Materials Science Forum Vols. 467-470

Abstract: Diffusion controlled creep in nanostructured materials is considered for the case when grain growth occurs concurrently. The Nabarro-Herring and Coble mechanisms that would predict creep rate reduction are re-considered to include the effect of grain-growth induced vacancy generation. It is shown that under such conditions creep is accelerated during an initial stage of grain growth as compared to the case of constant grain size. This creep enhancement stage is followed by a period of reduced creep rate. The predicted strain rate behaviour resembles primary and secondary creep.

Abstract: The paper considers the structure evolution during annealing of submicrocrystalline (SMC) titanium with a mean grain size of 0.2 µm, processed by severe plastic “abc” deformation. Electron microscopic studies have revealed that the microstructure of the SMC titanium is heterogeneous and of a mixed type. One can observe a high density of dislocations, grains, subgrains, elements of a banded structure and areas comprising coarse (up to 1-2µm) and fine (up to 0.1µm) grains. The grain boundary misorientation spectrum is presented by low angle and high angle boundaries of random and special types. Three stages of annealing of SMC titanium have been revealed. The second stage is connected with the intense SMC grain growth and the value of activation energy of grain growth at this stage is twice less than the value of activation energy of grain boundary diffusion in titanium. The latter is caused by acceleration of diffusion processes at SMC grain boundaries having an atom-disordered structure. The texture evolution with annealing temperature is characterized by weakening of predominantly basal texture in the as-received SMC state and strengthening of a prismatic component. The weakening of the basal component is connected with the growth of SMC grains first of all, as well as the increase of the volume fraction of special boundaries.

Abstract: IF steel processed by equal channel angular pressing to an equivalent strain of 9.2 via route BA was annealed for different times at 500°C and 600°C. At both temperatures the microstructural evolution shows continuous recrystallization followed by grain growth, in absence of primary recrystallization. At 600°C a slightly bimodal grain size distribution develops.

Abstract: Equal channel angular extrusion (ECAE) was used to obtain ultrafine grain structures in two aluminium- lithium alloys. The specimens were subjected to severe plastic deformation up to strain value of 9,2 at room temperature. After ECAE deformation, the grain size was reduced from an initial value of 300 µm to 1 µm. The main purpose of this study was to examine the recrystallization characteristics of ECAE treated specimens at temperatures of T = 0,5 and T = 0,7 Tm. At each temperature the specimens were heated for 1, 10, 100 seconds. The microstructure, texture and microhardness were examined in the as-deformed condition and after the annealing. The TEM observations indicate that in the Al-0.7 wt. % Li and Al-1.6 wt. % Li alloys, recrystallization has a continuous character. The microhardness results show that the lithium content increases, in an essential way, the mechanical properties of the alloys after severe plastic deformation and subsequent annealing. During ECAE process, a well pronounced texture with the orientation of the (441)[ 12 4 ], (145)[ 7 31] and (321)[ 3 4 6 ] types are formed within about ¼ volume of the material. After the annealing the grains acquire orientations other than those observed typically in these alloys when deformed by classical straining techniques followed by recrystallization.

Abstract: The crystallization and coarsening of pure and metal-doped (M: Ag, Fe, Co) TiO2 amorphous specimens were studied during in-situ synchrotron radiation diffraction experiments. The temperature and time dependence of the average grain-size of TiO2 anatase and rutile constituent phases was determined. Excess vacancies, secondary phases or solute elements cause the pinning of the grain-boundary motion, control the onset of the anataserutile transition and control the thermal stability of ultrafine microstructures in nanocrystalline TiO2 ceramics.

Abstract: In the present work, a nanocrystalline Invar alloy (Fe-36wt%Ni) foil was fabricated by using a continuous electroforming method. This material exhibited outstanding mechanical properties and a relatively low thermal expansion coefficient as compared to conventional Invar alloys. The as-deposited texture was of fibre-type characterized by strong <100>//ND and weak <111>//ND components. Grain growth occurred during annealing beyond 350°C and resulted in such texture change that the <111>//ND fibre texture strongly developed with the minor <100>//ND components. It was clarified using orientation imaging microscopy that abnormal growth of the <111>//ND grains in the early stages of grain growth plays an important role on the texture evolution. The mechanism of the abnormal grain growth has been discussed in terms of the orientation dependence of energy density.

Abstract: Samples of a hot-rolled AA1050 Aluminium alloy have been deformed by ECAE for 4 and 8 passes following route C. The structural stability of the samples after annealing treatments in the range 200°C to 350°C during 1 to 1000 minutes has been investigated. After 4 ECAE passes, the microstructure is not homogeneous. EBSD-scans show that the former grains can still be identified and that these grains show a different stage of subdivision. After 8 passes the structure is more homogeneous. During subsequent annealing the (sub)structure changes following a common pattern, but with different kinetics in all former grains. At first the dislocation density decreases, illustrated by an increase in image quality of the EBSD-scans and a sharpening of the (sub)structure can be observed. The (sub)grain size remains practically constant during this phase. At a given time, different from one (original) grain to another, some (sub)grains start to grow following an ‘abnormal grain growth’ pattern. This happens at all investigated temperatures. The kinetics and mechanisms of this microstructural evolution will be discussed.

Abstract: The amount of stored energy of cold work for the processing routes A and Bc of Equal Channel Angular Extrusion (ECAE) process has been measured using differential scanning calorimetry. The study is preceded by the effect of amount of ECAE deformation on materials of different stacking fault energies, namely Cu and Ag. The results indicate that the different processing routes have significant effect on the stored energy, which is likely to affect the grain refinement process.

Abstract: The properties of deposited metal films and interconnect structures at submicrometer scale are sensitive functions of microstructural features. Therefore, understanding of the factors which control microstructural evolution is necessary for the development and design of reliable, manufacturable interconnect structures, especially in copper damascene interconnects. The annealing textures of copper interconnects depend on their deposition textures and geometries. The copper interconnects are subjected to stresses even at room temperature, which in turn give rise to strain energies. The stress distributions in interconnects are not homogeneous, which in turn give rise to non-fiber type textures after annealing. The self-annealing textures of interconnects is discussed based on the strain-energy release maximization model, in which grains whose Young’s modulus direction is parallel to the absolute, maximum stress direction grow in preference to others.

Abstract: Cu interconnects are essential in advanced integrated circuits to minimize the RC delay. In manufacturing these devices, Cu is deposited electrochemically using a plating bath containing organic additives. The as-deposited nanocrystalline Cu films undergo self-annealing at room temperature to form a micronsized grain structure by abnormal grain growth. Systematic experimental studies of self-annealing kinetics on model Cu films deposited on a Au substrate suggest that the rate of grain size evolution depends primarily on the initial grain size of the asdeposited film. A model for the observed abnormal grain growth process is proposed. Assuming that desorption of the organic additives leads to mobile grain boundaries, the onset of abnormal grain growth is attributed to a sufficiently low additive concentration such that a full coverage of all grain boundaries cannot be maintained. The incubation time of abnormal growth is then a logarithmic function of the initial grain size. The probability to find a growing grain is proportional to the number of grains per unit volume. This assumption is seen to be in good agreement with the experimental observations for subsequent abnormal grain growth rates. The limitations of the proposed model and the challenges to obtain further insight into the complex microstructure mechanisms during self-annealing are delineated.