Investigation On Amorphization Process By Solid-state Deformation

Preparation and crystallization of amorphous been a major concern. In this experiment, NiTi amorphous alloy is prepared through the high pressure torsion. In this article, we analysis of the results through XRD, positron emission spectrum, DSC and TEM test method and explore the mechanism combined with thermodynamics and dynamics of the process. In addition, we explore the relationship between crystal and amorphous structures, and sum up inner link within the crystal and amorphous, laying the foundation for a unified system between crystal and amorphous.The crystallization of amorphous and amorphous preparation methods have received the widely attention. Severe plastic deformation is method of improving coarse crystal structure, and becoming a new method of preparation amorphous, and repeat cold-rolled.Observing XRD chart and TEM diffraction diagram of the original sample,5laps, ten laps,15laps and25laps samples, we can see that the original crystal structure change, along with the increase of the strain, from the coarse grain into nanocrystal, then to the amorphous structure change. In15laps, we can see obvious amorphous halo. But continuing to increase the strain, microstructure changes, appear crystallization, precipitation of the crystalline phase. This phenomenon is rarely recorded in the literature.Through the analysis of the positron life spectrum, it can be concluded that high pressure torsion deformation may introduce the defect. Along with the increase of the strain, defect increases ceaselessly, and on behalf of severe plastic deformation, high pressure torsion will introduce a great number of defects, and in five laps, the small defect is close to saturation state. Along with the strain increases further, continue to introduce defects and in the material, big defect volume increased significantly.Look at DSC curve, we can speculate that the stored energy of the dislocation defects is bigger than amorphous structure. That is, in the process of high pressure torsion, the introduction of a large number of dislocation change the crystal structure into amorphous. We can also see stability of dislocation crystal structure is poorer than amorphous structure.In this experiment, using the laws of thermodynamics, combined with the internal energy, potential energy and so on, we analysis amorphous causes from the angle of energy. We find this also is corresponding to the experimental results. High pressure torsion introduced dislocation and dislocation is a kind of potential energy. Therefore, HPT improve sample internal energy. In this paper, through the analysis of TEM, XRD spectrum of amorphous, it is concluded that "the crystal" spacing is not a certain value, but a rang, in which, the spacing corresponding to amorphous steamed peak is proportional to the spacing corresponding to weighted average of elements close packing. And the principal element decide amorphous short program. In addition, the incorporation of other elements cause distortion of short program structure. Therefore, amorphous and crystalline are closely linked. Combined with the experimental results, it is concluded that high pressure torsion process continuously introducing dislocation and the content of dislocation is very big, which making crystal confusion degree increased, then causing surface spacing change, becoming a range from a fixed value. When the degree of the disorder increased to a certain degree, we can see a amorphous steamed peak in the XRD results and amorphous halo in TEM diffraction peak, then we can say it is amorphous.