Effect of niobium on the directional solidification and properties of Alnico alloys

The effect of niobium on the extent of columnar growth of grains and on the magnetic properties in Alnico alloys has been studied. Alloys containing 1 % Nb show maximum growth of columnar crystals. In columnar alloys, coercivity increases slowly with increasing niobium content up to 1 %. With further increase in niobium, coercivity and remanence decrease. The maximum energy product (55 kJ m^–3) has been obtained at 1 % Nb. Niobium addition has also been found to suppress the precipitation of the undesirable phase.     

Pr对Sn-Cd包晶合金定向凝固组织的影响

为了解Pr对Sn-Cd包晶合金凝固组织的作用,在不同抽拉速率下对Sn-Cd-Pr合金进行定向凝固实验.采用光学显微镜、扫描电子显微镜(SEM)和能谱分析(EDS)研究了Pr对Sn-Cd包晶合金凝固组织的影响.研究发现:对于Sn-1.8%Cd过包晶合金,在2μm/s时0.5%的Pr增加了包晶相β的体积分数;在4μm/s时凝固组织明显细化.对于Sn-0.65%Cd亚包晶合金,在4μm/s时Pr的加入促进了平→胞界面的转变;在16μm/s时随着Pr含量的增大,凝固组织逐渐细化;在100μm/s时Pr含量的增大使凝固组织仅有微弱的细化.研究表明,Pr的加入可以改变组成相的体积分数、细化组织以及减小界面稳定性.     

Effect of silicon on microstructure of Ti3Al alloys containing niobium

Abstract

The effect of silicon content on the microstructure and phases present in Ti–(20–23)Al–11Nb alloys has been studied in the temperature range 800–1270°C.Four phases, β_o, α₂, O, and a silicide, are formed. The parent β_o is ordered at 1270°C. At 1050°C α₂ is formed which exhibits a higher silicon solubility than the parent β_o. A peritectoid transformation β₀+silicide→α₂ is proposed. Assuming that niobium substitutes for titanium and silicon for aluminium, energy dispersive X-ray spectroscopic data suggest that the α₂ phase, unlike that in binary Ti–Al alloys, is highly stoichiometric and of the form (Ti+Nb)₃(Al+Si). Similarly the silicide corresponds to binary Ti₅Si₃ with the same site substitution as in the α₂ phase. The O phase is orthorhombic and similar in composition to the α₂ which it replaces: its formation is promoted by silicon.

MST/3088     

Non-equilibrium effects on solid transition of solidification microstructure of deeply undercooled alloys

Integrated effects of undercooling and solute drag on recrystallisation mechanism of rapid solidification microstructure were investigated in highly undercooled Ni-Cu alloys. The equiaxed grained microstructures were prepared by fluxing method and subsequent quenching. Annealing the microstructures of the as-quenched alloys, substantial recrystallisation growth was observed. Applying solute trapping model of undercooled melt and solute drag model of solid-state transformation, it can be inferred that the microstructural evolution was dominated by nucleation and growth of recrystallisation process which is strongly dependent on the initial undercooling of the alloy melt and the solute drag force of the solid-state transition process.     

Experiments on the dynamic destruction of superconductivity in niobium and niobium alloys

The dynamic behaviour of the superconductive-normal transition in bulk wires of niobium and niobium alloys has been investigated under two experimental conditions. Firstly the influence of the magnitude of dl/dt on the critical current was studied, secondly, a dynamic critical curve was measured using magnetic field pulses of large dH/dt. For most of the samples the dynamic critical curve was found to be much lower than the static one. Furthermore a pronounced decrease of the transition time with increasing external field was observed. This decrease of the transition time levels off at very high fields.     

Superconductivity in deformed niobium alloys

A brief review of deformation structure in bcc metals is given and the main features of type II superconductivity are summarised. Microstructure and superconducting properties of deformed niobium-tantalum, niobium-titanium, and niobium-zirconium alloys were studied and compared with those of pure niobium. All these alloys are type II superconductors, which above a certain critical field, H _ic1, enter the mixed state, consisting of Abrikosov current vortices in a superconducting matrix. Dislocations introduced by deformation interact with current vortices to cause irreversible superconducting behaviour and contribute to a high current-carrying capacity. It is concluded from the results presented here that a strong vortex dislocation interaction is obtained only when a non-uniform dislocation distribution is present, and that a uniform distribution produces only weak pinning. Flux-pinning by dislocation tangles is treated as a special case of pinning by normal particles resulting from a variation in , the Ginzburg-Landau parameter.     

Rapid solidification of zirconium-based alloys

Zr based metal-metal binary and ternary alloys can be obtained in the amorphous state in very wide composition ranges. Several eutectic reactions and intermetallic compounds are present in these alloy systems which provide opportunities for examining the validity of different theories on glass formation. The amorphous phases in these alloys decompose by a variety of crystallization mechanisms. Instances of polymorphic, primary and eutectic crystallization have been encountered in these glasses. Zr-based metallic glasses possess excellent corrosion resistance and mechanical properties. In several studies their properties have been compared with that of their crystalline counterparts and interesting differences have emerged. In the solute lean Zr-based alloys very large freezing ranges are available for studying the liquid to solid transformation. It has been possible to study the formation of some of the low temperature phases directly from the liquid. This paper describes some of the aforementationed studies carried out on Zr-based amorphous and crystalline alloys.     

Modeling of gas porosity and microstructure formation during dendritic and eutectic solidification of ternary Al-Si-Mg alloys

A two-dimensional(2-D)multi-component and multi-phase cellular automaton(CA)model coupled with the Calphad method and finite difference method(FDM)is proposed to simulate the gas pore for-mation and microstructures in solidification process of hypoeutectic Al-Si-Mg alloys.In this model,the pore growth,and dendritic and eutectic solidification are simulated using a CA technique.To achieve the equilibrium among multiple phases during ternary Al-based alloy solidification,the phase transition thermodynamics and kinetics are evaluated by adopting the Calphad method.The diffusion equations of hydrogen and two solutes are solved by FDM.The developed CA-FDM coupled model can be used for sim-ulating the evolution of gas microporosity and microstructures,involving dendrites and irregular binary and ternary eutectics,of ternary hypoeutectic Al-Si-Mg alloys.It has the capability of reproducing the interactions between the hydrogen microporosity formation and the growth of dendrites and eutectics,the competitive growth among the growing gas pores of different sizes,together with the time-evolving concentration fields of hydrogen and solutes.The simulated morphology of gas pore and microstructure has a good agreement with the experimental observation.The influences of the initial hydrogen concen-tration and cooling rate on the microporosity formation are investigated.It is found that the main portion of porosity formation occurs in the eutectic solidification stage through analyzing the profiles of porosity percentage and solid fraction varying with solidification time.The varying features of simulated porosity percentage,the maximum and average pores radii indicate that increasing initial hydrogen concentration promotes the formation of higher final porosity percentage and larger pores,while the size of gas pores will significantly reduce with increasing cooling rate,leading to a lower final porosity percentage.     

Evolution of the microstructure and solute distribution of Sn-10wt% Bi alloys during electromagnetic field-assisted directional solidification

The effects of forced flows at different velocities on microstructure and solute distribution during the directional solidification of Sn-10 wt% Bi alloys under a simultaneous imposition of a transverse static magnetic field(TSMF) and an external direct current(DC) have been investigated experimentally and numerically. The experimental results show that the solid-liquid interface will gradually become sloping with the increase of the forced flow velocity when the thermoelectric magnetic convection(TEMC)dominates the forced flow at solidification front. However, the interface will gradually become planar as the flow velocity further increases when the electromagnetic convection(EMC) dominates the forced flow. Moreover, when the flow velocity gradually increases, the primary dendrite spacing decreases from384 to 105 μm accordingly. The simulation results show that the solute distribution at the two sides of the sample can be significantly changed by the forced flow at solidification front. The rejected solute will be unidirectionally transported to one side of the sample along the TEMC(a low-velocity forced flow),thereby causing the formation of a sloping interface. However, the rejected solute will be returned back along the EMC(a higher-velocity force flow), which results in a planar interface. Furthermore, the solute content at the two sides of the sample under the forced flows at different velocities was measured. The results are in good agreement with the simulation results, which shows that the solute content difference between the two sides of the sample reaches the maximum when a 0.5 T TSMF is applied, while the solute content difference decreases to zero with a simultaneous application of a 0.5 T TSMF and a 1.6 × 10~5 A/m~2 external DC.     

Kinetic aspects of the solidification of aluminium-zinc alloys

A method for determination of the kinetics of linear and volumetric dendritic growth on the basis of a model of dendritic solidification process referred to one mole of alloy has been presented. The model parameters have been derived using Krupkowski's equation of dendritic solidification for the case of Al-40 wt% Zn alloy solidified at a definite rate. The constructed kinetics curves have been analysed using the Johnson—Mehl relation and compared with that of Chalmers. The results of calculation concerning the maximum microsegregation have made it possible to define the linear and volumetric dendritic growth rates. The relationships between these rates and the experimental cooling rate of ingot have been discussed.     

Directional solidification of Co-Cu-R permanent-magnet alloys

Co-Fe-Cu-Ce permanent-magnet alloys have been prepared by directional solidification using a modified Bridgman technique. Samples melted at low superheat temperatures (DeltaT sim 20degC above the melting point of about 1100°C) and solidified at moderate rates (∼2.3 cm/h) resulted in a reasonably homogeneous columnar grain structure with a preferred crystallographic orientation. Thecaxis is generally aligned within 15° of the growth axis. Increasing the speed of solidification led to a fine-grained structure with no texture, while decreasing the speed led to coarse columnar grains with erratic orientation. A large superheat temperature (DeltaT sim 300-400degC resulted in a reaction of the liquid with the alumina crucible wall and led to the formation of face-centered cubic Co-rich dendrites. With the modified Bridgman technique, oriented samples 8 cm long and 2.54 cm in diameter have been prepared with good magnetic properties. After annealing at 1000°C followed by aging at 400°C, a Co3.5Fe0.5CuCe alloy exhibited values ofiHc= 7000 Oe, Br= 6100 G, and(BH)_{max} = 9.2MG.Oe. A Co3.6Fe0.5Cu0.9Ce alloy exhibited values ofiHc= 6000 Oe,B_{r} = 6250G, aud(BH)_{max} = 9.5MG.Oe after similar treatment.     

Unidirectional solidification of Ni-Mo-Al eutectic alloys

The composition triangle of the Ni-Mo-Al ternary system contains a monovariant trough between theγ′ (Ni₃Al)-α(Mo) pseudobinary eutectic and a guessedγ(Ni)-γ′ (Ni₃Al)-α(Mo) ternary eutectic. Alloys with compositions on this trough were directionally solidified at various growth rates. The microstructure of the alloys consists of fine Mo fibres of rectangular cross-section in an Ni₃Al/Ni matrix. The determined crystallographic relationship does not correspond to an interface of low lattice mismatch. The eutectic trough strongly extends towards increasing Mo contents so that the volume fraction of the Mo fibres varies between 18 and 25% dependent on composition. It was not possible to determine the position of a ternary eutecticγ-γ′-α because the liquidus temperatures along the trough are almost equal and the distribution of the three phases is strongly dependent on the growth rate.     

The directional solidification of Pb-Sn alloys

Directional solidification experiments have been carried out on different Pb-Sn alloys as a function of temperature gradient G, growth rate V and cooling rate GV. The specimens were solidified under steady state condition with a constant temperature gradient (50 °C/cm) at a wide range of growth rates ((10–400) × 10^–4 cm/s) and with a constant growth rate (17 × 10^–4 cm/s) at a wide range of temperature gradient (10–55 °C/cm). The primary dendrite arm spacing, ₁, and secondary dendrite arm spacing, ₂, were evaluated. This structure parameters were expressed as functions of G, V and GV by using the linear regression analysis. The results were in good agreement with the previous works.     

Coarsening during solidification of aluminium-copper alloys

Coarsening of directionally solidified -phase dendrites and of particulate -phase/liquid mixtures was investigated in Al-4, 10 and 20 wt% Cu alloys, as a function of temperature, composition and presence or absence of forced convection. Isothermal dendritic coarsening in the absence of convection operated in two stages. In stage I the dendritic structure broke down through remelting into fragments which spheroidized quickly; in stage II the spherical particles coarsened slowly. The coarsening rate of the dendritic or particulate solid increased with temperature and copper dilution. Alloy inoculation with titanium slowed coarsening, yielding finer dendritic microstructures. The effect of turbulent flow on coarsening was manifested only for longer holding times. At higher impeller angular velocities the dendritic structure breaks down into fragments which spheroidize rapidly. At lower shear rates (below 650 rev min^–1) solid particles in solid-liquid mixtures coalesce into clusters, whereas at higher rates the clusters break up again into individual particles. A coarsening model was introduced which showed that coarsening is faster in the presence of forced convection, because of the resulting decrease in solute diffusion-boundary layer thickness.