Materials Science Forum Vols. 561-565

Abstract: A simple method to extract the intrinsic mechanical properties of the soft metallic thin films on hard substrates by nanoindenation is presented. Utilizing the geometry relationship of residual impressions obtained by the SEM image and the cross-sectional profile, the pile up error in elastic modulus determination of soft thin films by the Oliver and Pharr analysis is first corrected. Knowledge of the ‘true’ elastic modulus, the ‘true’ hardness of thin film is then extracted from the measured contact stiffness data for an elastically homogeneous film-substrate system. The present method is applied for a 504 nm Au thin film sputter deposited on the glass substrate and the results show that the ‘true’ elastic modulus and hardness of Au film are 80 GPa and 1.3 GPa, which are in agreement well with the literatures.

Abstract: Structures and morphologies of Ge precipitates in an Al-Ge alloy were characterized by a combination of transmission electron microscopy and three-dimensional electron tomography. Faceting of the precipitates was clearly seen using transmission electron microscopy and varieties of precipitate morphologies were identified by three-dimensional electron tomography.

Abstract: An extra low carbon steel was cold rolled to 85% reduction and annealed at 680 °C to generate a microstructure containing ~2 % recrystallized grains. A partly recrystallized volume was analyzed using 3-D FIB-EBSD tomography. The results show that nucleation and subsequent growth of recrystallizing grains is more complex processes than that revealed using 2-D metallographic techniques. In the present steel, it was found that subgrains were found to be the origin of nucleation and these grains exhibit an internal structure similar to the surrounding deformation substructure. However, a certain subgrain keeps expanding to a stage where some part or parts of the boundary reach(es) and consume(s) a high stored energy deformation zone(s) to form (a) local dislocation free zone(s) having an orientation similar to the subgrain. After this stage, the residual dislocations in the original subgrain are annihilated and nuclei enter a well-defined growth stage. The overall growth of recrystallization nuclei was found to be controlled by the variation in both the stored energy and orientation of the surrounding deformation substructure that results in heterogeneous growth by so-called orientation pinning.

Abstract: A simple and material-independent indentation technique is presented to determine the hardness and elastic modulus of solid materials. This method requires only three experimentally measured quantities to determining the material properties, i.e., the peak load, corresponding displacements and the depth of residual indent, without curve fitting process and regression analysis. The results obtained from this simple technique were consistent with those obtained by using the conventional method. Furthermore, energy dissipation and the ability of elastic recovery of the ceramic materials were evaluated from the measured indentation data.

Abstract: An atomistic study of radiation-induced amorphization in the NiTi intermetallic compound was performed by using in-situ high-resolution high-voltage electron microscopy and molecular dynamics in conjunction with image simulations. Both theoretical and experimental results show that metastable nanometer-size inherent atomic clusters form and disappear during irradiation, so that a spatiotemporal fluctuation under amorphization is induced. The random formation and annihilation of such inherent nanoclusters are believed to be responsible for these fluctuations, which appear to be related to transitions between the ideal glass state and metastable, unrelaxed states in an energy-dissipative system under irradiation.

Abstract: Tungsten carbide based hardmetals with cobalt binder phase are widely used in engineering industries for their excellent mechanical properties and outstanding wear performance. Reciprocative sliding wear behaviour of a number of WC-Co based hardmetal grades was investigated using a small-scale pin-on-plate tribometer. Test samples were manufactured by electro-discharge machining (EDM) with various surface finishing regimes. SEM topographies and cross-section views of the cemented carbides were obtained both before and after dry friction tests, revealing distinctive wear mechanisms. The generated wear loss was quantified topographically using surface scanning equipment. Wear debris particles were collected and examined by EDX and TEM analysis. Based on experimental results, the execution of consecutive gradually finer EDM cutting steps was found to considerably enhance wear performance. Furthermore, a significant influence of contact load, sliding movement duration, application of lubricant and wear debris formation on wear rate and friction was established.

Abstract: Beryllium (Be) is susceptible to introduce stress because it is a brittle metal with a high elastic modular. The compact tension (CT) specimens of beryllium were designed to determinate stress and fracture behaviors. Stress distribution near notch in CT beryllium was measured by the combination of an X-ray stress analysis and a custom-designed load device. The results show that local stresses near notch tip are much higher than those on other area. Thus, stress concentration lead the CT specimens fracture along the notch direction. Residual stresses due to machining are remained. A finite element ( FE ) calculation on the same loaded geometry was made, and the result is agreement with the measured stress distribution near notch.

Abstract: Photo-oxidation of polyethylene (PE) has been studied thoroughly these years owing to its high output and various applications, which makes knowledge about its degradation of vital practical importance. Polyethylene products often suffer from service environment factors including ultraviolet, heat or chemicals, which may deteriorate their mechanical properties. The degradation behavior varies with the chain structure and morphology of polyethylene, e.g., branches, linear segments length and crystallization. Natural degradation of three kinds of polyethylene (HDPE, LDPE and LLDPE) has been examined in samples using different annealing conditions in this work. The extent of degradation, described as carbonyl index, was evaluated by Fourier transform infrared spectroscopy (FTIR). Crystallinity measurements were made using differential scanning calorimetry(DSC). The investigation indicates that the branch structure plays a dominant role in the photo-oxidation of polyethylene. Annealing will change the crystallinity of polyethylene before aging, but do not show obvious dependence on the oxidation.

Abstract: In this work, the physical aging and its effect on nonlinear creep behavior of poly(methyl methacrylate) are presented. After annealing above Tg to release the previous thermal and stress history, the samples were quenched to 60oC, aged for various times, and were then tested at three different stress levels (22MPa, 26MPa and 30MPa) at room temperature of 27oC. At each stress level, the creep strain was converted to compliance and measured as a function of test time and aging time. The test results show that higher stress accelerates creep rate of the material while physical aging plays a reverse role. The time-aging time superposition is applicable to build a master creep compliance curve at each stress level, and it is demonstrated that the shift rate deceases with increasing stress. Moreover, based on the time-stress superposition principle, a unified master curve was constructed by further shifting the sub-master curves at 30MPa and 26 MPa to a reference stress level of 22MPa.