By G.P. Thomas
Most metals, irrespective of their application, are composed of crystals or orderly, repetitive arrangements of molecules. The performance of the metals is largely determined by the size of the crystals.
A transition electron microscope image of the structure of the new tungsten-titanium alloy, after being exposed to a high temperature of 1,100 degrees Celsius for a week (Credit: Chookajorn, MIT)
Smaller the crystals, better is the performance. However, there is an accompanying problem of instability as the crystals exhibit a tendency to unite to form bigger crystals under conditions of heat and stress. In order to overcome this problem, researchers at MIT have designed a new class of alloys made up of nano-sized crystals. The alloy possesses great potential for application in high-strength materials owing to its ability to preserve its nanocrystalline structure even under conditions of high heat.
The study findings also include the theoretical premise for identifying alloys capable of forming nanostructures and the fabrication and testing details of such alloys. For decades, researchers have tried to develop alloys with smaller grain sizes but have not succeeded owing to the natural tendency of materials to adopt a lower energy state typical of big crystals. Also, the focus of these endeavors was based on the ability of the materials to mix together to form an alloy rather than on the grain boundaries. These grain boundaries are critical for the formation of stable nanocrystals and were incorporated into the research team’s calculations to determine which alloying materials are capable of stabilizing the grain boundaries of a given system.
The material developed by MIT researchers is an alloy of tungsten and titanium that possesses exceptional strength and can hence be used for protection of military and industrial machinery and in personal and vehicular armor. The material remained stable even after being subject to a temperature of 1,100oC for one week.