Recently, the material constitutive relationship and fatigue fracture team led by Professor Kang Guozheng of Southwest Jiaotong University has made important progress in the direction of magnesium alloy fatigue damage mechanism. The related work is entitled "In-situ synchrotron X-ray tomography investigation on damage mechanism of an extruded magnesium alloy in uniaxial low-cycle fatigue with ratchetting" and published in the top journal Acta Materialia in the field of metal materials (IF: 7.656, the first district of the Chinese Academy of Sciences), the link to the paper: https://doi.org/10.1016/j.actamat. 2021.116881. Doctoral student Wang Ziyi is the first author of the paper, researcher Wu Shengchuan is the second author, Philip J. Withers, a professor at the School of Materials Science, University of Manchester, and a foreign academician of the Chinese Academy of Sciences, and Shanghai Synchrotron Radiation Light Source Researcher Fu Yanan are the co-authors, and Professor Kang Guozheng is the co-author Corresponding Author.
Magnesium alloy is the lightest metal structural material, and its large-scale application in rail transit and aerospace will greatly improve energy efficiency, reduce carbon emissions, and provide assistance for the construction of Beautiful China. As a structural material, magnesium alloy will face the problem of fatigue failure during service, and its tension-compression asymmetry makes its fatigue mechanism more complicated, which will limit the further promotion and use of magnesium alloy on load-bearing components. At present, the research on the low-cycle fatigue mechanism of magnesium alloys is mostly limited to the surface damage of the material under strain-controlled loading, and there is still a lack of systematic research on the damage mechanism of the material under the stress control.
The asymmetry of tension and compression of magnesium alloys comes from the participation of multiple deformation mechanisms. For example, the dislocation slip mechanism dominates the deformation during tension, while the twinning mechanism is more likely to be activated during compression, and untwisting occurs when tensioning again after compression. The team conducted experimental studies on three loading conditions, corresponding to twinning/untwisting dominant deformation (TDD), twinning/untwining and dislocation slip co-dominant deformation (TDSD), and dislocation slip dominant deformation (SD). After fatigue loading of the three samples, imaging experiments were carried out at the Shanghai synchrotron radiation light source using the in-situ loading device independently developed by the team, and the imaged samples were further characterized by EBSD and TEM, and further explored the low-cycle fatigue damage mechanism of magnesium alloys when different deformation mechanisms dominate the deformation.
Figure 1: (a) Schematic diagram of sample size, sampling method and texture; (b) EBSD results of initial squeezed sample; (c) Shanghai synchrotron radiation source experimental site; (d-f) stress-strain under three loading conditions curve.
The experimental results show that the damage patterns of the samples under the three loading modes are very different: a large number of crack initiation can be observed in the TDSD sample, the number of cracks in the TDD sample is reduced, and only a single crack appears in the SD sample; in the TDSD sample, shear occurs between the cracks In the combined form, there is a tendency of shear merging between two surface cracks in the TDD sample, while the crack in the SD sample is basically parallel to the cross-section; in the SD sample, there are intergranular cracks and slip band cracks, while in TDSD and TDD The twin boundary cracks were also observed in the sample.
Further EBSD and TEM characterizations showed that there were a large number of twins in the TDD sample, the integral number of twins in TDSD was relatively reduced, while the number of twins in the SD sample was very small; the number of dislocation densities in the three samples was related to the number of twins. The distribution law of the integral number is just the opposite; TDD samples are dominated by high-density basal plane [c] dislocations and <a> dislocations, and higher density <c+a> dislocations are additionally found in TDSD and SD samples.
Based on the experimental results, the researchers proposed the damage mechanism of magnesium alloys when multiple deformation mechanisms are involved: twinning is more likely to occur than dislocation slip, so compared to SD samples, the generation of a large number of twins in TDD samples is more helpful For the TDSD samples, the generation of <c+a> dislocations in the grains can maintain the average intragranular stress at a high level, while the generation of twins in adjacent grains releases the intragranular stress, so the redistribution of intergranular stress occurs. The intergranular stress between the twinned grains will be redistributed between the untwinned grains and the twinned grains, resulting in additional intergranular damage in addition to twin boundary damage and slip band damage. This explains the largest number of cracks in the TDSD sample.
This research work has been strongly supported by the National Natural Science Foundation of China 11532010 and U2032121. Recent years ,the "Material Constitutive Relationship and Fatigue Fracture" research team led by Professor Kang Guozheng has been published 34 papers in top journals in the field of solid mechanics and metal materials such as Journal of the Mechanics and Physics of Solids, International Journal of Plasticity, International Journal of Solids and Structures , International Journal of Engineering Science and Acta Materialia, which have produced a large international academic influence.