Mechanical Properties,Dislocation Density and Grain Structure of Ultrafine-Grained Aluminum and Aluminum-Magnesium Alloys

The kind and amount of alloying elements strongly affects the formation of ultrafine-grained microstructures. Aluminum alloys with different amounts of the alloying element magnesium, and a commercially pure aluminum alloy, have been investigated in order to evaluate how the obtained microstructures will affect the mechanical properties. X-ray profile analysis has been used to determine grain size and dislocation density. With increasing amounts of alloying elements, a smaller grain size and a higher dislocation density after severe plastic deformation (SPD) are obtained, which lead to higher hardness and improved fatigue properties. This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: from Basics to Application”, which occurred September 25-27, 2006 in Kloster Irsee, Germany.

---

Grain Boundaries and Mechanical Properties of Ultrafine-Grained Metals

The concept of grain boundary (GB) engineering of ultrafine-grained (UFG) metals and alloys is developed for enhancement of their properties by tailoring different GBs (low-angle and high-angle ones, special and random, or equilibrium and nonequilibrium) and formation of GB segregations and precipitations by severe plastic deformation (SPD) processing. In this article, using this approach and varying regimes and routes of SPD processing, we show for several light alloys (Al and Ti) the ability to produce UFG materials with different GBs, and this can have a dramatic effect on the mechanical behavior of the processed materials. This article demonstrates also several new examples of attaining superior strength and ductility as well as enhanced superplasticity at low temperatures and high strain rates in various UFG metals and alloys.     

Microstructure and Mechanical Properties of Ultrafine-Grained Interstitial-Free Steel Processed by ECAP

Equal-channel angular pressing (ECAP) of interstitial-free (IF) steel at equivalent strain, ε_vm = 12 has been employed to develop ultrafine-grained (UFG) microstructure with high fraction of low angle grain boundaries, that enhances strength significantly with reduced tensile ductility. ECAPed IF steel has been deformed further by cold rolling/cryorolling at ?50 °C to >90 % reduction in area. It is observed that the UFG structure gets refined with an improvement in high angle grain boundary fraction and heavily stressed non-equilibrium grain boundaries in cryorolled state resulting in significant strengthening. However, the decrease in grain size to an ultrafine level with the increased lattice strain lowers the work hardening ability of the material that limits its ductility. Hence, the rolled samples are flash annealed at 675 °C in order to recover the ductility of the material by achieving partially recrystallized structures. Consequently, the increased subgrain size as well as the grain size, the reduced residual lattice strain, lower hardness and strength with marginal recovery of ductility is maintained in order to attain the yield strength 2–3 times compared to that of as-received coarse-grained IF steel.     

Dispersion-Strengthened Copper Alloys with Useful Electrical and Mechanical Properties

Abstract

Dispersion-strengthened alloys have been made which combine a high level of tensile strength at temperatures up to at least 600°C with an electrical conductivity better than that of most precipitation-hardened copper alloys. The reverse gel precipitation process has been used to co-precipitate hydroxides which were then selectively reduced in hydrogen, consolidated under an atmosphere of pure argon, and finally hot-extruded to bar. Copper?3 vol.-% zirconia alloys were prepared in which all the particles were <150 nm dia., while copper–1·5 vol.-% thoria and copper–3 vol.-% thoria alloys were prepared with most particles <50 nm dia. Although the dispersion in the Cu–zirconia alloys was somewhat inferior to that obtained in the Cu–thoria alloys, useful properties were obtained. The Cu–zirconia alloys were as strong as the commercial alloy Cu–1 wt.-%Cr at 500°C and twice as strong at 600°C. There was little difference in the strength of a Cu–1·5 vol.-% thoria alloy and the Cu–zirconia alloys but the former was more ductile. The most interesting properties were obtained from Cu–3 vol.-% thoria alloys which exhibited an electrical conductivity in excees of 90% IACS at 20°C and tensile strength five times that of Cu–1%Cr at 600°C, even after annealing at 600°C for 1 h. The Cu–3 vol.-% thoria alloys were readily cold-worked, exhibited exceptional stability, and were resistant to recrystallization up to 900°C. Grain sizes were of the order of 1·5 μm for unalloyed copper, 1 μm for Cu–1·5% thoria, and 0·5 μm for Cu–3% zirconia or Cu–3% thoria. Grain growth was severely restricted by the dispersions.     

Effects of Electromagnetic Continuous Casting on Structures,Element Distributions and Mechanical Properties of Cu-Mg-Te-Y Alloys

New Cu-Mg-Te-Y alloys with high strength and high conductivity were prepared by electromagnetic continuous casting（EMCC）and direct chill casting（DC）.The structures,patterns of elements distributions and mechanical properties of the alloys were investigated.It was found that the equiaxed crystal zone was enlarged and the element Mg and Cu₂Te phase were well-distributed by medium-frequency electromagnetic field during continuous casting method.Moreover,the defects and casting impurities were significantly reduced with the addition of electromagnetic,leading to the improvement of the mechanical properties.     

不同组织形态TC4钛合金力学性能研究

对常见几种组织形态的TC4钛合金的拉伸性能、硬度进行了测试.利用金相显微镜观察各试样的显微组织特点,利用扫描电子显微镜分析拉伸断口形貌,并对几种组织形态力学性能存在差异的原因进行了分析.实验结果表明:具有双态组织的3#试样的强度与塑性匹配最好,且强度最高;5#试样所具有的魏氏体组织,在提高了拉伸力学性能指标的同时,硬度...     

FeNiCrAl合金的显微组织和力学性能

对FeNiCrAl合金显微组织和力学性能的研究表明：[Ni]/[Cr]0.9（摩尔比）的合金以γ相为基体,[Ni]/[Cr]（摩尔比）＝0.6～0.8和0.6的合金分别以γ十α双相和α相为基体,合金在不同温度热处理后,由于相的析出或溶解,显示出不同的力学性能.     

Investigation of Mechanical Properties of High-Density Iron-Based Alloys with Nanosized Additions

Metallurgist - Detailed analysis is provided for results of studying mechanical properties of alloys based on iron and nanodispersed particles of nickel oxide, copper oxide, and silicon nitride...     

Mechanical Properties and Nanocrystallization Behavior of Al-Ni-La Alloys

Rapidly solidified Al₈₉Ni₆La₅ ribbons were obtained by induction melting and ejecting the melt onto a rotating Cu wheel in an Ar atmosphere. The ribbons were investigated by differential scanning calorimetry (DSC), X-ray diffractometry (XRD), transmission electron microscopy (TEM), microindentation, and nanoindentation techniques. The XRD and TEM studies revealed that the ribbon was fully amorphous; however, DSC did not show any glass transition. The alloy undergoes two stages of crystallization. The growth of fcc-Al is responsible for the first stage, and precipitation of Al₃Ni and Al₁₁La₃ is responsible for the second stage of crystallization. Microhardness of ribbons in the as-melt-spun, partially, and fully devitrified conditions was examined and subsequently correlated with evolved microstructure. Significant improvement in hardness was observed, with the progress of primary nanocrystallization, due to the effective barrier to shear band by a hard La-rich shell around the fcc-Al nanocrystals and enrichment of the remaining amorphous matrix by the solute elements. The pile up of materials in the form of semicircular shear bands was observed around all the indentations. During nanoindentation, it was observed that hardness and modulus values were initially increased and then decreased. The reasons for such observation were also discussed.     

Al-La过共晶合金的组织特征及力学性能

采用真空熔炼方法,在石墨铸型中制备了过共晶合金Al-30%La、Al-35%La和Al-40%La试样,研究了它们的组织形貌特征和力学性能,发现过共晶合金Al-30%La、Al-35%La和Al-40%La中的枝晶是不连续的,其中Al-35%La中的Al11La3是周期性双相枝晶。力学性能测试结果表明,过共晶合金Al-30%La、Al-35%La和Al-40%La的抗拉强度较低,但抗压强度较高,并具有一定的相对压缩率(7.06%~15.11%);由硬度测试结果可知,随La含量的增加,Al-La合金的硬度随之增大,但合金Al-35%La的硬度呈现出一个低谷。     

铈变质处理Fe-Cr-B合金组织和机械性能研究

研究了铈变质处理对Fe-Cr-B合金组织性能的影响.结果发现,处理后的Fe-Cr-B合金的铸态组织较之以前明显细化;原来在晶界处分布的连续的、网状的硼化物转变成为不连续的短杆状的和粒状的形态,表明稀土对Fe-Cr-B合金有明显的变质效果.但机械性能随变质剂加入量不同变化比较复杂.     

Solidification Microstructure and Mechanical Properties of Cast Magnesium-Aluminum-Tin Alloys

The solidification microstructure and mechanical properties of as-cast Mg-Al-Sn alloys have been investigated using computational thermodynamics and experiments. The as-cast microstructure of Mg-Al-Sn alloys consists of α-Mg, Mg₁₇Al₁₂, and Mg₂Sn phases. The amount of Mg₁₇Al₁₂ and Mg₂Sn phases formed increases with increasing Al and Sn content and shows good agreement between the experimental results and the Scheil solidification calculations. Generally, the yield strength of as-cast alloys increases with Al and Sn content, whereas the ductility decreases. This study has confirmed an early development of Mg-7Al-2Sn alloy for structural applications and has led to a promising new Mg-7Al-5Sn alloy with significantly improved strength and ductility comparable with commercial AZ91 alloy.     

Three-Dimensional Network Structure and Mechanical Properties of Al-Cu-Ni-Fe Cast Alloys with Gd Micro-addition

High-temperature tensile testing and X-ray microscope (XRM) characterization were performed to assess the effects of the micro-addition of Gd and the three-dimensional (3-D) network structure on the improvement of Al-6Cu-3.5Ni-0.8Fe alloy under as-cast conditions. Gd addition contributed to the modification of the microstructure, where a new thermally stable micro-sized Al₃CuGd phase was formed, and the refinement occurred in Al₃CuNi and Al₉FeNi. The tensile test results revealed that the alloy modified with 0.4 pct Gd exhibited optimal properties at 623 K (350 °C), with an ultimate tensile strength, yield strength, and elongation of 74.1 MPa, 61.2 MPa, and 15.5 pct, respectively. Fractographic analysis after the tensile tests indicated that at ambient temperature, brittle cleavage-type fracture of the precipitates and ductile fracture of the matrix were dominant, whereas the transformation from mixed fracture to fully ductile trans-crystalline fracture was detected at elevated temperatures. According to the CT characterization, there was no significant difference in the curvature or interconnectivity of the 3-D network structure formed by the aluminides between before and after the tensile test at 623 K (350 °C). It is believed that the 3-D continuous network structure of aluminides, equipped with excellent heat resistance, plays a pivotal role in the high-temperature performance of the studied alloys. This work provides a new and promising idea for solving the current heat resistance problems of cast Al alloys.

     

Biaxial Deformation Behavior and Enhanced Formability of Ultrafine-Grained Pure Copper

Coarse-grained commercially pure Cu was subjected to equal channel angular pressing at room temperature for 2 passes and 12 passes resulting in grain refinement down to the ultrafine scale. Uniaxial tensile testing revealed that as-ECAP Cu samples have very high strength, but low uniform elongation and elongation to failure, whereas small punch testing showed that strain in biaxial stretching of the as-ECAP Cu specimens was comparable to that in the coarse-grained Cu. Analysis of surface relief demonstrated extensive microlocalization of plastic flow into microshear bands during biaxial stretching of the as-ECAP Cu specimens. The effect of microstructure and stress state on formability of the material and the mechanisms governing its plastic deformation are discussed. It is suggested that although the high strength as-ECAP Cu exhibits poor ductility in uniaxial tension, in other strain paths such as biaxial stretching, it can show high formability which is sufficient for metal-forming processes.     

Mechanical Properties and Crystallization Behaviors of Microstructured Co-Fe-P Amorphous Alloys

Microstructure with microsized amorphous grains is found in thick-film Co-Fe-P amorphous alloys prepared by electrodeposition. The amorphous alloys exhibit different structure, mechanical properties, and crystallization behaviors compared with those of homogeneous amorphous alloys, e.g., bulk metallic glasses. The mechanical properties of these microstructured amorphous alloys are characterized by tensile tests, dynamic mechanical analysis, and micro- and nanoindentations. Using a differential scanning calorimeter, the activation energy of the crystallization process of the microstructured amorphous alloy is measured and is found to depend on its microstructure.