A new metallurgical strategy was introduced to develop advanced materials with outstanding performance—high-entropy alloys (HEAs). Today, HEAs contain five or more multiple principle metallic elements in equal or near-equal atomic percentages. HEAs’ four core effects—high configurational entropy, sluggish diffusion, severe lattice distortion, and the cock-tail effect—are mainly responsible for their various physical and mechanical properties. HEAs present promising properties, such as high strength and fracture toughness at room temperature and high temperatures and have excellent wear resistance, and corrosion resistance, along with high-temperature oxidation.
1. Introduction
A new metallurgical strategy was introduced to develop advanced materials with outstanding performance—high-entropy alloys (HEAs). Today, HEAs contain five or more multiple principle metallic elements in equal or near-equal atomic percentages
[1]. HEAs’ four core effects
[2]—high configurational entropy, sluggish diffusion, severe lattice distortion, and the cock-tail effect—are mainly responsible for their various physical and mechanical properties. HEAs present promising properties, such as high strength and fracture toughness at room temperature
[3][4][5][6] and high temperatures
[2][7] and have excellent wear resistance
[4], and corrosion resistance
[8][9][10][11], along with high-temperature oxidation
[8].
HEAs are more likely to generate a simple solid solution (typically body-centered cubic (BCC), face-centered cubic (FCC), or hexagonal closed-packed (HCP) phases) despite containing many components
[12][13][14][15]. Up to now, the development of HEAs has mainly gone through three stages: quinary equal-atomic single-phase solid-solution alloys; quaternary or quinary non-equal-atomic multiphase alloys; medium-entropy alloys, high-entropy fibers, high-entropy films and lightweight HEAs (LWHEAs)
[16][17]. HEAs mainly include two categories: refractory elements (such as V, Cr, Ti, Mo, Nb, Ta, W, Zr, and Hf) and commodity metals (such as Cr, Co, Fe, Ni, Mn, and Cu)
[18]. HEAs in the first category consist of many refractory elements, so-called refractory high-entropy alloys (RHEAs)
[13], and mostly form BCC structures
[19][20]. The second category of HEAs mostly forms FCC structures
[21][22], a combination of FCC and BCC structures
[23][24][25] or BCC structures alone
[26][27].
However, FCC structures or “FCC-based” structure HEAs show excellent ductility high plasticity, but its yield strength is not considerable in a large number of studies reported so far
[28][29]. It needs to be strengthened by a series of thermomechanical treatments before industrial application, which requires an additional cost. In contrast to FCC HEAs, BCC HEAs exhibit relatively high intrinsic yield strengths
[30][31]. Besides, RHEA systems have excellent high-temperature mechanical properties but insufficient toughness at room temperature
[32][33]. Developing BCC HEAs with high strength and good ductility is a key to making them potential structural materials in technological applications.
However, it is still weak to regulate the plastic deformation behavior of BCC HEAs via the strengthening methods of FCC HEAs. BCC HEAs can only deal with liquid-solid phase transformation by some processing technology, such as melt purification, homogenization, directional solidification (DS), powder metallurgy (PM) and additive manufacturing (AM).
Nowadays, BCC HEAs have typically been fabricated via simple arc-melting and casting, which has been confirmed to be a highly efficient technique for more than five metals. For instance, the AlCrFeCoNi HEA with a single-phase BCC solid solution exhibits excellent compressive properties of yield stress (1250.96 MPa) and plastic strain (32.7%)
[27]. VNbMoTa RHEAs exhibit excellent room-temperature ductility with a fracture strain > 25% and high-temperature strength (compressive yield strength of 811 MPa at 1000 °C)
[34]. However, the cast product has some drawbacks including many structural defects, such as voids, porosity, chemical segregation and grain coarsening.
Powder metallurgy (PM), a forming technology that allows significant compositional accuracy can completely prevent chemical segregation, can obtain a homogeneous microstructure, can produce nanocrystalline materials and can develop metal matrix composites
[35][36]. Lightweight RHEAs of CrNbVMo exhibited superior compressive specific yield strengths compared to cast RHEAs at 25 °C and 1000 °C
[37]. TiNbTa
0.5Zr and TiNbTa
0.5ZrAl
0.2 RHEAs were successfully prepared without any cracks or fractures by PM, and with Al addition, the compressive peak stress increased from 508 MPa to 603 MPa at 800 °C
[38]. The PM process is not only suitable for preparing RHEAs but is also suitable for preparing small and precise components.
In contrast to the conventional process, additive manufacturing (AM) is based on an incremental layer-by-layer fabrication process
[39]. Because local process control can be realized in the AM process, it has extremely rapid solidification cooling rates, production with unrivaled design freedom, and shorter production cycles
[40][41]. The AM process can overcome the inherent complexity and achieve the high levels of control required to produce homogeneous bulk alloys. Due to the immanent advantages of AM, it has attracted much attention in the last decade. At present, the process for preparing HEAs mainly includes direct laser deposition (DLD), selective laser melting (SLM) and selective electron beam melting (SEBM)
[42][43][44][45]. The phases, crystal features, mechanical properties, functionalities and potential applications of these products have been discussed. Because the high density, low ductility at room temperature and poor resistance to oxidation are the main drawbacks of RHEAs, it is a challenge to prepare powder for the AM process.
2. Mechanical Properties of BCC-Structured HEAs at Various Temperatures
= 2.1% at an ambient temperature and decreased in yielded strength to 561 MPa and 552 MPa when the samples deformed at 600 °C and 800 °C, respectively (
Figure 2)
[47]. Then, single-crystalline HEA pillars on samples’ surfaces were prepared with high compressive strength (~4–4.5 GPa) and lower size dependence by Zou et al. (
Figure 3)
[62], which could have been achieved by higher lattice friction caused by localized distortion at atomic length scales. Compared with the TaNbMoW HEA
[53], the specific yield strength (
SYS=σy/ρ
2.1.2. Processing by Powder Metallurgy
These samples in the reports mentioned above were prepared by using the vacuum arc-melting technique. Mechanical alloying (MA) with spark plasma sintering (SPS)
[64][65][66], as a typical technology of PM, can readily fabricate bulk high-density HEAs with ultrafine grains, excellent microstructural homogeneity, improved strength and hardness. Nb
25Mo
25Ta
25W
25 and Ti
8Nb
23Mo
23Ta
23W
23 HEAs
[67] were successfully prepared by MA with SPS technology. The compressive yield stress and fracture strain of Nb
25Mo
25Ta
25W
25 HEAs with average grain sizes ~0.88 µm are 2460 MPa and 16.8%, which are remarkably superior to those prepared by casting
[47]. However, coarse grain size is conducive to the improvement of high-temperature strength, which is mainly attributed to the fact that the grain boundary is the weak area and plays as the flow unit at high temperatures
[68]. Besides this, the addition of Ti can facilitate the grain refinement and Ti particles distributed at grain boundaries can improve the toughness of the Ti
8Nb
23Mo
23Ta
23W
23 HEA (2377 MPa, 26.3%). The WNbMoTaV HEA
[69], when sintered, shows an ultra-high compressive yield strength of 2612 MPa with a failure strain of 8.8% at room temperature, respectively. Meanwhile, Long et al.
[70] reported that Laves phase precipitated in the BCC matrix of NbMoTaWVCr HEA and enhanced its mechanical properties by introducing Cr to the WNbMoTaV HEA. Laves phase formed in the BCC matrix due to the smaller atomic radius of Cr compared with the other alloy elements in those RHEAs
[50][54][71]. The compressive yield strength (3416 MPa) of the bulk NbMoTaWVCr HEA is dramatically higher than those of the previously reported refractory HEAs fabricated by casting and powder metallurgy methods, as shown in
Figure 7. The outstanding mechanical properties of the WNbMoTaV HEA were attributed to fine-grain strengthening, intrinsic and interstitial solid solution strengthening and Orowan strengthening. The enhancement in yield strength of the NbMoTaWVCr HEA may result from the combined effects of finer grain size, a homogeneous microstructure and enhancement of atomic size misfit caused by the addition of Cr and interstitial solid solution strengthening from O (O was inevitably introduced into the mechanically alloyed powders).
In addition to the WNbMoTa HEA, BCC-structured HEA systems prepared by MA with SPS technology also include AlFeTiCrZnCu
[72][73][74], CrTiVTaW
[19][75], TiNbTaZr
[38] and FeCrMnV
[17][76][77]. The AlFeTiCrZnCu HEA
[74] prepared by MA with SPS technology can achieve a 99% density and homogeneous nanostructure (~10 nm) and its hardness can reach 2 GPa.
The effect of Ti on the phase structure and mechanical properties of TixWTaVCr HEA
[75] was studied: a single BCC solid solution could be formed when the Ti content was up to 7%. Compared with pure W and several other HEAs, Ti
7WTaVCr has higher room-temperature and high-temperature compressive yield strengths, owing to solid solution strengthening and the effects of Cr and V.
Cao et al.
[38] successfully prepared TiNbTa0.5Zr and TiNbTa
0.5ZrAl
0.2 HEAs with a single BCC phase using powder metallurgy technology. The compressive yield strength and strain for TiNbTa
0.5Zr and TiNbTa
0.5ZrAl
0.2 alloys at room temperature were 1310 MPa, 30% and 1500 MPa, 30%, respectively.
Figure 8 compares the compressive properties of these HEAs with those of available refractory HEAs. TiNbTa
0.5Zr and TiNbTa
0.5ZrAl
0.2 present a good combination of strength and plasticity, while most refractory HEAs still follow the strength-ductility trade-off. Besides this, TiNbTa
0.5Zr and TiNbTa
0.5ZrAl
0.2 HEAs show a compressed maximum engineering strain of 50% without any cracking or fractures at 800 °C. Moreover, the NbTaTiV HEA exhibits a compressive yield strength of 1.37 GPa and a high fracture strain of 23% at room temperature. When deformation occurs at 1000 C, it still exhibits a high yield strength of 437 MPa with a compression strain of over 40%. Its outstanding mechanical properties are mainly attributed to the homogeneous and fine microstructures and solid solution strengthening effect.
The volume fraction of the BCC2 phase gradually increased with the increase in the Al concentration in AlxCrFeMoV HEAs
[78]. The improvement of compressive yield strength from 2730 to 3552 MPa can be attributed to the solid solution strengthening of Al caused by the appearance of the BCC2 phase. The addition of Al dually influenced the properties of the CrFeMoV alloy by improving its strength and reducing the density of the system. The yield strength and hardness as a function of density were compared with data for previously reported HEAs (
Figure 9). AlxCrFeMoV HEAs with outstanding mechanical properties, a low cost and low density, which are better than those of any previously reported HEAs, suggested a promising future for the HEAs in many structural applications.
In another case, new phases appeared in the BCC matrix, such as the B2 phase, HCP phase and FCC phase, after SPS processing. As reported
[17], AlCuFeMnTiV HEA prepared by sintering powder containing only the BCC phase has the B2 phase, HCP phase and Cu-rich FCC phase precipitated at the grain boundary in addition to the BCC matrix. It exhibits the best comprehensive mechanical properties, with a density of 6.28 g/cm
3, compressive yield strength of 2060 MPa and plastic strain of 15.83%, which are superior to most LWHEAs and traditional lightweight alloys. The high strength and good plasticity of AlCuFeMnTiV HEAs are attributed to the strengthening effect of nano twins precipitated in the FCC phase on grain boundaries.
The above discussion demonstrates that powder metallurgy is a promising way of preparing ductile RHEAs with outstanding comprehensive mechanical properties.
In general, BCC structure HEAs can be successfully fabricated by arc-melting and PM (MA with SPS technology), but the preparation of pre-alloyed powder is still one of the difficult problems for the attempt of AM technology. BCC structure HEAs have ultrahigh strength, high hardness and room temperature plasticity, which has been gradually improved in recent studies. It has great potential in aerospace, military (high performance penetrator materials) and biomedical fields in future applications.