Modelling of rheological behaviour of semisolid metal slurries Part 3 – Transient state behaviour

Abstract

In this part of the current series, the rheological model developed in Part 1 is applied to study the transient state behaviour of semisolid metal (SSM) slurries under various deformation conditions, such as isothermal shearing, isothermal resting, isostructural shearing, and shear rate transient and cyclic shearing. The theoretical analysis demonstrated that there is a very close coupling between the slurry structure and the apparent viscosity. Irrespective of the flow conditions, the apparent viscosity of a SSM slurry with a specified solid fraction is exclusively determined by its structure, while the effect of shear rate and shearing time is reflected by their effect on slurry structure. For a SSM slurry with spherical solid particles, the deagglomeration time is of the order of a few seconds, while the agglomeration time is of the order of a few thousands of seconds, indicating that the deagglomeration kinetics is about three orders of magnitude faster than the agglomeration kinetics. The present model has also been successfully applied to predict the hysteresis loops under various cyclic deformation conditions. Theoretical predictions have shown that the physical origin of thixotropy lies in the fact that the deagglomeration kinetics of SSM slurries is much faster than the agglomeration kinetics. A suspension will not exhibit thixotropic characteristics if there is no structural change during the shear deformation, or even if there is structural change but the rates of agglomeration and deagglomeration are equal.     

Effect of hydrolysed aluminium treatment on rheological characteristics of α-alumina slurries

This paper describes the process for the hydrolysed aluminium treatment (HA) on alumina surface and its influence on the rheological characteristics of alumina slurries. Three different commercial grade aluminas were provided with the surface treatment with hydrolysed aluminium using aluminium nitrate and hexamethylenetetramine (HMTA) under controlled conditions. The HA treatment increased the concentration of Al-OH surface sites resulting in higher H⁺ adsorption on the alumina surface. A highly concentrated (>55 vol%) electrostatically stabilized alumina slurry was prepared from HA treated alumina powders. The rheology of such slurry was studied and the results on the viscosity and yield stress are presented. The alumina slurries followed the Casson Model flow behaviour. The wet and flow behaviour of the alumina with and without HA treatment was also studied and the results are compared. The surface treatment showed the advantage of maintaining low viscosity and yield stress of alumina slurries even at higher solids loading (>55 vol%) that are prepared in the acidic aqueous medium. The results on viscosity and yield stress were compared with that of the polyelectrolyte dispersed system.     

Influence of molybdenum on austempering behaviour of ductile iron Part 4 – Austempering behaviour of ductile iron containing 0·45%Mo

Abstract

Austempering kinetic measurements and mechanical property measurements are presented for a ductile iron of composition Fe–3·56C–2·77Si–0·25Mn–0·45Mo–0·43Cu–0·04Mg (wt-%) after austenitising at 870°C and austempering at 400, 375, 320, and 285°C. The austempering kinetic measurements show that increasing the Mo content of the iron, for example, to increase hardenability, does not delay the austempering reaction significantly and the processing window is open for all the austempering temperatures studied. The mechanical properties determined for austempering temperatures of 400 and 375°C show that the higher ductility grades of the austempered ductile iron standards can be satisfied as predicted by the open processing windows. The ductility of the 0·45%Mo austempered iron is reduced compared with that measured in 0·13%Mo and 0·25%Mo irons austempered under the same conditions. This is attributed to an increased amount and continuity of intercellular carbide as the Mo content increases.     

Modelling precipitation sequences in powerplant steels Part 2 – Application of kinetic theory

Abstract

New kinetic theory capable of dealing with the simultaneous precipitation of several phases has been applied to a variety of creep resistant power plant steels. It has been demonstrated that the model has the ability to predict the vast differences in precipitation kinetics reported in the published literature for power plant steels. New experimental results on precipitation in a 9Cr 1Mo type steel are reported and shown to be consistent with theoretical predictions.     

Effects of interphase strength on the damage modes and mechanical behaviour of metal–matrix composites

A numerical version of the generalized self-consistent method previously developed by the authors is combined with the Gurson model to undertake a parametric investigation of the damage mechanisms and their relations with the macroscopic tensile properties of SiC reinforced aluminium, for three different interphase strengths. The results show that the interphase strength is a governing factor for damage propagation in the composite. Thus, transformation of the failure mechanism from reinforcement fracture to void nucleation and growth can be achieved by reducing the interphase bond strength, although the strengthening effects on the composite decrease unfavourably.     

Controlling factors of microstructural evolution and corresponding deformation behaviour of Al–4Cu alloy in semisolid state

Abstract

The present work describes the microstructural evolution of an Al–4Cu alloy and its deformation response, as measured by the parallel plates technique. Specimens were heat treated to obtain a variety of Al₂Cu dispersions, and deformed by extrusion (1:16 ratio). Results indicated that microstructural coarsening took place initially by particle coalescence, but after 10 min soaking time Ostwald ripening became the controlling mechanism. It was also shown that texture accelerated particle growth rate by increasing the likelihood that chance encounters produced grain boundaries, and hence coalescence events. The mechanism of action of particle stimulated nucleation of recrystallisation was suggested, by observing that the particle growth kinetics of deformed specimens was much higher than that of non-deformed specimens with their larger initial particle size and slower initial growth rate. Finally, rheological experiments showed that deformation behaviour was mainly controlled by particle shape, and to a lesser extent by particle size.     

Influence of reinforcement morphology and matrix strength of metal–matrix composites on the cyclic deformation and fatigue behaviour

The cyclic deformation behaviour of three metal–matrix composites, namely AA6061-T6 reinforced with 20 vol.% alumina particles and short-fibres, respectively, and pure aluminium reinforced with 20 vol.% short-fibres, has been investigated at temperatures between T=−100°C and T=300°C in total strain controlled symmetrical push–pull fatigue tests. The cyclic stress response exhibits initial cyclic hardening, subsequent saturation and cyclic softening, depending on the test parameters for temperatures lower than T=150°C. Initial cyclic hardening is less pronounced with increasing temperature and decreasing applied strain amplitude. Short-fibre reinforced composites — both with alloyed and unalloyed aluminium matrix — harden cyclically more than the particulate-reinforced composite. The comparison of the cyclic with monotonic stress–strain curves indicates that, depending on the testing conditions, both cyclic hardening and cyclic softening can occur.     

Postbuckling behaviour of a pressurized stiffened composite panel – Part I: Experimental study

Results of an experimental study of the buckling and postbuckling behaviour until the collapse of a cylindrical stiffened composite panel are presented. The specimen is subjected to a uniform pressure on one of its faces using a combination of gas and liquid inside a hermetic box. The present analysis shows the postbuckling load carrying capacity of elements of this kind without developing failure mechanism. Due to the high sensitivity to geometric imperfections of these structures, a simple procedure to obtain their measurements once the specimen is placed in the experimental device is set out. The data registered in these tests will be used for the subsequent validation of the numerical model in order to develop more accurate solutions. This will produce a significant increment in the fidelity of those predictions, making possible a reduction in the number of tests to be performed in real applications.     

Mechanical behaviour of metal–matrix composite deposits

The electroplating technique is used for producing thin sheets of copper- or nickel-based composites containing different volume fractions of -alumina dispersions. The microhardness and tensile behaviour of such composites, in both the as-deposited and the annealed state, are characterized. The strengthening mechanism of electroplated composites is found to be a combination of Orowan-type strengthening and the Hall–Petch effect.     

Recrystallisation behaviour of supersaturated Al–Mn alloys Part 1 – Al–1.3 wt-%Mn

Abstract

The effects of concurrent precipitation on recrystallisation during the isothermal annealing of a supersaturated and deformed Al – Mn alloy have been investigated. At low annealing temperatures precipitation on the prior boundaries prevents recrystallisation whereas at high temperatures recrystallisation is complete before precipitation occurs. In the temperature range 375 – 500°C, recrystallisation is affected by precipitation and complex microstructures containing high fractions of low angle boundaries are formed. It is shown that strain induced boundary migration of very large boundary areas is an important recrystallisation mechanism in this temperature range. The interactions between precipitation and recrystallisation are analysed in terms of a simple model. The heating rate is shown to substantially affect the recrystallisation behaviour.     

Iron–phosphorus–carbon system: Part 2 – Metallographic behaviour of Oberhoffer's reagent

Abstract

An investigation of the microstructures found in phosphoric irons has been carried out as part of a larger archeomet allurgical investigation. Oberhoffer's reagent has been used traditionally to display the macroscopic distribution of phosphorus in steels but in order to determine the microscopic response of the reagent with certainty, a systematic study of its action has been undertaken. The reagent is in current use in the metallographic analysis of archeological iron artefacts and its action needed to be clarified before further use in the current work. A series of specimens with controlled microstructures and solute profiles has been examined when etched with nital and also with Oberhoffer's reagent. The solute distribution in some of the specimens has been confirmed using wavelength dispersive X-ray microanalysis. It has been shown that Oberhoffer's reagent responds to microstructural as well as macrostructural segregations of phosphorus and also to nickel and arsenic. The etch does not deposit copper at a significant rate on carbide or phosphide particles so that they remain bright after etching. In ferritic microstructures, there may be a slight response due to variation in grain orientation.     

Thermomechanical fatigue behaviour of nickel base superalloy IN738LC Part 2 – Lifetime prediction

Abstract

An experimental programme was carried out to study the thermomechanical fatigue life of the nickel base superalloy IN738LC used in gas turbines. First, out of phase and in phase thermomechanical fatigue experiments were performed on uncoated and air plasma spray coated materials. In the temperature range investigated, it was observed that deposition of a NiCrAlY coating did not affect the thermomechanical fatigue resistance. A physically based life prediction model that takes into account the contribution of different damage mechanisms was then applied. This model successfully reflected the temperature and strain rate dependences of isothermal cycling fatigue lifetimes, and the strain–temperature history effect on thermomechanical fatigue lifetimes.     

Modelling precipitation sequences in power plant steels Part 1 – Kinetic theory

Abstract

The ability of steels to resist creep deformation depends on the presence in the microstructure of carbides and intermetallic compounds which precipitate during tempering or during elevated temperature service. The precipitation occurs in a sequence which leads towards thermodynamic equilibrium. The present paper deals with an extension of the Johnson-Mehl-Avrami theory for overall transformation kinetics. The modification permits the treatment of more than one precipitation reaction occurring simultaneously, afeature which isfound to be essential for representing the reactions observed experimentally in a wide range of secondary hardening steels.     

Effects of momentum transfer on particle dispersions of dense gas–particle two-phase turbulent flows

A modified momentum transfer coefficient of dense gas–particle two-phase turbulent flows is developed and its effect on particle dispersion characteristics in high particle concentration turbulent downer flows has been numerically simulated incorporating into a second-order moment (USM) two-phase turbulent model and the kinetic theory of granular flow (KTGF) to consider particle–particle collisions. The particle fractions, the time-averaged axial particle velocity, the particle velocities fluctuation, and their correlations between gas and particle phases based on the anisotropic behaviors and the particle collision frequency are obtained and compared using traditional momentum transfer coefficients proposed by Wen (1966), Difelice (1985), Lu (2003) and Beetstra (2007). Predicted results of presented model are in good agreement with experimental measurement by Wang et al. (1992). The particle fluctuation velocity and its fluctuation velocity correlations along axial–axial and radial–radial directions have stronger anisotropic behaviors. Furthermore, the presented model is in a better accordance with Lu’s model in light of particle axial velocity fluctuation, particle temperature, particle kinetic energy and correlations of particle–gas axial–axial velocity fluctuation. Also, they are larger than those of other models. Beetstra’s model is not suitable for this downer simulation due to the relative lower particle volume fraction, particle collision and particle kinetic energy.     

Thermomechanical fatigue behaviour of nickel base superalloy IN738LC Part 1 – Comparison of two unified constitutive models for description of mechanical behaviour

Abstract

The capabilities of two unified constitutive models to predict the mechanical behaviour of nickel base superalloy IN738LC under uniaxial loading conditions have been investigated over the temperature range 450–850°C. The material parameters of each model have been identified from an experimental investigation and complemented by available data from the literature. Mechanical responses from isothermal tests performed at 450 and 850°C (such as creep, monotonic, and fully reversed isothermal cyclic tests) were used for the identification of model parameters, and model capabilities were examined by comparison with in phase and out of phase thermomechanical fatigue and stress relaxation tests. Both models were found to capture all features of the material responses under uniaxial loading, although with varying degrees of accuracy.     

Assessment and modelling of isothermal forging of intermetallic compounds Part 4 – NiAl

Abstract

An alloy based on NiAl has been deformed under isothermal conditions, to investigate the possibility of hot working as a processing route for such materials. Deformation has been shown to occur largely via dynamic recovery, although some evidence of recrystallisation has been noted. Material properties (strain rate sensitivity, activation energy of deformation, etc.) have been calculated and used in a constitutive algorithm to model successfully the forging behaviour of this material under all the conditions examined here.     

Characterization of high-velocity impact damage in CFRP laminates: Part II – prediction by smoothed particle hydrodynamics

High-velocity impact damage in CFRP laminates was studied experimentally and numerically. Part I of this study observed and evaluated near-perforation damage in the laminates and characterized the damage pattern experimentally. Part II predicts the extension of high-velocity impact damage based on smoothed particle hydrodynamics (SPH), which facilitates the analysis of large deformations, contact, and separation of objects. A cross-ply laminate was divided into 0° and 90° layers, and virtual interlayer particles were inserted to express delamination. The damage patterns predicted on the surfaces and cross-sections agreed well with the experiments. The analyzed delamination shape was similar to that resulting from a low-velocity impact, consisting of pairs of fan-shaped delaminations symmetric about the impact point. Finally, the mechanisms of high-velocity impact damage in CFRP laminates are discussed based on the observations and numerical analyses.     

Modelling the hot plane strain compression test Part 2 – Effect of friction and specimen geometry on spread

Abstract

A detailed finite element analysis of the hot plane strain compression test has been carried out to investigate the effect of various test parameters on the measured response and deformation. This paper (Part 2) reports the results of three-dimensional finite element simulations of the test, to study the influence on spread of specimen geometry and friction effects. It is found that the spread is more sensitive to specimen breadth than it is to the friction coefficient and initial specimen thickness. Spread is independent of the strain rate of deformation and of the material. It is shown that the empirical relationships used for correcting experimental data for spread during a test are in close agreement with the simulation results.     

Dielectric properties of chiral honeycombs – Modelling and experiment

Electromagnetic properties of mechanically chiral honeycomb structures are investigated. In extension to previous works on the subject, rigorous analysis is performed above the quasi-static frequency range. Theoretical considerations and full wave 3D electromagnetic simulations are conducted to prove that, for the honeycombs of interest, higher order harmonics due to structure periodicity are attenuated away from the panel surface at frequencies up to several GHz, which covers a number of popular ISM bands. As a consequence, only individual plane TEM waves are observable at practical locations of transmitters and receivers away from the panel. Under the same conditions, it is demonstrated that the structural chirality does not translate into chiral electromagnetic behaviour. In other words, orthogonal modes of the honeycomb scenarios are linearly polarised, and transformation of the electromagnetic energy into heat occurs purely as a result of classical conductivity or loss tangent, which are low for the low-density panels made of low-loss dielectric cores. This indicates that EMC or shielding characteristics can only be designed either by utilizing the phenomenon of wave reflections, or by equipping the panels with additional foils on surfaces or absorbing foams in air volumes. While precise measurements of final-sized honeycomb panels remain as a challenging task for further work, preliminary experiments have been performed showing good agreement with theoretical and computed predictions.     

Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel: Part 1—Mechanical behaviour

Abstract

A split Hopkinson bar is used to investigate the effects of prestrain and strain rate on the dynamic mechanical behaviour of 304L stainless steel, and these results are correlated with microstructure and fracture characteristics. Annealed 304L stainless steel is prestrained to strains of 0·15, 0·3, and 0·5, then machined as cylindrical compression specimens. Dynamic mechanical tests are performed at strain rates ranging from 10² to 5 × 10³ s^-1 at room temperature, with true stains varying from 0·1 to 0·3. It was found that 304L stainless steel is sensitive to applied prestrain and strain rate, with flow stress increasing with increasing prestrain and strain rate. Work hardening rate, strain rate sensitivity, and activation volume depend strongly on the variation of prestrain, strain, and strain rate. At larger prestrain and higher strain rate, work hardening rate decreases rapidly owing to greater heat deformation enhancement of plastic flow instability at dynamic loading. Strain rate sensitivity increases with increasing prestrain and work hardening stress (σ-σ_y). However, activation volume exhibits the reverse tendency. Catastrophic fracture is found only for 0·5 prestrain, 0·3 strain, and strain rate of 4·8 × 10³ s^-1. Large prestrain increases the resistance to plastic flow but decreases fracture elongation. Optical microscopy and SEM fracture feature observations reveal adiabatic shear band formation is the dominant fracture mechanism. Adiabatic shear band void and crack formation is along the direction of maximum shear stress and induces specimen fracture.