Papers by Keyword: Grain Boundary Microstructure

Abstract: The effects of the selective addition of Hafnium (Hf) on the grain boundary, phase, carbides and creep properties of experimented nickel superalloy after standard heat treatment and long-term exposure were investigated. Predicted by the Bayesian neural network, the creep life is prolonged with Hf content of 0-0.6 mass%, which is more effective at low stresses. The decrease of creep life of Hf free alloy after long term exposure was pronounced. Comparative study showed that the mainly small, coherent, blocky and closely spaced MC₍₂₎ and M₂₃C₆ carbides precipitated on the grain boundaries in the 0.4wt% Hf contained alloy, and that relatively larger, incoherent MC₍₁₎ carbides precipitated on the grain boundaries in the Hf free alloy. During long term thermal exposure, fine discrete M₂₃C₆ carbides decomposed from primary carbide, inducing a layer along the grain boundary, and the coarsening of grain boundary in Hf free alloy is more pronounced. At high stresses, the Hf-free alloy exhibited a stronger tendency of rafting than the 0.4Hf alloy, while the tendency of appearance of rafting was very similar at low stresses. However, Hf can render the alloy prone to the formation of σ phase, according to D-electrons method. Thus, the Hf content needs to be controlled to a suitable level.

Abstract: This paper gives an overview of our recent works on the effect of magnetic annealing, i.e. annealing in a magnetic field, on the evolution of texture and grain boundary microstructure in ultra-fine grained and nanocrystalline magnetic materials differently produced; rapidly solidified Fe-6.5mass%Si ribbons, electrodeposited nanocrystalline nickel, and nanocrystalline Fe78Si9B13 alloy ribbon crystallized from the amorphous state. It was found that the effect of magnetic annealing was powerful and useful for controlling grain growth resulting in the evolution of different types of texture and grain boundary microstructure, depending on the condition of magnetic annealing. In particular, the magnetic crystallization of amorphous Fe78Si9B13 alloy was found to be powerful for producing a nanocrystalline material with a sharp texture and a special grain boundary microstructure.

Abstract: In-situ observations of a/g phase transformation were made to study the effect of grain boundary microstructure of the generation of a new phase and the migration of a/g interphase boundaries in an Iron-4.2at.%Cr alloy. It was found that triple junctions with more random boundaries could be the primary nucleation sites, while triple junctions with low angle and low S coincidence boundaries did not play a role as preferential sites. The migration of a/g interphase boundaries during heating across the transformation temperature showed the two stage behaviour characterized first by a stage with a migration velocity of 0.33-0.75µm/s and secondly a stage with 3.7-7.6 µm/s. It was also found that abnormal grain growth and a high density of S3 coincidence boundaries could occur in the a/bcc phase after cycling of a/g/ a phase transformation.