Atomistic Scale Investigation of Thermal Stability, Cluster Dynamics and Microstructural Evolution of Immiscible Cu-Ta Alloys
Date
2019
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Abstract
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.