Interdiffusion and Atomic Mobility for Face-Centered-Cubic Co-Al Alloys

Interdiffusion in the face-centered-cubic (fcc) Co-Al binary alloys was studied by the diffusion-couple approach in the temperature range of 1173 K to 1573 K (900 °C to 1300 °C). Interdiffusion coefficients of the fcc Co-Al alloys were then evaluated by using the Sauer–Freise method. The effect of magnetic ordering on the Co-Al interdiffusion was observed at 1273 K (1000 °C) by examining the Arrhenius plots. The interdiffusion data were assessed to develop the atomic mobility for the fcc Co-Al alloys, and their validity was tested by simulating the diffusion-couple experiments.     

Microstructural Evolution in Niobium-Based Alloys

Nb-Ti-Cr-Si-Al-X alloys have been considered as candidate materials for high-temperature structural applications in aero engines. Using a multicomponent approach, attempts are being made to obtain good strength with adequate oxidation resistance at the expected operating temperatures (∼1073 through 1273 K) in these alloys. In the microstructure of these alloys, which essentially consist of the silicide phase distributed in the bcc β matrix, the presence or absence of the Laves phase can bring about considerable changes in the mechanical properties. The present article reports on the instability observed in the β phase and also on the formation of the Laves phase in the β matrix. The extent of the chemical separation and elements responsible for this separation has been determined. Different morphologies of the Laves phase have been obtained by aging at different temperatures for various durations and the structure, chemistry, and volume fraction of these phases have been determined. This article is based on a presentation given in the symposium entitled “Materials Behavior: Far from Equilibrium” as part of the Golden Jubilee Celebration of Bhabha Atomic Research Centre, which occurred December 15–16, 2006 in Mumbai, India.

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Microstructural Control of Ti-Al-Nb-W-B Alloys

In this work, new TiAl alloys containing W, B, and Nb have been developed. Fine uniform microstructures, with colony size smaller than 50 μm, can be conveniently obtained after hot-isostatic pressing (HIP) and homogenization treatment without any grain-refining processes using hot deformation. The effects of tungsten and boron on the microstructure of the TiAl alloys, including the colony size and lamellar spacing, were analyzed. This work shows that a small additional amount of W can refine the grain size of the Ti-Al-Nb-W-B alloys. The lamellar spacing also decreases with increasing W concentration. When the amount of W is greater than 0.4 at. pct, the β phase is stabilized and remains at room temperature, which can degrade the ductility of the alloy. Mechanical properties, such as the hardness of the alloy, can be increased by the addition of the alloying element through the solution strengthening and refinement of the grain size. This article is based on a presentation given in the symposium entitled “Deformation and Fracture from Nano to Macro: A Symposium Honoring W.W. Gerberich’s 70th Birthday,” which occurred during the TMS Annual Meeting, March 12-16, 2006 in San Antonio, Texas and was sponsored by the Mechanical Behavior of Materials and Nanomechanical Behavior Committees of TMS.     

Advances in Microstructural Understanding of Wrought Aluminum Alloys

Metallurgical and Materials Transactions A - Wrought aluminum alloys are an attractive option in the quest for lightweight, recyclable, structural materials. Modern wrought aluminum alloys depend...     

Effects of Titanium Addition on the Microstructural and Mechanical Property Evolution of FeCrB Alloys

Metallurgical and Materials Transactions A - Effects of Ti addition on the microstructural and mechanical property evolution of FeCrB alloys have been systematically studied through experiments and...     

Microstructural Investigation and Phase Relationships of Fe-Al-Hf Alloys

The effect of Hf addition on microstructures, phase relationships, microhardness, and magnetic properties of Fe₅₀Al_50?n Hf _n alloys for n = 1, 3, 5, 7, and 9 at. pct has been investigated. At all investigated compositions, the ternary intermetallic HfFe₆Al₆ τ ₁ phase forms due to the limited solid solubility of Hf in FeAl phase and tends to develop a eutectic phase mixture with the Fe-Al-based phase. The Hf concentration of the eutectic composition is found to be 7 at. pct from the microstructural examinations and the eutectic phase transition temperature is determined as 1521 K (1248 °C) independent of Hf amount by differential scanning calorimetry measurements. Furthermore, the enthalpies and activation energies (based on Kissinger and Ozawa methods) of eutectic phase transitions are reported. The minimum activation energy is calculated for the fully eutectic composition. Moreover, variation of the microhardness of Fe-Al-based alloys as a function of the Hf content is investigated, and its dependence on the thermal history of the alloys is explained.     

Fe3Al合金形变诱导有序显微组织变化及合金元素的作用（Ⅰ）

研究了Fe₃Al基金属间化合物合金的室温变形和高温蠕变特点。结果表明,Fe₃Al合金室温形变将发生诱导无序化现象.蠕变过程中不仅发生形变诱寻无序化现象,而且发生蠕变加速有序转变现象,形变诱导存序显微结构变化将淡化原始结构对蠕变断裂寿命的影响.     

Cr-Ge-Si Alloys for High-Temperature Structural Applications: Microstructural Evolution

The microstructural evolution of Cr-Si_1?x -Ge _x (0 < x < 15 at. pct) alloys was studied (in annealed state). The quasi-isothermal section of the ternary diagram was assessed by quantitative EPMA analysis. Morphology, phase formation, chemical distribution, and indentation hardness of the alloys were investigated as a function of Ge/Si ratio. The microstructure of all studied alloys consisted of Cr_ss solid solution and A15 Cr₃X intermetallic phases. Substitution of Si by Ge strongly altered the microstructure by transforming the morphology from a lamellar eutectic Cr-Cr₃Si system toward a peritectic one with dispersed Cr_ss phase in A15 matrix. EPMA chemical distribution maps and X-ray diffraction results prove the mutual solubility of Si and Ge in A15 phase by forming Cr₃(Si,Ge) as a complex A15 structure with Cr₃(Si_1?x Ge _x ) composition. Precipitates of the intermetallic phase within the Cr_ss phase was observed in Ge-alloyed samples. Indentation hardness results showed that upon Si-Ge substitution the hardness of both phases was reduced. However, Si substitution by Ge had a stronger influence on the hardness of the solid solution phase than on the intermetallic phase.     

Microstructural Model for the Deformation of Shape Memory Alloys

A model is proposed to describe the phase-structural deformation of shape memory alloys with allowance for the nonuniform strain hardening of the martensite part of representative volume. A scheme is developed to determine the volume fraction of martensite undergoing structural transformation during proportional nonreversible loading. The problem of reactive-stress generation in experiments on orientational transformation with constrained deformation after unloading is resolved.     

Microstructural Evolution During Friction Surfacing of Dissimilar Aluminum Alloys

The microstructural evolution during friction surfacing of an aluminum alloy 6082-T6 rod on an aluminum alloy 2024-T351 substrate was characterized using the electron backscatter diffraction technique. Crystallographic data were obtained from several regions in the consumable material and in the deposited material. From the results, it can be deduced that the grain structure formation was a complex process governed by the geometrical effect of strain and the superposition of continuous and discontinuous dynamic recrystallizations.     

Al-Bi过偏晶合金显微组织演变的数值模拟

基于Al-Bi过偏晶合金凝固通过难混溶区阶段产生的液-液相分解及分离的运动行为,采用两相体积平均法,对质量、动量、能量、组分及液滴密度守恒方程进行数值模拟,计算中考虑了形核、扩散长大、Stokes运动及Marangoni运动等多种复杂物理现象的耦合作用,分析了两相运动速度、第二相尺寸分布、第二相体积分数分布以及液滴密度分布对过偏晶合金凝固的显微组织演变及宏观偏析的影响。结果表明,过偏晶合金凝固过程中显微组织演化在不同阶段的主要影响因素不同:凝固初始阶段主要以形核和扩散长大为主;凝固中期和后期第二相迁移运动行为将逐渐占主导作用。凝固过程中,铸件顶角位置首先获得过冷度驱动形核,并以较快的形核速率达到最大形核密度。随着凝固过程不断地进行,第二相小液滴受到的Marangoni力约为Stokes粘滞阻力的两倍,开始由铸件顶角和边缘低温区向中心高温区聚集。凝固时间为1 s时,铸件顶角和边缘第二相小液滴的生长直径和第二相体积分数大于铸件中心位置,而凝固时间为5和7 s时,第二相小液滴直径随铸件中心距离变化的曲线斜率随凝固过程的进行而逐渐变缓,长大速率逐渐变慢。     

Microstructural Effects in Superplastic Deformation

TheresultsofrecentChineseinvestigationsonmicrostructuralaspectsofsuperplasticdeformationarereviewed.Attentionisgiventograingro-wth,precipitate-freezoneformation,grainboundaryslidingandmigration,anddislocation(bothintragranulardislocationandgrainboundarydisloca-tion)activity.Inthefinalpartofthearticle,anewmodelofmicrograinsuperplasticdeformationbasedonthemovementofgrainboundarydisloca-tionsandinvolvingbothgrainboundaryslidinganddiffusionisdescribed.Thepredictionsofthemodelarecomparedwiththedataexperimentallydetermined.     

Effects of Indium Content on Microstructural,Mechanical Properties and Melting Temperature of SAC305 Solder Alloys

The present study investigated the effects of indium (In) addition on the microstructure, mechanical properties, and melting temperature of SAC305 solder alloys. The indium formed IMC phases of Ag₃(Sn,In) and Cu₆(Sn,In)₅ in the Sn-rich matrix that increased the ultimate tensile strength (UTS) and the hardness while the ductility (% EL) decreased for all In containing solder alloys. The UTS and hardness values increased from 29.21 to 33.84 MPa and from 13.91 to 17.33 HV. Principally, the In-containing solder alloys had higher UTS and hardness than the In-free solder alloy due to the strengthening effect of solid solution and secondary phase dispersion. The eutectic melting point decreased from 223.0°C for the SAC305 solder alloy to 219.5°C for the SAC305 alloy with 2.0 wt% In. The addition of In had little effect on the solidus temperatures. In contrast, the liquidus temperature decreased with increasing In content. The optimum concentration of 2.0 wt % In improved the microstructure, UTS, hardness, and eutectic temperature of the SAC305 solder alloys.     

Mechanical Properties and Microstructural Evolution of Friction-Stir-Welded Thin Sheet Aluminum Alloys

Friction stir welding of thin aluminum sheets represents a potential goal for aircraft and automotive industries because of the advantages of using this new technological process. In the current work, the microstructural evolution and mechanical behavior of 6082T6-6082T6, 2024T3-2024T3, and 6082T6-2024T3 thin friction-stir-welded joints were investigated. Uniaxial tensile testing at room temperature, 443 K, 473 K, and 503 K (170 °C, 200 °C, and 230 °C) was used to determine the extent to which these ultra-thin joints can be used and deformed. The tensile stress–strain curves showed a decrease of the flow stress with increasing temperature and decreasing strain rate. The ductility of 6082T6-6082T6 joints generally improved when deformed at warm temperatures. It was almost constant for the 6082T6-2024T3 and reached the higher value in the 2024T3-2024T3 when deformed at 443 K and 473 K (170 °C and 200 °C) when compared with the room temperature value. Tensile specimens fractured in the middle of the weld zone in a ductile mode. The precipitation and growth of S’ type phases strengthens 2024T3-2024T3 joints during deformation. In the 6082T6-6082T6, β″ precipitates show some increase in size but give a lower contribution to strength. At 503 K (230 °C), recovery mechanisms (dislocation reorganization inside the deformed grains) are initiated but the temperature was not enough high to produce a homogeneous subgrain structure.