Hastelloy C-276合金多道次退火晶粒团簇组织的演变

Microstructure Evolution of Grain Cluster in Hastelloy C-276 Alloy during Multi-Path Annealing

采用电子背散射衍射(EBSD)技术研究了固溶处理后Hastelloy C-276合金,在5%变形条件下,不同温度多道次退火热处理中Σ3^(n)(n=1,2,3…)晶粒团簇及晶界特征分布演变规律。结果表明,两步退火热处理可以明显改善合金晶界特征分布,小变形样品经1000℃低温退火5 min后,再进行1100℃高温退火15 min热处理,晶粒组织发生再结晶,产生较多孪晶组织,形成内部互有Σ3^(n)取向关系的大尺寸晶粒团簇组织,Σ(9+27)/Σ3达到了0.125,特殊晶界比例达到78.8%,晶界特征分布得到优化。初始退火温度较高会使得变形储存能迅速释放,大角度晶界快速迁移,不利于产生Σ3退火孪晶晶界;而较长时间二次退火也不利于晶界反应,使得Σ9和Σ27晶界比例降低。退火过程中产生较多Σ9和Σ27晶界是晶界特征分布优化的关键。而过高的初始退火温度或过长的二次退火时间,抑制或消除了退火孪晶的产生,无法形成大尺寸晶粒团簇组织。

Hastelloy C-276 was a type of Ni-Cr-Mo superalloy,and had been widely used as one of critical components in aerospace,chemical and nuclear industries because of its excellent corrosion resistance to acids and high temperature strength.Intergranular corrosion was a serious damage phenomenon in nuclear industries under complex working environments.Especially,it usually occurred at intersection sites of grain boundaries in materials,and thus led to material failure.Since intergranular corrosion among grain boundaries was a key factor to impact the lifetime of high temperature service of superalloys,how to improve the resistance to intergranular corrosion in superalloys had become an important scientific research topic.As we had known,grain boundaries played an important role in material properties.Furthermore,a so-called“grain boundary engineering(GBE)”had aimed at improving the resistance to intergranular corrosion by optimizing the grain boundary character distribution(GBCD).For example,the so-called“coincidence site lattice(CSL)”boundaries with higher ordering degree,lower interface energy and stronger resistance to grain boundary failure etc.were increased by means of the thermal-mechanical processing in the GBE,in which the low-Σ CSL boundaries(Σ≤29)were called“special grain boundaries”.Therefore,the GBE had been regarded as an effective technology for various superalloys with low stacking fault energy by increasing the fraction of special grain boundaries in materials.Because of those special grain boundaries,some highly twinned grain clusters with large sizes were typical microstructures produced by the GBE,which was the main reason of enhancing the intergranular corrosion resistance.Thus,in superalloys,all of grains would possess Σ3^(n) mutual misorientations regardless whether they were adjacent or not.Moreover,along the penetration path in grain clusters,intergranular corrosion would stop its forward step if it encountered a triple junction composed of special grain boundaries.While it might penetrate into random boundaries outside grain clusters with large sizes,and spend more time in doping these grain clusters.Regarding grain clusters formed by the multiple twinning,they usually started from a single nucleus during the thermal-mechanical processing.And then,many of these nuclei would grow up along with the multiple twinning until they occupied the whole sample space.By means of recrystallization,randomly large-angled grain boundaries whose motions would consume the stored strain energy to generate more Σ3^(n) grain boundaries in the GBCDs could be found in samples.And,the high proportion of special grain boundaries could be observed from the formation of large grain clusters.According to these,for the Hastelloy C-276 alloy with 5% deformation under the multi-pass annealing at different temperatures in solution treatment,evolution of Σ3^(n) oriented grain-clusters and GBCDs were analyzed by means of the electron back scatter diffraction(EBSD).The results indicated that,for superalloys synthesized through recrystallization after the multi-path annealing,although the average size of grains had not changed significantly,characteristics of grain clusters had changed obviously.Thus,the GBCDs in superalloys could be improved by a reasonable two-step annealing treatment.However,a higher initial annealing temperature and a longer secondary annealing time were not good for reactions of grain boundaries since they might decrease the proportion of special grain boundaries in materials.Therefore,during the multi-path annealing,how to generate more Σ9 and Σ27 grain boundaries would be the key to optimize the GBCDs.For example,in a low-strain sample after the annealing treatment at a lower temperature of 1000 ℃ for 5 min and then a higher temperature of 1100 ℃ for 15 min,microstructures in superalloys through recrystallization would generate more twins,i.e.,the ratio of Σ(9+27)to Σ3 may reach a peak point of 0.125,special grain boundaries may reach 78.8% percentage,and large-sized grain clusters with inner mutual Σ3^(n) orientations was generated to reach the average size of 120 μm,meaning that the GBCDs were completely optimized to improve the intergranular corrosion resistance in superalloys.However,if the initial annealing temperature was pretty high,it would release the stored strain energy.And then,the large-angled grain boundaries would move quickly,but did not contribute to form Σ3 annealing twin boundaries in superalloys;If the secondary annealing time was longer,it could not promote interactions of special grain boundaries and decrease the proportion of Σ9 and Σ27 boundaries in superalloys.As results,more Σ9 and Σ27 boundaries would play key roles in disruption of connectivity for large-angled grain boundaries.So comprehensively,a higher initial annealing temperature or the longer secondary annealing time might inhibit or eliminate the generation of the annealing twins,and thus could not produce large-sized grain clusters.Thus,the initial annealing and the secondary annealing treatments should adapt to the evolution of GBCDs by optimizing some reasonable parameters to induce more interactions among special grain boundaries to produce more large-sized grain clusters.The final conclusion was that,the above thermal-mechanical processing might help the Hastelloy C-276 superalloy perform as one of nuclear critical components to enhance the safety of nuclear industry.