Tailoring Microstructure and Microsegregation in a Directionally Solidified Ni-Based SX Superalloy by a Weak Transverse Static Magnetic Field

A weak transverse static magnetic field (WTSMF, 0-0.5 T) is applied to the directional solidification process of a DD3 Ni-based SX superalloy, aiming to tailor the microstructure and microsegregation of alloys. The mechanisms of microstructural refinement and microsegregation distribution caused by a WTSMF during directional solidification are discussed. It is shown that the primary dendrite arm spacing is rapidly reduced from 181 to 143 μm, and the average size of γ′ phase is significantly refined from 0.85 to 0.25 μm as the magnetic field increases from 0 to 0.5 T. At the same time, the volume fractions of γ/γ′ eutectic and the segregation coefficient are also gradually decreased. The 3D numerical simulations of the multiscale convection in liquid phase show that the modifications of the microstructure and microsegregation in DD3 are mainly attributed to the enhanced liquid flow caused by thermoelectric magnetic convection (TEMC) at dendrite/sample scale under the WTSMF. The maximum of the TEMC increases with increasing the magnetic field intensity. This work paves a simple way to optimize the microstructure and microsegregation in directionally solidified Ni-based SX superalloys without changing the processing parameters and composition.     

Phase Selection and Microhardness of Directionally Solidified AlCoCrFeNi2.1 Eutectic High-Entropy Alloy

In the present work, the solidi?cation behaviors and microhardness of directionally solidi?ed AlCoCrFeNi_2.1 eutectic highentropy alloy (EHEA) obtained at different growth velocities are investigated. The microstructure of the as-cast AlCoCrFeNi _2.1 EHEA is composed of bulky dendrites (NiAl phase) and lamellar eutectic structures, indicating that the actual composition of the alloy slightly deviates from the eutectic point. However, it is interesting to observe that the full...     

Synthesis and properties of Ba3NaRu2O9−δ and Ba3(Na, R)Ru2O9−δ (R=rare earth) crystals

Crystals of Ba₃NaRu₂O_9−δ (δ≈0.5) and Ba₃(Na, R)Ru₂O_9−δ (R=Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) were grown by an electrochemical method, and their crystallographic, magnetic, and electric properties were studied. All crystals have a hexagonal structure of space group P6₃mmc. Ba₃NaRu₂O_9−δ and Ba₃(Na, R)Ru₂O_9−δ (except Ce) have a negative asymptotic Curie temperature suggesting the existence of an antiferromagnetic order; however, they are paramagnetic at temperatures above 1.7 K. Ba₃NaRu₂O_9−δ has an effective magnetic moment P_eff of 0.91 μ_B, while P_eff of Ba₃(Na, R)Ru₂O_9−δ (except Ce) reflects the large free-ion moment of the rare earth ions. Ba₃(Na, Ce)Ru₂O_9−δ shows peculiar magnetic behavior that differs from the magnetism of other Ba₃(Na, R)Ru₂O_9−δ crystals. The resistivity of all crystals exhibits an activation-type temperature dependence with an activation energy in the range of 0.10.2 eV.     

Molecular Dynamics Simulation of the Evolution of Interfacial Dislocation Network and Stress Distribution of a Ni-Based SingleCrystal Superalloy

The evolution of misfit dislocation network at γ/γ' phase interfaces and the stress distribution characteristics of Ni-based single-crystal superalloys under different temperatures of 0,100 and 300 K are studied by molecular dynamics(MD) simulation.It was found that a closed three-dimensional misfit dislocation network appears on the γ/γ' phase interfaces,and the shape of the dislocation network is independent of the lattice mismatch.Under the influence of the temperature,the dislocation network gradually becomes irregular,all[110]dislocations in the y matrix phase emit and partly cut into the γ′ phase with the increase in temperature.The dislocation evolution is related to the local stress field,a peak stress occurs at γ/γ' phase interface,and with the increase in temperature and relaxation times,the stress in the γ phase gradually increases,the number of dislocations in the y phase increases and cuts into γ' phase from the interfaces where dislocation network is damaged.The results provide important information for understanding the temperature dependence of the dislocation evolution and mechanical properties of Ni-based single-crystal superalloys.     

The Minor Precipitation at the Final Stage of U720Li Solidification

The minor precipitations caused by B and Zr which are the normal constituents of U720 Li alloy have been studied by analyzing the solidification process and the composition evolution. The present study aims to supply the elementary information about the existing form of B and Zr in the as-cast microstructure, which is helpful for the subsequent processing, such as homogenization treatment. The M_3B_2 and Ni_5Zr phases were observed in the U720 Li alloy in as-cast state, which were usually accompanying with each other together with g-Ni_3 Ti phase at the edge of eutectic(γ+γ'). Combining the DTA analysis and heating and quenching tests, the solidification sequence was determined to be the following: c matrix, eutectic(γ+γ'), g-Ni_3Ti, M_3B_2 and Ni_5Zr. The in situ composition analysis by EDS and EPMA revealed that the precipitation and microstructure were governed by the composition evolution in the liquids. The solidification of c matrix increased the Ti concentration in the residual liquids and resulted in the eutectic(γ+γ') formation; the(γ+γ') formation increased the Ti/Al radio in the liquids and the g-Ni_3Ti was formed in front of the eutectic(γ+γ'); the g-Ni_3Ti precipitation consumed up Al and Ti and increased the concentration of B, Mo and Cr, and M_3B_2 boride is formed;the previous precipitation of the phases consumed up most of the elements other than Ni and Zr, and Ni_5Zr is formed finally. The melting points are in the ranges of 1130–1140 °C for Ni_5Zr phase, 1180–1190 °C for M_3B_2 boride and1190–1200 °C for g-Ni_3Ti phase.     

Influence of Pre-strain on the Mechanical Properties of A6111-T4P Sheet with Bake Hardening

A6111 is an aluminum alloy, which exhibits good formability and excellent bake hardening property. This study aimed to reveal the influence of strain path, pre-strain orientation(α) as well as pre-strain level on the mechanical properties of A6111-T4 P sheet under bake treatment through uniaxial tension test.(0–5)% pre-strain, 150–170 °C bake temperature and 20–30 min bake time were considered in the study by referring to the actual production process. The results show that both pre-strain level and strain path play significant roles in improving the material properties. In the condition that tensile orientation(β) parallel with pre-strain orientation(β = α), the yield strength can be remarkably improved, and much higher parameter of n in Hockett–Sherby model can be obtained than those when tensile orientation non-parallel with pre-strain orientation(α≠β). In addition, when the pre-strain level, paint bake process were settled and β = α, the curves obtained in five tension orientations are similar in the plastic deformation stage.     

Influence of strong static magnetic field on intermediate phase growth in Mg–Al diffusion couple

Effects of strong magnetic field on intermediate phase growth in Mg–Al diffusion couples is addressed. It was found that both Al₃Mg₂ (β) and Al₁₂Mg₁₇ (γ) formed after annealing at different temperatures with or without the field. Systematic measurements showed that thickness of intermediate phases was reduced due to the application of the 10 T static magnetic field. Data analysis show that reduction of intermediate layer thickness is attributed to the decreasing of frequency factor (k₀) under the magnetic field, however the activation energy (Q) for layer growth is almost not changed irrespective of the application of the field. Layer thickness decreasing is suggested to be related with the retardation of atom diffusion resulting from the magnetic field and a possible theory based on ambipolar diffusion is discussed to explain this effect.     

强磁场下亚共晶Al-Cu合金初生相形核与生长行为(英文)

应用差热分析法研究了强磁场下Al-20.8%Cu(质量分数)亚共晶合金初生相形核与生长特性。差热分析曲线表明,初生相的形核温度随磁场强度的增大而降低,其生长速率则随磁场强度增大而增大。初生相枝晶由无磁场时无序生长转变为磁场下规则生长。研究表明,10T量级的磁场对Al晶体形核驱动力的影响可以忽略,初生相形核温度的降低主要归结为磁场下固液界面自由能的增加。枝晶形貌转变则源于磁场对熔体流动的抑制及铝晶体的磁各向异性。     

Nucleation and growth behaviors of primary phase in hypoeutectic Al–Cu alloy in high magnetic field

The nucleation and growth behaviors of primary Al phase in the hypoeutectic alloy of Al–20.8%Cu (mass fraction) in high static magnetic fields were investigated by differential thermal analysis (DTA). The DTA curves indicate that the nucleation temperature of primary Al phase decreases as the magnetic induction increases. The average growth rates of primary crystals increase with the increase of magnetic induction. The dendrite structures show that primary Al phase dendrites change from disorderly without the magnetic field to regularly with the field. The effect of magnetic field with the magnetic induction order of 10 T on driving force for the nucleation of Al crystals is negligible. The reduction of nucleation temperature of primary Al phase is mainly caused by the increase of the interfacial free energy between the melt and the nucleus. The change in dendrite morphology can be attributed to the suppression of melt flows in the magnetic field and magnetic anisotropy of Al crystals.     

Relationship Between Microstructures and Microhardness in High-Speed Friction Stir Welding of AA6005A-T6 Aluminum Hollow Extrusions

AA6005 A-T6 aluminum hollow extrusions were friction stir welded at a fixed high welding speed of 2000 mm/min and various rotation speeds. The results showed that the heat-aff ected zone(HAZ) retained the similar grain structure as the base material except some grain coarsening, and the density of dislocations and β′ precipitates were almost unchanged, indicating that the high welding speed inhibited the coarsening and dissolution of β″ precipitates via fast cooling rate. The thermo-mechanically aff ected zone(TMAZ) was characterized by elongated and rotated grains, in which a low density of β′ precipitates and the highest density of dislocations were observed. The highest heat input and severest plastic deformation occurring in the nugget zone(NZ) resulted in the occurrence of dynamic recrystallization and a high density of dislocations. Hence, all the β″ precipitates and most of the β′ precipitates dissolved into the matrix, and a few β′ precipitates were transformed into β precipitates. The microhardness was controlled by the precipitation and solution strengthening in the HAZ, by the dislocation and precipitation strengthening in the TMAZ, and by the fine-grain and dislocation strengthening in the NZ. With the increase in rotation speed, the peak and the lowest microhardness value increased monotonously.     

Dendrite fragmentation and columnar-to-equiaxed transition during directional solidification at lower growth speed under a strong magnetic field

The effects of strong magnetic fields on the columnar-to-equiaxed transition (CET) have been investigated experimentally. Six alloys have been directionally solidified at low growth speeds (1–10 μm s^?1) under magnetic fields up to 10 T. Experimental results show that the application of a strong magnetic field causes a dendrite fragmentation and then the CET. The thermoelectric magnetic force acting on cells/dendrites and equiaxed grains in the mushy zone has been studied numerically. Numerical results reveal that the value of the thermoelectric magnetic force increases as the magnetic field intensity and the temperature gradient increase. A torque is created on cells/dendrites and equiaxed grains. This torque breaks cells/dendrites and drives the rotation of equiaxed grains. The rotation of equiaxed grains in the mushy zone will further destroy cells/dendrites. Thus, with the increase of the magnetic field intensity and the temperature gradient, the volume fraction of equiaxed grains in front of columnar dendrites increases. When the magnetic field intensity and the temperature gradient reach a critical value, the growth of columnar dendrites is blocked and the CET then occurs. The present work may initiate a new method of inducing the CET via an applied strong magnetic field during directional solidification.     

Evolution of γ' Particles in Ni-Based Superalloy Weld Joint and Its Effect on Impact Toughness During Long-Term Thermal Exposure

Effects of long-term thermal exposure on γ' particles evolution and impact toughness in the weld joint of Nimonic 263(N263)superalloy were deeply studied at 750℃.Results showed that the precipitates in the weld metal were mainly composed of fine γ' particles,bulky MC carbides,and small M_(23)C_6 carbides.With the thermal exposure time increasing from o to 3000 h,γ' particles in the weld metal grew up from 19.7 nm to 90.1 nm at an extremely slow rate.After being exposed for 1000 h,γ' particles coarsened and some of them transformed into acicular η phase.At the same time,MC carbides decomposed to form η phase and γ' particles.This dynamic transition ensured the slight reduction in impact toughness of the weld metal after the thermal exposure,which indicated the stable serving performance of N263 weld joint.     

Phase Evolution and Thermal Expansion Behavior of a γ′ Precipitated Ni-Based Superalloy by Synchrotron X-Ray Diffraction

The phase evolution and thermal expansion behavior in superalloy during heating play an essential role in controlling the size and distribution of precipitates, as well as optimizing thermomechanical properties. Synchrotron X-ray diffraction is able to go through the interior of sample and can be carried out with in situ environment, and thus, it can obtain more statistics information in real time comparing with traditional methods, such as electron and optical microscopies. In this study, in situ heating synchrotron X-ray diffraction was carried out to study the phase evolution in a typical γ′ phase precipitation strengthened Ni-based superalloy, Waspaloy, from 29 to 1050 °C. The γ′, γ, M₂₃C₆ and MC phases, including their lattice parameters, misfits, dissolution behavior and thermal expansion coefficients, were mainly investigated. The γ′ phase and M₂₃C₆ carbides appeared obvious dissolution during heating and re-precipitated when the temperature dropped to room temperature. Combining with the microscopy results, we can indicate that the dissolution of M₂₃C₆ leads to the growth of grain and γ′ phase cannot be completely dissolved for the short holding time above the solution temperature. Besides, the coefficients of thermal expansions of all the phases are calculated and fitted as polynomials.     

Influence of A-site cation size-disorder on structural, magnetic and magnetocaloric properties of La0.7Ca0.3−xKxMnO3 compounds

The structural and magnetic properties of perovskite oxides La_0.7Ca_0.3−xK_xMnO₃ (0 ≤ x ≤ 0.15) have been investigated to explore the influence of the A-site cation size-disorder (σ²). The materials were prepared by the solid-state method and then characterized by X-ray diffraction (XRD). The XRD data have been analyzed by Rietveld refinement technique. For K doping concentration x ≤ 0.075, the samples crystallize in the orthorhombic structure, while for x ≥ 0.1, the structure becomes rhombohedral. The variation of the magnetization M as a function of the applied magnetic field μ₀H reveals the presence of a structural distortion leading to a reduction of the magnetization at low μ₀H values. When increasing μ₀H, the structural distortion decreases and for a high applied magnetic field, the M (μ₀H) curves saturate indicating the disappearance of the structural distortion. The influence of K doping concentration and the applied magnetic field on the magnetocaloric properties has been considered. A large magnetic-entropy change (|ΔS_M|  5 J/kg K) is obtained in all samples at Curie temperatures between 270 and 280 K for an applied magnetic field of 3 T. These results show that these materials can be used as candidates for magnetic refrigerants near room temperature.     

Microstructure,Mechanical Properties and Die-Filling Behavior of High-Performance Die-Cast Al-Mg-Si-Mn Alloy

This work aims to reveal the relationships between the microstructure,mechanical properties and flow behavior of die-casting AlMg_5Si_2Mn alloy.Results indicated that the microstructure of the die-cast AlMg_5Si_2Mn consists of α_1-Al grains,fine-size α_2~Al grains and(Al +Mg_2Si) eutectic.The surface layer observed has the thickness in a range of120-135 μm,while an ellipse-like surface layer edge is observed in the corner of the plate-like sample.Tensile strength and elongation(5) of the specimens are slightly decreased along the die-filling direction due to the backflow of melt.Pure(Al + Mg_2Si) eutectic layer and ultra-fine-size α_2-Al grains observed are around the overflow channels.Mass feeding is predominantly responsible for the superior mechanical properties of the round bars as compared to those of plate-like samples.     

Microstructural Evolution of Creep-Induced Cavities and Casting Porosities for a Damaged Ni-based Superalloy Under Various Hot Isostatic Pressing Conditions

The microstructural evolution of casting porosities and creep-induced cavities for a damaged nickel-based superalloy under different hot isostatic pressing(HIP) conditions was investigated in order to understand the effects of HIP parameters on the healing behavior of micropores. A number of small-sized creep cavities formed during long-term service and large-sized porosities formed during the casting process were observed. These microdefects were partially healed after treated at high temperature of 1100 °C combined with 150 MPa pressure for 2 h, together with the formation of the socalled concentrically oriented c0 rafting structure. When HIP temperature was increased to 1150 and 1175 °C, both the amount and the size of the microdefects were decreased. The concentrically oriented c0 rafting around creep cavities became more remarkable, and the primary c0 denuded zone was also formed between the raft structure and the cavity.Energy-dispersive X-ray spectroscopy analysis revealed that the c matrix solute atoms diffused toward the cavity under the concentration gradient, whereas the c0-forming elements diffused in a negative direction. When increasing HIP temperature up to 1200 °C, the micropores were hardly observed, indicating that both casting porosities and creep-induced cavities had almost been healed. Meanwhile, the c0 rafting structure disappeared since HIP temperature was beyond the c0 solvus temperature. It is revealed by the experimental results that the atomic diffusion could mainly dominate the healing process of micropores.     

2D12-T4铝合金微弧氧化膜的性能研究

对2D12-T4热处理状态铝合金微弧氧化防护膜层的拉伸强度和疲劳性能进行了研究.结果表明,随着氧化时间的增加,膜层厚度正比增加,膜层主要由γ-Al₂O₃和α-Al₂O₃及大量的非晶相构成;微弧氧化对基体的拉伸强度等力学性能影响较小,但会显著降低材料的疲劳性能,降低超过基体的100倍.     

纵向强磁场对MnBi/Bi共晶定向凝固组织的影响

进行了纵向强磁场下MnBi／Bi共晶定向凝固实验研究，并从热力学的角度分析了强磁场对MnBi／Bi共晶定向凝固组织的影响，发现磁场有利于纤维状MnBi／Bi共晶定向凝固组织的形成，扩大了形成纤维状共晶组织的速度范围；在同一生长速度下随着磁场强度的增加，MnBi／Bi共晶纤维组织变得更加规则，MnBi纤维粗化，纤维间距增大；而且强磁场的施加，使MnBi的形态发生了变化，小平面生长特性增强。     

Corrosion Behavior of Ψ and β Quasicrystalline Al-Cu-Fe Alloy

Two nanostructured Al-Cu-Fe alloys, Al₆₄Cu₂₄Fe₁₂ and Al_62.5Cu_25.2Fe_12.3, have been studied. Icosahedral quasicrystalline (ψ) Al₆₄Cu₂₄Fe₁₂ and crystalline cubic (β) Al_62.5Cu_25.2Fe_12.3 cylindrical ingots were first produced using normal casting techniques. High-energy mechanical milling was then conducted to obtain ψ icosahedral and β intermetallic nanostructured powders. Electrochemical impedance spectroscopy, linear polarization resistance, and potentiodynamic polarization were used to investigate the electrochemical corrosion characteristics of the powders in solutions with different pH values. Current density (i _corr), polarization resistance (R _p), and impedance modulus (|Z|) were determined. The results showed that regardless of pH value, increasing the solution temperature enhanced the corrosion resistance of the both phases. However, the electrochemical behavior of the ψ phase indicated that its stability depends on the submerged exposure time in neutral and alkaline environments. This behavior was related to the type of corrosion products present on the surfaces of the particles along with the diffusion and charge-transfer mechanisms of the corrosion process.     

Influences of the Welding Heat Input and the Repeated Repair Welding on Ti–3Al–2.5V Titanium Alloy

The primary purpose of this study was to determine the effects of gas tungsten arc welding heat input on the high-temperature tensile properties, toughness, and microstructural features of titanium alloy Ti–3Al–2.5V. The secondary objective was to examine the effect of the repeated repair welding on the properties of the alloy. It was also found that the mechanical properties progressively decreased with increasing the repair welding cycles, especially in the case of the weldment after the first welding repair. It was observed that the sizes of the acicular α' and prior β grain boundaries as well as the volume fraction of the acicular α' phases increased with increasing the welding heat input. In addition, the amount and size of the acicular α' phases were found to increase with increasing the repair welding cycles.