Programación del EKF para la fusión sensorial

This review has been devoted to a study of interface phenomena influencing advanced properties of nanoscale materials processed by means of severe plastic deformation, high-energy ball milling and their combinations. Interface phenomena include pro- cesses of interface defect structure relaxation from a highly nonequilibrium state to an equilibrium con-

dition, grain boundary phase transformations, en- hanced grain boundary and triple junction diffusivity. On the basis of an experimental investigation, a theo- retical description of the key interfacial phenomena controlling the functional properties of advanced bulk nanoscale materials has been conducted. An inter- face defect structure investigation has been per- formed by TEM, high-resolution X-ray diffraction, atomic simulation and modeling. The problem of the transition from a highly non-equilibrium state to an equilibrium one, which seems to be responsible for low thermostability of nanoscale materials, was studied. Also enhanced grain boundary diffusivity is addressed. Structure recovery and grain growth in nanocrystalline materials have been investigated by analytical methods and modeling.

Several major conclusions may be drawn from these investigations:

· Severe plastic deformation such as ECAP and HPT can be successfully applied both for grain refinement in pure metals and alloys and for cold consolidation of ball milled powders;

· A correlation between microstructure and me- chanical properties of Co foils prepared by ball milling and subsequent high pressure torsion has been presented. The crystallite size is found to be significantly reduced after HPT, much further than by subjecting Co powders to a long-term milling process alone. Simultaneously, large amounts of stacking faults, especially deforma- tion faults, are created during HPT. This is in contrast with BM, where the twin fault probabil- ity is significantly larger than the deformation fault probability;

· Partially glassy materials can be prepared by cold-compaction using high pressure torsion of ball-milled Fe77Al2.14Ga0.86P8.4C5B4Si2.6 amorphous

ribbons. The torsion straining process induces significant changes in the short-range order (both chemical and topological) of the multicomponent alloy which result in the formation of dispersed nanocrystallites inside the amorphous matrix and a reduction in the overall crystallization enthalpy. The HPT ribbons exhibit enhanced Curie tem- perature and microhardness with respect to the BM ribbons. This cold-compaction technique proves itself to be appropriate for the fabrication of thin bulk partially amorphous disks and it is particularly appealing for the consolidation of amorphous alloys with reduced supercooled liq- uid region.

· High-resolution X-ray diffraction and TEM experi- ments can be used to determine the microstruc-

ture and dislocation density in ultrafine-grained materials after processing by ECAP, HPT, cold rolling and their combinations. Assuming that the crystallite size distribution in all samples obeys a log-normal function and that the strains are caused by dislocations, the parameters of the crystallite size distribution and the disloca- tion structure were calculated by fitting the Fou- rier transforms of the experimental X-ray diffrac- tion profiles to physically well established theo- retical functions. The crystallite size values were compared to the grain sizes determined by TEM. It was found that the additional deformation after ECAP resulted in further grain refinement and an increase of the dislocation density. However, electrodeposition gives an even finer microstruc- ture and a higher dislocation density than the materials processed by SPD methods. The crys- tallite size values are lower than the grain size for all the specimens, since the former measures the dislocation cell size in SPD materials. For the elctrodeposited nickel, the crystallite size is close to the grain size determined by TEM; · By means of differential scanning calorimetry and

high-resolution X-ray diffraction, the microstruc- tural parameters and released enthalpy have been measured in ultrafine-grained nickel processed by ECAP, HPT and their combination. The differ- ence between the released DSC enthalpy and the elastic energy is attributed to the GB sur- face energy and the normalized surface energy has been evaluated. All nickel samples show GB surface energies for high-angle boundaries in the range of 1.0-1.2 J/m2 _{which is higher than}

reported in the literature. This is attributed to the non-equilibrium state of GBs existing in UFG ma- terials obtained by severe plastic deformation. · Severe plastic deformation by HPT of binary and

ternary Al-Zn, Al-Mg and Al-Mg-Zn alloys de- creases drastically the size of (Al) and (Zn) grains (below 100 nm) and particles of reinforcing β and τ phases (below 10 nm). Therefore, HPT leads to the formation of a nanostructure which is less equilibrium than that of the initial coarse grained material. At the same time, during HPT the Zn- rich supersaturated solid solution (Al) decom- poses completely and reaches the equilibrium corresponding to room temperature. This process is less pronounced for Mg-rich (Al). In ternary alloys Zn also leaves the supersaturated solid solution easier than Mg. Therefore, HPT leads to the formation of a phase structure which is more equilibrium than that of the initial coarse

grained material. The most probable mechanism of the equilibration of the (Al) supersaturated solid solution is the grain boundary diffusion ac- celerated by fluxes of vacancies due to the SPD and by the sweeping of Mg and Zn atoms from the bulk by moving GBs. It appears that the se- vere plastic deformation of supersaturated solid solutions can be considered, similar to irradia- tion at ambient temperature, as a balance be- tween deformation-induced disordering and de- formation-accelerated diffusion towards the equi- librium state;

· As a result of preliminary isothermal heat treat- ment of submicrocrystalline material, the nonequilibrium grain boundaries in the treated material pass to an equilibrium state, with the grain size remaining in the micron range. This causes a significant increase in the apparent activation energy of creep, which may be attrib- uted to the contribution of grain boundary sliding to the total deformation becoming less due to the enhanced resistance to shear over the equi- librium grain boundaries relative to the non-equi- librium grain boundaries. As the temperature of preliminary anneals increases and approaches the temperature of recrystallization (~ 673Ê), the activation energy of creep also tends to increase to become equal to the activation energy of vol- ume self-diffusion, e. g. in coarse-grain crystal- line Ni. At elevated temperatures, the creep in this material is controlled by the motion and climb of dislocations, the contribution of grain bound- ary sliding being negligible;

· The grain boundary recovery occurring simulta- neously during the diffusion experiment causes another significant effect, namely a drastic de- crease in the apparent activation energy of diffu- sion. This can explain the fact that the measured activation energy of grain-boundary diffusion in nanocrystals is approximately half that for usual polycrystals. In the case when the time of diffu- sion annealing is chosen without regard for the relaxation time and is the same for all tempera- tures of annealing, one cannot rule out the pos- sibility of an anomalous behavior of the diffusion coefficient exhibiting a negative activation energy. · Non-equilibrium grain boundaries are able to in- tensively emit perfect and partial lattice disloca- tions into grain interiors in nanocrystalline ma- terials fabricated by severe plastic deforma- tion. The emission of lattice dislocations by non- equilibrium grain boundaries gives rise to the softening and eliminates local stress concen-

trators - nuclei of nanocracks – in nanocrystalline materials under plastic deformation.