Full length articleA TRIP-assisted dual-phase high-entropy alloy: Grain size and phase fraction effects on deformation behavior
Graphical abstract
Introduction
High-entropy alloys (HEAs), also known as multi-principal element materials or compositionally complex compounds, have drawn significant attention during the last decade [1], [2], [3], [4], [5], [6], [7], [8], [9]. In the original HEA design concept, phase separation was regarded as an undesired phenomenon as it suggested that the configurational entropy was insufficient for stabilizing a single solid solution state. Specific concerns were that phase separation would lead to the formation of brittle intermetallic compounds or that partitioned alloying elements would reduce the targeted solid solution hardening effect [10]. In this context, several alloys have been proposed that develop single-phase face-centered cubic (FCC; e.g., FeMnNiCoCr [3], [5] and FeNiCoCrAl0.3 [11]), body-centered cubic (bcc; e.g., TaNbHfZrTi [12], VNbMoTaW [13], and HfMoTaTiZr [14]) and hexagonal close-packed (HCP; e.g., HoDyYGdTb [15]) crystal structures. Among these, the FeMnNiCoCr system, a typical single FCC phase solid solution HEA at room temperature that can be produced by conventional casting, has particularly excellent mechanical properties [2], [16], [17], including exceptional cryogenic fracture toughness [5].
However, a number of studies revealed that entropy-stabilized single-phase HEAs are often hard to realize and are also not necessarily equipped with superior properties [3], [4], [16], [18], [19], [20]. These observations have encouraged efforts to relax the strict restrictions on HEA design regarding single-phase stability. Motivated by this, we recently developed a new class of HEAs, namely, transformation-induced plasticity-assisted dual-phase (TRIP-DP) HEA [1]. The two high-entropy phases present in this TRIP-DP-HEA (i.e., FCC γ matrix and HCP ε phases) are compositionally equivalent [1]. The new alloy design concept was realized in the four-component FeMnCoCr HEA system. The new material combines the solid-solution strengthening effect inherent in HEAs with the TRIP effect known from certain high strength steels [21], [22], [23], [24], [25], resulting in improved strength and ductility compared to the above mentioned single-phase HEAs [1], [2], [5]. In the new TRIP-DP-HEAs, the micro-composite effect associated with its dual-phase microstructure and the displacive phase transformation upon deformation play key roles in enhancing the strain hardening potential of the material and hence its strength and ductility. The deformation-stimulated transformation behavior is influenced by the thermodynamic stability of the FCC matrix phase [1], which in turn is related to the initial FCC grain size, the alloy content and elemental partitioning, the HCP phase fraction present in the matrix prior to deformation and the load partitioning among the two phases.
We observed before that grain-refinement leads to substantial improvement in both strength and ductility in these materials [1], however, the underlying mechanisms enabling such behavior were not investigated. Here, we thus address these questions including the influence of the FCC grain size on the HCP phase fraction prior to loading, the effect of the initially available HCP phase fraction on the overall deformation response and kinetics, and the influence of the FCC grain size on its phase stability. For these reasons we produced DP-HEAs with varying FCC grain sizes and initially available HCP phase fractions by corresponding thermal and grain refinement processing. In the following we present these microstructures and the associated microstructure-mechanical property relations.
Section snippets
Alloys processing
The DP-HEA was first cast in a vacuum induction furnace using pure metals (>99.8 wt. % pure) to a predetermined composition of 50Fe-30Mn-10Co-10Cr (at. %). The as-cast ingot (10 × 50 × 150 mm3) was hot-rolled at 900 °C to a thickness reduction of 50%. Subsequently, the alloy sheets of 5 mm thickness were homogenized at 1200 °C for 2 h in Ar atmosphere followed by water-quenching. The exact composition (including the contents of residual elements) of the homogenized alloy was obtained by
Microstructure in the as-homogenized coarse-grained (CG) dual-phase HEA
Fig. 1 shows a typical analysis of the microstructure of the homogenized CG DP-HEA. Fig. 1a–c gives the BSE, ECCI and EBSD images. These data reveals that the alloy consists of two phases, namely, FCC γ matrix and HCP ε phase. The HCP ε phase is formed within the FCC γ matrix and mainly exhibits laminate morphology. According to the calculation from multiple BSE images and EBSD maps, the γ matrix has an average grain size of approximately 45 μm while the thickness of the ε laminate is varying
Micro-mechanisms and strain partitioning upon deformation
We first discuss the mechanisms of deformation in detail and also provide insights into the evolution of strain partitioning among the FCC and HCP phases at different deformation stages during room temperature tensile deformation. The CG DP-HEA in the as-homogenized state was selected as the representative case considering that the CG structure is more favorable for presenting the microstructural characteristics. This is also based on the observation that the basic sequence of microstructural
Conclusions
In this work, systematic microstructural-mechanical investigations of the newly designed TRIP-DP-HEAs with various FCC matrix grain sizes and initially available HCP phase fractions were presented. Deformation mechanisms and strain partitioning behavior in the DP-HEAs during room temperature tensile deformation were revealed in detail. The effects of FCC matrix grain size and HCP phase fraction prior to loading on the mechanical behavior were discussed. The main conclusions are as follows:
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Acknowledgements
This work is financially supported by the European Research Council under the EU's 7th Framework Programme (FP7/2007-2013)/ERC grant agreement 290998. The authors would like to gratefully acknowledge the kind support of H. Springer, S. Zaefferer, M. Nellessen, B. Breitbach, M. Adamek, F. Schlüter and F. Rütters at the Max-Planck-lnstitut für Eisenforschung.
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