Search

Disk of a Cu38Zr54Al8 bulk metallic glass was subjected to severe plastic deformation by high pressure torsion up to 25 revolutions. X-ray diffraction was utilized to investigate the microstructure of the deformed sample, while the thermal behavior of the glass was analyzed by differential scanning calorimetry. To examine the effect of plastic straining on structural changes, micropillars were fabricated on the cross-section of the disk along the radius at specific locations, i.e. center, half radius and perimeter. Compression tests of these micropillars were conducted in a scanning electron microscope with simultaneous recording of continuous acoustic emissions. Present paper captures the effect of competition between thermal relaxation and deformation induced structural changes. Specifically, probability distributions were constructed for a series of acoustic emission events, revealing that the occurrence of acoustic emission bursts at larger deformations is in correlation with the shift of the X-ray diffraction halos and change in thermal characteristics. Accordingly, a transition was identified from random to correlated acoustic emission behavior parallel with a progressive increase in the accumulation of excess free volume from negative to positive values.

Exceptional mechanical performance of dental enamel in the harsh environment of oral cavity can be preserved on decade timescale, which is a unique property in comparison with functional nanomaterials. Hierarchic architecture of enamel, based on site-specific structural organization of apatite nanocrystals has a key role in this durability.In the present study, a novel SEM imaging based method is presented for obtaining quantitative information on enamel prism orientation in sound primary dental enamel. This missing puzzle of quantification of the hierarchical enamel structure, along with spatial mechanical and chemical mapping, shed light on the optimum anisotropic gradient behaviour of elastic modulus of dental enamel.Specifically, orientation and composition dependent contributions in both the spatially changing hardness and elastic modulus were separated. Anisotropy of the enamel’s modulus was predicted and verified by the spatial variation of average prism orientation. Based on our results we conclude that the anisotropy of modulus for the bulk enamel arises from the elastic gradient in direction normal to the enamel external surface combined with the nearly constant value of modulus in the perpendicular cross section. This behaviour results in high surface strength and additionally can be responsible to the superior durability of human enamel.

Multiphase nanomaterials are of increasing importance in material science. Providing reliable and statistically meaningful information on their average nanostructure is essential for synthesis control and applications. In this paper, we propose a novel procedure that simplifies and makes more effective the electron powder diffraction-based Rietveld analysis of nanomaterials. Our single step in-TEM method allows to obtain the instrumental broadening function of the TEM directly from a single measurement without the need for an additional X-ray diffraction measurement. Using a multilayer graphene calibration standard and applying properly controlled acquisition conditions on a spherical aberration-corrected microscope, we achieved the instrumental broadening of ±0.01 Å in terms of interplanar spacing. The shape of the diffraction peaks is modeled as a function of the scattering angle using the Caglioti relation, and the obtained parameters for instrumental broadening can be directly applied in the Rietveld analysis of electron diffraction data of the analyzed specimen. During peak shape analysis, the instrumental broadening parameters of the TEM are controlled separately from nanostructure-related peak broadening effects, which contribute to the higher reliability of nanostructure information extracted from electron diffraction patterns. The potential of the proposed procedure is demonstrated through the Rietveld analysis of hematite nanopowder and two-component Cu-Ni nanocrystalline thin film specimens.