PhD Thesis Colloquium: Mr. Anuj Dash (24/02/23)

Thesis title:

Concept of Intersecting Dissimilar Constrained Diffusion Paths for Estimation of Diffusion Coefficients in Multi Principal Element Alloys: Solving the Unsolved in Multicomponent Diffusion

Faculty advisor(s):

Prof. Aloke Paul

When?

24^th February, 2022 (Friday), 03:00 PM (India Standard Time)

Where

KPA Auditorium, Dept of Materials Engineering

Abstract

Quantitative diffusion analysis in multicomponent metallic systems has been a formidable task historically despite decades of research. The in-depth studies were limited to interdiffusion, intrinsic diffusion coefficients mostly in binary systems and only interdiffusion coefficients in a few ternary systems until recently. The experimental complications associated with the need to intersect (𝑛−1) serpentine diffusion paths in the 𝑛 dimensional space led to unsolved problems and estimation of composition dependent diffusion coefficients was considered impossible until recently. The lack of diffusivity data in multicomponent systems has hampered the generation of mobility databases essential for microstructural simulation and correlation with various physical and mechanical properties of materials highlighting the performance and durability of the product.

The work first explores the additional advantages of practicing pseudo-binary (PB) and pseudo-ternary (PT) diffusion couple methods established recently in our laboratory. We demonstrate the importance of estimating tracer diffusion coefficients by proposing the augmented Darken-Manning correlations considering the vacancy wind effect. NiCoFeCr Multi Principal Element (MPE) system is considered as a model system for these analyses to compare the data estimated by the new methods proposed in this thesis with available radio tracer measurements. The contribution of vacancy wind effect is found to be very significant on certain cross intrinsic diffusion coefficients. Such analysis highlighting the role of vacancy wind effect was not reported earlier in multicomponent systems. The body diagonal method which was originally proposed for determination of interdiffusion coefficients only is extended in this thesis for determination of the tracer coefficients of all elements from only two diffusion profiles. This reduces the laborious task of producing (𝑛−1) diffusion paths within negligible but equal distances to each other. The requirement to produce (𝑛−1) diffusion paths passing within negligible distances to minimize errors as proposed in the original body diagonal method is often an impractical experimental target.

This work then explores the possibilities of intersecting dissimilar constrained diffusion paths. First, we demonstrate that intersecting a PT diffusion path with a PB diffusion path is easier and more precise compared to intersecting two similar PT diffusion paths. Further, we demonstrate that intersecting two dissimilar PT diffusion paths has the added benefits of estimation of diffusion coefficients of up to four elements. Following, we demonstrate that intersecting a conventional diffusion path in which all the elements produce the diffusion profiles with a rectilinear diffusion path of a PB couple is far easier for estimation of diffusion coefficient of all the elements in multicomponent system for any number of elements (𝑛≥3). This overcomes the limitations of all the methods demonstrated previously for estimation of diffusion coefficients of all the elements by intersecting only two diffusion paths at a desired composition. The equation scheme required for such analysis is established and described in detail.

Finally, a design strategy for intersecting the diffusion paths in Ni-Co-Fe-Cr-Al multi-principal element alloy is demonstrated. This is guided by the possibilities of producing PB diffusion profiles in the NiCoFeCr system, This, for the first time, facilitates the estimation of the tracer, intrinsic, and interdiffusion coefficients of all the elements purely experimentally in this quinary system. The tracer diffusion coefficients estimated in this quinary system are compared with the impurity and tracer diffusion coefficients in pure Ni (taken from literature), and at (or near) equiatomic compositions of binary Ni-Co, ternary Ni-Co-Cr, and quaternary Ni-Co-Fe-Cr systems estimated in this study. The relative mobilities of the elements are found to follow the trend 𝐷𝑁𝑖∗<𝐷𝐶𝑜∗<𝐷𝐹𝑒∗<𝐷𝐶𝑟∗<𝐷𝐴𝑙∗ in all the systems.

However, the tracer diffusion coefficients of the elements first decrease and then increase with the increasing order of the systems. The contribution of the vacancy wind effect on certain cross-intrinsic diffusion coefficients is found to be very significant and cannot be ignored. The influence of the vacancy wind effect on interdiffusion coefficients is found to be less significant compared to the intrinsic diffusion coefficients since interdiffusion coefficients are a kind of average of multiple intrinsic diffusion coefficients. We have further shown that describing the relative diffusion rates of the elements with main interdiffusion coefficients instead of the tracer diffusion coefficients can be misleading or confusing. Consideration of different elements as the dependent variable of interdiffusion coefficients may indicate a very different or opposite trend.