specimens were extracted from it. Specimens were exposed at 550 and 650 °C for 10 min, 1 and 10 h. Residual stresses were measured, and microstructure was observed before and after thermal exposure. Residual stress variations found after thermal exposure were the consequence of two factors: relaxation of strain bands during the early stage of exposure and diffusion-controlled creep. In addition, a modified Zener-Wert-Avrami model is proposed to predict residual stress relaxation caused by the diffusion-controlled creep. Once having fitted the modified Zener-Wert-Avrami model, the study was extended for a wider range of temperatures (400-650 °C). This analysis showed that surface residual stresses do not relax significantly at temperatures below 500 °C.     

Al-Zn-Mg-Cu铝合金变温双级蠕变时效模型

针对Al-Zn-Mg-Cu合金变温双级蠕变时效过程,建立了一种考虑蠕变应变与屈服强度的本构框架,通过实验数据的简单拟合方法获得了模型参数。模型不仅以简单的形式具备了处理蠕变时效过程中的应力松弛、强化响应和温度变化的能力,而且能够应用到有限元软件中模拟构件的蠕变量、屈服强度和回弹。模型结果不仅能够适应不同外加应力下实测的蠕变应变曲线,且有限元模拟结果与实测结果能够很好地吻合。     

CREEP DEFORMATION OF INTERMETALLIC ALLOYS

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Thermal evolution and grain boundary phase transformations in severely deformed nanograined Al–Zn alloys

The structure, phase composition and thermal evolution of binary Al–Zn alloys were studied before and after high-pressure torsion (HPT) in Bridgman anvils. On heating of HPT-deformed samples from room temperature to 300 °C, Zn grains dissolved, and a relatively fine-grained (Al) equilibrium solid solution formed. Differential scanning calorimetry curves reveal two-stage melting in the Al–Zn alloys studied, i.e., the melting of the (Al) solid solution starts 10–25 °C below the bulk solidus line. The effect is more pronounced in fine-grained samples. It is explained by the presence of layers of liquid-like phase in the (Al) grain boundaries (GB) between bulk and GB solidus lines. The new metastable GB solidus line appears in the (Al) single-phase region of the Al–Zn phase diagram, it can be compared with the metastable solvus lines for the formation of GP zones and α phases in the (Al)+Zn two-phase area.     

Optimization of the Mechanical Properties and Residual Stresses in 2024 Aluminum Alloy Through Heat Treatment

Residual stresses induced during quenching of aluminum alloys cause dimensional instability and distortion. In this study, the effects of different concentrations of polyalkylene glycol (PAG) quenchants on residual stresses and mechanical properties of 2024 aluminum alloy were investigated. Surface residual stresses were measured by using hole-drilling strain-gauge method. Also, mechanical properties and microstructure of the heat-treated samples were analyzed using hardness measurements, tensile tests, and transmission electron microscopy. Results showed that quenching into a 15% polymeric solution and aging at 190 °C for 12 h cause 50% reduction in residual stress as compared with quenching in water at 20 °C and naturally aging. Moreover, tensile strength decreased by 104 MPa (~?20%) in compared with the T6 sample.     

Mechanical properties of Nb25Mo25Ta25W25 and V20Nb20Mo20Ta20W20 refractory high entropy alloys

Two refractory high entropy alloys with compositions near Nb₂₅Mo₂₅Ta₂₅W₂₅ and V₂₀Nb₂₀Mo₂₀Ta₂₀W₂₀, were produced by vacuum arc-melting. Despite containing many constituents, both alloys had a single-phase body-centered cubic (BCC) structure that remained not only stable after exposure to 1400 °C, but also disordered, as confirmed by the absence of superlattice reflections in neutron diffraction data. Compressive flow properties and microstructure development of these alloys were determined from room temperature up to 1600 °C. Limited compressive plasticity and quasi-cleavage fracture at room temperature suggest that the ductile-to-brittle transition for these alloys occurs above room temperature. At 600 °C and above, both alloys showed extensive compressive plastic strain. The yield stress of both alloys dropped by 30–40% between room temperature and 600 °C, but was relatively insensitive to temperature above 600 °C, comparing favorably with conventional superalloys.     

The controlling creep processes in TiAl alloys at low and high stresses

The creep response of a nearly-lamellar Ti–47Al–4(W, Nb, B) alloy is studied at 760 °C in a wide stress range 100–500 MPa. The alloy exhibits excellent creep resistance with a minimum creep rate of 1.2×10⁻¹⁰/s at 100 MPa and the time to 0.5% creep strain of 1132 h at 140 MPa. The controlling creep process is probed by analysis of the post-creep dislocation structure and by observation of incubation period during stress reduction test. The results indicate that creep is controlled by dislocation climb at low stresses (Class II type) and by jog-dragged dislocation glide at high stresses (Class I type). The transition from Class II to Class I type creep occurs at about 180 MPa. The excellent creep resistance of the studied alloy compared to other W containing TiAl alloys is attributed to its highly stable lamellar microstructure consisting eventually of coarse gamma laths.     

Tensile creep of Mo–Si–B alloys

Multiphase Mo–Si–B alloys containing a Mo solid solution matrix and brittle Mo₃Si and Mo₅SiB₂ (T2) intermetallic phases are candidates for ultra-high-temperature applications_. The elevated temperature uniaxial tensile response at a nominal strain rate of 10^?4 s^–1 and the tensile creep response at constant load between 1000 °C and 1300 °C of a (i) single phase solid solution (Mo–3.0Si–1.3B in at.%), (ii) two-phase alloy containing ～35 vol.% T2 phase (Mo–6Si–8B in at.%) and (iii) three-phase alloy with ～50 vol.% T2 + Mo₃Si phases (Mo–8.6Si–8.7B in at.%) were evaluated. The results confirm that Si in solid solution significantly enhances both the yield strength and the creep resistance of these materials. A Larson–Miller plot of the creep data showed improved creep resistance of the two- and three-phase alloys in comparison with Ni-based superalloys. The extent of Si dissolved in the solid solution phase varied in these three alloys and Si appeared to segregate to dislocations and grain boundaries. A stress exponent of ～5 for the solid solution alloy and ～7 at 1200 °C for the two multiphase alloys suggested dislocation climb to be the controlling mechanism. Grain boundary precipitation of the T2 phase during creep deformation was observed and the precipitation kinetics appear to be affected by the test temperature and applied stress.     

不同体积无铅微尺度焊点的蠕变力学性能

采用基于动态力学分析仪的精密蠕变试验方法,对比研究了5.34×107 μm3与7.07×106 μm3两种体积相差近一个数量级的无铅Sn-3.0Ag-0.5Cu微尺度焊点的高温蠕变力学行为与蠕变性能.结果表明,所有微尺度焊点的蠕变曲线均呈现初始蠕变、稳态蠕变和加速蠕变阶段.虽然微尺度焊点的体积存在较大差异,但是它们的蠕变激活能与蠕变应力指数均非常接近.此外,在相同的试验温度与拉伸应力作用下,大体积微尺度焊点的稳态蠕变速率相对小体积焊点更大,而蠕变寿命则呈现完全相反的趋势.     

Pre-strain dependent relaxation behaviour of AA6016 during automotive paint drying processes

Heat treatable aluminium alloys are widely used in modern car bodies to achieve lightweight cars. Compared to steel, aluminium alloys are more sensitive to creep behaviour under high elastic stresses, since their melting point of about 600 °C is much lower. In order to predict permanent deformations of outer shell panels due to thermal stresses in drying processes and ensure highest quality standards regarding appearance to the customer, interdependencies with production process history must be evaluated. In this work, stress relaxation of the aluminium alloy AA6016 in initially T4 temper has been examined within the elastic regime with focus on the dependency of plastic pre-strain during primary creep phase. As a result, pre-strain becomes more important the higher the temperature was set. Hence, the former production process history of aluminium sheet metal parts must be taken into account when viscoelastic deformations are investigated. Based on the test results a pre-strain dependent material model for finite element simulations is pointed out, which can be used for novel deformation analyses of car bodies in automotive paint drying processes that contain aluminium parts out of AA6016.     

Creep of Polycrystalline Nickel

Minimum creep rates of nickel samples were measured in the stress region of 2.5×10⁷ to 2.8×10⁹ dyne per sq cm and the temperature region of 400° to 1100°C. The creep rate seems to be proportional to (stress)^4.6 at stresses below 7×10⁸ dyne per sq cm. The activation energy of creep is approximately 65,000 cal per mol.     

Effect of si addition on mechanical alloying behavior and creep properties of Al-10Ti-xSi alloys

Al-10Ti-xSi alloys (x=0∼6wt.%) have been mechanically alloyed under Ar atmosphere using an attritor and the alloying process has been investigated. From Al-10Ti composite powders, supersaturated Al(Ti) powders were obtained after mechanical alloying. In the ternary mixture, fine Si particles were observed to be distributed in the Al(Ti) matrix due to both the negligible solid solubility of Si in the Al matrix and the weaker chemical interaction of Si with Al, as compared with Ti. The sealed compacts were hot extruded to full density at 450°C with an extrusion ratio of 12:1. The microstructures and creep properties of the hot extruded alloys were examined. During consolidation, Si particles were dissolved in Al₃Ti up to 4 wt.% Si to form the (Al(Si))₃Ti phase, and the Ti₇Al₅Si₁₂ phase was formed beyond the solubility limit of Si in Al₃Ti. The transition from the Coble creep mechanism at low stresses and temperatures to dislocation one at high stresses and temperatures was observed. The stress and temperature of the transition from diffusional to dislocation creep became higher as Si concentration increased. This was due to an enhancement of Al₃Ti particle strength with increasing Si content as a result of Si incorporation. Thus, the addition of Si enhances the creep resistance of the MA Al-10Ti alloy.     

Creep-Rupture Characteristics of Al-Mg Solid-Solution Alloys

Three aluminum alloys of 0.94, 1.92, and 5.10 pct Mg, prepared from very high purity metals, were tested at 500°, 700°, and 900°F in creep rupture. The degree of strengthening through solid-solution alloying and the effects on the deformation characteristics and fracture were examined. The ductility of the alloys as a function of stress and temperature was closely followed.     

The mechanical properties of two uranium alloys and their role in the oxidation of the alloys

The creep and tensile properties of two uranium alloys (U-21 at.% Nb and U-16.6 at. % Nb-5.6 at. % Zr) were determined in the temperature range 750–900°C. The creep data were fitted to an equation of the form exp (-Q/RT)which in turn was used to calculate stresses in oxidizing alloy specimens on the basis of elongation measurements made on the specimens during oxidation. The variation of the average stress in the oxide and in the substrate metal is given as a function of time. The details of the mechanism by which stresses are generated during the oxidation of the alloys are discussed. Comparisons of the relative creep and oxidation rates of the alloys with the stress levels observed during oxidation lead to the conclusion that at least for certain specimen geometries the differences in the mechanical properties of the oxide scales and of the parent alloys account in large measure for the differences in the oxidation characteristics of the alloys.Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation.     

应力对蠕变时效2524合金微观组织的影响(英文)

研究了2524合金(Al-4.3Cu-1.5Mg)通过蠕变机器施加不同应力(0、173和250 MPa)在170℃时效的微观组织变化,测定了不同应力下的硬度曲线,并利用透射电镜(TEM)观察相应的微观组织。结果表明:施加应力的样品,时效后其峰值硬度增加,而达到峰值硬度所需的时间缩短;时效成形后,S(Al2CuMg)相长度变短,而密度增加;弹性应力下GPB区对峰值硬度起主要作用。