Effect of heat treatment on the microstructure,tensile properties,and fracture behavior of permanent mold Al-10 wt pct Si-0.6 wt pct Mg/SiC/10p composite castings

The present study was undertaken to investigate the effect of solution treatment (in the temperature range 520 °C to 550 °C) and artificial aging (in the temperature range 140 °C to 180 °C) on the variation in the microstructure, tensile properties, and fracture mechanisms of Al-10 wt pct Si-0.6 wt pct Mg/SiC/10_p composite castings. In the as-cast condition, the SiC particles are observed to act as nucleation sites for the eutectic Si particles. Increasing the solution temperature results in faster homogenization of the microstructure. Effect of solution temperature on tensile properties is evident only during the first 4 hours, after which hardly any difference is observed on increasing the solution temperature from 520 °C to 550 °C. The tensile properties vary significantly with aging time and temperature, with typical yield strength (YS), ultimate tensile strength (UTS), and percent elongation (EL) values of ∼300 MPa, ∼330 MPa, and ∼1.4 pct in the underaged condition, ∼330 MPa, ∼360 MPa, and ∼0.65 pct in the peakaged condition, and ∼323 MPa, ∼330 MPa, and ∼0.8 pct in the overaged condition. Prolonged solution treatment at 550 °C for 24 hours results in a slight improvement in the ductility of the aged test bars. The fracture surfaces exhibit a dimple morphology and cleavage of the SiC particles, the extent of SiC cracking increasing with increasing tensile strength and reaching a maximum in the overaged condition. Microvoids act as nucleation sites for the formation of secondary cracks that promote severe cracking of the SiC particles. A detailed discussion of the fracture mechanism is given.     

Microstructural effects on the tensile and fracture behavior of aluminum casting alloys A356/357

The tensile properties and fracture behavior of cast aluminum alloys A356 and A357 strongly depend on secondary dendrite arm spacing (SDAS), Mg content, and, in particular, the size and shape of eutectic silicon particles and Fe-rich intermetallics. In the unmodified alloys, increasing the cooling rate during solidification refines both the dendrites and eutectic particles and increases ductility. Strontium modification reduces the size and aspect ratio of the eutectic silicon particles, leading to a fairly constant particle size and aspect ratio over the range of SDAS studied. In comparison with the unmodified alloys, the Sr-modified alloys show higher ductility, particularly the A356 alloy, but slightly lower yield strength. In the microstructures with large SDAS (>50 μm), the ductility of the Sr-modified alloys does not continuously decrease with SDAS as it does in the unmodified alloy. Increasing Mg content increases both the matrix strength and eutectic particle size. This decreases ductility in both the Sr-modified and unmodified alloys. The A356/357 alloys with large and elongated particles show higher strain hardening and, thus, have a higher damage accumulation rate by particle cracking. Compared to A356, the increased volume fraction and size of the Fe-rich intermetallics (π phase) in the A357 alloy are responsible for the lower ductility, especially in the Sr-modified alloy. In alloys with large SDAS (>50 μm), final fracture occurs along the cell boundaries, and the fracture mode is transgranular. In the small SDAS (<30 μm) alloys, final fracture tends to concentrate along grain boundaries. The transition from transgranular to intergranular fracture mode is accompanied by an increase in the ductility of the alloys.     

Modeling the influence of fluid flow on the development of porosity and microstructure in castings

Fluid flow influences the development of porosity and microstructure in castings at several levels. Fluid flow affects the temperature distribution, and thus modifies the progression of solidification. Forced convection during mold filling may give way to thermal or solutal natural convection. Shrinkage may be fed, and in the final stages it occurs through interdendritic penetration. Macrosegregation is caused by fluid movement when the metal is partially solidified. Nucleation of the solid phase is affected by fragmentation of the dendrite branches and their transport to new sites where growth proceeds. A complete model of the solidification of castings must include all of these effects. This paper reviews programs that successfully compute each of these effects. Progress is being made on their integration into comprehensive modeling systems for foundry castings. It is important that work continue on modeling at both the macro-scale and micro-scale.     

C70250铜合金抗拉性能和导电性能研究

研究了铸态C70250铜合金的抗拉和导电性能,对合金的固溶与时效工艺进行了探讨,适当的加工和热处理工艺,可使合金力学性能和导电率都有很大的提高．     

The Role of the γ/ γeutectic and porosity on the tensile behavior of a single-crystal nickel-base superalloy

The microstructure of a single-crystal nickel-base superalloy, PWA 1480, has been varied by heat treatment and hot isostatic pressing in order to study the role of the γ/yγ′ eutectic and porosity on subsequent tensile behavior. The level of porosity was found not to affect any of the tensile properties, while the γ/γ′ eutectic strongly influenced ductility. Eliminating the γ/γ′ eutectic increased ductility which was attributed to the cleavage fracture of this constituent. It is proposed that such cleavage of the γ/γ′ eutectic is initiated by the stress created from impinging slip bands, promoting shear localization, and final fracture along {111} slip planes. The precise nature of this fracture process is discussed with emphasis on the role of the γ/′ micro-structure. The deformation structure of PWA 1480 was also studied, and while different in some respects from many other single-crystal superalloys, its fracture process appears to be similar. Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University.     

Effects of temperature on tensile properties and deformation behavior of GH4586A superalloy

With the rapid development of the aero-spaceindustry, the more excellent properties of the superal-loy used for turbine disk are required. GH4586 alloyis a new precipitation hardening Ni-base wrought su-peralloy which is developed independently in Chi-na[1,2]. GH4586 alloy has improved impact toughnessat RT(room temperature), high temperature stressrupture and creep properties with higher cleanlinessand it has already been applied on the turbine disk ofrocket with 120 Thigh driving force suc…     

温度对GH4586A合金拉伸性能及变形行为的影响

Effects of temperature on tensile properties and deformation behavior of the nickel-based superalloy GH4586A have been investigated. The results showed that deforming temperature has no effect on the microstructure of the alloy, while tensile properties are thermo-sensitive. With the increasing testing temperature the strength of the alloy decreased, and the ductility increased. While, the ductility of the alloy decreased weakly at the temperature range of 823 K to 923 K. And the main reason can be considered as the easily-broken of the MC type block carbides due to the stress concentration at the interface between the matrix and carbides to form the micro - cracks during the deforming process.     

A metallographic study of diffusion-induced grain boundary migration in the Fe-Zn system

Diffusion-induced grain boundary migration has been studied by zincification of pure iron. The temperature dependence of the rate was measured at 460–650°C A change in morphology was observed between 460 and 650°C and may explain the fact that the phenomenon changed character at higher temperatures. The temperature dependence of v/D^hδ may be decisive, v being the rate of migration. D^h the grain boundary diffusivity and δ the thickness of grain boundaries. The concentration behind the migrating boundaries was studied as function of the distance from the surface by microprobe measurements and allowed the grain boundary diffusivity to be evaluated as function of temperature.Fine zinc-rich grains of ferrite sometimes nucleate on the surface a form and complete fine-grained surface layer. After a while it disappears by abnormal grain growth but the increased zinc content remains.     

Heat treatment behavior and tensile properties of Cr−W steels

Ferritic steels containing Cr, W, and V are being developed for fusion reactor applications. These steels would be alternatives to the Cr−Mo steels that are being considered for structural components. Eight experimental steels were produced. Chromium concentrations of 2.25, 5, 9, and 12 pct were used. Steels with these chromium compositions and with 2 pct W and 0.25 pct V were produced. To determine the effect of tungsten and vanadium, 2.25Cr steels were also produced with 2 pct W and no vanadium, and with 0.25 pct V and zero and 1 pct W. A 9Cr steel containing 2 pct W, 0.25 pct V, and 0.07 pct Ta was also studied. For all alloys, carbon was maintained at 0.1 pct. Tempering behavior of the steels was similar to that of the Cr−Mo steels being considered. Tensile studies indicated that the 2.25Cr−2W−0.25V and 9Cr−2W−0.25V−0.07Ta steels had the highest strengths with properties similar to those of the 9Cr−1MoVNb and 12Cr−1MoVW steels, which are the strongest of the Cr−Mo steels of interest.     

A rationalization of tensile ductility and fracture in Alpha-Beta Ti-Mn alloys

Void nucleation and growth were studied in equiaxed α-gb Ti-Mn alloys, containing 1.8, 3.9, 5.8, and 8.0 wt pct Mn, in an attempt to understand why ductility tended to remain relatively constant, as both yield strength and fracture strength increased with increasing Mn content. In addition, measurements of volume fraction of voids,/,, and average void diameter,d _v, in the minimum neck section were made to determine whether catastrophic void coalescence was a possible mechanism of fracture. The values of_v andd _v at fracture were obtained by linear extrapolation of log_v andd _v vs true strain. These data indicated that the average distance between voids in the minimum neck section for Ti-3.9 Mn which contained the largest/, was most likely too large to permit catastrophic void coalescence to take place. These results supported earlier^1,2,3 and current observations that fracture occurred, during tensile straining, when a critical relationship was reached between void length and the fracture stress corrected for necking, σ_fc. It was shown that the changes in this critical relationship with both microconstituent size and volume fraction of phases were balanced by a change in void growth rate to critical size, with the result that strain to fracture remained nearly constant.     

Influence of cold forming on the tensile behavior and properties of low-carbon microalloyed steel

The tensile behavior and properties of cold formed low-carbon microalloyed steel with its microstructure of all ferrite and pearlite (F+P) were investigated.Bending and flattening deformations were carried out in the laboratory on hot-rolled sheets in order to simulate the cold forming process of steel sheets during pipe fabrication and sampling of high frequency straight bead welding pipes.A comparison of the tensile behavior and properties of the material made before and after cold forming indicates that cold deformation alters the tensile behavior and properties of the material to a certain degree depending on the manner of the cold deformation and the degree.The research on the Bauschinger effect indicates that for the steels investigated,when the plastic strain is small,the back stress increases rapidly with the increase of the plastic strain and then rapidly tends to saturation.The finite element analysis indicates that the change in the properties of the steel sheets due to cold forming is a result of the Bauschinger effect and work hardening.The mechanism of the change in the properties is also given in this study.     

Influence of neodymium addition on microstructure,tensile properties and fracture behavior of cast Al-Mg2Si metal matrix composite

The influence of Nd on the microstructures,tensile properties and fracture behavior of cast Al-18 wt.%Mg2Si in situ metal matrix composite was investigated.Experimental results showed that,after introducing a proper amount of Nd,both primary and eutectic Mg2Si in the Al-18 wt.%Mg2Si composite were well modified.The morphology of primary Mg2Si was changed from irregular or dendritic to polyhedral shape,and its average particle size was significantly decreased from 47.5 to 13.0 μm.Moreover,the morphology of the eutectic Mg2Si phase was altered from flake-like to a thin laminar,short fibrous or dot-like structure.Tensile tests revealed that Nd addition improved the tensile strength and ductility of the material.Compared with those of unmodified composite,the ultimate tensile strength and percentage elongation with 0.5% Nd were increased by 32.4% and 200%,respectively.At the same time,Nd addition changed the fracture behavior from brittle to ductile.     

A metallographic study of extruded reinforced composites based on iron and copper

Microprobe analysis has been applied to the boundary layers between fibers and matrix to examine the phase composition in relation to mode of extrusion for composite materials based on iron and copper as reinforced with molybdenum and steel fibers. The width of the interaction zone varies from 2 to 4 µm. The phases correspond to the phase diagrams for these systems. Fractography indicates the failure mechanism for reinforced composites under conditions of stress and strain. At the points of application of shock loads, there is planar transverse fracture in the fibers by the cleavage mechanism. The peripheral layers are subject to viscous failure in the fibers with the formation of necks. Extrusion produces plastic strain uniform throughout the fiber length, and there are no breaks in the fibers, which provides conditions for complete realization of the strength of the reinforcing phase.     

A metallographic study of extruded reinforced composites based on iron and copper

Microprobe analysis has been applied to the boundary layers between fibers and matrix to examine the phase composition in relation to mode of extrusion for composite materials based on iron and copper as reinforced with molybdenum and steel fibers. The width of the interaction zone varies from 2 to 4 µm. The phases correspond to the phase diagrams for these systems. Fractography indicates the failure mechanism for reinforced composites under conditions of stress and strain. At the points of application of shock loads, there is planar transverse fracture in the fibers by the cleavage mechanism. The peripheral layers are subject to viscous failure in the fibers with the formation of necks. Extrusion produces plastic strain uniform throughout the fiber length, and there are no breaks in the fibers, which provides conditions for complete realization of the strength of the reinforcing phase.     

热处理对Fe-Ni合金丝力学性能和膨胀特性的影响

分别采用X射线衍射、金相显微镜、原子力显微镜和透射电镜等组织表征手段以及室温拉伸和热膨胀系数测试等性能测试方法,对比研究了高温退火、低温时效处理对冷拔Fe-Ni合金丝微观组织演变、拉伸强度以及热膨胀系数的影响.研究结果表明:原始丝材强度高,但是室温热膨胀系数较大;而950℃退火导致晶粒粗化以及位错密度降低,虽然室温热膨胀系数低,但强度不足.相比之下,500℃较低温度的时效处理,能获得最优的强度/热膨胀系数组合(1189 MPa/0.2×10^-6℃^-1).对Fe-Ni合金丝相应的强化机制以及热膨胀系数的影响因素分别进行了讨论分析,分析结果表明:细晶强化与位错强化是该合金丝的主要强化机制,而热膨胀系数则主要受溶质原子-位错交互作用影响.揭示出,合适的热处理工艺选择对于Fe-Ni合金丝力学性能/热膨胀性能优化具有重要意义.