Microstructure analysis of GH2132 alloy band-typed mixed grain structure and its effect on mechanical properties

Band-typed mixed grain structure defect was an important reason affecting the stability of microstructure and properties of GH2132 alloy, so the band-typed mixed grain structure with cold drawn bars were experimentally studied in this article. By means of metallography, EPMA, EBSD and TEM, combined with thermodynamic equilibrium phase diagram and hardness test, the main causes of band-typed mixed grain structure were revealed, as well as the analysis of its internal microstructure and the effect on microhardness. The results show that the grain size of fine grain zone in band-typed mixed grain structure is generally less than 10 μm, while the maximum grain size in coarse grain zone is over 60 μm. It is analyzed that element segregation and cold drawing deformation are the causes for the difference of grain size, and the forming of band-typed mixed grain structure. Ti, Mo, C and B elements in the as-cast structure of the alloy show positive segregation, among which the segregation of C and Ti element is greater, leading to the precipitation of MC and M₃B₂ phases between dendrites, which will finally appear between the grain boundaries in austenite. The solute enrichment between grain boundaries not only plays a pinning role to prevent the grain growth during recrystallization, but also reduces the migration rate of grain boundary through solute drag, which hinders the grain boundary deformation. Finally, the solute enrichment area forms a fine grain zone, with the depleted region forming a coarse grain region. Both display band-typed mixed grain structure along the cold drawing direction. In addition, due to the different crystal orientation, the actual strains of grain deformation under the same drawing force are not consistent during cold drawing deforming, which will also aggravate the mixed grain phenomenon. The microscopic observation shows that after cold deformation, the deformation of fine grain region is more uniform compared with the coarse grain region, and the average dislocation density in the microstructure is higher. Annealing twins and deformation twins are observed in the mixed grain region, while the twin density in the fine crystal region is higher. To sum up, the different numbers of grain boundaries, with the change of dislocation density and the gradient structure of twin distribution finally lead to the microhardness gradient in the mixed grain region, where the hardness of fine grain region is significantly higher.