Pressure-induced nanocrystallization of Zr 55 Al 10 Ni 5 Cu 30 bulk metallic glass
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The effect of pressure on the crystallization behavior of Zr55Al10Ni5Cu30 bulk metallic glass was investigated by differential scanning calorimetry, x-ray diffraction, and transmission electron microscopy. Although the crystallization products under high pressure were about the same as those under ambient pressure, the evident changes in the relative crystallization fraction of each phase were observed. The applied pressure enhanced the crystallization temperature. Pressure annealing of the bulk metallic glass produced a composite with dispersion of very fine nanocrystallites in the amorphous matrix. A full nanocrystallization was obtained for the sample annealed under 5 GPa at 793 K. The mechanism for the pressure-induced nanocrystallization is discussed.
I. INTRODUCTION
Various kinds of metallic glasses with a large supercooled liquid region and high glass-forming ability, enough to enable fabrication of bulk samples by casting methods, have been discovered recently.1–4 Although bulk metallic glasses (BMGs) have shown potential as structural materials due to their unique mechanical properties, structural applications are currently limited by the lack of any significant plastic deformation.5,6 However, it was reported that amorphous/nanocrystalline composite alloys possess better mechanical and magnetic properties than their single amorphous counterparts.7–10 The excellent glass forming ability and the high thermal stability of BMGs are generally attributed to the difficulty of precipitation of crystalline compounds from the melt upon casting or from the supercooled liquid. Their typical crystallization mode is the single-stage eutectic reaction, which also implies that the amorphous phase containing homogeneously nanocrystalline particles is hardly formed in the BMGs. To obtain a nanoscale structure, the crystallization process should proceed with the largest nucleation rate and the slowest crystal growth. Such conditions can be obtained for alloys with some special compositions deviating from the three empirical rules supposed by Inoue11 or by applying particular methods of heat treatment.12 High pressure is a powerful tool that might be applied to control the nucleation and growth.13,14
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 17, No. 11, Nov 2002
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With the development of high-pressure technique, the influence of high-pressure research on the advancement of science has become more important, similar to that of temperature variable or that of composition variable. It is well known that the structures and properties of materials are profoundly altered under high pressure due to a larger change of atom spacing, chemical bonding, and Gibbs free energies. Therefore, materials with novel structure and properties can be created through a process known as pressure-induced phase transition.15,16 By applying high pressure during crystallization of ZrTiNiCuBeC bulk metallic glass, Wang et al. obtained very fine str
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