Design and manufacture of Nd-Fe-B thick coatings by the thermal spray process

Permanent magnetic coatings show potential in micro scale applications such as micro motors and micro generators. Nd-Fe-B magnetic coatings of average thickness 50 μm were produced via the flame spray method where the behavior of Nd-Fe-B particles in the thermal spray process was studied through single splat formation. There are generally five types of Nd-Fe-B splat morphology; which correspond to different solidification routes and varying degree of splashing. Microstructures of coating cross sections exhibit features such as cracks and porosity. The microstructural features were related to the physical properties and the brittle nature of the rare-earth alloy feedstock. Cross sections of the coatings also exhibited the presence of two distinct phases: Nd-rich and Fe-rich regions, which have been validated by EDS analysis. Metastable phase formation and decomposition followed by non-equilibrium solidification of the molten droplet prior to impact have been suggested to cause phase separation and were also identified in the formation of single splats. Hardness tests further confirmed the two distinct phases as Nd-rich and Fe-rich areas.     

液相分离对T2紫铜/316L不锈钢接头组织及性能的影响

采用钨极氩弧焊填充纯铜焊丝进行T2紫铜/316L不锈钢异种金属焊接工艺试验,分析了接头微观组织的形成机制. 结果表明,使用纯铜焊丝时,铁-铜液相分离对铜/钢焊缝组织的形成起主导作用. 以铜为基体的焊缝中分布着大量由铁-铜初次液相分离形成的富铁球,在其内部还分布有标志着铁-铜二次液相分离的富铜相. 富铁球内的析出相在表面能梯度和密度差的作用下,向富铁球中心呈球状聚集. 由于成分和所处区域的冷却速率不同,富铁球呈现不同的形貌. 基于熔池边界凝固理论,分析了接头铜/钢界面未混溶区宏观偏析机制. 填丝焊时,熔池边缘形成非等温边界. 纯铜焊丝制备接头铜/钢界面处液态钢母材的温度高于熔池主体,导致岛状和半未混溶区的形成. 接头的铜侧存在由粗大晶粒和正常尺寸晶粒组成的软化区,拉伸试样均断裂于此,抗拉强度达到铜母材的81.7%.     

定向凝固Ti-50Al合金组织演化及其片层取向控制

对Ti-50Al(原子分数,%)合金在较宽的生长速率范围内进行定向凝固实验,研究了生长速率对固/液界面形态、微观组织演化及片层结构形成的影响.发现合金在1-5μm/s的速率范围内均以α胞晶单相生长,最终形成全片层结构;当生长速率达到10μm/s时,在初始凝固的较长距离内为α胞品单相生长,随着凝固的进行,胞晶间溶质逐渐富集,晶间出现从液相析出的γ相,最终不能形成全片层结构;当生长速率大干15μm/s时,合金以α枝晶生长,枝晶间也出现γ相.对各生长速率下形成的片层结构取向的分析表明,片层结构取向与定向凝固启动界面处铸态品粒的取向的历史有关.根据上述规律,以Ti-50Al合金为籽晶和主体合金,选择确保α单相凝吲的生长速率8 μm/s,进行片层取向控制,最终扶得取向与生长方向一致的全片层结构.     

不同凝固条件下Cu80Fe20合金凝固组织以及磁性能研究

通过制备不同凝固条件下亚稳难混溶Cu80Fe20合金,系统的研究了不同凝固条件下合金凝固组织以及软磁性能的差异。结果表明:铜板模具冷却得到的Cu80Fe20合金凝固组织由细小的胞状富Fe枝晶以及基体组成,说明该凝固条件下合金凝固过程中仅仅发生平衡液-固相转变;然而石墨模具冷却得到的Cu80Fe20合金凝固组织由球状的富Fe相、发达的富Fe枝晶以及基体组成,说明该凝固条件下合金凝固过程中不仅发生平衡液-固相转变,而且发生了液-液相分离,并发现相分离形成的少量相富Fe液滴内部结构存在富Fe层的存在,且在迁移发生聚合、凝并以及碰撞;同时研究发现,不同的凝固条件下得到Cu80Fe20合金的饱和磁化强度和矫顽力存在差异,但综合来看制备的合金均呈现良好的软磁性能。     

Liquid phase separation of Cu-Cr alloys during rapid cooling

1 Introduction Cu-Cr alloys containing 20%- 50%Cr have been widely investigated because they are the most important contact materials used in high voltage vacuum switches. It has been well known that the smaller the Cr particles in Cu-Cr alloys are, the b…     

Effect of high magnetic field on solidification microstructure evolution of a Cu-Fe immiscible alloy

The liquid phase separation behavior and the evolution of the solidification microstructure of a binary Cu₅₀Fe₅₀ alloy were investigated under the conditions of without and with a 10 T magnetic field, with different undercooling during the solidification process. Results show that the combined effect of Stokes motion and Marangoni convection leads to the formation of the core-shell structure under the condition without the magnetic field. In addition, specific gravity segregation is reinforced by increasing the undercooling, resulting in Fe-rich phase drifts towards the sample edge. In the 10 T magnetic field, the Fe-rich phase is elongated in the parallel direction of the magnetic field under the action of demagnetization energy due to the difference of static magnetic energy and surface energy. In the vertical direction, through the action of Lorentz force, the convection in the melt is inhibited and Fe-rich phase becomes more dispersed. Meanwhile, the diffusion of the two phases and the coagulation of the Fe-rich phases are also restrained under the magnetic field, therefore, the phase volume fraction of the Fe-rich phase decreases at the same undercooling in the 10 T magnetic field. The magnetic field inhibits the segregation behavior in the vertical direction of the magnetic field, and at the same time, improves the gravitational segregation to a certain extent, which has a very important impact on microstructure regulation.

     

In Situ Observation of Growth Behavior and Morphology of Delta-Ferrite as Function of Solidification rate in an AISI304 Stainless Steel

It was presented the in situ observation of growth behavior and morphology of delta-ferrite as a function of solidification rate in an AISI304 stainless steel. The specimens have been solidified and observed using confocal scanning laser microscopy (CSLM). The δ-phase always appears like cells on the sample surface when critical supercooling occurs, during which the L→δ transformation starts. The solid-liquid (S-L) interface is found to be finger shaped and has no faceted shape. γ phase appears among δ grains due to partitioning of Ni into the melt during solidification, when solidification rate is higher. The mergence of observed δ cells is possible for the steel sample cooled at 7.5℃/min. The formation of dendrites can be observed on the free surface of the steel sample cooled at 150℃/min. The size of solidified delta grains decreases from 120 to 20-80μm, and the volume fraction of solidified austenite increases with increase in solidification rate from 7.5 to 150℃/min. The relation between the tip radius of δ cell and its growth rate is deduced, and the results agree with the experimental values.     

1J50锻棒车加工后出现的暗斑分析

1J50合金锻棒车加工成零件,1 180℃处理后表面中心处出现对称性"暗斑"。通过试验分析,确定为在冶炼浇注时不慎有微量的铝颗粒掉入钢液中,在钢液凝固结晶的作用下熔化及还没完全溶化的铝被逐渐推向钢锭最后凝固的中心部位,直至完全凝固。在后续的锻打过程中沿锻打方向变形延伸。车削加工时,这些由细小铝颗粒组成的夹杂被暴露在表面,在随后的热处理过程中发生了热腐蚀现象,从而形成了肉眼可见的"暗斑"。     

UNUSUAL SOLIDIFICATION MICROSTRUCTURE OF Fe_(70)Cr_(18)Mo_2Si_1B_9 SINGLE-ROLL QUENCHED RIBBON

The solidification microstructure of Fe_(70)Cr_(18)Mo_2Si_1B_9 single-roll quenched ribbons with dif-ferent rotating speed of roller and under different atmosphere conditions was investigated.Apart from usual crystallization by homogeneous and heterogeneous nucleation,it was foundthat α-iron cohnnnar grains with strong preferred orientation formed beneath free surface andthat spheres of α-iron single coastal with a size of several μm up to 20μm,which have rarelybeen reported,appeared inside the ribbons.There was experirnental evidence suggesting thatclusters of metal atoms formed from fluctuation in early undercooled liquid might play a roleas pre-existed nuclei and,in fact,control the formation of amorphous for this alloy.     

定向凝固Al-40%Cu合金三维微观组织重构及共晶间距演变

本文利用连续切片技术研究了定向凝固Al-40%Cu过共晶合金中金属间化合物Al₂ Cu初生相的三维微观组织,以及抽拉速率跃迁下,三维共晶组织形态的演变和间距调整.结果表明:抽拉速率为5μm/s时,初生Al₂Cu三维组织沿生长方向存在棱面和棱角,表现出明显的棱面相生长形态;凝固过程中初生Al₂Cu相释放的结晶潜热使得生长界面发生局部重熔,三维组织中出现孔洞,形成拓扑缺陷.在Al-40%Cu合金三维共晶组织中,Al₂Cu相和Al相的体积分数分别为56.8%和43.2%,且Al₂Cu相的生长方向与试样轴向夹角为5.1°.当抽拉速率从2μm/s突然跃迁到500μm/s时,三维共晶组织形态从层片向棒状转变,这种转变不是由合金中两相体积分数变化造成的.三维共晶组织间距调整机制不同于二维组织中的分叉、内凹和界面重新形核,而是通过三维空间非同一平面连续的分叉、分枝进行,在三维下并没有观察到二维下的重新形核共晶间距调整机制.     

磁场对Cu-Pb-La过偏晶合金液-液分离的作用

本实验在Cu-80%Pb过偏晶合金凝固过程中添加稀土元素La,研究不同冷却速度下稀土元素在过偏晶合金凝固制备中的作用,并研究施加磁场对Cu-Pb过偏晶合金液-液分离区凝固组织的影响,探讨了磁场下第二相的迁移和分布规律,为均质偏晶合金制备技术的改进提供参考。结果表明,添加La元素可显著降低Cu-80%Pb凝固组织中的宏观偏析,促进第二相颗粒的均匀分布和细化。较快的冷却速度下添加La元素的作用更加明显,偏析程度显著降低,在此冷速下进一步施加1 T水平方向磁场,Cu-Pb-La合金凝固过程中宏观偏析得到了抑制,富Cu相颗粒弥散,组织分布均匀。     

Pb-Bi-Sn准包晶合金定向凝固微观组织演化

采用液态金属冷却定向凝固结合液淬法,在温度梯度为18 K/mm、凝固速度为0.5～100 μm/s的条件下系统研究了Pb-30％Bi- 18％Sn准包晶合金的微观组织演化及溶质分布规律.主要从凝固路径、相组成、溶质分布及微观组织特征等几个方面来研究Pb-Bi-Sn准包晶合金定向凝固过程的组织演化.其中,准包晶合金的凝固路径通过DSC差热分析的方法进行研究,发现其凝固路径为:L→L+α-Pb→L+α-Pb+β-Pb7Bi3→L+α-Pb+β-Pb7Bi3+Sn→ (β-Pb7Bi3+Sn)共晶.结合XRD,分析定向凝固试样的相结构,结果表明:试样由具有面心立方结构的α-Pb相、密排六方结构的β-Pb7Bi3相和体心正方结构的(Sn)相构成.在Pb-Bi-Sn合金定向凝固过程中,当凝固速度为0.5～2 μm/s时,初生α-Pb相以胞状界面生长,而当凝固速度增大至5μm/s及以上时,初生α-Pb相将由胞状转变为枝晶状界面生长.利用能谱分析仪对定向凝固试样进行溶质分布测试,发现局部出现溶质偏析,导致共晶组织(β-Pb7Bi3+Sn)出现.     

过冷Ni-31．44％Pb偏晶合金凝固行为

采用熔融玻璃净化和循环过热相结合的方法研究Ni-31.44%Pb偏晶合金宽过冷区间凝固组织演化规律;结果表明,过冷偏晶合金在快速凝固阶段首先形成枝晶α骨架,再辉重熔后分布于枝晶间的残余液相按照平衡凝固模式进行后续反应;在0～286K过冷范围内,当ΔΤ＜50K时,合金凝固组织为粗大枝晶α+枝晶间Pb相;当70＜ΔT＜232K时,凝固组织为细密枝晶α+枝晶臂上细小的Pb颗粒+枝晶间Pb相;当ΔT＞242K时,凝固组织为过冷粒状晶+均匀细小的Pb颗粒+少量尺寸较大的枝晶间Pb颗粒,过冷粒状晶的粒化机制属于枝晶碎断-再结晶机制.     

焊丝对T2紫铜/316L不锈钢GTAW接头组织及性能的影响

采用钨极氩弧焊,分别填充纯铜焊丝和307Si焊丝对T2紫铜/316L不锈钢异种金属进行对接焊,探究焊丝对铜/不锈钢异种金属接头微观组织、力学性能和耐蚀性的影响.结果表明,铁-铜液相分离对焊缝组织的形成起主导作用,但由于过冷度不同,填充两种焊丝制备焊缝的液相分离程度不同:铜焊丝制备焊缝中生成了初次液相分离富铁球,其内部又...     

Microstructure Evolution of a Ternary Monotectic Alloy During Directional Solidification

A model was developed to describe the microstracture evolution in a directionally solidified ternary monotectic alloy.The directional solidification experiments were carried out on Al-3Pb-lSn(wt%) alloys by using a Bridgman apparatus.The microstracture evolution in the directionally solidified sample was calculated.The numerical results agree well with the experimental ones.It is demonstrated that the nucleation of the minority phase droplets occur at two different positions.One corresponds to the liquid-liquid decomposition,which occurs in front of the solidification interface.The other is at the liquid/solid interface.The nucleation rate of the minority phase droplets at the liquid/solid interface is significantly higher than at the position in front of the solidification interface.The characteristic of the nucleation process leads to a bimodal size distribution of the minority particles in the directionally solidified sample.     

Directional Solidification of TiAl-Based Alloys: Determination of the Primary Phase in Ti-50Al-5Nb Alloy

根据TiAl基合金片层组织的准三维金相形态准确地判断出其初生相类型。结果表明,具有不同片层取向的晶粒在Ti-50Al-5Nb合金定向凝固过程中其初生相均为β相。     

The microstructure of liquid immiscible Fe–Cu-based in situ formed amorphous/crystalline composite

In the microstructures of slowly and rapidly cooled liquid of the immiscible alloy Fe₃₀Cu₃₂Ni₁₀Si₁₃Sn₄B₉Y₂ two distinct regions were observed following arc melting and slow cooling, confirming that liquid/liquid phase separation had occurred. Rapid cooling from a temperature within the liquid immiscibility gap, melt spinning, resulted in an amorphous/crystalline composite, formed from the previously melted Fe- and Cu-rich regions, respectively. Transmission electron microscopic studies of this melt-spun ribbon revealed the glassy nature of the Fe-rich matrix, as well as of the Fe-rich spheres formed within the previously existing Cu-rich liquid.     

定向生长Bi2Te3-Sb2Te3三元合金热电材料的组织与性能

利用Bridgman定向凝固法,在大凝固速率范围内5～1000μm/s制备出Bi2Te3-Sb2Te3三元合金块体热电材料,并对其凝固组织和不同凝固速率下合金的热电性能进行研究。结果表明:高温度梯度和大凝固速率范围内制备的25%Bi2Te3-75%Sb2Te3合金定向凝固组织由Bi0.5Sb1.5Te3单相组织组成;在较低凝固速率5μm/s下,熔体生长平界面失稳形成胞状组织,而随定向凝固速率的增加,胞状组织减少,组织细化。不同定向凝固速率下25%Bi2Te3-75%Sb2Te3合金的Seebeck系数和电阻率随着凝固速率的增加而增大。50μm/s下300～450K范围内获得功率因子(PF)在4.6×10-3～5.01×10-3W/(K2.m),并在350K时PF值达到最大值5.01×10-3W/(K2.m);而在高凝固速率500μm/s下,其功率因子也可达4.5×10-3W/(K2.m),表明高温度梯度和大凝固速率制备热电材料是一种有效的制备工艺方法。     

Method for determining reaction rate of mild steel containers during melting of magnesium-aluminium alloys and effect of aluminium content on directionally solidified microstructures

A magnesium alloy of eutectic composition (33 wt-%Al) was directionally solidified in mild steel tubes at two growth rates, 32 and 580 μm s^?1 in a temperature gradient between 10 and 20 K mm^?1. After directional solidification, the composition of each specimen varied dramatically, from 32%Al in the region that had remained solid to 18%Al (32 fim s^?1 specimen) and 13%Al (580 (μm s^?1 specimen) at the plane that had been quenched from the eutectic temperature. As the aluminium content decreased, the microstructure contained an increasing volume fraction of primary magnesium dendrites and the eutectic morphology gradually changed from lamellar to partially divorced. The reduction in aluminium content was caused by the growth of an Al-Fe phase ahead of the Mg-Al growth front. Most of the growth of the Al-Fe phase occurred during the remelting period before directional solidification. The thickness of the Al-Fe phase increased with increased temperature and time of contact with the molten Mg-Al alloy. IJCMR/455