Dissertation/Thesis Abstract

Atomistic Scale Investigation of Thermal Stability, Cluster Dynamics and Microstructural Evolution of Immiscible Cu-Ta Alloys
by Koju, Raj Kiran, Ph.D., George Mason University, 2019, 131; 27666876
Abstract (Summary)

Nanocrystalline immiscible Cu-Ta alloys produced by mechanical alloying emerged as

a new class of material for high temperature and high strength applications. The extraordinary

structural stability of this alloy is believed to be due to the eect of precipitated

Ta nanoclusters on thermodynamics and mobility of internal interfaces, primarily grain

boundaries (GBs). In order to better understand this eect, we studied the interaction

of Ta clusters with individual GBs driven by applied shear stress and capillary force. The

atomistic simulations provided strong evidence that the experimentally found extraordinary

grain size stabilization in Cu-Ta alloys is dominated by the kinetic factor associated with

the Zener pinning of GBs by Ta clusters. Also, the eect of solute drag at GBs in a random

solution is not as strong as cluster pinning. A moving GB in the random solution displays

a stop-and-go character of motion precipitating a set of Ta clusters due to short circuit Ta

diusion in the GB core.

Moreover, atomistic simulations conrmed that small Ta clusters have FCC structure

and remain at least partially coherent with the Cu matrix. As the cluster size increases, it

becomes incoherent and emits mist dislocations into the matrix. At higher temperatures,

the lattice mist between the Ta clusters and the matrix decreases, promoting better coherency. The core-shell Ta clusters observed in experiments can be explained with the

redistribution and the crystallization of Cu and Ta atoms in a Ta rich amorphous solution.

Simulated deformation and creep tests conducted under tension and compression have

shown that the Ta clusters eectively suppress the grain boundary mechanisms of plastic

deformation, such as sliding and grain rotation. The Ta clusters also inhibit deformationinduced

grain growth and suppress the operation of dislocation sources inside the grains

leading to high strength and structural stability. The strain rate sensitivity parameter of

Cu-Ta alloy exhibits a limited rate of strain hardening even when subjected to temperatures

as high as 80% of melting point (1327 K) where pure nanocrystalline Cu becomes unstable

and undergoes rapid grain growth. We observed that creep in Cu-Ta alloy is governed by

atomic diusion with the stress exponent 3.78, while this value predicts dislocation

based creep mechanism in pure nanomaterials.

Indexing (document details)
Advisor: Mishin, Yuri
Commitee: Sheng, Howard, Vora, Patrick, Emelianenko, Maria
School: George Mason University
Department: Physics
School Location: United States -- Virginia
Source: DAI-B 81/8(E), Dissertation Abstracts International
Source Type: DISSERTATION
Subjects: Physics, Materials science, Atomic physics
Keywords: Computer modeling, Creep, Cu-Ta alloys, Mechanical behavior, Microstructural evolution, Zener pinning
Publication Number: 27666876
ISBN: 9781658430371
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