Effect of high pressure on the solid–liquid phase change of a nickel base superalloy during hot isostatic pressing

Effects of high pressure during a hot isostatic pressing (HIPing) on the solid–liquid phase change and the microstructure of a directionally solidified (DS) nickel base superalloy were studied. This alloy was treated under various HIP cycles at temperatures of 1330 or 1340 °C, higher than the liquidus of the alloy. The evolution of the microstructure, composition, crystallographic orientation of the texture and the hardness of the alloy was analyzed for the evaluation of the high isostatic pressure effect. The microstructural analysis revealed that high pressure around 120 MPa has a great influence not only on the metallography but also on the crystallography of the alloy at the processing temperature. The DS grain structure was preserved for some HIPed specimens, suggesting an increase in the melting temperature of the alloy at least by 10 K at 120 MPa. All the findings of this work were elucidated in terms of the change in the Gibbs free energy due to volume changes upon solid–solid and liquid–solid phase transformation and of the migration of constituent elements to mitigate the Gibbs free energy under a high isostatic pressure condition.