Sub-texture in adiabatic shear bands from arc additively manufactured stainless steel under dynamic loads
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摘要: 绝热剪切失效是增材制造金属材料在高应变率载荷下的重要失效方式。使用电火花从冷金属过渡电弧增材技术制备的316L不锈钢单壁上沿着制造方向和扫描方向割出动态加载圆柱试样（尺寸为
∅ 4 mm×4 mm）。采用分离式霍普金森杆对增材制造316L试样在应变率4000到6000 s−1下加载至绝热剪切状态，研究了其动态剪切变形行为特别是剪切带内微观组织特征结构。不同应变率动态加载下，电弧增材制造316L不锈钢的动态应力首先由于应变硬化而增大，随后绝热剪切热软化与应变硬化的平衡导致了动态变形最后阶段的应力平台效应。绝热剪切带中亚晶经历了动态再结晶过程，具有与基体完全不同的等轴晶形貌，晶粒尺寸大约在200~300 nm。动态剪切复杂热力过程导致剪切带内的亚晶形成了双重织构，既有与基体一致的沿着压缩方向的<110>丝织构，也有与宏观剪切方向相关的晶体学织构，即(111)沿着宏观剪切面，<112>沿着宏观剪切方向。不同剪切带的等轴亚晶都有大量残余Σ3 60°晶界，同时存在与基体相同的孪生织构，可以证明孪生再结晶是绝热剪切带内亚晶主要的动态再结晶机制。宏观绝热剪切带发展路径沿着压缩面35°的对称路径发展，这除了符合动态加载下试样中最大应变和热场分布的外加物理条件，还符合剪切面(111)与基体(110)面交角为35.2°的晶体学条件。此外，基体中存在大量微观局部变形带来承载应变，微观局部变形带内亚晶也具有与基体孪晶组织不同的位向和形貌。Abstract: Adiabatic shearing is a common failure mechanism for additively manufactured metals and alloys under dynamic loads. Cylindrical samples (∅ 4 mm×4 mm) along building and scanning directions were extracted from 316L stainless steel plate fabricated by cold metal transfer wire and arc additive manufacturing process (AM 316L). Cylindrical AM 316L samples were subjected to dynamic impacts to introduce adiabatic shear bands (ASBs) at high strain rates from 4000 to 6000 s−1 by using a split Hopkinson pressure bar. Deformed AM 316L samples were cut along compression direction. Multiple methods including scanning electron microscope, electron-back-scatter diffraction, focused ion beam, transmission electron microscope, transmission kikuchi diffraction were applied to characterize the microstructure of ASBs. The dynamic flow stress of AM 316L increases with forward strain due to strain hardening at first, and then comes an obvious flat stage for the balance between adiabatic thermal softening and strain hardening followed by adiabatic shearing prevailing causing the last failure. The sub-grains in ASBs experienced a dynamic recrystallization process, present fully distinct equiaxed crystal morphology with high angle grain boundaries from the matrix, of which the grain size is about 200−300 nm. The complex thermal and mechanical processes during adiabatic shearing lead to the formation of duplex components in sub-texture, which conclude not only the <110>-fiber along the compression direction similar with the matrix, but also the crystallographic texture related to shear direction with plane (111) along shear plane and orientation <112> along shear direction. The residual large amount of Σ3 60° grain boundaries and twin-symmetry texture in ASBs prove that twinning recrystallization is the main dynamic recrystallization mechanism. The ASB propagating paths of AM 316L along different directions under dynamic loadings are the similar, which is that both ASBs successively extend along the symmetrical path of angles 35° with respect to the loading surface. These two paths are the locations of the maximum strain and thermal distribution during the dynamic loadings consistent with previous simulation work. In addition to the external physical conditions of the maximum strain and thermal field distribution in the sample under dynamic loading, the paths conform to the crystallographic condition that the intersection angle between the shear plane (111) and the matrix (110) is 35.2°. Accompanied with macro adiabatic shear bands, micro-strain localization bands are formed to accommodate more strain, wherein the sub-grains take distinct orientation from matrix. -
图 1 电弧增材制造单壁316L圆柱试样取样示意图
Figure 1. Schematic diagram of how cylindrical arc additively manufactured 316L samples for impact tests were extracted
图 2 电弧增材制造316L试样动态压缩前后的宏观照片以及绝热剪切带EBSD扫描位置
Figure 2. Macrostructure of untested and incompletely fractured arc additively manufactured 316L samples as well as insert indicating facet of EBSD scanning for ASBs
图 3 电弧增材制造316L单壁不同视角EBSD反极图
Figure 3. EBSD IPF maps of as-built arc additively manufactured 316L plate from different planes
图 4 动态加载记录的波形图和换算的应力-应变曲线
Figure 4. Recorded wave and corresponding calculated stress-strain curves from dynamic compressions
图 5 沿着扫描方向的试样1在应变率4700 s−1的动态加载后初始绝热剪切带发展形貌
Figure 5. Initial ASB morphologies from 316L sample 1 in scanning direction under dynamic compression at 4700 s−1
图 6 沿着沉积方向试样2在应变率5700 s−1的动态加载后绝热剪切带整体及不同位置形貌
Figure 6. Typical morphology of ASBs from 316L sample 2 in building direction under dynamic compression at 5700 s−1
图 7 不同绝热剪切带微区晶粒EBSD形貌以及相应空间位向图
Figure 7. Multiscale EBSD IPF maps of different ASBs and corresponding calculated ODF maps from sub-grain orientation data
图 8 微观局部变形带D区域EBSD、TKD反极图和相应的TEM形貌和衍射图
Figure 8. EBSD IPF, TKD IPF and TEM maps from strain localization of area D
图 9 微观局部变形带E区域EBSD反极图以及TEM形貌和衍射图
Figure 9. EBSD IPF and TEM maps from strain localization of E area
图 10 不同绝热剪切带及应变局域带亚晶晶体学特征方向
Figure 10. Grain orientation in different areas from samples undergo adiabatic shearing
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