Molecular Dynamics Simulation Based Study for Creep Deformation Behaviour of Nanocrystalline Nickel and Nickel-Zirconium Alloys

Abstract

This thesis is an investigation of high temperature mechanical properties of nanostructure materials using atomistic simulation. Atomistic simulations are useful methods and techniques for obtaining beneficial information related to the study of materials phenomenon. The original contribution of this thesis is to provide an understanding of deformation behaviour of ultrafine-grained nanocrystalline (NC) material during creep process. The underlying deformation mechanism is revealed by dynamic characterization of structural evolution for ultrafine-grained NC Ni and ultrafine-grained NC NiZr alloys during creep process under various operative temperatures and applied stresses. The study of deformation mechanism for ultrafine-grained NC materials (grain size less than ten nanometres) by experimentation is difficult to perform, as it is very expensive as well as time consuming. Molecular dynamics (MD) simulation is a reliable and effective tool to identify the underlying deformation mechanism at nano scale. In this thesis, MD simulation based investigations have been carried out for a set of six research problems, and the first five are directly associated with the creep of ultrafine-grained NC Ni and NiZr alloys and the sixth one is the stress-induced solid-state amorphization (SSA) of ultrafine-grained NC Ni and NiZr alloys under static loading.

Creep deformation of ultrafine-grained NC materials controlled by lattice or grain boundary diffusion mechanism depends on the temperature, applied stress, grain size, and structural materials. The first work of the thesis is the study of the effect of different types of defects on creep properties of nano-sized single crystal and ultrafine-grained NC Ni. The detailed explanation of the nature of the creep curve and the structural evolution has been presented. The second work of this thesis is the investigation of the influence of bimodal grain size distribution on creep properties of ultrafine-grained NC Ni. The study of healing mechanism of nanocrack in ultrafine-grained NC Ni during creep process is discussed in the third chapter of the thesis. The fourth work of this thesis involves the study of the influence of Zr addition (accomplished in two different ways i.e. GB segregation and randomly distributed fashion in the specimen) on creep properties for ultrafine-grained NC Ni. The fifth work of the thesis includes the investigation of structural evolution and deformation features at the interface of the joint between two ultrafine-grained NC metallic systems at high-temperature. The sixth one encompasses the study of stress-induced SSA of NC Ni and NiZr alloys.

The principal deformation mechanisms promoting the creep process are dislocation motion, grain boundary diffusion, grain boundary sliding, lattice diffusion and grain rotation. In NC materials, either the diffusional flow of atoms or dislocation motion or both the above mentioned types of creep mechanisms are involved during deformation. An extensive investigation for ultrafine-grained NC Ni and ultrafine-grained NC NiZr alloys has been performed to establish the creep mechanism at various applied stresses and temperatures. Coble creep mechanism is dominant for both ultrafine-grained NC Ni and ultrafine-grained NC NiZr alloys specimens, (with grain sizes 4 nm, 6 nm, 7 nm, 8 nm, and 10 nm) during creep deformation occurring at low stresses (i.e., 0.5 GPa to 1.5 GPa) and high temperatures (i.e., 900 K to 1609 K). The various aspects of nanostructure models including bimodal structure, crack healing, NiZr alloys (Zr added in two different fashions such as GB segregation and random distribution in the specimen) and interfacial study between ultrafine-grained NC Ni and ultrafine-grained NC Fe-Cr-Ni system have been carried out to explain the underlying mechanisms during creep deformation process at various applied stresses and temperatures. Significant improvement in creep resistance for bimodal ultrafine-grained NC Ni specimens is observed with the increasing grain size at moderate creep temperatures (i.e. 900 K to 1300 K). Furthermore, applied stress is found to be a dominant contributory factor for crack healing during creep process. The creep properties of ultrafine-grained NC NiZr alloys having segregated Zr atoms at GB is found to be superior as compared to that of both ultrafine-grained NC Ni and ultrafine-grained NC NiZr alloy having randomly distributed Zr atoms. The calculation of activation energy for creep process and self-diffusion process has been studied to explain the possible controlling mechanism as well as the effect of external applied load on activation energy for ultrafine-grained NC materials. The stress-induced SSA and structural evolution of ultrafine-grained NC Ni and NiZr alloys under static loading as well as its underlying mechanism have also been studied using MD simulations. It is found that SSA for ultrafine-grained NC Ni and NiZr alloys are possible when specimens are subjected to high hydrostatic state of stress.

Hence, it can be encapsulated from the MD simulation results that the creep properties for ultrafine-grained NC Ni/NiZr alloys can be significantly altered by the variations in grain size, defects, applied stresses, operative temperatures and nature of solute atoms distribution. The creep resistant properties are found to be notably enhanced with the increasing size of coarse grain in bimodal structure. From the entire work of this thesis, it can be elucidated that the deformation of creep of ultrafine-grain is found to be majorly controlled by the grain boundary diffusion (i.e. Coble creep mechanism).