Materials Science Forum Vols. 773-774

Abstract: The mechanical properties of advanced composites are essential for their structural performance, but the surface finish on exterior composite panels is of critical importance for customer satisfaction. Previous work by the authors established the feasibility of wavelet texture analysis (WTA) for the task of automatically classifying the surface finish of carbon fibre reinforced polymer (CFRP) samples based on computer image processing. This paper presents an evaluation of the robustness of the WTA method to common process errors that can occur in the imaging of material samples. WTA creates a rich representation of the texture in an image that includes features related to both scale and orientation. Principal components analysis was used to reduce the dimensionality of the texture feature vector to a single principal component that could be used as the basis for discrimination between grades of CRFP sample surface finish quality. The results obtained indicate that the WTA method is robust to: significant horizontal and/or vertical translations of the sample being imaged; significant rotation of the sample being imaged; and significant dilation of the sample being imaged. The results obtained suggest that as long as reasonable precautions are taken in sample imaging, then the WTA method will yield repeatable results.

Abstract: Thermo-mechanical stresses have been considered one of the major concerns in electronic Packaging assembly structural failure. The interfacial stresses are often caused by the thermal mismatch stresses induced by the coefficient of thermal expansion (CTE) difference between materials, typically during the high temperature change in the bonding process. This research work examined the effect of bond layer on thermal mismatch interfacial stresses in a bi-layered assembly. The paper verified the existing thermal mismatch solder bonded bi-layered analytical model using finite element method (FEM) simulation. The parametric studies were carried out on the effect of change of bond layer properties in order to provide useful references for interfacial stress evaluation and the electronic packaging assembly design. These parameters included CTE, temperature, thickness, and stiffness (compliant and stiff bond) of the bond layer. The recent development on lead free bonding material was being reviewed and found to have enormous potential and key role to address the future electronic packaging assembly reliability.

Abstract: Periodic cellular metals (PCMs) based on lattice truss structure have been developed for a wide range of potential applications with their lightweight and multi-functionality. Although divers methods to fabricate truss PCMs, such as investment casting, perforation-bending, and extrusion-EDM, have been reported, the processes have been limited to fabricate flat plates. The paper proposes a new method, multiple pin stretch forming, to fabricate the truss PCM with various cross-sectional shapes. The process has some advantages to fabricate the truss architecture with locally different height and reduce the waste of material during perforation process compared with conventional processes. However, since a 3-D truss structure is built up by tensile deformation of truss members in the process, it is important to design the geometry of precursor sheet to avoid unexpected fracture and make full use of elongation of a given material. The paper focused on the design of precursor sheet through the investigation on failure limit in multiple pin stretch forming. The forming failure were predicted by FE simulations combined with a ductile fracture criterion for various geometrical conditions, and a forming failure diagram was constructed and verified with experiments. And finally, pyramidal truss PCMs with flat and stepped cross section were fabricated successfully without any defects.

Abstract: The aim of the present study was to investigate the role of deformation temperature on the active deformation mechanisms in a 0.6C-18Mn-1.5Al (wt%) TWIP steel. The tensile testing was performed at different temperatures, ranging from ambient to 400°C at a constant strain rate of 10^-3 s^-1. The microstructure characterization was carried out using a scanning electron microscopy. The deformation temperature revealed a significant effect on the active deformation mechanisms (i.e. slip versus twinning), resulting in different microstructure evolution and mechanical properties. At the room temperature, the mechanical twinning was the dominant deformation mechanism, enhancing both the strength and ductility. Dynamic strain aging (DSA) effect was observed at different deformation temperatures, though it was more pronounced at higher temperatures. The volume fraction of deformation twins significantly reduced with an increase in the deformation temperature, deteriorating the mechanical behavior. There was a transition temperature (~300°C), above which the mechanical twinning was hardly observed in the microstructure even at fracture, resulting in low ductility and strength. The current observation can be explained through the change in the stacking fault energy with the deformation temperature.

Abstract: In this work, in-situ BMG composites based on the Mg-Ni-Gd system with high Mg content (>80 at. %) were produced by copper mould gravity and injection casting methods. The morphology, distribution and volume fraction of the crystalline phases that form in the amorphous matrix was shown to be influenced by cooling rate, composition and casting parameters. Hence, the mechanical properties and deformation behaviour of Mg-Ni-RE BMG composites can be tailored by controlling the microstructure generated during casting.

Abstract: This paper presents an investigation of the capabilities of artificial neural networks (ANN) in predicting some mechanical properties of Ferrite-Martensite dual-phase steels applicable for different industries like auto-making. Using ANNs instead of different destructive and non-destructive tests to determine the material properties, reduces costs and reduces the need for special testing facilities. Networks were trained with use of a back propagation (BP) error algorithm. In order to provide data for training the ANNs, mechanical properties, inter-critical annealing temperature and information about the microstructures of many specimens were examined. After the ANNs were trained, the four parameters of yield stress, ultimate tensile stress, total elongation and the work hardening exponent were simulated. Finally a comparison of the predicted and experimental values indicates that the results obtained from the given input data reveal a good ability of the well-trained ANN to predict the described mechanical properties.

Abstract: Extrusion Mg-6Li-6.5Gd-1Dy-2Zn alloy was prepared successfully by vacuum melting. The microstructures and mechanical properties of as-cast and as-extruded Mg-6Li-6.5Gd-1Dy-2Zn alloys were investigated. Extruding process refines the α (Mg) phase and β (Li) phase, and makes the Mg3X phase crushed and dispersed well. After heat treatment, Mg3X phase particles became finer and much more RE enriched phase particles formed in the matrix. These effects result in the improvement of mechanical properties of the alloys. In this study, after solution treated at 510°C for 3h and aged at 215°C for 99h, the as-extruded alloy exhibits the best mechanical properties, and the microhardness, ultimate tensile and elongation reach 103.0, 249 MPa and 34.8%, respectively.

Abstract: The flow curve behavior and microstructure evolution of commercially pure titanium (CP-Ti) through uniaxial hot compression was investigated at 850 °C and a strain rate of 0.1/s. Electron back scattered diffraction (EBSD) was employed to characterize the microstructure and crystallographic texture development for different thermomechanical conditions. The stress-strain curves of CP-Ti alloy under hot compression displayed a typical flow behavior of metals undergoing dynamic recrystallization (DRX), which resulted in grain refinement. The critical strain for the onset of DRX was 0.13 using the double differentiation analysis technique. It was also revealed that the texture was markably altered during hot deformation.

Abstract: The un-doped zinc oxide (ZnO) films on the polyethylene terephthalate (PET) substrate at a low temperature (<75°C) by using cathode vacuum arc deposition (CVAD) system with different negative substrate bias voltage applied between 0 and -100 V. The material, mechanical, optical and electrical properties were investigated and discussed. The results show that all ZnO thin films have (002) preferred orientation, an average transmittance was over than 70 % in the visible region. Calculated band gap values are all around 3.2 eV with the different substrate bias voltages. The ZnO thin films with resistivities as low as 10^-3 Ω*cm have been obtained by adjusting the substrate bias voltage on the plastic substrates.

Abstract: This paper proposes a criterion for crack opening in FCC single crystals based on analyses of lattice orientation and interface energy of two adjacent crystals in a crystal plasticity finite element model (CPFEM). It also demonstrates the implementation of the criterion in Abaqus/Standard to simulate crack initiation and propagation in single-edged notch single crystal aluminium samples. Elements in the FEM mesh that have crystalline structures satisfying the crack opening criterion are removed from the mesh at the end of every loading step and FEM analyses are restarted on the new mesh in the next loading step. Removed elements effectively act as voids in the material due to crack nucleation. Similarly, the coalescence of newly removed elements at the end of a loading step with the existent ones simulates crack growth in the material. Two advantages of this approach are noted. Firstly, crack nucleation and its subsequent growth in the material is simulated solely based on lattice evolution history in the material without any presumptions of crack paths or regions where cracks are likely to occur. Secondly, as the criterion for crack nucleation is evaluated based on, and thus changes with, the lattice evolution during loading, a predefined energy criterion for crack opening, which could be erroneous, is avoided. Preliminary results of void nucleation and void growth around the notch tip in Cube and Brass oriented samples using CPFEM modelling appear to agree with molecular dynamics simulations of void growth in FCC single crystals.