Effect of Cerium and Magnesium on Surface Microcracks of Al–20Si Alloys Induced by High-Current Pulsed Electron Beam
Round 1
Reviewer 1 Report
This paper describes the addition of Ce and Mg to Al-Si alloys to minimize micro-cracks due to exposure to a high current pulsed electron beam.
There are some issues to address before acceptance.
Please provide phase diagrams and additional thermodynamic information on the various alloys they discuss in the paper. There are a lot of hypotheses in this paper that could be addressed with the addition of such information.
They need to discuss if this is a cost effective method given the price and rarity of Ce. The same applies to their earlier work on Nd in refences 1.
Mg is not a rare earth and this needs to be corrected int eh manuscript.
Error bars are given for the microcrack density in figure 4 but not in the text. Are the values of the density good to that many decimal places? Give the error bars in the text as well.
Define what they mean by workpiece as it does not make sense in the context of the paper.
In table 2, define Ecoor and Icorr and give error bars. Is there a typo in one of these? Make sure to be careful about accuracy in terms of the number of reported digits.
Overall, their method does improve the surface of Al-Si alloys but it may not be practical due to the cost of Ce. They need to discuss this. They also need to provide thermodynamic information and use that to explain their results to see if their approach can be applied to other alloys.
The corrosion analysis is the weakest part of the paper and needs some expansion to make it relevant to the paper. I do not quite see what they are getting at in this part. Corrosion resistance to what?
Author Response
Question 1:
Please provide phase diagrams and additional thermodynamic information on the various alloys they discuss in the paper. There are a lot of hypotheses in this paper that could be addressed with the addition of such information.
Answer: The phase diagram of Al-Si-Mg vertical section is added in manuscript, as shown in Fig.2.This phase diagram sufficiently illustrates that the refinement of primary silicon phase by Mg is due to the consumption of Si as a result of generating reaction of Mg2Si phase. Therefore, no additional thermodynamic information is required in this manuscript. For a detailed explanation of the phase diagram is seen in the lines 3-13 of paragraph 1 on page 5 as follows: According to the phase diagram of Al-Si-Mg vertical section in Figure 2 [32], for alloys with an Mg content of 5 wt.%, when the alloy melt cools below 575°C primary Si and Mg2Si phases start to form, while some proportion of the mixture remains in a liquid phase. The liquid phase then undergoes a eutectic reaction at 560°C to form the Al–Si eutectics. Finally, the microstructure of alloy consists of an Al-Si eutectic and the primary Si and Mg2Si phases at room temperature. Additionally, it can be seen from Figure 2 that as the Mg content increases, a larger proportion of the Si participates in the reaction described in Eq. (1) forming the primary Mg2Si phase; thus the primary Si phase disappears when the Mg content exceeds 6 wt.%. This suggests that the refinement of primary Si in the presence of Mg is due to the consumption of Si as a result of the reaction described by Eq. (1).
In addition, Li et al. indicated that the size of the brittle phase affected the formation of microcracks on the material surface after electron beam treatment. （Li, C.L.; Murray, J.W.; Voisey, K.T.; Clare, A.T.; McCartney, D.G. Amorphous layer formation in Al86.0Co7.6Ce6.4 glass-forming alloy by large-area electron beam irradiation. Appl. Surf. Sci. 2013, 280, 431–438. ）Because microcracks are prone to generate in the brittle phase, the refinement of the brittle phase effectively suppresses the propagation of microcracks. In this paper, microcracks occurs in the primary silicon phase, not extending into the aluminium matrix because of good plasticity of aluminium.( plastic deformation occurs in the aluminium matrix after electron beam treatment). From the above analysis, it can be deduced that the refinement of the primary silicon by Mg can inhibit the propagation of microcracks generated by the electron beam.
Question 2:
They need to discuss if this is a cost effective method given the price and rarity of Ce. The same applies to their earlier work on Nd in refences 1.
Answer: In fact, the Nd in reference [1] is replaced by Ce. About this point, the authors consider adequately the cost effectiveness of rare earths in this manuscript. For prices of the two rare earths, please refer to the website （https://hq.smm.cn/rare-earth/fullscreen）or the following price table of rare earths.
Table the prices of rare earths in Chinese market.
.It can be seen from the table that the price of Nd is 35 times higher than that of Ce. (The price of Nd is 164 USD per kilogram, while the price of Ce is 4.67 USD per kilogram, 1 USD = 6.3701 RMB.) Next, we calculate the cost prices of Ce and Nd in Al-20Si-5Mg-0.7RE alloy per kilogram, respectively. The price of Ce = 0.7wt. %×1kg×4.67USD/kg=0.03269USD; The price of Nd=0.7wt.%×1kg×164USD/kg
=1.148 USD. As we can see from the price list, the Ce has a big cost advantage. Therefore, the method used in this manuscript has potential applications in eliminating microcracks.
Question 3:
Mg is not a rare earth and this needs to be corrected int eh manuscript.
Answer: In reponse to this question, the revised parts are highlighted with yellow bright lines as follows: However, Mg has been rarely reported to affect the surface microcracks originating from the HCPEB irradiation. (See lines 5-6 of paragraph 1 on page 7 for details) The effect of Mg and Ce on microcracks is emphasized in this paper.These two elements can significantly refine the primary silicon phase in hypereutectic aluminium-silicon alloys, thus suppressing or eliminating surface microcracks after electron beam treatment. Of course, magnesium is not a rare earth.
Question 4:
Error bars are given for the microcrack density in figure 4 but not in the text. Are the values of the density good to that many decimal places? Give the error bars in the text as well.
Answer:
The authors have added error bars to the manuscript. The revised parts are highlighted with yellow bright lines on pages 7 and 8 of the manuscript.
Question 5:
Define what they mean by workpiece as it does not make sense in the context of the paper
Answer: The word “workpiece”appears in paragraph 3 on page 8 of the manuscript, which is replaced by the word “metallic components”. The revised parts are as follows: The most important application of surface modification technology based on HCPEB is surface modification of the metallic components such as the blades of engine and mould. This technology can prolong the service life of these metallic components and improve the surface properties of a material by optimising the surface microstructure. However, microcracks are inevitably introduced on the HCPEB-irradiated material surface, adversely affecting the surface properties and hindering the industrial application of this technology. In the present work, Mg and Ce were incorporated to Al–20Si alloy to better solve the aforementioned problem. The elimination of microcracks is important for surface modification based on HCPEB, thus facilitating its industrial application. ( See paragraph 3 on page 8) This paragraph highlights the industrial potential applications of electron beams. The solution of the microcrack problem promotes industrial applications of HCPEB.
Question 6:
In table 2, define Ecoor and Icorr and give error bars. Is there a typo in one of these? Make sure to be careful about accuracy in terms of the number of reported digits.
Answer: For each pulsed number sample, electrochemical test is performed once. Therefore, no error bars is added in this manuscript. The following literatures did not reveal the incorporation of error bars in the Ecoor and Icorr datas:
- Hao, S.Z.; Zhang , X.D .; Mei, X.X..; Grosdidier T, Dong C. Surface treatment of DZ4 directionally solidified nickel-based superalloy by high current pulsed electron beam. Mater. Lett. 2008, 62, 414-417.
- Cai, J.; Lv , P.;Zhang , C.L.; Wu, J.;Li, C.;Guan, Q,F. Microstructure and properties of low carbon steel after surface alloying induced by high current pulsed electron beam. Instrum. Meth. B. 2017, 410, 47–52.
In this manuscript, the units of the horizontal and vertical coordinates of the polarisation curve graphs as well as the units of the Ecoor and Icorr datas in the table of electrochemical parameters are consistent with those reported in the two aforementioned literatures. Therefore, the Ecoor and Icorr datas are correct. In addition, the Al-20Si-5Mg-0.7Ce alloy has good anticorrosion property.The corrosion current density of the Al–20Si–5Mg–0.7Ce alloy specimens shows a remarkable decrease from 15.40 μA/cm2 for the original specimen to 0.01021 μA/cm2 for the specimen subjected to 25 pulses, representing a decrease of three orders of magnitude. The corrosion potential of the alloy specimens also shifts toward more positive potentials with increasing number of pulses, implying that the metal might be stable in the corrosive medium, leading to reduced corrosion.(see lines 5-11 of paragraph 2 on page 11) The electrochemical datas in Table 2 reveals that the corrosion potential moves to the positive direction as the corrosion current decreases from original sample to 25-pulsed sample, indicating the improved corrosion resistance of the alloy surface. Therefore,the accuracy of the datas can be guaranteed.
Question 7:
Overall, their method does improve the surface of Al-Si alloys but it may not be practical due to the cost of Ce. They need to discuss this. They also need to provide thermodynamic information and use that to explain their results to see if their approach can be applied to other alloys.
Answer: Because the cost effectiveness of Ce has already been discussed in question 2, I will not repeat it here. The phase diagram has been added to this manuscript, as shown in Fig. 2. And no additional thermodynamic information is required. The results of this experiment are explained easily by phase diagram in this manuscript, which have already been discussed in question 1. About the applicability of the method, the authors reckon that it is applicable to other alloys. It is well known that rare earths, magnesium and other grain refiners can significantly refine the microstructure of materials. Particularly, brittle phases are refined in some materials by these chemical modifier. Because microcracks are easily generated in the brittle phase, refinement of the brittle phases can suppress the propagation of microcracks during the electron beam treatment. In addition, the effect of microcrack arrest or microcrack elimination can be achieved at appropriate parameters of electron beam. The authors believe that the method can be broadened to industrial applications. The aim of this study is to solve the problem of microcracks seriously hindering the industrial application of the HCPEB technology. Therefore, the method has potential advantages in industrial application .
Question 8:
The corrosion analysis is the weakest part of the paper and needs some expansion to make it relevant to the paper. I do not quite see what they are getting at in this part. Corrosion resistance to what?
Answer: The authors believe that the thought of corrosion analysis is clear in this manuscript. The SEM results after electrochemical corrosion reveal that the elimination of microcracks delays the occurrence of the pitting corrosion, thus improving the anticorrosion performance of the alloy surface. For alloys containing microcracks, the NaCl solution infiltrates the microcracks, accelerating the pitting corrosion of the alloy surface （lines 11-12 of paragraph 1 on page 13）. After the addition of Mg and Ce in alloys, the corrosion pits formed in the brittle phase were shallow due to elimination of microcrack as compared to alloy specimens containing Mg (Figs.10 and 11)，indicating less corrosion destruction. The above corrosion analysis reveals that the pitting corrosion is suppressed by the elimination of microcracks by Ce, improving the anticorrosion property of the alloy surface. In addition, for alloy specimens containing Mg, the corrosion current density is reduced by only one order of magnitude at 25 pulses as compared to the original specimens. But for alloy specimens containing Mg and Ce, this parameter is reduced by three order of magnitude at 25 pulses as compared to the original specimens. The above change of corrosion current also indicate that the anticorrosion performance of the alloy surface is significantly improved due to microcrack elimination.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
1. Large number of References appear through the whole length of the paper, while the literature analysis should be conducted in the Introduction section.
2. Figures 1a and Fig 1b are not informative, and I cannot see a clear difference between them. Please explain or replace them.
3. I cannot see any microcracks in Fig. 2 and Fig. 3 that is opposite to the author’s statements (page 11). Please replace the Figures with the magnified views.
4. Fig. 5 shows strong non-uniformity of Si and Al, and this fact is explained by the tendency of Mg and Si to diffuse outwards the area affected by HCPEB. At that, the authors state that the surface components of the alloys become homogenous at high pulse numbers, thus improving the surface properties of the materials. However, I doubt that the number of pulses was large enough in the experiments discussed in the paper. Please, explain.
5. References are a little bit old and need to be updated by the last two years' literatures.
Author Response
Question 1:
Large number of References appear through the whole length of the paper, while the literature analysis should be conducted in the Introduction section.
Answer: The authors believe that the phenomenon of this experiment can best be explained by introducing appropriate references in the section of Results and Discussion. Of course, the authors have added some literature analysis in the introduction section, please see lines 16-24 of paragraph 1 on page 1 as follows: For example, Fu et al.[7] discovered the formation of an amorphous phase on the surface of HCPEB-irradiated AISI 1045 steel, and concluded that the amorphous phase could be used as a homogeneous passive film to significantly improve the anticorrosion properties of the material. Yan et al.[9] utilised the HCPEB equipment to prepare a Cu-enriched Al supersaturated solid solution on the top surface of a 2024 Al alloy, and the supersaturated solid solution was found to improve the anticorrosion properties of alloy surface. Zhang et al.[19]discovered a nano graphite phase on the YN13 hard alloy surface after irradiation with a pulsed electron beam; the phase was observed to improve the tribilogy performance of the hard alloy.
Question 2:
Figures 1a and Fig 1b are not informative, and I cannot see a clear difference between them. Please explain or replace them.
Answer:The authors reckon that Figs 1a and 1b are informative. Compared to Fig. 1a, the size of the primary silicon is further refined significantly due to the addition of Ce (Fig. 1b). This can be found in the manuscript as follows: In the case of the Al–20Si–5Mg–0.7Ce alloy, the primary Si phase is further refined and uniformly dispersed in the alloy as a result of the incorporation of Ce and the average size of the phase changes from 61 to 48 μm (Figs. 1a and 1b) （lines 1-3 of paragraph 2 on page 5）
In addition, the refinement mechanism of primary silicon by rare earths are also elaborated as follows: As previously reported, the RE-rich intermetallic compound is uniformly distributed near the primary Si phase, implying that the RE atoms are clustered at the crystallographic front of the growing primary Si phase [1]. This atomic arrangement induces a compositional supercooling of the RE atoms, ultimately refining the primary Si phase. （lines 8-12 of paragraph 2 on page 5）
Question 3:
I cannot see any microcracks in Fig. 2 and Fig. 3 that is opposite to the author’s statements (page 11). Please replace the Figures with the magnified views.
Answer: Figs. 2 and 3 have been replaced according to the reviewers' comments, please see Figs. 3 and 4 for details.
Question 4:
Fig. 5 shows strong non-uniformity of Si and Al, and this fact is explained by the tendency of Mg and Si to diffuse outwards the area affected by HCPEB. At that, the authors state that the surface components of the alloys become homogenous at high pulse numbers, thus improving the surface properties of the materials. However, I doubt that the number of pulses was large enough in the experiments discussed in the paper. Please, explain.
Answer: The authors reckon that the inhomogeneity of Si and Al in local areas is independent with the number of pulses, which depends upon the morphology of the original microstructure. In our previous work（Gao, B.; Hu , L.; Li, S.W.; Hao, Y.; Zhang, Y.D.; Tu, G.F.; Grosdidier T.Study on the nanostructure formation mechanism of hypereutecticAl–17.5Si alloy induced by high current pulsed electron beam. Appl. Surf. Sci. 2015, 346, 147–157）, the presense of Si-rich and Al-rich zones on the surface of the alloy was reported at 100 pulses. The Si-rich zone is evolved from the primary Si phase in the original microstructure, and the alloy surface is irradiated by each pulse, inducing the melting of the surface and the diffusion of Si inside the primary Si phase towards the exterior. While the Al-rich zone is formed due to eutectic Si dissolving in the Al matrix after electron beam treatment. But the content of each element on the surface of the alloy tends to the composition of the alloy bulks as the number of pulses increases. For this experiment, the EDS analysis is carried out for the HCPEB treated alloy samples. Please see the following table for details:
Table Surface components of HCPEB-irradiated and HCPEB-unirradiated Al–20Si–5Mg and Al–20Si–5Mg–0.7Ce alloys via SEM-EDS analysis (wt.%).
|
Alloy |
Number of pulses |
Al |
Si |
Mg |
Ce |
|
Al–20Si–5Mg alloy |
0 |
71.32 |
24.12 |
4.56 |
-- |
|
5 |
69.71 |
22.84 |
7.45 |
-- |
|
|
15 |
70.40 |
22.69 |
6.91 |
-- |
|
|
25 |
72.34 |
21.67 |
5.99 |
-- |
|
|
Al–20Si–5Mg–0.7Ce alloy |
0 |
71.42 |
24.83 |
3.63 |
0.12 |
|
5 |
72.33 |
24.55 |
2.93 |
0.19 |
|
|
15 |
72.64 |
23.67 |
3.29 |
0.40 |
|
|
25 |
71.99 |
23.37 |
4.14 |
0.50 |
From the table，the results of the surface component analysis of the alloys show that the content of all of the elements trends toward that in the alloy bulk with incremental number of pulses (as shown in table 1 of this manuscript), implying that the surface components of the alloys becomes homogenous at high pulse numbers.
Question 5:
References are a little bit old and need to be updated by the last two years' literatures.
Answer: In this manuscript, the authors have replaced some of the references highlighted with yellow bright lines. Please see reference section for details. In addition, the remaining references have not been replaced because our previous works could explain some of the experimental phenomena in this manuscript. And the development of microcrack arrest technology is introducted in the introduction section,requiring some of old references. Therefore, the above old references are useful for this manuscript. I hope that the reviewers will understand the citation of these references. Thank you very much to this expert for reviewing this paper.
Author Response File: Author Response.pdf
Reviewer 3 Report
The Paper is well written and considers the effect of Ce and Mg on surface microcracks of Al–20Si alloys induced via high-current pulsed electron beam (HCPEB) were studied. Mg was revealed to refine the primary Si phase in the pristine microstructure by forming a Mg2Si phase, leading to the suppression of microcrack propagation within the brittle phase after HCPEB irradiation. The incorporation of Ce into the Al–Si–Mg alloys further refined the primary Si phase and reduced the local stress concentration in the brittle phase induced by HCPEB irradiation. Ultimately, the surface microcracks were observed to be eliminated by the synergistic effects between the two elements. For Al–20Si–5Mg–0.7Ce alloys, Ce demonstrated a homogeneous distribution in the Al matrix on the HCPEB-irradiated alloy surface, while the Mg and Si exhibited a certain degree of aggregation in the Mg2Si phase.
I recommend the paper for printing and publishing
Author Response
The Paper is well written and considers the effect of Ce and Mg on surface microcracks of Al–20Si alloys induced via high-current pulsed electron beam (HCPEB) were studied. Mg was revealed to refine the primary Si phase in the pristine microstructure by forming a Mg2Si phase, leading to the suppression of microcrack propagation within the brittle phase after HCPEB irradiation. The incorporation of Ce into the Al–Si–Mg alloys further refined the primary Si phase and reduced the local stress concentration in the brittle phase induced by HCPEB irradiation. Ultimately, the surface microcracks were observed to be eliminated by the synergistic effects between the two elements. For Al–20Si–5Mg–0.7Ce alloys, Ce demonstrated a homogeneous distribution in the Al matrix on the HCPEB-irradiated alloy surface, while the Mg and Si exhibited a certain degree of aggregation in the Mg2Si phase.
I recommend the paper for printing and publishing
Answer: Thank you very much to this expert for review of this paper.
Author Response File: Author Response.pdf
