Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Zr-4 coarse-grained specimens were corroded in static autoclave at 360 ℃, 18.6 MPa in 0.01 mol/L LiOH aqueous solution for 70 and 160 d exposure. EBSD, SEM and HRTEM were used to investigate the microstructures and crystal structures of oxide layers, and the relationships between the oxide thickness and the grain orientation of the metal matrix. The results showed that the oxide layers formed on the grain surfaces with the orientations nearby basal plane (0001) were thicker, and exhibited a prominent anisotropic for the oxide growth when Zr-4 specimens were corroded in LiOH aqueous solution for 160 d, but this was not the case for 70 d. The grains with the surface orientation nearby (0001), (1010) and (1120) were selected from the specimens corroded for 70 d to investigate the effect of metal grain orientation on the microstructure of oxide layers. The results showed that the crystal structure and microstructure of oxide layers formed on different metal grains were obviously different, and the scattering of m-ZrO2 columnar grain orientations in the oxide layers formed on near basal plane (0001) was wider than that on near prismatic plane (1010) and (1120). Besides the majority of m-ZrO2, c-ZrO2, t-ZrO2 and sub- oxide phase Zr3O were also detected at the oxide/metal interface, and it showed that the microstructure and crystal structure of oxide layers were very complex. The microstructural evolution of oxide layers will affect the diffusion of oxygen and subsequently the growth of oxide. Therefore, the microstructural evolution of oxide layers, which was affected by the different microstructure of oxide layers formed initially on grains and the water chemistry of corrosion tests, resulted in the anisotropic growth of oxide layers when Zr- 4 specimens were corroded in LiOH aqueous solution in subsequent corrosion tests.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Alloy 825 is widely used for chemical and petrochemical applications due to its good combination of mechanical properties and corrosion resistance. However, intergranular corrosion (IGC) is one of the serious problems for alloy 825 exposed to aggressive environments, which could result in unexpected failures and lead to huge losses. The grain boundary structure, which can partly be described by coincidence site lattice (CSL) model, can influence the grain boundary chemistry and the susceptibility to intergranular corrosion. The field of grain boundary engineering (GBE) has developed a lot over the last two decades since the concept of grain boundary design was proposed. The aim of GBE is to enhance the grain-boundary-related properties of materials by increasing the frequency of low ΣCSL (Σ≤29) grain boundaries (GBs) and tailoring the grain boundary network. It was reported that in some fcc materials with low stacking fault energy, such as Ni-based alloys, lead alloys, austenitic stainless steels and copper alloys, the frequency of low ΣCSL GBs can be greatly increased by using proper thermomechanical processing (TMP), and as a result the grain boundary related properties were greatly enhanced. In this work, GBE is applied to the manufacture of Ni-based alloy 825 tubes by cold drawing using a draw-bench on a factory production line and the subsequent annealing. The effect of thermomechanical processing on the grain boundary character distribution (GBCD) of alloy 825 was studied by means of the EBSD technique and orientation image microcopy (OIM). The results show that the proportion of low ΣCSL grain boundaries increase to more than 75% by the TMP after 5% cold drawing and subsequent annealing at 1050 ℃ for 10 min, and simultaneously the large- size highly-twinned grain- cluster microstructure is formed. The size of the grain- cluster and proportion of low ΣCSL grain boundaries decrease with the increase of pre-strain. The proportion of low ΣCSL grain boundaries decreases with the increase of the mean grain size. The annealing temperatures in the range of 1050~1125 ℃ have no obvious effect on the GBCD of the specimen with 5% cold drawing deformation; while the proportions of low ΣCSL GBs of the sample with 3%, 7% and 10% cold drawing deformation decrease with the increase of annealing temperature.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Zirconium alloys with low alloying content are mainly used in the nuclear industry as structural materials because of their superior properties in terms of thermal neutron transparency, mechanical strength and corrosion resistance. They are used for fuel cladding tubes and channels. The reaction between zirconium and water at high temperature forms oxide film on the surfaces. In order to further improve the corrosion resistance of Zrbased cladding tubes, research has continued on developing new zirconium alloys. The corrosion resistance of Zr-0.72Sn-0.32Fe-0.14Cr-xNb alloys (x=0, 0.12, 0.28, 0.48, 0.97, mass fraction, %) was investigated in a superheated steam at 500 ℃ and 10.3 MPa by autoclave tests. All the plate specimens of zirconium alloys with thickness of 2.8 mm have a similar texture. The microstructure of alloys and oxide films on the corroded specimens were observed by TEM and SEM. The results showed that no nodular corrosion appeared on these alloys for 500 h exposure. The thickness of oxide layers developed on the rolling surface (SN), the surface perpendicular to the rolling direction (SR) and the surface perpendicular to the transversal direction (ST) after 500 h exposure was close to each other. There was no anisotropic corrosion resistance for these alloys. The corrosion rate of the alloys increased with the increase of Nb content after 250 h exposure when the Nb content exceeded 0.28%. In the alloy with low Nb content, the fcc-Zr(Fe, Cr)2 or fcc-Zr(Fe, Cr, Nb)2 precipitate was mainly formed, while the hcp-Zr(Fe, Cr, Nb)2 precipitate was frequently observed in the alloy with high Nb content. The corrosion resistance of Zr- 0.72Sn- 0.32Fe-0.14Cr-xNb alloys was improved by decreasing the Nb/Fe ratio. From a point of view for the improving corrosion resistance, the addition of Nb no more than 0.3% is recommended.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： The heat-affected zone (HAZ) produced by welding in stainless steel has higher susceptibility to intergranular corrosion, which is attributed to the Cr depletion induced by grain-boundary carbide-precipitation. The grain boundary engineering can be used to control over the grain boundary structure, which has significant influence on the carbide precipitation and the associated Cr depletion and hence on the susceptibility to intergranular corrosion. The grain boundary network in a 304 austenite stainless steel can be controlled by grain boundary engineering (GBE) with 5% tensile deformation and subsequent annealing at 1100 ℃ for 30 min. The total length proportion of Σ3n coincidence site lattice (CSL) boundaries was increased to more than 75%, and the large-size highlytwinned grain-cluster microstructure was formed through the treatment of GBE. Specimens were welded by gas tungsten arc- welding. Then the microstructure and the corrosion resistance of HAZ were characterized. The result showed that the high proportion of low ΣCSL boundaries and the optimum grain boundary character distribution were stable in the HAZ of the grain boundary engineered stainless steel, and the grain size was nearly the same. The weld-decay region of GBE samples performed better intergranular corrosion resistance during the intergranular corrosion immersion experiment and electrochemical potentiokinetic reactivation (EPR) test. The reported results indicated that the grain boundary engineering can effectively improve the intergranular corrosion resistance of the heat-affected zone in 304 austenitic stainless steel.
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-15 Cooperative journals: 《金属学报》
Abstract：采用工厂生产线上的冷拔机对镍基825合金管材进行冷拔加工然后再进行退火，作为晶界工程（GBE）处理。利用EBSD和取向成像显微技术（OIM）研究了不同冷拔变形量和不同退火温度对825合金晶界特征分布（GBCD）的影响。结果表明，合金在冷拔变形5%，1050℃退火10min时，低∑CSL（coincidence site lattice,∑≤29）晶界的比例可提高到75%以上（Palumbo-Aust标准），同时形成大尺寸的“互有∑3n取向关系晶粒的团簇”显微组织。随着再结晶退火前冷拔变形量的增加，晶粒团簇的尺寸减小，同时低∑CSL晶界的比例也下降，并且低∑CSL晶界的比例随晶粒尺寸的增加而下降。当合金经过5%的冷拔变形后，在1050~1125℃的温度范围内退火处理10min时的晶界特征分布无明显变化，退火温度对合金的低∑CSL晶界比例影响较小；当经过3%，7%，10%的冷拔变形后，合金的低∑CSL晶界比例随着退火温度的升高不断下降。
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-15 Cooperative journals: 《金属学报》
Abstract：选用无织构的Zr-0.72Sn-0.32Fe-0.14Cr和 Zr-0.85Sn-0.16Nb-0.37Fe-0.18Cr合金大晶粒片状样品，利用静态高压釜在500 ℃，10.3 MPa过热蒸汽中进行500 h的腐蚀试验，采用EBSD，SEM和TEM等方法研究了合金的显微组织，以及氧化膜的厚度与金属晶粒表面取向的关系。结果表明，Nb对第二相的晶体结构产生影响，Zr-0.72Sn-0.32Fe-0.14Cr合金中的第二相主要为面心立方的Zr(Fe,Cr)2，Zr-0.85Sn-0.16Nb-0.37Fe-0.18Cr合金中的第二相为面心立方和密排六方的Zr(Nb,Fe,Cr)2；两种合金均未出现疖状腐蚀，并且不同金属晶粒取向上的氧化膜厚度没有明显差别，即没有表现出腐蚀各向异性特征。
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-12 Cooperative journals: 《中国腐蚀与防护学报》
Abstract：摘 要:利用高压釜腐蚀实验研究了Zr-1Nb-xGe(x=0, 0.05, 0.1, 0.2，质量分数，%)合金在400℃/10.3MPa过热蒸汽中的耐腐蚀性能；利用SEM和TEM分别观察了合金和氧化膜的显微组织。结果表明：添加Ge可以改善Zr-1Nb合金的耐腐蚀性能，当Ge含量为0.05%时效果最佳. 在Zr-1Nb-xGe合金中存在4种第二相，分别是β-Nb，Zr(Nb,Fe,Cr)2，Zr(Nb,Fe,Cr,Ge)2第二相，和尺寸较大的Zr3Ge第二相. Zr-1Nb-xGe合金α-Zr基体中的最大固溶含量在0.05%---0.1%之间，固溶在α-Zr基体中的Ge可以有效延缓氧化膜中显微组织的演化，从而改善合金的耐腐蚀性能，当添加Ge含量超过合金的固溶含量，会形成Zr(Nb,Fe,Cr,Ge)2以及尺寸较大的Zr3Ge第二相，Zr3Ge会使耐腐蚀性能降低。
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-04 Cooperative journals: 《金属学报》
Abstract：用高压釜腐蚀实验研究了Zr-1Nb-xBi（x =0.05%---0.3%, 质量分数）系列合金在360 ℃/18.6 MPa去离子水中的耐腐蚀性能, 结果表明在Zr-1Nb合金的基础上添加Bi对其耐腐蚀性能有明显的改善作用, 且随着Bi含量的增加改善作用愈明显. 合金显微组织的TEM和EDS分析表明, 合金中存在ZrNbFe型和β-Nb第二相, Bi含量对第二相的种类, 尺寸和数量没有明显的影响; 0.3%的Bi可全部固溶在α-Zr基体中, 且不影响Nb的固溶含量. 氧化膜断口和内表面形貌的SEM观察表明, 固溶在?-Zr基体中的Bi能够明显延缓氧化膜显微组织结构的演化, 包括孔隙发展成为微裂纹的过程和柱状晶向等轴晶的转变. 可见, 增加α-Zr基体中Bi的固溶含量可以改善合金的耐腐蚀性能.
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-04 Cooperative journals: 《金属学报》
Abstract：通过冷轧5%变形及1100℃退火30min的晶界工程（GBE）处理工艺可将304奥氏体不锈钢低ΣCSL晶界比例提高到75%（Palumbo-Aust标准）以上，并形成了大尺寸的“互有Σ3n取向关系晶粒的团簇”显微组织 。采用钨极气体保护焊焊接样品，对焊接后样品的焊接热影响区进行显微组织表征和耐腐蚀性能测试。结果表明GBE处理过的304奥氏体不锈钢具有较好的晶界网络稳定性，焊接热影响区仍具有高比例重位点阵晶界，并且晶粒尺寸并未明显变大。在晶间腐蚀实验和EPR测试中，GBE样品HAZ敏化区都表现出了更好的耐腐蚀性能，因此表明晶界工程可以有效改善304奥氏体不锈钢焊接热影响区耐晶间腐蚀性能。
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