Metallic glass material is tougher than metal

Researchers in the United States have developed a new material that is more resilient than today's best steel and titanium alloys.

Block metal glass materials have shown promise in engineering for more than 15 years but often have poor flexibility and toughness, and make them brittle and fragile. But recently, Douglas Hofmann, William Johnson and colleagues at the California Institute of Technology (Caltech, Pasadena, California) have found a way to transform the structure of block metal glass materials to make their toughness significantly improved.

The metallic glass material (Bulk Metallic Glassy - BMGs) is like metal in that they contain conductive and metallic bonds, but the atoms have the same disordered nature as glass. An disorderly structure cannot contain defects, meaning that BMGs withstand heavy loads before being broken. This is at least true when they are bent or pressed, but when they are stretched, their toughness is not enough to make them quickly break. One way to improve toughness and fracture resistance according to the results of the Caltech team is to add a number of elements in the process of creating the initial metallic glass alloy, which will produce fine particles. can disperse or ' branches ' when they are cooled . These ' branches ' will act as barriers to the development of slip bands and limit them from all developments leading to faults.

The idea of ​​combining ' tree branches ' was first tested by Johnson with members of another team who created BMGs alloys from the molten state. This process creates branched crystals that are quite small and only increase toughness at the edges. However, also from these results, Hofmann, Johnson and colleagues have found two properties of tree branches that can optimize toughness: they must be as wide as the length of the branches. Sliders, and must be softer than the BMGs themselves .

Figure 1. Structure of the observed material on high-resolution electron microscopy images (Nature, 451 1085).

To create this property, the group began from alloys of Titanium (Ti), Zirconium (Zr), Niobium (Nb), copper (Cu) and Beryllium (Be). By holding the molten alloy at temperatures between 800 ^° C and 900 ^° C, the branched crystals of Ti, Zr and Nb start to grow to large sizes. In the process of growing, a ratio of Be and Cu in the remaining mass does not form crystal fish will increase gradually. And Caltech's team found that when the ratio of Be in the block is about 1/3, the aggregate properties will be balanced and the branched crystals will not be any bigger. And by adding less Be to the original alloy, they were able to produce a lower percentage of glass phases and larger amounts of branched crystals. 'What we found was remarkable' - Hofmann said.

With three samples of BMGs cooled with different tree branch proportions, the team found samples with the largest proportions of branched crystals (67%) for the best toughness, K = 173 MPa m1 / 2 - comparable to the best Ti, iron and steel alloys available today. However, the value of strength - defined by the largest shear stress, G sets a new record.'We have put these alloys from the most fragile metal group into the toughest type' - Hofmann said. Hofmann explained that although used in some applications, his team wanted to exploit the toughness found in structural applications, such as in the airline industry. 'We hope to bring them into the market of high quality Ti alloy' - Hofmann added.

Figure 2. Comparison of tough alloys with new alloys (Nature, 451 1085).

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