In Situ Al₃BC/Al Composite Fabricated via Solid-Solid Reaction: An Investigation on Microstructure and Mechanical Behavior

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

A manuscript entitled “In-situ Al3BC/Al composite fabricated via solid-solid reaction: An investigation on microstructure and mechanical behavior” is very well written and structured. The manuscript needs some minor modifications before acceptance. The changes required in the manuscript are as follow:

• Author must incorporate need/motivation/application (2-3 sentences) in abstract to attract the attraction of readers before explaining their work.
• Author needs to give full name rather abbreviations like “Ceramic reinforcements with high modulus 38 and strength, such as SiC [9], Al2O3 [10], B4C [11], TiB2 [12], AlN [13,14], ZrB2 [15], Al3Ti [16], 39 Mg2Si [17], etc.,” (Line 39).
• Give full form at first places then abbreviations can be used. SiC, and Al₂O₃ used without mentioning full form. Add their full forms at appropriate places. Check this for all other abbreviations in manuscript.
• Author needs to divide experimental section into three different parts giving details of Materials, Processing technique and characterization equipment’s.
• There are some typo and grammatical errors which needs to be modified during revision stage of the manuscript.
• Why author choose only compressive testing while there are many other mechanical properties like tensile, impact and fracture toughness. Explain why?
• Rewrite the conclusions precisely by removing repetition as discussed in abstract and discussion part. Only give the crux/key outcomes having significance for scientific community along with future road map.

Author Response

1. Why was the molar ratio of B, GNFs and Al powder of 1:2:4 used to prepare the Al3BC/Al composite?

The addition of AlBGNPs powders in a 4:2:1 ratio promotes a more homogeneous microstructure, which stabilizes the flow of Al matrix during hot-press deformation, enabling better distribution of Al₃BC reinforcements across the grain boundary interface of Al matrix.

2. Does the mixture not oxidize during preheating (overnight at 80°C)?

The mixture was preheated in vacuum and therefore did not oxidize. No diffraction peaks from oxides in the XRD pattern indicates that the mixture did not oxidize during preheating.

3. In Figure 2c, only one peak is assigned to AlB₂ to state that such a phase exist, at least 2 peaks should be assigned.

We are thankful to the reviewer. In the revised version of the manuscript we have shown that there are more than one peak (approx. 4 peaks) of hexagonal-AlB₂ (space group: P6/mmm; PDF-2-2023#03-65-3381) exists in the XRD pattern. The diffraction peaks appear at 27.38°, 34.42°, 44.53° and 61.68°, corresponds to the peak Miller indices of (001), (100) (101), and (110).

4. The text must include JCPDS card numbers of all phases.

The XRD pattern [Figure 1a] of the composite powder with different milling times (180 min-420 min) showed only 5 major peaks from FCC phase of Aluminium (FCC-Al) [card number 98-008-4181]. No intermetallic compounds such as Al₄C₃ and Al₂O₃ could be identified. As is evident from Fig. 1a. the yield of the powder with milling duration 7-h is the highest. Therefore, milling duration 7-hr (420 min) could be the most suitable option. Fig. 1b represents the conventional XRD patterns recorded at room temperature for the 7-hour milled products obtained after thermal treatment at sintering 1000°C and subsequent hot-press (40 MPa/400°C). It indicates with the increase in temperature at 1000°C, the diffraction peaks appear at 30.5°, 37.8°, 43.3°, 49.6° and 52.3°, corresponds to the peak Miller indices (101), (102), (103), (104), (006), (105) and (110) expected for hexagonal Al₃BC (space group: P63/mmc; PDF-2-2023#00-047-1628). Moreover, peaks from FCC-Al (space group: Fm-3m; PDF-2-2023#00-004-0787), and hexagonal-AlB₂ (space group: P6/mmm; PDF-2-2023#03-65-3381) also appear in the XRD pattern. This finding suggests that thermal treatment at 1000°C promotes Al₃BC as primary in-situ reinforcing phase. Furthermore, presence of graphite peak (space group: P63/mmc; PDF-2-2023#00-041-1487) appearing at 26.4° corresponds to the peak Miller indices (002) suggests that unreacted graphite is present.

5. How were the volume/ Mass fraction in Table 1 calculated to be 10%? Why such a large amount?

The vol. fraction/ or mass fraction of AlB₂ with 10  0.1 was calculated using the Rietveld refinement of XRD peak profile analysis. The area fraction  of the phases was quantitatively measured (including > 5000 particles) using the ImageJ software which determined area fraction of AlB₂ as  173 %. The XRD peaks in Fig. 1 suggests AlB₂ exits in such a high amount. The reaction kinetics (DSC figure) suggests that there is a simultaneous reaction of Al, B and GNPs during the solid-solid reaction that synthesis in-situ Al₃BC; 3Al(s) + [B] + [C] → Al₃BC(s). Concurrently, several Al-B compounds such as AlB2, AlB10, AlB12 also forms in the Al-B system [1-3]. As AlB2 is the most stable compound below 980°C [in this case solid-solid reaction temperature 1000°C], corresponding amount of AlB2 (10 ± 0.1 wt.%.) will form while reacting remaining un-reacting B with the Al matrix; Al(s) + 2B(s) → AlB₂ (s). Which remain in the microstructure adjacent to or surrounded by the Al₃BC phase.

6. The authors states (lines 145-147) that the addition of GNPs and B promotes the Al3BC formation. Are the results of other studies known? Justify your statement.

No

7. Why are the values of hardness and modulus of FCC-Al and AlB₂ phases taken from (Table 1)? Determined or taken from literature?

The values of the hardness of FCC-Al, Al₃BC and AlB₂ were measured from the Vickers hardens experiment with a value of 3 1 GPa,  22 7 GPa and 2 0.5 GPa, respectively. The hardness was measured using a Vickers hardness tester at a load of 100 g [ASTM E10-14 standard]. Which was useful to determine the composite hardness of the Al3BC/Al composite as a bulk theoretically predicted using the ROM approach [33] with a value of 14.8 ± 5 GPa. The modulus of FCC-Al matrix was determined from the nanoindentation techniques with a value of 52 GP, whereas the modulus values of Al₃BC and AlB₂ was taken from literature references of [20], and [ref-1], respectively. The reference of modulus is mentioned from ref-1, which has been incorporated in the modified manuscript.

References:

ref-1. V.I. Ivaschenko, P.E.A. Turchi, R.V. Ivaschenko. L. Gorb, J. Leszczynski, Amorphous AlB2, AlBC, and AlBN alloys: A first-principles study. J. Non-Cryst. Solid. 577 (2022) 121325.

8. Unify the sizes of the numbers in Table 2.

Modified as requested.

9. The values presented in line 181-183 do not correspond to the data in lines 144-145, because 65±2 wt.% are assigned to the Al3BC phase there. And in line 183 it is said that ~65 wt.% is Al2O3, which was not mentioned at all before. Please provide an explanation and correct the errors in the manuscript.

We are very sorry for the typos mistake. It should be the Al3BC phase. There is no such Al₂O₃ phase formed during the process [XRD pattern in Fig. 1].

Reviewer 2 Report

Comments and Suggestions for Authors

In this research, the authors focused on a straightforward method for fabricating Al₃BC/Al composite using mechanical alloying (ball milling) combined with a solid-state reaction technique (pressing and sintering). Al-matrix composites are very important because of superior specific strength and stiffness, enhanced wear resistance, and good structural integrity with property design flexibility.

 

Comments:

1. Why was a molar ratio of B, GNFs, and Al powders of 1:2:4 used to prepare the Al₃BC/Al composite?
2. Does the mixture not oxidize during preheating (overnight at 80 ^oC)?
3. In Figure 2c, only one peak is assigned to the AlB₂ In order to state that such a phase exists, at least 2 peaks should be assigned.
4. The text must include the JCPDS card numbers of all phases.
5. How were the volume? mass fractions in Table 1 calculated? AlB₂ mass fraction is calculated to be 10±1 wt.%. Why such a large amount?
6. The authors states (lines 145-147) that the addition of GNFs and B promotes the Al₃BC formation. Are the results of other studies known? Justify your statement.
7. Where are the values of hardness and modulus of fcc-Al and AlB₂ phases taken from (Table 1)? Determined or taken from the literature?
8. Unify the sizes of the numbers in Table 2.
9. The values presented in lines 181-183 do not correspond to the data in lines 144-145, because 65±2 wt.% are assigned to the Al₃BC phase there. And in line 183 it is said that ~65 wt. % is Al₂O₃, which was not mentioned at all before. Please provide an explanation and correct the errors in the manuscript.

Author Response

1. Authors must incorporate need/motivation/application (2-3 sentences) in abstract to attract the attention of readers before explaining their work.

Thank you for the suggestions. We have modified the abstract accordingly which have been incorporated in the revised version of the manuscript.

Al₃BC with their remarkable high modulus of elasticity (326 GPa) and hardness (14 GPa), coupled with a low density (2.83 g/cc), stands out as a promising reinforcement material for Al matrix composite. To study feasibility of solid-solid reaction (SSR) by forming in-situ Al₃BC reinforcing phase within the matrix, this study developed Al₃BC/Al composite via mechanical alloying and followed by sintering - 1000 °C/1 h and subsequent hot pressing - 400 °C/40 MPa. The reaction kinetics and corresponding Electron microscopy images suggests that the aluminum (Al) – boron (B) reacts with graphene nanoplates (GNPs) to form both clusters, and heterogeneous multi-structured Al₃BC reinforcements network dispersed within fine grain (FG) Al matrix. The heterostructure contributes to a good balance between strength (~284 MPa) and ductility (~17%) and stiffness (~212 GPa). Superior strain hardening ability (n = 0.3515) endorses remarkable load-bearing capacity (σ_c = 1.63) and thereby promotes excellent strength-ductility synergy in the composite. The fracture morphology reveals that reasonable ductility primarily relies on the crack deflection by the FG-Al matrix, playing a critical role in delaying fracture. The potential importance of the matrix microstructure in the overall fracture resistance of the composite has been highlighted.

2. Authors need to give full name rather than abbreviations like …line 39.

Thank you for the suggestions. We have modified the manuscript accordingly which have been incorporated in the revised version of the manuscript.

3. Give full form at first places than abbreviations can be used. SiC and Al2O3 used without mentioning full form. Add their full forms at appropriate places. Check this for all other abbreviations in manuscript.

Thank you for the suggestions. We have modified the manuscript accordingly which have been incorporated in the revised version of the manuscript.

4. The author needs to divide experimental section into three different parts giving details of Materials, Processing technique and characterization equipment’s.

Experiment work:

2.1 Materials:

Investigating Al3BC/Al composite was fabricated via ball milling and vacuum sintering (1000°C/1 h) to initiate solid-solid reaction of constituent elements of commercial Al powder (particle size 250-450 μm, HiMedia TM), GNPs (average diameter 1.65 nm, Alfa Aesar TM) aggregates, and amorphous B (Sigma Aldrich TM). B and GNP powders (2:1 molar ratio) were mechanically alloyed in a high-energy ball mill (Retsch PM 400 MA).

2.2. Processing technique:

Milling was carried out for 2 h at a speed of 200 rpm in an Argon atmosphere to ensure uniform element distribution. The ball milling parameters have been reported elsewhere [27]. The milled B+GNPs powder was mixed with Al powder (overall molar ratio 1:2:4) for an additional 5 h using a ball mill and subsequently pre-heated (overnight at 80°C) and green compacted (60 vol.%). To prepare the corresponding Al3BC/Al composite the green compact was then sintered in a vacuum sintering furnace (tubular furnace) at 1000°C. The sintering process maintained a slower heating and pressurization process, e.g., heated at a rate of 5°C/min, kept for 1 h (soaking time) below 5 × 10^-2 mbar pressure, and then slowly cooled to room temperature. It ensures Al will maintain formability favorably and even become liquid at 680°C. Finally, a low-pressure (40 MPa) and high temperature (400°C) hot-press was applied for 20 min in a graphite mold (φ 20 mm) to achieve final consolidated specimens. During hot-pressing, the heating rate was maintained at 25°C/min followed by water cooling. The experimental procedure employed in this research is shown schematically in Fig. 1.

2.3. Characterization:

The composite samples were subjected to standard metallurgical procedures including grinding and polishing and subsequently cut (as cylindrical compression test specimens) using wire-EDM following standard procedure. The overall phase composition was evaluated with X-ray diffraction (XRD) using RigakuTM DMAX X-ray diffractometer [operating at 40 kV - 40 mA and Cu-Kα radiation]. The reactions during the composite formation were investigated using differential scanning calorimetry (DSC). Microstructure and composition area mapping were then investigated using an electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS). The SEM device fitted with an electron backscattered diffraction (EBSD) detector was used to obtain inverse pole figure maps (IPF) with step size 0.5 μm by an EDAX TridentTM system along with system integrated FEI Scios™ field emission gun (FEG) scanning electron microscope (FE-SEM). The EBSD data was processed using an EDAX OIM Analysis™ software and a detailed description of the technique has been reported elsewhere [27]. The hardness was measured using a Vickers hardness tester at a load of 100 g [ASTM E10-14 standard]. The room temperature (RT) compression test (strain rate 0.5×10^-3 s^-1) specimens with dimensions 20 × 5.6 × 5 mm3 was evaluated using a servo-hydraulic UTM (100 kN load cell) [ASTM–E08 standard]. The tests continued until the failure of the specimen. The fracture surfaces of the post-compressed specimens were examined using SEM to understand the mode of failure.

5. There are some typos and grammatical errors which needs to be modified during the revision stage of the manuscript.

Thank you for the suggestions. We have modified the manuscript accordingly which have been incorporated in the revised version of the manuscript.

6. Why authors choose only compressive testing while there are many other mechanical properties like tensile, impact and fracture toughness. Explain why?

The investigating alloys were synthesized via prolonged 7-hour milling of amorphous boron (B), GNP (C), and aluminum (Al), followed by high temperature at 1000°C for 1-hr thermal treatment (sintering) and subsequent hot-press under 40 MPa at 400°C. Because the hot-press limited the sample dimensions, we could not get tensile specimens. We were able to perform only the room temperature compression test with a specimen dimension of 20 × 5.6 × 5 mm³ [ASTME08 standard]. It’s unfortunate that the simple dimension constraints prevented us from performing other mechanical properties like tensile, impact and fracture toughness to explore in this study. However, we plan to investigate these properties in our future work.

7. Rewrite the conclusions precisely by removing repetitions as discussed in the abstract and discussion part. Only give the crux/key outcomes having significance for scientific community along with future road map.

In summary, the as-fabricated Al3BC/Al composites involving solid-state reactions exhibit improved mechanical properties. This reaction kinetics suggests that that prolonged milling duration (7-hr) accelerates the solid-solid reaction to initiate in the modified alloy. Structure and microstructure investigation reveals that hot-pressing (400°C/40 MPa) effectively promotes multi-scale heterostructure in the composite via size reduction by breaking down and dispersing agglomerates within the composite from coarse-grain CG-Al₃BC to sub-micron FG-Al₃BC dispersed within FG-Al matrix. Strong interfaces between FG-Al matrix promote crack bridging-deflection through the plastic flow, reducing crack propagation, delaying fracture, and ultimately maintaining adequate ductility.