Orientation Dependence of Stress Rupture Properties of a Ni-based Single Crystal Superalloy at 760℃

The orientation dependence of creep rupture lives of a single crystal superalloy at 760℃/760 MPa was investigated.The orientations of the specimens tested were about 30°away from [001].The results showed that specimens with orientations on the [001]-[011] boundary had the longest rupture life.The deformation of these specimen were controlled by a/2110 slip and a few stacking faults with two orientations were observed.On the other hand,specimens with orientations near the [001]-[011] boundary or on the [001]-[111] boundary showed short rupture lives,and stacking faults with single orientation were observed in these specimens.The rupture properties and the deformation mechanisms were discussed based on the dislocation pattern and the calculated Schmid factors for different specimens.     

偏离〈001〉取向15°的三种镍基单晶高温合金试样的中温蠕变变形行为

研究了一种偏离<001>取向15^o的镍基单晶高温合金的中温蠕变(760℃/793 MPa)性能和变形组织。结果表明：取向靠近<001>-<101>边界合金的蠕变寿命最长,而取向靠近<001>-<111>边界合金的蠕变寿命最短。虽然三种试样取向偏离<001>的夹角都约为15^o,但是其微观变形组织明显不同。取向靠近<001>-<101>边界试样的变形主要受控于{111}<110>滑移系,而取向靠近<001>-<111>边界试样变形主要受控于{111}<112>滑移系。     

Effect of Threshold Stress on Anisotropic Creep Properties of Single Crystal Nickel-Base Superalloy SRR99

The influence of orientation on the creep properties of the nickel-base single crystal superalloy SRR99 was investigated at 760℃ and various levels of applied stress.It was found that the effect of anisotropy was strikingly prominent at this temperature.Deformation mechanisms of single crystals with three principal orientations at 760℃/790 MPa were explained according to detailed observations of dislocation arrangements by means of transmission electron microscopy (TEM).Liability to shearing γ' precipitates for [011] orientation and co-planar slip for [111] orientation resulted in poor creep strength and short stress rupture life.It was also found that the apparent primary creep strain could be measured when the applied stress was increased to 565 MPa.Modified power law equation was adopted and the concept of a threshold stress σ 0 which determines dislocation looping or shearing to be activated was then involved.Through detailed calculations,the threshold stress was obtained to further analyse the distribution of the applied stress and better rationalize the anisotropic creep behaviour in the stereographic triangle in combination with TEM observations.     

Investigation the effect of crystal orientation of nickel‐based single crystal superalloys on bending creep tests

The relationship between the three points bending creep test and the uni‐axial creep test on the single crystal superalloy was investigated by using crystal plasticity slip theory with a three‐dimensional (3D) finite element model. The purpose of the present work is to build the relationship between bending creep and conventional uni‐axial tensile creep in determining crystallographic creep parameters for face centered cubic (FCC) nickel‐based single crystal superalloys. To this aim, the bending creep performed on [001]‐, [011]‐, and [111]‐oriented nickel‐based single crystal superalloys were respectively investigated, and the data was compared with those obtained with uni‐axial tensile creep counterparts. It shows that the determination of crystallographic creep stress exponent is independent of crystallographic orientations, and the results agree reasonably well between bending creep test and uni‐axial tensile creep test. The findings may shed some light on understanding of the crystal structures and its time‐dependent deformation mechanisms with the bending creep method.     

A quantitative metallographic assessment of the evolution of porosity during processing and creep in single crystal Ni‐base super alloys

The present work reviews previous research on the evolution of porosity. It presents new results from a detailed study on the evolution of porosity during casting, heat treatment and creep of a single crystal Ni‐base superalloy subjected to uniaxial tensile creep at 1050 °C and 160 MPa in [001] and [110] directions. A quantitative metallographic study was performed on carefully polished metallographic cross sections, monitoring sampling fields of 4500 × 1000 µm² using the back scatter contrast of an analytical scanning electron microscope; evolutions of pore sizes and pore form factors were analyzed and all important details which were previously revealed in a synchrotron study could be reproduced. In addition, it was observed that micro cracks form at larger cast pores. They interlink and thus initiate final rupture. The [110] tensile creep tests showed lower rupture strains than the [001] experiments. In agreement with earlier work, this can be rationalized on the basis of aligned porosity along primary dendrites.     

Electromechanical coupling properties of [001], [011] and [111] poled Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 single crystals

The electric field induced “butterfly” curves and polarization loops, and the stress induced strain and polarization responses of [001], [011] and [111] oriented Pb(Mg_1/3Nb_2/3)O₃-0.32PbTiO₃ (PMN-0.32 PT) relaxor ferroelectric single crystals have been systematically investigated by experiment study. The focus is on the effect of constant compressive bias stress on the electromechanical coupling behavior along three crystallographic directions of PMN-0.32 PT single crystals. Dependence of the coercive field, remnant polarization, dielectric constant, and piezoelectric coefficient on the bias stress has been quantified for PMN-0.32 PT single crystals oriented in three different directions. Obtained results show that the large piezoelectric responses under zero compressive stress in [001] and [011] orientation are dominated by intrinsic crystal lattice while the engineered domain structure has a relatively minor effect. It is found that observed responses under stress cycle for [001] oriented crystals are due to polarization rotation and phase transformations. However, those for [011] and [111] oriented crystals are due to domain switching. The “butterfly” curves and polarization loops driven by electric field under different bias compression are described by two non-180° domain switching.     

EBSD characterization of an ECAP deformed Nb single crystal

A niobium single crystal was subjected to equal channel angular pressing (ECAP) at room temperature after orienting the crystal such that [1 −1 −1] ∥ ND, [0 1 −1] ∥ ED, and [−2 −1 −1] ∥ TD. Electron backscatter diffraction (EBSD) was used to characterize the microstructures both on the transverse and the longitudinal sections of the deformed sample. After one pass of ECAP the single crystal exhibits a group of homogeneously distributed large misorientation sheets and a well formed cell structure in the matrix. The traces of the large misorientation sheets match very well with the most favorably oriented slip plane and one of the slip directions is macroscopically aligned with the simple shear plane. The lattice rotation during deformation was quantitatively estimated through comparison of the orientations parallel to three macroscopic axes before and after deformation. An effort has been made to link the microstructure with the initial crystal orientation. Collinear slip systems are believed to be activated during deformation. The full constraints Taylor model was used to simulate the orientation evolution during ECAP. The result matched only partially with the experimental observation.     

气膜孔分布对DD6单晶高温合金高周疲劳断裂行为的影响

利用带不同电液束加工气膜孔分布的DD6单晶气冷叶片模拟试样,研究其在常温下的高周疲劳性能,并对试样断口及断口侧面形貌进行宏观与微观观察。结果表明:在相同的实验条件下,气膜孔的存在对试样高周疲劳寿命的影响较大,无孔试样的平均寿命约为带3排孔试样的4倍,但是气膜孔布局对疲劳寿命的影响相对较小。通过断口宏观与微观观察发现,无孔试样呈线源特征,而1~3排孔试样裂纹均从气膜孔附近起源,呈多源特征。根据断口和晶体学理论推测,对于无孔、1排孔和多排孔试样的中间部位,裂纹沿{001}滑移面扩展;而对于多排孔试样的上下2排孔孔周的裂纹沿{111}滑移面扩展。采用有限元方法分析4种不同试样孔边应力场的分布规律,数值模拟分析结果与试样的断裂位置及形貌吻合。     

Studies of optical anisotropy in opals by normal incidence ellipsometry

Anisotropy of thin opal films was studied by ellipsometric technique in a visible spectral range. At normal light incidence, the ellipsometric data were directly related to anisotropy parameters measured by polarization modulation technique. In the (111)-oriented thin films, the optical anisotropy was mainly caused by internal strain-induced birefringence with anisotropy axes oriented along [110] and [-112] directions. The deviation from 180°-symmetry, which has been observed for ellipsometric parameters in the in-plane sample rotation experiments at normal incidence, was enhanced at oblique incidence and assigned to particular properties of opal. Experimental data were discussed in the model of stacked anisotropic layers.     

基于EBSD技术的P91钢蠕变过程中小角度晶界演化行为表征

在620℃、145 MPa条件下对给定的P91钢进行高温蠕变持久与间断试验,采用电子背散射衍射(EBSD)技术研究其在蠕变过程中小角度晶界的演化行为。通过引入EBSD图像中的取向差分布来表征小角度晶界处(0.5~5°)的边界位错密度,分析了边界位错密度在蠕变过程中与小角度边界的数量、塑性应变以及内部微观组织演化之间的关系。此外,通过改变EBSD像素点与像素点之间的计算步长,探讨了步长选择对边界位错密度计算结果的影响。结果表明,小角度晶界处的位错密度在蠕变过程中先迅速上升,在最小蠕变率处达到极值后缓慢下降,直到最后基本保持不变;同时,EBSD的计算步长越小,得到的位错密度值越准确。     

Continuous X-ray diffraction measurement of lattice rotation during tensile deformation of aluminium crystals

Results are reported of experimental measurements in which crystallographic orientation changes due to lattice rotation were continuously monitored during uniaxial tensile tests on 99.99% pure aluminium crystals. The use of an image intensifier tube, lens coupled to a fluorescent screen, permitted Laue transmission X-ray diffraction patterns to be recorded with a motion picture camera at rates as fast as 24 frames per second. In all tests, the lattice rotation caused the tensile axes to overshoot the [1 0 0]-[1 1 1] symmetry line contrary to the classical view. In general, the experimentally measured lattice rotation values lagged behind the theoretical values as derived from the idealized single slip model. No clear dependence of lattice rotation on strain rate was found, although in every test the lattice rotated fast initially with no bending evident and then the lattice rotation slowed down accompanied by considerable lattice bending.     

Effects of low- to medium-angle grain boundaries on creep properties of superalloy

The creep properties of Ni-based bicrystal specimens with low- to medium-angle grain boundaries are investigated at 1100°C and 130?MPa with two misorientation angles prepared by double-seed solidification. Experimental results show that the angle of the grain boundary with respect to the stretching direction has a strong effect on creep life. Molecular dynamics simulations show that the effect of the grain boundary on the neighbouring dislocation density is related to the grain boundary angle. A creep constitutive model with the grain boundary angle and the initial damage rate can be adopted to describe the creep damage evolution. The model is then modified according to the molecular dynamics simulation and experimental results to give more accurate predictions of creep life.     

The micro-Vickers hardness of TiC single crystals up to 1500° C

The behaviour of mechanical properties of TiC single crystals grown by r. f. floating zone process with nearly stoichiometric composition and low dislocation density were studied by the measurement of microhardness up to the temperature of 1500° C. First, anistropy in the microhardness of (100), (110) and (111) planes at room temperature, second the variation of indentations on (100) [001] orientation, (110) [001] orientation, and the (111) [1¯10] orientation at high temperature were described. Temperature dependency of the relaxation behaviour of indentation (900 to 1200° C) yields the activation energy for creep which is close to that obtained for self-diffusion of carbon in TiC.     

Dendritic Growth Pattern and Dendritic Network Distortion in the Platform of a Ni-based Superalloy

The pattern of dendritic growth and distortion of dendritic network in the platform have been investigated by one mold casting with different platform length during directional solidification. As the platform length elongates, the symmetry of dendritic growth along left and right edges gradually worsens in platform base. While the platform length reaches 14 mm, the distortion of dendritic network is first observed in outward platform. It is found that the distortion of dendritic network along platform inside is more serious than that along platform edges. Both [001] deviation and accumulated misorientation along platform inside, up to 9~ and 16.3~, respectively, are far greeter than those along left-outward-right edges. The deformation of dendritic network in a platform may be caused by the asymmetry of the solidification front at the mush zone.     

Creep deformation and rupture behaviour of directionally solidified GTD 111 superalloy

The creep behaviour of directionally solidified (DS) Ni‐base superalloy GTD 111 has been investigated at various temperatures (649 °C to 982 °C) and stresses (124 MPa to 896 MPa). Specimens machined in longitudinal and transverse directions with respect to the grain orientation from three batches of the material were tested. The specimens in the longitudinal direction consistently exhibited higher creep rupture life and creep ductility than specimens from the transverse direction. There were some systematic variations in creep deformation and rupture behaviour among specimens from different batches. Optical and scanning electron microscopy investigations were conducted to understand the creep rupture behaviour. Various deformation and rupture models were evaluated for representing the creep behaviour of the alloy and a neural network model was applied to creep rupture data to assess its predictive capability.     

Accumulation of coincidence site lattice boundaries during grain growth

Abstract

Monte Carlo simulations were used to investigate the effect of grain growth on the coincidence site lattice (CSL) boundary content of randomly textured polycrystals. Each grain was assigned an orientation, and grain boundary properties were dependent on both the boundary misorientation and the CSL character. While low misorientation angle boundaries (LABs) increase during growth, the fraction of CSL boundaries does not change with time. Decreasing CSL boundary energy and mobility did not alter these results. In contrast with LABs, which are characterised by a scalar misorientation angle, a particular combination of three independent rotation variables is required to create a low energy CSL boundary; thus, these boundaries are unlikely to form or to persist in a random polycrystal. While texture influences boundary formation, a texture that can enhance CSL boundaries is not apparent. Boundary plane effects should not increase CSL fraction during grain growth.     

Ductility and fracture of copper bicrystals with boundary SiO2 particles

Abstract

High temperature deformation andfracture ofCu-SiO₂ bicrystals with [001] twist boundaries of various misorientation angles were investigated under the condition of non-activation of grain boundary sliding. As the misorientation angle increases, the bicrystals became more susceptible to intergranular brittle fracture. Clear intermediate temperature embrittlement was observed in bicrystals with a random high angle boundary. The boundary segregation of O atoms was found to enhance intergranular fracture. Although the boundary SiO₂ particles provide stress concentration sites which cause early formation of boundary cavities, the boundary dependent deformation and fracture behaviour is essentially determined by inherent boundary strength, which is afunction of the misorientation angle.

MST/1969     

Orientation dependent behavior of tensile-creep deformation of hot rolled Ti65 titanium alloy sheet

The mill products like sheet always have one or more severe textures inevitably,and its effect on mechan-ical properties is not a negligible issue.The orientation dependent tensile-creep behavior induced by rolling texture of Ti65 titanium alloy sheet has been systematically investigated at 650℃.There are some anisotropic characteristics between TD and RD of Ti65 sheet.The UTS and TYS of TD are higher than RD at 650℃.Besides,the creep endurance time of TD(172.6-174.5 h)is about three times longer than RD(55.6-65.1 h)at 650℃and 240 MPa.Moreover,the grains are inclined to form Texture Ⅲ(1 2(1)6)[1(2)11]and(01(1)3)[1(2)11]after creep along with TD,but to form Texture I((1)2-(1)0)[10-(1)0]after creep along with RD.Finally,the crack initiation site is different during creep in TD and RD.The reason for anisotropic properties of tensile and creep has been summarized in two aspects:(ⅰ)the change of the SFs(Schmid factors)value between TD and RD;(ⅱ)the difference of creep mechanism between TD(grain boundary sliding)and RD(dislocation slip).Anisotropy of Ti65 sheet should be fully considered to increase structural efficiency in the engineering design and application.     

DD6单晶高温合金振动疲劳性能及断裂机理

研究[001]取向的DD6单晶高温合金的室温振动疲劳S-N曲线,并获得了其室温振动疲劳极限。利用体视显微镜、扫描电子显微镜、背散射衍射等手段对DD6单晶高温合金振动疲劳断裂机制进行分析。结果表明:采用S-N法估算得到的[001]取向的DD6单晶高温合金室温振动疲劳极限约为337.5MPa。振动疲劳裂纹断口呈现单个或多个沿{111}晶体学扩展平面组成的形貌特征,断口上分为疲劳源区和疲劳扩展区两个阶段,裂纹在应力最大截面处的表面或内部缺陷处萌生,呈单源特征,疲劳扩展区呈现类解理断裂特征,未出现典型的疲劳条带特征。说明沿{111}晶面滑移是DD6单晶高温合金室温振动疲劳断裂的主要变形机制,断口上的类解理扩展平面以及微观上类解理花样是DD6单晶高温合金室温振动疲劳断裂的主要特征。     

Effect of the grain boundary on the evolution of deformation in a bicrystal

The role of grain boundary constraint in strain localization and concomitant constitutive response was examined by performing a series of uniaxial compression tests on a tantalum bicrystal. Tantalum single crystals were diffusion bonded to form a (011) 90^∘ twist boundary that was compressed along the common [011] direction. The plastic deformation resulted in the creation of deformation bands away from the highly constraining grain boundary, resembling those bands known from single crystal plastic deformation. Near the grain boundary, such deformation band formation could not be detected. Instead a distinctive pattern of crystal lattice rotation was observed that filled a rather large volume (several millimeters in size) around the bicrystal grain boundary. The internal deformation band structure as well as the crystal lattice rotation pattern near the bicrystal grain boundary were characterized and found to give greater rates of work hardening in the neighborhood of the grain boundary.