基于CAFE法模拟易切削钢9SMn28凝固显微组织和从柱状晶到等轴晶转变的变化

基于铸造中的对流计算，用有限元-元胞自动机（CAFE）模型模拟了易切削钢9SMn28的凝固显微组织，模拟结果与试验结果一致，模拟的结晶过程与实际相符，9SMn28合金在慢冷条件下以体积凝固方式凝固。用CAFE模型也研究了从柱状晶到等轴晶转变机理（CET），CAFE模型中的CET机理是相互的热作用。CET不是突变而是渐变产生，长柱状晶的生长首先被较长晶粒阻挡，随凝固过程的发展热梯度降低，晶粒逐渐变为等轴晶。

通过钛脱氧细化低碳高锰钢显微组织-夹杂物沉淀和凝固结构

在C（0．07％）-Mn（0．9％）钢中，研究用钛作为脱氧剂代替铝有2个目的，第一个目的是确认凝固过程中（二次脱氧）能否促进夹杂物沉淀，第二个目的是调查初始氧含量和钛含量以及凝固过程中冷却速率发生变化时，二次夹杂物对钢凝固结构的后序演变影响。在钛镇静钢试样中，当试样初始氧含量较高时（全氧（T．O）=50—80ppm），氧化物夹杂组成为MnO—TiO2（0．5—5μm）和MnS（1—3μm），而在低氧含量试样中（T．O=7—10ppm），夹杂物组成为Ti—Al-（Mg）-O（0．3—1μm）。与热力学计算结果相比，认为后者说的夹杂物为在中间枝晶区域的单独二次脱氧沉淀产物。对于较高氧含量试样来说，发现其夹杂物组成为初生和二次脱氧产物的共同体。通过控制冷却速率介于3-10K／之间，在真空感应炉中采用不同模型，研究了凝固过程中冷却速率的影响。除此之外，将在金像炉中重熔的试样，在冷却过程中，冷却速率分别控制为1．1，14和84K／s，此炉放于共焦扫描激光显微镜中（CSLM）。随着冷却速率的增加，钛镇静钢试样中的夹杂物密度也随之增加，而在铝镇静钢中观察不到这种现象。在凝固过程中，通过溶质偏析模型预测钛镇静钢试样中二次粒子的尺寸与观察到的平均粒子尺寸相吻合。在钛镇静钢试样中，随着夹杂物密度的增加，凝固组织较细，夹杂物尺寸小于1μm，然而，在铝镇静钢试样中观察不到此现象。

高温热处理对(DS)NiAl-Cr(Mo)-Hf共晶合金显微组织和显微硬度的影响

采用高温度梯度定向凝固装置制备NiAl-28cr-5Mo-1Hf(原子分数,％,下同)共晶合金,研究了高温热处理对合金显微组织和显微硬度的影响。结果表明,热处理后,NiAl和Cr(Mo)的层片状组织形貌基本没有变化,而合金中分布于胞界的半连续的Heusler(Ni_2AlHf)相部分或大部分消失,并以弥散的Heusler颗粒形式在NiAl基体中重新析出。此外,Cr(Mo)相中的NiAl微粒粗化,在Cr(Mo)相中出现了位错线,定向凝固NiAl-28Cr-5Mo-1Hf合金的显微硬度明显高于NiAl-28Cr-6Mo合金,热处理后,合金的硬度值基本保持不变。

通过钛脱氧细化低碳高锰钢显微组织：奥氏体晶粒长大和分解

本文研究了脱氧夹杂物对低碳钢显微组织演变的影响。将低碳（0．07％wt％）、高锰（0．9wt％）钢在A2O3或MgO坩埚中通过添加铝（0．05wt％）或钛（0．05、0．03或0．015wt％）进行脱氧。试验在容量400g真空炉中完成，然后在铜模中浇铸，冷却速率介于2．0～6．0K／s之间。为了研究冷却速率的影响，将这些浇铸试样重新熔化并且以各种不同冷却速率进行冷却，冷却速率从1—100k／s，采用高温共焦激光扫描显微镜（CSLM）予以分析。 在所有钛镇静钢试样中，氧化物夹杂尺寸均较小并且夹杂物密度高于那些铝镇静钢试样。在钛镇静钢试样中，夹杂物尺寸和密度随氧含量的增加而增加，夹杂物尺寸随着冷却速率的增加而减少，夹杂物密度随冷却速率的增加而增加。 采用共焦激光扫描显微镜（CSLM）研究钛镇静钢和铝镇静钢试样两者之间固态显微组织变化的差别。研究了等温条件下奥氏体晶粒的长大，并且从中发现：钛镇静钢试样中的晶界迁移率和最终晶粒尺寸均低于其它镇静钢。至于奥氏体分解，在从相似的奥氏体晶粒结构连续冷却过程中，由于沉淀反应导致较高的韦氏板条密度，最终导致钛镇静钢试样中的奥氏体分解，使其结构更为细小。夹杂物尺寸对奥氏体晶粒尺寸和奥氏体分解结构均有显著影响，在钛镇静钢试样中可观察到来自夹杂物的铁素体沉淀物不同取向。在最小平均夹杂物尺寸试样中获得最高板条浓度，而不是在最高密度的亚微米夹杂物中获得最高板条浓度。

通过钛脱氧细化低碳高锰钢显微组织以及二次脱氧颗粒的特征和作用

本文研究了脱氧夹杂物对低碳（0．07％wt％）、高锰（0．9wt％）碳钢显微组织的影响。通过在容量400g真空炉中的熔铁添加铝（0．05wt％）或钛（0．05wt％、0．03wt％或0．015wt％）完成脱氧试验。通过对凝固试样的再熔化和凝固过程中淬火，采用共焦激光扫描显微镜（CSLM）评价冷却速率的影响。在钛镇静钢试样中获得了细小二次脱氧粒子，粒子密度随氧含量的增加而增加，尺寸随凝固过程中的冷却速率增加而减小。发现了二次钛脱氧颗粒对显微组织演变的影响，例如凝固显微组织、奥氏体晶粒长大和奥氏体分解。通过FE—TEM检测脱氧颗粒，并分别在低氧（[O]=7×10^-6）和高氧（[O]=56．81×10^-6）钛镇静钢中确定为TiO、MnTiO3和Mn2TiO4，与那些通过热力学计算预测的结果定性相同。TiO、MnTiO3和Mn2TiO4的稳定性受Mn存在影响，通过热力学计算估算组成变化和氧化物分解。通过热机械处理评价了粒子对铁素体的形成影响，TiO最能有效通过异相形核促进铁素体的形成。粒子对铁素体形成的作用依如下顺序，TiO〉TiN〉MnS〉MnTiO3〉Ti2O3。结果发现二次钛脱氧颗粒产品通过凝固过程中前进的固相被卷吸，以PET（推进卷吸转变）速度、粒子尺寸和凝固速率为基础。枝晶末端和枝晶间区域的粒子很可能受钢水流动抑制，导致细小的凝固显微组织，

Effects of heat treatment on microstructure of directionally solidified Ti-45Al-8Nb-(W,B,Y) alloy

The effects of heat treatments on typical microstructures of directionally solidified（DS） Ti-45Al-8Nb-（W,B,Y）（molar fraction,%） alloys prepared by the Bridgeman method were studied.Two typical DS microstructures including full lamellae with cellular growth morphology and massive structure with dendritic growth morphology were examined.The results show that the heat treatment of 1250 ℃ for 24 h ＋ 900 ℃ for 30 min＋air cooling can efficiently eliminate the B2 phase in the DS alloys and change the massive structure of the rapid DS alloy into lamellar microstructure.Columnar lamellar colonies with widths of 150-200 μm and 50-100 μm respectively were observed in intercellular and dendritic arm regions.The heat treatment of 1 400 ℃ for 12 h＋900 ℃ for 30 min＋air cooling could simultaneously remove the B2 phase,massive structure and solidification segregations from the DS alloys,however,it caused severe growth of grains.

Effect of spark plasma sintering temperature on microstructure and mechanical properties of melt-spun TiAl alloys

A TiAl alloy from pulverized rapidly solidified ribbons with the composition of Ti-46Al-2Cr-4Nb-0.3Y（mole fraction,%） was processed by spark plasma sintering（SPS）.The effects of sintering temperature on the microstructure and mechanical properties were studied.The results show that the microstructure and phase constitution vary with sintering temperature.Sintering the milled powders at 1200 ℃ produces fully dense compact.Higher sintering temperature does not improve the densification evidently.The dominant phases are γ and α2 in the bulk alloys sintered at 1200 ℃.With higher sintering temperature,the fraction of α2 phase decreases and the microstructure changes from equiaxed near γ grain to near lamellar structure,together with a slight coarsening.The bulk alloy sintered at 1260 ℃ with refined and homogeneous near lamellar structure reveals the best overall mechanical properties.The compressional fracture stress and compression ratio are 2984 MPa and 41.5%,respectively,at room temperature.The tensile fracture stress and ductility are 527.5 MPa and 5.9%,respectively,at 800 ℃.

Effect of pre-annealing on microstructure and compactibility of gas-atomized Al-Si alloy powders

Effect of pre-annealing treatment temperature on compactibility of gas-atomized Al-27%Si alloy powders was investigated. Microstructure and hardness of the annealed powders were characterized. Pre-annealing results in decreasing Al matrix hardness, dissolving of needle-like eutectic Si phase, precipitation and growth of supersaturated Si atoms, and spheroidisation of primary Si phase. Compactibility of the alloy powders is gradually improved with increasing the annealing temperature to 400 ℃. However, it decreases when the temperature is above 400 ℃ owing to the existence of Si-Si phase clusters and the densely distributed Si particles. A maximum relative density of 96.1% is obtained after annealing at 400 ℃ for 4 h. In addition, the deviation of compactibility among the pre-annealed powders reaches a maximum at a pressure of 175 MPa. Therefore, a proper pre-annealing treatment can significantly enhance the cold compactibility of gas-atomized Al-Si alloy powders.

Effect of liquid diffusion coefficients on microstructure evolution during solidification of Al356.1 alloy

（The effect of liquid diffusion coefficients on the microstructure evolution during solidification of primary （Al） phase in Al356.1 alloy was investigated by means of the phase-field simulation using two sets of diffusion coefficients in liquid phase, while fixing other thermophysical and numerical parameters. The first set is only with impurity coefficients of liquid phase in Arrhenius formula representing only the temperature dependence. While the second set is with the well-established atomic mobility database representing both temperature and concentration dependence. For the second set of liquid diffusion coefficients, the effect of non-diagonal diffusion coefficients on the microstructure evolution in Al356.1 alloy during solidification was also analyzed. The differences were observed in the morphology, tip velocity and composition profile ahead of the tip of the dendrite due to the three cases of liquid diffusivities. The simulation results indicate that accurate databases of mobilities in the liquid phase are highly needed for the quantitative simulation of microstructural evolution during solidification.

Comparison of dendrite and dispersive structure in rapidly solidified Cu-Co immiscible alloy with different heat flow modes

Rapid solidification of Cu-Co immiscible alloy was investigated by glass-fluxing, spray casting and melt-spinning techniques. Both the transition from dendrite to dispersive structure and corresponding scale evolution were revealed and further elucidated in terms of the heat flow mode, nucleation and growth processes under different solidification conditions. With the increase of undercooling, columnar dendrite is replaced by dispersive structure due to the immiscible effect. In contrast, equiaxed dendrite forms in spray cast alloy due to multiple nucleation events and becomes thinner for the case of higher cooling rate. Ascribed to the enhanced non-equilibrium effect and insufficient period for collision and coagulation processes between separated droplets, fine globular dispersion appears upon the diameter of spray casting reaching 4 mm. As for the melt-spun ribbon with the highest cooling rate, a single-phase solid solution microstructure with refined grain of cellular morphology can be obtained, which is attributed to the suppression of liquid phase separation by instant solidification.

Microstructure and room temperature mechanical properties of directionally solidified Mg-2.35Gd magnesium alloy

Directional solidification of Mg-2.35Gd （mass fraction, %） magnesium alloy was carried out to investigate the effects of the solidification parameters （growth rate v and temperature gradient G） on microstructure and room temperature mechanical properties under the controlled solidification conditions. The specimens were solidified under steady state conditions with different temperature gradients （G=20, 25 and 30 K/mm） in a wide range of growth rates （v=10-200 μm/s） by using a Bridgman-type directional solidification furnace with liquid metal cooling （LMC） technology. The cellular microstructures are observed. The cellular spacing 2 decreases with increasing v for constant G or with increasing G for constant v. By using a linear regression analysis the relationships can be expressed as 2=136.216v^-0.2440 （G=30 K/mm） and 2=626.5630G^-0.5625 （v=10 μm/s）, which are in a good agreement with Trivedi model. An improved tensile strength and a corresponding decreased elongation are achieved in the directionally solidified experimental alloy with increasing growth rate and tempertaure gradient. Furthermore, the directionally solidified experimental alloy exhibits higher room temperature tensile strength than the non-directionally solidified alloy.

A review of particulate-reinforced aluminum matrix composites fabricated by selective laser melting

Selective laser melting(SLM)is an emerging layer-wise additive manufacturing technique that can generate complex components with high performance.Particulate-reinforced aluminum matrix composites(PAMCs)are important materials for various applications due to the combined properties of Al matrix and reinforcements.Considering the advantages of SLM technology and PAMCs,the novel SLM PAMCs have been developed and researched in recent years.Therefore,the current research progress about the SLM PAMCs is reviewed.Firstly,special attention is paid to the solidification behavior of SLM PAMCs.Secondly,the important issues about the design and fabrication of high-performance SLM PAMCs,including the selection of reinforcement,the influence of parameters on the processing and microstructure,the defect evolution and phase control,are highlighted and discussed comprehensively.Thirdly,the performance and strengthening mechanism of SLM PAMCs are systematically figured out.Finally,future directions are pointed out on the advancement of high-performance SLM PAMCs.

Microstructural and mechanical properties assessment of transient liquid phase bonding of CoCuFeMnNi high entropy alloy

The transient liquid phase(TLP)bonding of CoCuFeMnNi high entropy alloy(HEA)was studied.The TLP bonding was performed using AWS BNi-2 interlayer at 1050℃ with the TLP bonding time of 20,60,180 and 240 min.The effect of bonding time on the joint microstructure was characterized by SEM and EDS.Microstructural results confirmed that complete isothermal solidification occurred approximately at 240 min of bonding time.For samples bonded at 20,60 and 180 min,athermal solidification zone was formed in the bonding area which included Cr-rich boride and Mn3Si intermetallic compound.For all samples,theγsolid solution was formed in the isothermal solidification zone of the bonding zone.To evaluate the effect of TLP bonding time on mechanical properties of joints,the shear strength and micro-hardness of joints were measured.The results indicated a decrement of micro-hardness in the bonding zone and an increment of micro-hardness in the adjacent zone of joints.The minimum and maximum values of shear strength were 100 and 180 MPa for joints with the bonding time of 20 and 240 min,respectively.

New insights into microstructural changes during transient liquid phase bonding of GTD-111 superalloy

The effects of joining temperature(TJ)and time(tJ)on microstructure of the transient liquid phase(TLP)bonding of GTD-111 superalloy were investigated.The bonding process was applied using BNi-3 filler at temperatures of 1080,1120,and 1160℃ for isothermal solidification time of 195,135,and 90 min,respectively.Homogenization heat treatment was also applied to all of the joints.The results show that intermetallic and eutectic compounds such as Ni-rich borides,Ni−B−Si ternary compound and eutectic-γcontinuously are formed in the joint region during cooling.By increasing tJ,intermetallic phases are firstly reduced and eventually eliminated and isothermal solidification is completed as well.With the increase of the holding time at all of the three bonding temperatures,the thickness of the athermally solidified zone(ASZ)and the volume fraction of precipitates in the bonding area decrease and the width of the diffusion affected zone(DAZ)increases.Similar results are also obtained by increasing TJ from 1080 to 1160℃ at tJ=90 min.Furthermore,increasing the TJ from 1080 to 1160℃ leads to the faster elimination of intermetallic phases from the ASZ.However,these phases are again observed in the joint region at 1180℃.It is observed that by increasing the bonding temperature,the bonding width and the rate of dissolution of the base metal increase.Based on these results,increasing the homogenization time from 180 to 300 min leads to the elimination of boride precipitates in the DAZ and a high uniformity of the concentration of alloying elements in the joint region and the base metal.

Effects of solidification rate and excessive Fe on phase formation and magnetoclaoric properties of LaFe_(11.6x)Si_(1.4)

The effects of solidification rate and excessive Fe on phase formation and magnetocaloric properties of LaFe11.6xSi1.4(x=1.1,1.2)were investigated by XRD,SEM and VSM measurements.The XRD results show that the amount of LaFeSi phase in the as-cast melt-spun ribbons prepared by a copper wheel at a speed of10m/s is less than that in the as-cast arc melting buttons with the same x values.The annealed melt-spun ribbons contain smaller amount of La(Fe,Si)13(1:13)phase than the corresponding annealed arc melting buttons.Although the melt-spun sample has finer crystalline grains ofα-Fe,as indicated by SEM analysis,its crystalline size has not reached nano-scale.Therefore,the magnetic exchange-coupling between1:13phase andα-Fe phase has not been observed in melt-spun ribbons.Further,the maximum negative magnetic entropy change(?SMax)and relative cooling power(RCP)of annealed melt-spun ribbons under a field change of0?2T are weaker than those of the corresponding annealed arc melting buttons.

Effect of elevated-temperature annealing on microstructure and properties of Cu−0.15Zr alloy

Cu−0.15Zr(wt.%)alloy with uniform and fine microstructure was fabricated by rapid solidification followed by hot forging.Evolution of microstructure,mechanical properties and electrical conductivity of the alloy during elevated-temperature annealing were investigated.The alloy exhibits good thermal stability,and its strength decreases slightly even after annealing at 700℃ for 2 h.The nano-sized Cu_(5)Zr precipitates show significant pinning effect on dislocation moving,which is the main reason for the high strength of the alloy.Additionally,the large-size Cu_(5)Zr precipitates play a major role in retarding grain growth by pinning the grain boundaries during annealing.After annealing at 700℃ for 2 h,the electrical conductivity of samples reaches the peak value of 88%(IACS),which is attributed to the decrease of vacancy defects,dislocations,grain boundaries and Zr solutes.

Microstructure and tensile property of SLM 316L stainless steel manufactured with fine and coarse powder mixtures

Selective laser melting(SLM)technology is the prevailing method of manufacturing components with complex geometries.However,the cost of the additive manufacturing(AM)fine powder is relatively high,which significantly limits the development of the SLM.In this study,the 316L fine powder and coarse powder with a mass ratio of 80:20,70:30 and 60:40 were mixed using a ball milling and the samples with a relative density greater than 97%were prepared by SLM.The results show that the intricate temperature gradients and surface tension gradients in SLM will produce Marangoni flow,forming a typical molten pool morphology,cellular and strip subgrain structures.And as the proportion of coarse powder increases,the scanning track morphology changes from smooth to undulating;the morphology of the molten pool and subgrain structure are weakened.Meanwhile,the unmelted particles appear on the surface of the SLM sample.On the premise of an introducing appropriate amount of large particle size powder(20%),the SLM samples still have good mechanical properties(662 MPa,47%).

Microstructure and mechanical properties of AlCuFe eutectic alloy

In the production of AlCuFe alloy for a special application,the growth rate was changed and the results were evaluated.Changes in the eutectic spacing(microstructure)of a material due to the growth rate are known to affect its mechanical,electrical and thermal properties.To evaluate its microstructure,the eutectic composition of Al−32.5wt.%Cu−0.5wt.%Fe was prepared and directional solidification experiments were conducted using a Bridgman-type furnace at a constant temperature gradient(G=8.50 K/mm)and five growth rates(V=8.25,16.60,41.65,90.05,164.80μm/s).The effect of the growth rate on the eutectic spacing was then determined,and the resulting microhardness and ultimate tensile strength were obtained based on the change in the microstructure by regression analysis and Hall−Petch correlations.Despite the fact that the growth rate increased by approximately twenty times,the eutectic spacing decreased by a factor of approximately 5,and these changes in the growth rate and microstructure caused the mechanical properties to change by a factor of approximately 1.5.

Hot tearing behaviors and in-situ thermal analysis of Mg-7Zn-xCu-0.6Zr alloys

Thermal analysis was used to investigate the microstructural evolution of Mg-7 Zn-x Cu-0.6 Zr alloys during solidification. The effect of Cu content（0, 1, 2 and 3, mass fraction, %） on the hot tearing behavior of the Mg-7 Zn-x Cu-0.6 Zr alloys was investigated with a constrained rod casting（CRC） apparatus, equipped with a load sensor and a data acquisition system. The thermal analysis results of Mg-7 Zn-x Cu-0.6 Zr alloy revealed that the alloy consisted of two distinct phases： α-Mg and Mg Zn2. Three distinct peaks were observed in the alloys with Cu addition, which were identified as α-Mg, Mg Zn Cu and Mg Zn2. In addition, the reaction temperature of α-Mg decreased and the reaction temperatures of Mg Zn2 and Mg Zn Cu increased as the Cu content increased. The experimental results of hot tearing demonstrated that the addition of Cu significantly reduced the hot tearing susceptibility（HTS） of Mg-7 Zn-x Cu-0.6 Zr alloys due to the higher eutectic temperature and the shorter solidification temperature region.

Cellular/dendritic transition,dendritic growth and microhardness in directionally solidified monophasic Sn-2%Sb alloy

Horizontal directional solidification experiments were carried out with a monophasic Sn-2%Sb（mass fraction） alloy to analyze the influence of solidification thermal parameters on the morphology and length scale of the microstructure. Continuous temperature measurements were made during solidification at different positions along the length of the casting and these temperature data were used to determine solidification thermal parameters, including the growth rate（VL） and the cooling rate（TR）. High cooling rate cells and dendrites are shown to characterize the microstructure in different regions of the casting, with a reverse dendrite-to-cell transition occurring for TR5.0 K/s. Cellular（λc） and primary dendrite arm spacings（λ1） are determined along the length of the directionally-solidified casting. Experimental growth laws relating λc and λ1 to VL and TR are proposed, and a comparative analysis with results from a vertical upward directional solidification experiment is carried out. The influence of morphology and length scale of the microstructure on microhardness is also analyzed.