From Ni–P Metastable Alloy Nanoparticles to Bulk Submicrometer Grain-Sized MMCs with Tunable Mechanical and Magnetic Properties

Round 1

Reviewer 1 Report

The authors report about synthesis of bulk nanostructured MMCs with tuneable mechanical and magnetic properties.

The manuscript contains a lot of weak points and outstanding issues in the experimental, results and discussion section, some misspellings and vague formulations. For example:

-Extensive editing of English language and style required. The paper is not written in proper scientific English. For example:

“True stress vs. true strain curves obtained from compression tests of the Ni-P samples sintered at 10 min under 53 MPa for holding time 600 °C range and starting from various grain sizes of the initial powder are shown in Figure 4.”

“These two parameters make the MMC  material an increasingly hard structure.”

-Abbreviations are not consistently used: XRD, SPS, etc.

-Although the final Ni grain size in the MMCs is well above the nanocrystalline regime (250-640nm), the authors title their work “nanostructured” MMCs.

-The authors claim that the can tune the composite from a soft magnetic behaviour to a hard magnetic behaviour. That is not accurate since the values for the coercivity reported in the paper are in the best case suitable for a “semi-hard” material.

-It is stated that the MMCs are fully dense, which is not the case, since the density reported range from 94.5-97.8%.

-No details about the grain size determination by TEM are given. The grain size in the BF images looks similar for NiP130SPS and NiP220SPS, although a huge difference in the grain sizes are reported and also the mechanical and magnetic properties differ a lot for both specimes.

In summary, the discussion section leaves many questions open, the conclusions are often not supported by the experimental data and the work is viewed as not being of the same quality of general articles published in metals. Based on these collective findings, I must reject the article from publication.

Author Response

Reviewer #1: Comments and Suggestions for Authors

The authors report about synthesis of bulk nanostructured MMCs with tuneable mechanical and magnetic properties.

The manuscript contains a lot of weak points and outstanding issues in the experimental, results and discussion section, some misspellings and vague formulations. For example:

1) Extensive editing of English language and style required. The paper is not written in proper scientific English. For example:

Response 1: The manuscript has been reviewed and corrected by Mdpi service editing (see certificate attached).

2) Abbreviations are not consistently used: XRD, SPS, etc.

Response 2:  This has been fixed in the revised version.

3) Although the final Ni grain size in the MMCs is well above the nanocrystalline regime (250-640nm), the authors title their work “nanostructured” MMCs.

Response 3: Thank you for this important remark. Indeed, if we refer to the size of grain, the MMCs are above the nanostructured regime. But their properties are similar to that of nanostructured ones that appear in our published works (M.A. Bousnina, F. Schoenstein, L. Smiri, N. Jouini, Solid State Sci. 40 (2015) 13-19, Bousnina M.A.; Turki F.; Schoenstein F.; Têtard F.; Rabu P.; Smiri L.S.; Jouini N. J. Alloys Compd. 686 (2016) 252-266) and thus they were called nanostructured. However, we agree with the reviewer and we changed in all the manuscript, the term into submicrometer grain-sized a term used in the literature beside ultra-fine grained to characterize materials with grain size below the micrometer.  

4) The authors claim that the can tune the composite from a soft magnetic behaviour to a hard magnetic behaviour. That is not accurate since the values for the coercivity reported in the paper are in the best case suitable for a “semi-hard” material.

Response 4: Thank you for this pertinent remark. We did not use this term in the body of the text. It was only used in the abstract and also in the conclusion to emphasize the importance of the results obtained, namely the control of the coercive field in a wide range. We agree with the reviewer that the MMCs are in the best case semi-hard materials. Changes have been made both in the abstract and in the conclusion.

Abstract: lines 25-26; Conclusion: lines 388-389

5) It is stated that the MMCs are fully dense, which is not the case, since the density reported range from 94.5-97.8%.

Response 5: We agree with the reviewer. Our materials are not fully dense. They present a residual porosity varying in the range 5.5%-2.2%. The term fully dense appears in the first version only in the introduction when the advantages of SPS are cited. To avoid any ambiguity, the term has been removed in the new version.   Also a discussion on the porosity of the obtained MMCs was added in several parts in the new version and particularly its influence on the microhardness :

Part 1 in the paragraph result: 2. Density and Vickers microhardness Part 2 in the paragraph discussion- lines: 268-277 Part 3 in the paragraph discussion- lines: 288-296

6) No details about the grain size determination by TEM are given. The grain size in the BF images looks similar for NiP130SPS and NiP220SPS, although a huge difference in the grain sizes are reported and also the mechanical and magnetic properties differ a lot for both specimens.

Response 6:  The grain size of Nickel and Ni₃P was determined by a statistical study of 200 particles using the Image Tool program. This information was added in the revised version (lines 81-82).

We agree with the remark of the reviewer , the TEM image of the NiP130SPS sample was modified by another which clearly shows that the nickel grain size is around 383 nm and a new image of the NiP100SPS samples was added (suggested by  reviewer 2) (See now Figure 3).

7) In summary, the discussion section leaves many questions open, the conclusions are often not supported by the experimental data and the work is viewed as not being of the same quality of general articles published in metals.

Response 7: We hope that the new version is much improved along all these dimensions. We clearly show that the mechanical and magnetic properties of this submicrometer grain-sized MMCs depend on 3 main parameters: the grain size, the presence of Ni3P as reinforcement and the porosity. Among these parameters, in the new version we have established more clearly that the grain size plays the major role. Yield strength, coercive field increase when the grain size decrease whatever the amount of porosity (5.5%-2.2%) and the amount of the reinforcement phase (varying in a relative narrow domain (4%-9% in weight)).  The presence of Ni3P enhances the mechanical behavior in comparison with similar materials based on pure nickel. More interesting, it was shown that mechanical characteristics (Yield strength, plastic strain) along with coercive field behave similarly in function of Ni grain size with a crossover for a critical size around 350 nm. Below this size, the MMCs have as a hard brittle character and semi-hard magnetic one. Above this size, the MMCs evolve to ductile and soft magnetic characters. Finally we agree with the referee that some questions remain open and particularly theoretical predictions and calculations in magnetism and mechanic in order to understand the crossover observed for both properties for the same critical size.

All changes are indicated in red color.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript titled “From Ni-P Metastable Alloy Nanoparticles to Bulk Nanostructured MMC with Tunable Mechanical and Magnetic Properties” is presenting investigations on the effect of grain and crystallite size of Ni-based MMCs (Ni – matrix, Ni3P reinforcing particles), on some mechanical and magnetic properties. This paper is not recommended for publication in the current state. Following are only some of the issues in the manuscript which are to be seriously addressed: 1. The English language and grammar used in the present manuscript is generally poor. There are plenty of instances where mistakes, misspelled words and/or poorly chosen words (ambiguous) are present. I strongly suggest that the paper should be proofread and double-checked concerning the spelling and phrasing. 2. It is not clear why the authors choose to use the “metastable” term in regards to the original nanoparticles. What makes them metastable? 3. Row 82, correct mm3 to mm. 4. The article might benefit from adding XRD patterns for the original nanoparticles, along with related discussions. 5. Please add the corresponding images to figure 2, related to sample NiP100SPS, even if they are used in a different article. 6. The discussion related to the differences of micro-hardness (especially) is unconvincing. Apart from the differences in grain and crystallite size, the Ni3P content also differs between samples. Considering this phase is considered as reinforcing, I would expect that the content will change the mechanical behavior. Can the authors comment on this issue? 7. Table 2: missing error for hardness; last column is not necessary.

Author Response

Reviewer #2: Comments and Suggestions for Authors

The manuscript titled “From Ni-P Metastable Alloy Nanoparticles to Bulk Nanostructured MMC with Tunable Mechanical and Magnetic Properties” is presenting investigations on the effect of grain and crystallite size of Ni-based MMCs (Ni – matrix, Ni3P reinforcing particles), on some mechanical and magnetic properties.

This paper is not recommended for publication in the current state.

Following are only some of the issues in the manuscript which are to be seriously addressed: 1. The English language and grammar used in the present manuscript is generally poor. There are plenty of instances where mistakes, misspelled words and/or poorly chosen words (ambiguous) are present. I strongly suggest that the paper should be proofread and double-checked concerning the spelling and phrasing.

Response 1: The manuscript has been reviewed and corrected by Mdpi service editing (see certificate attached)

It is not clear why the authors choose to use the “metastable” term in regards to the original nanoparticles. What makes them metastable?

Response 2: The use of the “metastable” term in regards to the original nanoparticles was well explained in our previous work: [ref 1]

“The phosphorus is inserted in the face-centered cubic cell of nickel forming a solid solution Ni-P with a weight percentage of phosphorus close to 4%. Such solid solution cannot be explained in the basis of the stable equilibrium diagram of Ni-P system wherein the solid solution Ni-P is very limited with phosphorus content not exceeding 0.17% by weight (Figure a) [ref 2]. To account for solid solution with higher phosphorous amount, a metastable equilibrium diagram was established by Duncan (Figure b) [ref 3] with a percentage of phosphorus up to 15% by weight. In this diagram we note the presence of two phases namely β and γ ones. The β phase is a crystalline solid solution of phosphorus in nickel with phosphorous content varying up to 4.5%, and γ phase is an amorphous solid solution with phosphorous amount ranging from 11% to 15%. One can note that the nanoparticles prepared in this work have the composition up to 4% phosphorus. They appear to be only constituted of the β phase; since, the X-ray diffraction shows no amorphous phase.” Article Solid State Sciences_2015

(a) Ni-P stable equilibrium diagram [ref 2] (b) Ni-P metastable equilibrium diagram [ref 3]

[Ref 1]: M.A. Bousnina, F. Schoenstein, L. Smiri, N. Jouini, Solid State Sci. 40 (2015) 13-19.

[Ref 2]: C. Schmetterer, J. Vizdal, H. Ipser, Intermetallics 17 (2009) 826-834.

[Ref 3]: R.N. Duncan, Plat. Surf. Finish. 83 (1996) 65-69.

In conclusion, we have included in the revised version a short discussion on this point and a reference has been added (C. Schmetterer, J. Vizdal, H. Ipser, Intermetallics 17 (2009) 826-834) (see  3.1. X-ray diffraction and TEM characterizations :  lines 98-103)

Row 82, correct mm3 to mm.

Response 3:  1.5 × 1.5 × 2.4 mm3 is corrected to 1.5 mm × 1.5 mm × 2.4 mm (line 85 in the new version)

The article might benefit from adding XRD patterns for the original nanoparticles, along with related discussions.

Response 4: According to the reviewer suggestion, the XRD patterns of as-prepared powders were added and discussed in the revised version (Results: 3.1. X-ray diffraction and TEM characterizations

Paragraph 1). All prepared nanoparticles have cell parameter almost identical to that of pure Ni despite the presence of an important amount of phosphor.  Also, the particle size for each sample was added in table 1. Furthermore, TEM images and chemical composition are given in supplementary material S1.

Please add the corresponding images to figure 2, related to sample NiP100SPS, even if they are used in a different article.

Response 5: The TEM image and the grain size histogram for the sample NiP100SPS is added in figure 3 (Figure 2 in the first version).

The discussion related to the differences of micro-hardness (especially) is unconvincing. Apart from the differences in grain and crystallite size, the Ni₃P content also differs between samples. Considering this phase is considered as reinforcing, I would expect that the content will change the mechanical behavior. Can the authors comment on this issue?

Response 6: Thank you for this important remark. This led us to deepen and enrich the discussion of the relationship between properties and microstructure. It appears that alongside the two parameters mentioned in the first version (grain size and presence of the Ni3P phase), the porosity must be considered to explain the different evolution of Vickers hardness in comparison with the other mechanical characteristics inferred from compression test (Yield strength, maximum stress, plastic strain).
From our recent work (Bousnina et al. J. Alloys Cmpds 2016), it clearly appears that the grain size is the main parameter governing the mechanical characteristics deduced from the compression test. We have gathered in the table below the necessary data extracted from our previous work to illustrate our remarks:

 As it can be seen:
- whatever the Ni₃P composition between 12 and 9%, YS for example,  increases when the grain size decreases
- despite its highest porosity, the material (Np2-600) with the smallest grain size has the highest value of YS,  the highest value of maximum stress and the lowest value of plastic strain.

In contrast, Vickers hardness, which corresponds to a more location-specific analysis, has a more subtle behavior. It increases with the grain size to reach a maximum value when the grain size is close to 350 nm, and then it decreases. This shows that the hardness depends of several parameters including mainly the porosity and the grain size. A balance between these two parameters makes it possible to obtain the highest hardness (P % 4.4, grain size 330 nm). The samples with a smaller grain size correspond to high porosities which are harmful for obtaining high hardness. Beyond the critical size of 330 nm, the samples have a very low porosity but have a low hardness similar to solid nickel due to the large grain size.

In conclusion, the revised version includes several parts taking into account with these points:

Part 1 in the paragraph result: 2. Density and Vickers microhardness Part 2 in the paragraph discussion lines: 268-277 Part 3 in the paragraph discussion lines: 288-296

Table 2: missing error for hardness; last column is not necessary.

Response 7: The hardness error was added in table 2 and the last column was deleted.

All changes are indicated in red color

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The manuscript has been significantly improved and now warrants publication in Metals.

Reviewer 2 Report

My comments were answered satisfactorily, the manuscript was significantly improved.