Investigation on the Durability of Ti-6Al-4V Alloy Designed in a Harmonic Structure via Powder Metallurgy: Fatigue Behavior and Specimen Size Parameter Issue

Round 1

Reviewer 1 Report

The manuscript is well written and clear and the topic is for sure of interest of the readership. There are few comments that I ask the authors to address before the manuscript is accepted for publication:

The manuscript is lengthy and could be surely more concise in some chapters, starting from the abstract. The results section already includes some sort of discussion. It would be best if the data interpretation is all included in the discussion section.

The introduction provides a good view of what is mot well investigated in the state of the art regarding the scale effect of bimodal samples; however, what is missing and not discussed is the effect of the fabrication process. The introduction does not refer to the fabrication process used in this study and refers just to powder metallurgy. I suggest the authors include a brief list of other technologies (such as severe plastic deformation based techniques, additive manufacturing and spray deposition, etc.. ) and include the intrinsic differences. You can check - Proc. Natl. Acad. Sci., 112 (47) (2015), pp. 14501-14505; Nature, 419 (6910) (2002), p. 912 and Additive Manufacturing, 33, (2020) 101131.

In the caption of figure 1, specify the design refers to the miniaturized or standard specimen. It would be best of both designs are included in this figure for a direct comparison.

Page 2, line 88 mentions that milling process induced grain refinement at the powder surface. This implies that the individual nano regions are actually comprised of micrograin cores. The definition of bimodal and separation between the regions seems a bit confusing to me as if the nano surface of particles could be congruously disrupted with their micro-core, impeding having a continuous nano matrix. While in Fig. 2 we see a continuous nano phase. Please clarify.

Please explain how many samples were used for fatigue tests at each stress level and provide the standard deviation of the calculated fatigue lives. Since the difference between the fatigue life of the MM and IP series is not huge, it would be interesting to include the standard deviation to verify whether the difference is statistically significant or not.

Please comment on the possible contribution of the volume fraction of the two phases to the fatigue performance of the bimodal structures.

Author Response

Manuscript ID: metals-802060

 Title: Investigation on the durability of Ti-6Al-4V alloy designed in harmonic structure via powder metallurgy: fatigue behavior and specimen size parameter issue

 Metals, special issue

“New trends in Powder Metallurgy: Microstructure, Properties, Durability”

 Dear Editor,

First of all, we would like to thank the Reviewer and the Editor for their comments. In the following we listed the replies to these comments. The requested changes appear in red in the track changed revised manuscript.

Reply to the reviewers’ comments: the comments are in normal style, while the answers are in bold.

Reviewer #1

1. The manuscript is lengthy and could be surely more concise in some chapters, starting from the abstract. The results section already includes some sort of discussion. It would be best if the data interpretation is all included in the discussion section.

In accordance with the wishes of the reviewer, abstract of the manuscript has been shorten. Concretely the following sentences have been deleted: “In the latest decade, metallic materials designed in heterogeneously distributed grain size microstructure revealed superior static strength than homogeneous counterparts. Due to its flexibility, powder metallurgy is a fantastic tool to design unprecedented crystallographic structures in a sustainable way. Therefore, numerous attempts were made worldwide to create innovative structures ensuring advantageous mechanical properties. Such efforts lead to the conception of the “harmonic structure”, which possesses quasi-spherical large grain regions (“cores”) embedded in a continuous fine grain region (“shell”). This structure highlighted higher mechanical strengths than homogeneous counterparts for various metallic materials used in structural applications. Nevertheless, examination of the fatigue behavior is essential for structural usage purposes.” For the sake of comprehension, some other slight changes have been conducted on the abstract content.

Furthermore, some very limited points of result analyses (lines 174~179, 207~209, 331~333 of track changed manuscript) are indeed inside the experiment result section. These extremely quick deductions are written in the experimental results section for the sake of continuity with these experimental results. Displacement of these sentences to the discussion section is neither advantageous from a viewpoint of manuscript length (since reintroduction of experimental points should be needed), nor favorable from a viewpoint of reader understanding, according to the authors. Therefore, authors do think that such very uncomplicated deductions are better suited in the continuity of the related experimental results.

2. The introduction provides a good view of what is mot well investigated in the state of the art regarding the scale effect of bimodal samples; however, what is missing and not discussed is the effect of the fabrication process. The introduction does not refer to the fabrication process used in this study and refers just to powder metallurgy. I suggest the authors include a brief list of other technologies (such as severe plastic deformation based techniques, additive manufacturing and spray deposition, etc.. ) and include the intrinsic differences. You can check - Proc. Natl. Acad. Sci., 112 (47) (2015), pp. 14501-14505; Nature, 419 (6910) (2002), p. 912 and Additive Manufacturing, 33, (2020) 101131.

Since the present manuscript is intended to be published in a special issue dealing with powder metallurgy technique, authors have introduced this theme in particular in the original manuscript. Nevertheless, acknowledging the reviewer’s though, authors have decided to insert new references related to other fabrication processes, like severe plastic deformation and cold spray additive manufacturing. These new references, numbered [12-14] in the revised manuscript, have been inserted in the introduction, lines 52~53 (track changed manuscript).

3. In the caption of figure 1, specify the design refers to the miniaturized or standard specimen. It would be best of both designs are included in this figure for a direct comparison.

Following the reviewer’s comment, Fig. 1 of the manuscript has been corrected, showing the dimensions of the three types of specimens (tensile, fatigue SS and fatigue MS). Proportional sizes of the drawing corresponding to fatigue specimens have been set to figure out the scale change between MS and SS configuration.

4. Page 2, line 88 mentions that milling process induced grain refinement at the powder surface. This implies that the individual nano regions are actually comprised of micrograin cores. The definition of bimodal and separation between the regions seems a bit confusing to me as if the nano surface of particles could be congruously disrupted with their micro-core, impeding having a continuous nano matrix. While in Fig. 2 we see a continuous nano phase. Please clarify.

Authors apologize for the misleading sentence in line 88 of the original manuscript. Indeed, mechanical milling conditions are set to generate a fine grain region layer from the powder surface. However, this layer’s thickness reaches several tens of microns [18], keeping a micrograin region at the center of the powder. Consequenctly, consolidation by SPS of such prepared powder, operated at suitable sintering conditions, leads to a microstructure with a continuous nanograin region (“shell”) and spherical micrograin regions (“cores”), i.e. the harmonic structure as defined in the manuscript. To clarify this point, manuscript has been revised in Section 2.1 (line 96 of track change manuscript) as follows: the sentence “This powder is subjected to mechanical milling (MM) for 90 ks in argon atmosphere, to generate a layer of fine grains at the powder surface.” has been substituted by “This powder is subjected to mechanical milling (MM) for 90 ks in argon atmosphere, to generate a several tens micron-thick layer of fine grains from the powder surface, resulting in 397 and 369 Hv average hardness values inside fine grain layer and powder center region, respectively [18].”

5. Please explain how many samples were used for fatigue tests at each stress level and provide the standard deviation of the calculated fatigue lives. Since the difference between the fatigue life of the MM and IP series is not huge, it would be interesting to include the standard deviation to verify whether the difference is statistically significant or not.

The S-N diagram presented in the present work accepts the usual convention, which stipulates that one drawn point corresponds to one specimen datum. The present work is designed with a limited number of specimens, due to the compact initial size. Nevertheless, number of specimens to draw the S-N curves is based on a minimum of 8 points, which is sufficient for primary analysis of the fatigue behavior of metallic materials, according to ISO 12107 standard (mentioning at least 6 specimens needed).

Under such a circumstance, the standard deviation of the fatigue limit at 10⁷ cycles may be determined by the stress range increment used for fatigue testing campaign. Based on this assumption, the fatigue limits of respective fatigue testing data series are:

IP, MS: 683 ± 14 MPa  /  IP, SS: 680 ± 20 MPa

MM, MS: 738 ± 15 MPa  /  IP, SS: 570 ± 10 MPa

These fatigue limit expressions have been inserted in the revised manuscript, in Sections 3.3.1/3.3.2 (lines 223, 232 of track changed manuscript) and in the second point of the conclusion (lines 580~584 of track changed manuscript). Even though fatigue limit discrepancies are not “huge”, the gap between fatigue results underlines an effect of the microstructure on the fatigue behavior.

6. Please comment on the possible contribution of the volume fraction of the two phases to the fatigue performance of the bimodal structures.

Some additional considerations of the contribution of the core and shell volume fraction to the fatigue performance of the harmonic structure have been inserted in the conclusion (lines 607~611 of tack changed manuscript) as follows: The sentence “In particular, the simplified model suggests that the elevated shell areal fraction of the harmonic structured material investigated here is a key parameter of the size effect emergence” has been replaced by “In accordance with the simplified model, fatigue performance from harmonic structure materials with a large shell areal fraction should be altered, if the smallest dimension of the critical volume is out of the same order to core / shell unit size. On the contrary, fatigue strength of harmonic structure materials possessing a low shell areal fraction should hardly be affected, due to the limited influence of the critical volume size on the active interface between core and shell regions.”

Reviewer 2 Report

The paper « Investigation on the durability of Ti-6Al-4V alloy designed in harmonic structure via powder metallurgy: fatigue behavior and specimens size parameter issue » is a study on harmonic core /shell microstructure of Ti64 and the mechanical properties of the obtained material. The core /shell structure is obtained by milling the powder and sintering by SPS. The microstructures obtained and the mechanical results are interesting and may be published in the journal « metals » after minor but important revision listed below.

• More details should be added to the microstructure obtained, like the porosity and typical hardness of the phases.
• in the abstract: It should be checked in the context if the so-called crystallographic structure is not in fact microstructure characteristics because the study is on core/shell microstructure
• line 86: What is the microstructure of the raw starting material and after MM IP and SPS? the SPS treatment is below the transus, then it is interesting to know the microstructure before and after SPS.
• line 114: please name the reference of the testing machine.
• Figure 2, these microstructures are shown in the inner and outside region. It would also be interesting to see the microstructure in the center and the edge of the SPS specimen has the heating is often higher in the center for SPS of metals.
• figure 4a and 4b: the difference between a) and b is not clear, I would suggest merging them.
• line 217: the difference between MS and SS is still not clear

Author Response

Manuscript ID: metals-802060

Title: Investigation on the durability of Ti-6Al-4V alloy designed in harmonic structure via powder metallurgy: fatigue behavior and specimen size parameter issue

Metals, special issue

“New trends in Powder Metallurgy: Microstructure, Properties, Durability”

Dear Editor,

First of all, we would like to thank the Reviewer and the Editor for their comments. In the following we listed the replies to these comments. The requested changes appear in red in the track changed revised manuscript.

Reply to the reviewers’ comments: the comments are in normal style, while the answers are in bold.

Reviewer #2

The paper « Investigation on the durability of Ti-6Al-4V alloy designed in harmonic structure via powder metallurgy: fatigue behavior and specimens size parameter issue » is a study on harmonic core /shell microstructure of Ti64 and the mechanical properties of the obtained material. The core /shell structure is obtained by milling the powder and sintering by SPS. The microstructures obtained and the mechanical results are interesting and may be published in the journal « metals » after minor but important revision listed below.

• More details should be added to the microstructure obtained, like the porosity and typical hardness of the phases.

In the present work, such analyses on the microstructure were not operated. Nevertheless, such experiments have been conducted in previous published studies on harmonic materials. Based on the results published in [18] of the revised manuscript, using the same initial powder and mechanical milling conditions, average hardness of the powder before mechanical milling is 326 Hv, whereas powder prone to mechanical milling revealed hardness of 397 Hv and 369 Hv in refine grain region and in its center.

Such experimental results have been notified in the revised manuscript (Section 2.1, lines 95 and 98) as follows. The sentence “This powder is subjected to mechanical milling (MM) for 90 ks in argon atmosphere, to generate a layer of fine grains at the powder surface.” has been replaced by “Material inside initial powder presents an acicular microstructure, which is a typical characteristic of α’ martensitic transformation induced by PREP rapid cooling [18], with an average hardness of 326 Hv. The chemical composition of this powder is introduced in Table 1. This powder is subjected to mechanical milling (MM) for 90 ks in argon atmosphere, to generate a several tens micron-thick layer of fine grains from the powder surface, resulting in 397 and 369 Hv hardness values inside fine grain layer and powder center region, respectively [18].”

Concerning the porosity of the compact, relative density of the compacts after SPS gave a value of 99.9%. Furthermore, previous work on the harmonic structure [15] outlined a very limited number of porosities into the microstructure. Therefore, the following sentence has been inserted in the revised manuscript: “Post SPS examination indicates a relative density of 99.9% of the compacts, which suggest a limited number of pores inside the examined materials, as already reported [15].”

• in the abstract: It should be checked in the context if the so-called crystallographic structure is not in fact microstructure characteristics because the study is on core/shell microstructure

Authors agree the wrong use of “crystallographic structure” in the abstract of the original manuscript. Since the abstract has been drastically shorten through the revision process, the occurrence of “crystallographic structure” has been deleted, thus is no more in the abstract of the revised manuscript.

• line 86: What is the microstructure of the raw starting material and after MM IP and SPS? the SPS treatment is below the transus, then it is interesting to know the microstructure before and after SPS.

In a similar way to the hardness values pointed out by the reviewer earlier, the microstructure of the very same initial powder has been investigated in [18]. Microstructure found shown acicular shape grains, very likely caused by α’ martensitic transformation induced by the rapid cooling process for powder fabrication (PREP).

Therefore, the sentence “Material inside initial powder presents an acicular microstructure, which is a typical characteristic of α’ martensitic transformation induced by PREP rapid cooling [18], with an average hardness of 326 Hv.” Has been added in the revised manuscript (Section 2.1, lines 94~96 of change tracked manuscript).

• line 114: please name the reference of the testing machine.

In accordance with the reviewer wish, reference of the testing machines (tensile loading: AG-X plus, Shimadzu Corp., fatigue loading: EMIC 9514-AN/SD and SAGINOMIYA SMH201) have been listed in revised manuscript (Section 2.2, lines 110, 123, 130 of change tracked manuscript).

• Figure 2, these microstructures are shown in the inner and outside region. It would also be interesting to see the microstructure in the center and the edge of the SPS specimen has the heating is often higher in the center for SPS of metals.

Authors acknowledge the reviewer’s point of view, regarding the microstructure analysis of the SPS compact. In the present investigation, the microstructure analyses were undertaken in location close to the center and the edge of the sintered compact. Since the actual positions of the observed samples were not exactly located at the center and the edge of the SPS compact, authors denoted these locations as “inner” and “outer” regions. Nevertheless, these regions are located at a maximum distance of 3 mm away from the respective center and edge positions of the compact. Therefore, the microstructure analysis in the present investigation aims to compare the microstructures at the center and the edge of the SPS compact, as suggested by the reviewer.

Therefore, it appears that “inner” and “outer” region expressions need to be clarified in the revised manuscripts. To do so, the following sentence has been inserted in Section 3.1 (line 167 of track changed manuscript): “These regions are located at a maximum distance of 3 mm from the center and the edge of the sintered compact, respectively.”

• figure 4a and 4b: the difference between a) and b is not clear, I would suggest merging them.

In accordance with the reviewer’s comment, Figs. 4(a) and (b) have been merged (Section 3.3.1, line 210 of track changed manuscript)

• line 217: the difference between MS and SS is still not clear

NB: due to additional citations in the revised manuscript, reference [18] of the original manuscript becomes reference [21] in the revised manuscript.

As mentioned in the introduction, previous work conducted by the authors [21] is dealing with the fatigue behavior of Ti-6Al-4V designed in harmonic structure. However, experimental results in [21] involve only fatigue test conditions equivalent to MS condition. Therefore, line 217 of the original manuscript consists only in a straightforward comparison of the fatigue crack initiation features from our previous work [21] and the present work in MS configuration.

To clarify this point, authors have revised the manuscript as follows: (Section 3.3.3, line 237 of track changed manuscript): the sentence “IP compound, in a way similar to the observations carried out in our previous work [18], showed a crack initiation at the specimen’s tensile surface revealing clear facets, as depicted in Fig. 5(a).” is replaced by “IP compound, in a way similar to the observations carried out in our previous work [21] involving only experiments on MS configuration, showed a crack initiation at the specimen’s tensile surface revealing clear facets, as depicted in Fig. 5(a).”