The present paper reports the synthesis of AlCoCuFeMnNi high entropy alloy (HEA) with arc melting process. The as-cast alloy was heat treated at 900°C for 8 hours to investigate the effect of heat treatment on the structure and properties. Microstructural and mechanical properties of the alloy were analyzed together with the detailed phase analysis of the samples. The initially as-cast sample was composed of two separate phases with BCC and FCC structures having lattice parameters of 2.901 Å and 3.651 Å, respectively. The heat-treated alloy displays microsized rod-shaped precipitates both in the matrix and within the second phase. Rietveld refinement has shown that the structure was having three phases with lattice parameters of 2.901 Å (BCC), 3.605 Å (FCC1), and 3.667 Å (FCC2). The resulting phases and distribution of phases were also confirmed with the TEM methods. The alloys were characterized mechanically with the compression and hardness tests. The yield strength, compressive strength, and Vickers hardness of the as-cast alloy are 1317 ± 34 MPa, 1833 ± 45 MPa, and 448 ± 25 Hv, respectively. Heat treatment decreases the hardness values to 419 ± 26 Hv. The maximum compressive stress of the alloy increased to 2123 + 41 MPa while yield strength decreased to 1095 ± 45 with the treatment.
Traditional alloy design approach is based on mixing of one or two primary elements with known properties and minor elements are added in order to improve their properties. According to conventional strategy for developing new alloy systems, multiprincipal elements can lead to the formation of intermetallic compounds and complex microstructures. Yeh et al. [
Within the high entropy alloy systems, AlCoCrFeNi alloy is the one which has extensively been studied [
Equiatomic amount of high purity Al (99.9%), Co (99.9%), Cu (99.9%), Fe (99.9%), Ni (99.9%), and Mn (99.9%) was melted in an arc furnace under argon atmosphere. Approximately batches of 3 grams were melted in a water-cooled copper mold. The ingot was remelted three times in order to increase homogeneity. The specimen was evaluated at as-cast condition and heat treated at 900°C in an argon atmosphere for 8 hours and furnace cooled.
The phases are characterized with X-ray diffraction method using a Bruker D8 ADVANCE X-ray diffractometer (XRD) with copper target operated at 40 kv 30 mA. The lattice parameters and crystal structures as well as volume fractions were obtained from the Rietveld refinement of X-ray data with MAUD program [
Thin-foil specimens were prepared by mechanical thinning followed by ion milling and were observed under a transmission electron microscope (200 kV TEM, JEM-2100F, JEOL, Tokyo, Japan).
Mechanical properties were evaluated by uniaxial compression tests on samples with 3 mm diameter and 4.5 mm length by using an Instron 5582 testing system with a strain rate of 10−3 s−1. Three compression tests were performed to obtain average value. The hardness values were measured using 4.903 N load for 10 sec. The reported hardness value was an average of at least 10 measurements.
Figure
X-ray diffractograms for AlCoCuFeMnNi (a) before heat treatment and (b) after heat treatment.
The alloy was heat treated at 900°C for 8 hours. After heat treatment, a second FCC phase with similar lattice parameter appeared on XRD data with BCC phase seen in as-cast sample remaining in the structure (Figure
Rietveld Refinement results of the as-cast and heat-treated sample.
Phase | Lattice parameter (Å) | Phase fractions (%) | |
---|---|---|---|
As-cast | BCC | 2.901 | 65.66 |
FCC | 3.651 | 34.34 | |
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Heat-treated | BCC | 2.893 | 53.64 |
FCC-1 | 3.667 | 26.83 | |
FCC-2 | 3.605 | 19.53 |
Rietveld refined X-ray diffractogram of heat-treated sample.
Figure
Chemical composition of the as-cast and heat-treated alloys.
Al | Mn | Fe | Co | Ni | Cu | |
---|---|---|---|---|---|---|
As-cast | ||||||
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12,04 | 17,28 | 20,32 | 16,40 | 13,94 | 20,04 |
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21,49 | 15,36 | 15,12 | 16,07 | 17,35 | 14,61 |
Heat-treated | ||||||
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9,92 | 15,67 | 14,87 | 12,51 | 12,76 | 34,28 |
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19,12 | 15,53 | 17,12 | 16,00 | 16,41 | 15,82 |
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16,25 | 15,41 | 17,12 | 14,96 | 14,19 | 22,08 |
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21,78 | 15,28 | 17,07 | 18,03 | 18,25 | 9,60 |
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17,92 | 16,70 | 16,19 | 15,87 | 16,27 | 17,07 |
Backscattered SEM images of the (a) as-cast and (b) heat-treated sample. (c) A closer look to the heat-treated sample.
Heat treatment of the sample leads to the formation of more complex microstructure, which is supported by X-ray diffraction analysis, Figure
As it can be seen from the SEM image there seems to be four regions. These regions can be named as (1) the dark matrix phase (MP) and (2) bright precipitates within matrix phase (PM) and (3) bright secondary phase (SP) and (4) dark precipitates within secondary phase (PS). To understand the nature of these regions, point elemental analyses are applied to each region and these values are also included in Table
In order to verify the compositional variation within the microstructure, the elemental mapping was carried out and the colored maps are given in Figure
Elemental mapping of the heat-treated high entropy alloy.
The obtained phase distribution has further been verified with TEM analysis. The similar microstructure obtained from SEM analysis is observed in TEM. Bright field TEM image for the heat-treated sample is shown in Figure
TEM image and corresponding SAED patterns of bulk AlCoCuFeMnNi HEA after heat treatment. (a) Bright field image showing the primary and secondary phases. (b) SAED pattern of the matrix from
Figure
Engineering stress-strain curve of bulk AlCoCuFeMnNi HEA under compression.
The microhardness value of the AlCoCuFeMnNi alloy was 448 ± 25 Hv while with heat treatment it was measured to be 419 ± 26 Hv. A decrease was observed in the microhardness values with heat treatment since there is a decrease in the volume fraction of BCC phase, which exhibits higher hardness/strength than that of the FCC phases, consistent with what has been previously reported [
In the current study, we investigate the effect of heat treatment on the microstructure and phase distribution and on the mechanical properties of the alloy. We observe that two phase structures which were initially FCC and BCC structure were converted into three phase structures with one BCC and two FCC structures with heat treatment. The SEM micrographs show that the phases of HEA exhibit rod-shaped precipitates within both the matrix and second phase. TEM analyses verify that these three phases were distributed as the FCC precipitates (PM) in BCC matrix (MP) and as BCC precipitates (PS) in FCC second phase (SP). These two FCC phases have close lattice parameters as 3.605 Å and 3.667 Å. This change in microstructure and phase constitution was accompanied by a corresponding progressive decrease in microhardness with decreasing BCC content, clearly indicating that the BCC microstructure is substantially harder than the FCC microstructure. The corresponding increase on mechanical strength value was thought to be related to small sized precipitates within the structure.
The author declares that there are no conflicts of interest regarding the publication of this paper.