双轴比例加载条件下含氢化物Zr-4合金变形行为的数值分析

采用有限元数值分析方法,模拟了含氢化物Zr-4合金拉-扭双轴比例加载下的变形行为.相同外加等效应变下,加载路径对含氢化物Zr-4合金的应力、应变分量的大小和分布有明显的影响,而对其等效应力-应变曲线影响较小.不同加载路径下, Zr-4合金基体的最大静水拉应力区位于氢化物与基体的界面附近.随着拉伸-扭转应变比的增加,基体中最大静水拉应力增加,而最大静水拉应力位置与外加剪切方向的夹角由45°逐渐转向90°.     

渗氢处理对冷变形Zr-4合金双轴疲劳行为的影响

研究了不同程度渗氢处理Zr-4合金的双轴循环变形行为。结果表明：当氢含量为CH=400μg/g时，在相同等效应变幅和相同相位角下，渗氢Zr-4合金的疲劳寿命高于原始冷变形状态Zr-4合金。当氢含量增加到CH=580μg/g时，渗氢处理Zr-4合金的双轴疲劳寿命却低于原始冷变形状态的Zr-4合金。透射电镜和金相显微镜观察表明，冷变形Zr-4合金渗氢后析出的氢化物具有良好的塑性变形能力，渗氢过程中中温条件下长时间保持导致基体产生去应力退火效应是Zr-4合金疲劳寿命提高的主要原因。     

Pd添加对Zr-4合金在LiOH水溶液中耐腐蚀性能的影响

采用静态高压釜腐蚀实验研究添加微量Pd元素的Zr-4合金在(360℃,18.6 MPa)条件下0.01 mol/L Li OH水溶液中的耐腐蚀性能,采用TEM和SEM研究合金和氧化膜的显微组织。结果表明:添加0.05%~0.2%Pd(质量分数)均可明显改善Zr-4合金在Li OH水溶液中的耐腐蚀性能,其中添加0.1%Pd时改善作用最显著,随着添加Pd含量的进一步增加,对Zr-4合金耐腐蚀性能的改善作用减弱。在Zr-4合金中Pd元素以Zr2(Fe,Cr,Pd)、Zr2(Pd,Fe)形式的第二相析出,其中Zr2(Pd,Fe)第二相尺寸较大。固溶在α-Zr中的Pd和尺寸较小的Zr2(Fe,Cr,Pd)型第二相可以提高Zr-4合金的耐腐蚀性能;析出尺寸较大的Zr2(Pd,Fe)第二相后会对Zr-4合金的耐腐蚀性能产生不利的影响。     

添加Bi对Zr-4合金在400℃/10.3 MPa过热蒸汽中耐腐蚀性能的影响

在Zr-4合金基础上添加0.1%—0.5%Bi（质量分数）制备成Zr-4+xBi合金,用高压釜腐蚀实验研究了Bi含量对Zr-4+xBi合金在400℃/10.3 MPa过热蒸汽中耐腐蚀性能的影响;用TEM,EDS和SEM观察了合金和合金腐蚀后氧化膜的显微组织.结果表明:随着Bi含量的增加,Zr- 4+xBi合金中第二相的尺寸和形状变化不大,但数量增多,并出现了不同成分的第二相,包括Zr（Fe,Cr）₂,Zr-Fe- Cr-Bi.Zr-Fe- Sn -Bi和Zr-Fe- Cr-Sn-Bi.在Zr -4+0.1Bi合金中检测到了含Bi的第二相,这说明580℃时Bi在Zr- 4+xBi合金α- Zr基体中的固溶度小于0.1%.另外,适量Bi的添加促进了原先固溶在α-Zr基体中Sn的析出.与Zr-4合金相比,在Zr-4中添加0.1%—0.5%Bi后合金的耐腐蚀性能反而下降,并随着Bi含量的增加耐腐蚀性能恶化趋势越显著,这说明Zr-4合金中添加Bi并不能改善合金的耐腐蚀性能,反而产生有害的影响.这应该与含Bi第二相和同时含有Bi,Sn第二相的析出有关.     

Zr-4+xCu+xGe合金在LiOH水溶液中耐腐蚀性能的研究

对添加少量合金元素Cu和Ge的Zr-4+xCu+xGe(x=0、0.05、0.1、0.2,质量分数,%)合金在360℃/18.6 MPa/0.01 M LiOH水溶液中进行静态高压釜腐蚀试验。利用TEM和SEM研究了合金和氧化膜的显微组织。结果表明:添加适量Cu和Ge可以延缓氧化膜中微裂纹的形成,显著提高Zr-4合金在360℃/18.6 MPa/0.01 M LiOH水溶液中的耐腐蚀性能;在Zr-4+xCu+xGe合金中主要析出密排六方结构的Zr(Fe,Cr)_2和Zr(Fe,Cr,Cu,Ge)_2型第二相,随着Cu和Ge添加量的进一步提高,还会有粗大的四方结构的Zr_2Cu和Zr_3Ge第二相析出,第二相的氧化易导致应力集中并促进微裂纹形成,不利于Zr-4合金耐腐蚀性能的改善。     

Zr-1Nb-xCu合金在400℃过热蒸汽中的耐腐蚀性能

采用静态高压釜腐蚀试验研究了在Zr-1Nb锆合金基础上添加Cu的Zr-1Nb-xCu(x=0~0.5,质量分数,%)合金在400℃/10.3 MPa过热蒸汽中的耐腐蚀性能,用TEM和SEM分别观察了合金的显微组织和氧化膜的断口形貌。结果表明,当Zr-1Nb合金中添加的Cu含量不超过0.2%时,大部分Cu都固溶在α-Zr中,合金中析出的第二相主要为尺寸细小的β-Nb,这时合金在400℃/10.3 MPa过热蒸汽中的耐腐蚀性能会随着合金中Cu含量的增加而得到明显的提高;当Zr-1Nb合金中添加的Cu含量超过0.2%时,合金中析出了Zr2Cu型第二相,且析出的Zr2Cu型第二相会随着Cu含量的增加而数量增多,尺寸增大。在Zr-1Nb-0.35Cu合金中,析出了适量的Zr2Cu型第二相,这对改善合金在400℃/10.3MPa过热蒸汽中的耐腐蚀性是有利的;但是在Zr-1Nb-0.5Cu合金中,由于析出了数量较多且尺寸较大的Zr2Cu型第二相,在400℃过热蒸汽中腐蚀时将诱发疖状腐蚀,对合金的耐腐蚀性能是有害的。     

径向压扁变形对Zr-4合金管材氢化物取向的影响

通过对规格为Φ10 mm×0.7 mm的Zr-4合金管材施加径向单次及多次压扁变形试验,研究压扁量和压扁次数对氢化物取向的影响。研究表明,压扁量大小对氢化物取向有明显影响,当压扁量≤0.3 mm时,氢化物取向基本不发生改变;当压扁量>0.3 mm时,随着压扁量的增大氢化物取向因子明显增大,这主要是由于该区域内形成的拉应力导致氢化物产生了应力再取向析出效应。压扁次数对氢化物取向的影响较小,随着压扁次数的增加,在大压扁量下氢化物取向因子略有增加,而压扁量较小时(0.3 mm)不发生改变。     

Zr-4合金板材微观组织异常条线现象的研究

采用金相显微镜、扫描电镜等检测手段对Zr-4合金板材微观组织中出现的异常条线现象进行研究,分析了Zr-4合金板材条线现象的本质、成因以及对其使用性能的影响。研究表明,Zr-4合金板材微观组织中出现的异常条线现象并不是裂纹、夹杂、氢化物、织构等缺陷的反映,而是由沿轧向成带状分布的粗大Zr（Fe,Cr）2第二相更容易遭受腐蚀造成的。此外,Zr-4合金板材微观组织中出现的条线现象对均匀腐蚀性能无明显影响,但带状分布的粗大第二相不利于板材的抗疖状腐蚀,并且会对板材的弯曲性能产生影响。     

Ge含量对Zr-4合金在LiOH水溶液中耐腐蚀性能的影响

采用堆外高压釜腐蚀试验研究添加0.1%Ge和0.5%Ge(质量分数)对Zr-4合金在360℃、0.01 mol/L LiOH水溶液中耐腐蚀性能的影响,用TEM和EDS研究合金的显微组织和第二相成分。结果表明:与Zr-4合金相比,添加Ge后合金的耐腐蚀性能显著提高,但随着Ge添加量的增加,对耐腐蚀性能的提高作用减弱。当Ge含量为0.1%时,析出密排六方结构的Zr(Fe,Cr)2和Zr(Fe,Cr,Ge)2型第二相;当Ge含量达到0.5%时,还会析出Zr-Sn-Fe-Cr-Ge和尺寸较大的四方结构Zr3Ge型第二相,添加Ge会使α-Zr基体中固溶的Sn含量降低。固溶在α-Zr中的Ge和尺寸较小的Zr(Fe,Cr)2、Zr(Fe,Cr,Ge)2型第二相可以提高Zr-4+xGe合金的耐腐蚀性能;析出Zr-Sn-Fe-Cr-Ge和尺寸较大的Zr3Ge第二相后会对耐腐蚀性能产生不利的影响。     

合金成分对锆合金焊接区腐蚀时吸氢性能的影响

采用Fe含量相同，但Cr含量不同的锆合金板与Zr-4板对接焊，腐蚀后不同样品中的氢含量差别很在，与氧化膜的厚度不成比例，说明合金元素Cr对锆合金的吸氢性能有着非常明显的影响，当不含Cr时，焊接样品腐蚀后的氢含量明显减少：Zr．4经不同热处理后Zr(Fe，Cr)2第二相的大小和多少与氢含量有关，说明合金中存在Cr时形成的Zr(Fe，Cr)2第二相是引起吸氢量增加的主要原因。在不含Cr的锆合金中添加合金元素Nb，不仅改善了焊接区的耐腐蚀性能，而且不会引起腐蚀时吸氢增加。氯化锆会在对接焊样品中未焊透的前端析出，这是张应力引起氢的扩散富集并诱发氢化物析出的结果。氢化锆还会沿着熔区边界集中析出，这将影响锆合金力学性能的均匀性。     

渗氢后再结晶状态Zr-4合金拉伸变形的原位观察

利用扫描电镜原位观察和相应的多相有限元数值分析方法，研究了渗氢后再结晶状态Zr-4合金拉伸过程中氢化物的变形行为以及其对基体变形行为的影响。结果表明，氢化物自身具有一定的塑性变形能力，在具有良好塑性的基体约束下，可以与基体一起发生较大的变形，但是氢化物的变形能力和其在Zr-4合金中的分布和形貌密切相关：稀疏分布的氢化物，易发生变形，不易出现裂纹；单个片状的氢化物可以被基体中的滑移线穿过；而在大块氢化物团聚体内部易出现裂纹。氢化物的形貌不仅影响基体的变形行为；而且对基体的断裂也有显著的影响。渗氢后Zr-4合金的断裂属于韧性断裂，断裂时材料内部出现多裂纹源，无主裂纹出现。     

Hydride reorientation in Zr2.5Nb studied by synchrotron X-ray diffraction

The crystallographic texture and stress state of hydride platelets of two different orientations precipitated in Zr2.5Nb pressure tubes were studied by synchrotron X-ray diffraction experiments using an 80 keV photon beam and an area detector in transmission geometry. Circumferential and radial hydrides with platelet normals along the tube radial and circumferential directions, respectively, were precipitated in tube material loaded up to H contents of 130 wt.ppm. This macroscopic hydride orientation was controlled by application of a load along the tube hoop direction during hydride precipitation. The experiments show that both circumferential and radial platelets are composed of δ-hydrides precipitated in α-Zr grains from a wide range of orientations, but with a clear preference for Zr crystals with their c-axes at an angle of ～15–20° from the hoop direction. Some moderate differences between the crystallographic texture of the two hydrides result from application of the load during precipitation. The results are explained in terms of an autocatalytic nucleation process and the Zr2.5Nb microstructure. A careful stress analysis revealed that both hydride types are compressed by the matrix on the plane of the platelet, with the largest stresses always found along the axial direction of the tube. Determination of the stress state of the hydride could be exploited as a diffraction signature of the hydride orientation.     

IN-SITU ELECTRON MICROSCOPY STUDY ON PRECIPITATION OF ZIRCONIUM HYDRIDES INDUCED BY STRESS AND STRAIN IN ZIRCALOY-2

The precipitation process of zirconium hydrides induced by stress and strain was investigatedby means of electron microscopy in-situ.The precipitating hydrides induced by stress werefound to be γ phase with orientation relationship of (110)_γ‖(110)_(αZr),(001)_γ‖(0001)_(αZr) between γ-hydrides and surrounding matrix.The growth rate of γ-hydrides whichwas much faster along [110] direction brought them in taper shape.After fracture ofy-hydrides,a new one will precipitate at the tip of cracks.This is the essential process ofhydrogen-induced delayed cracking in Zircaloy.The precipitating hydrides induced by strainwere found to be δ phase with both orientation relationships of(111)_δ‖(0001)_(αZr),(110)_δ‖(110)_(αZr) or (010)_δ‖(0001)_(αZr),(001)_δ‖(110)_(αZr)between δ-hydride and surroundingmatrix.The δ-hydrides become much finer as the strain rate increased.     

Elastic fields generated by a semi-spherical hydride particle on a free surface of a metal and their effect on its growth

The formation of a metal hydride is associated with a large increase of volume relative to the parent metal and therefore in large strain energies. Effects of elastic energy on the hydriding of metals are revealed in the microstructural evolution and kinetics of hydride growth on free surfaces. In the present work, we study in detail the elastic fields set up by a semi-spherical hydride particle growing at a free surface of metal with cubic symmetry, with and without an oxide layer. These systems combine geometric (structural) and material anisotropies.Three stages along the microstructural evolution on the surface of some hydride forming metals exposed to hydrogen at constant pressure were described experimentally. For these stages along hydride growth, correlations with the elastic fields are suggested as follows. (a) A hydride particle at the free surface generates regions of tensile and compressive hydrostatic stress in the surrounding matrix. This may induce a preferred nucleation of new hydrides and formation of clusters of hydrides precipitates, which is indeed observed experimentally. (b) Clustering, on the other hand, may contribute to the cease of growth due to competition on hydrogen. In addition, as the particle grows, changes in the stress fields may retard further diffusion from the surface and be another contribution to the cease of growth. (c) A growing hydride increases the stress in the oxide layer and may finely break it. Then the elastic energy per unit volume drops to its minimum value and the growth may accelerate. The formation of such “growth center” is favored for that hydride precipitate that grow alone and not in a cluster.     

A theoretical and experimental study of hydrides in zirconium alloys

A method developed for computing the critical length and thickness of hydride plates formed in delayed hydride cracking (DHC) in zirconium alloys is considered. The model is based on analyzing the distribution of tensile stresses in the plane of a sharp normal tensile crack. The characteristics of hydrides formed due to DHC in reactor tubes produced from alloy Zr-2.5% Nb are determined experimentally. The results of the computation agree well with experimental data. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 5–9, April, 2006.     

Hydride precipitation and stresses in zircaloy-4 observed by synchrotron X-ray diffraction

The grain stresses within hydrides precipitated in rolled zircaloy-4 plates were determined by synchrotron X-ray diffraction experiments using an 80 keV photon beam and a high-speed area detector placed in transmission geometry. Results showed large compressive stresses (360 ± 20 MPa) in the hydrides along the plate rolling direction. The origin of these stresses was investigated by performing hydride dissolution/precipitation in situ for thermal cycles between room temperature and 400 °C. A large stress hysteresis was observed, with a steady decrease on heating and an abrupt change on cooling. The observed stresses are explained by the constraint imposed by grain boundaries on the growth of hydride platelets on the rolling–transverse plane of the rolled plates.     

AZ31镁合金摩擦搅拌焊在慢拉伸应力腐蚀过程中的组织演化和氢致延迟开裂

研究AZ31镁合金摩擦搅拌焊组织的演化,包括织构和断口变化。利用中子衍射仪测量织构。利用光学显微镜和扫描电子显微镜观察应力腐蚀开裂（SCC）样品的金相及断口形貌。利用X射线衍射仪研究SCC样品的断口表面结构。结果表明,在母材表面形成了明显的基面织构。然而,搅拌区晶粒发生了基面旋转,大多数晶粒的基面沿着焊接方向倾斜25°。在慢拉伸应力作用下,在空气和侵蚀性溶液中分别形成了羽毛状孪晶和氢化物。在溶液中,穿晶裂纹扩展,最终断裂在回转侧。在断口表面存在的氢化物表明镁合金应力腐蚀可能存在氢致延迟开裂机制。     

电子束辐照诱导Al2Ni3相的析出

用４００ｋＶ透射电镜观察由Ｎ９Ｒ及２Ｈ马氏体组成的淬火Ｃｕ－１１．２Ａｌ－２．９Ｎｉ合金样品时，发现大量弥散相因电子束辐照而析出。电子衍射分析证实析出相为Ａｌ２Ｎｉ３。暗场像体视测量与高分辨显微术研究表明，辐照诱导析出不仅是表面反应。Ａｌ２Ｎｉ３粒子择优在晶体缺陷及马氏体界面处析出。延长辐照时间，析出相长大并粗化。     

残余应力对Super304H焊缝金属再热裂纹敏感性的影响

采用预压缩CT试样的方法研究了残余应力对Super304H奥氏体不锈钢焊缝金属再热裂纹敏感性的影响。通过ABAQUS模拟结合EBSD表征分析了不同残余应力下的焊缝金属的塑性应变分布和位错密度分布,并从残余应力诱导焊缝金属晶内析出相的方面解释了残余应力对焊缝金属再热裂纹敏感性的影响。结果表明：高预压缩载荷的CT试样具有更高的残余应力,更容易产生再热裂纹。随着残余应力的增加,焊缝金属拉伸试样的抗拉强度逐渐降低。焊缝金属CT试样靠近U型口的高残余应力区域具有更高的位错密度和更高比例的亚晶,促进了晶内小颗粒相的析出,使得晶内硬化更为严重,再热裂纹更容易产生。     

Tensile behaviour of different pretreated alloy 718 sheets welded with electron beam welding (EBW)

Abstract

The present paper investigates the tensile behaviour of alloy 718 sheets processed after electron beam welding (EBW). The alloy 718 sheets were pretreated in three conditions: as received, solution and precipitation pretreatments. Metallographic and fractographic examination on the specimens were conducted with optical microscope, scanning and transmission electron microscopes. Experimental results show that cracks propagate through all of the fusion zones during tensile test. The intermetallic laves phase, which precipitates along the fine columnar grain boundary in the fusion zone, offers a preferred crack propagation path, especially for the precipitation pretreated weld. The static ductility of precipitation-treated weld decreases significantly after EBW. This was attributed to high precipitation density of laves phase along the columnar grain boundary. Relatively high tensile strength and ductility of as-received and solution-pretreated welds can be obtained by using two sequential welding passes. The first pass is full penetration welding with high heat input, and the secondary pass uses cosmetic welding to reduce the surface undercut resulted from the first pass.