Materials Science Forum Vol. 1016

Abstract: In the Li₂O-M₂O₅-TiO₂ system, Li_1+x-yM_1-x-3yTi_x+4yO₃ (M = Nb, or Ta, 0.06 ≤ x ≤ 0.33, 0 ≤ y ≤ 0.175 (LMT) forms a superstructure, known as smart material. The superstructure is formed by periodical insertion of a corundum-type intergrowth layer of [Ti₂O₃]²⁺ in a matrix having a trigonal structure during the grain growth. To apply this unique structure as a host material of phosphor, new phosphors doped with Mn⁴⁺ ion with a red emission colour, which had a broad peak around 685 nm excited by 493 nm. In order to improve the PL intensity, we investigated the compositions, Mn⁴⁺ ratio and crystal structure. Results showed that PL intensity was closely related to Mn⁴⁺ ratio and its crystal structure.

Abstract: Acicular ferrite (AFα) formed on oxide particles in steel weld metals has a positive effect on toughness at low temperature. Good lattice coherency between AFα and oxide is one of the proposed reasons for promotion of AFα formation. Lattice coherency is affected by crystal structure of oxide and crystal orientation relationship (OR) between oxide and AFα. In the present study, ORs among AFα, oxide and γFe are investigated in a low carbon steel weld metal. Cube-cube (C-C) OR is observed between γFe and the oxide. It is probable that the oxide liquefied at high temperature, and then crystalized having the C-C OR with the surrounding γFe during cooling in welding process. Near Kurdjumov-Sachs (K-S) and near Baker-Nutting (B-N) ORs are observed between γFe/AFα and oxide/AFα, respectively. The misorientation from the B-N OR is larger than that from the K-S OR just after nucleation of AFα. This implies that AFα forms satisfying a near K-S OR with γFe essentially. It is supposed that formation of both the C-C (γFe/oxide) and near K-S (AFα/γ) ORs results in apparent formation of the near B-N OR between oxide and AFα.

Abstract: Kink-strengthening for mille-feuille structures has attracted many attentions in recent years. This study aims at identifying the kink formation/strengthening mechanisms via numerical reproductions of emerging kink-like morphologies based on FTMP (Field Theory of Multiscale Plasticity)-incorporated FE simulations, considering the Rank1 connection, where the incompatibility-based relevant underlying microscopic degrees of freedom for kinking are introduced. The targeted phenomena here include an experimentally-observed unique feature recently reported based on the combined ND–AE (neutron diffraction - acoustic emission) technique, i.e., scale-free-like energy release before (precursor) and during kink formations. This study uses a Mg single crystal model with alternatingly aligned soft and hard layers in parallel to the basal plane under c-axis plane-strain compression, where the soft/hard regions are controlled by the values of the hardening ratio. Also, we assume that the kink mode is only active, while the basal, prismatic and pyramidal slip and the twin systems are not operative associated with the layered structure. From the simulated results, we confirm kink-like morphologies and the attendant significant misorientation in the basal plane angles. Also, the simulated results are demonstrated to exhibit power-law type distributions in the strain energy fluctuation from the early stage of deformation even before the massive emergence of kink-like regions, which are analogous to the above-mentioned ND–AE observations.

Abstract: The studies showed that as a result of radial-shear rolling (RSR) in the temperature range of 850-650°C the formation of a quasi-globular ultrafine grain-subgrain structure with a size of structural elements of about 0.5-0.7 μm is observed in near β titanium alloy Ti-5Al-5V-5Mo-1Cr-1Fe. Subsequent annealing (aging) in the temperature range of 450°C for 5 hours leads to the decomposition of the β-phase matrix deformed during the rolling process with the formation of nanosized acicular precipitates of the martensitic α" phase along with finely dispersed inclusions of α phase particles preserved after rolling. The formation of such ultrafine-grained (UFG) structure leads to a significant (more than 25%) increase in the strength properties of the alloy compared to the initial state while maintaining sufficient ductility (~5%). Also obtained by the method UFG titanium alloy exhibits a high fatigue strength under very high cycle loading (fatigue strength based on 10⁸ cycles exceeds 700 MPa).

Abstract: Metal additive manufacturing based on powder bed fusion processes is increasingly important. However, highly transient physical phenomena that occur in these processes at different length scales are difficult to observe. Challenging and costly experiments are usually needed to obtain data for process understanding and improvement. Computational modelling of powder-bed fusion processes is therefore important from several points of view. These include better process understanding, optimisation of process parameters and component designs, prediction of component properties, qualification of components and to assist process control. Several physical processes have to be treated to develop a complete model, namely the raking of the powder bed surface, the transfer of energy from the laser or electron beam to the metal, the melting and solidification of the powder, the flow of liquid metal in the melt pool, the heat transfer from the melt pool to the surrounding powder and solid metal, the evolution of the microstructure, and the residual stress and deformation of the component. These processes occur at very different scales, and have to be treated using several different computational techniques. In addition, the interdependency of some of the processes has to be accounted for. This paper discusses the rationale for developing a complete model, progress in developing sub-models of the different physical processes, and the framework that is envisaged to combine the sub-models into a predictive model of the additive manufacturing process.

Abstract: Medium-carbon steel with approximately 0.6 wt% carbon is widely used for mechanical applications, because the martensite contained in it is harder than that of low-carbon and high-carbon steel. The microstructure of steel martensite varies depending on the carbon content, and the microstructure of medium-carbon steel martensite are a mixture of lath martensite (LM) and butterfly martensite (BM). The geometry of LM grains are thin plates having thicknesses of 0.2–0.5 μm. As for BM, some researches demonstrated that the region of martensite spreads in the depth direction of the observation surface. However, the three-dimensional geometry of BM grains remains unclear. Through cross-sectional observation using focused ion beam (FIB), the present study demonstrates that one BM grain is formed via collision of two plates. Previous research shows that the hardness of LM is inversely correlated with the thickness of the grains. The FIB observation result indicates that geometry of LM and BM grains are both plate-like. In the present study, whether the same relationship is valid for the mixture of LM and BM grains is investigated. The results show that the hardness of medium-carbon steel martensite increases according to the decrease in thickness of constituent grains of the two-phase structure of LM and BM.

Abstract: We investigate the effect of the cold reduction rate on ferrite recrystallization behavior during the annealing of low-carbon steel with different initial microstructures. Three types of hot-rolled sheet specimens are prepared: specimens P, B, and M, which consist of ferrite and pearlite, bainite, and martensite, respectively. To evaluate the effect of the cold reduction rate on ferrite recrystallization behavior, hot-rolled sheet specimens are cold-rolled at cold reduction rates of 40% and 67%. The cold-rolled sheet specimens are heated to the target temperature, and then water-quenched to room temperature. Irrespective of the initial microstructures, the ferrite recrystallization is accelerated by increasing the cold reduction rate. In addition, the dislocation densities of specimens P and B increase at the larger cold reduction rate, which accelerates ferrite recrystallization in these specimens. In the case of specimen M, the dislocation arrangement parameter remarkably decreases at the larger cold reduction rate, whereas the dislocation density hardly changes. Thus, we conclude that the accelerated ferrite recrystallization at the larger cold reduction rate for specimen M can be mainly attributed to an increase in the amount of interactions between dislocations in the specimen.

Abstract: The technology has been developed for the production of low-carbon and microalloyed steel for the production of strips in order to manufacture small and medium-diameter pipes for the extraction and transportation of petroleum products. The developed technology allows to obtain stable corrosion properties, according to the NACE Standard TM 0284 technique, in a hot-rolled condition from slab with thickness more than 200 mm and with a content of more than 0.03% of carbon, which is very difficult in terms of ensuring the availability of inclusions and heterogeneity in the structure - as the main reasons for the reduced fracture toughness of steels working in acid media (in environments saturated with H2S). The obtained results allowed, with minimum costs, to prepare the previously developed integrated computer model (STAN 2000) for calculating the structure and mechanical properties. Calculations using the model made it possible to select such temperature-deformation regimes, which would be the minimum structural inhomogeneity over the section of the thickness of the hot-rolled strip. The results obtained: corrosion resistance repeatedly confirmed by the absence of cracks at the beginning after testing in accordance with the procedure Standard TM 0284.

Abstract: It is expected that the material ratio of steel for an automobile will decrease rapidly in next decade due to the rise of electric vehicles, and multi-materialization of parts will be promoted consequently. Hence, technologies for dissimilar materials joining have been studied by researchers successfully for recent years. The authors have studied dissimilar materials joining with utilizing friction-stir forming (FSF) approach. The FSF is a friction-stir process invented by Nishihara in 2002. In FSF, a substrate material was put on a die firstly. Next, friction stirring was conducted on the back surface of the material. The material then deformed and filled the cavity of the die due to high pressure and heat caused by the friction stirring. The authors utilized the FSF approach to generate mechanical joints between dissimilar materials. In this presentation, the author introduces various techniques for joining dissimilar materials with employing the FSF.

Abstract: In this paper, we have investigated the threshold voltage (V_TH) control of metal-oxide-nitride-oxide-Si (MONOS) nonvolatile memory (NVM) with high-k HfN/HfO₂ stacked layers for analog memory application. The Si surface atomically flattening was found to significantly improve the V_TH controllability of the MONOS NVM with high-k HfN/HfO₂ stacked layers. The multi-level-cell (MLC) operation by controlling the program voltage at the source and drain was demonstrated utilizing MONOS NVM with high-k HfN/HfO₂ stacked layers.