Microstructures of Thermomechanically Treated Eutectoid Zn-Al Alloy

A continuously cast eutectoid Zn-Al alloy was extruded at about 250℃ and the complex microstructures of three phases (α. ε and η'E) were observed in the extruded eutectoid Zn-Al alloy.The fcc Al-rich α phase appeared as isolated particles with clear boundaries instead of the α phase occurring with diffuse boundary within the dendrites of the as-cast structure. A new unstable phase η'E was determined to have hexagonal close packed crystal structure. The decomposed β's, phase comprised the matrix. The new phase η'E decompoeed and a four phase transformation α+ε T'+η occurred afterwards during the isothermal holding. The extruded alloy tended to be stable after these two phase transformations. The early shrinkage and the following increase in dimension were related to the decomposition of the new phase η'E and the four phase transformation.     

High-Curie-temperature ferromagnetism in self-organized Ge1-xMnx nanocolumns

The emerging field of spintronics would be dramatically boosted if room-temperature ferromagnetism could be added to semiconductor nanostructures that are compatible with silicon technology. Here, we report a high-TC (>400K) ferromagnetic phase of (Ge,Mn) epitaxial layer. The manganese content is 6%, and careful structural and chemical analyses show that the Mn distribution is strongly inhomogeneous: we observe eutectoid growth of well-defined Mn-rich nanocolumns surrounded by a Mn-poor matrix. The average diameter of these nanocolumns is 3nm and their spacing is 10nm. Their composition is close to Ge(2)Mn, which corresponds to an unknown germanium-rich phase, and they have a uniaxially elongated diamond structure. Their Curie temperature is higher than 400K. Magnetotransport reveals a pronounced anomalous Hall effect up to room temperature. A giant positive magnetoresistance is measured from 7,000% at 30K to 200% at 300K and 9T, with no evidence of saturation.     

双模CNT/Al复合材料变形过程的代表性体积单元模拟

目的 研究双模CNT/Al复合材料在塑性加工过程中的微观结构演变行为。方法 通过ABAQUS/Python二次开发,对双模CNT/Al复合材料代表性体积单元(RVE)进行参数化建模,并基于显式动力学求解器和欧拉-拉格朗日耦合技术,对双模CNT/Al复合材料RVE模型的单向压缩过程和挤压过程进行数值模拟分析。结果 单向压缩模拟结果显示,结构心部异质区两侧的等效塑性应变较大,结构边缘基体区域的等效塑性应变相对较小;最大等效应力位于结构边角处的基体区域,而结构心部异质区的等效应力则相对较小。通过对异质区材料和形态的影响规律进行分析发现,当异质区为粗晶材料时,其变形后的形态更趋于扁平,而当异质区为细晶材料时,其变形后的形态更接近椭圆。通过挤压过程的模拟,实现了对变形过程中双模组织结构变化的分析,模拟结果显示,材料表面与模具贴合良好,异质区随变形的进行被不断拉长,部分异质区由于拉伸程度过大,甚至出现了分离的情况。结论 提出的模拟方法为双模CNT/Al复合材料的挤压变形工艺设计和变形后组织结构预测提供了有效手段。     

Solidification and microstructure of ZA27/SiCp composite fabricated by mechanical–electromagnetic combination stirring process

Abstract

The solidification behaviours and microstructural characteristics of both ZA27/SiC_p composites and monolithic ZA27 alloy were studied by using differential scanning calorimetry, scanning electron microscopy, transmission electron microscopy, electron probe microanalysis, and X-ray diffraction. It was found that there were differences in the transformation temperature and volume fraction of the phases, although the solidification process was almost identical for the composite and the monolithic alloy. The incorporation of SiC particles in the ZA27 alloy led to slight refinement of primary grains and reduced volume fraction of eutectic-like phase. The SiC particles obstructed Zn diffusion in the residual melt during the formation of proeutectic β phase, but promoted Zn diffusion from (Al) to η (Zn) phase during eutectoid transformation. During solidification, Cu was mainly segregated in the final solidification regions; Mg was present not only in the matrix but also on SiC particles; and oxide inclusions were mainly distributed around SiC particles. The matrix microstructure for both materials mainly consisted of primary cores of Al rich +η eutectoid; β′ phase resulting from the eutectoid transformation of the proeutectic β phase; and Zn rich +η eutectoid resulting from the eutectoid transformation of the eutectic-like phase. The SiC particles were mainly distributed around the primary grains. Several new phases based on the Al–Zn–Mg–Cu system and interfacial reaction products, including Al₂₁Fe₃Si, Cu₅Zn₈, Mg₆Cu₃Al₇, MgAl₂O₄, and amorphous oxide inclusions, were identified in the final solidification regions. The nucleation of both primary phase and eutectic-like +η phase at the surface of SiC particles and their crystallographic orientation relationships were investigated theoretically and experimentally. No distinct crystallographic orientation relationship between the matrix and SiC has been identified, although the mismatch between (0001)_SiC and (111)_ was calculated to be as small as 7·6%.     

Composites of mesoporous Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Fe- or Co-containing nanoparticles

Composites of mesoporous alumina and Fe- or Co-containing ferromagnetic nanoparticles have been prepared from appropriate aqueous metal chloride solutions by a sol-gel process. The synthesis products were heat-treated at different temperatures in air or vacuum and then characterized by a variety of techniques. The results indicate that the mesoporous structure of the nanostructured composites has a large specific surface area, a narrow, unior bimodal pore size distribution, and high thermal stability. The phase composition of the magnetic particles in the X-ray amorphous oxide matrix strongly depends on the heat-treatment conditions.     

Preliminary investigations of the phase composition and fine pore structure of super-critically carbonated cement pastes

Super-critical carbonation of cement-based materials can lead to significant improvements in their properties. Preliminary investigations suggested that processing should be aimed at producing a matrix material with minimal amounts of Ca(OH)₂, anhydrous material and C—S—H gel along with a controlled pore structure. Using ²⁹Si MAS NMR and TGA as the principal investigative techniques it has been shown that moisture content during carbonation is a major factor in determining the phase composition and pore structure of the resulting matrix. Of the drying regimes studied, 60% DOD gave the greatest amount of conversion to calcium carbonate and silica gel.     

Effect of Solid Solution Supersaturation on Precipitation of γ′ in Rapidly Quenched Ni-Al Binary Alloys

The effect of solid solution supersaturation on the precipitation of γ′ in rapidly quenched Ni-Al binary alloys containing 11.6, 14.2 and 16.5 at. pct Al was investigated. The samples were solutioned at 1250°C, quenched in iced brine, and then analyzed by electron microscopy and XRD (X-ray diffraction) techniques. In Ni-11.6 at. pct Al alloy, ordering and phase separation took place simultaneously, resulting in a uniform distribution of γ′ . A transition from uniform to bimodal γ′ phase distribution occurred in the composition range between 14.2 and 16.5 at. pct Al. This transition was accompanied by changes in the morphology of γ′ precipitates. The microstructural observations were discussed in view of both kinetics and crystallographic considerations.     

Fe和C元素对铜合金组织结构及力学性能的影响

目的 增强铜合金的强度、硬度,拓展该合金的应用范围.方法 在铜粉中添加不同比例共析成分的铁粉和石墨粉,利用真空热压烧结法,制备Cu-Fe-C合金.结果 热压烧结可以原位生成渗碳体;随着铁粉、石墨粉添加量的增加,铜基体的硬度持续增加,Cu-Fe-C合金的抗拉强度先上升后下降;当铁粉和石墨粉的质量分数为5％时,抗拉强度由197 MPa增大至281 MPa,提升了43％.结论 强度和硬度的升高是第二相引起的晶粒细化和第二相强化所致,强度先升后降的原因是随着石墨粉的增加,石墨粉聚集区逐渐增多,割裂了铜基体.     

On the thermal conductivity of bimodal SiC/A356 composites fabricated via powder metallurgy route

The present study investigates the thermal conductivity of bimodal SiC particulate distribution in aluminum matrix composites fabricated via powder metallurgy route. The effects of the SiC_p reinforcement size distribution and processing parameters such as sintering time and temperature on the thermal conductivity have been examined. The Box–Behnken experimental array was employed to identify the effects of selected variables on the thermal conductivity of the composite. A reasonable augmentation in the thermal conductivity was observed with an increase in sintering time and %volume fraction of fine SiC particulates. It has been demonstrated that the matrix doped with fine SiC particulates (37?µm) occupied interstitial positions and formed continuous SiC–matrix network resulting in minimizing the micropores that contributed for good thermal conductivity, that is, 235?W/mK. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) were conducted to evaluate the microstructure architecture and interfacial phase formation.     

Creep-induced phase transformations in furnace cooled Zn-Al alloy

Phase transformation and microstructural change of a furnace-cooled eutectoid Zn-Al alloy specimen (76 wt% Zn-22 wt% Al-2 wt% Cu) were investigated during creep testing by using scanning electron microscopy and X-ray diffraction techniques. Creep-induced decomposition of a metastable η′T phase and a four-phase transformation, α+ε→T′+η, occurred during the creep testing. Also microstructural change was observed from a lammellar structure partially into a spheroidized structure in the rupture part of the furnace-cooled alloy. The mechanism of the creep rupture of the furnace-cooled eutectoid Zn-Al alloy was also discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Surface alloying of magnesium and AZ91 by thermochemical treatment in a medium containing zinc chloride and potassium chloride

Surface layers enriched with zinc were formed on pure magnesium and AZ91 alloy by heating the specimens in contact with paste containing zinc chloride and potassium chloride at 440 °C for 2 h, using no protective atmosphere. The study involved determining the microstructure, phase composition and microhardness of the resulting layers. During the layer formation process, a transient liquid phase occurred at the substrate/paste interface. The layers fabricated on both substrates were about 200 μm in thickness and they were metallurgically bonded to the substrate material. The same process conditions were used for both types of substrates. From the experimental data it is clear that the layer fabricated on magnesium differed in microstructure from that formed on AZ91. The alloyed layer on magnesium was characterised by dendrites of a solid solution of zinc in magnesium surrounded by a lamellar eutectoid structure composed of an MgZn intermetallic phase and a solid solution of zinc in magnesium. In the layer formed on AZ91, aluminium was detected in all the structural constituents. The layer was composed of a solid solution of zinc and aluminium in magnesium and Mg₁₇(Al, Zn)₁₂ and Mg₅Al₂Zn₂ intermetallic phases. The alloyed layers had much higher hardness than the substrate materials.     

Effect of Solid Solution Supersaturation on Precipitation of γ'''' in Rapidly Quenched Ni-Al Binary Alloys

The effect of solid solution supersaturation on the precipitation of γ' in rapidly quenched Ni-Al binary alloys containing 11.6, 14.2 and 16.5 at. Pct Al was investigated. The samples were solutioned at 1250℃, quenched in iced brine, and then analyzed by electron microscopy and XRD (X-ray diffraction) techniques. In Ni-11.6 at. Pct Al alloy, ordering and phase separation took place simultaneously, resulting in a uniform distribution of γ'. A transition from uniform to bimodal γ' phase distribution occurred in the composition range between 14.2 and 16.5 at. Pct Al. This transition was accompanied by changes in the morphology of γ' precipitates. The microstructural observations were discussed in view of both kinetics and crystallographic considerations.     

Influence of Preparation Technology on the Structure and Phase Composition of MoSi2-Mo5Si3/SiC Multi-coating for Carbon/Carbon Composites

Oxidation protective MoSi₂-Mo₅Si₃/SiC multi-coatings for carbon/carbon composites were prepared by chemical vapor reaction and slurry-sintering method. The influence of preparation technology on the structure and phase composition of the coating was investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analyses, and then their relationship was discussed. The results indicate that the Si/Mo ratio of the slurry and sintering processing were two main factors that significantly affected the structure and phase composition of the multi-coating. Appropriate sintering process and relatively high Si/Mo ratio were essential for preparing the multi-coating with dense structure and favorable phase composition. After being sintered at 1723 K for 2 h and with the Si/Mo ratio of the slurry being 4.5 (weight ratio), a dense structure accompanied by favorable phase composition of the coating can be obtained. When heat treated at 2373 K for 1 h, this coating became more compact and continuous. Oxidation tests (performed at 1623 and 1823 K) demonstrated that both of these two obtained multi-coatings exhibited better anti-oxidation property than single layer SiC coating.     

Eutectoid transformation in Au–39 at.-%In

Abstract

The kinetics of the isothermal eutectoid transformation in a eutectoid Au–39 at.-%In alloy has been studied in the temperature range 393–473 K. The maximum extent of growth of the eutectoid colonies and the lamellar spacing were determined through optical and scanning electron microscopy. The analysis of the kinetic data using existing classical and ledge growth models of boundary diffusion controlled eutectoid transformation yielded a low activation energy of 111 kJ mol^-1, indicating that the eutectoid transformation in this alloy is an interface diffusion controlled process. Comparison with interface boundary diffusivity data appears to suggest that the ledge growth model may be valid for the present system. The microscopy observations indicated that the parent ψ phase experiences a continuous precipitation of γ′ phase. This precipitation reaction precedes and accompanies the eutectoid decompos ition of the alloy, and the separate γ′ precipitates become embedded in the eutectoid colonies.     

Bimodal grain size distribution: an effective approach for improving the mechanical and corrosion properties of Fe–Cr–Ni alloys

The effect of nanocrystalline grain size and bimodal distribution of nano- and microcrystalline grain sizes on the oxidation resistance and mechanical properties of Fe-based alloys has been investigated. Nanocrystalline and bimodal Fe–10Cr–5Ni–2Zr alloy pellets, prepared by mechanical alloying route, have been compared with conventional microcrystalline stainless steel alloys having 10 and 20 wt% Cr. Zr addition has been shown to improve the grain size stability at high temperatures. A significant improvement in the ductility of bimodal alloys with respect to nanocrystalline alloys was seen presumably due to the presence of the microcrystalline grains in the matrix. The high temperature oxidation of nanocrystalline and bimodal alloys at 550 °C shows superior oxidation resistance over microcrystalline alloy of similar composition (Fe–10Cr–5Ni) and comparable to that of microcrystalline alloy having twice as much Cr (Fe–20Cr–5Ni). Secondary Ion Mass Spectroscopy depth profiling confirms the hypothesis that nanostructure facilitates the enrichment of Cr at the oxide metal interface resulting in the formation of a passive oxide layer.     

Microstructural investigation of a Co-base alloy processed by liquid phase sintering

The microstructure of samples processed by liquid phase sintering of prealloyed atomized powders is investigated by microprobe analysis, X-ray and electron diffraction. Materials obtained from thin powders are shown to consist of a dispersion of M₇C₃ in a Co matrix; the composition of the phases corresponds to the thermodynamic equilibria. A coarser powder provides a cellular structure containing islands of very thin spheroidal structure as well as M₇C₃ and a Co matrix. The W level is very high in these islands (star phase). A comparison of compositions indicates these islands correspond to the massive solidification of the undercooled liquid phase strongly saturated in W, Cr and C. For all the samples, during ageing, intense precipitation of M₂₃C₆ occurs in coherency with the Co matrix. The structure of the star phase coarsens and the W level decreases. After long ageing the star phase consists of only M₇C₃ and a Co matrix.     

Eutectoid decomposition in Fe2Si5 based alloys with a small amount of 10(Pd, Pt) and 11(Cu, Ag, Au) group elements

The addition of a small amount of Cu is effective in accelerating the + Si eutectoid decomposition. Some elements (Pd, Pt, Ag and Au) that are expected to have similar chemical properties were added to an Fe₂Si₅ based alloys up to 1.0 at.% to examine whether a similar effect could be revealed. The microstructures, X-ray diffraction and differential thermal analysis (DTA) of slowly solidified or heat treated specimens were investigated in detail. The solubility of all containing elements into the phase was less than 0.2 at.% for the slowly solidified specimen. The excess of the additive solidified as a phase of a eutectic with the Si phase. On the other hand, the solubility of these additives in the phase was classified into two groups. The first group had higher solubility in the phase than that in the phase. The solubility of the other group was as low as that in the phase. Pd and Au belonged to the former and Pt and Ag belonged to the later. The acceleration of the eutectoid decomposition was only observed in the former group but it was negligible in the later group. The existence of the eutectic melt at the annealing temperature was effective on the coarsening of the eutectoid structure but not essential for the acceleration. The eutectoid decomposition strongly depended on the solubility of these elements into the phase.     

High temperature structural evolution of a CuAlNi alloy studied by in situ X-ray diffraction and isothermal mechanical spectroscopy

The purpose of this work is to study the microstructural mechanisms associated with the eutectoid transition in a ternary Cu–12 wt.% Al–3 wt.% Ni alloy. The samples have been initially annealed at 850 °C, then slowly cooled down to room temperature. The experiments have been carried out both on cooling and on heating above 500 °C using isothermal mechanical spectroscopy and X-ray diffraction (fitted with a temperature camera). On heating, a relaxation peak with a high intensity rises up above 600 °C, then on cooling, the peak totally disappears below 580 °C, the effect being reproducible. The structural analysis, undertaken in the same temperature domain, has clearly evidenced each step of the evolution, particularly the eutectoid transformation. Consequently, the damping effect seems to be associated to the presence of the high temperature β phase.     

Effect of Barothermal Processing on the Solid-State Formation of the Structure and Properties of 16 at % Si–Al Hypereutectic Alloy

We describe barothermal processing (hot isostatic pressing) of a 16 at % Si–Al binary alloy for 3 h at a temperature of 560°C and pressure of 100 MPa for 3 h, in combination with measurements of heat effects during cooling. The results demonstrate that this processing leads to the fragmentation of the silicon structural constituent and ensures a high degree of homogenization of the as-prepared alloy. Heat treatment of the 16 at % Si–Al alloy at 560°C and a pressure of 100 MPa leads to a thermodynamically driven enhanced silicon dissolution, up to ~10 at %, in the aluminum matrix, resulting in the formation of a supersaturated solid solution, which subsequently decomposes during cooling. We analyze the complete porosity elimination process, which makes it possible to obtain a material with 100% relative density. According to differential barothermal analysis, microstructural analysis, and scanning and transmission electron microscopy data, barothermal processing of the 16 at % Si–Al alloy produces a bimodal size distribution of the silicon phase constituent: microparticles 3.6 μm in average size and nanoparticles down to ~1 nm in diameter. The Al matrix has been shown to contain a high density of edge dislocations. Barothermal processing reduces the thermal expansion coefficient and microhardness of the hypereutectic alloy. We conclude that solid-state barothermal processing is an effective tool for completely eliminating microporosity from the 16 at % Si–Al alloy, reaching a high degree of homogenization, and controlling the microstructure of the alloy, in particular by producing high dislocation density in the aluminum matrix.     

Tensile Stress Induced Phase Transformations in Zn-Al Alloy

Both furnace cooled and as-cast eutectoid Zn-Al alloys were investigated under external tensile stress at 100℃. It was observed that the external tensile stress caused decomposition of two metastable phases η'T and η'S which derived from both original state of the alloy, and a phase transformation, αf +ε→T' +η, in both furnace cooled and as-cast eutectoid Zn-Al alloys. Also spheroidized structure formed partially during tensile testing. Superplasticity of the alloy has been discussed correlating with the phase transformations and microstructural changes.