Microstructure Evolution And Property Regulation Of Gradient Structured Inconel 625 Superalloy

When Inconel 625 superalloys are used in high-temperature,high-pressure,and complex corrosive environments,premature alloy failure occurs due to the mismatch between strength,plasticity/ductility,and corrosion resistance.This may cause safety issues and economic losses,which can significantly limit the use of Inconel 625superalloys in extreme conditions.Therefore,the production of alloys with a high strength,good plasticity and ductility,and excellent corrosion resistance is a key scientific problem that needs to be solved.It is worth noting that the gradient structure induced by surface mechanical treatment is an effective means for increasing the corrosion resistance and improving the strength-plasticity-ductility matching of materials.In view of this,based on the concept of plainified materials,this study used SMGT to build gradient structures and regulate microstructures in alloys without changing the chemical composition of Inconel 625 superalloys.The microstructure evolution and stability of the gradien t structure in alloys and its action mechanism on the mechanical and corrosion resistance properties of the alloy were examined,which provided a theoretical basis and technical support for the integration of strength,plasticity/ductility,and corrosion r esistance of alloys.The main conclusions are as follows:（1）Surface mechanical grinding treatment（SMGT）technique was used to construct gradient nanotwinned（GNT）structure in Inconel 625 superalloys to explore the morphology and organization evolution of alloy microstructures during SMGT.The results showed that a 600μm thick gradient structure layer could be successfully constructed in the superficial layer of the alloy by adjusting the process parameters（v1=400 rpm,v2=8 mm/min,ap=20μm,and P=10）.The TEM characterization revealed that the grain size of the outermost layer of the alloy could be refined to 18 nm,with a thickness of up to 30μm and a bar surface hardness of 7.4 GPa.In addition,with an increase in the layer depth,a high-density twin/matrix（T/M）lamellar structure was formed in the microstructure at high strain rates during the process of plastic-induced grain refinement.Moreover,the original coarse grains were refined.During further deformation,due to the increasing di fficulty of work hardening and the reactions between dislocations and twin boundaries,the lamellar structure was sheared and broken into equiaxed nanocrystals.Subsequently,the nanocrystals were further refined to a smaller size through twinning.During this process,twinning is more likely to occur for a grain size of 71 nm.Finally,a gradient change from nano-grain（NG）and nanotwinned（NT）to dislocation structure（DS）was formed in the alloy structure,and twinning played a major role in the refineme nt.（2）The microstructure evolution,phase precipitation behaviour,and microhardness distribution in different regions of Inconel 625 alloys with gradient structures under thermal action were investigated using SEM,EBSD,TEM,and a microhardness tester to examine the thermal stability of the gradient structures in Inconel 625 superalloys.The results showed that the deformation induction promotes the precipitation of carbides in alloys,and that the carbides have a gradient distribution according to the change in microstructure gradient.Among them,the MC-type carbide precipitation temperature was reduced by 150°C in the superficial NG of alloys.In addition,the alloy gradient structure was destabilized at 800°C and the thermal stability of the subsurface and deeper layers followed a common rule that the recrystallization temperature decreases as the layer depth decreases（strain increases）,and the microstructure becomes more unstable.However,for superficial NG microstructures,there is an anomalous thermal stability due to the pinning effect of several precipitated fine carbides with close spacing and the low-energy grain boundaries generated by the grain boundary relaxation.Their thermal stability is better than that of subsurface NG.（3）The effect of gradient structure and its volume fraction（30–100%）on the mechanical properties of Inconel 625 alloys was investigated using quasi-static tensile experiments,and the strengthening mechanism was revealed.The results showed that the volume fraction of gradient structures has a significant effect on the yield strength and work hardening behaviour of Inconel 625 superalloys.At low strains,the work hardening rate（Θ）of the gradient nanotwinned-coarse grain（GNT-CG）structure is significantly higher than that of the GNT and CG structures alone.Moreover,a higherΘcan be maintained during the subsequent plastic deformation.Meanwhile,the increase in yield strength is mainly attributed to the strengthening of fine grains in the superficial GNT layer and the synergistic strengthening of GNT-CG structures.The high plasticity is maintained mainly due to the inhibition of strain localization in superficial NG microstructures by CG structures and the excellent plasticity of high proportion of NT in the alloy.（4）The microstructure of gradient-structured Inconel 625 superalloys was regulated by a high-temperature short-time annealing process to further improve the corrosion resistance of alloys while maintaining its excellent mechanical properties.Moreover,the influencing mechanism of the boundary regulation on its corrosion resistance was explored.The results showed that hybrid gradient structured Inconel625 superalloys with appropriate annealing twin densities and grain sizes can be successfully prepared by combining surface nanocrystallization and high-temperature short-time annealing（SMGT+1150℃/1-10 min）.At the same time,excellent strength,plasticity,and corrosion resistance（strength-plasticity-corrosion resistance matching）can be obtained for Inconel 625 superalloys at an annealing temperature of1150℃for 1 min.The yield strength,elongation,and corrosion current density are925 MPa,26%,and 3.02×10^-8 A/cm²,respectively.The integration of strength,plasticity/ductility,and corrosion resistance of alloys is realized.