Materials Science Forum Vols. 638-642

Abstract: Grain size dependence of yield strength was reviewed for polycrystalline ferritic iron and low carbon steel. Yielding of polycrystalline low carbon steels was characterized by a clear yield point (upper yield point) and such a yielding behavior is taken over to ultra fine grained steel with the grain size below 1m. Yield strength (y) of polycrystalline low carbon steels obeys the Hall-Petch relation: y[MPa]=+600×d[m]-1/2 . The Hall-Petch coefficient ky is around 600 MPa･m1/2 for the commercial low carbon steels but it is lowered to about 100 MPa･m1/2 for interstitial fee steel. Besides, it is known in industrial pure iron (Fe-30ppmC) that ky increases with aging at 363K. The value of ky is also increases with increasing the amount of solute carbon content. The ky is enlarged from 100 MPa･m1/2 to 550 MPa･m1/2 by adding 60ppm of solute carbon and then levels off at around 600 MPa･m1/2 in the carbon concentration region above 60ppm. On the other hand, nitrogen hardly influences the ky value. Difference between C and N in the contribution to ky is probably due to the difference in grain boundary segregation behavior. Macroscopic yielding of polycrystalline ferritic iron is reasonably explained by the Hall-Petch model considering dislocation pile-up against grain boundary and dislocation emission from the grain boundary where stress concentration has been generated by piled up dislocations. It is seemed that the segregated carbon stabilized the dislocation emission site at grain boundary and this leads to the increase in ky.

Abstract: Basically, thermal spray and laser processing can be considered as half brothers since they show many common features due to the use of a (more or less) high-energy source for both. Their combination can therefore be very fruitful and prominent to achieve coatings, which results in their most recent and advanced applications. In the materials processing development story, the laser will thus have moved from cutting to coating. This keynote presentation focuses on the recently-developed coupling of laser processing to cold spray). In this dual process, a cold spray gun is combined to a laser head in a single device, e.g. on a robot. Series of coating experiments using various laser irradiation conditions, primarily pulse frequency, were carried out for Al-based and Ni-based alloys. Laser pre-treatment of the substrate just prior to cold spray, was shown to be beneficial for adhesion of cold-sprayed coatings. Adhesion improvement was exhibited and studied from LASATesting (LASAT for “LAser Shock Adhesion Test”). Incidentally, through LASAT also, the role of lasers in the development of thermally-sprayed coatings can be considered as major. Results are discussed in the light of a TEM (Transmission Electron Microscope) study of the coating-substrate interface with and without laser pre-treatment.

Abstract: The oriented-nucleation and oriented-growth for recrystallization (Rex) textures of electrodeposits, vapor deposits, and plastically deformed metallic materials have been discussed based on the strain-energy-release-maximization (SERM) model. When the Rex orientation predicted by the SERM model from major components of the deformation texture is the same as a minor component in the deformation texture that is calculated to be thermally stable by the SERM model, the calculated Rex orientation is sure to become the main component of the Rex texture. This implies that the oriented-nucleation and the oriented-growth affect the evolution of the Rex texture. For polycrystalline materials, the Rex orientation predicted by the SERM model is likely to be measured because heterogeneous nucleation can occur in grain boundaries even when shear bands are absent. In other words, the grain boundaries are unlikely to control the Rex texture and the oriented growth dominates the Rex texture.

Abstract: A CPFE model was used for an assessment of the assumptions used by the ALAMEL model concerning grain interactions. A finite element mesh was constructed for a multicrystal consisting of four grains. There were 17496 integration points per grain. The main goal was to capture the complex nature of the plastic fields in the vicinity of the grain boundaries. The distribution of strain rates, both along and perpendicular to the grain boundaries, confirms that the basic assumptions of the ALAMEL model are qualitatively correct, except at triple junctions. Splitting of one of the grains was occasionally observed, which has also been observed experimentally.

Abstract: Recently the distribution of grain boundary planes in metals and other materials has begun to be studied in addition to the misorientation distribution. It has been found that a ‘grain boundary texture’, i.e. a non-random distribution of grain boundary planes, often exists. The present paper will show examples of grain boundary texture taken from austenitic stainless steel and rocksalt. The characteristics of the grain boundary texture will be described in terms of low-index boundary planes and tilt/twist descriptors. Reasons for the formation of the particular grain boundary texture and possibilities for exploitation via grain boundary engineering will be discussed.

Abstract: In this paper, some of our recent results in phase equilibrium, microstructure, texture and precipitation resulting from the application of an external high magnetic field during diffusional phase transformation in both medium carbon and high carbon steels have been summarized Their potential engineering applications are foreseen.

Abstract: High power pulsed magnetron sputtering (HPPMS) is an emerging thin film deposition technology that generate high ionization plasma by applying a very large amount of peak power to a sputtering target for a short period of time. HPPMS is also known as High Power Impulse Magnetron Sputtering (HiPIMS). However, HPPMS/HiPIMS exhibits decreased deposition rate as compared to continuous dc magnetron sputtering. Modulated pulse power (MPP) magnetron sputtering is an alternative HPPIMS deposition technique that overcomes the rate loss problem while still achieving a high degree of ionization of the sputtered material. In the present work, the principles and some important characteristics of MPP technology were presented. Technical examples of CrN coatings were deposited using MPP and continuous dc sources. The positive ion mass distributions were characterized using an electrostatic quadrupole plasma mass spectrometer. The structure and properties of MPP and dc CrN coatings were characterized using x-ray diffraction, scanning electron microscopy, nanoindentation tests, and ball-on-disc wear test. It was found that the MPP CrN coating exhibits denser microstructure and improved mechanical and tribological properties as compared to the dc CrN coating.

Abstract: In attempts to improve the performance of dissimilar joints between AZ31 Mg alloy and different Al alloys, solid state joining processes such as Magnetic Pulse Welding (MPW) and Friction Stir Welding (FSW) were applied for minimizing the formation of brittle intermetallic phases. MPW process has been concentrated mainly on round section tube to tube and tube to bar welds. Mg alloy AZ31 has been successfully welded to pure Al A1070 as well as to Al alloy A3003. Tensile test clearly showed the MPW welds were stronger than the weaker of the base metal so failure occurred in aluminum base metal. While FSW process for the dissimilar joint between AZ31B/A6061 alloys with a thickness of 2mm revealed optimum weldability under the conditions of travel speed of 0.8mm/sec and tool rotation speed of 850rpm. For the sound dissimilar joint, the maximum tensile strength of 179 MPa, which was about 80 % of the Mg base metal tensile strength, has been obtained.

Abstract: Microstructure evolution during ring rolling process of a large-scale Ti-6Al-4V ring was investigated with the combined approaches of three dimensional finite element method (FEM) simulation and microstructure prediction model. A microstructure prediction model was established by considering the volume fractions and grain size of  and  phases varying with process variables, and grain growth. In order to perform FE simulation for ring rolling process of Ti-6Al-4V alloy, a constitutive equation was generated by utilizing the flow stress data obtained from hot compression tests at different temperature and strain rate conditions. The volume fraction and grain size of  and  phases during ring rolling were calculated by de-coupled approach between FEM analysis and microstructure prediction model. The prediction results were compared with the experimental ones. Our proposed microstructure simulation module was useful for designing hot forming process of Ti-6Al-4V alloy

Abstract: Standard and high resolution transmission electron microscopy (TEM) and advanced post processing of the TEM images have been applied for quantitative characterization of the hardening particle structure of an Al-Mg-Si alloy optimized for formation of the metastable phase β’. The relation between the structural characterization and the mechanical properties has been developed. A first attempt is presented on visualization and quantification of the coherency strain field of β’, based on a combination of Vienna Ab-initio Simulation Package (VASP) calculations and continuum mechanical modelling.