Submicroscopic Defects in Metals and their Influence on HVEM In-situ Experiments

Recent high-voltage electron microscopy has provided evidence that substantial submicroscopic changes may occur in specimens examined in the HVEM under conditions where no visible radiation damage occurs. These are the introduction of vacancies by plastic deformation, rapid accumulation of invisible vacancy clusters by electron irradiation, and radiation-induced contamination and refining of materials. These processes are analysed in terms of defect interactions and the consequences for in-situ experiments are discussed.     

In-situ HVEM Observations of Dislocation Processes during High Temperature Deformation of Silicon Crystals

The characteristics in the plastic deformation of silicon crystals are first reviewed. Such characteristics have been interpreted quantitatively on the basis of some models on the velocity and the multiplication of dislocations during deformation. The results of the in-situ observations of silicon crystals deformed at elevated temperatures in a HVEM are presented. The slowness and the smoothness in the dislocation motion, the dynamic pile-up as a general mode of the collective motion of dislocations, the formation processes of multiplication centers of dislocations observed during the deformation all support the validity of the models adopted. Dislocation dipoles and Lomer-Cottrell sessiles are observed not to act as strong obstacles which play important roles in the work hardening of the crystals.     

In-situ Observations on Dynamic Processes Associated with Stacking Faults and Deformation Twins in Silicon Crystals

Stacking faults and thin deformation twins are observed to develop during the in-situ deformation of silicon crystals at elevated temperatures in a HVEM. Both the nucleation and the motion of partial dislocations take place under the applied stress in the regions of the stress concentration. Twinning dislocations moving on twin boundaries are extremely flexible. The spontaneous nucleation of loops of twinning dislocation on twin boundaries is observed rather frequently. No pole mechanism is observed to be operating. Stacking faults and deformation twins interact with glide dislocations moving along the intersecting planes by various ways.     

Relation between the Activation Parameters of the Plastic Deformation in MgO Crystals Determined by Macroscopic and by In-situ HVEM Experiments

The results of macroscopic strain rate and temperature cycling tests on MgO single crystals are compared with the results of in-situ deformation tests in a HVEM on the basis of the basis of the thermally activated dislocation glide model. The activation volume estimated by strain rate cycling tests is compatible with the mean obstacle distance of the screw dislocations. However, the local stress estimated by in-situ deformation tests was found to be larger than the macroscopic yield stress.     

High Temperature In-situ Investigation in HVEM — Deformation,Irradiation, and Recrystallization. Instrumentation and Applications to Materials Science

High temperature in-situ experiments not only require a conventional equipment, but also special devices such as a double tilting straining holder, a double tilting heating holder (1500 K), a high vacuum specimen chamber (a few 10⁻⁶ P ≅ 10⁻⁸ Torr) and a low light level, high resolution, visualization and recording system. Some preliminary conclusions, regarding the representativeness of the in-situ experiments in HVEM, may be derived from the recrystallization and deformation tests performed at ONERA. The fact that the results of these experiments are in good agreement with those obtained through macroscopic tests, a posteriori demonstrates that the former experiments are representative of the bulk material.     

In-situ Straining Experiments on Glass Foils in the HVEM

Crack propagation in Na₂O CaO SiO₂ glass foils with thicknesses between 0.4 μm and 4 μm was studied by HVEM in-situ straining experiments. The propagation of cracks could be achieved by a stepwise loading of the glass foils in a quantitative HVEM tensile tilting stage. The contrasts observed in front of the crack tips indicate a plastic deformation of the glass foils. The influence of the imaging electrons on the mechanical properties of the foil material is discussed.     

In-situ Study on the Fatigue Process of Metals by HVEM

In-situ fatigue test on pre-fatigued aluminum single crystals and also in-situ tensile test of pre-fatigued copper-2 at.% aluminum alloy single crystals were carried out in an HVEM. In the Al crystals, forward and backward motion of dislocations, change of cell structures, formation of voids and crack propagation were directly observed in-situ by an HVEM under alternating stress. In the Cu-Al crystals, it was observed that slip occurred only in the part of the persistent slip bands during in-situ tensile deformation in an HVEM.     

Methods for studying the coherent 4D structural dynamics of free molecules and condensed state of matter

Studies in the coupled 4D spatial and temporal continuum are necessary for understanding the dynamic features of molecular systems with a complex profile of the potential energy surface. The introduction of time sweep into diffraction methods and the development of principles for studying coherent processes have revealed new approaches to the analysis of the dynamics of wave packets, the intermediate products and the transition state of the reaction center, and short-lived compounds in gaseous and condensed media. The use of picosecond and femtosecond electron probe pulses, synchronized with excitation laser pulses, determined the development of ultrafast electron crystallography, time-resolved X-ray diffraction, and dynamic transmission electron microscopy (DTEM). One of the most promising applications of the developed diffraction methods is the characterization and visualization of the processes occurring upon the photoexcitation of free molecules and biological objects and the analysis of surface and thin films. The whole set of spectral and diffraction methods based on different physical principles, which are complementary and make it possible to perform the photoexcitation of nuclei and electrons and carry out diagnostics of their dynamics at ultrashort time sequences, reveal new possibilities for studies with the necessary integration of the “structure-dynamics-function” triad in chemistry, biology, and materials science.     

Investigation of Crystalline Polymeric Solids at Reduced Beam Damage in the High Voltage Electron Microscope

A special technique was used for investigating polymers in the electron microscope at essentially reduced radiation damage. The method is based on a high voltage electron microscope (accelerating voltage of 1 MV) and the use of highly sensitive X-ray films. Compared with the conventional transmission electron microscopy this technique reduces the specimen damage by a factor of 50 … 100. Diffraction contrast can therefore be used to investigate the structure of semi-crystalline polymers. Using samples of polyethylene, the arrangement of the lamellae was determined by bright field and dark field images and diffraction patterns. There are further advantages concerning the improved possibility of investigating very thick specimens (thicknesses above 5 μm). Thus, a better stereoscopic analysis of extended structures is possible, and in-situ deformation tests of polymers can be performed in the electron microscope.     

In-situ investigations on the deformation,crack, and fracture behaviour of carbon materials in the scanning electron microscope

An attachment for scanning microscopes was developed in order to study the deformation, crack, and fracture behaviour of carbon materials during their electron optical observation. Thus the simultaneous registration of electron microscope images, stress-strain-curves and acoustic emission rates at different load regimes is possible. The combination of the mentioned methods allows a correlation between the macroscopic material behaviour and the microscopic processes taking place in the material under load. From the submitted experimental results it can be concluded that the fracture toughness of graphitic materials rises with a growing part of lamellar structure and with grain coarsening.     

Structural Changes During Cutting of Mo Crystals

Structural changes occurring in a shaving and in product during cutting of Mo single crystals as well as softening processes during annealing after cutting are investigated. The extremal deformation are created when the shaving is removed and as a result the dynamic recovery process with lattice reorientation takes place. The strong deformation heterogeneity, which is revealed as the two-scale recrystallisation processes during isothermal annealings, and its localisation in separate sections of the specimens was observed. During high-temperature annealings of the product and shaving the matrix-crystal orientation recovery occurs by means of absorption of the recrystallised layer by undistorted matrix.     

Electron Beam Induced Changes of the Real Structure of Semiconductors

The formation of irradiation defects produced in a high voltage electron microscope (HVEM) has been studied in Ge and other semiconductor crystals in dependence on various thermal treatments and on covering of the specimen surface. It has been concluded that the defect formation depends — besides general features known for pure metals — also on the state of electrically neutral impurities. Besides, various interactions between dislocations and irradiation defects were observed.     

Electron Beam-Specimen-Environment Interactions in the HVEM

Possible interactions between the electron beam, specimen and environment surrounding the specimen are categorised and discussed. A number of examples which illustrate the possible of such interactions on the results of an in-situ experiment are given, and the importance of accurate knowledge of the environment is stressed.     

Zum Bildkontrast kristalliner Objekte in der Rasterelektronenmikroskopie

Electron channelling patterns originally found by COATES and interpreted for the first time by BOOKER , SHAW , WHELAN , HIRSCH are increasingly used for characterizing the orientation and perfection of crystal surfaces in scanning electron microscopy. This field of application of the scanning electron microscope is explained and illustrated by micrographs considering especially the question of the co-ordination of electron channelling patterns to the perfection of the specimen regions near the surface. The possibilities of scanning electron microscopy for the imaging of individual crystal defects are pointed out.     

Surface flattening by annealing after molecular beam epitaxy growth revealed by in-situ secondary electron microscopy

The consecutive evolution of the configurations of islands, steps and terraces on a GaAs(001) surface during annealing after growth was studied by in-situ observation using a secondary electron microscope. Three unexpected processes were revealed: some large two-dimensional islands grow, although most small ones shrink, step distances becomes more uniform rather than step bunching occurring, and multilayer high islands shrink and disappear. The latter two processes play an important role in surface flattening. The growth of islands is explained by thermodynamic equilibrium.     

CMTD晶体的分解过程原子力显微镜实时观测研究

用原子力显微镜实时观测了有机晶体CMTD(CdHg(SCN)4(H6C2OS)2)的表面分解过程,这一分解过程是以表面形貌变化来表征的,并且能反映分解过程中的一些固态反应的物理一化学变化机制.X射线能谱分析(EDAX)表明,相对于新鲜的CMTD晶体,在分解物中Hg低而S高;X射线衍射(XRD)表明分解物为非晶态;红外吸收光谱(IR)表明在CMTD晶体结构中的(H6C2OS)结构基元被分解为H2O和酸;量热分析(DSC)表明CMTD晶体在室温空气中确实有化学反应发生.本文用原子力显微镜实时记录了这一分解过程的形貌变化,发现这种非晶态的分解物的产生过程为:成核一核生长一核分散等.这是首次用原子力显微镜实时观测到这种分解过程.     

CVD Processes with Direct and Indirect Incorporation

Indirect incorporation via adsorption, surface diffusion, and reaction at half crystal sites can be treated by the BCF theory of layer-by-layer growth in simple cases of CVD, even if there is strong surface coverage by adsorbed species. I that case, however, the parallel path by direct reactive incorporation of atoms into half crystal sites from the gas phase may play an important role. In any case, direct incorporation becomes the sole possibility for (normal) growth if the surface fraction y_k of half crystal sites (kinks) becomes I. The continuous transition from singular faces (y_k = 0) to half crystal faces (y_k = l) is covered by a simplified analytical expression and can be included into an overall circuit representation of stationary CVD processes which holds for in-situ doping, too. The model results in a good compromise between the conventional BCF treatment and Chernovs treatment of CVD surface processes.     

In-situ Observations on Cu-SiO2 and Stainless Steels

Specimens of Cu SiO₂ and of several high purity stainless steels have been observed during deformation in an HVEM. The behaviour of the Cu SiO₂ specimens is dominated by the free surfaces, even in the thickest regions which can be examined, but in the stainless steels it has been found that if the specimen is thicker than about 0.5 μm, bulk behaviour is observed. The formation of ϵ- and α-martensite has been directly observed and essentially identical observations of martensite formation have been made during cooling. It has been shown that ϵ-martensite is always formed in regions where appropriately, but usually irregularly spaced faults, are generated and detailed analyses of isolated and overlapping faults show that these faults contain close-packed planes of the appropriate eph spacing. α-martensite is formed in association with dislocation pile-ups.     

HVEM In-situ Straining Experiments on MgO Specimens Predeformed at High Temperature

A deformation of MgO crystals at high temperature significantly increases the plasticity in a subsequent deformation at room temperture. It was shown by deformation experiments inside the high voltage electron microscope the this effects is most probably due to the homogeneous dislocation structure formed by straining at high temperature, but not to change in the processes controlling the mobility of individual dislocations.     

The electron-dislocation interaction and electronic effects at low-temperature deformation

The effect of the electronic state of crystals on the processes of their plastic flow is considered. Presented are the results of the observation and study of the effects of the electrondislocation interaction at a low-temperature deformation of metals.