Precipitation Hardenable High Entropy Alloy for Tooling Applications
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.146
Precipitation Hardenable High Entropy Alloy for Tooling Applications O. Stryzhyboroda1, U. Hecht1, V. T. Witusiewicz1, G. Laplanche2, A. Asabre2, M.B. Wilms3, A. Weisheit3 1
ACCESS e.V., Intzestraße 5, D-52072 Aachen, Germany
2
Institut für Werkstoffe, Ruhr-Universität Bochum, D-44780 Bochum, Germany
3
Fraunhofer-Institut für Lasertechnik ILT, Steinbachstraße 15, 52074 Aachen, Germany
ABSTRACT
We present a high entropy alloy (HEA) from the system Al-Co-Cr-Fe-Ni with small additions of W, Mo, Si and C which was designed to allow for precipitation hardening by annealing in the temperature range from 600 to 900 °C. The alloy development was supported by thermodynamic computations using ThermoCalc software and the specimens were produced by arc melting. The microstructure of one selected sample in as-cast and annealed conditions was analysed using SEM/EDS, SEM/EBSD and TEM. The as-cast microstructure consists of spinodally decomposed BCC dendrites enveloped by FCC+Cr23C6 eutectic. Upon annealing at 700 °C for 24 h nanoscale precipitates form within the spinodal BCC as well as from FCC. Precipitation is exquisitely uniform leading to an increase in microhardness from 415 HV0.5 in the as-cast state to 560 HV0.5 after annealing. We investigated coarsening of this microstructure using varying holding time for a constant temperature of 700 °C. The microstructure evolution during coarsening and the corresponding mechanical properties obtained from instrumented indentation experiments are presented in this work.
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INTRODUCTION High entropy alloys have attracted broad scientific interest aiming at finding novel alloy compositions and materials properties that would outperform conventional materials in terms of corrosion, wear and fatigue resistance etc. [1][2][3]. Among other HEAs, alloys from the Al-Co-Cr-Fe-Ni system offer means to tailor the microstructure and mechanical properties by tuning the alloy composition. The Al content in the range from 12 to 20 at%, for instance, allows balancing FCC/BCC phase fractions [4] [5] [6] [7] while other alloying elements can be introduced to promote precipitation hardening [8][9][10]. The Vickers hardness of spinodally decomposed BCC in Alx(CoCrFeNi)1-x alloys reaches values comparable with hot tool steels [11],[12] while promising improved resistance to thermal softening [13]. However, alloy design towards hot tool steel applications will have to meet further requirements regarding thermal conductivity, abrasive and adhesive wear along with a reasonable compromise between strength and toughness. Whether additive manufacturing (AM) e.g. by laser cladding can contribute to property improvement by structure refinement is a subject of current research. In this paper we
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