Role of Ordering Energy in Formation of Grain Structure and Special Boundaries Spectrum in Ordered Alloys with L12 Superstructure

It was revealed that an average energy of special boundaries is proportional to APB energy in the alloys with the L12 superstructure. This fact proves the appearance of the GAPBs in the planes of location of special boundaries in coincidence sites of ordered alloys. It was determined that the more energy of special boundaries in ordered alloys, the more energy of complex stacking fault. There is a correlation between the distribution of special boundaries as a function its relative energy and ordering energy: the more ordering energy, the more degree of washed away of distribution. The correlation between average relative energy of special boundaries and ordering energy was detected: the more ordering energy, the more average energy of special boundaries. The reverse dependence between ordering energy and average number of special boundaries in grains limited by boundaries of general type was discovered.

Special nanostructures in Al-Mg alloys subjected to high pressure torsion

Deformation twins and stacking faults were observed in nanostructure A1-Mg alloys subjected to high pressure torsion. These observations are surprising because deformation twinnings have never been observed in their coarse-grained counterparts under normal conditions. Experimental evidences are introduced on non-equilibrium grain boundaries, deformation twinnings and partial dislocation emissions from grain boundaries. Some of these features can be explained by the results reported from molecular-dynamics simulations of pure FCC metals. Special emphasis is laid on the recent observations of high density hexagonal and rhombic shaped nanostructures with an average size of 3 nm in the A1-Mg alloys processed by high pressure torsion. A possible formation process of these nanostructures is proposed based on molecular-dynamics simulations.

Intergranular Fatigue Cracking Behaviour in Aluminium

In order to understand the basic mechanism of intergranular cracking in pure metals during fatigue, stress-controlled push-pull fatigue tests were carried out with high purity aluminium. Tests were interrupted frequently so as to study the grain boundary (GB) cracking behaviour by the surface observation. The results show that crack initiation at GB was a process controlled by multi-factors, such as boundary structure, GB-slip interaction, GB sliding and so on. If these factors are varied so that the incompati- bility at a GB increased, the possibility of cyacking at the boundary will be raised. Some inteygranular cracking phenomena are not able to be explained by the GB stepping mechanism.

Computation of Five-Dimensional Grain Boundary Energy

Abstract A broken-bond type computational method has been developed for the calculation of the five-dimensional grain boundary energy. The model allows quick quantification of the unrelaxed five-dimensionally specified grain boundary energy in arbitrary orientations. It has been validated on some face-centred cubic metals. The stereo projections of grain boundary energy of ∑3,∑5,∑7, ∑9, ∑11, ∑17b and ∑31a have been studied. The results of Ni closely resemble experimentally determined grain boundary energy distribution figures, suggesting that the overall anisotropy of grain boundary energy can be reasonably approximated by the present simple model. Owing to the overlooking of relaxation matter, the absolute values of energy calculated in present model are found to be higher than molecular dynamic-based results by a consistent magnitude, which is 1 J/m2 for Ni. The coverage of present method forms a bridge between atomistic and meso-scale simulations regarding polycrystalline microstructure.

M5B3 Boride at the Grain Boundary of a Nickel-based Superalloy

The grain boundary microstructures of a heat-treated Ni-based cast superalloy IN792 were investigated. The results show that M5B3 boride precipitates at the grain boundary. A special orientation relationship between M5B3 phase and the matrix at one side of the grain boundary is found. At the same time, two M5B3 borides with different orientations could co-exist in a single M5B3 particle as an intergrowth besides existing alone, thus forming orientation relationship between the two M5B3 phases and matrix. This phenomenon could be attributed to the special orientation relationship between M5B3 phase and the matrix.