Fraction Of Special Grain Boundaries Research Articles

Combined effect of special grain boundaries and grain boundary carbides on IGSCC of Ni–16Cr–9Fe– xC alloys

Susceptibility to intergranular stress corrosion cracking in Ni–16Cr–9Fe– xC alloys in 360°C primary water is reduced with increasing fraction of special grain boundaries, i.e. coincident site lattice boundaries (CSLB) and low angle boundaries, and grain boundary carbides. Intergranular stress corrosion cracking (IGSCC) was investigated using interrupted constant extension rate tensile tests in a primary water environment at 360°C. Thermal–mechanical treatments were used to increase the fraction of special boundaries from approximately 20–25% to between 30 and 40%. In a carbon-doped heat, further heat treating was used to precipitate grain boundary carbides preferentially on high-angle boundaries (HAB). Orientation imaging microscopy was used to determine the relative grain misorientations and scanning electron microscopy (SEM) was used to identify specific grain boundaries after each interruption. After each strain increment, the same regions in each sample were examined for cracking. Results showed that irrespective of the microstructure condition, CSLBs always cracked less than HABs. Results also showed that IGSCC is reduced with increasing solution carbon content, and for the same amount of carbon in solution, the addition of grain boundary carbides reduced IGSCC still further. The best microstructure was the one consisting of an enhanced CSLB fraction and chromium carbides precipitated preferentially on high-angle boundaries.

Toward Optimization of the Grain Boundary Character Distribution in OFE Copper

The authors used a two-step (low and high temperature) strain-annealing process to evolve the grain boundary character distribution (GBCD) in fully recrystallized oxygen-free electronic (OFE) Cu bar that was forged and rolled. Orientation imaging microscopy (OIM) has been used to characterize the GBCD after each step in the processing. The fraction of special grain boundaries, special fraction, was {approximately} 70% in the starting recrystallized material. High, moderate, and low temperature processing conditions were employed. The high-temperature process resulted in a reduction in the fraction of special grain boundaries while both of the lower temperature processes resulted in an increase in special fraction up to 85%. Further, the lower temperature processes resulted in average deviation angles from exact misorientation, for special boundaries, that were significantly smaller than observed from the high temperature process. Results indicate the importance of the low temperature part of the two-step strain-annealing process in preparing the microstructure for the higher temperature anneal and commensurate increase in the special fraction. The authors approach is aimed at testing a hypothesis for the mechanism for GBCD optimization in Cu, i.e., it is possible to optimize the GBCD in Cu by selectively removing random grain boundaries through a strain-annealing process. Theymore » begin with a fully recrystallized material, deform it by a crucial amount, then apply a specific heat treatment schedule. The deformation is not intended to be sufficient to induce full recrystallization upon heating, but is intended to localize the deformation energy at random boundaries (since random boundaries are expected to be less efficient at transmitting dislocations than special boundaries). Upon heating, the stored energy near these random boundaries is expected to provide sufficient driving force to rearrange these boundaries into special types.« less

Toward Optimization of the Grain Boundary Character Distribution in Copper by Strain Annealing

ABSTRACTWe have used a two-step (low and high temperature) strain-annealing process to evolve the grain boundary character distribution (GBCD) in fully recrystallized oxygen-free electronic (OFE) Cu bar that was forged and rolled. Orientation imaging microscopy (OIM)[1–4] has been used to characterize the GBCD after each step in the processing. The fraction of special grain boundaries, “special fraction,” was ∼70% in the starting recrystallized material. Three different processing conditions were employed: high, moderate, and low temperature. The high-temperature process resulted in a reduction in the fraction of special grain boundaries while both of the lower temperature processes resulted in an increase in special fraction up to 85%. Further, the lower temperature processes resulted in average deviation angles from exact misorientation, for special boundaries, that were significantly smaller than observed from the high temperature process. Results indicate the importance of the low temperature part of the two-step strain-annealing process in preparing the microstructure for the higher temperature anneal and commensurate increase in the special fraction.

Effect of grain boundary characteristics on mechanical behaviour of quenched aluminium

Tests concerning the mechanical properties of quenched aluminium were carried out on samples having different grain boundary characteristics. In comparison with aluminium of equilibrium structure, the quenched aluminium was found to be characterised by: a greater value of yield point in samples having a low fraction of special grain boundaries, and a lower value of yield point and a reduction of the Lüders strain in samples having a high fraction of special grain boundaries. The effects can be ascribed to the activity of dislocation loops as sources of dislocations or to a change in grain boundary characteristics as a result of annihilation of non-equilibrium vacancies on the grain boundaries.MST/965

Effect of grain boundary characteristics on mechanical behaviour of quenched aluminium

Tests concerning the mechanical properties of quenched aluminium were carried out on samples having different grain boundary characteristics. In comparison with aluminium of equilibrium structure, the quenched aluminium was found to be characterised by: a greater value of yield point in samples having a low fraction of special grain boundaries, and a lower value of yield point and a reduction of the Lüders strain in samples having a high fraction of special grain boundaries. The effects can be ascribed to the activity of dislocation loops as sources of dislocations or to a change in grain boundary characteristics as a result of annihilation of non-equilibrium vacancies on the grain boundaries.MST/965