work

Work in Corteks Group, University of Hyderabad

In my Masters work titled "Calphad and experimental studies on Co-Cu-Fe-Ni immiscible high entropy alloys", I dealt with three things:

1. High entropy alloys

We designed an alloy and studied the immiscibility among the component elements. Unlike conventional alloys which have one principal element and other element in trace amount, ours system was different in a way that every element was in a concentrated amount. These kind of alloys are called high entropy alloys, owing to the high configurational entropy they have.

The interesting thing about mixing whether its metals or anything else, is we get something extra over what  we mixed.

2. Calphad

Calphad  (Calculation of Phase Diagram) method uses Gibbs energy functions of lower order systems (which are easy to find using experiments) to extrapolate Gibbs energy of complex systems and thus calculate properties from these.

3. Phase diagrams

Phase diagrams contain information about state of the system at all compositions. Through our work, we investigated the questions like:

Is the single phase region really a single phase ?

Isn't there any difference in the arrangement of atoms near to the single phase-two phase boundary and away from it even though phase diagram shows none?

Does randomness have a degree?

We intend to add more detail to phase diagram regarding the nature of phase shown in it. this was done through synthesis of HEAs. System selection and composition selection was done using Thermocalc. Alloys were synthesized using Vacuum Arc melting and characterized.

Work in CMEG, IITB

Phase field modelling of immiscible system.

Phase field modelling is used for microstructural evolution of metallic systems. Microstructure is defined in terms of phase field variables, which are continuous functions of space and time. Differential equation involving these phase field variables are solved numerically for evolution of microstructure. Interface is implicitly represented by the continuously changing value of phase field variables hence there is no need to keep track of interfaces like in sharp interface models. Hence, simulating complex morphology is easier.

To simulate real systems we need to enter material properties in the phase field. CALPHAD data can be added in the phase field model to get results for real systems.

MicroSim

MicroSim is software stack consisting of various phase-field codes developed under the National Supercomputing mission. We are using Cahn-Hilliard module in MicroSim (which uses FFTW numerical technique to solve the Cahn-Hilliard Equation) to get spinodal microstructure for real systems like FeCr. To get quantitative results we need to couple real data as input in phase-field. Property descriptions generated using CALPHAD method (which are based on experimental information of simple systems and DFT calculations) can also be used. 

Gibbs energy description from Thermocalc is extracted using TC-Python SDK and used as bulk free energy density in Cahn-Hilliard Equation.

Composition dependent gradient energy coefficient (kappa)

Classical Cahn-Hilliard equation for a regular solution model evaluates the gradient energy coefficient to be constant using summation of pairwise interaction.

We intend to study methodically the effect of composition dependent gradient energy coefficient on microstructural evolution. For this we assume the linear and quadratic composition dependence of gradient energy coefficient (kappa) in Cahn-Hilliard equation and solve it for spjnodal system.

Formulation: Free energy functional was written in the same fashion as defined by Cahn-Hilliard as made up of two parts: first defines the free energy of the uniform composition and the other due to composition gradient. Driving force, i.e., chemical potential was found out by solving Euler-Lagrange equation for variational derivative of free energy functional with respect to composition.

Chemical potential on substituting in modified diffusion gives the evolution equation. Code was written in python using Numpy library.

Implementation: Fourier spectral technique was used for solving the differential equation.

Semi-implicit technique was used for discretization to avoid unstability of solution at larger timesteps.

Phase-field modelling of multiferroics (current work)

Multiferroics are materials which exhibit more than one ferroic order such as ferromagnetism,  ferroelectricity and ferroelasticity. Phase-field modelling is a relevant tool to model such systems as bulk energy is described by Landau's polynomial which assumes continuum and is relevant description for mesoscale modelling.