Determination of volume fraction of bainite in low carbon steels using artificial neural networks

Artificial neural networks have been used to estimate the volume fraction of bainite in low carbon steels containing various alloying elements. The network predicts the volume fraction for a given composition, isothermal transformation temperature and isothermal transformation time. Additionally, the maximum transformation temperature at which bainite formation takes place is also provided as an input to the neural network. The network was trained using the experimental data from three low carbon steels and it was found to perform quite well in predicting the volume fraction of bainite. The impact of the composition of alloying elements on the volume fraction of bainite was also studied and the results were in agreement with the known metallurgical theory.     

Microstructural Aspects of Damage and Fracture in AZ31 Sheet Materials

There are considerable data in the literature dealing with deformation mechanisms in AZ31 sheets. However, there is little information on the damage and fracture processes in this material. In this contribution, digital image correlation is used to follow deformation patterns occurring during tensile and v-bending tests at room temperature. A variety of surface analysis techniques and three-dimensional x-ray tomography have been used to examine the relationship between deformation, damage initiation, and the final fracture processes. The results show that premature diffuse necking occurs in the tensile tests without transit into localized necking. Deformation twins cluster by an autocatalytic process to form shear bands serving as preferential sites for strain localization and crack initiation. Damage appears in the form of microcracks within the shear bands at a late stage of necking and lead to the final fracture. The presence and the distribution of second-phase particles and their distributions help accelerate the final fracture processes.     

Effect of deformation on the austenite-to-ferrite transformation in a plain carbon and two microalloyed steels

Isothermal compression tests were carried out on three steels: (i) a plain C, (ii) a Mo, and (iii) a Mo-Nb-V microalloyed grade in order to study the effect of deformation on the austenite-to-ferrite transformation. Dynamic TTT (DTTT) curves were determined, which show clearly the extent to which deformation accelerates the decomposition of austenite. The latter effect is diminished by the addition of the alloying elements Mo, Nb, and V and is further reduced as the temperature is increased. The substitutional elements Mo, Nb, and V appear to reduce the nucleation rate through reduction of the austenite grain boundary energy. The growth rate is also reduced by these elements, apparently through the solute drag-like effect. The microstructural results indicate that the ferrite formed under dynamic conditions becomes more homogeneous and finer when the strain rate or the temperature is increased. Under static conditions, increasing the prestrain or the prestraining strain rate accelerates the γ-to-α transformation and reduces the mean grain size of the ferrite, although the highest transformation rate is still associated with the dynamic case.     

An analytical model to calculate the current–voltage characteristics of AlGaN/GaN HEMTs

Journal of Computational Electronics - In this paper, a simple analytical model is presented to determine the sheet carrier density and the current–voltage characteristics of different...     

Texture weakening and static recrystallization in rolled Mg–2.9Y and Mg–2.9Zn solid solution alloys

The microstructure and texture evolution in the Mg–2.9Y and Mg–2.9Zn solid solution alloys were investigated following rolling and subsequent isothermal annealing. The Mg–2.9Y alloy was hot rolled, and the Mg–2.9Zn alloy was rolled at room temperature in order to evaluate the possibility of attaining texture weakening by the suppression of dynamic recrystallization (DRX) and promotion of static recrystallization (SRX). It was found that texture weakening can be attained in Mg even in the absence of Y when there is no DRX, and SRX occurs during annealing. In solid solution, Y suppresses DRX during hot rolling, and retards the kinetics of SRX and grain coarsening in Mg. In the two alloys, the orientation of statically recrystallized grains at bands/twins (TSRX grains) is close to that of double and compression twins, exhibiting a much more evenly distributed and slightly wider orientation than that of basal parent grains and twins. In both Mg–2.9Zn and Mg–2.9Y alloys, a continuous texture weakening is observed with the progress of SRX, which results in a bimodal microstructure consisting of small TSRX grains and larger ones. With the increase in grain size during coarsening, the maximum intensity of basal pole figures rises linearly, with the slope of the lines being nearly identical in the two alloys. This texture strengthening was ascribed to the consumption of small TSRX grains by larger ones.     

Kinetics of Delta-Ferrite to Austenite Phase Transformation in a Two-Phase Fe-Al-C Alloy

The kinetics of delta-ferrite to austenite phase transformation was investigated using a quenching dilatometer in a Fe-Al-C alloy. The results showed that the austenite phase nucleated along the delta grain boundaries. The transformed austenite morphology changed from cellular to Widmanstätten pattern when the holding temperature decreased from 1398 K to 1123 K (1125 °C to 850 °C). Full partitioning of the substitutional alloying elements was observed and the spacing of the austenite plates was controlled by the diffusing distance of the substitutional elements. Interestingly, growth of the austenite front was controlled by the long-range diffusion of carbon from the center of the delta grains to the growing front. Deformation of the delta phase enhanced the nucleation of austenite at existing grain boundaries and newly formed subgrain boundaries.     

Microstructure and temperature monitoring during the hot rolling of AZ31

This study details the microstructural evolution during hot rolling of AZ31 alloy sheet using a pilot-scale rolling mill. The aim is to understand the deformation mechanisms leading to grain refinement under industrial processing conditions and to design and optimize the hot rolling schedule for AZ31 in order to produce sheet with a fine and homogeneous microstructure. The study examined three different hot rolling temperatures, 350, 400, and 450°C, and two rolling speeds, 20 and 50 rpm. A total thickness reduction of 67% was obtained using multiple passes, with reductions of either 15% or 30% per pass. It was found that the microstructure of the AZ31 alloy was sensitive to the rolling temperature, the reduction (i.e., strain) per pass and the rolling speed (i.e., strain rate). The results show that the large cast grain structure is broken down by segmentation of the cast grain through localized deformation in twin bands, where dynamic recrystallization occurs in these bands as well as at the grain boundaries (necklacing).