铁素体-贝氏体双相钢韧性断裂过程中的夹杂物临界尺寸及孔洞生长

Critical Inclusion Size and Void Growth in Dual-Phase Ferrite-Bainite Steel During Ductile Fracture

利用多模态关联成像方法研究了铁素体-贝氏体双相钢韧性断裂过程中局部微结构对孔洞生长的影响。首先使用X射线CT成像技术,从宏观层面量化分析了变形过程中孔洞的生长,并定位典型孔洞的空间坐标。之后对选定的目标孔洞,采用等离子体聚焦离子束(PFIB)进行连续切片三维电子背散射衍射(3D-EBSD)扫描成像,从介观层面研究孔洞周围微观组织对孔洞形核与生长的影响。结果显示,夹杂物周围和贝氏体中均有孔洞形成。尽管有时促使大尺寸孔洞生长的应变比小尺寸孔洞处的应变更小,但相比于在小尺寸夹杂物或贝氏体中形核的孔洞,大尺寸夹杂物导致的孔洞体积更大。进一步对孔洞周围的位错密度研究显示,上述现象可能是由于不同尺寸的夹杂物周围应变梯度不同造成的。孔洞周围的位错密度与诱发孔洞的夹杂物尺寸成反比,存在明显的尺寸效应,表明影响孔洞生长的夹杂物存在一个临界尺寸。利用解析理论模型推测出夹杂物临界尺寸范围为1.85~2.86μm,小于该临界尺寸的夹杂物诱发的孔洞,由于局部变形梯度效应,位错塞积会阻碍孔洞的生长。孔洞的生长是非均匀的且其形状表现出各向异性,孔洞生长形貌与周围晶粒的可变形性相关,可用晶粒尺寸加权的Schmid因子描述。

Ferrite-bainite dual-phase steel is widely used in the automotive industry owing to its high strength and excellent ductility.The impact of inclusions and void growth behavior in dual-phase steel is a major concern among researchers seeking to achieve better mechanical properties.To investigate this,a cross-length-scale multimodal method was employed to study the influence of local microstructures on void growth during ductile fracture of a dual-phase ferrite-bainite steel.During tensile testing,laboratory X-ray computed tomography(XCT)was used to measure the evolution of void volume.3D-electron back scatter diffraction(3D-EBSD)provided information about the voids nucleated at both inclusion particles and bainite phases or their boundaries.Carefully controlled,broad-focused ion beam excavation was performed to reveal a new interface at a specific depth of the voids.Results showed that voids resulting from large inclusions are significantly bigger than either small inclusions or the bainite phase.Large inclusions lead to large voids even when the strain correlated with the growth of those voids is lower.An investigation of the dislocation densities surrounding the voids suggested that they may be related to the strain gradient around the different inclusion sizes.A critical inclusion size estimated to be around 1.85-2.86μm was found below which nucleation occurs but with limited growth.The elevated rate of local dislocation multiplication due to local deformation gradient effects can impede the growth of smaller voids.The growth of voids is heterogeneous,and their shape correlates well with the deformability of the surrounding grains,as indicated by a Schmid factor weighted using the grain size.This weighted Schmid factor explains not only the shape of the voids but also sheds light on the ease of void coalescence based on the microstructures separating the voids.