Modification of Cantor High Entropy Alloy by the Addition of Mo and Nb: Microstructure Evaluation, Nanoindentation-Based Mechanical Properties, and Sliding Wear Response Assessment
Abstract
:1. Introduction
2. Materials and Methods
3. Results and Discussion
3.1. Microstructure Assessment
3.1.1. XRD Analysis
3.1.2. Microstructural Analysis and Parametric Model Assessment
3.1.3. Possible Solidification Sequence
- At the initial stages of solidification, upon cooling, Mo is the element with the highest melting point and as such, drives the solidification process. By combining it with Cr, it forms the primary BCC solid solution phase, which also contains considerable amounts of the other elements being dissolved within it.
- Once the primary phase is formed, the last to solidify the liquid forms the FCC phase.
- Caution should be taken as potential σ-sigma phase formation may also take place, which is, however, difficult to be ascertained through SEM-EDS analysis.
- Nb and Cr seem to control the initial stages of solidification. The primary light phase, according to the EDS analysis, is rich in Nb and Cr with their relative ratio of almost 2:1. Nevertheless, the overall actual composition of the alloy (Table 1) indicates Nb and Cr at a ratio of almost 1:2. It is thus obscure to expect the formation of a Nb rich phase in a system where Cr dominates. Nevertheless, the Cr-Nb phase diagram (Figure 5) [58] may enlighten this grey zone and assist in the solidification of the primary phase. According to the phase diagram, for the relative overall composition of the system where the ratio of Nb to Cr is 1:2, the Laves phase C14 of a cubic crystal structure and with a stoichiometry of Cr2Nb can be formed. It was also interesting to note that from both sides of the Laves phases, a gap in the BCC structure also existed, the presence of which is very crucial. Based on these observations, it can be proposed that the solidification of the primary phase commences with the formation of C14 Laves. Once the C14 phase is formed, it locally depletes the remaining melt form Cr, and as such, it may shift the composition to the right area of the Laves region and upon temperature decrease, the system can be located within the BCC phase area where a BCC phase rich in Nb can be formed. This scenario of the formation of C14 and BCC phases is in agreement with the data presented in the XRD analysis.
- Once the primary phase is formed, the remaining melt seems to follow the postulates of He et al. [48] and behaves as a eutectic system with FCC and C15 Laves as the involved phases. As the temperature decreased, the established undercooling conditions led to the formation of the first eutectic structure, which was fine. As the development of the eutectic structure proceeded due to recalesence and the related heat release, the kinetics of the eutectic development became slower and as such, the eutectic gradually obtained a coarser structure. At the very last stages of the eutectic solidification, the kinetics were significantly retarded and the characteristic lamellae morphology vanished, giving space to more independent decoupling growth of the involved phases. Similar observations can be found in other works by Karantzalis and co-workers [49,50,51].
3.2. Nanoindentation Based Mechanical Property Assessment
Basic Mechanical Property Calculation
- (1)
- Modulus of elasticity (Eit): It can be seen that there was a slight increase in Eit after the modification of the basic Cantor alloy by Mo and Nb. More specifically, the Eit values were 200 ± 7, 213 ± 10, and 230 ± 10 for the plain Cantor alloy, the Mo modified system, and the Nb modified system, respectively. A possible explanation for this even slight increase can be associated with the microstructure and the involved phases in each case. By recalling the observations in the microstructure session, the plain Cantor alloy is a single-phase FCC solid solution with a lattice distortion δ value of 3.26, as calculated by the various parametric models. Lattice distortion mirrors the stress field experienced within the lattice and as such, can be associated with the modulus of elasticity measured in each case. The modification of the basic alloy by the addition of Mo led to phase segregation and the presence of two main phases. As shown in Table 3, the phases that were formed in this system had δ values of 3.80 (primary phase) and 3.40 (secondary phase or matrix). The nano-indentation tests were randomly performed on the specimen surface and as such, the calculated values were the average of the contribution of both segregated phases. Both the δ values of these phases were slightly higher than that of the plain Cantor alloy and as such, their overall contribution provided a δ value higher than the monolithic alloy, resulting in a slightly higher Eit value (213 GPa). The same approach also stands for the Nb modified alloy. The situation was even more intensive as the microstructure consisted of various segregated phases with their δ values ranging from 3.83 up to 6.60. These values were even higher and as such, their overall contribution resulted in an even higher average δ value, which was finally depicted by an even higher Eit value (230 GPa). Similar observations can also be found in other experimental efforts [29,59,60].
- (2)
- Hardness HV: Table 3 clearly shows that the modification of the monolithic Cantor alloy by Mo and Nb led to a significant alteration in the initial hardness HV. More specifically, the HV values were found to be 271 ± 10, 468 ± 30, and 490 ± 35. It can also be observed that the hardness alterations were by far more significant than the modulus Eit. Since hardness is considered to be the resistance of the system to plastic deformation, this means that the modification by Mo and Nb caused a significant negative effect on the dislocation mobility and plastic deformation, leading to a significant increase in the hardness values. The reasons for such an increase can be as follows: (a) Lattice distortion: The higher the lattice distortion, the higher the stress field within the lattice, and the more restricted the dislocation movement. (b) Multiple phases of different crystal structure: The Cantor alloy is an FCC alloy whereas the Mo modified system, according to both the XRD and the SEM analyses, apart from FCC, also contained the BCC and σ phases, which by nature exhibit low dislocation mobility and lower plastic deformation. This trend becomes more severe in the case of the Nb modified system, where additionally to the previous, intermetallic phases may also being present. (c) Increase of phase and grain boundaries: By recalling the microstructures of the examined system, it was observed that from the simple one phase grains of the monolithic Cantor alloy, at least two segregated phases where observed in the case of the Mo modified alloy, whereas in the case of the Nb addition, the microstructure became even more complicated with the presence of primary phases and eutectic morphologies. Thus, a sequence of the progressive development of more complicated microstructures associated with a progressive formation of more phase and grain boundaries was observed. This extensive boundary network also constitutes significant obstacles to the dislocation movement, and therefore to plastic deformation. All of these remarks are also in agreement with other research works [23,24].
- (3)
- nit ratio: The nit ratio, by definition, follows the trend of the hardness values. Since the hardness is increased by the addition of Mo and Nb (i.e., the resistance in plastic deformation is accordingly increased), the energy absorbed in the elastic region is increased (i.e., the nit ratio is altered in the same manner as the hardness).
3.3. Creep Assessment
3.3.1. Calculation Frame
3.3.2. Creep Results
3.4. Sliding Wear Response
4. Conclusions
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
References
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Composition at.% | ||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Alloy | Cantor | Cantor + Mo | Cantor + Nb | |||||||||||||||
Phase | ||||||||||||||||||
Cr | Mn | Fe | Co | Ni | Cr | Mn | Fe | Co | Ni | Mo | Cr | Mn | Fe | Co | Ni | Nb | ||
Nominal | 20.0 | 20.0 | 20.0 | 20.0 | 20.0 | 24.8 | 16.8 | 16.8 | 16.8 | 16.8 | 8.0 | 24.8 | 16.8 | 16.8 | 16.8 | 16.8 | 8 | |
Actual | 20.1 | 19.2 | 20.5 | 20.8 | 19.4 | 24.7 | 15.3 | 17.5 | 17.8 | 16.8 | 7.9 | 24.6 | 15.7 | 17.0 | 16.4 | 16.4 | 9.1 | |
Matrix | 25.1 | 14.1 | 19.0 | 18.3 | 17.4 | 6.1 | ||||||||||||
Primary phase | 29.3 | 12.9 | 16.0 | 16.5 | 11.8 | 13.5 | ||||||||||||
Light phase | 20.4 | 5.9 | 12.3 | 13.1 | 8.4 | 40 | ||||||||||||
Gray phase | 21.2 | 14.9 | 15.1 | 17.9 | 15.1 | 15.8 | ||||||||||||
Dark phase | 26.2 | 16.8 | 18.6 | 17.9 | 17.1 | 3.38 | ||||||||||||
Parametric Models | ||||||||||||||||||
Alloy | Cantor | Cantor + Mo | Cantor + Nb | |||||||||||||||
Phase | Nominal | Actual | Nominal | Actual | Matrix | Primary | Nominal | Actual | Light Phase | Gray Phase | Dark Phase | |||||||
Model | ||||||||||||||||||
δ | 3.27 | 3.22 | 3.65 | 3.58 | 3.40 | 3.79 | 4.58 | 4.69 | 6.60 | 5.40 | 3.83 | |||||||
ΔSmix [J/K mol] | 13.38 | 13.38 | 14.52 | 14.51 | 14.33 | 14.44 | 14.52 | 14.58 | 13.20 | 14.82 | 14.03 | |||||||
ΔHmix [kJ/mol] | −4.16 | −4.09 | −3.96 | −3.99 | −4.01 | −3.51 | −8.06 | −8.56 | −16.44 | −11.43 | −5.80 | |||||||
ΔG | −31.76 | −31.73 | −35.59 | −35.62 | −35.05 | −35.15 | −39.52 | −40.33 | −49.33 | −44.53 | −35.65 | |||||||
VEC | 8 | 8 | 7.68 | 7.71 | 7.76 | 7.41 | 7.60 | 7.58 | 6.63 | 7.43 | 7.73 | |||||||
Ω | 5.76 | 5.86 | 6.99 | 6.94 | 6.76 | 8.19 | 3.41 | 3.25 | 1.78 | 2.54 | 4.49 | |||||||
γ | 1.096 | 1.096 | 1.107 | 1.107 | 1.107 | 1.107 | 1.167 | 1.167 | 1.164 | 1.166 | 1.168 | |||||||
Tm [K] | 1790 | 1793 | 1905 | 1908 | 1892 | 1994 | 1893 | 1906 | 2218 | 1960 | 1854 | |||||||
ΔHIM/ΔHmix | 1.66 | 1.65 | 2.35 | 2.29 | 2.13 | 2.75 | 2.75 | 2.83 | 3.60 | 2.99 | 2.36 | |||||||
k1cr | 3.30 | 3.34 | 3.80 | 3.78 | 3.71 | 4.28 | 2.36 | 2.29 | 1.71 | 2.02 | 2.79 |
Element | Cr | Mn | Fe | Co | Ni | Mo | Nb | |
---|---|---|---|---|---|---|---|---|
Element | ||||||||
Cr | 0 | −110 | −8 | 5 | −30 | 42 | −47 | |
Mn | −110 | 0 | 9 | −19 | −115 | −136 | −153 | |
Fe | −8 | 9 | 0 | −60 | −97 | −484 | −2505 | |
Co | 5 | −19 | −60 | 0 | −21 | −52 | −150 | |
Ni | −30 | −115 | −97 | −21 | 0 | −100 | −316 | |
Mo | 42 | −136 | −484 | −52 | −100 | 0 | 0 | |
Nb | −47 | −153 | −2505 | −150 | −316 | 0 | ||
|
System | Eit (GPa) | HV | nit (%) |
---|---|---|---|
Cantor | 200 ± 7 | 271 ± 10 | 11.7 ± 2 |
Mo modified | 213 ± 10 | 468 ± 30 | 18.6 ± 4 |
Nb modified | 230 ± 10 | 490 ± 35 | 22.0 ± 7 |
System | n Extra | n Actual | m Extra | m Actual | Vcr Extra (nm3) | Vcr Actual (nm3) | hcreep (nm) | τmax (GPa) |
---|---|---|---|---|---|---|---|---|
Cantor | 32 ± 8 | 34 ± 9 | 0.031 ± 0.008 | 0.029 ± 0.009 | 0.196 ± 0.05 | 0.208 ± 0.06 | 38 ± 8 | 0.674 ± 0.05 |
Mo modified | 53 ± 9 | 63 ± 10 | 0.019 ± 0.007 | 0.018 ± 0.007 | 0.227 ± 0.06 | 0.269 ± 0.07 | 33 ± 10 | 0.960 ± 0.07 |
Nb modified | 107 ± 12 | 109 ± 12 | 0.0133 ± 0.009 | 0.0115 ± 0.09 | 0.62 ± 0.08 | 0.414 ± 0.07 | 23.5 ± 8 | 1.071 ± 0.09 |
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Karantzalis, A.E.; Poulia, A.; Kamnis, S.; Sfikas, A.; Fotsis, A.; Georgatis, E. Modification of Cantor High Entropy Alloy by the Addition of Mo and Nb: Microstructure Evaluation, Nanoindentation-Based Mechanical Properties, and Sliding Wear Response Assessment. Alloys 2022, 1, 70-92. https://doi.org/10.3390/alloys1010006
Karantzalis AE, Poulia A, Kamnis S, Sfikas A, Fotsis A, Georgatis E. Modification of Cantor High Entropy Alloy by the Addition of Mo and Nb: Microstructure Evaluation, Nanoindentation-Based Mechanical Properties, and Sliding Wear Response Assessment. Alloys. 2022; 1(1):70-92. https://doi.org/10.3390/alloys1010006
Chicago/Turabian StyleKarantzalis, Alexandros E., Anthoula Poulia, Spyros Kamnis, Athanasios Sfikas, Anastasios Fotsis, and Emmanuel Georgatis. 2022. "Modification of Cantor High Entropy Alloy by the Addition of Mo and Nb: Microstructure Evaluation, Nanoindentation-Based Mechanical Properties, and Sliding Wear Response Assessment" Alloys 1, no. 1: 70-92. https://doi.org/10.3390/alloys1010006
APA StyleKarantzalis, A. E., Poulia, A., Kamnis, S., Sfikas, A., Fotsis, A., & Georgatis, E. (2022). Modification of Cantor High Entropy Alloy by the Addition of Mo and Nb: Microstructure Evaluation, Nanoindentation-Based Mechanical Properties, and Sliding Wear Response Assessment. Alloys, 1(1), 70-92. https://doi.org/10.3390/alloys1010006