A method to predict the orientation relationship,interface planes and morphology between a crystalline precipitate and matrix. Part I. Approach

A model based on the near-coincidence of diffraction intensity weighted reciprocal-lattice spots is proposed to determine preferred orientation relationships between two crystalline phases. The preferred orientation is found by minimizing the three-dimensional lattice mismatch, i.e. by maximizing the total overlapping intensity, between the diffraction spots of two crystals. The procedure is biased towards matching reciprocal lattice sites with high structure-factor values, which is physically equivalent to matching planes of high atomic density. In contrast to previous reciprocal-lattice models, including the diffraction intensity in the present method makes it sensitive to the types of atoms in (chemistry of) crystals. The preferred orientation relationship is then used to identify the orientations of low-energy interfaces using a Δ g approach. A Voronoi (or Wigner–Seitz) construction based on the Δ g values is further used to qualitatively estimate the equilibrium shape of the precipitate in the matrix. The model was tested by performing calculations on hypothetical Au–Cu crystals to investigate the effects of chemistry and fcc truncated-octahedral precipitates in fcc matrices in Al–Ag, Al–Xe and Al–Pb alloys. The present model has the ability to sample the entire orientation space and rationalize and compare alternate orientation relationships in a reasonable timeframe, thereby providing insight into the formation of precipitate orientation relationships and shapes.     

Tilt of InGaN layers on miscut GaN substrates

We report on the crystallographic orientation of InGaN layers grown on GaN substrates with a miscut with respect to c ‐planes up to 2.5°. The samples were examined using high‐resolution X‐ray diffraction (HRXRD) and atomic force microscopy (AFM). Because of the large (up to about 2% in this study) lattice mismatch between InGaN and GaN, an additional tilt between the c lattice planes of InGaN and GaN was observed and explained by using the Nagai model [J. Appl. Phys. 45 , 3789 (1974)]. We observed that for part of the samples, this tilt is about 10% smaller compared to the one predicted by the model. The experimental data are important for understanding the microstructure of InGaN layers grown on substrates of non‐perfect morphology. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of thermomechanical processing on the creep behaviour of Udimet alloy 188

Udimet alloy 188 was subjected to grain-boundary engineering involving thermomechanical processing in an attempt to improve the creep performance and determine the effects on creep deformation processes. The as-received sheet was cold-rolled to either 10, 25 or 35% reduction per pass followed by a solution treatment at 1191°C for 1 h plus air cooling. This sequence was repeated four times and the resultant microstructure and grain-boundary character distribution were described using electron backscatter diffraction. The fraction of general high-angle grain boundaries tended to increase with increased cold rolling. The 10 and 25% cold-rolled materials exhibited lower creep rates than the 35% cold-rolled material. The measured creep stress exponents and activation energies suggested that dislocation creep with lattice self-diffusion was dominant at 760°C for stresses ranging between 100 and 220 MPa. A transition in the creep exponent below the applied stresses of 100 MPa indicated that a different secondary creep mechanism was rate-controlling at low stresses. A significant amount of grain-boundary cracking was observed both on the surface and subsurface of deformed samples, but surface cracks were greater in number and size than those within the bulk. The cracking behaviour was similar in both vacuum and air environments, indicating that grain-boundary cracking was not caused by environment. To assess the mechanisms of crack nucleation, in situ scanning electron microscopy was performed during elevated-temperature (T ≤ 760°C) tensile-creep deformation. Sequential secondary electron imaging and electron backscatter diffraction orientation mapping were performed in situ to allow the evolution of crack nucleation and linkage to be followed. Cracking occurred preferentially along general high-angle grain boundaries and less than 15% of the cracks were found on low-angle grain boundaries and coincident site lattice boundaries. A fracture initiation parameter analysis was performed to identify the role of slip system interactions at the boundaries and their impact on crack nucleation. The parameter was successful in separating the population of intact and cracked general high-angle boundaries at lower levels of strain, but not after crack coalescence dominated the fracture process. The findings of this work have significant implications regarding grain-boundary engineering of this alloy and potentially for other alloy systems.     

La0.8Ca0.2MnO3/SrTiO3薄膜厚度对其结构及磁学性能的影响

采用多种X射线衍射技术和磁电阻测量技术研究了不同厚度的La_0.8Ca_0.2MnO₃/SrTiO₃ (LCMO/STO)薄膜的应变状态及其对磁电阻性能的影响.结果表明，在STO(001)单晶衬底上生长的LCMO薄膜沿［00l］取向生长.LCMO薄膜具有伪立方钙钛矿结构，随着薄膜厚度的增加，面内晶格参数增加，垂直于面内的晶格参数减小,晶格参数a和b相近，略小于c.LC 关键词： X射线衍射 微结构 应变 物理性能     

Mechanisms of high-temperature creep of nickel-based superalloys under low applied stresses

[001] single-crystal specimens of the superalloys CMSX-4 and CMSX-10 were tested for creep at 1100°C under tensile stresses between 105 and 135?MPa, where they show pronounced steady creep. The deformed superalloys were analysed by density measurements, scanning electron microscopy and transmission electron microscopy which supplied information about porosity growth, evolution of the γ–γ′ microstructure, dislocation mobility and reactions during creep deformation. It is shown that, under the testing conditions used, steady creep strain mostly results from transverse glide–climb of (a/2) ?011? interfacial dislocations. A by-product of the interfacial glide–climb are vacancies which diffuse along the interfaces to growing pores or to a ?100? edge dislocations climbing in the γ′ phase. Climb of a ?100? dislocations in the γ′ phase is a recovery mechanism which reduces the constraining of the γ phase by the γ′ phase, thus enabling further glide of (a/2) ?011? dislocations in the matrix. Moreover the γ′ dislocations act as vacancy sinks facilitating interfacial glide–climb. The creep rate increases when the γ–γ′ microstructure becomes topologically inverted; connection of the γ′ rafts results in extensive transverse climb and an increase of the number of a?100? dislocation segments in the γ′ phase.     

Theory of diffusional rotation about the common boundary of a bicrystal

During the creep of polycrystals, individual grains may undergo shape changes, grain boundary sliding and grain rotation. Theoretical studies have focused on the first two of these processes but only recently has the theory of rotation received detailed attention. Diffusional rotation was analysed by Burton [Phil. Mag. A 82 51 (2002); Phil. Mag. 83 2715 (2003)], for a bicrystal with orthorhombic grains of dimensions X, Y and Z with the common boundary in the yz plane and with Z???X,Y. Rate equations were derived and the stress profile over the common boundary predicted, for cases where grain boundary and lattice diffusion predominate. In this paper, the analyses are extended using numerical methods, to the full two- and three-dimensional cases for boundary and lattice diffusion, respectively. For boundary diffusion, the results for Z/Y???1 reproduce those obtained by analytical means and this is regarded as a verification of the numerical method. When Z/Y?=?1, the rotation rates are shown to be about 30% faster, due to the additional diffusion contribution in the z direction. This contribution increases with decreasing values of Z/Y. The stress patterns at the rotating boundary are derived. For lattice diffusion, the stress pattern at the boundary, the shapes of the vacancy potential contours and the variation of the rotation rate with the ratios X/Y and Z/Y are presented.     

中子衍射分析时效处理对镍基单晶高温合金相结构的影响

采用φ振荡和φ固定两种数据采集模式的中子衍射实验结果表明较高的时效温度对消除枝晶最有效,微应变(晶粒区域间的变形不协调性)主要存在于γ'相.利用中子衍射结合扫描电子显微镜对合金的微观组织形貌进行了细致观察,给出了时效温度和时间对γ'相的影响状况,超晶格测量发现了γ'相晶粒之间出现的独特取向差.由不同晶面的中子衍射结果判断时效后合金出现了轻微的四方对称性(a < c)畸变,对这种畸变起主要作用的是基体相.实验结果同时证实了不同方向的应变差异,因而为筏化驱动模型的定量 关键词： 单晶高温合金 中子衍射 超晶格 时效处理     

Shape of the magnetic resonance line in a thin film on the surface of an anisotropic superconductor

The shape of the EPR line in a thin (=λ/2, where λ is the London penetration depth of the magnetic field in the superconductor) paramagnetic film deposited on the surface of an anisotropic superconductor is calculated in an oblique magnetic field with allowance for the inhomogeneity of the local magnetic field of the Abrikosov vortex lattice. It is shown that, as the tilt angle of the external magnetic field is varied, the shape of the EPR line changes noticeably. This fact can give additional information about the superconductor parameters (the symmetry type of the vortex lattice and the anisotropy parameter of the superconductor). Fiz. Tverd. Tela (St. Petersburg) 41, 386–388 (March 1999)     

Factory-roof-shaped phase front at the paraelectric-ferroelectric transition in KD PO : an incoherent interface

The phase front during the 218 K transition in KD₂PO₄ crystals under a thermal gradient perpendicular to the c ferroelectric axis is observed to have a factory-roof shape. This shape is studied versus the magnitude of G_e in samples cut with faces in (100), (010), (001) planes or in ( 0), (110), (001) ones. A geometric approach as well as the calculation of the elastic-strain energy caused by lattice misfits along the phase front demonstrate the incoherent interface nature of the phase front. Furthemore, the results and their interpretation allow to predict the sign of the lattice deformation u _xx ( > 0). Received 25 April 2002 Published online 29 November 2002     

Lattice parameters of relaxed wurtzite indium nitride powder obtained by MOCVD

We have grown by metal–organic chemical vapour deposition a thick layer of indium nitride (1.4 μm) on a sapphire substrate using appropriate growth parameters. The substrate was then removed and the layer reduced to powder by a soft mechanical method. The resulting material was used to obtain an X-ray powder diffraction profile for completely relaxed indium nitride, i.e. without strain due to the lattice mismatch with sapphire. The diffraction measurement was made with a Phillips XPERT-Pro diffractometer, without slits, and collected using radiation at 300 K.The powder pattern contains many (hkl) peaks, depending upon two lattice parameters, a and c, for the wurtzite crystal structure. A two-dimensional least squares procedure was adapted to minimize the fitting errors on the experimental data. The lattice parameters were accurately extracted and will be given here.     

In situ synchrotron radiation topography of NaCI during high temperature creep

High temperature plastic deformation is associated with large changes in the microstructure of single crystals. To observe this microstructure during the creep test, we have performed X-ray reflection topography, taking advantage of the high intensity of the synchrotron radiation. A special creep machine was designed which permits in situ observation.

Creep tests and microstructural observations were performed on NaCl single crystals compressed along <100> at about 600°C. As soon as the deformation started, subgrains appeared within the crystal, independent of the initial microstructure. Migration of the subboundaries during transient creep is followed by stabilization during steady state creep where a well developed subgrain structure keeps constant while new appearing subboundaries migrate. Misorientation between sub-grains increases progressively although more slowly in the steady state creep. A correlation between the microstructure evolution and the changes in the creep curves has been attempted.     

High‐energy transmission Laue micro‐beam X‐ray diffraction: a probe for intra‐granular lattice orientation and elastic strain in thicker samples

An understanding of the mechanical response of modern engineering alloys to complex loading conditions is essential for the design of load‐bearing components in high‐performance safety‐critical aerospace applications. A detailed knowledge of how material behaviour is modified by fatigue and the ability to predict failure reliably are vital for enhanced component performance. Unlike macroscopic bulk properties (e.g. stiffness, yield stress, etc.) that depend on the average behaviour of many grains, material failure is governed by `weakest link'‐type mechanisms. It is strongly dependent on the anisotropic single‐crystal elastic–plastic behaviour, local morphology and microstructure, and grain‐to‐grain interactions. For the development and validation of models that capture these complex phenomena, the ability to probe deformation behaviour at the micro‐scale is key. The diffraction of highly penetrating synchrotron X‐rays is well suited to this purpose and micro‐beam Laue diffraction is a particularly powerful tool that has emerged in recent years. Typically it uses photon energies of 5–25 keV, limiting penetration into the material, so that only thin samples or near‐surface regions can be studied. In this paper the development of high‐energy transmission Laue (HETL) micro‐beam X‐ray diffraction is described, extending the micro‐beam Laue technique to significantly higher photon energies (50–150 keV). It allows the probing of thicker sample sections, with the potential for grain‐level characterization of real engineering components. The new HETL technique is used to study the deformation behaviour of individual grains in a large‐grained polycrystalline nickel sample during in situ tensile loading. Refinement of the Laue diffraction patterns yields lattice orientations and qualitative information about elastic strains. After deformation, bands of high lattice misorientation can be identified in the sample. Orientation spread within individual scattering volumes is studied using a pattern‐matching approach. The results highlight the inability of a simple Schmid‐factor model to capture the behaviour of individual grains and illustrate the need for complementary mechanical modelling.     

Controllable growth of nanocomposite films with metal nanocrystals sandwiched between dielectric superlattices

Nanocomposite films, with Ni nanocrystals (NCs) (5–8 nm) embedded within SrTiO₃/BaTiO₃ superlattice (period: 30 nm), were prepared by laser molecular beam epitaxy. In situ reflection high-energy electron diffraction was employed to study the role of lattice strain in the self-organization of NCs and the epitaxial growth of SrTiO₃/BaTiO₃ superlattice. It was found that the strain from large lattice mismatch drove the self-organization of Ni NCs. Also the SrTiO₃/BaTiO₃ epitaxial growth was achieved on the NC-dotted irregular interface, because the epitaxial growth occurred preferably at sites with low strain. The fine alternation of the two processes would provide a possible route to engineer controllably the nanocomposite microstructure.     

High-temperature creep of single-crystal nickel-based superalloy: microstructural changes and effects of thermal cycling

Creep tests were performed on MC2 single crystal superalloy at 950°C/200?MPa and 1150°C/80?MPa under isothermal and thermal cycling conditions with a tensile axis along the [0?0?1] direction. It was found that the thermal cycles strongly affect the creep behavior at 1150°C but not at 950°C. This was related to the repetitive precipitation and dissolution of small γ′ rafts at the higher temperature, as revealed by quantitative characterization of the γ/γ′ microstructure. The dislocation microstructure exhibits similar trends in all the tested conditions, with a very high activity of a[1?0?0]-type dislocations climbing through the rafts. Such climbing dislocations constitute a recovery process for the deformation active system. It appears that the density of a[1?0?0] dislocations, and not their climb velocity or diffusion rate, is the key parameter for the control of creep rate. The thermal cycles, which imply the creation and subsequent dissolution of rafts, provided new dislocations, which explains the acceleration of creep observed under such conditions.     

Molecular-Dynamics Simulation of Grain-Boundary Diffusion Creep

Molecular-dynamics (MD) simulations are used, for the first time, to study grain-boundary diffusion creep of a model polycrystalline silicon microstructure. Our fully dense model microstructures, with a grain size of up to 7.5 nm, were grown by MD simulations of a melt into which small, randomly oriented crystalline seeds were inserted. In order to prevent grain growth and thus to enable steady-state diffusion creep to be observed on a time scale accessible to MD simulations (of typically 10^-9s), our input microstructures were tailored to (i) have a uniform grain shape and a uniform grain size of nm dimensions and (ii) contain only high-energy grain boundaries which are known to exhibit rather fast, liquid-like self-diffusion. Our simulations reveal that under relatively high tensile stresses these microstructures, indeed, exhibit steady-state diffusion creep that is homogenous (i.e., involving no grain sliding), with a strain rate that agrees quantitatively with that given by the Coble-creep formula.     

High resolution x-ray diffractometry of the structural characteristics of a semiconducting (InGa)As/GaAs superlattice

A statistical approach is used to construct a kinematic theory of x-ray diffraction on a semiconducting superlattice with a two layer period. This theory takes two types of structural deformations into account: crystal lattice defects caused by microdefects distributed chaotically over the thickness of the superlattice, and periodicity defects of an additional superlattice potential owing to random deviations in the thicknesses of the layers of its period from specified values. Numerical simulation is used to illustrate the effect of structural defects on the development of the diffraction reflection curve. The theory is used to analyze experimental x-ray diffraction spectra of semiconducting In_xGa_1−x As/GaAs superlattices. Zh. Tekh. Fiz. 69, 44–53 (February 1999)     

CoSi2/Si (111) Interface study by X-ray standing waves

Summary The CoSi₂/Si (111) interface has been studied with the X-ray standing-wave technique. The interface (a 49? thick CoSi₂ layer) has been epitaxially grown on Si (111) under ultrahigh vacuum andin situ characterized with Auger spectroscopy and low-energy electron diffraction. The perpendicular lattice mismatch between epilayer and substrate has been measured with double-crystal X-ray diffraction. The X-ray standing-wave analysis gives clear indication that the Co atoms are fivefold coordinated at the interface. This work has been partially supported by Progetto Finalizzato ?Materiali e dispositivi per l'Elettronica a Stato Solido?.     

Effects on the evolution of microstructure and properties of low-silicon cast aluminum alloys in service

ABSTRACT

The microstructure evolution and property change of four kinds of low silicon cast aluminum alloy exposed to heat for 0–50?h at 200°C were studied by means of Brinell hardness test, tensile property test, friction and wear property test and XRD analysis. The results show that with increasing thermal exposure time, the tensile strength of each group of samples decreased and the amount of wear increased. The tensile strength of samples with more Si content decreased slowly. When the time increased to 50?h, the increase of wear loss was the largest. The hardness of samples after thermal exposure increases compared with that before thermal exposure. The residual stress of (311) diffraction crystal surface of AlSi3.5Mg0.66 under different thermal exposure time was measured. The type of residual stress changed from residual tensile stress to residual compressive stress after thermal exposure. There is an abnormal phenomenon that the hardness of the sample increased and the amount of wear increased, and it is evident that the distribution of residual stress was inhomogeneous after thermal exposure. It is found that with increasing thermal exposure time to 50?h, the average lattice distortion ε of the low-index crystal plane and the high-index crystal plane in the aluminum alloys gradually increased.     

Regularities in the variation of the rate of stepped creep in polyethylene with different supermolecular structure

The study of stepped creep, previously discovered with micron-size deformation increments (ɛ) of polymers, in the form of a variation of the rate near the average value is continued. A scheme based on a laser interferometer was used to record the creep; this made it possible to perform precise measurements. Attention was focused on the degree of scatter of the rate h in the process of deformation of polyethylene fibers. It is shown that the creep rate of textured fibers is extremely nonuniform and pulsates continuously, forming beats of different periods, i.e., deformation jumps of different height. The ratio of h of the highest to the lowest rate for arbitrarily chosen small increments of the deformation has a maximum near the start of the “flow” stage and prior to fracture. The h-ɛ curve shifts along the deformation scale as the polymer structure changes, but the form of the curve and the overall level of h change very little. It is also established that the value of h for identical deformations is higher in more highly oriented polymers, and the value of h is higher in cross-linked structures than in unmodified structures. It is proposed that h reflects not only the deformation heterogeneity, but also influences crack formation during the creep process. Fiz. Tverd. Tela (St. Petersburg) 39, 580–585 (March 1997)     

Highly oriented La2/3Ca1/3MnO3 films grown on silicon-on-insulator substrates by pulsed laser deposition

High quality La_2/3Ca_1/3MnO₄(LCMO) thin films have been deposited on silicon-on-insulator (SOI) substrates only buffered by yt tria-stabilized zirconia (YSZ) by using the pulsed laser deposition (PLD) technique. The results obtained from X-ray diffraction (XRD), reflection high energy electron diffraction (RHEED), scanning electron microscopy (SEM) and magnetization investigations indicate that the LCMO films are highly oriented both in-plane and out-of-plane. The Curie temperature T _c is close to 260 K and the insulator–metal (I–M) transition appears around 220 K. The conducting mechanism at low temperatures is dominated by the electron–magnon scattering. A tensile stress from the film–substrate lattice mismatch results in magnetic ‘easy axes’ in the film plane and the magnetic anisotropy energy increases with cooling. A maximum magnetoresistance (MR) is observed near 190 K, with the external magnetic field either parallel or vertical to the LCMO film plane. Moreover, the large intrinsic high-field magnetoresistance (HFMR) and the very small extrinsic low-field magnetoresistance (LFMR) again reveal that the LCMO films on SOI substrates are highly oriented thin films of good crystallinity.