1,2-Diphenyl-1H-imidazo[4,5-f][1,10]phenanthroline

In the title compound, C25H16N4, the fused ring system is essentially planar [maximum deviation = 0.1012 (15) Å]. The imidazole ring makes dihedral angles of 77.41 (8) and 56.26 (8)° with the phenyl rings attached to nitro­gen and carbon, respectively. The dihedral angle between the two phenyl rings is 65.50 (8)°. Weak C—H⋯π inter­actions are found in the crystal structure

Effect of roll diameter on the thermal-mechanical behaviour of AZ31 strip during twin roll casting

Although, the feasibility of producing AZ31 strip and other magnesium alloys via laboratory and pilot scale Twin Roll Casting (TRC) facilities has proven successfully [1, 2], key questions remain in terms of the changes in the thermal mechanical history experienced by the strip and mill force and power requirements as the process is scaled up to a larger TRC machine suitable for industrial production of magnesium strip. A powerful tool to help understand and quantify the effect of this TRC scale up on both the mill requirements and solidification and thermal mechanical history experienced by the strip is to develop and validate a mathematical model of the process. In this study a Thermal-Fluid-Stress model has been developed for TRC of AZ31 magnesium alloy to investigate the effect of roll diameter (355mm, 600mm and 1150mm) using the FEM commercial package ALSIM. Figure 1 illustrates the schematic of the TRC process. The mathematical model includes heat transfer and fluid flow in the liquid metal, heat transfer, fluid flow and latent heat of fusion release in the mushy zone and deformation in the material once the coherency point is reached, Details of the model development and its validation are provided in [3] and a schematic of the TRC process is shown in Figure 1. As expected, the roll diameter has a significant effect on the pressure distribution in the roll bite. The larger diameter roll will lead to an increased arc of contact (L) between the magnesium and the roll. The model-predicted effect of roll diameter on the surface normal stress is shown in Figure 2. By scaling up the caster the amount of solid material in the roll bite region which experiences plastic deformation increases which leads to development of higher level of normal stress on the strip surface. Copyright © 2013 MS&T\u2713®

Inverse and centreline segregation formation in twin roll cast AZ31 magnesium alloy

Two microstructure defects formed in the twin roll cast AZ31 strips were investigated: inverse and centreline segregations. A two-dimensional finite element thermal-fluid-stress model was employed to study the thermomechanical response of the AZ31 strip during twin roll casting process. The results showed that the key parameter for centreline segregation is the mushy zone thickness at centreline. For inverse segregation, the interaction between the yy peak stress at the centreline in the mushy zone and the solidified shell on the roll surface is the determinant parameter. The modelling results suggested increasing the setback distance decreases the risk of both defects. Moreover, scaling up the caster reduces the propensity to inverse segregation but appears to have a minor effect for centreline segregation formation

Thermal fluid mathematical modelling of twin roll casting (TRC) process for AZ31 magnesium alloy

A two-dimensional computational fluid dynamics (CFD) thermal fluid model has been developed and validated for twin roll casting (TRC) of AZ31 magnesium alloy using the commercial package ANSYS CFX. The model was developed to represent the steady state part of the TRC process. The thermal history of the strip was studied in terms of the temperature gradient through the strip thickness at the exit of the caster, the sump depth and mushy zone thickness at the centreline for different casting speeds, final strip thicknesses and heat transfer coefficients between the rolls and the surface of the strip. Moreover, the effect of these parameters on the average solidification cooling rate and secondary dendrite arm spacing in the solidified structure was investigated. Model validation was done by comparing the predicted and measured exit strip surface temperature as well as the secondary dendrite arm spacing (SDAS) through the thickness of the sheet to those measured using the Natural Resources Canada Government Materials Laboratory, CanmetMATERIALS located in Hamilton, Ontario, Canada. Analysis of the effect of TRC conditions on the SDAS showed that more uniform SDAS through the thickness is obtained when casting thinner strips at higher casting speeds. © 2013 W. S. Maney & Son Ltd

Twin roll casting (TRC) of magnesium alloys - Opportunities and challenges

Twin Roll Casting (TRC) has been successfully employed for the past sixty years to produce aluminum, steel and, in the past ten years, magnesium sheet. Although the TRC process is relatively simple, its application for commercial-scale magnesium strip production has proven difficult. This is primarily due to inherent characteristics of magnesium alloys, such as their high reactivity to oxygen, low specific heat and latent heat of fusion, and large freezing ranges, which can induce formation of casting defects if various TRC processing parameters, such as metal delivery design, heat transfer in the roll gap, and casting speed, aren\u27t tightly controlled. Research is underway worldwide to concurrently gain a better understanding of TRC processing variables in order to provide optimum casting conditions which will reduce defects, and develop new magnesium alloys with properties tailored to the TRC process. The opportunities and challenges associated with magnesium TRC will be outlined and include: 1) defect formation during TRC of magnesium alloy AZ31, 2) the feasibility of producing clad magnesium strip via TRC and 3) the effect of scale-up (moving from a laboratory unit to commercial production) will have on the TRC process for magnesium. © (2014) Trans Tech Publications, Switzerland

Solidification behavior of dilute Mg-Zn-Nd alloys

A combination of calculations using the FactSage software and measurements using a number of experimental techniques was explored to assess the solidification characteristics of ternary Mg-Zn-Nd alloys along with the commercial grade ZEK100 and determine the role of rare earth metal neodymium. For each chemical composition tested, the solidification under equilibrium and non-equilibrium conditions affected the type and volume fractions of phases formed. Thermal analysis identified two major reactions during solidification: formation of α-Mg dendrites followed by the eutectic transformation. For a constant Zn content of 2% and 4%, an increase in Nd content in the range of 1–2% caused a reduction in liquidus temperature but increase in solidus and eutectic temperatures. As verified by controlled solidification experiments the cooling rate during solidification affected the refinement of alloy microstructure, a volume fraction of intermetallic precipitates and their distribution. There was no obvious influence of Nd content on the value of secondary dendrite arm spacing for all cooling rates examined. However, there was an influence of Zn where an increase of its content from 1% to 4% halved the average secondary dendrite arm spacing. The beneficial role of Zr is confirmed and a presence of 0.25%Zr in ZEK100 caused a dendrite refinement comparable to that achieved through an increase in a cooling rate from 30 °C/s to 110 °C/s. The role of small-additions of Nd and Zn in design of new magnesium alloys, specifically optimized for twin roll casting, is discussed

Analysis of the hot deformation of ZK60 magnesium alloy

Hot deformation of cast-homogenized and extruded (in both the extrusion and transverse directions) ZK60 magnesium alloy was conducted using the Gleeble® 3500 thermal-mechanical simulation testing system. A new approach to model the high temperature constitutive behavior of the alloy was done using two well-known equations (i.e. hyperbolic sine and Ludwig equations). For this approach, the deformation conditions were divided into regimes of low and high temperature and strain rate (four regimes). Constitutive model development was conducted in each regime and the material parameters (P) were evaluated as strain, strain rate and temperature-dependent variables; P(ε, ε˙, T). Using this approach, the flow curves were predicted with high accuracy relative to the experimental measurements. Moreover, detailed information on the evolution of hot deformation activation energy was obtained using the modified hyperbolic sine model. Using the modified Ludwig equation, details of strain hardening and strain rate sensitivity of the ZK60 material during hot deformation were obtained. Keywords: ZK60 magnesium alloy, Hot deformation, Constitutive modeling, Zener–Hollomon, Hyperbolic sine, Ludwig equatio

Development of a mathematical model to study the feasibility of creating a clad AZ31 magnesium sheet via twin roll casting

A previously developed and validated thermalfluid mathematical model of the twin roll casting (TRC) process for magnesium alloy AZ31 was used to quantitatively study the feasibility of producing a clad magnesium strip via the TRC process. The clad material was varied to identify the effect of material composition on the feasibility of producing a clad strip. The clad alloys chosen included pure Zn, pure Al, AA3003, and AA5182 aluminum alloys. In the analysis, the effect of casting speed and clad sheet thickness (100 and 500 μm) on the thermal history in the magnesium strip and clad layer was analyzed. Assessment of the process feasibility was determined based on the exit temperature of the clad strip at the centerline, temperature of the clad sheet prior to the roll bite entry, and fraction solid of both the core (magnesium sheet) and clad along the core/clad interface. The results indicated that using pure Zn as a clad material is not feasible due to premelting of the clad strip prior to introduction into the TRC apparatus. All three aluminum alloys studied proved to be feasible in terms of a cladmaterial, and it was found that the effect of clad thickness and clad material chemical composition on the thermal history (temperature distribution) of the clad strip was negligible. It was also predicted using the thermodynamics package FactSageTM that the intermetallic phase at the core/clad interface will be primarily α-Mg (Mg17Al12). For AA5182 clad material, formation of β-Mg (Al3Mg2) is also possible. © Springer-Verlag London 2014

Warm and Hot Deformation Behavior of As-Cast ZEK100 Magnesium Alloy

Isothermal warm and hot deformation behavior of as-cast ZEK100 (Mg-1.2Zn-0.25Zr-0.17Nd, in wt.%) magnesium alloy was studied using the Gleeble® 3500 thermal-mechanical simulation testing system. The study was conducted over a wide range of temperatures (100 °C-500 °C) and strain rates (5.0 × 10−4 s−1–1.0 × 10 s−1), for a low true strain regime (i.e. 0.2) associated with general deformation level during twin roll casting process. For the range of studied strain, steady state flow stress was obtained at low strain rates (5.0 × 10−4 and 1.0 × 10−3 s−1) and high deformation temperatures (350, 400 and 450 °C). At medium strain rates of 1.0 × 10−2 and 1.0 × 10−1 s−1, steady state flow stress was observed for deformation temperatures of 400–450 °C and 450 °C, respectively. In the rest of conditions, strain hardening was the dominant deformation mechanism. A modified Ludwig equation was successfully used to develop constitutive model for the ZEK100. The model was validated by compression of material under continuous cooling or an abrupt change in the strain rate conditions. Deformed microstructure was consisted of twinning at low temperature deformation; while, at higher temperatures discrete regions of discontinuous dynamic recrystallization (by bulging at serrated boundaries) were observed. The former is associated with the strain hardening observed in the stress strain curves while the latter is the main reason of steady state flow stress. Strain hardening sensitivity and strain rate sensitivity exponents were also correlated to the deformed microstructure of the alloy

Scale-up modeling of the twin roll casting process for AZ31 magnesium alloy

In the present study, a 2-D finite-element method (FEM) thermal-fluid-stress model has been developed and validated for the twin roll casting (TRC) of AZ31 magnesium alloy. The model was then used to quantify how the thermo-mechanical history experienced by the strip during TRC would change as the equipment was scaled up from a laboratory size (roll diameter = 355 mm) to a pilot scale (roll diameter =600 mm) and to an industrial scale (roll diameter = 1150mm) machine. The model predictions showed that the thermal history and solidification cooling rate experienced by the strip are not affected significantly by caster scale-up. However, the mechanical history experienced by the strip did change remarkably depending on the roll diameters. Casting with bigger rolls led to the development of higher stress levels at the strip surface. The roll separating force/mm width of strip was also predicted to increase significantly when the TRC was scaled to larger sizes. Using the model predicted results, the effect of both casting speed and roll diameter was integrated into an empirical equation to predict the exit temperature and the roll separating force for AZ31. Using this approach, a TRC process map was generated for AZ31 which included roll diameter and casting speed