The Massachusetts Institute of Technology, Cambridge, announces that researchers have developed a tungsten-chromium-iron powder metallurgy alloy that could replace depleted uranium in armor-piercing projectiles. The W-7Cr-9Fe alloy is significantly stronger than commercial tungsten alloys. Tests show a nanoindentation hardness of 21 GPa, about double the nanoindentation hardness of nanocrystalline iron-based alloys or coarse-grained tungsten.
The material is made by a powder metallurgy process of high-energy ball milling. This process involves repeated shearing of the metal powders, with the shearing driving the alloying elements to intermix while competing thermally activated recovery processes allow the alloy to return to its equilibrium state.
“So there is this competition between these two processes,” says materials science and engineering graduate student Zachary C. Cordero. He reported results in a paper with senior author and Department of Materials Science and Engineering head Christopher A. Schuh and colleagues in the journal Metallurgical and Materials Transactions A.
His paper proposed a simple model to predict chemistries in a given alloy that will form a solid solution, and validated it with experiments. The improvement was achieved by compacting metal powders in a field-assisted sintering hot press, with the best result, measured by the fine grain structure and highest hardness, achieved at a processing time of one minute at 1200°C. Longer processing times and higher temperatures led to coarser grains and weaker mechanical performance.
“The way that I make my materials is with powder processing, where first we make nanocrystalline powder and then we consolidate it into a bulk object. But the challenge is that consolidation requires exposing the material to higher temperatures,” Mr. Cordero says. Heating the alloys to high temperatures can cause the grains within the metal to enlarge, which weakens them. Cordero was able to achieve ultrafine grain structure of about 130 nanometers in the W-7Cr-9Fe compact, confirmed by electron micrographs.
“Using this powder processing route, we can make big samples up to 2 centimeters in diameter, or we could go bigger, with dynamic compressive strengths of 4 GPa. The fact that we can make these materials using a scalable process is maybe even more impressive,” he says.
http://newsoffice.mit.edu/2014/alloying-tougher-tungsten-zack-cordero-1202#.VH5Abu6skNM.email
