【作者】 方晓英；

【导师】 周邦新； 王卫国；

【作者基本信息】 上海大学 ， 材料学， 2009， 博士

【摘要】 本文利用电子背散射衍射（EBSD）技术、扫描电子显微镜（SEM）、光学显微镜研究了轧制和退火以及初始状态对304奥氏体不锈钢晶界特征分布（GBCD）的影响。采用基于EBSD的单一截面迹线分析法对合金GBCD中的∑3晶界进行了共格与非共格的区分测定。在了解合金轧制退火过程中合金GBCD演变基本规律的前提下,通过对合金组织“原位”跟踪观察,进一步阐明了合金实现GBCD优化的微观机制。最后通过晶界腐蚀实验比较了各种晶界的腐蚀抗力。冷轧退火对合金的特殊晶界比例f_SBs和GBCD产生重要影响。研究发现,小形变冷轧后较比中等形变冷轧后的合金样品在随后的退火过程中更容易诱发高比例（超过80%）的∑3ⁿ（n=1,2,3）晶界,而且f_∑9+∑27晶界与f_∑3的比值f_{∑9+∑27/∑3}超过0.1,而中等形变（20%～50%）后的样品退火后该值不超过0.05。表明小形变量是促进304奥氏体合金在后续退火过程中广泛发生∑3ⁿ孪晶反应的必要条件。对于小形变后在相对低温长时间退火处理（GBE1）,随着退火时间的延长,合金中∑3ⁿ（n=1,2,3）晶界比例f_SBs和f_{∑9+∑27/∑3}逐渐提高,最高值分别达80%和0.2。而相同形变冷轧的样品经高温短时退火（GBE2）后,上述两值随退火时间的延长先增后减。由∑3ⁿ晶界构成的特殊晶界团是GBCD优化了的合金的典型组织特征,小形变冷轧后的样品经低温长时间退火（GBE1）和高温短时间退火（GBE2）处理后,前者的特殊晶界团尺寸明显大于后者,且其一般大角度晶界网络连通性被阻断的效果也优于后者。在已实现GBCD优化的合金样品中,其特殊晶界团内存在大量的非共格∑3晶界。另外还发现合金经小形变冷轧后在两步退火GBE工艺中,样品首先在相对低温下短时退火有利于合金在第二步退火过程中GBCD的再优化,而多道次循环冷轧退火处理对合金的GBCD优化没有显著影响,仅造成残留亚结构分布的变化。利用SEM和EBSD技术研究了合金的初始状态包括晶粒尺寸和碳化物对合金最终GBCD优化效果的影响。研究结果表明,时效状态或碳化物的存在通过影响晶界的迁移行为对合金的GBCD优化具有负面效应,碳化物的充分溶解或防止碳化物的晶界析出是合金实现GBCD优化的必要条件之一。原始晶粒尺寸对GBE处理后合金的GBCD也产生一定的影响。当原始晶粒取向随机分布时,细晶组织的GBCD优化效果好于粗晶组织。通过对合金组织“原位”跟踪观察,了解了合金GBCD演变过程,进而分析阐明了合金实现GBCD优化的微观机制。小形变冷轧后在相对低的温度下退火可诱发某些原有晶界的优先迁移,并伴以特殊晶界团的形成,特殊晶界团内部通过非共格∑3晶界的迁移反应促进∑3ⁿ晶界的大量增殖,其周围的大角度晶界在应力（应变）梯度作用下的优先迁移以及通过与附近晶界的相互作用形成∑3ⁿ（n=0,1,2）和非∑3ⁿ的低Z-CSL晶界,从而实现一般大角度晶界网络连通性的阻断。进一步的分析得出非共格∑3晶界的大量形成和某些晶界面的优先迁移是合金实现GBCD优化的主要微观机制。最后通过晶界腐蚀实验评价了各种晶界的腐蚀抗力,发现∑1和∑3（包括共格与非共格）晶界表现出优异的腐蚀抗力。约有50%的∑9晶界和少数一般大角度晶界在腐蚀抗力上表现出特殊性,分析表明晶界的这种特殊性不仅和构成该晶界两侧晶粒的取向差有关还和晶界所处的晶面位置密切相关。本文的创新点主要为:（1）利用基于EBSD的单一截面迹线分析法对合金GBCD中的∑3晶界进行了共格与非共格的区分判定,发现在被优化的合金GBCD中,弯曲的非共格∑3晶界是主体,其它∑3ⁿ（n=2,3...）晶界是非共格∑3晶界迁移反应所致。（2）通过合金显微组织的原位跟踪观察,了解了合金在冷轧退火过程中GBCD演变以及特殊晶界团形成和扩大的规律。认为某些晶界面在应力作用下的优先迁移并伴以非共格∑3晶界的形成是特殊晶界团不断长大和∑3ⁿ晶界不断增加的重要原因。进而提出了非共格∑3晶界的形成和某些原来的晶界面在应力作用下的优先迁移是合金实现GBCD优化的主要微观机制。更多还原

【Abstract】 The effects of strain-annealing treatments and initial microstructures on the grain boundary character distribution（GBCD） in 304 stainless steel were studied by electron backscatter diffraction（EBSD）,scanning electron microscopy（SEM） and optical microscopy（OM）.The single-section trace analysis method based on EBSD was employed to distinguish statistically the incoherent and coherent∑3 boundaries in the samples as processed.The mechanism of GBCD optimization was clarified based on the in-situ observation on the GBCD evolution at the different stages of strain-annealing treatments.Finally,the corrosion characteristics of various grain boundaries were assessed and compared by intergranular corrosion testing.The processing varieties of cold-rolling and annealing have a significant influence on the fraction of special grain boundaries(f_SBs) and the GBCD.It is found that low strain（6～10%） followed by annealing can produce more∑3ⁿ（n=0,1,2,3...） grain boundaries（more than 70%） than for intermediate strain（20～50%） followed by annealing does.The trend is also true for the ratio of fraction summation of∑9+∑27 boundaries(f_∑9+∑27) to the fraction of∑3 boundaries (f_∑3),i.e f_{（∑9+∑27/∑3）}.The averaged f_{（∑9+∑27/∑3）} is 0.13 in the case of low strain（6%） followed by annealing and only 0.05 in the case of intermediate strain（20～50%） followed by annealing.As to the dependence of the f_SBs and f_{（∑9+∑27/∑3）} on the annealing time,it shows some variations for different grain boundary engineering（GBE） processes.In the GBE1 process which involves low strain（6%） and subsequent long-time（24～96 h） annealing at relatively low temperature（900℃）,the f_SBs and f_{（∑9+∑27/∑3）} are increasing with the annealing time prolonged. Upon annealing for 96 h,their value hit the record of 80%and 0.2,respectively.However,in the GBE2 process which involves low strain（6%） and subsequent short-time annealing at high temperature（1050℃）,the f_SBs and f_{（∑9+∑27/∑3）}were increasing firstly and then decreasing with the annealing time.The special grain boundary cluster with large Size（more than 200μm） appeared as the typical microstructure feature in the optimized GBCD.The larger cluster（nearly 1mm in dimension） has developed with connectivity of HABs network intorrupted substantially in the 304 stainless steel specimens treated by GBE1 method.In such cluster,densely-populated∑3ⁿ（n=1,2,3...） grain boundaries emerge.As the majority of∑3ⁿ grain boundaries,∑3 boundaries have been measured by the single-section trace analysis method based on EBSD.It is found that the major part of∑3 boundaries is incoherent.The two-step annealing treatment applied to the 304 stainless steel specimens previously treated by 6%cold-rolling is evidenced to be effective for GBCD evolution.The first annealing at relatively low temperature（900℃） for short time（1 h） contributes a lot to the final optimization of GBCD.However,multi-cycle treatment didn’t show different effects on the GBCD apart from singe cycle treatment except it brings some change to the residual sub-structure population.The effects of carbide and grain size in the starting state on the GBCD are also investigated by SEM and EBSD technology.The results show that presence of carbide in the initial specimen plays a negative role in the GBCD optimization.When the initial random orientation distribution was obtained,it is suggested that the finer microstructure is better for GBCD optimization.Based on the in-situ microstructure observations,the GBCD evolution process is revealed and the mechanism of GBCD optimization clarified as well.The strain-induced grain boundary migration occurs during annealing in GBE1 treated 304 stainless steel.Accompanying with this process,the∑3ⁿ grain boundary clusters come into being,within which∑3ⁿ grain boundaries are multiplied significantly through the migration and interaction of incoherent∑3 grain boundaries. Meanwhile,the preferentially migrated grain boundaries can modify the neighboring grain boundaries and promote∑3ⁿ（n=0,1,2,3） and non-∑3ⁿ low∑CSL boundaries,which in return break the connectivity of the HABs network effectively.Further discussion points out that the formation of a large number of incoherent∑3 grain boundaries and strain-induced grain boundary migration might be the mechanism of GBCD optimization based on the twin-induced GBE.Finally,the corrosion behaviors of random and special grain boundaries are compared in a sensitized type 304 stainless steel previously treated by particular GBE process.The results show that∑1 and∑3（incoherent and coherent） grain boundaries exhibit better intergranular corrosion resistance.Meanwhile,nearly half of∑9 grain boundaries and a small number of random HABs possess good intergranular corrosion resistance,which indicates that the specialness of grain boundary is not only related to the misorientation of the neighboring grains but also related to the grain boundary plane location.The main achievements include:（1） The incoherent and coherent∑3 boundaries are distinguished statistically by the single-section trace analysis method based on EBSD.It is found that the major part of ∑3 boundaries is incoherent in the optimized GBCD.The high order∑3ⁿ boundaries are produced via the migration and interaction of incoherent∑3 boundaries.（2） Based on the in-situ microstructure observations,the evolution process of GBCD are elucidated in detail.It is pointed out that the formation of incoherent∑3 boundaries and the strain-induced preferential migration of particular grain boundaries are responsible for the GBCD optimization based on twin-induced GBE.更多还原