Thermodynamics and Dynamics of Metallic Liquids Studied by NMR: Liquid-Liquid Transition and Dynamic Crossover
Abstract
Mechanical properties of solid metallic alloys such as metal castings depend crucially on the characteristics of the morphology and the microstructure formed in the solidification process. Here, properties of the metallic alloy melt play a direct role and the understanding and control of the liquid phase properties of metallic alloys are of crucial importance for alloy development and materials optimization. This requires information on the structure and dynamics of metallic alloy melts, especially the detection of their changes with modifications such as minor alloying element additions. Sensitive measurement techniques for detecting subtle property changes in the liquid phase of metallic alloys are extremely important but are quite challenging due to the complexity of liquid structures. Subtle structural changes could lead to major changes in liquid properties. One of the potential mechanisms is the liquid-liquid transition (LLT). LLT is a phase transition from one liquid phase to another liquid phase with identical composition and could occur in metallic alloy melts above the liquidus temperature with a major influence on the solidification process. Detection of LLT could be quite challenging since large discontinuous changes in thermodynamic properties are not expected at the transition point. Recently, it was shown that high-temperature nuclear magnetic resonance (NMR) is an effective tool for revealing LLT in La-based metallic alloy melt. In this proposed work, a systematic NMR study of LLT and qualitative property changes will be carried out in La-based and Al-based metallic alloys including commercial alloys that are important for lightweight vehicles and aerospace industry. Our objectives are: 1.Investigate LLT and crossover behaviors in Al-based and La-based metallic alloy melts. 2.Identify the origin of the overheating effects of metallic alloy melts above the liquidus temperature Tliq. 3.Understand the NMR signatures associated with LLT and crossover behaviors in metallic alloy melts and establish the NMR method for studying metallic alloy melts. Methods:Nuclear spin such as that of Al-27 is a natural atomic scale probe for local structures and dynamics of matters including metallic alloy melts. Subtle changes in local structures and dynamics in metallic alloy melt can be detected by the sensitive change of the NMR resonance frequency via the Knight shift mechanism as well as the spin-lattice relaxation. High-temperature NMR is shown to be a sensitive and convenient tool for studying metallic alloy melts capable of detecting events such as LLT. Broad impact: LLT is a phenomenon challenging the fundamental understanding of phase transitions based on symmetry breaking. From the practical point of view, it could provide a new avenue for optimizing alloy properties by introducing qualitative changes in metallic alloy melts. Such qualitative changes in the liquid state of metallic alloys could be quite common including in alloy systems that are of great importance for developing lightweight vehicles and aerospace applications. The proposed work is directly related to ARO s mission, in particular, for developing new design principles for the optimization of advanced materials properties.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Oct 23, 2018
- Source ID
- W911NF1710224
Entities
People
- Yue Wu
Organizations
- Army Contracting Command
- United States Army
- University of North Carolina at Chapel Hill