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No description available.

No description available.

Distinct serrations had been observed on the stress-strain curves of various steels tested previously at high temperatures (950--1100°C) at McGill University. An explanation proposed for this behavior was that dynamic strain aging (DSA), caused by the presence of substitutional elements, was taking place. To investigate the possibility that the jerky flow was caused by an interaction between dislocations and substitutional elements, the conditions of temperature and strain rate under which serrated yielding had previously been observed were explored. In addition, some of the same material was utilized in the testing. / Much of the previous work on DSA in steel has focused on the effect of interstitials, namely, carbon and nitrogen, rather than that of substitutional elements. These studies have been conducted in the blue brittle region (i.e. 100--400°C), where the diffusivity of the interstitial elements is sufficiently rapid for them to keep up with the moving dislocations. However, for substitutional elements to obtain enough mobility to induce DSA, the temperature range must be significantly higher. / The effect of substitutional elements on DSA in steel was examined in torsion and, although numerous tests were formulated and carried out in an attempt to gather evidence for this phenomenon, no firm data for the occurrence of DSA were obtained. Further experiments and analysis will be required to gain a better understanding of the behavior of DSA at elevated temperatures, particularly for the case where dynamic recrystallization is taking place. A testing method might then be devised that could make the effect of DSA more evident.

In the presence of air and oxygen, oxidation of a molten aluminum alloy will easily take place. The oxidation not only harms the operation by contributing to melt losses, but the processing of the oxides present in the melt and refractory is also costly in time and money. Refractory accretions formed due to the interactions between the oxide-layer and base refractory material, especially alumina-lined ones, are a significant problem for the industry. Aluminum alloys containing magnesium are known to oxidize much more easily and rapidly. In the foundry, oxidation of molten aluminum-magnesium alloy is more prominent than that of pure aluminum. Magnesium has a higher affinity for oxygen, causing it to oxidize more easily than aluminum. In previous studies of oxidation of aluminum alloys, it has been found that strontium additions to the alloy reduce the amount of oxidized layer in an undisturbed melt. / The effects of strontium additions on the oxidation behavior of commercial A356, A357 and 5182 aluminum alloys were investigated by monitoring sample weight gains with time with a thermo-gravimetric balance at 700, 750, and 800°C. Sample surfaces were examined using electron microscope and x-ray diffraction techniques. It was found that in the absence of Sr, the A356 and A357 samples gained substantial amounts of weight through the preferential oxidation of magnesium. Samples containing strontium had significantly lower weight gains. For the high magnesium-containing 5182 alloy, an increase of incubation period before the onset of significant oxidation is associated with the presence of strontium. This change in oxidation behavior was linked to the presence of strontium containing oxide species in the oxidizing surface.

The effect of chemical composition and cooling rate on the static strain aging (SSA) and dynamic strain aging (DSA) behaviors of four IF steels was investigated after reheating to different temperatures and cooling to room temperature at various rates. In the case of DSA, tensile tests were carried out over the range from room temperature to 450°C and at strain rates of 10-4 to 10-1 s-1 . The DSA behavior was also studied in torsion in the austenite and ferrite regions. Aging index (AI) tests were carried out to evaluate the response to SSA and to estimate the amount of carbon in solution after employing various cooling rates. In all cases, although the atomic Ti/C ratio was greater than one, even still air cooling (3°C/s) led to a supersaturated/unstabilized material and to the occurrence of both DSA and SSA. By contrast, there were no signs of SSA and DSA after furnace cooling (0.05°C/s). / The three Ti and the Ti-Nb IF grades studied displayed serrated flow behavior at all strain rates in the blue brittleness temperature range (100 to 300°C). A simple model is described that predicts whether or not DSA will occur at the strain rates and temperatures involved in the processing of IF steels. It was found that the higher the Ti/S ratio, the higher the solute C. Steels with different chemistries but equal Ti/S ratios displayed the same aging behavior. Internal friction measurements were executed to establish a calibration between the AI values and solute C levels. Two novel techniques for the strengthening of IF steels by dynamic bake hardening (BH) are presented. These techniques lead to much higher BH values compared to those produced by conventional BH methods. The observations also show that when C supersaturation results from cooling after coiling, lower annealing temperatures after cold rolling lead to higher solute C levels.

Grain refinement of Al-Si casting alloys is commonly assessed by the presence of Ti and B in the melt, but in the last decade, thermal analysis has become an alternative control tool for the determination of the degree of refinement in the melt prior to casting. The objective of this work is to determine the best optimum method to predict the grain size in 319 and 356 Al-Si casting alloys by the use of the thermal analysis technique. Different time and temperature parameters from the cooling curve and its derivatives have been analyzed for a variety of grain refined samples. Four different master alloys (Al-6%Ti, Al-5%Ti-1%B, Al-2.5%Ti-2.5%B and Al-5%B) and two salt fluxes (AlTab-75%Ti and TiLite75BC-75%Ti-l.5%B) were used as grain refiners and samples were frozen at two different cooling rates (1.0 and 0.1°C/s). The effect of type of refiner and cooling rate on the thermal analysis parameters has been analyzed. / A time parameter, t1, which is the duration of the recalescence period, and the maximum undercooling and recalescence temperatures, TU and TR respectively, yield the best correlation with grain size. These results are consistent irrespective of the type of grain refiner, for both 319 and 356 alloys, but only when the alloy solidifies at a cooling rate of 1.0°C/s). Lower cooling rates produce scattering in the results. / Grain growth velocity, as calculated from the dendrite coherency point, correlates well with grain size for both alloys. A grain growth model is proposed to explain the effectiveness of these thermal parameters, where the duration of the recalescence period is related to a free growth period of the grains. Thermal analysis parameters related to the nucleation period seem to be sensitive to the type of grain refiner used and do not show good correlation with grain size.

TiC reinforcement particles were incorporated into a Ti-6 %Al-4%V matrix and processed by two powder metallurgy techniques, namely elevated temperature pressureless sintering and hot deformation-assisted sintering (also known as hot pressing). For these composites, processing by sintering alone necessitated high temperatures (>1500°C) for near-complete density consolidation, whilst the conditions for temperature and hold time were reduced (i.e. 1000°C and 1/2 hour) through deformation-assisted sintering. During high temperature processing in the absence of deformation, considerable coarsening of the lamellar matrix microstructure occurred. The interfacial reaction between the reinforcement and matrix was characterized by in situ neutron diffraction sintering studies at temperatures between 1100°C and 1350°C. Initial reaction occurred by carbon diffusion from the TiC particle to the titanium alloy, as evidenced through the increase in the lattice parameter of the matrix phase with holding time at the various sintering temperatures. Beyond the carbon solubility limit of the matrix phase, a stable stoichiometric phase formed as shown by the appearance of distinct peaks in the neutron diffraction patterns. Room temperature lattice parameter measurement gave a value of 4.290 A with a fractional occupancy of carbon of 0.45 +/- 0.04, which corresponds to a stoichiometry of Ti2C. For the various isothermal sintering temperatures, change in the Ti2C volume fraction with hold time was determined and growth of this interfacial phase was reasoned to occur by carbon diffusion from the TiC particles, through the reaction zone and to the Ti-6%Al-4%V alloy. Transformation of the entire TiC particle to Ti2C occurred in the composites sintered at 1500°C. For the composites processed by sintering only, the mechanical properties determined by shear punch testing indicated that the strength and ductility are limited at low temperature sintering because of high por

A numerical model is developed concerning the motion of particles in current-carrying liquid metals in a cylindrical coordinate system. The fluid flow is obtained by solving Navier-Stokes equations, and particle trajectories by equations for the motion of particles which incorporate the drag, added mass, history, fluid acceleration and electromagnetic force, with correction factors for particle shape and orientation. Wall effects and the flow conditions in the entrance region are considered. Dimensionless numbers Re, RH, gamma, and k are introduced to represent the fluid velocity, electric current, particle density and particle size, respectively. Electromagnetic force squeezes non-conducting particles away from, while pushes more conductive particles towards the symmetric axis. In a cylindrical pipe or ESZ orifice, particles follow the fluid flow closely in the axial direction. In the radial direction, at low current, non-conductive particles move towards the central axis first and then to the sidewall, while at high current, directly to the wall because of the competition between the fluid acceleration and the electromagnetic force which increases with particle size, electric current, and distance from the central axis. Lighter and larger particles move faster towards the wall. The dominating increase in added mass over electromagnetic force on oblates than on prolates, the smaller drag force and the lower added mass on prolates, with their symmetric axes perpendicular to the transverse axis of the ESZ, move prolates faster towards the wall. In parabolic ESZ orifice, bubbles lead and heavier particles lag behind the fluid flow in the axial direction, and the transient time difference makes particle discrimination realizable. The conditioning effect is attributed to the dramatic increase in fluid velocity near the parabolic orifice wall upon current surge. Designs are proposed for improving the conditioning effect in steel LiMCA, for reducing the background nois

Manganese and iron are two major alloying elements in various aluminum products. Since these elements have high melting points and low diffusivities in molten aluminum, their dissolution rates are very slow, when they are added to aluminum melts. In order to improve the kinetics of dissolution, several alloying methods have been introduced. All methods of alloying use mechanical stirring of some form or other to enhance dissolution rates by promoting forced convective mass transfer. In the present study, a comparison between the kinetics of dissolution of iron and manganese when added to the melt as discrete alloying particles or as compact briquettes (consisting of alloying elements and aluminum particles) was carried out. This study revealed that exothermic reactions and the local accumulation of heat within the briquettes can accelerate the kinetics of the alloy-making process. / Using the results obtained, a model for the dissolution of briquettes in molten baths of aluminum is proposed. Following disintegration of the briquettes into the melt, fine particles disperse freely into the bath, while their dissolution being controlled by mass transfer. To predict the dissolution rates, the relative velocity between fine particles entrained in the turbulent field and the fluid must be known. However, such velocities are extremely difficult, if not impossible, to clarify, since the particles will be moving erratically with a turbulent liquid, in which, random velocity fluctuations will be superimposed on mean bulk flow velocities. As such, the hydrodynamic interactions which control particle-fluid mass transfer in an agitated vessel are highly stochastic and impossible to predict, ab initio. In order to quantify such phenomena two theoretical approaches have been introduced by aqueous model researchers. In this investigation, for the first time, these theories have been evaluated for a high temperature liquid metal system involving the dissolution of alloying particles. / In the evaluation of the terminal velocity theory in high temperature systems, water modelling experiments in conjunction with dimensional analysis between cold model and hot temperature system were carried out to study the entrainment behaviour of (heavy) alloying particles. Then, through continuous sampling of the melt, mass transfer coefficients and rates of dissolution of suspended particles in the melt were measured. A comparison between predicted values and measured results indicated that the terminal velocity theory, in conjunction with correlations proposed for aqueous systems, was not applicable to the high temperature alloying systems studied. / In Kolmogoroff's theory of local isotropy, mass transfer in a turbulent system can be treated by a single parameter, i.e. energy dissipation rate. High temperature experiments revealed that this approach could be applied to stirred alloying particles-molten aluminum systems. However, at higher rates of mixing, when a fully suspended condition is reached, further increases in input mixing energy have negligible effects on dissolution rates. Thus, as a practical measure, very high mixing rates are not recommended. In this regard, a correlation for mass transfer coefficient as a function of the rate of input energy was presented.