2.1. Background and Conventional Methods
Brian Cantor estimated the total number of possible metallic alloys with different compositions to be up to around 1078
[9]. This means many new alloys are yet to be discovered. For the manufacturing of HEAs, the initial synthesis strategy was to choose equi-atomic concentration of principle elements to maximize the entropy of the system. However, later, HEAs in non-equi-molar ratios were also developed for various applications. Arc melting was mostly preferred to produce HEAs thanks to its convenience, availability and simplicity. Furthermore, developing a HEA became more complex as more non-equi-atomic compositions were considered and several other manufacturing techniques were used. Alshataif et al.
[10] covered almost all kinds of processing techniques used so far for HEAs synthesis. They detailed solid state processing (i.e., powder atomization methods, ball milling, cold/hot pressing, sintering, spark plasma sintering), liquid state processing (i.e., arc melting, vacuum induction melting, directional solidification, infiltration, electromagnetic stirring), thin film deposition (i.e., magnetron sputtering, pulsed laser deposition, plasma spray deposition) and additive manufacturing. Most of these manufacturing techniques are commercially available. That means most HEAs would not require a special manufacturing process and mass-producing HEAs would be possible with the existing alloying technologies and facilities.
The influence of process parameters, such as temperature and pressure, on the properties of HEAs were also studied. The effects of temperature on the properties of HEAs were studied through processes such as: annealing and heat treatments
[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] and thermomechanical processing
[31][32][33][34]. A number of research groups reported how temperature affected the microstructures and mechanical properties of HEAs in various manufacturing processes
[22][35][36][37][38]. Moreover, the physical or chemical responses of various HEAs under a variety of thermal histories during manufacturing were studied: thermal aging behavior
[12][39][40][41], TaNbHfZrTi synthesis by hydrogenation–dehydrogenation reaction and thermal plasma treatment
[42], martensite formation
[43][44][45][46], Al
xCoCrFeNi formation with high gravity combustion from oxides
[47], laser surface melting
[48], precipitation behavior
[49][50][51][52] and WTaMoNbV synthesis using inductively coupled thermal plasma
[53].
Researchers have also attempted to alter the microstructures and properties of HEAs by high pressure treatments. Regulating pressure during fabrication of HEAs can considerably alter the interaction between the atoms by changing the interatomic distance, bonding nature and packing densities. These changes often convert the microstructures and affect the mechanical and structural properties. Dong et al.
[54] reviewed the applications of high pressure technology for HEAs. They reviewed the use of dynamic high pressure, diamond anvil cells, high pressure torsion and hexahedron anvil press. Zhang et al.
[55] reviewed high pressure induced phase transitions in HEAs. Application of high pressure torsion
[56][57][58][59][60][61][62][63][64][65][66][67][68][69][70] is more frequent than other pressure techniques
[64][71][72][73][74][75][76][77][78].
Furthermore, various researchers successfully welded/brazed HEAs
[79][80][81][82][83]. Guo et al.
[79] reviewed arc welding, laser welding, electron beam welding, friction stir welding to join HEAs and conducted the microstructural analysis on the welded structures. Filho et al.
[81] gave a general review on the properties of welded HEAs parts and Tillmann et al.
[83] reviewed HEAs brazing. Lopez et al.
[80] reviewed fusion based welding (i.e., for CoCrFeNiMn and other related HEA systems) and solid state welding. Scutelnicu et al.
[82] reviewed friction stir, electron and laser beam, tungsten inert gas welding techniques for CoCrFeMnNi, AlCoCrCuFeNi, AlCrFeCoNi and CoCrFeNi alloys.
2.2. Additive Manufacturing of HEAs
3-D printing in manufacturing industries, when properly applied, not only makes a design phase more efficient and economic but also brings thoughtful impacts on product design. Recent advances in additive manufacturing (AM) made it more influential throughout the supply chain which generates revenue as well
[84]. The additive manufactured HEAs showed improvement in their mechanical properties in comparison to as-cast HEAs
[85][86][87][88][89][90][91][92]. Higher cooling rates in AM processes help suppress diffusional phase transformation and increase the chemical homogeneity of HEAs
[93]. Under certain circumstances, AM gives a better control over the material processing and helps tailor application-specific microstructures which become more important for the parts for applications under extreme environments. For example, it was demonstrated that fine and tailorable microstructures in HEAs were obtained using AM techniques
[94][95][96][97][98][99][100][101], which implies AM can improve the mechanical performance of at least some HEAs. However, this may not be a trivial task as a good understanding of the AM technique and material behavior during the AM process is required
[102].
AM of HEAs has been discussed briefly in a few review papers
[103][93][104][105] and books
[106][107]. Xiaopeng Li
[93] discussed the requirements and challenges of AM of HEAs and bulk metallic glasses. Chen et al.
[103] examined the microstructural evolution and mechanical properties of AM-processed CoCrFeNi, Al
xCoCrFeNi, CoCrFeMnNi and Ti
25Zr
50Nb
50Ta
25. Fabricating HEAs by spark plasma sintering (SPS) and their property analyses were discussed in the book chapter “Spark Plasma Sintering of High Entropy Alloys” of
[108]. SPS followed by mechanical alloying has largely been used to develop HEAs, which was reviewed in detail by Vaidya et al.
[109]. Therefore, SPS studies are not included here.
Here, studies on the AM of HEAs are tabulated and the mechanical properties of these HEAs are discussed. Table 1, Table 2 and Table 3 detail the HEAs synthesized by selective laser melting (SLM), electron beam melting (EBM) and direct energy deposition (DED), respectively. The performances of these HEAs are discussed in terms of their composition, their microstructure and their mechanical properties, such as ultimate tensile strength (UTS), % elongation at fracture (ε), yield strength (YS), hardness (H), compressive strength (CS), compressive yield strength (CYS), bending strength (BS), bending elongation (δb) and % compression at fracture (C).
Cantor alloy (i.e., CoCrFeMnNi) and its variants have been largely investigated. Apart from SPS, SLM is the most widely studied AM technique for HEAs
[130][131][132][133][134][135][136][137][138][139][140][141][142][143][144][145][146][147][148][149][150][151][152][153][154][155][156][157][158][159][160][161][162][163][164][165]. HEAs that were successfully fabricated by SLM include CoCrFeNiMn
[85][86][101][110][111][112][113][114][115][116][117][118][119][122][136][137][138][139][140][143][146][148][149][151][154][155], AlCrFeNiV
[127], AlCoCrFeNi
[102][124], AlCoCrCuFeNi
[128], CoCrFeNi
[24][87][120][121][123][142][147][156], CoCr
2.5FeNi
2TiW
0.5 [157], Fe
40Mn
20Co
20Cr
15Si
5 [131], Al
xCoCrFeNi
[89][90][92] AlCoCuFeNi
[38][126], Al
xCrCuFeNi
[141][144], AlCrCuFeNi
x [201], AlCrFe
2Ni
2 [152], Al
0.2Co
1.5CrFeNi
1.5Ti
0.3 [150], Ni
6Cr
4WFe
9Ti
[134], Ti
1.4Nb
0.6Ta
0.6Zr
1.4Mo
0.6 [145], Al
0.5FeCrNi
2.5V
0.2 [153] and NbMoTaW
[132][133].
Meanwhile, EBM was used to manufacture CoCrFeNiMn
[159], AlCoCrFeNi
[160][161], CoCrFeNiTi
[162] and Al
0.5CrMoNbTa
0.5 [163]. DED techniques were used to fabricate CoCrFeNiMn
[164][165][166][167][168][169][170][171][172][173][174][189][190][191][192][200], CoCrFeNi
[192][193][196], Al
0.3CoCrFeNi
[195], CoCrFeNiMo
0.2 [175], CoCrFeNiNb
x [176], Al
xCoCrFeNi
[177][178][179][193][198], AlCoCrFeNi
2.1 [180][198], AlCrFeMoV
x [181], AlCoCrFeNiTi
0.5 [182], AlCrCuFeNi
[183][197], AlCoCrFeNiCu/AlTiCrFeCoNi
[184][185], NbMoTaW
[186], TiZrNbMoV
[187], NbMoTaTi
xNi
x [194], CoCrFeNb
0.2Ni
2.1 [199] and TiZrNbHfTa
[188].
The microstructures and mechanical behaviors of the HEAs produced by different AM processes are still under investigation by several research groups. The HEAs listed in
Table 1,
Table 2 and
Table 3 mainly have either FCC or BCC microstructures except Co
20Cr
15Fe
40Mn
20Si
5 which has HCP. Improvement in mechanical properties was reported when HEAs were fabricated with AM
[90][101][111][114][122][127][202]. These improvements are mostly attributed to grain refinement. Grain refinement in HEAs is claimed to be due to the high cooling rates as it happens in various other materials
[90][134][164]. Moreover, the wear behavior
[123][183], thermo-mechanical analysis
[133][180], effect of annealing
[24], creep behavior
[117], residual stresses
[166], corrosion behavior
[110][132][160][162][165][175][183], strengthening mechanisms
[85] and deformation mechanism
[171] of additive manufactured HEAs have also been reported.
Particle reinforcement in a HEA matrix with AM has been an area of interest for many researchers lately who expect microstructure refinement and mechanical properties enhancement
[49][111][142][146][154][155][203][204][205][206][207][208][209][210][211][212][213][214]. Li et al.
[111] introduced nano TiN ceramic particles in a CoCrFeMnNi matrix, which led to equiaxed grains of 5 µm. The same group
[113] also fabricated the same composition with SLM followed by laser remelting and obtained ultrafine grains (80% grains less than 2 µm and 90% grains less than 3.5 µm). Song et al.
[122] showed that the YS and ductility of CoCrFeNi increased by 25% and 34%, respectively, when doped with 1.8 at% nitrogen. Fu et al.
[213] noticed that adding Ti-C-O particles into NbTaTiV increased the UTS, YS and fracture strain up to 2270 MPa, 1760 MPa and 11%, respectively. Amar et al.
[173] added TiC into CoCrFeNiMn and found the YS and UTS increased from 300 MPa to 385 MPa and from 550 MPa to 723 MPa, respectively. Similarly, Li et al.
[172] embedded WC particles into CoCrFeNiMn alloy and observed improvement in YS from 300 to 675 MPa and UTS from 550 to 845 MPa due to the formation of Cr
23C
6 precipitates. Li et al.
[115] noticed that TiC reinforcement CoCrFeNiMn gave the UTS of around 1100 MPa. Rogal et al.
[205] increased the UTS of CoCrFeNiMn up to 1600 MPa by introducing nano-Al
2O
3 particles. Carbon doping was attempted
[86][87][114][120] to enhance the mechanical properties of HEAs. Peng et al.
[142] added diamond particles into CoCrFeNi and found out the bending strength was 925MPa. Park et al.
[146] added carbon into CoCrFeNiMn ((CoCrFeNiMn)
99C
1) and noticed that the YS and UTS were ~741 MPa and ~874 MPa, respectively. Similarly, Kim et al.
[155] also added carbon into CoCrFeNiMn in a ratio (CoCrFeNiMn)
100−xC
x (x = 0.5–1.5). The YS for x = 0.5, 1, 1.5 was measured to be 653, 752 and 753 MPa respectively. The UTS for x = 0.5, 1, 1.5 was found to be 766 ± 318.5, 895 ± 22.3 and 911 ± 125.1 MPa, respectively. Shen et al.
[192] discussed the effect of SiC particles added to CoCrFeNi. They noticed that adding SiC particles changed the microstructure from the FCC phase to the FCC/Cr
2C
7 dual phase. The hardness and YS improved significantly from ~139 HV to ~310 HV and ~142 MPa to ~713 MPa, respectively.
Various HEAs have exhibited significant improvement in their mechanical properties after AM synthesis as compared to the as-cast structures of the same compositions
[85][86]. Zhou et al.
[87] reported that arc-melted CoCrFeNi had the YS of 225 MPa whereas SLM-manufactured CoCrFeNi had the YS of 656 MPa. Brif et al.
[88] observed that SLM-manufactured CoCrFeNi showed noticeable improvement in YS from 188 MPa (as-cast) to 600 MPa and in UTS from 457 MPa (as-cast) to 745 MPa. Peyrouzet et al.
[89] showed that the YS of Al
0.3CoCrFeNi increased from 275 MPa (as-cast) to 730 MPa and the UTS from 502 MPa (as-cast) to 896 MPa when manufactured with SLM. The UTS of as-cast Al
0.3CoCrFeNi was 522 MPa and it was increased to 878 MPa with SLM processing
[90]. Arc-melted Al
0.5CoCrFeNi had the YS of 334 MPa and the UTS of 709 MPa
[91]. SLM increased the YS up to 579 MPa and the UTS up to 721 MPa
[92].
Moreover, the CS of AlCrCuFeNi was 2052 MPa when fabricated with SLM and 1750 ± 15 MPa
[215] with arc-melting. The hardness of AlCoCrCuFeNi improved from 500 to 710 Hv
[128] by using SLM. The YS of AlMgScZrMn manufactured with arc melting, SPS, and SLM is 188 ± 2.3 MPa, 231 ± 3 Mpa and 394 Mpa respectively
[129]. Agrawal et al.
[131] reported that the YS of as-cast and SLM-printed Fe
40Mn
20Co
20Cr
15Si
5 was 420 ± 20 Mpa and 530 ± 40 Mpa, respectively. The YS of CoCrFeNiMn was 2.5 times higher (around 518 Mpa)
[170] with DED in comparison to that of cast parts (209 Mpa)
[216] at room temperature (RT). Furthermore, the as-cast AlCoCrFeNi had the UTS of 956 MPa, and the EBM specimen had the UTS of 1073 MPa
[160]. Similarly, Fujieda et al.
[162] reported that EBM-synthesized CoCrFeNiTi showed the improved tensile strength of around 1178 MPa, which is much stronger than various commercial high corrosion resistant materials such as duplex stainless steel: 655 MPa, super duplex stainless steel: 750–800 MPa and Ni-based super alloys (i.e., Alloy C276: 690 MPa, Alloy 718: 1275 MPa).
Refractory HEA NbMoTaW has shown a drastic reduction in grain size when made with AM. The average grain size of BCC phase was 200 μm in as-cast sample
[217] and 13.4 μm in SLM-processed sample. Additionally, this alloy did not follow the rule of mixtures. Instead, it showed the cocktail effect for the hardness of the final structure. The hardness of Nb, Mo, Ta and W was in the range of 85–410 HV but the final hardness of SLM processed NbMoTaW was measured to be 826 HV
[132]. Senkov et al.
[218] commented that NbMoTaW did not have any abrupt hardness changes at high temperatures, consistently exhibiting better hardness properties than superalloys. Moreover, SLM-processed Ni
6Cr
4WFe
9Ti (UTS = 972 MPa, YS = 742 MPa, ε = 12.2%) had ~93% increase in YS, ~50% increase in UTS, and ~77% increase in tensile ductility as compared to the vacuum arc melted samples (UTS = 649 MPa, YS = 385 MPa, ε = 6.9%)
[134][135].
In summary, various studies have successfully manufactured SLM, EBM and DED techniques. They have also shown that the properties of HEAs could be altered by changing the input parameters for AM process. For example, CoCrFeNiMn was manufactured with SLM by multiple researchers
[85][86][101][110][111][112][113][114][115][116][117][118][119][122][136][137][138][139][140][143][146][148][149][151][154][155] and many of them acquired different mechanical properties for CoCrFeNiMn by changing input parameters in AM processes (refer
Table 1,
Table 2 and
Table 3).