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Thursday, 7 November 2013

A new understanding of metallic glass - MIT News Office

A new understanding of metallic glass

I have relayed this MIT new article due to the growing importance of metallic glasses which I first learned of due to the interest and investment in R&D in my old outfit the famous"Imphy SA, Imphy, France." The site situated on the banks of the Loire river, 15kms from Nevers was originally chosen by the famous Colbert and is a proud manufacturer of one of the 4 base blocks supporting La Tour Eiffel. Now unfortuneatly, this once proud company has become a hotch-potch consortium (Belgian,French,Indian)with the outlandish name of Aperam, composed of  Eramet,Aubert &Duval & Arcelor-Mittal.

Back to the scientific nitty-gritty:

Simulations reveal that the formation of some glassy materials is like the setting of a bowl of gelatin.

In a 50/50 mix of copper and niobium, regions that are richer in copper separate from regions that are richer in niobium. The interface between these two kinds of regions forms an irregular sponge-like surface, shown in this visualization in green. While most of the material is disordered (making it a glass), small collections of atoms at the boundary zone (shown in gray) form a stiff interconnected network, giving the material greater strength. 

Gelatin sets by forming a solid matrix full of random, liquid-filled pores — much like a saturated sponge. It turns out that a similar process also happens in some metallic glasses, substances whose molecular behavior has now been clarified by new MIT research detailing the “setting” of these metal alloys.

The research is published this week in the journalPhysical Review Letters, in a paper 
co-authored by assistant professor of materials science and engineering Michael Demkowicz and graduate student Richard Baumer. It addresses one of the “grand challenges” in physics, Demkowicz says: understanding what happens during what is known as the “glass transition” in materials, when their molecular structure settles into a disordered, yet solid, state.

“It was a serendipitous discovery,” Demkowicz says, after Baumer “started out working on something completely different, studying the radiation response of amorphous metallic alloys.” But in the course of that research, while conducting simulations of the behavior of these alloys, Baumer found something unexpected: a series of brief events in which tiny pockets of the alloyed metals melted and then solidified again. 

Certain metallic alloys are known to form glasses — materials in which the atoms are distributed in a disordered way (unlike crystalline metals, which form perfectly regular arrays). While the alloy Baumer was studying was not of this type, its behavior provided hints that it might be capable of forming glasses.

The alloy, a 50/50 blend of copper and niobium, is “unlike other glass-formers,” Demkowicz says: Normally its two constituents are like oil and water, and don’t mix. (Typically, alloys that form glasses are composites of materials with a strong affinity for one another.)

But as the copper-niobium blend is quenched — that is, cooled quickly to below its melting point — a brief phase-separation occurs, then suddenly stops. But instead of separating out into adjacent, pure crystals of the two components, the alloy’s structure remains disordered. “There are regions enriched in copper, and regions enriched in niobium, and interfaces between them,” Demkowicz says. The regions themselves are too small to allow for the formation of a crystalline structure: “You can’t make a perfect repeating structure out of any of them.”

The boundary between the copper-rich and niobium-rich zones turns out to be crucial. This zone is similar to the spongelike structure that gives gelatin its stiffness, even though gelatin is mostly liquid. In this case, the pockets between the boundary regions are initially also liquid, but gain strength from the framework around them.

This “gelation” process, Demkowicz says, “may be more common than people think.” As a result, this work could lead to reevaluation of a variety of metal alloys not previously considered good candidates for glass formation.

While the work is so far theoretical, Demkowicz says that this better understanding of the formation process may improve the mechanical properties of glasses — such as by creating new glass materials whose brittleness is minimized. “This may be a new way of controlling the distribution of components” within glass, he says.

Evan Ma, a professor of materials science and engineering at Johns Hopkins University who was not involved in this research, says, “Their findings in this metallic system are remarkably similar to gelation processes in polymeric and colloidal gels, and thus point to significant common features that bridge different kinds of amorphous materials and glasses.” Yunfeng Shi, an assistant professor of materials science and engineering at Rensselaer Polytechnic Institute, adds that the work could lead to “understanding of the commonality in glass transition among all glass-formers.”

The work was supported by the National Science Foundation.

Materials physics for materials design_The attractions of computor simulation ref. MIT's Michael Demkowicz Grp.

The attractions of computor simulation although mostly for resolution of complex multi-variable and multi-dimentional problems they are also an excellent tool for visulisation and highly attractive communication.

Here are some examples taken from MIT's  Demkowicz Group 


Computational materials design using Reduced Order Mesoscale Models (ROMMs)


2. Radiation-resistant amorphous materials.
3. Materials design through interface engineering.
4. Bayesian inference applied to grain boundaries.
5. Prediction and prevention of wear.

 Mitigating hydrogen- and helium- induced damage

Although they may "seem benign", hydrogen (H) and helium (He) can both cause severe embrittlement in structural materials. H embrittlement is especially problematic in acidic environments, such as those encountered in deep oil wells, while He embrittlement may occur in nuclear reactors. By controlling how H and He atoms diffuse, cluster, and interact with crystal defects, we are designing new embrittlement-resistant materials for energy applications
NB. Hydrogen embrittlement is a well known phenomena and has long been documented. He damage came to the fore due to the extensive use of Ar and/or  He in powder metallurgy where one or other of these inert gases are used to atomise liquid metal streams from typically VIM-Vacuum Induction Furnaces.

Crystalline materials are tough, but susceptible to radiation damage. To create materials with significantly improved safety, performance, and reliability for advanced nuclear reactors, we are investigating the radiation response of amorphous (non-crystalline) materials. Combining both crystalline and amorphous solids into composites may lead to materials that capitalize on the strengths of both while compensating for their weaknesses.

Although they are sometimes viewed as no more than dividing surfaces between neighboring constituents, solid-state interfaces (most usually called grain boundaries) in fact have distinct physical properties of their own. We are using the properties of interfaces to design new materials with radically enhanced performance under extreme irradiation and mechanical loading.

Many materials are so complex, that we can only gain partial understanding of their structures and properties using individual experimental or modeling methods. However, we can gain a more complete picture by combining results from many kinds of different techniques. Bayesian inference provides a rigorous mathematical framework for intelligently combining vastly different types of data such that the whole is greater than the sum of the parts. We are applying this technique to infer the properties of grain boundaries as a function of their crystallographic character.

Wear is a life-limiting materials degradation mechanism in many applications. For example, wear by grid-to-rod fretting (GTRF) is the most common cause of fuel leaks in current nuclear reactors. We are developing microscale simulations for modeling wear at the level of single asperities and debris particles. These simulations will lead to better predictions of wear and to the design of materials with improved wear resistance.
1. Projects.

Although they are sometimes viewed as no more than dividing surfaces between neighboring constituents, solid-state interfaces in fact have distinct physical properties of their own. We are using the properties of interfaces to design new materials with radically enhanced performance under extreme irradiation and mechanical loading.

High Purity Cr sources for Superalloys

Energy for th Future:Phil.Trans.A-Vol. 365, N° 1853 / April 15, 2007, curtesy The Royal Soc. London

Engineered foams and porous materials: Phil Trans A. Vol 364, N° 1838 / 06 curtesy_The R Soc. Lond