Yulai Xu

Thermodynamic study of the critical nucleus size for metadynamic recrystallisation

On the basis of thermodynamics, it is shown that the critical nucleus size for metadynamic recrystallisation is smaller than that for dynamic recrystallisation. It follows that the nucleation rate of metadynamic recrystallisation is not zero. A Cr25Ti steel was hot deformed and its recrystallised structures were examined after delayed quench. It was observed that after the deformation was terminated, both the grain size and the subgrain size decreased at first, although the grains eventually coarsened at longer times. This is attributed to the conversion of potential embryos formed during dynamic recrystallisation into true nuclei in metadynamic recrystallisation. The results from the experiment appear to be consistent with the theoretical analysis.

Improvement of Stress-rupture Life for Modified-HR6W Austenitic Stainless Steel

Stress-rupture life of HR6W austenitic stainless steel modified with B and Mg additions was measured, and the microstructures were analyzed by optical microscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy equipped with energy dispersive spectroscopy The results indicated that the enhancement of the stress-rupture life was mainly due to the precipitation with B in the elemental form at the grain boundaries, and the improvement of the form of carbides at grain boundaries and the removal of O and S elements by addition of Mg. The micro-alloying elements have a beneficial effect on stress-rupture life of the modified-HR6W austenitic stainless steel at high temperature.

Effects of Al on Coherent γ′, Lattice Misfit of γ′/γ and Stress‐Rupture Property of Nimonic 80A

Effects of Al on microstructures and stress-rupture property of Nimonic 80A had been investigated. The γ′ presents a coherent orientation relationship with γ on {100}γ′/γ and {110}γ′/γ. Al increases the fraction of γ′ and lattice misfit of γ′/γ, but the fraction and lattice misfit decrease after stress-rupture tests compare with that after full heat treatment for each alloy, which is primarily due to increased width of γ matrix channel and probably lower thermal expansion coefficient of γ′, respectively. The significant increase of stress-rupture life is mainly due to the increased fraction of γ′ and the strengthening can be explained by Orowan looping mechanism. Increased lattice misfit may partly contribute to the growth of stress-rupture life. Precipitate of discrete Cr23C6 benefits stress-rupture properties, however, continuous grain boundary Cr23C6 can provide easy paths for crack propagation.