Loss of High Angle Boundary Area during Annealing a Cryo-SPD Processed Al-Alloy with a Nano-Scale Lamellar Grain Structure

The grain structure and texture evolution during annealing an Al-0.13%Mg submicron grained alloy, deformed by plane strain compression (PSC) at cryogenic temperatures, has been investigated. On annealing the grain structure coarsened and transformed from lamellar to equiaxed. But, remarkably, the fraction of low angle boundaries (LABs) increased, from less than ~ 25% to ~50% above 300 °C, leading to instability and discontinuous coarsening at higher temperatures. The surprisingly large increase in LAB fraction on annealing is shown to be related to orientation impingement originating from the strong texture present after PSC in liquid nitrogen.

Study of Refined Microstructures after Two-Step Heat Treatments by Means of EBSD Technique on γ-TiAl Alloys

In this work, significant modification of the microstructure of a rolled Ti-45Al-5Nb (at.%) alloy has been achieved by a two-step heat treatment. A combination of high cooling rates for the first step of the treatments with a subsequent annealing at high temperatures, results in refined microstructures where substantial discontinuous coarsening seems to play an important role for the grain refinement. In order to fully understand the transformation mechanisms responsible for such a refinement, Electron Back Scattered Diffraction (EBSD) technique has been extensively used. In particular, a study of orientation relationships between the phases which are present in the transformed microstructures, was performed by this technique. Besides discontinuous coarsening process, the obtained results also suggest that a modified discontinuous coarsening mechanism is operative, in order to justify the formation of novel microstructures, whose development could not be explained by the classical discontinuous coarsening.

Effects of Heat Treatment Condition on the Microstructure Evolution of Inconel 690

The effect of heat treatment on the morphologies of precipitates and grain boundaries of Inconel 690 alloy was studied. When the specimens were slowly cooled from the solutionizing temperature, typical discontinuous coarsening of Cr-rich carbides in grain boundaries was observed. As the cooling rate decreased, Cr-rich carbides grew and wider Cr depletion zone was created between the carbide precipitates, which resulted in wavy grain boundaries. The driving force of the wavy grain boundary was explained by the coherent strain energy.

Effect of B Addition on Thermal Stability of Lamellar Structure in Ti-47Al-2Cr-2Nb Alloys

ABSTRACTThermal stability of fully lamellar microstructures in Ti-47Al-2Cr-2Nb (at.%) alloys with and without B additions has been evaluated in the temperature range of 800 to 1200°C. The alloy with 0.15B exhibits an α2 + γ fully lamellar microstructure containing ribbon-like structures (TiB2+β) inside the lamellar colonies. During aging at 800 and 1000°C, β particles surrounded by γ grains form adjacent to the ribbon-like structures. The formation of these grains is attributed to the precipitation of thin plate-like β phase along {1100} within the TiB2 ribbon particles, resulting in the loss of α2 plates around the ribbon-like structure by scavenging Ti. This allows coarsening γ lamellar plates to form new γ grains, and then the ribbon-like structure completely dissolves to leave only β particles. At 1200°C, discontinuous coarsening (DC) of the lamellae becomes the dominant mode of microstructural change for all the alloys. However, the ribbon-like structures in the alloy with B additions play a role in pinning the DC cells, which leads to higher thermal stability of the lamellar microstructure in that alloy.