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Designing Nanolayered Metal Composites: Mimicking Nature

Name: Sumit Maurya (Ph.D, IITB -Monash)

Department: Mechanical Engineering

Supervisors: Prof. J. F. Nie at Monash University and Prof. Alankar at IIT Bombay

 

Designing Nanolayered Metal Composites: Mimicking Nature

Layered materials are abundant in nature e.g. sea shells, wood and many types of rocks. These materials have exceptional mechanical properties due to the effect of layer thickness and interfaces. Multilayer metal composites are the materials with one-dimensional compositional periodicity or composition and structural periodicity. Such composites can be formed using dissimilar metals as the constituent materials without altering the individual chemistry of constituent metals. The interfaces between dissimilar metals can be designed based on many factors e.g. the crystallographic orientations of the constituent metals in each layer. Aluminum (Al) and titanium (Ti) are used extensively in the automotive and aircraft industries. Various experimental studies on Al-Ti multilayer show hardness values up to 7 GPa. These multilayers have a wide range of potential applications as protective coatings. The hardness of the Al-Ti multilayers increases as the layer thickness decreases and exceptional hardness is observed below 100 nanometers. As we decrease the layer thickness to nanometers, the effect of surfaces and interfaces becomes more pronounced, which triggers many metastable microstructural transformations. It has been reported that HCP (hexagonal close packed) Ti transforms to FCC (face entered cubic) Ti at the layer thickness of less than 22 nm, whereas FCC Al transforms to HCP Al at the layer thickness of less than 10 nm. To optimize the mechanical properties of these Al-Ti multilayers, it is important to understand the kinetics and mechanism of such metastable microstructural transformations. We have used classical molecular dynamics simulation to understand the microstructural transformation in the Al-Ti multilayer system which will eventually help us in optimizing the mechanical properties by tailoring the microstructure. 

Fig 1

Representative volume element of Al-Ti multilayer used in this study.

 

We have performed a series of atomistic simulations to examine the roles of interface characteristics, layer thickness, and heating rate on the microstructural transformation of Al-Ti multilayers. HCP-FCC transformation occurs by creating a series of stacking faults of atomic planes, and the density of these faults decreases as the layer thickness increases. The nature of the stacking faults depends on the individual layer thickness and heating rates. The HCP-FCC transformation results in a decrease in energy, and an increase in the volume of the multilayer. Reoriented HCP Ti is equivalent to  a parent HCP Ti crystal rotated by 90° around a fixed axis. The main feature of our study is the revelation that it is not a rigid body rotation but a result of the coordinated displacement of atoms. Based on the atomic displacements and simulated microstructure, we have proposed a two-stage displacement mechanism of reorientation of HCP Ti, which consists of the formation of an intermediate BCC (body centered cubic) phase. Interface between parent and reoriented HCP Ti is formed between two different types of atomic planes, the first plane which is equivalent to the face of a hexagonal prism is termed as prismatic plane and the second plane which is equivalent to the base of the hexagonal prism is termed as basal plane, so the interface formed between these two types of planes are called as PB and BP interfaces. A PB interface is formed where the prismatic plane of parent Ti is parallel to the basal plane of reoriented Ti and vice-versa. Reorientation results in the change of c/a ratio from 1.69 to an ideal value of 1.63, which decreases the overall strain the Ti layer. 

We are currently investigating the energetics of these transformations using Density Functional Theory (DFT) based Nudge Elastic Band (NEB) analyses. In order to establish a complete structure-property correlation, we have extended this study to understand the role of layer thickness and deformation rate on  tensile, compressive and fracture behavior of Al-Ti multilayers. 

Fig 2

The proposed displacement based mechanism of transformation of HCP Ti to FCC Ti.

 

This work was performed by IITB-Monash PhD scholar Sumit Maurya under the supervision of Prof. J. F. Nie at Monash University and Prof. Alankar in the Department of Mechanical Engineering at IIT Bombay. Complete Article: S. K. Maurya, J. F. Nie, A. Alankar, Atomistic analyses of HCP-FCC transformation and reorientation of Ti in Al-Ti multilayers, Computational Materials Science, 2021, p. 110329, https://doi.org/10.1016/j.commatsci.2021.110329