GH2132合金混晶条带组织分析及其对力学性能影响

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

混晶条带是影响GH2132合金组织及性能稳定性的重要因素,因此针对合金冷拉棒材出现的混晶条带缺陷进行了试验研究。利用金相、EPMA、EBSD和TEM手段,结合热力学平衡相图与硬度测试,揭示了混晶条带的主要成因,并分析了其内部微观组织状态及混晶组织对显微硬度的影响。结果表明,混晶条带组织中细晶区晶粒尺寸普遍小于10μm,粗晶区晶粒最大可超过60μm,而元素偏析与冷拉变形是造成晶粒大小差异并形成混晶条带的原因。合金铸态组织中Ti、Mo、C、B元素均表现出正偏析,其中C、Ti元素的偏析程度较高,能够在枝晶间析出MC与M_(3)B_(2)相,并最终遗传至奥氏体晶界;晶界溶质富集不仅起到钉扎作用,阻止再结晶过程晶粒长大,还能够通过溶质拖拽作用降低晶界的迁移速率而阻碍晶界变形,最终溶质富集区域形成细晶区、贫化区域形成粗晶区,并沿合金棒材组织冷拉方向表现出混晶条带。另外,在冷拉变形过程中,由于晶体取向不一,在受同样拉拔力条件下晶粒变形的实际应变并不一致,这也会加剧混晶现象。微观组织观察显示冷变形后细晶区与粗晶区相比形变更为均匀,组织内平均位错密度更高,混晶区域内存在退火孪晶与形变孪晶,但细晶区孪晶密度更高。最终混晶区存在的晶界数量不同、位错密度变化、孪晶分布的梯度组织,导致细晶区显微硬度明显高于粗晶区的硬度梯度。

Band-typed mixed grain structure defect was an important reason affecting the stability of microstructure and properties of GH2132alloy,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 10μm,while the maximum grain size in coarse grain zone is over 60μm.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 M_(3)B_(2)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.