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Lecture

Control of precipitation kinetics in equiatomic CoCrFeMnNi high-entropy alloy by microstructural design: On the role of grain boundaries and interfaces

Wednesday (01.01.2020)
00:15 - 00:30
Part of:


Multi-principal-element CoCrFeMnNi high-entropy alloys solidifying as a single-phase solid solution have attracted great attention as a promising material for various engineering applications due to their exceptional mechanical properties and thermal stability in a wide range of application temperatures. It has been shown that especially CoCrFeMnNi alloys with equiatomic compositions remain a single-phase solid solution and do not undergo any significant structural changes (except for grain coarsening) upon annealing of several hundreds of days at temperatures above 900°C. On the other hand, the alloy decomposes into multiple phases at intermediate temperatures ranging from about 500 to 800°C. A complex phase decomposition and formation of second-phase precipitates have been observed preferentially at grain boundaries or at intragranular defects such as inclusions and pores [2]. In this study, equiatomic CrCoFeMnNi alloy was synthesized by physical vapor deposition as an epitaxial single-crystalline film at 650°C, which corresponds to a temperature at which the single-phase solid solution was found to be unstable and at which Cr-rich phase and traces of NiMn and FeCo precipitates have been observed [1]. For comparison, a polycrystalline CoCrFeMnNi film was synthesized at the identical process conditions and characterized by transmission electron microscopy and X-ray diffraction. The results revealed the dominant role of grain boundaries on the precipitation kinetics. While the epitaxial single-crystalline CoCrFeMnNi alloy remained a single-phase solid solution, the microstructure of the polycrystalline alloy consisted of Cr-, Co- and Ni-rich precipitates formed preferentially at the grain boundaries. These findings were further validated by annealing of the single-crystalline alloy at various temperatures to induce its decomposition. The grain boundary-free single-crystalline alloy was, however, found to preserve its single-phase microstructure irrespective of the thermal loading. The fact that the activation barrier for nucleation is much higher in the grain interior than at the grain boundaries suggests a great potential of the microstructural design to supress phase decomposition of the alloy by controlling the grain size and distribution.

Speaker:
Prof. Dr. Rostislav Daniel
Montanuniversität Leoben
Additional Authors:
  • Dr. Jakub Zalesak
    Montanuniversität Leoben
  • Dr. Igor Matko
    Slovak Academy of Sciences
  • Walter Baumegger
    Montanuniversität Leoben
  • Prof. Dr. Easo P. George
    Oak Ridge National Laboratory
  • Prof. Dr. Christian Mitterer
    Montanuniversität Leoben
  • Prof. Dr. Jozef Keckes
    Montanuniversität Leoben