Abstract
In today's diverse and demanding application environments, there is a growing need for materials that possess a unique combination of mechanical and physical properties, which are often absent in conventional alloys. Since 2004, the concept of high entropy alloys (HEAs) has emerged as a promising solution, garnering significant attention in the materials science community owing to their extraordinary structural and functional characteristics. By incorporating multiple principal elements into a single lattice, it becomes possible to create a single-phase crystal structure with highly distorted characteristics. During my PhD research, I focused on the development of a single-phase high-entropy intermetallic alloy (HEIA) with a composition of (CoNi)50(TiZrHf)50 (at. %), leveraging the substantial size and property differences among these five principal elements. Compared with HEA, HEIA is featured by the strong chemical long-range order as defined in conventional intermetallic alloys, which manifests as the formation of sublattices. Through extensive experimental and computational investigations, encompassing elastic, plastic, electrical, and thermal properties, as well as underlying mechanisms, I explored the potential of this HEIA. The outcomes of my research showcased the profound impact of severe lattice distortion on the alloy's exceptional strength and plasticity (Chapter 2), its remarkable Elinvar effect (Chapter 3), and its ultralow thermal conductivity (Chapter 4). These findings shed light on the unique properties and behaviors of the (CoNi)50(TiZrHf)50 HEIA, providing valuable insights for future material design and application development.Intermetallic alloys are known for their ordered crystallographic structure and strong bonding. Nevertheless, their brittleness at room temperature is often attributed to insufficient slip systems. Due to the lack of strain hardening, intermetallic alloys tend to soften and even fail upon yielding. However, our research has led to the development of a single-phase B2 HEIA - (CoNi)50(TiZrHf)50 - that defies these conventional limitations. This HEIA exhibits impressive strength and plasticity, distinguishing it from traditional intermetallic alloys. The key to its exceptional mechanical properties lies in its highly distorted crystalline lattice with complex chemical ordering, which enables the presence of multiple slip systems and high flow stress. Notably, the alloy demonstrates an unprecedented dynamic hardening mechanism initiated by dislocation gliding, which effectively preserves its strength across a wide temperature range. Consequently, this alloy exhibits remarkable resistance to thermal softening, maintaining extensive plastic flow even at elevated homologous temperatures. It surpasses various body-centered cubic and B2 alloys, opening up new possibilities for the development of intermetallic alloys with broad engineering applications. These findings present a promising pathway for advancing the field of intermetallic alloys and expanding their potential for practical engineering applications.
In addition to investigating the mechanical properties of the (CoNi)50(TiZrHf)50 high-entropy intermetallic alloy (HEIA), my research also delved into exploring the Elinvar effect exhibited by this alloy, as outlined in the third chapter of my study. To initiate this investigation, we employed micro-alloying techniques to fabricate a series of severely distorted high-entropy Elinvar alloys, specifically (CoNi)50-x(TiZrHf)50Fex (in atomic percentage). By meticulously designing and conducting experiments, we were able to establish a direct correlation between the overall lattice distortion in these B2 HEIAs and the tunability of the Elinvar effect. Moreover, we propose a concise physical model that effectively captures the general trend observed in our experimental findings. Remarkably, the non-magnetic nature of this Elinvar effect not only expands its temperature range but also overcomes the limitations typically associated with conventional Elinvar alloys utilized in magnetic environments. These results signify a significant advancement in the understanding and application of the Elinvar effect, offering broader possibilities for its utilization across various fields.
In the fourth chapter of my research, I focused on investigating the influence of lattice distortion on the transport properties of our (CoNi)50(TiZrHf)50 high-entropy intermetallic alloy (HEIA). Prior studies on FCC HEAs in the Fe-Co-Ni-Cr-Mn system indicated that the high entropy mixing strategy primarily affects electron transport rather than phonon behaviors. However, in our B2 structured (CoNi)50(TiZrHf)50 HEIA, we observed a noticeable impact on phonon behavior as well. By employing both geometric phase analysis (GPA) and molecular dynamics (MD) simulations, we discovered that the lattice distortion in our HEIA surpasses that observed in the FCC FeCoNiCrMn HEAs. Specifically, the lattice contraction is more pronounced around the Ti atoms. The large mass mismatch in (CoNi)50(TiZrHf)50 results in Ti contributing to the high-frequency portion of the density of states (DOS), while the other elements dominate at lower frequencies. This starkly contrasts with the closely distributed partial DOS observed in FeCoNiCrMn. Consequently, phonons experience increased scattering, leading to diminished lattice thermal conductivity. These findings position our HEIA as a promising material for thermal insulation applications in industries such as aerospace and nuclear, where low thermal conductivity is desired.
| Date of Award | 28 Aug 2024 |
|---|---|
| Original language | English |
| Awarding Institution |
|
| Supervisor | Yong YANG (Supervisor) |