Microstructure and Properties of Single Crystal L12 Phase Precipitation Strengthened Multi-component High Entropy Alloy

Abstract

In the course of human industrialization, there is an urgent and growing demand for advanced structural materials capable of delivering exceptional high-temperature performance. These materials are critical for enhancing energy efficiency, ensuring structural reliability, and meeting the increasingly stringent operational requirements of next-generation turbines and engines. The newly developed high entropy alloys (HEAs) usually contain three or more different elements, where no single element dominates. This alloying strategy offers greater possibilities for compositional design to achieve desired performance combinations. Among these, bulk HEAs strengthened by multi-component L1₂ precipitates have garnered significant attention, offering a promising approach to addressing persistent challenges in structural materials through flexible compositional control and a unique dual-phase structure. In this thesis, we designed an L1₂-strengthened Co-rich HEA, which can effectively eliminate the negative impact of grain boundaries in high-temperature applications, allowing multi-component alloys to be more widely used in high-temperature applications. This work comprehensively examines the alloying effects, microstructural evolution, mechanical properties, oxidation behavior, creep properties, and other relevant characteristics, aiming to advance their potential for high-temperature applications.

We first conducted a systematic review of the research progress of superalloys, including the development history, different materials systems, alloying effects, and fabrication methodologies. Furthermore, we provided an overview of the newly developed multi-component alloy, including its microstructure and comprehensive performance.

We then designed and fabricated a single-crystal multi-component alloy with the Bridgman technique. The master alloy with a nominal composition of Co₄₁Ni₃₅Al_1·5Ta_2·5Cr₄Ti₆ (at.%) was produced in a vacuum induction furnace and then directionally solidified into [001] single-crystal rods at a constant withdrawal rate of 3.5 mm/min. By adjusting the heat treatment conditions, a homogeneous and high-density L1₂ precipitate-strengthened structure was acquired. With multiscale characterization, the precipitation behavior, including size and density, morphology, and composition of the L1₂ precipitates, was carefully investigated. And the mechanical properties were systematically evaluated, and the yield strength of our single-crystal alloy can still be maintained at ~800 and ~500 MPa when tested at 900 and 1000 °C. Compared to commercial single-crystal superalloys, the single-crystal alloy fabricated in this work demonstrates superior performance.

Under high-temperature service conditions, severe oxidation can induce substantial degradation of structural material properties, ultimately leading to catastrophic failures. Consequently, in addition to mechanical performance, oxidation resistance emerges as a pivotal property for high-temperature structural applications. We conducted systematic investigations on the oxidation behavior of our single-crystal superalloy at 800 °C, 900 °C, and 1000 °C. After 120 hours of oxidation exposure, the synergistic interaction among multiple alloying elements facilitated the formation of complex multilayer oxide scales across all test temperatures. We used TEM to analyze it in detail, identify its composition, and analyze the mechanism of its joint action. In summary, the complex oxide layer produced under this new alloy greatly improves its oxidation resistance.

Third, for superalloys, creep is one of the most important demands for high-temperature applications. We further investigated its creep performance and deformation mechanisms. We found that this lightweight alloy exhibits exceptional creep resistance at 850-900 °C under stresses of 500-400 MPa, surpassing most existing Co-based superalloys. This excellent creep resistance can be attributed to a synergistic combination of enhanced coherent interface strengthening, antiphase boundary (APB) strengthening, and local strengthening related to twin boundaries.

In summary, the precipitation-strengthened single-crystal alloys we designed and developed were thoroughly evaluated in terms of mechanical properties, oxidation resistance, and creep performance. These findings establish a significant framework for advancing the design of next-generation structural materials with enhanced performance across a broader temperature range.

Date of Award	13 Aug 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Tao YANG (Supervisor) & Ji-jung KAI (Co-supervisor)