Structure-Corrosion Property Correlations in Al-based Amorphous Alloys

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Alloys are created by combining any one metal with one or more other elements, metal or otherwise. Mankind has, for long, used the technique to create a variety of materials suitable for specific purposes. Alloys usually turn out to be greater than the sum of their parts; they are usually stronger than the materials used to create them. The individual materials combine to enhance the strengths of one another while negating inherent weaknesses.

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Metallurgy as a science has advanced to a stage where it is possible to create alloys of almost any desired physical and mechanical property. This is done by changing the composition and microstructure of a given metal by combining it with judiciously chosen substances. For example, the strength of many engineering alloys is improved by appropriate heat treatment (that is, age hardening), so that they can be employed in structural applications. The heat treatment creates micro-structural modifications in the matrix by introducing precipitates of sizes varying from a few nm to hundreds of nm. These heterogeneities in the material adversely affect the electrochemical properties of the material by promoting galvanic interaction, thus causing localized corrosion. Interestingly, it has been shown that as the precipitate size decreases, there is a limit below which the precipitates do not seem to act like separate electrochemical entities, while simultaneously improve the strength. This is of significant interest, since it offers a potential route to help design strong and corrosion-resistant alloys in the future.

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However, in practice, there are difficulties in studying this effect. One is caused by the grain boundaries and other defects present in normal crystalline alloys. These defects can make it difficult to isolate the effect of the precipitates on electrochemical behavior. As a result, it is extremely difficult to study and understand how the individual components and the way they combine together contribute to the corrosion behavior of the alloy. However, in this context metallic glasses, the precipitation that can be made to occur in them, represents a good, ‘model’ system to study the effects of precipitation on electrochemical corrosion behavior, since other defects are absent in the matrix of metallic glass.

At the IITB-Monash Research Academy in Mumbai, research scholar Rinkel Jindal has found a way to circumvent this problem. He uses amorphous alloys to study the behavior of individual elements when combining to form alloys. Rinkel chose this method because it is easier to introduce elements and the precipitates into an amorphous alloy in a controlled manner.

Using Al-Ni-Y amorphous alloys as the model system, Rinkel has studied the mechanism(s) through which precipitates affect corrosion resistance. The processes that have been examined in relation to corrosion tendency of amorphous alloys are: (a) Structural relaxation, (b) Surface diffusion of elements during annealing, (c) Partial crystallization and (d) Complete crystallization. Apart from the work carried out by Rinkel results published in relevant literature were also employed in the study.

Picture Credit :by Danny Fowler

A new approach for improving the corrosion resistance of Al-based alloys has been developed from the study based on model Al-Ni-Y amorphous alloys. Based on the results of this investigation, Rinkel has proposed a new hypothesis: the surface segregation of the solute (alloying element) during annealing is highly beneficial for improving corrosion resistance. This study has helped to understand the dominant role of surface segregation over structure relaxation in the context of corrosion. This will provide the basis for an alternative approach for the improvement in corrosion properties or to design new corrosion resistant materials. In addition, the mechanistic role of various phases introduced in the amorphous matrix during controlled annealing on the corrosion properties is also examined in the present study.

The groundbreaking work carried out by Rinkel will have a direct and beneficial impact on the materials used in a wide range of industries, such as: Aerospace and Automobile, Oil & Gas Pipelines/Extraction/Storage, Engineering/Architecture/Design/Construction, Construction (Bridges & Highways).

In short, given the fact that corrosion is a concern wherever metals or alloys are used, there would be very few facets of human existence and endeavor that do not reap the benefits of the pioneering work done by this young research scholar.

Rinkel works under the guidance of Prof. V. S. Raja of IIT Mumbai, Prof. Christopher Hutchinson of Monash University and Dr. Mark Gibson, CSIRO, Australia.

The IITB-Monash Research Academy operates a graduate research program in Mumbai. The IITB-Monash Research Academy is a Joint Venture between the IIT Bombay and Monash University. It fosters research partnerships between Australia and India. Research is conducted by scholars in both countries, whilst studying for a dually-badged PhD from both organisations.

Research scholar: Rinkel Jindal, IITB-Monash Research Academy

Project title: Structure-Corrosion Property Correlations in Al-based Amorphous Alloys

Supervisors: Prof. V. S. Raja, Prof. Christopher Hutchinson, Dr. Mark Gibson

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