Microstructural Evolution And Electron Microscopic In Situ Study Of The Nickel-based Single Crystal High Temperature Alloy CMSX-4

Nickel-based single crystal high temperature alloys have many excellent properties and are widely used in the advanced manufacturing industry and in the aero-engine sector.In recent years,the design requirements of advanced manufacturing industries have become increasingly demanding and the performance requirements of aero-engines have also increased.In order to meet the requirements of modern high-tech development,to meet the leading needs of the industry and to be able to design alloys with better performance in the future,this paper selects a nickel-based single crystal high-temperature alloy of type CMSX-4 as the research material and uses transmission electron microscopy combined with three-dimensional reconstruction,in-situ by simulating the contact diffusion process between the cooling medium Sn elements and the casting in the liquid metal cooling method,the microstructural changes of the diffusion phenomenon and the improvement of the mechanical properties were analysed,and the evolution of the alloy with temperature during in-situ heating and the elemental segregation phenomenon were studied,providing some key experimental data,which are summarised as follows:1.The effect of heat treatment processes on the organisation of the alloy.With 1280°C/1h+1290°C/2h+1300°C/6h AC（air cooling）+1140°C/4h AC+870°C/16h AC multi-stage solution heat treatment,the alloy produces holes,which tend to accumulate and increase in size as the ageing time increases;with 1320°C/3h AC+1140°C/4h AC+870°C/16h AC With the super-primary melting point heat treatment system,the alloy avoids holes but precipitates TCP（Topologically closed-packed）phases with a predominantly plate-like and needle-like morphology,which are identified by selected area electron diffraction.With the heat treatment system,the alloy effectively avoids the precipitation of the TCP phase and improves the tendency of the holes to grow in an irregular direction when the aging time is prolonged in the multi-stage solution heat treatment system;all three heat treatment systems are able to obtain 60%-70%of the strengthened phases after two-stage aging treatment,and the degree of cubicization and homogenization of the strengthened phases is higher.2.Selective diffusion of the element Sn.The single crystal alloy has two microstructures,theγphase and theγ’phase,which are co-located.Its preparation is mainly through a liquid metal cooling oriented solidification process,while Sn,as a cooling medium,diffuses in contact with the casting and is distributed in theγ-phase.According to calculations related to the first nature principle,the energy barrier for Sn diffusion inγ’-Ni₃Al（0.56 e V）is approximately 2.4 times higher than inγ-Ni（0.23e V）when Sn atoms occupy the octahedral gap between theγandγ’phases.In addition,the lattice distortion of the Sn atoms diffusing in theγ-Ni phase is smaller,resulting in a weaker barrier and easier diffusion.As a result,Sn exhibits a diffusion tendency towards theγ-phase of the alloy.3.The effect of Sn element diffusion on the mechanical properties of the alloy.After Sn diffusion into theγmatrix,theγphase region becomes amorphous and the atoms are in a disordered state because of rapid cooling and solidification,which in turn improves the toughness of the alloy,which affects the mechanical properties of the alloy.Compared to the original CMSX-4 alloy sample,after macroscopic nanoindentation tests,the hardness and elastic modulus of the areas with higher Sn concentration at the alloy interface are increased after Sn diffusion.From microscopic in-situ compression experiments,the CMSX-4 alloy underwent elastic-plastic deformation first and had a lower yield strength.In addition,the alloy after Sn diffusion was able to improve to some extent the process hardening phenomenon that occurred first in the original CMSX-4 alloy at an advance displacement of 400 nm.4.The effect of temperature on the evolution of the alloy structure and the enrichment of elemental segregation.In combination with macroscopic ex situ heat treatment experiments,the alloy exhibits three main temperature intervals of tissue changes during in situ heating.At 600°C-700°C,the increase in Al,Ta,etc.content at the channel and the decrease in Cr,Re,etc.content lead to the formation of high supersaturation and the precipitation of secondaryγ’phases;at 750°C to 850°C,large size polymers enriched in Cr,Re,W,Ta,etc.penetrate the matrix and strengthening phases of the alloy,and in addition,the choice of a drastic heating rate,e.g.100°C/min.The alloy precipitated pin-rod-like morphology and plate-like morphology at 500°C-700°C,which were further identified as TCPμ-phase andσ-phase;at 950°C,the diffusion of alloy elements was intense,and the reinforcing phase underwent directional coarsening due to high temperature thermal activation,resulting in the formation ofγ’phase andγ-phase inversion,and the alloy underwent rafting.The above experimental phenomena and conclusions,using transmission electron microscopy to study the microstructure of the alloy,can help to understand the structure of the alloy more finely and provide more meaningful data to support the regulation and improvement of alloy properties.