Thermodynamic instability of a nanocrystalline, single-phase TiZrNbHfTa alloy and its impact on the mechanical properties

Abstract

An equiatomic, single-phase TiZrNbHfTa high-entropy alloy was subjected to high-pressure torsion,leading to a grain size below 100 nm. Introducing a nanocrystalline microstructure to the material shouldhelp to accelerate a possible phase decomposition of the material by having a high amount of fastdiffusion pathways and possible nucleation sites in the form of grain boundaries. In order to test thematerials thermodynamic stability the nanocrystalline high-entropy alloy. was subjected to various heattreatments for temperatures between 300 C and 1100 C. Isochronal heat treatments (1 h) resulted in ahardness increase from 420 HV1 for the as-processed state to 530 HV1 for an annealing temperature of500 C, while for temperatures of 700 C and higher a softening compared to the as-processed stateoccurred. In order to clarify this unexpected annealing response, analysis of selected microstructuralstates was performed utilizing electron microscopy, x-ray diffraction as well as mechanical testing to gainfurther information on microstructure-property relationships. Complementary, thermodynamic simulationswere performed via the Calphad approach and compared to the experimental results. A phasedecomposition of the originally equimolar single-phase high-entropy alloy into a NbTa-rich bodycenteredcubic phase and ZrHf-rich phases, which occurred in two different crystal structures dependingon the annealing temperature, was the main reason for the property changes. The obtained results notonly give valuable new insights into the phase stability of the TiZrNbHfTa alloy, but also demonstrate theimpact of the newly forming phases in regards to mechanical properties and its implication for a possiblepractical application of this alloy