Understanding the mechanical behaviour and the large strength/ductility differences between FCC and BCC AlxCoCrFeNi high entropy alloys
Graphical abstract
Introduction
High entropy alloys (HEAs) are a relatively new concept of alloy design deriving their properties from five or more metallic constituent elements, with the concentration of each element being between 5 and 35 at.% [1], [2], [3]. Typically, a disordered solid solution structure is stabilised in these multicomponent alloys by the lowering of Gibbs free energy due to the high configurational entropy of mixing (ΔSconf) [1], low enthalpy of mixing and the chemical compatibility among the constituent elements [4], [5]. However, in some cases ordered phases have been found to form [2]. The HEA class exhibits promising mechanical properties with strengths of up to 3 GPa being reported [6]. The AlxCoCrFeNi HEA system shows a unique combination of physical and mechanical properties, and has been chosen for further study in the present manuscript. Depending on the Al concentration, this system is known to have either a single phase FCC structure, or a duplex BCC structure comprising both a conventional disordered BCC (ferritic) phase in addition to a related superlattice structure of either L12 or B2 crystal structure [7], [8], [9]. A combination of FCC, BCC and superlattice structures can also form [8], [9], [10], [11], [12], [13], [14], [15], [16].
The FCC alloys of the AlxCoCrFeNi system such as Al0.1CoCrFeNi [17] and Al0.3CoCrFeNi [18], [19] show fairly low yield strengths of just a few hundred MPa, but show exceptionally high work hardening rates (WHR) which are very similar to TWIP steels [20]. The plasticity mechanisms in FCC HEA's have been well studied and it is known that deformation occurs by normal FCC slip on the {111} plane in the 〈110〉 direction [21], [22]. It was shown in the literature that high strength can be achieved in FCC HEAs by grain refinement [23], dislocation hardening [19], [24] and precipitation hardening [9]. However, the unusually high WHR is yet to be fully explored. Although we suggested in a previous paper that the high WHR was due to mechanical twinning [18], the precise mechanism is yet to be examined in detail. Work hardening is a balance between the accumulation of dislocations and their concurrent annihilation by dynamic recovery. It is not yet clear if this process of dynamic recovery is retarded in HEA's – this may be the key to their high WHR's. This is one of the aspects of HEA deformation behaviour studied in the present work.
In stark contrast to the mechanical behaviour of the FCC low-aluminium alloys, those alloys from the AlxCoCrFeNi family with high Al concentrations show extremely high yield strengths of up to 1400 MPa, but very poor ductilities [25], [26]. This transition has been suggested in literature [27], [28] to be the result of the formation of hard B2 phase (ordered BCC phase enriched in Ni- and Al-) in the BCC alloys, but this has not been studied in detail. It remains unclear why two very similar HEA's, one with a FCC structure and one with a BCC structure would show such markedly different properties. The micro-composite effect associated with the BCC/B2 microstructure may be one of the reasons for both the high strength and low ductility, but the distribution of strain within the different phases has not before been studied, and is therefore not yet understood. The plastic deformation mechanisms of the BCC alloys have not been treated in any detail, and may be more complex than the FCC case suggests. The deformation behaviour of the B2 phase formed in the BCC alloys is not entirely clear, there seems to be consensus that NiAl deforms by slip on the {011} plane, but the direction of slip could be either 〈111〉 or 〈100〉 [29], [30], [31], [32]. In the present case it may be that the B2 phase do not plastically deform at all, and simply act as rigid particles which are bypassed by dislocations by Orowan bowing. Clearly, we must examine this in more detail before we can understand the transition in behaviour of these alloys as a function of Al concentration. In this manuscript the plastic deformation behaviour and strength-ductility combinations of the AlxCoCrFeNi family with a range of Al concentrations and phase distributions is studied, with particular emphasis on a high-aluminium BCC HEAs.
Section snippets
Materials and methods
The compressive and tensile properties of three selected alloys were compared: FCC- Al0.3CoCrFeNi alloy (Al0.3), dual phase- Al0.6CoCrFeNi alloy (Al0.6) and BCC- Al0.85CoCrFeNi alloy (Al0.85) containing B2 phase. These HEAs were fabricated using a TRUMPF TruLaser Cell 7040 direct laser fabrication (DLF) apparatus from spherical gas-atomized powders of Co, Cr, Fe, Ni and Al as discussed elsewhere [27]. Columns were built with a cross-section of 15 × 15 mm2 and 100 mm in length. The bulk chemical
Phase and microstructure
X-ray diffraction spectra of the three selected alloys produced by DLF were shown in our previous publication [27] and is replotted and shown in Fig. 1. The XRD pattern of Al0.3 alloy showed it to be a single phase FCC structure and a strong (200)FCC diffraction peak was observed. The DLF Al0.6 sample exhibited diffraction peaks corresponding to both FCC and BCC phases. The Al0.85 alloy showed diffraction peaks corresponding to the BCC phase, with a strong (200)BCC diffraction peak. The
Strengthening model
It is commonly found in metallurgical systems that the strength of an alloy is directly proportional to the dislocation density, and that this is a first order effect [44], [45]. Fig. 13 shows the dislocation density data plotted according to strength rather than plastic strain for the FCC alloy. Also on this plot is data for three different austenitic stainless steels subject to cold rolling by S.K. Paul et al. [Unpublished]. It can be seen that there is excellent agreement between the three
Conclusions
The deformation mechanisms of additively manufactured FCC, FCC + BCC and BCC HEAs of the AlxCoCrFeNi alloy system under tensile and compressive loading were studied leading to the following key insights:
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The FCC Al0.3 alloy showed a yield strength of 194 MPa in tension and compression. The alloy displayed a high work hardening rate in compression and this was correlated with an increase in the dislocation density with strain. Benchmarking the compressive strength against three austenitic steel
Acknowledgements
The financial support of the Australian Research Council through the Industry Transformation Research Hub for Additive Manufacturing (IH130100008) is gratefully acknowledged. Also the authors are grateful to the Deakin Advanced Characterisation Facility for the technical support.
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