锆合金的织构及其对性能的影响

锆合金被广泛应用于核反应堆中，作为燃料元件的包壳材料或堆内结构部件，织构会影响其众多的应用性能，因此织构的研究及控制在锆合金的开发利用中具有重要作用。综述了，锆合金塑性变形的滑移和孪生系统；锆合金管板材的织构特点及控制方法；热轧温度对锆合金板材织构的影响以及退火处理锆合金织构的演化；并分别总结了织构与锆合金屈服强度、蠕变强度、碘致应力腐蚀开裂、氢化物取向分布以及辐照生长等性能的相互关系。     

N36锆合金管材的织构研究

利用X射线衍射测试技术,对N36锆合金成品管材的织构进行了测量。利用极图、织构取向因子,特别是采用三维晶体学取向分布函数等表征手段,系统分析了N36锆合金成品管材的织构特征。研究结果表明,N36锆合金成品管材的主要织构组分为基面型(0002)1120;管材径向上织构取向因子最大,大多数晶粒的[0002]基极集中在与管材的轴向垂直的ND-TD面上,并且偏离径向30°左右呈现典型双峰分布;除基面类型织构组分外,N36锆合金管材还存在一些相对强度比(0002)基面类型织构更高的锥面类型织构,而柱面类型织构较弱。     

锆合金变形机理及其板材织构演化规律

锆是核工业的重要结构材料,又是优秀的化工耐蚀结构材料.锆合金的织构会对它的屈服强度、蠕变和疲劳强度、应力腐蚀开裂行为以及辐照尺寸变化等产生很大影响,因此变形机理的研究和织构控制在锆合金的开发利用中有十分重要的地位.综述了锆合金的变形机理,介绍了锆合金板材在不同轧制温度下的织构演化规律,以及退火温度对锆合金板材织构的影响,并总结了织构对锆合金板材力学性能的影响.最后指出,目前对锆合金板材加工后的织构进行精确预测还十分困难,需进行详细深入的研究,同时在加工中产生的织构对加工过程的影响以及与温度、应力分布、合金成分和组织的关系还需进一步认识.     

退火态及马氏体态M5锆合金管轴向拉伸性能研究

M5锆合金经β相区淬火后会发生马氏体相变,为了解具有马氏体相的M5锆合金管的轴向拉伸性能并研究马氏体相对锆合金管轴向拉伸性能的影响,通过静态拉伸试验对退火态及马氏体态的M5锆合金包壳管室温轴向拉伸性能进行了研究,并采用扫描电镜对静态拉伸断口进行了观察。结果表明,马氏体态M5锆合金管比退火态M5锆合金管强度高,但塑性和韧性低;从微观断口形貌来看,两种状态的M5锆合金管轴向拉伸断口形貌均为韧窝,断裂机理均为微孔聚集型断裂,断裂类型均为韧性断裂。     

温度和应力对Ti-47.5Al-6(Cr,Nb,W,Si,B)%合金蠕变性能的影响

研究了Ti-47.5Al-6(Cr,Nb,W,Si,B)%合金在不同试验温度和试验应力下的蠕变性能,并分析了不同试验条件下的蠕变机制.试验结果表明,该合金在760℃,100～150MPa具有良好的蠕变性能,在200MPa,700～800℃温度范围内蠕变激活能为U≈299kJ·mol-1,蠕变机制受原子扩散过程控制.在760℃和100～200MPa应力范围内,蠕变应力指数n从2.1变到4.2,故蠕变变形由高密度界面滑移控制变为位错攀移控制的回复蠕变.     

锆合金变形机理及其板材织构演化规律

锆是核工业的重要结构材料,又是优秀的化工耐蚀结构材料。锆合金的织构会对它的屈服强度、蠕变和疲劳强度、应力腐蚀开裂行为以及辐照尺寸变化等产生很大影响,因此变形机理的研究和织构控制在锆合金的开发利用中有十分重要的地位。综述了锆合金的变形机理,介绍了锆合金板材在不同轧制温度下的织构演化规律,以及退火温度对锆合金板材织构的影响,并总结了织构对锆合金板材力学性能的影响。最后指出,目前对锆合金板材加工后的织构进行精确预测还十分困难,需进行详细深入的研究,同时在加工中产生的织构对加_丁过程的影响以及与温度、应力分布、合金成分和组织的关系还需进一步认识。     

锆合金包壳管涡流检测的影响因素分析

理论分析了检测线圈、激励频率、填充系数等参数的选择对锆合金包壳管涡流检测的影响,以及在包壳管制造过程中尺寸均匀性、残余应力、缺陷的形状等因素对涡流检测的影响.针对规格为Φ9.50×0.57mm的锆合金包壳管,给出了检测参数的选择范围,通过合理的选择检测参数,优化管材制造工艺,正确的识别缺陷信号,可有效地提高涡流检测的可靠性.     

基于DOE的锆合金包壳管冷轧质量分析及改善

为了提高锆合金包壳管的冷轧质量,通过统计过程控制技术和工序能力分析研究了锆合金包壳管冷轧后的壁厚偏差问题,并基于试验设计(design of experiment, DOE)技术对皮格尔冷轧工艺进行了优化。包壳管冷轧质量分析和工艺优化试验的结果表明,轧制前管材的壁厚偏差和送进量对轧制后的管材壁厚偏差有显著影响;当轧制前管材壁厚偏差<0.3 mm、壁厚变形量为65%、送进量为1.0 mm/次时,轧制后的管材壁厚偏差最小;通过轧制工艺优化后,反映壁厚偏差离散性的极差平均值由0.036减小到0.018,极差波动也明显减小,轧制质量显著提高;当轧制管材壁厚变形量一定时,对轧制前壁厚偏差较大的管材,采用小送进量轧制,可减小轧制后管材的壁厚偏差,达到提高锆合金包壳管材质量的目的。     

少量Mg,Zr对一种Ni—Cr—Ti型变形合金晶界滑移行为的影响

采用板状试样,沿垂直于应力轴方向预制刻线作为标记,在750℃、392 MPa真空蠕变条件下测定了含少量镁、锆与无镁、锆的Ni-Cr-Ti型变形合金的晶界滑移量,表明在稳态蠕变阶段两种合金没有多大差异,但进入第三蠕变阶段后,合镁、锆合金晶界滑移量和晶界滑移速率逐渐加大,明显超过无镁、锆合金。加入少量镁、锆对晶内滑移也起较大作用,晶内滑移随时间迅速增加,致使晶界滑移与总应变的比值逐渐降低。无镁、锆合金晶内滑移很少,只在断裂前在断口附近有较密滑移带。两合金的晶界滑移-时间曲线与总应变-时间曲线对应相似,晶界滑移受控于晶内滑移。     

980℃/230 MPa下IC6SX单晶合金蠕变行为的各向异性研究

研究了[001]和[111]不同取向的IC6SX单晶合金在980℃/230MPa下的蠕变性能,探明了蠕变性能与取向的相关性,研究了蠕变组织的演变,确定了合金蠕变断裂机制.不同取向IC6SX单晶合金的蠕变性能研究表明:单晶合金的蠕变寿命存在显著的各向异性.在980℃/230MPa条件下,[001]和[111]取向的蠕变寿...     

Texture and anisotropy of zirconium in relation to plastic deformation

Abstract

Possible modes of plastic deformation of tubular materials are analyzed generally. Based on symmetry considerations applied to known textures of zirconium wire and strip, textures of zirconium tubes are predicted as resulting from a wide range of deformation modes. This is made possible by focusing on the main feature of texture, Le., the orientation distribution of basal poles. A relation between texture and the mechanical anisotropy of zirconium is developed, and rules are suggested for softening and hardening caused by texture change. Experimentally, Zircaloy tubes were deformed by various techniques. The resulting textures were determined by X-ray diffraction and the mechanical anisotropy by Knoop-hardness measurement. Each determination is condensed into two representative figures, one describing the type of texture or anisotropy and the other describing the sharpness of texture or the degree of deviation from isotropy.

The experiments verify the predicted relation between type of texture and mechanical anisotropy, and also indicate how degree of anisotropy depends on sharpness of texture. Changes in texture are investigated in relation to plastic deformation, and further refined rules are derived for the direction and rate of these changes. The initially predicted textures are concluded to be stable, i.e., they are reached only after a certain degree of deformation. Zircaloy tubes of three different textures were subjected to tensile testing, closed-end burst testing and open-end burst testing. The observed texture-softening phenomenon corroborates the rules suggested and is considered to commonly occur, as opposed to texture hardening, which requires special combinations of texture and testing method.

Résumé

Les différents modes de déformation plastique possibles dans les tubes ont été analysés d'une façon générale. En se basant sur des considérations de symétrie appliquées aux textures connues pour le zirconium en fil et en bandes, la texture des tubes de zirconium resultant de divers modes de déformation a été prédite. Ceci est possible si l'on tient surtout compte des principes de la texture, c'est à dire, la distribution des orientations des poles de base. Une relation entre la texture et l'anisotropie du zirconium est développée, et des règles sont suggérées pour l'adoucissement et la consolidation dûs à un changement de texture. Dans les expériences, les tubes de zirconium ont été déformés par plusieurs modes. Les textures résultantes ont été déterminées par la diffraction des rayons-X. et l'anisotropie par des mesures de dureté Knoop. Chacune de ces déterminations est représentée dans deux figures, l'une décrivant le genre de texture ou anisotropie, l'autre décrivant la netteté de la texture ou l'écart du caractère isotrope. Les expériences vérifient la relation prévue entre la texture et l'anisotropie. Les changements de texture sont examinés en fonction de la déformation plastique, et d'autres règles plus rigoureuses sont dérivées pour la direction et la vitesse de ces changements. On conc1ut que les textures prévues initialement sont stables, c'est à dire qu'elles ont lieu apres une certaine déformation. Des tubes de Zircalloy de trois textures différentes ont été soumis à des essais detraction, des essais d'éc1atement à extrémités ouvertes ou fermées.

Le phénomène d'adoucissement observé, causé par la texture, est en accord avec les règles suggérées et est généralement présent, ce qui n'est pas le cas du durcissement causé par la texture, durcissement nécessitant des combinaisons spéciales de texture et de méthodes d'essai.     

Relationship between contractile strain ratioR and texture in zirconium alloy tubing

The crystallographic textures of zirconium alloy tubing used as cladding in nuclear reactor fuel are commonly characterized by the quantitative texture numbersF (Källström) and f_r (Kearns) which are derived from the direct and inverse pole figures. The texture numbers of zircaloy 2 and 4 tubes have been correlated experimentally with the value of the contractile strain ratioR which is a measure of the plastic anisotropy of the tube. The correlations were based on the results of 20 different tubing lots. Thef _r-R correlation shows much less data scatter than theF-R correlation. By assuming a simple plastic deformation model for zirconium alloys the following relations between texture and anisotropy are obtained:F=R- 1/R+1 and f_r = R/R+1 The theoretically derived relations are in good agreement with the experimental data. The procedure of correlating texture with plastic anisotropy is not limited to zirconium alloy tubing, but should be equally applicable to textured sheet and plate materials and other alloys with a limited number of slip systems.     

Effect of process variables on texture and creep of pressurised heavy water reactor pressure tubes

Zr-2.5 wt%Nb pressure tubes are used for coolant channels of Pressurised Heavy Water Reactors (PHWRs). These pressure tubes are lifetime components of the reactor and have to sustain extremely harsh conditions of temperature, pressure and neutron irradiation during service. One of the major life limiting factors for pressure tubes is diametral creep. This causes dilation of the pressure tube leading to flow bypass and inefficient heat removal from fuel bundles. This underscores the importance of producing pressure tubes with higher creep resistance. The primary metallurgical parameters controlling the creep strength of Zr-2.5Nb pressure tubes are texture, microstructure, grain size, dislocation density and alloying additions. This includes the strengthening of beta phase due to niobium enrichment. Heat treated Zr-2.5Nb pressure tubes have been reported to have the highest creep strength. This paper briefly discusses the effects of various processing parameters on the microstructure and texture of pressure tubes, which enhance their creep resistance.     

Analysis of steady-state thermal creep of Zr-2.5Nb pressure tube material

The steady-state thermal creep rate in the axial and transverse directions of Zr-2.5Nb of pressure tubes, used in CANDU nuclear reactors, was determined. The data were obtained both from tensile samples having their tensile axes cut along the axial and transverse directions of the pressure tubes and from small-sized, thin-walled tubes, i.e., “mini” tubes stressed either in torsion or by internally pressurizing capsules manufactured from the mini tubes, or by additionally applying an external, axial load on these internally pressurized capsules. The temperature range of the data was from 373 to 596 K (100 °C to 323 °C) and the duration of the tests was from about 1500 hours to over 12,000 hours. The tests were carried out over a sufficiently long time for the creep rate to be measurable in the steady-state creep regime. It was found that the steady-state creep rate depends on stress in a nonlinear fashion and the stress exponent over the entire temperature range was about four. This value is consistent with the values measured earlier on other zirconium alloys. The activation energy Q was found to be about 21 and 10 kcal/mol for temperatures above and below 475 K (∼ 200 °C), respectively. These values are lower than those measured by other investigators on the same material at higher temperatures but similar to values found on other Zr alloys at low temperatures. It appears that Q is dependent on temperature and its value is consistent with the presence of dynamic strain aging (DSA). The results of this study were analyzed with a polycrystalline, nonlinear self-consistent model that take into account the crystallographic texture of the material. This model was used to derive the values of critical resolved shear stress (CRSS), which are consistent with prismatic, basal, and pyramidal glide. By using these values and the apparent temperature dependence of Q, it was shown that this model predicts well the steady-state creep rate over the entire temperature range and under very different stress states.     

Creep of zirconium and zirconium alloys

Cumulative zirconium and zirconium alloy creep data over a broad range of stresses (0.1 to 115 MPa) and temperatures (300 °C to 850 °C) were analyzed based on an extensive literature review and experiments. Zirconium obeys traditional power-law creep with a stress exponent of approximately 6.4 over stain rates and temperatures usually associated with the conventional “five-power-law” regime. The measured activation energies for creep correlated with the activation energies for zirconium self-diffusion. Thus, dislocation climb, rather than the often assumed glide mechanism, appears to be rate controlling. The common zirconium alloys (i. e., Zircaloys) have higher creep strength than zirconium. The stress exponents of the creep data in the five-power-law regime were determined to be 4.8 and 5.0 for Zircaloy-2 and Zircaloy-4, respectively. The creep strength of irradiated Zircaloy appears to increase relative to unirradiated material. It was found that the creep behavior of zirconium was not sensitive to oxygen in the environment over the temperature range examined.     

The influence of crystallographic texture and interstitial impurities on the mechanical behavior of zirconium

Development of physically based constitutive models capable of simultaneously describing slip, twinning, and anisotropy requires knowledge of the coincident influence of each on mechanical response. In this article, the influence of interstitial impurities and texture on twinning in zirconium (Zr), in addition to variations in strain rate and temperature, are examined, to probe their effects on substructure evolution and mechanical behavior. The compressive-yield responses of both high-purity (HP) crystal-bar and lower-purity (LP) zirconium were found to depend on the loading orientation relative to the c-axis of the hcp cell, the applied strain rate, which varied between 0.001 and 3500/s, and the test temperature, which varied between 77 and 298 K. The rate of strain hardening in zirconium was seen to depend on the controlling defect-storage mechanism as a function of texture, strain rate, and temperature. The substructure evolution of HP zirconium was also observed to be a function of the applied strain rate and test temperature. The substructure of HP zirconium was seen to display a greater incidence of deformation twinning when deformed at a high strain rate at 298 K or quasi-statically at 77 K.     

Biaxial creep testing of textured Ti-3AI-2.5V tubing

Creep anisotropy of annealed Ti-3A1-2.5V tubing has been studied under biaxial stress conditions at 673 K using internal pressurization combined with axial loading. Biaxial strains were measuredin situ during creep using laser and linear variable differential transformer (LVDT) extensometers. Creep data were obtained for different stress ratios (α = σ_θ/gs_z @#@), and the steady-state creep rates were found to obey power law with a stress exponent of 4.5 ±0.2 essentially independent of the stress state. Trie experimentally determined creep locus constructed at a constant value of the dissipative work function(W) deviated significantly from isotropy, indicating anisotropy of the material caused by crystallographic texture. The anisotropy parameters(R andP) in the modified Hill’s equation were obtained from the locus fitted to the experimental data to be 5.9 and 1.0, respectively. The crystallographic texture of the material was characterized through inverse and direct pole figures using X-ray diffraction techniques. The crystallite orientation distribution function (CODF) was derived from the pole figure data and represented graphically in the form of Euler plots. This CODF, along with the lower-bound plasticity model, was employed for model predictions of the anisotropy parameters and the creep loci assuming the dominance of basal, prismatic, and pyramidal slip systems. The texture-based predictions differ from the experimental results, and probable reasons for the discrepancy are discussed.     

Behavior and rupture of hydrided ZIRCALOY-4 tubes and sheets

The mechanical behavior and rupture mechanisms of ZIRCALOY-4 guide tubes and sheet containing 150 to 1200 wt ppm hydrogen have been investigated at room temperature. Sheets were notched to study the influence of geometrical defects on rupture. It is shown that hydrides strengthened the material, as maximum stresses sustained by the material are increased with increasing hydrogen contents. On the other hand, ductility is reduced. The material also exhibits a strong anisotropy due to its pronounced texture. Metallographic examinations have shown that damage by hydride cracking is a continuous process that starts after the onset of necking. Notches reduce ductility. A modified Gurson-Tvergaard model was used to represent the material behavior and rupture. Numerical simulation using the finite element method demonstrates the strong influence of plastic anisotropy on the behavior of structures and rupture modes.