Rapid solidification of steels

Solidification at very high rates of cooling results in considerable refinement in the microstructure of alloys. It enables the formation of extremely fine grains, extends solid solubilities and leads to the nucleation of many metastable crystalline phases. We discuss briefly the methods of rapid solidification and their impact on microstructure and structure of alloys. Each of the aspects of structure development is illustrated with examples from steels.     

Modelling of convection during solidification of metal and alloys

The role of convection during solidification is studied with the help of a mathematical model. The effect of various mush models on convection and consequent macrosegregation is examined with the help of numerical simulations. The predicted macrosegregation profiles are compared with published experimental data. Subsequently, the importance of proper auxiliary relationship for thermo-solutal coupling in the mushy region is highlighted through some careful numerical simulations. Finally, the role of material parameters on double-diffusive convection is illustrated through comparative study of solidification of aqueous ammonium chloride, iron-carbon and lead-tin binary systems. Important results of these studies are presented and discussed.     

Al-Pb合金快速定向凝固的数值模拟

在考虑凝固界面前沿第二相液滴形核、长大以及迁移综合作用的基础上 ,提出了描述偏晶合金在快速定向凝固条件下微观组织形成过程的数学模型 ,并对Al-Pb轴承合金在垂直Bridgeman定向凝固条件下的凝固组织进化过程进行了计算分析 .结果表明 :在大的凝固速度条件下 ,凝固界面前沿存在成分过冷区 ,液 -液相分解在此区域内进行 ;在恒定的温度梯度条件下 ,凝固速度越快 ,第二相液滴的形核速率越大 ,液滴的数量密度越高 ,平均半径越小 ;凝固界面前沿液滴的平均半径 (R)与凝固速度 (v)之间存在如下指数关系 :R(z =0 ) =C2 v-0 .3 9± 0 .0 1     

Some amorphous alloys in Zr-Ru-Mo and Zr-Ru-W systems prepared by rapid solidification

All binary systems of the ternary systems Zr-Mo-Ru and Zr-W-Ru show at least one eutectic point. This indicates the existence of low melting eutectic alloys in these ternary systems (∼ 1200–1300°C). As starting materials, homogeneous Zr-Mo-Ru and Zr-W-Ru samples of different compositions were prepared by rapid solidification (∼10⁶Ks⁻¹) in a splat-cooling apparatus with a rf levitation coil and a high-velocity two-piston arrangement driven by solenoids. The possibility of obtaining alloys in amorphous state from low-melting areas of these ternary systems with the addition of boron or silicon has been investigated.     

Reducing porosity and optimizing performance for Al-Cu-based alloys with large solidification intervals by coupling travelling magnetic fields with sequential solidification

Porosity is a major casting defect in alloys with large solidification intervals due to the disordered microstructure and broad mushy zones,which decreases badly the mechanical performance.Hence,finding ways to effectively reduce the porosity,further to optimize microstructure and mechanical per-formance is of great significance.In this regard,the Al-Cu-based alloys with large solidification intervals are continuously processed by coupling the travelling magnetic fields (TMF) with sequential solidifica-tion.Additionally,experiments combined with simulations are utilized to comprehensively analyze the mechanism of TMF on the reduction in porosity,including shrinkage porosity and gas porosity,from dif-ferent perspectives.Current findings determine that downward TMF can effectually optimize together the porosity,microstructure and performance,by inducing the strong long-range directional melt flows,stabilizing the mushy zones,and optimizing the feeding channels and exhaust paths,as well as increas-ing the driving force of degassing process.Eventually,downward TMF can increase the ultimate tensile strength,yield strength,elongation and hardness from 175.2 MPa,87.5 MPa,13.3 ％ and 80.2 kg mm-2 without TMF to 218.6 MPa,109.3 MPa,15.6 ％ and 95.5 kg mm-2,while reduce the total porosity from 0.95 ％ to 0.18 ％.However,Up-TMF exerts negative effects on the optimization of porosity,microstruc-ture and performance due to the opposite strong directional magnetic force and melt flows.Overall,our study provides an effective way to optimize together the porosity,microstructure and mechanical performance,and reveals their relationship,as well as details the relevant mechanisms of TMF on the porosity reduction from different perspectives.     

Optimizing the microstructures and mechanical properties of Al-Cu-based alloys with large solidification intervals by coupling travelling magnetic fields with sequential solidification

Alloys with large solidification intervals are prone to issues from the disordered growth and defect formation;accordingly, finding ways to effectively optimize the microstructure, further to improve the mechanical properties is of great importance. To this end, we couple travelling magnetic fields with sequential solidification to continuously regulate the mushy zones of Al-Cu-based alloys with large solidification intervals. Moreover, we combine experiments with simulations to comprehensively analyze the mechanisms on the optimization of microstructure and properties. Our results indicate that only downward travelling magnetic fields coupled with sequential solidification can obtain the refined and uniform microstructure, and promote the growth of matrix phase -Al along the direction of temperature gradient.Additionally, the secondary dendrites and precipitates are reduced, while the solute partition coefficient and solute solid-solubility are raised. Ultimately, downward travelling magnetic fields can increase the ultimate tensile strength, yield strength, elongation and hardness from 196.2 MPa, 101.2 MPa, 14.5 % and85.1 kg mm-2 without travelling magnetic fields to 224.1 MPa, 114.5 MPa, 17.1 % and 102.1 kg mm-2,and improve the ductility of alloys. However, upward travelling magnetic fields have the adverse effects on microstructural evolution, and lead to a reduction in the performance and ductility. Our findings demonstrate that long-range directional circular flows generated by travelling magnetic fields directionally alter the transformation and redistribution of solutes and temperature, which finally influences the solidification behavior and performance. Overall, our research present not only an innovative method to optimize the microstructures and mechanical properties for alloys with large solidification intervals,but also a detailed mechanism of travelling magnetic fields on this optimization during the sequential solidification.     

Effect of rapid solidification on giant magnetostriction in ferromagnetic shape memory iron-based alloys

Ferromagnetic shape memory Fe–29.6 at.% Pd alloy ribbons prepared by the rapid solidification, melt-spinning method, showed a giant magnetostriction of 830 microstrain when an external magnetic field of 7 kOe was applied nearly normal to the ribbon surface at room temperature. This ribbon’s magnetostriction was several times as large as conventional polycrystalline bulk’s one before rapid solidification. The magnetostriction in the rolling direction depended strongly on a direction of applied magnetic field. We considered that this phenomenon is caused by a rearrangement of activated martensite twin variants just below the austenite phase transformation temperature. We investigated their basic material properties, i.e. the dependencies of magnetostriction on temperature as well as on magnetic angular orientation to the surface, magnetic properties, crystal structure, surface texture morphology and shape memory effect of Fe–29.6 at.% Pd ribbon samples by comparing with conventional bulk sample. It can be concluded that the remarkable anisotropy of giant magnetostriction of ribbon sample is caused by the unique uniaxial-oriented fine grain structure formed by the melt-spinning method. In addition, we confirmed the possibility of rapidly solidified Fe–Pt ribbon as a new kind of iron-based ferromagnetic shape memory alloys for magnetostrictive material.     

Substantial grain refinement of Al-Mn-Si alloys mediated by collaborative effect of Al-5Ti-1B refiner and sub-rapid solidification

The mechanical properties of as-cast metallic materials depend strongly on the size and shape of grains,which are critical microstructural parameters dictated by the interplay of nucleation and growth of crys-talline solids during solidification.In our experiments,the microstructure transition from coarse colum-nar crystals into fine equiaxed crystals for dilute Al-Mn-Si alloys was achieved by using sub-rapid so-lidification with the addition of Al-5Ti-1B grain refiner.The average grain size of Al alloy was reduced from a millimeter size to 73 pm.Through temperature gradient calculation,we found that the acqui-sition of fine equiaxed crystals could be attributed to the existence of a high number density of TiB2,acting as effective nucleation sites with an increase in total undercooling.Furthermore,the curvature su-percooling,constitutional undercooling,thermal undercooling,and kinetic undercooling during sub-rapid solidification were quantitatively determined for given solidification rates.Our results showed that con-stitutional undercooling,rather than thermal undercooling,was primarily responsible for the formation of fine equiaxed grains,with the assistance of Al-5Ti-1B grain refiner.This work provides a new insight into the grain refining mechanism under sub-rapid solidification.     

Optimizing microstructure,shrinkage defects and mechanical performance of ZL205A alloys via coupling travelling magnetic fields with unidirectional solidification

ZL205A alloys tend to form disordered and defective microstructure due to the large solidification intervals and multi-phase.Accordingly,finding ways to effectively optimize the microstructure and mechanical performance is of great significance.In this regard,the coupling of travelling magnetic fields (TMF) with unidirectional solidification was used to continuously regulate the mushy zones of ZL205A alloys.Additionally,experiments are combined with simulations to systematically reveal the mechanisms on the optimizations at each stage of solidification process.Current findings demonstrate that different directional strong melt flows generated by TMF are responsible for these optimizations.Additionally,the effects of TMF on microstructure are different at each stage of solidification process.Specifically,downward TMF coupled with unidirectional solidification can refine and uniform the microstructure,decrease the formation of precipitation,promote the growth consistency of matrix phase α-Al growing along the <001 > crystal orientation,reduce the secondary dendrites and overlaps between dendrites,eliminate the shrinkage defects,and increase the ultimate tensile strength,yield strength,elongation and hardness from 198.3 MPa,102.2 MPa,7.5 ％ and 82.3 kg mm-2 without TMF to 225.5 MPa,116.1 MPa,13.6 ％ and 105.2 kg mm-2.Contrastively,although upward TMF can reduce Al3Ti and refine α-Al,it increases the formation of Al6Mn,Al2Cu,secondary dendrites,overlaps between dendrites,and shrinkage defects;then it deflects and disorders the growth of α-Al,further to decrease the overall performance of alloys.     

Formation Mechanism of Microstructure of Melt Spun Al—In Immiscible Alloys

Immiscible alloys are attractive for their valuable physical and mechanical properties. In this paper, Al-ln immiscible alloy is prepared by melt spinning process and its morphological evolution is studied at various indium contents. The results show that the morphologies of the matrix phase depend on the indium content. Different morphologies lead to different distribution of the second phase particles. Due to a particular solidification mechanism of immiscible alloys, even under the melt spinning rapid solidification condition, it is still impossible to produce homogeneous Al-In hypomonotectic alloy ribbons. But for Al-In hypermonotectic alloys, there is almost no segregation of the second phase throughout the cross section of the ribbons.     

Microstructure,phase formation and properties of rapid solidified Al–Fe–Cr–Ti alloys

Improvements in the mechanical strength of Al–Fe–Cr–Ti alloys have been demonstrated when non-equilibrium microstructures are developed. This paper investigated the effect of cooling rate and composition on the phase formation, microstructure and properties of new Al_96.6Fe_1.5Cr_1.7Ti_0.2 and Al_91.6Fe_4.9Cr_2.2Ti_1.3 (at.-%) alloys. Wedge-shaped samples produced by suction casting were characterised by optical, scanning electron microscopy, differential scanning calorimetry, X-ray diffraction, energy dispersive X-ray spectroscopy and microhardness. The results showed that the morphology and size of the phases precedent of the flower-like phases change from small, spherical particles to large flower-like phases with decreasing the cooling rate. The presence of intermetallic phases Al₁₃Fe₄, Al₁₃Cr₂ and Al₃Ti in the Al_91.6Fe_4.9Cr_2.2Ti_1.3 alloy, resulted in a hardness 1.6 times higher compared to the Al_96.6Fe_1.5Cr_1.7Ti_0.2 alloy.     

Rapid solidification and magnetism characteristics of Co-11.8 at%Gd and Co-7.5 at%Gd dual-magnetic ribbons at different wheel speeds

The rapidly solidified ribbons of Co-7.5 and 11.8 at%Gd alloys were obtained by the melt spinning technique. The magnetic properties were measured and the effects of the wheel speed and the heat treatment on the coercivity were examined. SEM and EDS results show that the microstructures of melt-spun ribbons vary at different wheel speeds and after the heat treatment (1073 K, 30 min), and the soft magnetic properties of annealed ribbons are better than those of quenched ones. For Co-11.8 at%Gd ribbons, the maximum saturation magnetization reaches to 65.11 emu/g at the wheel speed of 20 m/s, while the minimum value of coercivity is 75.32 Oe. For Co-7.5 at%Gd ribbons, the minimum value of coercivity is 43 Oe in the annealed ribbons at 30 m/s and the maximum value of saturation magnetization is 106.93 emu/g from the annealed ribbons at 40 m/s. Theoretical analysis indicates that the exchange coupling length of (Co) and Co₁₇Gd₂ phases is 65.3 nm, and the exchange coupling coefficient of Co-7.5 and 11.8 at%Gd ribbons is in the range of 0.023–0.089. The exchange coupling effect in Co-11.8 at%Gd ribbons is stronger than that in Co-7.5 at%Gd ribbons.     

Structural investigations of mechanical properties of Al based rapidly solidified alloys

In this study, Al based Al–3 wt.%Fe, Al–3 wt.%Cu and Al–3 wt.%Ni alloys were prepared by conventional casting. They were further processed using the melt-spinning technique and characterized by the X-ray diffraction (XRD), scanning electron microscopy (SEM) together with energy dispersive spectroscopy (EDS), transmission electron microscope (TEM), differential scanning calorimetry (DSC) and the Vickers microhardness tester. The rapidly solidified (RS) binary alloys were composed of supersaturated α–Al solid solution and finely dispersed intermetallic phases. Experimental results showed that the mechanical properties of RS alloys were enhanced, which can be attributed to significant changes in the microstructure. RS samples were measured using a microhardness test device. The dependence of microhardness H_V on the solidification rate (V) was analysed. These results showed that with the increasing values of V, the values of H_V increased. The enthalpies of fusion for the same alloys were determined by DSC.     

Joining of zirconia using zirconium-based alloys

The contact angles of Zr-Ni, Zr-Cu and Zr-Co alloys against PSZ were measured by the sessile drop method. Each alloy wetted PSZ very well. Zr-Co alloys showed a different behaviour. Joints of PSZ plates were obtained using Zr-17Ni alloy. At the joint interface, internal oxidation of zirconium occurred. The fracture shear strength of this joint was 55 MPa.     

Some aspects of solidification and homogenisation of Mg–Ag alloys

The model of non-equilibrium solidification based on assumption of full diffusion in liquid and its lack in the solid state has been adopted in Krupkowski's evaluation to Mg–Ag alloys to describe maximum microsegregation of components in the solid solution resulting in appearance of maximum amounts of non-equilibrium phases. The results were then compared with experimental in MgAg 2.5 wt.% alloys with and without addition of 0.6 wt.% zirconium and 2.5 wt% neodymium (RE) by means of scanning and transmission electron microscopy equipped with an energy dispersive spectrometer. It was found that zirconium entered the solid solution, while neodymium appeared as a net of the Mg–Ag–Nd ternary eutectic.The homogenisation process has been studied based on hardness measurements and structure analysis. The times and temperatures of homogenisation to receive uniform distribution of components in the solid solution have been chosen neglecting the remaining eutectic precipitates due to economical reasons.     

快速凝固Al－Li合金塑韧性的改进研究

本文从合金化、粉末颗粒界面强化和断裂行为三个方面探讨了快速凝固粉末冶金（ＲＳＰ）Ａｌ－Ｌｉ合金塑韧性改进的途径，并取得了新的进展。     

Rapid solidification in thermal spray deposition: Microstructure and modelling

Mechanical thermal and adhesive properties of thermal spray coating are primarily determined by the phase and microstructure of single splats, which ultimately depend on rapid solidification of each splat and on the interactions between the splats and between the splat and the substrate. Significant efforts are being made to develop a better understanding of the physical mechanisms underlying these phenomena. This paper reviews a series of work in the area of mathematical modeling of phase and microstructure formation during the rapid solidification of single splats and coatings. The model development has been complimented by special experiments. Conditions under which plantar interface solidification occurs, columnar celluar or dendritic growth takes place, or banded structure forms, have been identified. A microstructure map can therefore built using the model presented here. The process parameters that promote crystalline nucleation and grain structure formation can be isolated and the effect of interfacial heat transfer, splat substrate temperature difference, and substrate melting and resolidification can be examined using the model. The model prediction agree qualitatively well with the experimental data for alumina, yttria, partially-stabilized zirconia, and molybdenum.