Brazing of UNS S32101 and UNS S32304 lean duplex stainless steels and UNS S32750 super duplex with AWS A5.3 BNi-7 (Ni–Cr–P) filler metal

Summary

Weld metals solidified in the ferritic-austenitic solidification mode (FA mode) have dual phases of ferrite and austenite in their as-solidified condition, where ferrite exhibits different morphologies depending on the chemical composition and welding conditions. This paper describes an investigation of the effect of the solidification and transformation sequence on the formation of final ferrite morphologies. Austenite is formed through either a eutectic reaction or peritectic reaction at the dendrite boundaries after the primary formation of ferrite. During the eutectic formation of austenite, the <100>_δ direction of the primary ferrite and the <100>_γ direction of the eutectic austenite are parallel to each other and lie along the growth direction of the primary dendrites. However, any specific lattice plane relationship between the two phases is not identified. During cooling after solidification, the austenite extends into the primary ferrite via solid-state transformation, and the final morphology of the ferrite is vermicular without any coherent orientation relationship between the primary ferrite and eutectic austenite. During peritectic formation of austenite, the Kurdjumov-Sachs orientation relationship is established between the primary ferrite and peritectic austenite, and the <100>_γ direction of the peritectic austenite is not parallel to the growth direction of the primary dendrites. During cooling after solidification, the primary ferrite transforms into austenite, and the final morphology of the ferrite is lathy, since the primary ferrite and peritectic austenite have a favourable coherent orientation.     

X70微合金管线钢组织中针状铁素体细化机制的研究

以微合金管线钢XTO为研究对象，在Gleeble 1500热模拟机上，进行了不同形变量和冷却速度对合金相变行为及组织细化影响的研究。实验结果表明，形变量和冷却速度的增加，有助于针状铁素体的形成及细化。管线钢中针状铁素体组织典型的形貌为非常微细的亚结构、高位错密度以及部分细板条铁素体，基体上弥散分布着M／A岛和渗碳体。同时可以观察到在铁素体晶粒边界和铁素体晶粒内部有相的析出。可见，针状铁素体组织细化的机制主要有：形变诱导铁素体、铁素体的动态再结晶和相的析出抑制晶粒长大。     

MORPHOLOGY AND PHASE TRANSFORMATION OF GRANULAR BAINITE AND GRANULAR STRUCTURE

本文研究了低碳合金钢奥氏体在连续冷却或等温转变过程中形成的组织形态和转变机理。结果表明在中温贝氏体区得到“粒状贝氏体”组织,在先共析转变区得到“粒状组织”。两者都有“铁素体基体+小岛”形貌。但前者有表面浮凸现象,铁素体呈长条状,与母相维持K-S关系,惯习面为{111}_γ,小岛也多星长条状平行排列。后者无表面浮凸,铁素体呈无规则形状,与母相无严格位向关系,小岛亦呈无规则排列。然而这两种组织都是由扩散型相变机制形成的。最后,提出了形成这两种组织的相变模型。     

Effects of applied stress on coherent precipitates via a discrete atom method

Morphological evolution of coherent precipitates under an applied stress is analyzed by means of a discrete atom method, which is predicated upon Hookean atomic interactions and Monte Carlo diffusion, and mates no assumption of a specific shape. Precipitates having elastic constants different from those of the matrix phase art treated in dislocation-free, anisotropic elastic systems under a plane strain condition with a purely dilatational misfit. Under an applied tensile stress, soft particles with a positive misfit strain tend to become plates perpendicular to the applied stress axis, while hard particles elongate along the stress direction. If the elastic interaction between the applied stress and the coherency strain is strong enough, precipitates often split into smaller particles and then follow coarsening. Even in the absence of a coherency strain, particles are shown to undergo morphological evolution through Eshelby’s inhomogeneity effects. A particle shape depends on the following variables: the sign and magnitude of the coherency strain, the sense and magnitude of the applied stress, its stiffness relative to the matrix phase, and the magnitude of the interfacial energy.     

RESEARCH ON TRANSFORMATION OF GRANULAR STRUCTURE IN LOW CARBON Mn–Si STEEL

对低碳Mn--Si钢中的粒状组织进行了实验研究, 考察了粒状组织中小岛的形状及其分布. 结果表明, 不含Cr时, 空冷后形成了由先共析铁素体基体+岛状物所组成的粒状组织,其中的岛状物形态及分布可以是无规则的, 也可以是长条状的小岛沿一定方向分布于基体上. 对于这2种形态的粒状组织, 尤其是对后者形态的形成和铁素体基体的分析表明, 小岛的形状和分布是先共析铁素体形态的反映, 条形规则排列的小岛可能受先共析铁素体基体台阶长大机制控制, 粒状组织小岛在一定条件下也可能具有方向性. 加适量Cr后, 连续冷却后的组织中不出现粒状组织.     

PRECIPITATION IN A Cu-BASED SMA

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Ductility Anisotropy Induced by Ferrite in Direct Laser Deposited 17-4 PH Steel: Combined Microstructure and Dislocation Density Simulation

The anisotropic ductility of a direct laser deposited 17-4 PH cubic part was investigated. Anisotropic elongations in the specimens from varied surfaces of the part were obtained: ～ 6.2%, ～ 1.5%, and ～ 4.5% in XY, YZ, and XZ samples, respectively. Furthermore, various orientations of ferrite were found in different specimens, taking the loading direction as reference. A finite element analysis depending on actual microstructures and dislocation density revealed that the orientation of ferrite caused the ductility anisotropy. The orientation of ferrite affected its plastic deformability and the deformation compatibility between phases during the uniaxial loading. The ferrite parallel to the tensile direction in the YZ sample had the worst deformability and induced severe strain localization and stress triaxiality, which resulted in inferior ductility. The ferrite perpendicular to the tensile direction showed the best deformability, whereas strain localization remained intense in the XZ sample owing to the unmatched deformability of martensite. The inclined ferrite in the XY sample exhibited moderate deformability and was found to enhance the plastic fl ow of martensite, leading to the best deformation compatibility and ductility.     

Formation of aligned two-phase microstructures by applying a magnetic field during the austenite to ferrite transformation in steels

Application of a magnetic field during the ferrite to austenite transformation in Fe–C alloys was found to yield a two-phase microstructure with the paramagnetic austenite grains aligned as chains or columns along the direction of the field in the matrix of ferromagnetic ferrite phase. The underlying mechanism of dipolar interactions suggests that similar alignment of microstructures should take place during the austenite to ferrite transformation under a magnetic field. In the present investigation, an experimental setup has been designed to study the magnetic alignment. Its concept is characterized by deforming steels prior to the austenite to ferrite transformation to introduce ample nucleation sites in addition to applying magnetic fields up to 12 T. Experiments have revealed successful conditions for aligned two-phase microstructures in carbon steels. The formation mechanism of the aligned structures is discussed from the viewpoint of the nucleation and growth of ferrite grains in austenite phase under a magnetic field. Furthermore, it is shown that the shape of the aligned ferrite grains is determined by a balance of the magnetostatic and the interfacial energies.     

Precipitation of Icosahedral Quasicrystalline Phase, R-phase and Laves Phase in Ferritic Alloys

Ferritic heat resistant steels involving precipitation of intermetallic phases have drawn a growing interest for the enhancement of creep strength, while the brittleness of the intermetallic phases may lower the toughness of the alloy.Therefore, it is necessary to optimize the dispersion characteristics of the intermetallics phase through microstructural control to minimize the trade-off between the strength and toughness. The effects of α-Fe matrix substructures on the precipitation sequence, morphology, dispersion characteristics, and the stability of the intermetallic phases are investigated in Fe-Cr-W-Co-Si system. The precipitates of the Si-free Fe-10Cr-I.4W-4.5Co (at%) alloy aged at 873K are the R-phase but those of the Si-added Fe-10Cr-1.4W-4.5Co-0.3Si (at%) alloy are the icosahedral quasicrystalline phase. The precipitates in both the Si-free and Si-added alloys aged at 973K are the Laves phase. Matrix of the alloys is controlled by heat treatments as to provide three types of matrix substructures; ferrite, ferrite/martensite mixture and martensite. The hardening behavior of the alloys depends on the matrix substructures and is independent of the kinds of precipitates. In the alloys with ferrite matrix, the peak of hardness during aging at 873K shifts to longer aging time in comparison with that in the alloys with lath martensite matrix which contain numbers of nucleation sites.     

含Mg铝合金牺牲阳极的电流效率及其影响因素

在铝合金牺牲阳极中添加Mg、Ti、Nb等合金元素,研究它们的含量与存在形式对牺牲阳极性能的影响,分析第二相、晶粒细化等的作用和影响。结果表明：在所讨论的含Mg铝合金牺牲阳极中,会形成一定类型的第二相,进而影响牺牲阳极的电化学性能;理想的Mg含量为2%;加入微量Ti、Nb可以有效细化晶粒,对提高牺牲阳极的电流效率有很好的促进作用,而且单独加入Ti要比同时加Ti、Nb的效果好。     

低碳Mn-Si钢中粒状组织转变研究

对低碳Mn-Si钢中的粒状组织进行了实验研究,考察了粒状组织中小岛的形状及其分布.结果表明,不含Cr时,空冷后形成了由先共析铁素体基体+岛状物所组成的粒状组织,其中的岛状物形态及分布可以是无规则的,也可以是长条状的小岛沿一定方向分布于基体上.对于这2种形态的粒状组织,尤其是对后者形态的形成和铁素体基体的分析表明,小岛的形状和分布是先共析铁素体形态的反映,条形规则排列的小岛可能受先共析铁素体基体台阶长大机制控制,粒状组织小岛在一定条件下也可能具有方向性.加适量Cr后,连续冷却后的组织中不出现粒状组织.     

Magnetic micro- and nanostructures of unalloyed steels: Domain wall interactions with cementite precipitates observed by MFM

The morphology and content of a cementite phase controls the macroscopic, mechanical and magnetic properties of steels. The influence of the cementite content on the bulk magnetic properties in unalloyed steels was observed in hysteresis loop and Barkhausen noise signals. Globular cementite embedded in a ferrite matrix is characterized by atomic force microscopy and magnetic force microscopy. Size, shape and orientation of the grains influence the domain configuration. When an external magnetic field is applied, the magnetization process occurs mainly in the ferrite matrix. The Bloch walls in the ferrite matrix move, and they are pinned by the cementite precipitates. This microscopic observation correlates with the macroscopic magnetic properties of the investigated material.     

Oxide Layers on (0001) Plane of Zircaloy-4 After Corrosion at 360 °C in Lithiated Aqueous Solution

The microstructures of the oxide layers formed on near (0001) plane of Zircaloy-4 were investigated by autoclave tests at 360 °C in lithiated aqueous solution. Oxygen-rich regions with hcp structure were observed at the undulating O/M interface, and the inner surface morphology of the oxide layers formed on (0001) was only concave-convex. Monoclinic, tetragonal and cubic phases and a kind of zirconium sub-oxide with bcc structure were detected in the oxide layer near the metal matrix. This zirconium sub-oxide layer had a coherent relationship with α-Zr matrix, and the growth direction of the zirconium sub-oxide layer was nearly parallel to the [0001] direction of α-Zr regardless of the orientation of metal matrix. The orientations scattering of columnar grains formed on near (0001) plane differ from that formed on near () plane.     

低氮低钒D36船板钢中析出相的特征及沉淀强化作用

通过透射电镜观察了低氮低钒D36船板钢的析出相特征,并讨论了其沉淀强化作用。结果表明:实验用钢的组织均为铁素体-珠光体组织,珠光体中未见析出相,仅在铁素体中观察到析出相。VCx析出相为矩形片状形貌,在铁素体基体上均匀弥散沉淀,析出相以{100}α-Fe为惯习面,与基体存在Baker-Nutting取向关系;少量球状V6C5析出相(单斜结构)与铁素体基体存在一定取向关系,可能是共格VCx相通过碳原子重新分布转变而成。低氮低钒钢中加入0.05%V,沉淀强化值约为63 MPa,加入微量铌后在铁素体内形成一定密度的位错能有效发挥钒的沉淀强化作用。     

强磁场对硅锰铸钢等温珠光体粒化的影响

对硅锰铸钢在相变温度(A1)附近进行等温处理的研究表明,经磁场处理后的试样组织中存在更多的粒状珠光体组织。此粒状珠光体由两部分构成:一部分来自块状铁素体中直接析出粒状的不连续渗碳体;一部分来自等温过程中片状渗碳体的熔断。磁场通过促进铁素体的形核与长大,使较多的碳原子"陷落"在铁素体内以沉淀析出方式形成粒状渗碳体,处理温度越低,此作用越明显。此外,磁场下珠光体组成相磁致伸缩率存在差异,引起珠光体两组成相之间的应变能变化,有利于渗碳体以球状析出于奥氏体或铁素体中,也对粒状珠光体分数的增加作出贡献。     

钒、钛元素对Fe-C合金等温转变动力学和形成组织的影响

研究了含钒、钛元素的Ｆｅ－Ｃ合金等温转变动力学和组织转变。等温相变时,铁素体以激发形核方式成核,以扩散型的台阶侧向生长而长大,岛状组织（Ｍ＋Ａ）是中间相变产物,最终产物为铁素体与碳化物。分析了钒、钛元素的碳（氮）化物析出的相形貌、结构和化学组成的关系,提出了碳（氮）化物析出相的在奥氏呈立方形态析出,在铁素体中呈片状形态析出,延长等温转变时间、上两种碳（氮）化物发生球化。讨论了硅、锰元素对Ｆｅ－Ｃ合     

Icosahedral quasicrystalline phase in an as-cast Mg-Zn-Er alloy

The microstructure of an as-cast Mg-Zn-Er alloy was investigated through scanning electron microscopy (SEM) and transmission electron microscopy (TEM) equipped with energy dispersive spectroscopy (EDS). The results indicate that two different second phases, one with eutectoid-lamellar morphology and the other with granular shape, distribute in the α-Mg matrix. The coexistence of the face-centered icosahedral quasicrystalline phase (I-phase) and W-phase with the face-centered cubic structure is found in the as-cast alloy. The coexistence of I-phase and W-phasc in the Mg-Zn-Er alloy is because the W-phase is the primary phase and the I-phase forms by peritectic reaction during solidification.     

Effect of Second Phase on the Deformation and Fracture Behavior of Multiphase Low-Density Steels

The effects of the second-phase characteristics on the deformation and fracture behavior in low-density steel are investigated in Fe-0.2C-(4, 8)Mn-5Al-1Si alloys, where the second phase can be retained austenite or a mixture of κ-carbide and ferrite depending on the processing condition. With austenite as second phase, the mechanical stability of the austenite is critical regarding ductility because the fracture is likely to occur before the onset of necking when the austenite transforms into martensite in the early stage of deformation. In the case of the mixture of ferrite and κ-carbide, the round shape of κ-carbide is more favorable than the dense lamellar structure. The prominent influence of the second phase on the deformation and fracture behavior is attributable to the propensity of matrix ferrite for cleavage fracture due to its large aluminum concentration.     

定向凝固含Y镁合金的枝晶形态及微观组织研究

采用液态金属冷却高温度梯度定向凝固炉制备了含稀土元素Y的镁合金,研究了定向凝固镁合金的枝晶形态及微观组织特征。结果表明,镁合金的一次枝晶沿着热流方向生长,且一次枝晶周围含有六个二次枝晶呈花瓣形状。通过扫描电镜及能谱分析发现,凝固组织中除了a(Mg)固溶体及枝晶间的?(Mg17Al12)相组成外,还存在稀土相。稀土相呈粗大的杆状和细小的粒状形态,主要由Y、Mg、Al和Mn元素组成,其中稀土Y的质量分数占22%左右。     

Selective variant growth of coherent precipitate under external constraints

An anisotropic distribution of coherent precipitate variants may result in anisotropic behavior of a two-phase material. The distribution of the coherent precipitate variants can be controlled using constrained aging. This article reports our experimental and computational studies of the stress effect on the spatial arrangement of coherent precipitate variants. The research demonstrates that the anisotropic elastic coupling between applied stress/strain and the local strain caused by the lattice mismatch between different phases makes the growth of differently oriented phase variants selective. Ti₁₁Ni₁₄ precipitation in a Ti-51.5at.%Ni alloy was investigated as a particular example. It was demonstrated that the constrained aging strongly affected the distribution of Ti₁₁Ni₁₄ precipitate variants. The resulting selective variant growth of Ti₁₁Ni₁₄ precipitates can be predicted based on the symmetry analysis and the elastic energy calculation.