1. Introduction
High-entropy alloys (HEAs), also known as concentrated multi-component alloys (CMCAs), have attracted significant attention from the metallic materials community due to their unique properties and related scientific value.1–3) Compared with conventional alloys, HEAs containing four or more principle elements exhibit unique properties, including high strength and hardness, outstanding corrosion resistance, excellent cryogenic behavior, and good resistance to irradiation and hydrogen embrittlement.4–10) Recently, HEAs were reported to possess outstanding mechanical properties under dynamic loading,11–13) and it is of great value to understand the dynamic behavior of the alloys in view of their potential applications such as armor protection, aerospace, and automotive industry.14)
With regard to the industrial application of HEAs, well-controlled solidification at low cost plays a significant role in achieving desired microstructure and properties. However, numerous studies so far have only focused on the effects of thermomechanical processing on microstructure and mechanical properties of HEAs,5,15–18) very limited work has been done on the solidification control and its influence.19–21) Especially, an ever-increasing interest has been drawn recently to improve the mechanical properties in nickel-based alloy, bulk metallic glasses, and aluminum alloys using directional solidification.22–24) Compared to the conventional casting in which the specimen suffers inhomogeneous cooling rates from surface to the center, directional solidification methods such as Bridgman solidification (BS) can achieve uniform cooling rates all over the samples, enabling homogeneous and well-controlled microstructure of the alloy.
With these in mind, in the present work, the single body-centered cubic (BCC) phase AlCoCrFeNi HEAs, which show excellent performance in quasi-static compression25) and can be successfully manufactured by BS,26) were prepared with three different solidification rates using BS. The effects of varied solidification rates on the microstructure and strain-rate-sensitive mechanical behavior were systematically investigated, especially at dynamic strain rate range.
3. Results and Discussion
3.1 Microstructure
Figure 1 shows the optical micrographs of H200, H1200, and H2400 alloys, which all have single BCC phase as experimentally proved in previous study under different cooling rates.26) The H200 alloy exhibits columnar crystals with grain size of ∼200 µm in the central region of the samples as shown in Figs. 1(a1) and (a2). The trunks of those columnar crystals grow along the solidification direction (indicated by arrow), while the branches originates from the trunk and then grow perpendicular to the solidification direction, which are in line with the classical solidification theory in conventional alloys.27) With increasing solidification rate, the microstructure of H1200 alloy changed to equiaxed dendrite, as shown in Fig. 1(b1) and (b2), and the grain size is ∼120 µm. With further increase of the solidification rate, as seen in Figs. 1(c1) and (c2), the H2400 alloy exhibits equiaxed dendrite with size of ∼80 µm. Higher magnification images shown in the insets of Fig. 1(b1-1) and (c1-1) confirmed the equiaxed dendrite microstructure formed under high solidification rate in the AlCoCrFeNi HEA. It is worth noting that, compared to the case when the solidification rate increases from 1200 µm/s to 2400 µm/s, the alloys exhibit more significant changes in microstructure when the solidification rate increases from 200 µm/s to 1200 µm/s.
From Fig. 1, with increasing solidification rate, the microstructure evolves from columnar crystals to equiaxed grains accompanied with grain size decrease, indicating that the microstructure of the HEA is sensitive to the solidification condition. This could be attributed to the sluggish diffusion effect in HEAs, which could impair the nucleation and growth of crystal during solidification by reducing the undercooling degree and slowing down solute diffusion or distribution.27–29)
In the equilibrium solidification state, temperature gradient and freezing rate have been confirmed to be important factors in determining the solidification morphology of the alloy. It is also known that the microstructure evolution can be evaluated using the parameter of G/V during solidification, in which G is the temperature gradient and V is the growth rate. When increasing the G/V ratio, the solidification microstructure would transform from columnar dendrites to equiaxed grains.30) In Bridgman directional solidification, cooling rate (or solidification rate) increases with the withdrawal rate; i.e., higher degree of supercooling leads to the reduction of critical nucleation energy and the increase of nucleation ratio.31,32) Therefore, more grains can be formed in the transition region and grow freely, while the growth of grains in the solid phase region along the direction of flow is suppressed. The microstructure evolution trends are consistent with conventional alloys reported in literature.24,31) It should be noted that the microstructure is different from Zhang et al.’s work26) with similar withdrawal velocities, which could be related to superheat and temperature gradient. The melting temperatures of the elements in HEAs are significantly varied, which leads to a wide solidification range, and thus large superheat is required. In addition, the HEA samples could not be heated into complete melting condition, resulting in the formation of dendrite instead of equiaxed grains, especially at the bottom of the sample which is the closest to the water-cooled In–Ga–Sn liquid alloy surface. In addition, Zhang et al. adopted a higher temperature gradient (∼70 K/mm), which is more beneficial to the formation of equiaxed grains.26)
Figure 2 shows the typical SEM images of H200, H1200, and H2400 alloys in longitudinal sections, demonstrating that the increased solidification rate induces microstructural refinement. As seen in Fig. 2(a1), the H200 alloy exhibits coarse columnar crystals (Fig. 2(a1)), while the H1200 alloy and H2400 alloys have equiaxed grains (Fig. 2(b1) and (c1)). Figures 2(a2)–(c2) are the local detailed views of H200, H1200, and H2400 alloys, indicating that typical nano-scale spinodal decomposition occurs in those alloys. Figure 2(a2) shows microstructure walls consisting of coherent and alternating platelets, and the spinodal structure size of H200 is ∼50 nm. When the solidification rate increases to 1200 µm/s, the size of spinodal structure decreases to ∼2 nm (Fig. 2(b2), (b3)). With further increase in the solidification rate, the size of the spinodal structure in the H2400 alloy becomes even smaller (Fig. 2(c2), (c3)).
High solidification rate is beneficial to the uniformity of the composition and structure of the alloy. With the solidification rate increases, the size of spinodal structure decreases and the morphology changes from wall to honeycomb-like structure, which is similar with Zhang et al.’s work.26) The local chemical compositions derived from EDS analysis are given in Table 1. The matrix can be distinguished into two parts: the white plate and the black plate. Local chemical compositions are consistent in the three samples such that the white plate is enriched with Al–Ni and the black plate is enriched with Cr–Fe. In addition, the distribution of Co in these alloys is homogeneous.
Table 1 Local chemical compositions of the AlCoCrFeNi HEAs (at%).
Quasi-static compression was employed to investigate the mechanical behavior of the HEAs, and the true stress vs. strain curves are shown in Fig. 3. The 0.2% yield stress (YS) and fracture strain can be obtained and are listed in Table 2. It is apparent that both the ductility and strength of AlCoCrFeNi HEAs are improved with increasing solidification rate, which could be attributed to the decrease in the sizes of grain and spinodal structure. The YS increases obviously when the withdrawal velocity increases from 200 µm/s to 1200 µm/s, but barely alters with further increase to 2400 µm/s. This is because the increment between H200 and H1200 is related to the microstructure transition from columnar dendrites to equiaxed dendrites, while the one from H1200 to H2400 is only from the reduction in the size of equiaxed grain. In addition, the stress-strain curves of H1200 and H2400 are nearly the same except for the slight difference in fracture strain, which suggests that the size of spinodal structure does not significantly affect the YS. With the solidification rate increases, more grain boundaries are generated which hinder the movement of dislocations, while the disappearance of the dendrites and more homogeneous microstructure facilitate the movement of dislocations, leading to the increase in both YS and fracture strain simultaneously. The nanoscale spinodal structure also releases the stress caused by the pile-up of the dislocations and decreases the chance of cracking induced by the stress concentration, improving the plasticity of alloys.26)
Table 2 Mechanical properties and grain size of AlCoCrFeNi Alloys by BS with three withdrawal velocities.
Figure 4 shows the fracture morphology of the alloys under quasi-static compression in which the loading direction is marked by arrow. It is obvious that axial-directional cracks distribute on the fracture surface of H200 alloy as shown in Fig. 4(a). The morphology characterized by river-like patterns and cleavage steps can also be observed, suggesting a typical cleavage fracture. In Fig. 4(b), the shearing surface of H1200 alloy exhibited appreciable amounts of cleavage steps and river-like patterns. As seen in Fig. 4(c), H2400 alloy presents complex morphology consisting of three different distinct characteristics: river-like patterns, cleavage steps, and dimples, indicates a mixture of axial splitting and shearing fracture. Therefore, it can be concluded that the fracture mode changes from brittle to ductile fashion with the solidification rate increases.
Figure 5 shows the dynamic compressive true stress vs. true strain curves of H200, H1200, and H2400 alloys at the strain rate of 2.5 × 103 s−1 by SHPB experiments. The fluctuations in the stress-strain curve at high strain rate are due to the nature of elastic wave propagation in cylinder bars, or the dispersion effect.33) It should be noted that no cracks were observed on the surface of all three samples after the tests. As seen in Fig. 5, the H200 alloy shows nearly no work-hardening behavior, suggesting a balance between thermal softening and work hardening during the dynamic compression. The fracture strain of the H200 alloy is remarkably greater than those of the H1200 and H2400 alloys. With increasing solidification rate, the work hardening tendency under high strain rate enhances and the YS also increases. The finer grains and decreasing size of spinodal structure contribute to the improvement of YS in HEA with lower solidification rate as analyzed above. The significant improvement in fracture strain of the H200 alloy may be the result of preferred orientations of columnar crystals. As shown in Fig. 1, when the withdrawal velocity is 200 µm/s, the growth direction of grains is parallel to the predominant heat flow direction, developing into columnar grains with preferred orientations. Correspondingly, these columnar crystals in the direction of loading are in a favorable condition for slip during plastic deformation, which results in better plasticity.
To compare the quasi-static and dynamic compressive mechanical behavior of the HEAs, Fig. 6 summarizes the true stress vs. strain curves of different alloys. Strain-rate sensitivity (SRS),21) as defined by the slope of the log flow stress versus the log strain rate, is employed to characterize the strain-rate effect on the mechanical responses. The SRS parameter is calculated using the data from Fig. 6 in the form of m = ∂ log σ/∂ log ε, and the result are shown in Table 2. Obviously, the flow stress increases with the strain rate in all three alloys, exhibiting a positive strain-rate effect. Generally, HEAs have been reported to show higher SRS than conventional FCC metals.19,34,35) In HEAs, both multi-element solid solution strengthening controlled by thermal activated deformation and low stacking fault energy (SFE) that causes dislocations to spilt into wide partials can promote SRS.16,35) Furthermore, SRS is a structurally-sensitive parameter,19) and the relatively large grain size may also contribute to the high SRS value, similar to what was found in Al0.3CoCrFeNi.34) It should be noted that the H200 alloy shows a larger m value (∼0.016) than the other two rapid solidified alloys suggesting an exceptional SRS. As explained by Sindhura et al., the change in SRS due to grain size refinement would be inversely proportional to the associated strength gain.34) In AlCoCrFeNi HEAs, as shown in Fig. 1 and Fig. 3, with withdrawal velocity enhanced from 200 µm/s to 1200 µm/s, the microstructure transforms from columnar dendrites to equiaxed dendrites accompanied by grain refinement, and YS is dramatically improved. Consequently, H200 alloy exhibited higher SRS.
