交变磁场作用下特种合金的凝固特性

以高温合金(K403)为实验材料,对中、小尺寸特种合金在交变磁场作用下成形过程的凝固特性进行研究,以期加深对凝固组织细化机理的认识,进而实现对凝固组织的优化控制。在研究过程中首先试验、研究了高温合金无接触和软接触电磁成形的凝固特性,探讨了电磁致流、磁化套以及抽拉速度对凝固组织的影响,进而分析了该特种合金电磁成形凝固组织的细化机理。结果表明:使用磁化套可以获得平直、细化的定向枝晶组织;在该文实验条件下,当抽拉速度为9.8mm/min时,一次枝晶间距能减小到80μm;电磁致流导致二次枝晶臂脱落是无接触电磁成形凝固组织细化的主要机理,而枝晶尖端开裂是软接触电磁成形凝固组织细化的主要机理。     

Dephosphorization of silico-manganese alloys under reducing atmosphere with metallic calcium

Dephosphorization of silico-manganese ferroalloy with metallic Calcium was investigated under sealed and open conditions. Effects of the initial Si content in silico-manganese melts, the adding amount of Ca chips and flux CaF₂, the time of dephosphorization and the initial C content in silico-manganese melts upon dephosphorization efficiency were ascertained by experiment. The dephosphorization product was identified as Ca₃P₂ upon stoichiometric composition. Experimental results show that dephosphorization efficiency, rather high, from 95 % to 99 % could be obtained by adding Ca under sealed conditions and a high Ca utilization ratio was obtained.     

Solidification of undercooled molten Ni-based alloys

The effect of undercooling on grain structure is investigated in pure nickel, Ni₇₅Cu₂₅, and DD3 singlecrystal superalloy by employing the method of molten salt denucleating combined with thermal cycling. Meanwhile, a comparison of factors that may be related to structure formation is performed and the difference in the refined structure between Ni₇₅Cu₂₅ alloy and DD3 single-crystal superalloy is explained. Only one grain refinement occurs at the critical undercooling in pure nickel, whereas two take place at both low and high undercoolings in Ni₇₅Cu₂₅ and DD3 single-crystal superalloy melts. The first grain refinement at low undercoolings mainly originates from dendrite remelting driven by the chemical superheating produced in recalescence, and the second one at high undercoolings is due to the recrystallization process as a result of the high stress provided in the rapid solidification after high undercooling. Dislocation morphology evolution in as-solidified structure is also provided by the transmission electron microscopy (TEM) technique to further verify the recrystallization mechanism.     

Solidification of undercooled dilute tin-lead alloys

The rate of solidification of dilute tin-lead alloys has been measured as a function of the initial undercooling (up to 45°C) and the solute content (up to 2 wt pct lead). Solidified specimens were examined by metallography and X-ray diffraction to obtain information on the solidification process and the resulting grain structure. Over an intermediate range of undercoolings, it was found that dendrites grow in the tin-lead alloys as much as four times faster than in pure tin at the same undercooling. This result is inconsistent with any present theories for dendrite growth kinetics in binary alloys. At both lower and higher undercoolings there is no evidence for growth by simple extension of dendrites along the specimen, and solidification rate measurements made under these conditions are probably not indicative of normal dendrite growth kinetics. A. W. Urquhart and G. L. F. Powell were formerly at the Thayer School of Engineering.     

Solidification of highly undercooled Fe-P alloys

Rapid solidification behavior of highly undercooled iron-phosphorus alloys was investigated by using a high-speed optical temperature measurement system. The experimental results on solidification rate as a function of bulk undercooling agree well with a model which includes a treatment of nonequilibrium effects during the solidification process. The model is based on an earlier analysis by Boettinger, Coriell, and Trivedi_[81] (BCT) and employs temperature-dependent values of equilibrium liquidus slope, equilibrium solute distribution coefficient, and solute interdiffusion coefficient. Values of the kinetic parameters,a ₀ andV ₀ , in the analysis which best fit the experimental results are 5 x 10_-10 m and 600 m/s, respectively. Comparison of experimental results and theory suggests that a transition from local equilibrium to nonequilibrium solidification takes place with increasing undercooling and that interface kinetic effects become predominant at higher undercooling (or growth velocity).     

基于均匀形核的金属液滴凝固过程

研究了均匀形核的金属液滴凝固过程,应用渐近分析法求得金属液滴内晶核生长数学模型的渐近解,分析了表面张力、界面动力学参数、初始晶核尺寸和过冷度对晶核界面生长速度、晶核半径以及液滴凝固时间的影响.在一定的过冷条件下,表面张力和界面动力学参数显著减缓了晶核界面生长速度.在凝固开始的很短时间内晶核界面生长速度迅速上升,当速度上升到最大值后,随着晶核半径的增大,界面生长速度逐渐减慢,表面张力和界面动力学参数对晶核生长速度的作用也逐渐减小.过冷度越大,液滴凝固时间越短.经过在开始的瞬变凝固阶段之后,温度场从设定的初始分布迅速地调整为由过冷度、表面张力、界面动力学参数等所确定的特定温度分布.     

感应加热熔炼Cu-Cr合金的凝固行为

采用电弧熔炼和感应加热熔炼研究了不同组成Cu-Cr合金的凝固行为.结果表明:由于高温下Cu-Cr合金与坩埚材料发生反应,氧化铝坩埚中感应加热熔炼Cu-Cr合金受到污染,澄清了Müller与Li的争议;当合金中Cr的摩尔分数大于40%时,感应加热熔炼Cu-Cr合金出现了液相分离,合金与坩埚材料的反应生成物促进了Cu-Cr合金的液相分离.     

Solidification of undercooled Sn-Bi and Pb-Sb alloys

In hypoeutectic and eutectic alloys of Sn-Bi and Pb-Sb, the first phase to solidify from the melt is the nonfaceted phase, tin and load, respectively. Since primary tin and lead are poor nucleants for the second phase of the eutectic, the faceted phase, substantial undercooling below the equilibrium eutectic temperature occurs before the eutectic is nucleated. Once nucleated the faceted phase does not grow preferentially to return the melt composition to the equilibrium eutectic composition. Instead growth of a complex regular structure takes place from a melt whose composition is displaced from the equilibrium eutectic composition towards the faceted component. Reheating experiments have confirmed that the complex regular structure is rich in the faceted component. This structure grows at a temperature depressed several degrees below the equilibrium eutectic temperature. An hypothesis is presented to explain the growth of a structure rich in the faceted component at a depressed temperature.     

Microhardness of amorphous metallic alloys Fe-Cr-B

Translated from Poroshkovaya Metallurgiya, No. 8(332), pp. 49–53, August, 1990.     

Thermodynamic properties of liquid metallic alloys

Experimental data on the activity of components in concentrated (nondilute) liquid metallic alloys must be approximated by characterizing the alloy’s thermodynamic properties on the basis of logγ/(1 ? x ²) rather than logγ. Here γ and x are, respectively, the activity coefficient and molar fraction of the component in the alloy. In that case, the error in calculating the alloy’s integral thermodynamic characteristics—the mixing energy, the regular function—is reduced.     

Solidification structures in rapidly quenched Cu-Ti-Zr alloys

The melt-spinning and pulsed laser-quenching techniques were used to generate fast cooling rates necessary to form metallic glass in ternary Cu-Ti-Zr alloys. Even though the cooling rates generated during melt-spinning were much lower than those generated during pulse laser-quenching, the amount of glass obtained in the melt spun alloys was much larger than in the laser-quenched alloys. The microstructure of the laser-quenched alloys consisted of metallic glass and a fine microcrystalline fcc phase, whereas the melt-spun ribbons of the same compositions were completely amorphous. Glass transition temperature trends in the ternary system were determined from alloys of composition traversing linearly from the binary Cu-Ti side to the Cu-Zr side. These trends showed a nearly linear change of T_g with composition.     

Solidification of hastelloy alloys: an alternative interpretation

HASTELLOY C-22 and C-276 are engineering nickel-based alloys that served as the focus of a comprehensive examination of the microstructural development of this class of materials presented previously by Cieslak et al. The work presented here provides a re-examination of this study. The use of computational thermodynamic algorithms combined with solidification-path calculations based on experimentally determined solute-partition data eliminates the need for many assumptions and serves as the basis for the solute-redistribution model that is presented here. A new series of solidification sequences for HASTELLOY C-22 and C-276 is proposed based on this model, and these conclusions are supported by the microstructural characterization presented in the previous work.     

Solidification of Nb-Ge alloys in long drop tubes

The 30 m and 100 m long drop tubes at the Marshall Space Flight Center have been used to obtain large undercooling in Nb-Ge alloys. Electron beam melting has been used to obtain drops approximately 2.5 mm in diameter. In the 30 m tube many specimens fell the length of the tube without solidifying, and were ultimately liquid quenched in oil. The amount of undercooling prior to the quench was usually around 0.13T;m. In the 100 m tube, freezing generally initiated during free fall, and the maximum undercooling was around 0.22Tm. Microstructures were characterized by a combination of X-ray diffraction, optical microscopy, and scanning electron microscopy with energy dispersive analysis by X-rays. A variety of interesting microstructures was observed.     

Diffusion-coefficient measurements in liquid metallic alloys

The value of the diffusion coefficient in the liquid (D _l ) is generally obtained from the measurement of composition profiles ahead of a quenched planar interface. The experimental results show significant scatter. The main reason for this scatter will be shown to be due to the presence of fluid flow in the liquid. Directional-solidification studies in the Al-Cu system have been carried out to first establish the experimental conditions required for diffusive growth. The composition profiles are then measured to obtain the values of D _l for alloy compositions ranging from 4.0 to 24.0 wt pct Cu. The value of D _l =2.4×10⁻³ mm²/s was obtained along the liquidus line, and this result is significantly smaller than the values reported in the literature, which vary from 3.0 to 5.5 × 10⁻³ mm²/s. It is shown that the scatter in the reported values can be correlated with the diameter of the sample used and, thus, with the fluid flow present in their experiments. Detailed experimental procedures to obtain and verify diffusive-growth conditions are outlined, and appropriate analyses of the data are discussed.     

Electrochemical oxidation of nickel sulfide metallic alloys

The sequence of the electrochemical oxidation of phase components of the converter matte is confirmed and variations in the composition of its surface during the anode oxidation are revealed. It is shown that regularities of this process that are characteristic of its phase components, namely, nickel and its sulfides, are inherent to the converter matte. The oxidation of nickel sulfides in sulfide-metallic alloys starts at lower potentials (1.1–1.2 V) than that of synthesized nickel sulfide (1.2–1.3 V). During the oxidation of the converter matte, metal nickel initially oxidizes and, only as this stage finishes, the oxidation of sulfides accompanied by the isolation of elemental sulfur is developed. Copper impurities weakly affect the oxidation of the converter matte, although the shift of potentials takes place as the Cu/Ni ratio increases.