Advanced Materials Research Vol. 409

Abstract: The quality of multi-crystalline silicon ingot from casting process by heat exchange method (HEM) is significantly affected by the cooling condition and the design of the hot-zone. The shape of the liquid-solid interface has great impact on the direction and orientation of grain growth and the occurrence of defects such as dislocations, impurities segregation, and residual thermal stresses. In this study, the temperature variation/distribution of crystallization process of silicon ingot is investigated through numerical simulation and compared with experimental measurements. In HEM system, the temperature variation/distribution is affected by the adiabatic condition of the furnace and temperature curves of the heaters in the furnace. Incorporated with the Cellular Automaton (CA) method and residual thermal stress computation, the grain structures are also simulated. The different slices of the practical silicon ingot are then compared with the results of grain growth simulation to verify the accuracy of the numerical system. With the numerical system validated, various designs and operating conditions can then be numerically evaluated to obtain the optimal design and operation.

Abstract: The phase transformation during continuous cooling in low carbon steel has been widely measured by dilatometer using the lever rule. However, the concept of lever rule has several limitations. In low carbon steels, it is observed that overlapped transformation region of multi-phase and inflection point of small amount of low temperature phase is hardly differentiated. First derivatives of LVDT during continuous cooling could be better way to identify the inflection point and transformation region of phases (especially low temperature phase). Furthermore, first derivative of LVDT could be expressed as the lattice parameter and phase fraction due to temperature. Therefore, phase transformation behavior is simulated by the analysis of first derivative of LVDT using Avrami equation from experimental LVDT. As a result, the start, finish temperature and the amount of each phase are determined. The method is also confirmed by OM and SEM.

Abstract: The influence of the microstructure of metallic materials on its creep behavior is complex. Besides the chemical composition, the distinct configuration of the microstructural elements has a major influence on the deformation processes. In the presented work a physically based Finite Element model has been applied to study creep behavior on a microstructural level. The main focus is set on the local influence of grain boundaries and triple points on creep straining. The results indicate that such microstructural configurations lead to a highly heterogeneous creep strain distribution. Thus, this study is an important step to a deeper understanding of complex local interactions of creep phenomena.

Abstract: The effect of B, Si, P, Cr, Ni, Zr and Mg on cohesive properties of Al and the special grain boundary (GB) Σ5 (210)[100], as well as their segregation behavior at the GB and the (210) surface are studied by first principles method. The analysis of these parameters allows us to single out Ni as the best and phosphorus as the worst interatomic bond strengthening alloying elements.

Abstract: Numerical simulations of phase separation in Fe-Cr-Mo and Fe-Cr-Ti ternary alloys and Fe-Cr-Mo-Ti quaternary alloys were performed with use of the Cahn-Hilliard equation for ternary alloys and quaternary alloys. We obtained that the asymptotic behaviours of minor element X(Mo, Ti) in Fe-Cr-X ternary alloys and Fe-Cr-Mo-Ti quaternary alloys along a trajectory of a peak top of the major element Cr was classified into three groups according to the sign of the second derivative of the chemical free energy with respect to the compositions of Cr and X(Mo, Ti). Theoretical analysis for the symptotic behavior Mo in Fe-Cr-Mo ternary has been formed in order to discuss the simulation results. Effect of the other elements, such as Ti on separation behaviours of Cr in Fe-Cr-Ti ternary alloys were also investigated. A simple theory for describing the effect of subsutitutonal element in Fe-Cr-X ternary alloys on the basis of simulations and theoretical analyses was proposed.

Abstract: The knowledge of the flow behavior of metallic alloys subjected to hot forming operations is of particular interest for designers and engineers in the practice of industrial forming processes simulations (i.e. rolling mill). Nowadays dynamic recrystallization (DRX) is recognized as one of the most relevant and meaningful mechanisms available for the control of microstructure. This mechanism occurs during hot forming operations over a wide range of metals and alloys and it is known to be as a powerful tool which can be used to the control of the microstructure and properties of alloys. Therefore is important to know, particularly in low stacking fault energy (SFE) materials, the precise time for which DRX is available to act. At constant strain rate such time is defined by a critical strain, εc. Unfortunately this critical value is not directly measurable on the flow curve; as a result different methods have been developed to derive it. Focused on steels, in the present work the state of art on the critical strain for the initiation of DRX is summarized and a review of the different methods and expressions for determining εc is included. The collected data is suitable to feeding constitutive models.

Abstract: An investigation of molten metal droplet impingement on substrate was conducted. Micro droplets of molten lead-free solder; Sn_3.0wt%Ag_0.5wt%Cu, were ejected at 230°C by using a piezoelectric inkjet printing process. The droplets were squeezed out from nozzle with orifice diameter of 50 µm by applying a voltage pulse of bipolar waveform. In this study, micro droplets with various velocities were deposited onto flat substrate. Spread factors of solders are at the range of 1.23-1.25. With the aid of numerical simulation, interfacial heat transfer coefficients between molten solders and substrate were observed to rise with droplet impact velocity. Though analyzing the data of simulation results, the mathematical relation between interfacial heat transfer coefficient and potential and kinetic energy shows the trend of direct proportion.

Abstract: Thermomechanical Controlled Processing (TMCP) including accelerated cooling after the final hot rolling pass is a well-established technology, widely applied in HSLA steel plate production. However, there are still certain limitations, especially for thicker plate. The rolling schedule includes a long holding period (HP) after the roughing stage to allow the temperature to fall sufficiently for optimised TMCP during finishing. Intermediate Forced Cooling (IFC) applied during the HP can increase productivity by decreasing the required hold time, can restrict austenite grain growth, and can also improve the subsequent strain penetration in thick plate with further metallurgical benefits. Multi-pass plane strain compression (PSC) tests have been performed on the thermomechanical compression (TMC) machine at Sheffield University including different severities of IFC. Clearly it is impossible to simulate all aspects of the temperature and strain gradients present in thick plates in laboratory specimens, and most of the tests were conducted at temperatures and strains calculated by Finite Element modelling as relevant to specific positions through the plate thickness. However, some aspects of the gradients were addressed with tests using cold platens. The results have indeed shown that IFC can shorten the HP and reduce austenite grain growth and its variation across thick plate.

Abstract: Numerical simulation of phase separation in Fe-Cr-Mo and Fe-Cr-Ni ternary alloys and Fe-Cr-Mo-Ni quaternary alloys were performed with use of the Cahn-Hilliard equation for ternary and quaternary alloys. A new numerical model based on the Gauss-Seidel and Newton Raphson methods was utilized to obtain efficient and accurate solution.

Abstract: The GB embrittlement mechanism of Fe enhanced by P segregation has been investigated by first-principles tensile tests because a P atom is a famous GB embrittler in Fe. The first-principles tensile tests have been performed on Fe with two P-segregated GBs, where P atoms are located at the different sites, and with a nonsegregated GB. The tensile strength and the strain to failure in the P-segregated GBs were lower than those in the nonsegegated GB. The first bond breaking occurred at the Fe-P bond owing to the covalent-like characteristics, although the charge densities were high at the Fe-P bonds even just before the bond breaking. This premature bond breaking of Fe-P was independent of the location of the P atom.