Materials Science Forum Vols. 783-786

Abstract: The aim of this work was to analyze the evolution of texture and the deformation anisotropy of a AA1050 after ECAP and further recrystallization. The initial material was processed by two different routes: the first set of samples came from a hot rolled band with a mixture of cube, copper and brass textures and the second from roll casting and presented a strong brass texture. The ECAP deformation followed routes A, Ay (no rotation between passes but keeping the normal to rolling direction parallel to the transverse extrusion axis) and Bc. Texture evaluation was performed by x-ray analysis. Results were analyzed by regular texture and orientation distribution function calculations. The deformation anisotropy was measured by tension tests with specimens oriented 0, 45 and 90^o to the extrusion direction.

Abstract: The problem of the onset of the Portevin-Le Chatelier (PLC) effect is revised by combining a study of the kinetics of the flow stress evolution upon abrupt changes in the applied strain rate and acoustic emission (AE) accompanying plastic deformation of an AlMg alloy. The kinetic measurements allow evaluating the strain-rate sensitivity of the flow stress and the time characteristics of transient processes as functions of plastic strain. Using known criteria of plastic instability, domains of instability are constructed in the (strain, strain rate) plane. A particular accent is put on the strain-rate range corresponding to the so-called “inverse” behavior. The comparison of such maps with experimental data on the critical strain testifies to the insufficiency of these criteria for explaining the onset of the PLC effect. Moreover, the slow transient kinetics contradicts observations of the fast development of stress drops. The AE measurements bear witness that the stress serrations are associated with bursts in duration of acoustic events generated by the collective motion of dislocations. The possible role of synchronization of dislocation dynamics on the onset of plastic instability is discussed.

Abstract: Recent studies of plastic deformation using high-resolution experimental techniques bear witness that deformation processes are often characterized by collective effects emerging on an intermediate scale between the scales describing the dynamics of individual crystal defects or the macroscopic plastic flow. In particular, the acoustic emission (AE) reveals intermittency of plastic deformation in various experimental conditions, which is manifested by the property of scale invariance, a characteristic feature of self-organized phenomena. Some materials, e.g., Al or Mg alloys, display a macroscopic discontinuity of plastic flow due to the Portevin-Le Chatelier effect or twinning. These materials are therefore of special interest for the study of collective effects in plasticity. The present work reviews the results of a multiscale investigation of AE accompanying plastic deformation of such model alloys. The AE is analyzed by methods borrowed from the theory of nonlinear dynamical systems, including statistical and multifractal analyses.

Abstract: In the present study, a systematic study of nanoscale precipitates in an Al-7.5Zn-1.7Mg-1.4Cu-0.12Zr alloy have been carried out for various typical aging tempers, including T6, T76, T74, T73 and RRA treatments, by combining synchrotron-based small-angle x-ray scattering (SAXS) and transmission electron microscopy (TEM) techniques. Based on the TEM observations, quantitative and statistical structural information, including precipitate size, volume fraction, number density and inter-precipitate distance have been extracted from SAXS data through model fitting. The results show that the T6 peak-aged alloy with the smallest precipitate size has the highest number density and lowest volume fraction of precipitates. Under two-step T7X over-aged tempers, with the deepen of aging degree, the precipitate size, volume fraction and inter-precipitate distance increases, but the number density decreases. The size distribution of precipitates for the RRA-treated alloy is in between that of T6 and T76. The results also show that as the degree of over-aging deepens, the precipitate size distribution interval becomes broader.

Abstract: The purpose of this study is to develop an Al-Mg-Zn series aluminum alloy casting suitable for the plating. We evaluated the influence of the metal structure on the plating process of both AC4CH and a developed aluminum alloy and then examined their suitability to plating. With the AC4CH-T6 casting containing the silicon, it was difficult to obtain a smooth surface by pretreatment process because of the surface irregularity between the hard and chemically stable eutectic silicon and the aluminum matrix. On the other hand, with the developed silicon-free alloy casting, it was possible to produce a smooth surface by pretreatment process, and this glossiness was superior to that of the AC4CH-T6 casting after electroless Ni-P plating. Furthermore, the plating property of the developed casting was also superior to that of the AC4CH-T6 casting, because the zinc deposit on the developed casting from the zincate treatment became uniformly, depending on the uniform displacement reaction.

Abstract: At the micro scale, dense cortical bone is structurally comprised mainly of Osteon units that contain Haversian canals, lacunae, and concentric lamellae solid matrix. Osteons are separated from each other by cement lines. These microfeatures of cortical bone are typically captured in digital histological images. In this work, we aim to automatically segment these features utilizing optimized pulse coupled neural networks (PCNN). These networks are artificially intelligent (AI) tools that can model neural activity and produce a series of binary pulses (images) representing the segmentations of an image. Two segmentation methods were used: one statistical and another based on the physical attributes of the microfeatures. The first, statistical-based segmentation method, cost functions based on entropy (probability of gray values) considerations are calculated. For the physical-based segmentation method, cost functions based on geometrical attributes associated with microfeatures such as relative size, shape (i.e., circular or elliptical) are used as targets for the fitness function of network optimization. Both of these methods were found to result in good quality segregation of the microfeatures of micro-images of bovine cortical bone.

Abstract: The aim of the present study was to characterize the high temperature tensile behaviour of the C355 (Al-Si-Cu-Mg) alloy produced under controlled casting condition so as to obtain different secondary dendrite arm spacing (SDAS). C355 samples were produced through a gradient solidification equipment able to produce microstructures with fine (20-25 μm) and coarse (50-70 μm) SDAS values. The as-produced specimens were subjected to hot-isostatic pressing and then T6 heat treated. Microstructural characterization, room and high temperature (200 °C) tensile tests were carried out on the heat treated specimens. The tensile behaviour was related to the different SDAS value of the samples.

Abstract: Solidification and microstructural characteristics of Al-5wt.%Mg-1.5wt.%Si-0.6wt.%Mn-0.2wt.%Ti alloy have been investigated in high pressure die casting. The average size of dendrites and fragmented dendrites of the primary α-Al phase formed in the shot sleeve is 43 m, and the globular α-Al grains formed inside the die cavity is 7.5 m. Solidification inside the die cavity also forms the lamellar Al-Mg2Si eutectic phase and the Fe-rich intermetallics. The size of the eutectic cells is about 10 m, in which the lamellar α-Al phase is 0.41 m thick. The Fe-rich intermetallic compound exhibits a compact morphology and is less than 2 m. Calculations using the Mullins and Sekerka stability criterion reveal that the solidification of the primary α-Al phase inside the die cavity has completed before the spherical α-Al globules begin to lose their stability, but the α-Al grains formed in the shot sleeve exceed the limit of spherical growth and therefore exhibit a dendritic morphology.

Abstract: Mean free path of dislocation motion have a significant effect on mechanical properties because dislocation motion is blocked by eutectics and intermetallics in Al-Si casting alloys. In this study, we proposed a statistical method to evaluate mean free path of dislocation motion by using image processing, and investigated relation between the measured mean free path and mechanical properties of Al-Si casting alloys.

Abstract: Direct quenching (DQ) is one of the latest process routes in production of ultra-high strength, high performance steels and Ruukki one of the pioneering companies in the utilization of direct quenching. Ruukki has applied direct quenching for the production of ultra-high-strength structural steels in the form of hot-rolled strip and plate. The paper briefly summarizes the physical metallurgy fundamental of direct steels and shows some selected examples of the microstructures and properties of steels produced by direct quenching. In addition, a brief review on the usability properties and design rules of ultra-high strength structural steels is made.