Abnormal L1₂-LPSO phase transformation and its influence on mechanical properties and deformation mechanisms in a Ti-alloyed medium-entropy alloy

A comprehensive understanding of precipitation behaviors, including phase transformation, morphology and distribution of precipitates, and their impacts on mechanical properties and deformation mechanisms is very important for the development of precipitation-hardened high/medium-entropy alloys (HEAs/MEAs). In this work, a Ti-alloyed MEA was successfully fabricated and systematically investigated. Experimental results reveal that two types of precipitates, i.e. intragranular spherical L1₂ and intergranular lamellar long-period stacking ordered (LPSO) precipitates with same stoichiometry, are coexist at early aging state in this Ti-alloyed MEA, which result in superior mechanical properties. With aging process continuous, the L1₂ phase could abnormally transforms to the LPSO phase, and this abnormal transformation originates from geometrical modification of the compact planes stacking sequences. Furthermore, the effects of abnormal phase transformation on mechanical properties and deformation mechanisms are systematically investigated. This work not only provides a comprehensive understanding of precipitation behaviors and their impacts on mechanical properties in a Ti-alloyed MEA, but also an interesting phase transformation path for the design of precipitation-hardened HEA/MEAs.