Rediscovery of Hall-Petch strengthening in bulk ultrafine grained pure Mg at cryogenic temperature: a combined in-situ neutron diffraction and electron microscopy study
Rediscovery of Hall-Petch strengthening in bulk ultrafine grained pure Mg at cryogenic temperature: a combined in-situ neutron diffraction and electron microscopy study
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DOI：
10.1016/j.actamat.2022.118243
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发表时间：
2022-08
期刊：
影响因子：
9.4
通讯作者：
Ruixiao Zheng;W. Gong;Jun-ping Du;Si-feng Gao;Maowen Liu;Guodong Li;T. Kawasaki;S. Harjo;Chao-Li Ma;S. Ogata;N. Tsuji
中科院分区：
文献类型：
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作者：
Ruixiao Zheng;W. Gong;Jun-ping Du;Si-feng Gao;Maowen Liu;Guodong Li;T. Kawasaki;S. Harjo;Chao-Li Ma;S. Ogata;N. Tsuji
根据Hall-Petch关系，细化可以导致金属材料的强化。然而，我们最近的结果表明，块体超细晶(UFG)纯镁在室温下的变形方式主要是晶界滑移，导致软化。在这里，我们首次报道了在低温下，块状UFG纯镁可以恢复Hall-Petch强化。77K时，平均晶粒度为0.6μm的超细镁粉具有超高的抗拉屈服强度和最大抗拉强度，分别为309 Mpa和380 Mpa。电子显微镜和原位电子衍射相结合的研究表明，在298K拉伸试验中，UFG样品中几乎没有形成残余位错结构和形变孪晶。77K时，晶格缺陷的快速积累和明显的重取向现象明显，表明晶界过程受到抑制，塑性变形以位错滑移和形变孪晶为主。此外，所有纯镁样品在77K都表现出明显的应变硬化，这主要是由于晶界滑移和动态回复受到抑制所致。从中子衍射数据定量地确定了77K形变的UFG样品的平均位错密度和位错与不同Burgers矢量的相对分数。尽管UFG具有很强的应变硬化能力，但在77K时，伸长率为5%的UFG试件过早断裂。77K塑性显著降低的原因可能是其较高的拉伸强度，产生较高的沿晶应力，并在达到塑性失稳点之前发生沿晶断裂。提出了提高超细镁合金低温强度和延性协同效应的可能途径。
Grain refinement can lead to the strengthening of metallic materials according to the Hall-Petch relationship. However, our recent results suggested that grain boundary sliding is the dominant deformation mode in bulk ultrafine grained (UFG) pure Mg at room temperature, leading to softening. Here, for the first time, we report that the Hall-Petch strengthening can be regained in bulk UFG pure Mg at cryogenic temperature. At 77K, the UFG pure Mg with a mean grain size of 0.6 μm exhibited ultrahigh tensile yield strength and ultimate tensile strength of 309 MPa and 380 MPa, respectively. Combinedin-situneutron diffraction and electron microscopy investigation indicated that residual dislocation structures and deformation twins hardly formed in the UFG specimen during tensile test at 298K. In contrast, fast accumulation of lattice defects and remarkable reorientation were evident at 77K, suggesting that the grain-boundary-mediated process was suppressed and the plastic deformation was dominated by dislocation slip and deformation twinning. In addition, all the pure Mg specimens exhibited pronounced strain hardening at 77 K, which was mainly attributed to the suppressed grain boundary sliding and dynamic recovery. The mean dislocation density and relative fractions of dislocations with various Burgers vectors of the UFG specimen deformed at 77K were determined quantitatively from neutron diffraction data. Despite the strong strain hardening capacity, the UFG specimen fractured prematurely at an elongation of 5% at 77K. The significantly reduced ductility at 77K was possibly caused by its higher tensile strength, which generated high intergranular stress and resulted in intergranular fracture before reaching the point of plastic instability. Potential ways for improving the strength and ductility synergy of the UFG Mg at cryogenic temperature were proposed.