Control of solidification structure of wear-resistant austenite-bainite polyphase steel with nodular eutectic

A new austenite-bainite polyphase steel with nodular carbides can be obtained by controlling the solidification structure of the steel melt, which only contains manganese and silicon, with modification of Si-Ca-B compound and air-hardening. The result indicates that the nodular carbide is in the eutectic form of austenite and (Fe, Mn)3C, which is formed between the austenitic dendrites during solidification due to element segregation. The modifying elements (calcium, silicon, etc.) have the following functions: (1) their chemical compounds (CaS, SiO2) are formed preferentially during solidification to act as heterogeneous nuclei for nodular eutectic crystallization, (2) the eutectic can be turned into the nodular shape after modification because of the decrease in the amount of the adsorbed impurity elements (oxygen and sulphur) and silicon enriched on the eutectic growth interface. The quantity of nodular eutectic makes up 10%–20%, with a size of 15–25 μm. The hardness and the toughness of this steel are 40–50 HRC and 20–40 J, respectively, and hence its wear-resistance can be more greatly increased than that of the austenite-manganese steel and the austenite-bainite steel. This revised version was published online in November 2006 with corrections to the Cover Date.     

Strain rate effects on tensile deformation behavior of Fe-3.5Mn-0.3C-5Al ferritic based lightweight steel

Tensile deformation behavior of Fe-3.5Mn-0.3C-5Al ferritic based lightweight steel was studied in a large range of strain rate (0.001 s⁻¹–1200 s⁻¹). Microstructures of the steel before and after tension were observed. The results show that Fe-3.5Mn-0.3C-5Al lightweight steel has a good strength (820 MPa) and plasticity (40 %) and exhibits excellent combinations of specific strength and ductility (>32000 MPa %) at the strain-rate of 0.001 s⁻¹ after annealing at 850 °C for 5 minutes then directly quenching into water. The austenite in the steel tested was transformed into α′-martensite during the tensile deformation process. With an increase in strain rate from 0.001 s⁻¹ to 1200 s⁻¹, tensile strength of the steel investigated increased from 820 MPa to 932 MPa, while its elongation first decreased from 40 % to 15 %, and then increased from 15 % to 29 %. At the strain rate of 1200 s⁻¹, adiabatic heating resulted in temperature rising in matrix, suppressed the transformation of austenite to α′-martensite. Comparing with transformation induced plasticity steel, the austenite in 3.5Mn lightweight steel is obviously unstable and cannot provide progressive phase transition.     

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.     

Segregation and precipitation formation for in situ oxidised 9Cr steel powder

Powder and sintered bulk materials of a reduced activation steel with oversaturated Ti and Y are prepared by gas atomisation and sintering. Segregation on both the surface and interior of the powder particles are observed after an thermal treatment. The oxidation of the segregates promotes formation of clusters of large Y oxide crystals and thick continuously distributed Ti oxide layers on the surface of the powder particles. The oxidation also occurs within the powder particles and mainly forms larger Ti oxide dispersoids in the grain boundaries of the powder. The coarsening and aggregation of the oxide dispersoids occur with increasing duration of the thermal treatment. Yttrium is also segregated and precipitated in the grains of the powder particles.     

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.     

Unidirectional solidification of ternary eutectic Al-Ni-Si alloys

Two monovariant alloys — type I (Al/6%Ni/0–2.5%Si) and type II (Al/12%Si/0–1.5%Ni) — and the ternary (Al-Ni-Si) eutectic have been studied. For the type I alloys particular attention was given to the effect of silicon concentration on the microstructural stability of the binary eutectic Al-Al₃Ni. Three distinct morphologies were observed, according to the solidification parameters. These were: well-aligned; aligned/cellular transitional; and cellular. The morphology of the silicon phase in the type II alloys was found to be very sensitive to any change in the solidification rate. The presence of very small amounts of nickel resulted in the formation of Al₃Ni; its growth was influenced by the kinetics and growth of the silicon phase. In the ternary eutectic, two of the three phases — Al₃Ni and Si — developed rod-like morphologies at the lowest solidification rates, with almost planar solid/liquid interfaces. As in the type II alloys, the silicon phase was sensitive to the solidification rate and it affected the growth of the Al₃Ni.     

Controlled solidification of the eutectic LiF-MgF2

A two-phase model composite material of LiF and MgF₂ has been prepared by unidirectional solidification of the mixed eutectic fluoride. The microstructure has been examined by optical and electron microscopy. The morphology is that of regularly arranged rods of the MgF₂ phase (rutile structure type) in a continuous LiF matrix. This microstructure is restricted to high purity materials with a planar solid-liquid interface and progressively degrades into a cellular lamellar structure associated with constitutional supercooling. The crystallographic orientation relation between the phases for the regular morphology has been determined as: Growth axis //[001]MgF₂//[001]LiF, Preferred interfaces //(110)MgF₂//(100)LiF and (1¯10)MgF₂//(010)LiF.     

Effects of Re on surface eutectic formation for Ni-base single crystal superalloys during directional solidification

Effects of Re on the formation of surface eutectics have been investigated by using Ni-base single crystal superalloys with different Re additions. It was found that Re promotes the segregation of Al and Ta to the eutectic melt, leading to an increase of the surface and internal eutectics. In addition, the addition of Re also increased the freezing range, the local solidification time, and the permeability of the dendritic network within the mushy zone. These factors ultimately promoted the outflow of the interdendritic residual liquid with the action of solidification shrinkage, and led to the formation of more surface eutectics. In contrast, the addition of Re had no obvious influence on the surface eutectic microstructures.     

Recalescence behaviour of rapid solidification in the deeply undercooled Ni-B hypereutectic alloy and formation mechanism of anomalous eutectic

以Ti--Al--Si合金作为合金化填充材料, 用氮氩混合等离子气体对SiCp/Al基复合材料进行等离子弧原位焊接, 研究了Ti-Al-Si对焊缝的组织和性能的影响. 结果表明: 填加Ti-75Al-5Si合金时, 熔池中Si和Ti的联合作用有效抑地制了针状脆生相Al4C3的生成, 形成了稳定的熔池, 得到了以TiN、AlN 、TiC和Ti5Si3等为二次增强相的焊缝. 焊缝的组织致密, 结合良好, 其最大拉伸强度为225 MPa     

Segregation in Pb1−xSnx Te during solidification by the Bridgman technique

The influence of the growth parameters on the concentration gradient of Pb_1–xSn_xTe monocrystals grown by the vertical Bridgman technique is described. The concentration profiles of all three constituents (Pb, Sn, Te) have been analysed in ingots having different Pb-Sn ratios and grown under different conditions. The presumed role played by free convection in the Pb-Sn ratio is shown. Te was found to be constant in all ingots and not dependent upon the growth conditions.     

Pattern formation in solidification

Regular patterns form in many solidification processes. Examples occur during lamella and rod-like eutectic growth and when single-phase cells or dendrites are formed. The scale and regularity of the microstructure can determine the properties of the cast materials and is thus important practically. The purpose of the present paper is to show that common features occur in all processes.

Steady-state analysis indicates that a wide range of possible spacings could occur during eutectic, cellular or dendritic growth. The degree of freedom is removed by considering the mechanism determining the minimum and maximum spacing on a specimen. It is found that the minimum spacing occurs when the array first becomes stable for a lamella or rod-like eutectic, for cell growth and for some dendrites. For low temperature gradient, high-velocity dendrites, the minimum spacing is determined by thespacing when the dendrites irst become near enough to interact. The maximum spacing for eutectics andfor cells is determined by tip splitting. The maximum spacing for dendrites occurs when a tertiary arm becomes a new primary. Good agreement is obtained between theory and experiment using this approachto predict spacing limits. The average spacing on a specimen can approach either limit depending o past history. The two extreme spacings are found to span the spacing of the minimum undercooling foreutectic and cellular growth and this allows an average spacing to be estimated using a single condition.

It is concluded that three conditions are necessary to form regular structures. A mechanism must exist to eliminate members of the array when the spacing is too small. A mechanism must exist to form new members of the array when the spacing is too wide. The structure must be stable to fluctuations in the range of spacing between the two limits.     

Pattern formation in solidification

Regular patterns form in many solidification processes. Examples occur during lamella and rod-like eutectic growth and when single-phase cells or dendrites are formed. The scale and regularity of the microstructure can determine the properties of the cast materials and is thus important practically. The purpose of the present paper is to show that common features occur in all processes.Steady-state analysis indicates that a wide range of possible spacings could occur during eutectic, cellular or dendritic growth. The degree of freedom is removed by considering the mechanism determining the minimum and maximum spacing on a specimen. It is found that the minimum spacing occurs when the array first becomes stable for a lamella or rod-like eutectic, for cell growth and for some dendrites. For low temperature gradient, high-velocity dendrites, the minimum spacing is determined by the spacing when the dendrites first become near enough to interact. The maximum spacing for eutectics and for cells is determined by tip splitting. The maximum spacing for dendrites occurs when a tertiary arm becomes a new primary. Good agreement is obtained between theory and experiment using this approach to predict spacing limits. The average spacing on a specimen can approach either limit depending on past history. The two extreme spacings are found to span the spacing of the minimum undercooling for eutectic and cellular growth and this allows an average spacing to be estimated using a single condition.It is concluded that three conditions are necessary to form regular structures. A mechanism must exist to eliminate members of the array when the spacing is too small. A mechanism must exist to form new members of the array when the spacing is too wide. The structure must be stable to fluctuations in the range of spacing between the two limits.     

SCN-Cam共晶合金的定向凝固研究

采用实时观测装置和定向凝固系统研究了SCN-Cam(Succinonitrile-wt%Camphor,wt%为质量分数)模型合金的凝固过程.实验结果表明,SCN-23.6%Cam共晶合金在常规条件下形成规则的共晶组织,共晶间距随界面推移速度的增大而减小;加入超声振动时,共晶合金生长出初生相;SCN-21%Cam亚共晶...     

Kinetics of triple-junctions in eutectic solidification: a sharp interface model

The kinetics of triple-junctions (TJs) in eutectic solidification is modeled by the thermodynamic extremum principle (TEP). It consists of two parts. First, TJs as the interaction of interfaces follow the interface kinetics according to which the temperature and concentration at the TJs are determined. This interface part of TJ kinetics is closely related to the eutectic point in the kinetic phase diagram. Second, TJs have their specific kinetics according to which their morphology (e.g., the contact angles in two dimensions) is determined. Using a new solution of solute diffusion in liquid, the TJ kinetics is incorporated into the current lamellar eutectic growth model. The model is applicable to the concentrated alloy systems and can be extended to any kind of eutectics. Simulation results of the rapid solidification of a lamellar Ni₅Si₂–Ni₂Si (γ–δ) eutectic show that both parts of TJ kinetics can play important roles in eutectic solidification and need to be considered to improve the current eutectic theory.     

超音速电弧喷射Ag—Cu共晶熔滴的快速凝固

采用超音速电弧喷射雾化制备Ag-40％（原子分数）Cu共晶合金粉末，用X射线衍射、扫描电镜等对其凝固行为进行研究。结果表明：粉末为球形颗粒，粒径分布90％集中在15～60μm之间、平均粒径为35gm；冷却速率在10^5～10^7K／s范围，最大过冷度不小于232K；粉末富Ag相的平均点阵参数为0．401877nm，Cu的平均扩展固溶度为20．15％（原子分数），直径≤50μm时，点阵参数为0．39252nm，Cu的固溶度达到40％（原子分数），凝固组织为单相亚稳固溶体；对快速凝固行为进行了分析讨论。