In Situ Synchrotron X-ray Study of the Mechanical Properties of Pure Mg Produced by Powder Metallurgy
Round 1
Reviewer 1 Report
The manuscript regards the evaluation of dislocation slip systems acting in pure magnesium processed through a powder metallurgy (PM) route using synchrotron measurements. The use of this technique allows calculating the contribution of the different slip systems in the course of deformation. While this is an excellent contribution, there are other points that should be improved in a revised version of the manuscript.
1) Basically, there is no comparison with the results reported in the literature for pure PM-magnesium. Why the data reported in this work are so different to those reported in the literature? Although the size of original magnesium powders is different (i.e. also the grain size of the consolidated materials), the tensile curves of both materials are almost identical, only being different the ductility. How can it be true when there is a large deviation in the grain size of both materials? The yield stress and yield strength of both alloys is much lower than those reported for pure PM-magnesium when the original size of the powders is lower than those reported in other works. This should lead to very fine-grained pure magnesium, i.e. with higher yield stresses but the opposite is found.
2) Why there is a different evolution of <a> and <c+a> slip systems in DH and FPS materials when all microstructural parameters are essentially the same, excepting the grain size?
3) Why texture is almost identical in spite of the different processing route?
4) It is not clear how the grain size was measured. It seems that grain size coincides with the size of original magnesium powders, but TEM images reveals that the grain size is smaller than the calculated values. More clear images, if possible corresponding to etched samples, should be provided to check the grain size of the alloy.
Author Response
We appreciate the Key Reader’s positive opinion towards this paper. The reviewers’ comments are addressed below.
Indeed, the yield strength of both materials are lower than some PM pure Mg extrusion bars. The reason could be due to the texture effect or different defects (MgO decorated around grain boundary, high initial dislocation density) introduced. No MgO is detected in our samples because of powder sheathed under the Ar atmosphere and compacted before cans are stripped off. The yield stresses with no MgO reported but similar textures would be the same level as ours (Thein et al., 2010, Meenashisundaram et al., 2016, Pérez et al., 2007, Gupta & Wong, 2015).
The DH material with a grain size ~3m shows similar yield strength as the FPS material with a grain size ~10m, which has similar tendency as reported in ref.(Pérez et al., 2007). A possible explanation is that the heat treatment at 500oC not only increased the grain size but also eliminated some porosity in FPS. The latter effect may offset the Hall-Petch effect, so that the two materials show similar strength.
The DH material shows a higher fraction of <c+a> dislocations during its deformation. The smaller grain size would cause more dislocation pile-up at grain boundaries, which tends to induce <c+a> dislocations. Similar result was reported in [28].
The two specimens do show similar texture. The figure below compares their original diffraction patterns（see attachment in the word version）, which also look like each other. The only difference between these two materials is that the powders was heat treated at 500 oC for 1h before extrusion for the FPS material. This step increases the grain size, but is unlikely to change the texture significantly.
The grain size was measured using the linear intercept method with a software named Nano-measurement. For both DH and FPS, at least 3 pictures were used to measure the grain size. For the DH sample, the grain size is close to the size of the original powders, which has a lognormal grain size distribution.
Thanks again for all your kind and helpful comments and if any question, pls. not hesitate to contact us.
Yours sincerely,
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The work is very interesting. However, considering the high number of variables that can influence, some aspects should be commented:
1- At what speed were the tensile tests carried out? How would this variable influence the study?
2- In the sample with "equiaxial" grain size ¿How can this influence the behavior described?
3- How was porosity evaluated? The reported differences are very small. Some significant figures of reported results should be corrected
4- How many tests were done?
5- Where was the study done, on the surface or in the core of the samples? The tensile behavior depends fundamentally on the core of the sample
6- How does the observed anisotropy influence the mechanical behavior?
7- Magnesium is used in biomedical applications. However this is biodegradable (in physiological fluid) ¿This type of study could be implemented? How is the behavior of dislocations expected to change?
Author Response
We appreciate the Key Reader’s positive opinion towards this paper. The reviewers’ comments are addressed below. 1. The strain rate is 2*10-4 /s. For pure Mg at this strain rate around 10-4, it will not cause much difference in dislocation activity.
2. The aspect ratio in both materials is close to 1. Since plastic deformation occurs by dislocation activity inside grains, we don’t expect the grain shape to affect the dislocation activity significantly.
3. The porosity was evaluated by SAXS. The new Fig. 4 shows how we estimated it.
4. For the in situ test, we only uses one specimen for each material. This is often the case for in situ synchrotron X-ray studies. We did some ex situ tensile and compression tests, and similar findings were made
5. The tensile samples were cut from the core of the extrusion bars, as shown in Fig. 1 (step 5).
6. The activation of different dislocation slip systems is common for hexagonal materials. It makes the material more ductile than the activation of single slip system.
It will influence the mechanical property depending on the texture and the direction of applied force. Here in our paper the differences are grain size and initial dislocation constitutions. That is why with different yield stress and elongation.
7. The biodegradability is another advantage of Mg. However, this is not the focus of the present work. We will study this topic later. This type of study would be implemented via making the test samples sealed together with a physiological fluid using a plastic container outside. It is hard to expect different behaviors of dislocations in physiological fluid or without it, if only tensile test at normal strain rate performed. Because the test time then is too short to make the degradable happen unless creep test there. But it is also not suitable to deduce the behavior of dislocation evolvement during creep test because the initial position of dislocation is still in controversial. The dislocation will be increased a little if dislocation stems from grain boundary but will have little difference to the phenomenon observed in test without physiological fluid while stemming from intral grain. Thanks again for your kind comments
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The authors have addressed the comments marked by this reviewer in their response but they have not included it in the new version of the manuscript. The manuscript will be suitable for publishing once these points would be included in the manuscript
Author Response
We add the contents of the reply comments into this updated manuscript. The added texts are highlighted in yellow.
Reviewer 2 Report
The authors have answered correctly in a general way. Now this manuscript can be accepted in its present form.
Author Response
We thank the reviewer for having provided useful comments.
