Papers by Author: Leng Chen

Abstract: Free-standing CVD diamond films were prepared under the substrate temperature in the range of 850-1050oC. Macro- and micro-textures of the films were investigated based on the SEM observation as well as on the ODF and EBSD analysis. It was found that certain growth selection process appeared during diamond deposition which, however, did not lead to a strong film texture. It is indicated that strong fluctuation of growth ratio V<100>/V<111> and frequent growth twinning during film deposition resulted in randomization effect of grain orientations, which can be transformed by adjusting the parameters of film preparation.

Abstract: The residual stress and crystallographic texture of diamond films were investigated in the present work. The diamond films were synthesized on (100) silicon wafer by Microwave Plasma Chemical Vapor deposition (MPCVD). Then the residual stresses of the films were measured by X-ray diffractometer equipped with the two-dimensional detector. The residual stresses can be classified into two categories, i.e., the intrinsic stresses and the thermal stresses. It was shown that the thermal stresses were compressive in the temperature range studied and the intrinsic stresses were tensile. The crystallographic textures of the films were measured by X-ray diffractometer with the method of pole figure and orientation distribution function (ODF). The experimental results suggest that the crystallographic textures of the films depend upon the deposition temperature and methane flow rates, and the components and intensity of crystallographic textures have effect on the residual stresses in diamond films to a certain extent.

Abstract: Orientation mapping based on EBSD technique was applied to reveal the orientations of new grains and their relationships to the surrounding matrices, to analyze Kikuchi band contrast and the influence of strain rates on local orientation evolution. This information is used to understand the dynamic recrystallization mechanism and the relative contribution of plastic slip versus grain boundary glide or grain rotation related with super-plasticity. For this purpose samples with different initial textures were deformed by (quasi-)plain strain compression at two strain rates. It is suggested that the dynamic recrystallization in this alloy proceeds in continuous way by progressive subgrain rotation. No evident non-basal slip of was observed by referring texture evolution in the sample with initial basal texture. A high strain rate promotes more contribution of plastic slip accompanied by fast orientation changes. The fact that groups of grains with very similar orientations in basal oriented samples is discussed in terms of viscous flow.

Abstract: The crystallographic textures of the free-standing diamond films which were synthesized by the chemical vapor deposition (CVD) using a CH4/H2/Ar gas mixtures have been investigated. Texture components of the diamond films are analyzed with pole figures and orientation distribution function (ODF). The surface morphology of the diamond films grown at different substrate temperatures as well as different gas concentrations is observed with scanning electron microscopy (SEM). The experimental results indicate that the intensity of {110} fibre texture becomes much higher with the increase of the substrate temperature. It is considered that the higher multiplicity of <110> crystallographic direction in diamond octahedron leads to the higher appearance possibility of {110} fibre texture and the final texture components of the diamond films depend upon the CVD deposition parameters.

Abstract: A new model describing the reaction stresses during plastic deformation of metals is proposed in which the reaction stresses among the grains and their accumulation are calculated. The model could overcome the shortages of the Sachs and Taylor deformation model. According to the model, the plastic strain tensor induced by activation of slip systems will produce certain elastic reaction stress in the surrounding matrix, which influences the choice of further activation of slip systems as well as the orientation evolution. The model gives more attention to both of the stress and strain compatibility among the grains. The simulation on the tensile deformation of pure copper indicates that the model could exhibit the main characteristics of the real deformation process.

Abstract: A new approach to simulating the orientation distribution function (ODF) using one diffraction frame collected by X-ray two-dimensional detector is proposed. Based on the concept that the texture component presents a normal distribution form, the volume fraction and the orientation spread of texture component are obtained using curve fitting method from diffraction profile and, the pole figures as well as ODF can be calculated. Thus the texture of grain-oriented silicon steel is determined rapidly and conveniently. A comparison of the simulation ODF with that reconstructed from experimental pole figures by series expansion is presented and the results show that the new approach works correctly.