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New method for making super-plastic glasses

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16/04/2007

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It was a long-cherished dream for materials scientists to find a nearly ideal metallic alloy with high strength and super-plasticity concurrently as a super-material both extremely strong and exceptionally hard for human use. Due to the limitation of the deforming mechanism, a material's plasticity usually has to turn down or even to disappear when its strength is raised. This is because the same properties that give metallic glasses their strength also contribute to their propensity to fracture.

[2007-3-19]

A research team headed by WANG Weihua from the CAS Institute of Physics has successfully developed a new kind of super-plastic bulk metallic glasses. The work is an important advance in the exploration of plasticity of amorphous alloys as the new material developed by CAS scientists features an unprecedented integration of high strength and super-plasticity at room temperature. Based on its structural analysis, the CAS scientists proposed a new method for making plastic bulk amorphous crystals in big sizes. More importantly, they demonstrate the possibility in the combination of two most important properties for a construction material: high strength and super-plasticity. Besides, the new material is expected to find wide prospects in industrial application because of its outstanding mechanical properties plus its cheap components which are ordinary metals readily available at marketplaces with low prices.Their feat was reported at the 9 March issue of Science.
In a conventional metal, atoms line up in a uniform array known as the crystalline grain, forming a great number of tiny crystals packed next to one another in a neat formation. When strain is applied, these grains slip past one another, allowing the metal to be bent. In bulk metallic glasses, however, atoms are typically caught in a random order, like those piling up in a liquid. In this case, strain becomes localized in a single plane of atoms when slipping past its neighbors. Just like a fault in geologic structure where an earthquake occurs, ruptures come into being in a series of snap-offs, leading to a catastrophic failure of the material. So, in daily life, we cannot find a window pane that flexes and bends like a copper plate.

Such a scenario is not finished hereby. Actually, the trend tends to become more apparent as soon as the material's grain size is reduced accordingly. When a metallic alloy reaches its amorphous state as its crystalline structure is in sheer disorder, its plasticity is wiped out step by step until its plastic deformation is completely lost. The main reason responsible for the loss lies in the lack of dislocated crystalline lattices. This is because the plastic stress is burdened by highly localized shear bands in the course of deformation, finally leading to the amorphous material's brittle breakdown. So, a metallic glass cannot be universally used as a high-capacity engineering materials due to its high brittleness.

The research team led by Prof. Wang has conducted a systematic research on the plastic deformation of a metallic glass during the past years, further deepening our understanding of the breakdown mechanism in some brittle materials by making clear some key problems in the process. (Phys. Rev. Lett. 94, 125510, 2005) In looking for a simple solution, the team hit upon a simple recipe that yielded a mixture of hard and dense regions of the material surrounded by less dense but soft zones. The result was: when the material was bent, fractures that began in one zone didn't propagate through the neighboring zones. Instead of one major crack resulting in the material, the glass dissipated the force into a crowd of mini-cracks and in this way, the material could bend even more than itself in the former state.

Based on this, the CAS scientists suggested a new configurational model for the shear band, using the Poisson Ratio and defined the room temperature plasticity of the majority of bulk amorphous metallic alloys, providing a solid foundation for improving the metallic glass's plasticity and designing new non-crystalline materials with super-plasticity and high toughness. Their first success was in developing the CuZr-based single-phased glasses. (Phys. Rev. Lett. 94(2005)205501.)

By using the common metallic elements such as Zr, Cu, Ni and Al, the CAS researchers recently succeeded in synthesizing amorphous alloys with super-plasticity through the present-day means of fast-speed metallurgy. Unlike other metal glasses, the new material is noted for its powerful capacity of compressed deformation in a room temperature as pure copper and pure aluminum do when they are bent into a certain shape. What is more, it has high toughness. The analysis of its structure shows that it has a special micro-structure responsible for its new property. This creative work indicates that by rationalizing the composition and structural regulation, it is possible for us to effectively control the formation, movement and expansion of the shear band, so that the metal glass's plasticity might be dramatically improved without reducing its high toughness. In the same time, the CAS scientists suggested a new method for making bulk metal glasses in large sizes. Because they discovered the glass is very sensitive to its composition when changing its own plasticity; by expressing the sensitivity with the Poisson Ratio, we can probe new ways for developing metal glasses with super-plasticity. In this new class of glasses, the imposed strain is relaxed by a host of mini-fractures instead of a single major rupture, allowing the material to bend rather than break.

The work is of special significance in understanding the mechanism of plastic deformation in amorphous metal alloys and providing a solution to the poser of the brittleness in them. "It's a very important result," comments Reinhold Dauskardt, a materials scientist at Stanford University in California. Scientists believe, the advance achieved by CAS researchers could potentially usher in a new family of wonder materials. According to well-informed sources, the creative work has been jointly funded by the CAS and the National Natural Science Foundation of China.

Source: http://english.cas.ac.cn/eng2003/news/detailnewsb.asp?infono=26449

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