Study of the Hydrolytic Stability of Fine-Grained Ceramics Based on Y2.5Nd0.5Al5O12 Oxide with a Garnet Structure under Hydrothermal Conditions

The hydrolytic stability of ceramics based on Y2.5Nd0.5Al5O12 oxide with a garnet structure obtained by the spark plasma sintering (SPS) method has been studied. The tests were carried out in distilled water under hydrothermal conditions in an autoclave and, for comparison, in a static mode at room temperature. The mechanism of leaching of Y and Nd from the ceramics was investigated. It has been shown that at “low” temperatures (25 and 100 °C), the destruction of pores occured, and the intensity of the leaching process was limited by the diffusion of ions from the inner part of the sample to the surface. At “high” test temperatures (200 and 300 °C), intense destruction of the ceramic grain boundaries was observed. It was assumed that the accelerated leaching of neodymium is due to the formation of grain-boundary segregations of Nd3+ in sintered ceramics.

Grain Boundary Design of Bulk Nanomaterials for Advanced Properties

Nanostructuring of metals and alloys by severe plastic deformation techniques is an effective way of enhancing their mechanical and functional properties. The features of the nanostructured materials produced by severe plastic deformation (SPD) are stipulated by forming of ultrafine-sized grains as well as by the state of grain boundaries. The concept of grain boundary (GB) design of ultrafine-grained metals and alloys is developed for enhancement of their properties by tailoring grain boundaries of different types (low-angle and high-angle ones, special and random, equilibrium and nonequilibrium) and formation of grain boundary segregations and precipitations by SPD processing. The paper presents experimental data demonstrating the super-strength and “positive” slope of the Hall-Petch relation when passing from micro-to nanostructured state in a number of metallic materials subjected to severe plastic deformation. The nature of the superior strength is associated with new strengthening mechanisms and the difficulty of generation of dislocations from grain boundaries with segregations. This new approach is used for achieving the enhanced strength in several commercial Al and Ti alloys as well as steels subjected to SPD processing.

SPD-Induced Grain Boundary Segregations and Superior Strength in UFG Al Alloys

Two Al alloys (AA1570 and AA6061) in the solutionized state have been processed by HPT at room temperature to achieve a homogeneous UFG structure. After HPT, the grain size was found to have a mean value about 100 nm for both alloys. Measured yield stress values of HPT-produced UFG alloys being plotted in terms of the Hall-Petch relationship were found to exceed the plot predictions for the range of ultrafine grain size. For both alloys, Atom Probe Tomography measurements allowed to reveal segregation of solute elements along grain boundaries. The origin of the extremely high strength of the alloys nanostructured by HPT is discussed with a special attention to the influence of such segregations on the emission and the mobility of dislocations.