一种新型Ni-Cr-Co基变形高温合金的均匀化工艺

Homogenization of a New-Type Ni-Cr-Co Based Wrought Superalloy

利用光学显微镜(OM)、扫描电子显微镜(SEM)、电子探针(EPMA)、能谱仪(EDS)和JMatPro热力学软件对采用真空感应熔炼(VIM)和电渣重熔(ESR)双联工艺冶炼得到的Ni-Cr-Co基变形高温合金铸锭进行微观组织、析出相和元素偏析研究,并且运用JMatPro计算评价不同的均匀化处理工艺,结合均匀化后的组织分析、元素偏析消除程度,最终确定了最佳的均匀化制度。结果表明,铸锭存在明显的树枝晶和元素偏析,各元素的偏析程度大小为:Ti>W>Al>Co>Cr,其中Ti元素偏析最严重,W元素次之,偏析系数(K)分别为2.37和0.62;Ti元素在枝晶间大量富集促进针状η相的形成,并且枝晶间处的γ′相尺寸明显大于枝晶干处,其大小分别为96和55 nm。JMatPro理论计算表明,在1180℃保温24 h后元素基本已经扩散均匀;在均匀化退火过程中,随保温时间的延长,树枝晶和元素偏析逐渐被消除,η相回熔,并且合金塑性大幅度提高,其延伸率达到40%左右,约为铸态合金的两倍;实验结果证明JMatPro能够很好地预测退火处理消除枝晶偏析的程度,在均匀化处理24 h之后元素的偏析已经大部分被消除,KTi和KW分别为1.11和0.87;将保温时间延长至40 h后,Ti和W元素基本扩散均匀,KTi=1.03,KW=0.93,合金晶粒尺寸均一且无异常长大。

The development of highly alloyed tendency of nickel-based wrought superalloys is prone to bring disadvantages such as poor microstructure stability and large deformation resistance while improving the strength of the alloy.Therefore,a new-type Ni-Cr-Co based wrought superalloy with low alloying and low stacking fault energy was designed based on the concept of plainification,which was used as a synergistic strengthening model alloy for studying plainification on the basis of structure of"phase and nanotwin".Consequently,the combination of strength and plasticity of the alloy could be achieved instead of alloying method.The alloy contained arelatively high content of Tielement that is easy to segregate.The m phases could be precipitated when Ti is enriched seriously during solidification,which would accelerate the formation and propagation of cracks and greatly reduce the service life,and indirectly weaken the strengthening effect of alloying elements.Therefore,the ingot might be homogenized to eliminate dendrite segregation and improve the plastic processing ability of the alloy,which was prepared for controlling high density twins of the new-type Ni-Cr-Co based wrought superalloy.This work was combined JMatPro simulation calculation with systematic experiments to explore the appropriate homogenization treatment.The microstructure and microsegregation of Ni-Cr-Co based wrought superalloy ingot prepared by vacuum-induction-melting(VIM)and electroslag-remelting(ESR)were studied by optical microscopy(OM),scanning electron microscopy(SEM),electron probe micro analysis(EPMA),energy-dispersive X-ray spectroscopy(EDS)and JMatPro thermodynamic software.The segregation degree of each element was characterized by the segregation coefficient(K).The homogenization module of JMatPro was used to predict the elimination degree of element segregation after homogenization at 1180°C for different time.Based on the calculation results,the homogenization treatments were carried out at 1180℃ for different time with the interval of 8 h,the evolution law of dendrite structure,precipitated phases,and the elimination of element segregation were observed.In addition,the effect of homogenization treatment on the strength and plasticity of as-cast alloy was investigated by comparing the tensile properties of homogenized specimens with as-cast ones.Finally,the most appropriate homogenization annealing process was determined by combining grain size and the elimination of element segregation.The results showed that dendrite and element segregation existed in the ingot,and these elements displayed different segregation degree as the following sequence:Ti>W>Al>Co>Cr.Ti was the principal segregated element,and the segregation coefficients of Ti and W elements were 2.37 and 0.62,respectively.There were nubbly TiC and needle-like m phases in the microstructure.Moreover,the average size of'phases at interdendritic region(96 nm)was significantly larger than that at dendritic region(55 nm).JMatPro calculation results indicated that the elements segregation were basically eliminated at 1180℃ for 24 h.With the increase of homogenization time,the contrast of dendrites decreased and n phases dissolved to the matrix,and the elements also diffused more evenly.In addition,the plasticity and deformability of alloy could be improved obviously after homogenization treatment,and the elongation was about 40%,which was about twice as much as that of as-cast alloy.The fracture morphology of as-cast alloy was changed from cleavage fracture into dimple fracture with the increase of homogenization time.The experimental results showed that the element segregation was basically eliminated after homogenization for 24 h,and Kr,and Kw were 1.11 and O.87 respectively.This experimental value of segregation coefficient was essentially in accordance with the theoretical value.The errors of Kr,and Kw were 11%and 4%respectively,which indicated that JMatPro thermodynamic software could well predict the effect of homogenization treatment on eliminating element segregation.The elements of Ti and W almost diffused uniformly with K.=1.03 and Kw=0.93 after annealing at 1180°C for 40 h.Besides,grain size was uniform without abnormal coarsening after homogenization treatment for 40 h.From the analysis results of the law of element segregation in as-cast alloy,the effect of homogenization treatments on the microstructure and properties of as-cast alloy,and evaluation of JMatPro calculation results of homogenization treatments,this work could be summarized as the following three important conclusions:(1)JMatPro simulation calculation and experimental results showed that Ti element was seriously enriched in the interdendritic region,which promoted the formation of needle-like n phases.Additionally,the average size of phases at interdendritic was larger than that at dendritic region due to the high content of Ti element in the interdendritic region.(2)After homogenization treatment,the element segregation of as-cast alloy was eliminated gradually and the plasticity of the homogenized alloy could be greatly improved.Considering grain size and the elimination of element segregation,the appropriate homogenization treatment was finally determined as 1180°Cx40 h.(3)JMatPro could well predict the segregation of element in as-cast alloy and the effect of homogenization treatment on eliminating element segregation.