Very "magnesium" is very strong, crack plastic poor!

Application of pure magnesium 

As the lightest metal structure material, magnesium has broad application prospects in aerospace, automotive, high-speed rail, electronic products and medical and other fields.  However, compared with traditional metal materials, magnesium has poor plasticity, difficult deformation processing of profiles and parts, and high process cost.  This severely restricts the wide application of magnesium as a structural material.  

The current mainstream view is that poor plasticity is an intrinsic property of magnesium, because the conical dislocation (a crystal defect) in magnesium spontaneously decomposes into non-slippable structures and cannot coordinate plastic deformation.  Therefore, increasing plasticity requires adjusting the behavior of cone dislocations by adding some specific elements.  However, some scholars hold a different view that the cone dislocation is an effective plastic deformation carrier, and as long as the nucleation and slip of the cone dislocation can be promoted, the plasticity of magnesium can be improved.  

The above disputes directly affect the design ideas and technical route of the next generation of high plasticity magnesium alloys, so it becomes a scientific problem that needs to be solved urgently.  However, because the geometry and structure of cone dislocations are very complicated, it is difficult to analyze them comprehensively through experiments.  Previous studies have generally focused on computer simulations, and the ideas and inferences lack strong experimental evidence.  

Therefore, the team of Zhiwei Shan of Xi 'an Jiaotong University recently found that poor plasticity is not an inherent property of magnesium, and that increasing the flow stress (for example, by refining the grain or increasing the strain rate) to promote dislocation nucleation and slip may be an effective plasticizing method.  

After extensive investigation and in-depth discussion, The team of Professor Shan Zhiwei of Xi 'an Jiaotong University decided to adopt the following strategies:  

1) Solve the problem of one-to-one correspondence among geometrical deformation, microstructure evolution and mechanical curve of samples by in-situ electron microscopy nanomechanics testing technology;  

2) Select the appropriate loading direction to eliminate the interference of other dislocation;  

3) Gradient sample design was used to solve the difficulty of capturing and characterizing single dislocation;  

4) Use 3D image reconstruction technology to solve the difficult problem of dislocation slip surface;  

5) The influence of electron beam is clarified by comparing mechanical curves.  

Thanks to these targeted experimental designs, the team demonstrated convincingly that, at least for pure magnesium at the submicron scale, various types of cone dislocations (edge, screw, mixed type) can not only slip, but also lead to very large plastic deformation.  Compared with bulk materials, the yield strength and flow stress of micro/nano samples are higher.  Therefore, the team speculated that the high stress promoted the nucleation and slip of the cone dislocations, which in turn improved the plasticity of the test samples.  Through further analysis, not only the slip planes of the dislocations were identified, but also the intersecting slip of the conical dislocations, the formation of the dislocation dipoles and the reciprocating motion of the dislocations were clearly observed, which had not been reported before.  

This study provides important experimental data for improving the plastic deformation theory of magnesium and brings new inspiration for the development of high plastic magnesium alloys.