Mg对Zn--11%Al合金镀层凝固组织及合金层生长的影响

将工业纯铁分别在510℃的Zn-11% Al、Zn-11% Al-1.5% Mg、Zn-11% Al-3% Mg和Zn-11% Al-4.5% Mg合金熔池中进行不同时间的热浸镀,使用X射线衍射仪、扫描电子显微镜、能谱仪等仪器设备,研究Mg含量对Zn-11% Al合金镀层凝固组织和镀层中Fe-Al合金层生长的影响.结果表明:Zn-11% Al合金镀层凝固组织由富Al相和Zn/Al二元共晶组成;随着Zn-11% Al-x% Mg合金中Mg含量的增加,合金镀层的凝固组织中逐渐出现Zn/Al/MgZn₂三元共晶、块状MgZn₂相和Al/MgZn₂二元共晶.四种合金镀层中合金层主要由Fe₂Al₅Zn_x和FeAl₃Zn_x相组成,合金层的厚度随浸镀时间的增加而增加,Mg含量的增加使Fe-Al合金层生长速率指数和生长速率降低.在Zn-11% Al合金镀层中Fe-Al合金层形成的初期,可形成致密稳定的Fe-Al化合物层;热浸镀120 s后,扩散通道的移动使Fe-Al化合物层失稳破裂.Zn-11% Al-x% Mg合金中Mg元素可明显推迟液Zn进入镀层中Fe-Al合金层的时间,使Fe-Al合金层更加稳定和致密.      

Solidification of an alloy 625 weld overlay

The solidification behavior (microsegregation, secondary phase formation, and solidification temperature range) of an Alloy 625 weld overlay deposited on 2.25Cr - 1Mo steel by gas metal arc welding was investigated by light and electron optical microscopy, electron microprobe, and differential thermal analysis techniques. The overlay deposit was found to terminate solidification at ≈ 1216 °C by aγ/Laves eutectic-type reaction. The Laves phase was highly enriched in Nb, Mo, and Si. The solidification reaction and microsegregation potential of major alloying elements in the overlay deposit are compared to other Nb-bearing Ni base alloys and found to be very similar to those for Alloy 718. Solidification cracks observed in the overlay were attributed to the wide solidification temperature range (≈170 °C) and formation of interdendritic (γ+Laves) constituent. Reasonable agreement is obtained between the calculated and measured volume percent (γ+Laves) constituent with the Scheil equation by treating the overlay system as a simpleγ-Nb “binary” and using an experimentally determinedk ^Nb value from electron microprobe data.     

Solidification Analysis of an Al-19 Pct Si Alloy Using <Emphasis Type="Italic">In-Situ</Emphasis> Neutron Diffraction

In-situ neutron diffraction and thermal analysis techniques were used simultaneously to evaluate the kinetics of the nonequilibrium solidification process of an Al-19 pct Si binary alloy. Feasibility studies concerning the application of neutron diffraction for advanced solidification analysis were undertaken to explore its potential for high resolution phase analysis coupled with fraction solid/liquid analysis of phase constituents. Neutron diffraction patterns were collected in a stepwise mode during solidification between 983 K and 793 K (710 °C and 520 °C). The variation of intensity of the diffraction peaks was analyzed and compared to the results of conventional cooling curve analysis. Neutron diffraction was capable of detecting nucleation of the Si phase (primary and eutectic), as well as the Al phase during Al-Si eutectic nucleation. Moreover, neutron diffraction indicated the possibility of detecting the presence of Si peaks at near liquidus temperature and premature nucleation of α-Al prior to Al-Si eutectic temperature. The solid and liquid volume fractions were determined based on the change of intensity of neutron diffraction peaks over the solidification interval. Overall, the volume fraction determined was in good agreement with the results of the cooling curve thermal analysis, as well as calculations using the FactSage software. The potential of neutron diffraction for high resolution melt analysis required for advanced studies of grain refining, eutectic modification, etc. was illustrated. This study will help us better understand the solidification mechanism of Al-Si alloys used for various casting component applications.     

Influence of Ti and C on the Solidification Microstructure of Fe-10Al-5Cr Alloys

The influence of Ti and C on the solidification microstructure of Fe-10Al-5Cr (all composition values in weight percent) alloys was examined with solidification modeling and a variety of experimental techniques. Several Fe-10Al-5Cr-Ti-C alloys were fabricated using the arc button melting process and characterized using quantitative image analysis and electron microscopy techniques. The experimental alloys exhibited primary ferrite dendrites with an interdendritic ferrite/TiC eutectic constituent, and the amount of eutectic was affected by the Ti and C concentrations. A liquidus projection and primary solidification paths were calculated for the Fe-10Al-5Cr-Ti-C system in order to estimate the amount of TiC that is expected to form during solidification. The range in the calculated amount of TiC-containing eutectic matched the experimentally measured values reasonably well. The ability to control the amount of TiC that forms during solidification of an Fe-10Al-5Cr-Ti-C-based alloy shows promise for developing corrosion-resistant weld overlay claddings with resistance to hydrogen cracking.     

Solidification behavior,microstructure and silicon twinning of Al-10Si alloys with ytterbium addition

The solidification behavior, micro structure and silicon twinning of Al-10 Si alloys with Yb addition were investigated by thermal analysis, optical microscopy, X-ray diffraction. scanning electron microscopy, and transmission electron microscopy. The results indicate that the nucleation temperature, minimum temperature, and growth temperature of Al-lOSi alloys decrease with increasing Yb content. The cooling curves of the Yb-modified alloys exhibit marked recalescence. The recalescence of the modified alloy peaks at 2.3 ℃ when the Yb content is 0.7 wt%. The 3 D morphologies of eutectic Si in Yb-modified alloys change from a coarse plate-like structure to a honeycomb structure with many fine fibrous structures.The Al-Si-Yb intermetallic compound is observed in the 1.0 wt% Yb-modified alloy. Meanwhile, XRD analysis and TEM results indicate that the average twin spacing in the Yb-modified alloys is 18-46.2 nm.The average twin spacing of eutectic Si decreases with increasing Yb content. When the Yb content in the modified alloy is increased to 0.7 wt%, the average twin spacing value of eutectic Si reaches to 18 nm,which promotes the formation of twins and refinement of eutectic Si.     

Density and solidification shrinkage of hypoeutectic aluminum-silicon alloys

The density of liquid and solid hypoeutectic aluminum-silicon alloys has been measured with high accuracy in the temperature range 400 °C to 800 °C by using the indirect Archimedian method. A eutectic mixture of KCl and LiCl salts was used as reference liquid. This method allowed measurements of density for both liquid and solid in the same experiment and thus reduced the systematic error in estimating the solidification shrinkage. The results show that the density of liquid aluminum-silicon alloy increases with increasing silicon content, while silicon reduces the density in the solid state. Silicon content reduces the solidification shrinkage from 6.6 pct for pure aluminum to 4.4 pct for Al-11.6 pct Si.     

Microstructure and mechanical properties of unidirectionally solidified Al-Si-Ni ternary eutectic

Al-10.98 pct Si-4.9 pct Ni ternary eutectic alloy was unidirectionally solidified at growth rates from 1.39μm/sec to 6.95μm/sec. Binary Al-Ni and Al-Si eutectics prepared from the same purity metals were also solidified under similar conditions to characterize the growth conditions under the conditions of present study. NiAl₃ phase appeared as fibers in the binary Al-Ni eutectic and silicon appeared as irregular plates in the binary Al-Si eutectic. However, in the ternary Al-Si-Ni eutectic alloy both NiAl₃ and silicon phases appeared as irregular plates dispersed in α-Al phase, without any regular repctitive arrangement. The size and spacing of NiAl₃ and Si platelets in cone shaped colonies decreased with an increase in the growth rate of the ternary eutectic. Examination of specimen quenched during unidirectional solidification indicated that the ternary eutectic grows with a non-planar interface with both Si and NiAl₃ phases protruding into the liquid. It is concluded that it will be difficult to grow regular ternary eutectic structures even if only one phase has a high entropy of melting. The tensile strength and modulus of unidirectionally solidified Al-Si-Ni eutectic was lower than the chill cast alloys of the same composition, and decreased with a decrease in growth rate. Tensile modulus and strength of ternary Al-Si-Ni eutectic alloys was greater than binary Al-Si eutectic alloy under similar growth conditions, both in the chill cast and in unidirectionally solidified conditions.     

Microstructure and mechanical properties of unidirectionally solidified Al-Si-Ni ternary eutectic

Al-10.98 pct Si-4.9 pct Ni ternary eutectic alloy was unidirectionally solidified at growth rates from 1.39μm/sec to 6.95μm/sec. Binary Al-Ni and Al-Si eutectics prepared from the same purity metals were also solidified under similar conditions to characterize the growth conditions under the conditions of present study. NiAl₃ phase appeared as fibers in the binary Al-Ni eutectic and silicon appeared as irregular plates in the binary Al-Si eutectic. However, in the ternary Al-Si-Ni eutectic alloy both NiAl₃ and silicon phases appeared as irregular plates dispersed in α-Al phase, without any regular repctitive arrangement. The size and spacing of NiAl₃ and Si platelets in cone shaped colonies decreased with an increase in the growth rate of the ternary eutectic. Examination of specimen quenched during unidirectional solidification indicated that the ternary eutectic grows with a non-planar interface with both Si and NiAl₃ phases protruding into the liquid. It is concluded that it will be difficult to grow regular ternary eutectic structures even if only one phase has a high entropy of melting. The tensile strength and modulus of unidirectionally solidified Al-Si-Ni eutectic was lower than the chill cast alloys of the same composition, and decreased with a decrease in growth rate. Tensile modulus and strength of ternary Al-Si-Ni eutectic alloys was greater than binary Al-Si eutectic alloy under similar growth conditions, both in the chill cast and in unidirectionally solidified conditions.     

The composition of interdendritic eutectic

A theory is given which relates the composition of interdendritic “eutectic” forming during unidirectional solidification to solidification variables. This theory shows that the composition of the solid freezing from the interdendritic liquid is not equal to the eutectic composition but is exactly equivalent to the composition of the alloy which would just grow with a planar front at the same growth rate and temperature gradient. It is shown that this leads to a method for determining the critical values of temperature gradient and growth rate for interface stability of off-eutectic composites. Experimental results for the binary Al-Cu system and the ternary Al-Cu-Ni system give good agreement with theory.     

冷却速度对Zn--5 Al--0.1 RE--x Si合金显微组织及耐蚀性能的影响

运用扫描电子显微镜/能谱仪、X射线衍射、盐雾实验、电极化曲线等手段,研究冷却速度和Si对Zn-5Al-0.1RE合金组织及耐蚀性能的影响.结果表明,Zn-5Al-0.1RE-xSi合金由先析出的η-Zn和η-Zn+α-Al共晶组织组成,前者均匀分布在相邻的η-Zn+α-Al共晶胞的边界上.降低冷却速度和Si的加入,均使Zn-5Al-0.1RE-xSi合金单位面积的晶粒增大,晶界减少,合金耐蚀性能提高.Zn-5Al-0.1RE-xSi合金耐蚀性能的差异与合金凝固组织及合金腐蚀产物中Zn₅(OH)₈Cl₂·H₂O和ZnO的相对量有关.      

Evolution of Secondary Phases Formed upon Solidification of a Ni-Based Alloy

The solidification of UNS N08028 alloy subjected to different cooling rates was studied, where primary austenite dendrites occur predominantly and different amounts of sigma phase form in the interdendritic regions. The solidification path and elemental segregation upon solidification were simulated using the CALPHAD method, where THERMO-CALC software packages and two classical segregation models were employed to predict the real process. It is thus revealed that the interdendritic sigma phase is formed via eutectic reaction at the last stage of solidification. On this basis, an analytical model was developed to predict the evolution of nonequilibrium eutectic phase, while the isolated morphology of sigma phase can be described using divorced eutectic theory. Size, fraction, and morphology of the sigma phase were quantitatively studied by a series of experiments; the results are in good agreement with the model prediction.     

The role of iron in the formation of porosity in Al-Si-Cu-based casting alloys: Part III. A microstructural model

Iron has been shown to have a significant effect on the formation of porosity and shrinkage defects in Al-Si-Cu-based foundry alloys. This is not simply a direct consequence of the physical presence of the β-Al₅FeSi platelets in the microstructure, but is also due to the effect that these platelets have on the nucleation and growth of eutectic silicon. The alloy-dependent critical iron content determines when the β phase first solidifies and, hence, when it can participate in the silicon nucleation event. At critical iron contents, the β phase solidifies as the initial component of the ternary eutectic. However, at supercritical iron contents, the β phase is already well developed when ternary eutectic solidification begins, while, at subcritical iron contents, the β phase forms as a component of the ternary eutectic only after the binary Al-Si eutectic is well established. Each of these paths of microstructural evolution leads to different variations in microstructural permeability and, hence, interdendritic feedability and porosity formation. The actual porosity-forming response to these alloy-induced microstructural changes is influenced by the solidification conditions in the casting.     

Formation of silica and silicates during the solidification of Fe−Si−O alloys

Small (35 g) ingots of Fe?Si?O alloys have been solidified under controlled conditions and the form, distribution, and compositions of the oxides in the cast structures are reported. The composition of a melt was established by charging a preselected amount of silicon, 0.002 < pct Si < 0.7, and equilibrating the melt with the silica crucible at 1550° or 1650°C. The concentration of FeO and the morphology of the oxide particles in the cast structures indicate that during solidification the process of segregation causes the oxygen potential in the interdendritic liquid to increase over that in equilibrium with the silicon content of the liquid. It appears that nuclei for the formation of the oxide phases, which probably are in the liquid prior to the onset of solidification, become surrounded by solid iron and are made ineffective. It is possible at lower silicon levels that homogeneous nucleation may result by the melt becoming sufficiently supersaturated in oxygen to form FeO or an iron-silicate liquid phase.     

Influence of Phosphorus on the Nucleation of Eutectic Silicon in Al-Si Alloys

The role of phosphorus (P) in the heterogeneous nucleation of eutectic silicon (Si) and the evolution of eutectic grains in hypoeutectic aluminum-silicon alloys were investigated. Systematic additions of P in the range of 0.5 to 20 ppm to Al-7 wt pct Si alloys of different purities have shown that the morphology of the eutectic Si changes from a fine plate- to a coarse flake-like structure. The growth of eutectic grains was investigated by interrupting the eutectic reaction by quenching experiments. Moreover, the macroscopic growth mode of the eutectic grains was characterized by electron backscatter diffraction. An increase in P concentration from 2 to 3 ppm resulted in a transition of the macroscopic growth mode of the Al-Si eutectic in high purity alloys from growth with a planar front with a strong dependence of the thermal gradient, to nucleation in the vicinity of the primary Al dendrites and subsequent growth of distinct eutectic grains. It is suggested that AlP particles are the key impurities acting as potential nucleation sites for eutectic Si. This is further substantiated as with increasing P concentration nucleation and growth of the Al-Si occurred at higher temperatures close the equilibrium Al-Si eutectic solidification temperature at 850 K (577 °C). In addition, the recalescence undercooling ΔT _R,eu was reduced from 4.5 K (0.5 ppm P) to 1.5 K (20 ppm P) in high purity alloys. This was accompanied by a drastic increase of the nucleation rate of the eutectic grains.     

Solidification characteristics of the Al-8.3Fe-0.8V-0.9Si alloy

Studies of solidification behavior have been conducted on cast Al-Fe-V-Si alloys. The first phase to precipitate during solidification of an Al-8.3Fe-0.8V-0.9Si alloy is Al₃Fe(V,Si), which is isostructural with the Al₃Fe phase. Thereafter, the solidification proceeds through several invariant reactions. The final invariant reaction is associated with a pronounced arrest. The temperature of this arrest is a function of the cooling rate and modification treatment, with magnesium added as an Al-20 pct Mg or Ni-20 pct Mg master alloy. The coarse iron aluminide precipitates in a slow-cooled (>1 °C/s) cast structure transform to a ten-armed, star-like morphology upon chill casting the melt (cooling rate >10 °C/s) from 900 °C or upon water quenching from above 800 °C. Treatment with magnesium refines the morphology, size, and distribution of iron aluminide precipitates in slow-cooled alloys.     

Solidification and Microstructural Evolution of Hypereutectic Al-15Si-4Cu-Mg Alloys with High Magnesium Contents

The low coefficient of thermal expansion and good wear resistance of hypereutectic Al-Si-Mg alloys with high Mg contents, together with the increasing demand for lightweight materials in engine applications have generated an increasing interest in these materials in the automotive industry. In the interests of pursuing the development of new wear-resistant alloys, the current study was undertaken to investigate the effects of Mg additions ranging from 6 to 15 pct on the solidification behavior of hypereutectic Al-15Si-4Cu-Mg alloy using thermodynamic calculations, thermal analysis, and extensive microstructural examination. The Mg level strongly influenced the microstructural evolution of the primary Mg₂Si phase as well as the solidification behavior. Thermodynamic predictions using ThermoCalc software reported the occurrence of six reactions, comprising the formation of primary Mg₂Si; two pre-eutectic binary reactions, forming either Mg₂Si + Si or Mg₂Si + α-Al phases; the main ternary eutectic reaction forming Mg₂Si + Si + α-Al; and two post-eutectic reactions resulting in the precipitation of the Q-Al₅Mg₈Cu₂Si₆ and θ-Al₂Cu phases, respectively. Microstructures of the four alloys studied confirmed the presence of these phases, in addition to that of the π-Al₈Mg₃FeSi₆ (π-Fe) phase. The presence of the π-Fe phase was also confirmed by thermal analysis. The morphology of the primary Mg₂Si phase changed from an octahedral to a dendrite form at 12.52 pct Mg. Any further Mg addition only coarsened the dendrites. Image analysis measurements revealed a close correlation between the measured and calculated phase fractions of the primary Mg₂Si and Si phases. ThermoCalc and Scheil calculations show good agreement with the experimental results obtained from microstructural and thermal analyses.     

Microstructural Evolution of 11Al-3Mg-Zn Ternary Alloy-Coated Steels During Austenitization Heat Treatment

This study details the microstructural evolution of a commercial hot-dip 11Al-3Mg-Zn-coated steel during austenitization. After 5 minutes of austenitization at 1173 K (900 °C), the ternary alloy coating transformed to consist of a nearly pure Zn as the major layer, a Fe-Al alloy layer at the interface, and a thin oxide overlay. The Fe-Al alloy layer effectively acted as the inhibition layer to prevent Fe from diffusing and reacting with Zn, which in turn retained the molten Zn layer and the integrity of the surface oxide layer. Moreover, the potential difference between the 11Al-3Mg-Zn coating and the steel substrate remained similar after austenitization, signifying the resulting coating kept its sacrificial protection capability.     

Ir-Nb-Si ternary refractory superalloys with a three-phase Fcc/L12/silicide structure for high-temperature applications: Phase and microstructural evolution

To find a new phase with the potential to improve the high-temperature strength of Ir-based superalloys, the novel idea of introducing silicides into the Ir-Nb binary was implemented. Hypoeutectic Ir-10Nb, eutectic Ir-16Nb, and hypereutectic Ir-25Nb alloys were used as bases, and 5 mol pct Si was added through the removal of Ir. XRD (XRD), scanning electron microscopy (SEM), and electron-probe microanalysis (EPMA) revealed the formation of a three-phase fcc/L1₂/silicide microstructure in the Ir-Nb-Si ternary after Si addition. The type of silicide formed was dependent on heat-treated temperatures and Nb content. After heat treatment at 1750 °C and 1600 °C, a tie-triangle composed of fcc/L1₂/silicide (Ir₂Si) appeared in the Ir-10Nb-5Si and Ir-16Nb-5Si alloys; in the Ir-25Nb-5Si alloy, an L1₂ and silicide (Ir,Nb)₂Si tie-line was observed. In the as-cast and 1300 °C heat-treated samples, the Ir-10Nb-5Si microstructure changed to a two-phase fcc/silicide structure, while the Ir-16Nb-5Si alloy maintained a three-phase fcc/L1₂/silicide structure. The Ir-25Nb-5Si alloy, however, had the same phases as that at 1600 °C. Silicides typically continuously or discontinuously distribute along the interdendritic regions or grain boundaries of the fcc or the L1₂ phase. With the addition of Si, it was found that both the eutectic point and solid solubility of Nb in Ir would shift toward Ir.     

The solidification behavior of 8090 Al-Li alloy

In this work, the solidification and segregation behaviors of 8090 Al-Li alloy have been investigated with differential thermal analysis (DTA) and the metallographic-electron microprobe method. The results show that 8090 Al-Li alloy has a much more complex solidification path than Al-Li binary alloy due to the addition of many alloying elements and the presence of impure elements. Solidification begins at about 635 °C with the reaction of L → α-Al + L′, and this reaction goes on to termination. The alloying element Cu and impure elements Fe and Si have a strong segregation tendency. During solidification, Cu segregates to the interdendrite and finally forms α-Al + T₂ eutectic. As a result, the solidification temperature range is greatly extended. Iron and Si form the insoluble constituents Al₇Cu₂Fe, AlLiSi,etc., although their concentrations in the alloy are quite low. With the increase of Fe content, there is a eutectic reaction of α-Al/Al₃Fe at about 595 °C. The formation of insoluble constituents is influenced by both concentrations of impure elements in the alloy and the cooling rate.