Effect of Pre-Stretching on Microstructures and Mechanical Behaviors of Creep-Aged 7055 Al Alloy and Its Constitutive Modeling
Round 1
Reviewer 1 Report
Comments;
1) The effective digits in Table 2 are quite curious. Please reconsider these values in the table.
2) The value of Q in Table 2 is quite important in high temperature creep experiments. How do they think about the relation between Q value and elementary process of dislocation behaviors during the creep behaviors? They should be mentioned it in the text.
Although authors explain the results in Fig.7, I am wondering if the explanation could be reasonable or not, since it seems to me that the behaviors of the creep strain curves in Fig.5 look like almost the same except the spontaneous strain just after the applied stress and little strain rate chang.
3) Authors should mentioned in the text how they identified the difference between η and η'.
Author Response
REPLY AND REVISIONS
The comments of reviewer are gratefully acknowledged. The manuscript has been revised in accordance with the advice of the reviewer #1. We appreciate the positive comments and constructive suggestions of the reviewer #1.
Reply to Reviewer #1:
1. The effective digits in Table 2 are quite curious. Please reconsider these values in the table.
We are grateful for the reviewer’s suggestion. According to the reviser’s suggestion on effective digits in Table 2, we have revised the effective precisions of s0 and n0, which are given three digits, the original values of 1.67e5 and 1.42e3 are individually replaced by the values of 1.670e5 and 1.420e3, and marked by red in the revised manuscript.
2. The value of Q in Table 2 is quite important in high temperature creep experiments. How do they think about the relation between Q value and elementary process of dislocation behaviors during the creep behaviors? They should be mentioned it in the text.
We are grateful for the reviewer’s questions. This paper presents that the Q value is a constant, which ranges from 5.832 to 90.921 KJ/mol when the variety of n1 is from 1.9 to 2.6. This conclusion was obtained from reference [24] and reference [25]. Meanwhile, the relation of Q value and dislocation behaviors during the creep behaviors had been clearly elaborated in the references [24] and [25]. In addition, this introduced constitutive equation is mainly used to reflect the influences of different pre-stretching amounts, stress levels and creep-aging system, rather than the effect of Q value. So, we think that the Q value, which is quite important in high temperature creep experiments, does not need to be elaborated in detail, it just needs to give the cited literature in the text.
As for the explanation of this result in Fig. 7, it is reasonable. Because in Fig. 5, we can conclude that the creep strain increases with the degree of intensity of pre-stretching at a certain time using the tensile test. We explain the reasons for the mentioned result based on the aging reinforcement theory. One is that the pre-stretching treatment introduces many kinds of defects such as dislocations and vacancies in the matrix as a result of work hardening. The other one is that more dislocation slip and the climb in the steady-state creep stage occur. It is easy to know that this explanation is just an assumption or a guess, it still needs to be further verified and analyzed from the microscopic perspective, so the TEM experiment and the corresponding results are made in Fig. 7.
3. Authors should mentioned in the text how they identified the difference between η and η'.
We are grateful for the reviewer’s question. Yes, in the paper, we had mentioned how the difference of between η and η' can be identified. We can identify them by their shape. Page 7, the twenty-fourth line in the manuscript for revisions (metals-506136), the shape of η' is rod-like and the shape of η is plate-like.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
General comment: the work is very interesting and well conducted. There some main questions to clarified before acceptance:
line 14-16: please read carefully and re-edit: the goal for creep is obtaining a lower creep strain not "more creep strain" explain or re-edit.
line 26-30: this general sentence is misliding: the AA7055 alloy belongs to the 7XXX high strength alloys family.. this is all.
line 90: Probably you miss the verb in this sentence.
Please add a table for explaining the three different treatments with: solution, ageing, and stretching details for each-one.
line 134 143: there are some trivial information, please, re-edit and consider that this is a scientific paper not a lesson or a PhD thesis.
line 182-192: this figure show the fact that if you are work-hardening an alloy you gain some points in tensile strength but you pay in ductility...no news. Consider to cut most of the trivial information.
line 197-206: please, in order to facilitate the reader, insert a table with all the stress, strains and main information and results
Fig. 5: I see your point and your interesting results, but 12 h of creep test duration are more similar to a low-rate tensile test, because you miss most of the secondary creep stage (you are not aware about the total duration of the second stage, and all the tertiary stage. The Al-alloys creep curve if you look at the creep tests curve of many of the Al-alloy the tertiary stage has a not negligible duration. This means that the starting condition curves cut at 12 h are just a actual-photo of a more long and unknown deformation process.
This fact suggest that you insert a clear comment describing why you are interested in a short-creep duration (aeronautical-creep forming process.. i.e.) .
Obviously, re-edit some of your conclusion because the effected you are claiming are proved only for the primary and early secondary creep stage.
line 326: Probably "activation energy" is better than “excitation energy”.
line 383-387: suggestion: cancel these lines-the comment is really not important and well known, probably trivial
line 398-399: even if I can agree with your comment, you have not reported the phase-density measurements of eta-prime phase for each of the three treatments . So, in order to improve greatly your work, I strongly suggest to use your TEM images for a phase identification density in order to prove your statement in line 399.
Author Response
REPLY AND REVISIONS
The comments of reviewer are gratefully acknowledged. The manuscript has been revised in accordance with the advice of the reviewer. We appreciate the positive comments and constructive suggestions of the reviewers.
Reply to Reviewer #2:
1. line 14-16: please read carefully and re-edit: the goal for creep is obtaining a lower creep strain not "more creep strain" explain or re-edit.
We are grateful for the reviewer’s suggestion. What we need to explain here is that, in general, the goal of creep is to obtain a lower creep strain. But in this text, it is worth noting here that all creep-strain profiles show an increasing trend with the increase in stress, and the creep rate is small under lower stress condition, but lasts longer at the second stage. The maximum creep strain of 7055 plate in solid solution state is just 0.09 %. Therefore, the creep aging performance of 7055 plate in solid solution state is extremely inferior, and it cannot be used in the later experimental tests. In other words, to satisfy the actual production, relatively higher stress level should be selected so as to obtain more creep strain without reducing its mechanical properties.
Furthermore, the creep tensile behaviors of the specimens experienced by T6 are further investigated. In Fig. 3b, it may be noted that the materials treated by T6 have greater creep strain than that of solution-quenched counterparts. The reasons for this involve the greater creep strain in first stage and also the longer duration for the creep rate during second stage. For instance, the maximal strain shown in Fig. 3b in the primary-creep stage is significantly larger than that shown in Fig. 3a (note that the creep strain of the former is 3 times more than that of the latter in terms of detail). Then, the creep curves of solution-quenched specimens enter into the second stage (separated by the gray dotted line) and the maximum creep strain accumulating at that period is up to 0.065 % (see the pink line of Fig. 3a) after 1.5 h of ageing. But it only takes 3.5 h for samples treated by T6 to reach the maximum—0.165% of creep strain in the same stage. This shows that compared with solid-quenched treatment, the effect of T6-treated 7055 alloy is extremely beneficial because the creep strain of the first stage generates more and creep time of the second stage lasts longer. The maximal amount of creep strain in the third stage of Fig. 3a seen in pink line accumulates up to 0.09%, but it is in the steady-state creep stage that maximum creep strain of 0.274% has been achieved (as shown the pink line of Fig. 3b). This phenomenon can also be used to demonstrate the fact that 7055 alloy treated by T6 has positive effects on the produce of creep strain. In a word, compared with the solution-quenched alloy plate, the 7055-T6 alloy shows better secondary creep after being loaded within a short duration of primary creep.
So we think that compared with solid-quenched alloy, the 7055-T6 alloy is the optimal scheme to attain more creep strain in line 14-16. Such a statement does not contradict the conventional expression of “the goal for creep is obtaining a lower creep strain”.
2. line 26-30: this general sentence is misliding: the AA7055 alloy belongs to the 7XXX high strength alloys family.. this is all.
Many thanks for the reviewer’s question. As we all know, AA stands for American standard aluminum alloy, and 7055 is just a kind of aluminum alloy brand. Alloy material used by us is provided by an aviation research institute in China. According to the reviser’s suggestion, for convenience of the reviewer to read without ambiguity, “Aluminum alloy 7055, as a kind of Al-Zn-Mg-Cu alloy with the highest alloy forming properties, exhibits best comprehensive performance such as high-strength,” is revised to be “Aluminum alloy 7055, as a kind of Al-Zn-Mg-Cu alloy with the higher alloy forming properties, exhibits better comprehensive performance such as high-strength,” seen in line 26-30 of “1. Introduction”.
3. line 90: Probably you miss the verb in this sentence.
Please add a table for explaining the three different treatments with: solution, ageing, and stretching details for each-one.
We are grateful for the reviewer’s question. First of all, I am very sorry for the missing the verb. The “Three groups that corresponded to three kinds of experimental schemes (signed by ①, ② and ③), one is an Al 7055 plate treated with solid solution and the other two are …” is revised to be “Three groups correspond to three kinds of experimental schemes (signed by ①, ② and ③). One is an Al 7055 plate treated with solid solution and the other two are …” in line 90-91 of the Revised Manuscript.
As for a table for explaining the three different treatments, we think that it is not necessary because the Figure 2 has presented the corresponding information. Meanwhile, in line 90-101, we have explained it in detail. And we think that the Figure 2 is clearer than the table required to be added. From Figure 2, we can see that three groups correspond to three kinds of experimental schemes (signed by ①, ② and ③). Among them, one is an Al 7055 plate treated with solid solution and the other two are Al 7055 plates treated with T6, both, with or without pre-stretching treatment. These were creep-tested to investigate their properties of creep-ageing. The samples prepared in scheme ① (see the blue line of Fig. 2) were held at 470 °C for 1 hour in the vacuum resistance furnace and then the specimens were immediately quenched in ambient temperature water. Meanwhile, some samples among them that respectively corresponded to schemes ② and ③ were designed to be held at 120 °C for 24 hours in the drying oven during T6 treatment (see the red line of Fig. 2) and then part of the samples were still subjected to pre-stretching treatment (see the green line of Fig. 2). Next, the creep-aging tests were conducted on all of samples under stress range (e.g. 150 to 240 MPa) for a controlled amount of time (e.g. 12 h) at 155 °C. Finally, the tensile tests were further conducted at room temperature with a constant speed of 3 mm/min.
4. line 134 143: there are some trivial information, please, re-edit and consider that this is a scientific paper not a lesson or a PhD thesis.
Many thanks for the reviewer’s question. In line 133-149 of the Revised Manuscript, “It is worth noting here that all creep strain profiles show an increasing trend with the increase in stress, and the creep rate is small under lower stress condition, but lasts longer at the second stage. The creep tensile behaviors for 7055 alloys in solid solution state under different stress levels are presented in Fig. 3a. It can be observed that the creep rate at the first stage (named as transient creep period) decreases gradually, and the corresponding duration is shorter. The creep strain of the following second stage (called steady-state creep period) presents a trend of linear increase. Its creep rate is generally thought as having a complicated relationship with the applied stress. Then the creep rate of the third stage, known an accelerated creep period, increases dramatically for a short time until a fracture appears. Obviously, compared with the third stage, the creep strains at second stage are almost unchanged and the corresponding creep time was relatively short. As time goes by, the creep curves of 7055 plate in solid solution state enter into the third stage in around 4.25 h (separated by the orange dotted line) and the creep strain in the third stage only accelerates to a maximum of 0.09 %. This indicates that the creep aging performance of 7055 plate in solid solution state is extremely inferior, which cannot be used in the later experimental tests. The reason is that, to satisfy the actual production, relatively higher stress level should be selected so as to obtain more creep strain without reducing its mechanical properties [12].” is revised to be “For 7055 alloys in solid solution state in Fig. 3a, it is worth noting that all creep-strain profiles show an increasing trend with the increase in stress. The creep rate is generally thought as having a complicated relationship with the applied stress. The lower the stress condition is, the smaller the creep rate is in the first stage (named as transient creep period), but the longer the creep duration is in the second stage (called steady-state creep period). Then the creep curves enter into the third stage in around 4.25 h (separated by the orange dotted line, known an accelerated creep period), and creep strain of this stage only accelerates to a maximum of 0.09 %. This indicates that the creep aging performance of 7055 plate in solid solution state is extremely inferior, which cannot be used in the later experimental tests. The reason is that, to satisfy the actual production, relatively higher stress level should be selected so as to obtain more creep strain without reducing its mechanical properties [12].”
5. line 182-192: this figure show the fact that if you are work-hardening an alloy you gain some points in tensile strength but you pay in ductility...no news. Consider to cut most of the trivial information.
We are grateful for the reviewer’s suggestion. In order to make readers can clearly understand the literature contents of this paper, in line 182-192 of the Revised Manuscript, “The true stress-strain curves for the studied 7055-T6 alloy under different pre-stretching conditions at atmospheric temperature are shown in Fig. 4a. The corresponding mechanical properties including the tensile strength, yield strength and elongation at break of the investigated material, are presented in Fig. 4b. The initial tensile and yield strengths of 7055-T6 are immediately measured after pre-stretching while it is about 509.35 MPa and 411.42 MPa for the un-stretched specimens, respectively. The strengths of samples with pre-stretching of 6.7% increase significantly to 540.76 MPa and 463.52 MPa, respectively. It can be seen clearly that the mechanical behaviors of material treated by T6 are tremendously influenced by pre-stretching treatments prior to the creep-aging process. This reason can be ascribed to work-hardening effect, with greater pre-stretching, resulting in higher tensile and yield strengths of the studied alloy before creep-aging process, where the strengths show significantly monotonous trend.” is revised to be “The true stress-strain curves for the studied 7055-T6 alloy under different pre-stretching conditions at atmospheric temperature are shown in Fig. 4a. The corresponding mechanical properties including the tensile strength, yield strength and elongation are presented in Fig. 4b. The initial tensile and yield strengths of 7055-T6 are immediately measured after pre-stretching while it is about 509.35 MPa and 411.42 MPa for the un-stretched specimens, respectively. And the strengths of pre-stretched samples of 6.7% increase significantly to 540.76 MPa and 463.52 MPa, respectively. It can be seen clearly that the mechanical behaviors of material treated by T6 are tremendously influenced by pure pre-stretching treatment at room temperature. This reason can be ascribed to work-hardening effect, with greater pre-stretching, resulting in higher tensile and yield strengths of the studied alloy before creep-aging process, where the strengths show significantly monotonous trend.”.
6. line 197-206: please, in order to facilitate the reader, insert a table with all the stress, strains and main information and results.
The reviewer’s suggestion is appreciated. After our discussion, we decide that there is no need to insert a table with all the stress, strains and main information and results. The reason is that we only care about the change trends and the corresponding maximums of both the tensile and yield strengths of the investigated alloy. If the strengths of all the pre-stretching are newly added, this section will be too tedious and lead to a lack of focus.
7. Fig. 5: I see your point and your interesting results, but 12 h of creep test duration are more similar to a low-rate tensile test, because you miss most of the secondary creep stage (you are not aware about the total duration of the second stage, and all the tertiary stage. The Al-alloys creep curve if you look at the creep tests curve of many of the Al-alloy the tertiary stage has a not negligible duration. This means that the starting condition curves cut at 12 h are just a actual-photo of a more long and unknown deformation process. This fact suggest that you insert a clear comment describing why you are interested in a short-creep duration (aeronautical-creep forming process.. i.e.) .
Many thanks for the reviewer’s question. As we all know, for ageing strengthening, in creep process, the second stage is the most important because of its longer duration. For the study on corrosion or fatigue resistance, it is more meaningful to study the creep tertiary stage. In addition, this work focuses on the effect of pre-stretching on creep-aged 7055-T6 alloys. In the creep second stage, the mechanical properties of 7055-T6 alloy will be improved according to the book wrote by YC Lin (Yongcheng Lin, Mingsong Chen, Yuchi Xia, et al. The technological basis of plastic forming and creep aging forming of typical aero-aluminum alloys [M]. Science Press, 2014.). So, we choose 12h as the creep time.
8. line 326: Probably "activation energy" is better than “excitation energy”.
Many thanks for the reviewer’s suggestion. In line 326 of the Revised Manuscript, the “excitation energy” is revised to be “activation energy”.
9. line 383-387: suggestion: cancel these lines-the comment is really not important and well known, probably trivial.
Many thanks for the reviewer’s suggestion. According to the suggestion of reviewer, the “primary-creep” is revised to be “primary creep”, as shown in Revised Manuscript.
10. line 398-399: even if I can agree with your comment, you have not reported the phase-density measurements of eta-prime phase for each of the three treatments. So, in order to improve greatly your work, I strongly suggest to use your TEM images for a phase identification density in order to prove your statement in line 399.
Thanks for the reviewer’s suggestion. We have marked the dislocation morphology in Figures 6 and 7. So we can distinguish the existence, number and distribution density of dislocations.
Author Response File: Author Response.pdf
