Age Heat Treatment of Al_0.5CoCrFe_1.5NiTi_0.5 High-Entropy Alloy

Abstract

In this study, Al_0.5CoCrFe_1.5NiTi_0.5 high-entropy alloy was heat-treated from 500 °C to 1200 °C for 24 h to investigate age-hardening phenomena and microstructure evolution. The as-cast alloy, with a hardness of HV430, exhibited a dendritic structure comprising an (Fe,Cr)-rich FCC phase and a (Ni,Al,Ti)-rich B2 phase, and the interdendrite exhibited a spinodal decomposed structure comprising an (Fe,Cr)-rich BCC phase and a (Ni,Al,Ti)-rich B2 phase. Age hardening and softening occurred at 500 °C to 800 °C and 900 °C to 1100 °C, respectively. We observed optimal age hardening at 700 °C, and alloy hardness increased to HV556. The hardening was attributed to the precipitation of the σ phase, and the softening was attributed to the dissolution of the σ phase back into the matrix and coarsening of the microstructure. The appearance of fine Widmanstätten precipitates formed by the (Al,Ti)-rich BCC phase and (Ni,Al,Ti)-rich B2 phase at 1200 °C led to secondary hardening.

1. Introduction

The Industrial Revolution catalyzed the development of metals and alloys. Even after half a century, research into metals and alloys is still continuing. High-entropy alloys (HEAs) constitute a novel class of materials. Yeh et al. discovered the original concept [1]. HEAs integrate at least five principal metallic elements, with the concentration of each element ranging from 5 to 35 at.%, and configurational entropies in a random state being greater than 1.5R [2,3]. Four classic core effects, namely high entropy, severe lattice distortion, sluggish diffusion, and cocktail effects, impart superior properties to HEAs [4,5]. These effects have been shown not to apply to the extent originally suggested, especially true of the concept of high entropy and the cocktail effect [6]. Furthermore, some researchers suggested the diffusion coefficients of the component elements have minor effects on sluggish diffusion [7,8].

Previous studies [9,10,11,12,13,14,15,16,17] have reported that some HEAs provide a correlation between age hardening and microstructure. AlCoCrCuFeNi as-cast alloy mainly comprises a BCC phase and a minor amount of the FCC phase. By the increase in the aging temperature, the structure is gradually transformed to mainly the FCC phase and partly the BCC phase [18]. After annealing of the alloy at 1000 °C, it exhibits high compression strength and good ductility of 1.63 GPa and 34%, respectively [19]. After annealing of the Al_0.3CrFe_1.5MnNi_0.5 alloy at 700 °C for 2 h, it exhibits a high hardness value of HV840. After annealing at 700 °C, the BCC phase dendrite is transformed to the σ phase, leading to the strengthening of alloy hardness [20,21]. The aging hardness of CuCr₂Fe₂NiMn alloy [22] exhibits a peak value of HV450 at 800 °C. This age hardening effect is mainly attributed to the precipitation hardening of the σ phase. The Al_0.5CoCrNiTi_0.5 alloy [23] exhibits a dendritic structure, and the hardness of the as-cast alloy is HV743. Apparent age hardening is observed from 600 °C to 900 °C, and age softening does not occur even after aging at 1200 °C. The age hardening of this alloy is attributed to the transformation of the BCC phase to the σ phase. The Al₂₀Co₂₀Cu₂₀Ni₂₀Zn₂₀ alloy after solution treatment [24] exhibits a single FCC phase. The hardness of the aged alloy reveals a peak age hardening value of 6.2 GPa at 773 K, corresponding to the (Ni,Al)-rich L1₂-phase precipitate in the FCC matrix. The HfNbTaTiZr alloy [25] was homogenized at 1200 °C and annealed at 600–1000 °C. The maximum precipitation hardening is caused by the precipitation of HCP phase particles in the BCC matrix after annealing at 600 °C. Cr₁₅Fe₂₀Co₃₅Ni₂₀Mo₁₀ as-cast alloys [26] exhibit a single-phase FCC structure. Alloys strengthened by the thermal-mechanical process via a Mo-rich nanoscale μ phase precipitates in the FCC matrix.

A previous study has reported that an Al_0.5CoCrFe_1.5NiTi_0.5 as-cast high-entropy alloy exhibits a high compressive strength of 1843 MPa and a fracture strain of 0.38 [27]. This alloy may be a new choice for development into a ductile high-strength alloy although the value of the tensile strain has not been reported until now. To further understand the age-hardening effect and microstructure evolution of this alloy, effects of age heat treatment on the hardness and microstructure of the Al_0.5CoCrFe_1.5NiTi_0.5 alloy were investigated.

2. Materials and Methods

Al_0.5CoCrFe_1.5NiTi_0.5 high-entropy alloy were synthesized by vacuum arc melting in a copper mold under argon. The alloys were remelted five times to ensure their homogeneity. Commercial-grade elements, Al, Co, Cr, Fe, Ni, and Ti with 99.5% purity were used as the raw materials. Dimensions of each resulting alloy ingot were 62 mm × 39 mm × 11 mm. The ingot was then cut to 10 mm × 10 mm × 2 mm flakes for analysis of the crystal structure and microstructure. The heat-treatment temperature ranged from 500 °C to 1200 °C, with a temperature gap of 100 °C, and aging was conducted for 24 h followed by water quenching. The heating rate was conducted at 10 °C/min. The samples were ground using SiC sandpaper and polished using an Al₂O₃ suspension to achieve a flat surface. Next, the samples were etched using aqua regia for scanning electron microscopy (SEM; S3400N, Hitachi High-Tech Corporation, Tokyo, Japan) analysis. For transmission electron microscopy (TEM; Tecnai™ G² F20, FEI Technologies Inc., Hillsboro, OR, USA) analysis, the samples were ground to 0.1 mm thick using SiC sandpaper, followed by punching to a 3 mm diameter disc and grinding to 0.03 mm thick using SiC sandpaper for twin-jet electropolishing using 5% perchloric acid at -20 °C. The microstructure was investigated by SEM and TEM, and the composition was determined by energy-dispersive spectrometry (EDS; XFlash^® 6|100, Bruker, Billerica, MA, USA). The crystal structure was examined by X-ray diffraction (XRD; D2 PHASER, Bruker, Billerica, MA, USA) using Cu K_α radiation at a scanning rate of 2°/min and a 2θ range of 20–100 degrees. The hardness was measured under a load of 1 kg for a duration of 15 s by using a Vickers hardness tester (FM-300e, FUTURE-TECH CORP., Kawasaki, Japan). Ten different locations were measured to calculate the average hardness and standard deviation.

3. Results and Discussion

Figure 1 shows the hardness test results of the as-cast and aged alloys. The hardness values of the as-cast alloy and 500–1200 °C aged alloys are HV430, HV483, HV519, HV556, HV473, HV414, HV325, HV303, and HV460, respectively. With the increase in the temperature from 500 to 700 °C, hardness increases and reaches the maximum value of HV556 at 700 °C. At 800–1000 °C, hardness decreases with the increase in the temperature. The minimum hardness of HV303 is observed at 1100 °C. At 1200 °C, the hardness increases to HV460 again and exhibits a secondary hardening effect. In the next sections, the variation of hardness is described by the crystal structure and microstructure analyses.

Figure 2 shows the XRD patterns of as-cast and aged alloys. Three phases are observed in the as-cast alloy and alloy aged at 500 °C, i.e., FCC (a = 3.600 Å), BCC (a = 2.869 Å), and B2 (ordered BCC, a = 2.924 Å) phases. Four phases are observed in the alloy aged at 600–1000 °C, i.e., FCC, BCC, B2, and σ (a = 8.816 Å, c = 4.553 Å, c/a = 0.516, PDF# 05-0708) phases. Three phases remain in the alloy aged at 1100 °C, i.e., FCC, B2, and BCC phases. The disappearance of the σ phase at 1100 °C indicates that the stable temperature of the σ phase is less than 1100 °C. When the temperature of 1200 °C is reached, three phases, i.e., FCC, BCC, and B2, are detected.

Previously, Guo et al. [28] have reported that the valence electron concentration (VEC) can be utilized to predict the formation of FCC and BCC phases in HEAs. When VEC ≥ 8, the alloy exhibits an FCC-phase single solid solution. For VEC < 6.87, the crystal structure of an alloy forms a single BCC-phase solid solution. At 6.87 ≤ VEC < 8, the alloy comprises FCC and BCC phases. According to the criterion proposed by Tsai et al. [29], the σ phase is generated at a VEC of between 6.88 and 7.84. For this alloy, the VEC value is 7.35. Therefore, the presence of the FCC, BCC, ordered BCC, and σ phases in alloys aged at 600–1000 °C is expected. With the increase in the temperature, the hardness of the as-cast alloy to alloys aged at 700 °C increases. This age hardening effect can be explained by Figure 2: With the increase in the temperature from 600 °C to 700 °C, the σ phase forms, and the XRD peak intensity of the σ phase increases. However, the σ phase is not apparent in the alloy aged at 500 °C, and it still exhibits an age hardening effect. This phenomenon can be described by the following microstructural analysis. At 800–1100 °C, the hardness decreases with the increase in the temperature, and an age-softening effect is observed at 900–1100 °C. The comparison of the alloys aged at 700 °C, 800 °C, and 900 °C reveals that the XRD peak intensity of all four phases does not exhibit a distinct change, i.e., the volume fraction of each phase does not exhibit any clear difference, and the hardness still decreases considerably. These results can be explained by the following microstructural analysis. For the alloy aged at 1000 °C, the hardness significantly decreases because of the considerable decrease in the XRD peak intensity of the σ and BCC phases. From 1000 °C to 1100 °C, the σ phase disappears, leading to the further decrease in the hardness. At 1200 °C, a secondary hardening effect is observed because of the considerable increase in the XRD peak intensity of BCC and B2 phases, as well as a considerable decrease in the XRD peak intensity of the FCC phase.

Figure 3 shows the SEM-BSE (back-scattered electron) photographs depicting the microstructures of the Al_0.5CoCrFe_1.5NiTi_0.5 as-cast alloys.

Table 1. EDS analysis (at.%) of the SEM-BSE image of FCC, BCC, and B2 phases.

Phases	Al	Co	Cr	Fe	Ni	Ti
FCC	3.1	18.3	19.0	31.8	19.3	8.5
BCC	3.5	16.5	30.3	35.3	10.1	4.3
B2	19.7	19.0	5.4	10.9	26.5	18.5

lists the chemical composition as determined by EDS of the interior structures shown in Figure 3. The FCC phase in dendrite (DR) and BCC phase in the interdendrite (ID) are both (Fe,Cr)-rich phases. Comparison of the element content of these two phases reveals a higher Fe and Cr content of the BCC phase, albeit lower Ni, Ti, and Co content. The B2 phase is a (Ni,Al,Ti)-rich phase. Figure 4 shows the TEM bright-field images and selected-area electron diffraction (SAED) patterns of the as-cast alloy.

Table 2. EDS analysis (at.%) of the TEM image of the Al_0.5CoCrFe_1.5NiTi_0.5 as-cast alloy.

Phases	Al	Co	Cr	Fe	Ni	Ti
FCC	3.6	19.9	23.1	30.1	17.3	6.0
BCC	4.3	14.1	33.4	37.2	8.5	2.5
B2	22.9	18.7	4.2	10.2	24.4	19.6

shows the EDS analysis of FCC, BCC, and B2 phases in Figure 4. The SAED patterns reveal that the dendrite consists of (Fe,Cr)-rich FCC and (Ni,Al,Ti)-rich B2 phases and that the interdendrite is a spinodal decomposed structure [30,31] comprising the BCC and B2 phases.

Figure 5 shows the SEM-BSE photographs depicting the microstructures of Al_0.5CoCrFe_1.5NiTi_0.5 aged alloys.

Table 3. EDS analysis (at.%) of the SEM-BSE image of the Al_0.5CoCrFe_1.5NiTi_0.5 alloy aged at 500 °C.

Phases	Al	Co	Cr	Fe	Ni	Ti
FCC	2.8	18.7	18.1	31.2	20.0	9.2
BCC	3.3	15.3	31.6	36.0	10.6	3.2
σ	1.4	15.8	37.9	31.5	8.4	5.0
B2	22.0	18.7	4.9	9.2	25.8	19.4

shows the EDS analysis of the FCC, BCC, σ, and B2 phases in Figure 5b. The as-cast alloy and alloys aged at 500–1000 °C exhibit a dendritic structure. This dendritic structure disappears at 1100 °C and converts into a dense Widmanstätten structure at 1200 °C. By aging at 500 °C (Figure 5a,b), the σ phase precipitates in the dendrite region and exhibits a black particle shape. It describes the age hardening effect at 500 °C. The σ phase is not detected by XRD because of the extremely low volume percent of the σ phase. Figure 5a–c shows the microstructure of alloys aged at 500 °C to 700 °C. Combine with the qualitative analysis from intensity of XRD peaks in Figure 2, the volume percentage of the σ phase increases with the increase in temperature, leading to the increase in hardness. Several studies on similar composition on phase transformation have been reported [32,33]. Comparison of the alloys aged at 700 °C and 800 °C as shown in Figure 5c,d reveals that the dendrite and interdendrite structures become coarse with the increase in temperature. Hence, hardness decreases at 800 °C. Figure 5d,e show the microstructures of the alloys aged at 800 °C to 900 °C. The volume percentage of the σ phase decreases, and the coarsening of structure increases, leading to the decrease in the hardness. At 1000 to 1100 °C (Figure 5f,g), the σ phase disappears at 1100 °C, and the structure continuously becomes coarse, leading to the decrease in the hardness. Figure 5h shows the microstructure of the alloys aged at 1200 °C. The microstructure transforms into a dense Widmanstätten structure, leading to a secondary hardening effect. This Widmanstätten pattern has been reported in some high entropy alloys, such as Al_0.5CoCrCuFeNi, Al_0.3CoCrCu_0.5FeNi, and Al_0.6CoCrFeNi alloys [34,35,36]. Figure 6 shows the TEM bright-field image and SAED patterns of the alloy aged at 1200 °C.

Table 4. EDS analysis (at.%) of the TEM image of the Al_0.5CoCrFe_1.5NiTi_0.5 alloy aged at 1200 °C.

Phases	Al	Co	Cr	Fe	Ni	Ti
FCC	2.0	17.8	24.4	39.3	13.9	2.6
BCC	15.5	17.6	15.2	22.7	18.3	10.7
B2	22.3	19.1	4.9	11.4	24.0	18.3

shows the EDS analysis of the FCC, BCC, and B2 phases in Figure 6. The SAED patterns suggest that the Widmanstätten structure is composed of the (Fe,Cr)-rich FCC phase matrix and rod-like (Al,Ti)-rich BCC + (Ni,Al,Ti)-rich B2 precipitate phases. The width and length of the rod-like precipitate phases are 100–200 nm and 0.5–3 μm, respectively. In the precipitate phase, BCC and B2 phases are alternately formed. Comparison with the SEM images in Figure 5h reveals that the dark gray plate-like phase in the SEM images consists of BCC and B2 phases. The microstructure of the Widmanstätten structure is seldom reported in high-entropy alloys, especially at a high aging temperature. This fine Widmanstätten structure provides good precipitation strengthening. Recently, the annealed Ni_1.5Co_1.5CrFeTi_0.7 alloy prepared by powder metallurgy [37] showed another fine structure of titanium oxide nanoparticles formed in FCC matrix that also exerted a strengthening effect.

4. Conclusions

Al_0.5CoCeFe_1.5NiTi_0.5 as-cast alloy and alloys aged at 500–900 °C exhibit a dendritic structure, the dendrite of which consists of the (Fe,Cr)-rich FCC phase and (Ni,Al,Ti)-rich B2 phase, with the interdendrite exhibiting a spinodal decomposed structure comprising an (Fe,Cr)-rich BCC phase and a (Ni,Al,Ti)-rich B2 phase. This dendritic structure disappears at 1000 °C and transforms into a Widmanstätten structure at 1200 °C. The Widmanstätten structure formed by the (Fe,Cr)-rich FCC matrix and (Al,Ti)-rich BCC + (Ni,Al,Ti)-rich B2 phases precipitate at 1200 °C. Age hardening and softening occur at 500 °C to 800 °C and at 900 °C to 1100 °C, respectively. The maximum hardness is apparent at 700 °C, and hardness increases from HV430 to HV556 for the as-cast ally to the alloy aged at 700 °C. Hardening is related to the precipitation of the σ phase, and softening is attributed to the dissolution of the σ phase back into the matrix and coarsening of the microstructure. A secondary hardening effect is attributed to the dense Widmanstätten precipitates.