Advanced Materials Research Vols. 15-17

Abstract: Rapidly solidified industrially stripcast Pb-Sb-Sn triboalloys showed microstructures typical for the eutectic decomposition of metallic glasses. Evidence was found of crystallisation above the glass transition temperature and ageing at room temperature. The observation supports a previous hypothesis by the authors on the wear properties of soft tribological alloys.

Abstract: The non-homogeneous metal flow during the extrusion process is well controlled by introducing a reflecting non-equilibrium parameter of metal flow —— the mean-square deviation of velocity ( SDV ) in this paper. The finite deformation elastoplastic finite element method is used to carry out the numerical simulation research on the profile extrusion process with the different extrusion parameters (such as extrusion ratio, frictional factor). The varied laws of the extrusion pressure, SDV and stress-strain fields with the extrusion parameters are obtained. It is a theoretic consideration to optimize the technological parameters in the profile extrusion process.

Abstract: Board-level reliability of conventional Sn-37Pb and Pb-free Sn-3.0Ag-0.5Cu solder joints was evaluated using thermal shock testing. In the microstructural investigation of the solder joints, the formation of Cu6Sn5 intermetallic compound (IMC) layer was observed between both solders and Cu lead frame, but any crack or newly introduced defect cannot be found even after 2000 cycles of thermal shocks. Shear test of the multi layer ceramic capacitor (MLCC) joints were also conducted to investigate the effect of microstructural variations on the bonding strength of the solder joints. Shear forces of the both solder joints decreased with increasing thermal shock cycles. The reason to the decrease in shear force was discussed with fracture surfaces of the shear tested solder joints.

Abstract: In this study, we investigate the crystalline states and conditions for the grain-refinement of Bi70Sb30 (at.%) alloy. It was considered under an ultra strong gravity field, the crystals were fine-grained from the primary grain sizes of several mm to several tens of mm, and the crystal growth followed with formation of graded-composition structure due to sedimentation of atoms along the direction of gravity. It was found that for the crystal-grain refinement in Bi70Sb30 alloy the minimum gravitational field and the minimum time duration were at least 160,000 G and 10 hours, respectively at about 200 °C.

Abstract: In order to solve the contradiction found in the Hall-Petch behavior in the ultra-finegrained- and nano-grained- materials, reinvestigation of the physical meaning of Hall-Petch constants has become necessary. The present work is hence attempted to theoretically construct the Hall-Petch slope constant, KH-P. This was carried out based on the principle of image analysis and previous internal energy approach. After analyzing several influencing factors on the KH-P, a model was established with 95% accuracy in predicting the KH-P value. In other words, this model, for the first time, has related the KH-P value to grain boundary surface energy,γ , shear modulus, G, and lattice constant, a0, via 1/ 2 0 K β G(2a γ ) H P = − .

Abstract: Microstructural evolution in pure copper during multi-directional forging (MDF) at temperature of 77 K was studied. Flow stress during MDF at 77 K showed a monotonical increase at all strain. Ultra fine (sub)grains of 0.15μm in diameter were evolved, which was accompanied by deformation twinning, at strain of Σε = 2.4. In higher strain region, Σε = 6.0, lamellar-look structure of twins extensively appeared. The lamellar spacing was 10-100nm. For comparison, samples were also MDFed at 300 K. The flow stress curves showed an apparent steady state flow at above strain of Σε =2.0, which implies occurrence of dynamic recovery. The evolved (sub)grain size was 0.3 μm at high strain of Σε = 6.0. Therefore, grain refinement seems to take place more easily by MDF at 77 K compared with that at 300 K due to effect of deformation twin. Microstructures evolved under MDF at 77 K and 300 K showed different annealing behavior. Static recrystallization started earlier and faster in the samples MDFed at 77K than those MDFed at 300K.

Abstract: Forming of metallic parts by the application of high intensity transitory magnetic pulses or shock waves is a challenge task from industrial perspectives as this offers extended scope of forming highly precise parts that result from material behavior at high deformation rates. Electromagnetic forming requires that the part must be intrinsically very conducting. The electrohydraulic forming is exempt from this material constraint as the deformation is generated by a shock wave in a fluid through electric discharge in between the electrodes. The application of a static pressure during forming is used to reduce the discharge energy for a given deformation. Work has been conducted to form different parts through these two techniques involving aluminum, copper and steels. The paper presents the technical obstacles still facing the electromagnetic techniques and gives examples of formed parts and joints in relation with microstructures.

Abstract: It was limited to 60/40 brass and the adjustment of grain size was performed, thereafter, it was processed by one way torsion working and back-torsion working. The effect of both workings on the work hardening and the surface roughness of 60/40 brass were investigated. The quenched material has excellent surface roughness in comparison with the furnace-cooled material both after and before workings. It seemed that a little difference in their grain size (15-55μm) affects the restoration ratio of surface roughness. For the back-torsion worked material, as the grain size is coarse, the amount of hardening increases. If the β phase ratio is more, it is easy to become hardened. Inversely, if the β phase ratio is less, it obtains the maximum amount of hardening.

Abstract: The α-phase in 60/40 brass added Si was investigated by transmission electron microscope (TEM) in order to understand the relationship between the ability of hardening during annealing and phase decomposition. The ability of hardening ( Δ HV), which is the value of difference between the maximum hardness during annealing and the value of as-quenched sample, was obtained measurement from the alloy. As a result, the Si-bearing alloy is higher in ΔHV than a base alloy without Si. The α-phase in the Si-bearing alloy showed fringes in its TEM image like as stacking faults, and extra reflected spots in its selected area diffraction pattern (SADP), which is corresponding to the 9R structure. Streaks were also observed together with spots in its SADP. Extra reflected spots and streaks in SADP became weak or disappeared when the annealing time increased. After 600 ks annealing, SADP obtained from the α-phase could be indexed as FCC lattice. Change in chemical composition of α-phase was also measured by the energy dispersive X-ray spectroscopy (EDS).

Abstract: The microstructure simulation of spinodal decomposition was carried out in the isothermally-aged Cu-Ni and Cu-Ni-Fe and Cu-Ni-Cr alloys using the phase field method. The numerical simulation was based on a solution of the Cahn-Hilliard partial differential equation by the finite difference method. The calculated results were compared to those determined by atom-probe field ion microscope analyses of the solution treated and aged alloys. Both the numerically simulated and experimental results showed a good agreement for the concentration profiles and microstructure in the aged Cu-Ni, Cu-Ni-Fe and Cu-Ni-Cr alloys. A very slow growth kinetics of phase decomposition was observed to occur in the aged Cu-Ni alloys. The morphology of decomposed phases consists of an irregular shape with no preferential alignment in any crystallographic direction at the early stages of aging in all the aged alloys. In the case of the aged Cu-Ni-Fe alloy, a further aging caused the change of initial morphology to an equiaxial shape of the decomposed Ni-rich phase aligned in the elastically-softest crystallographic direction <100> of Cu-rich matrix. The growth kinetics rates of phase decomposition in Cu-Ni-Fe and Cu-Ni-Cr alloys are appreciably faster than that in Cu-Ni alloys.