Ramped oscillatory testing: A new technique to examine dynamic strain ageing

A new experimental technique for examining dynamic strain ageing in solid solutions is proposed. It is based on the imposition of an oscillatory perturbation on a tensile specimen monotonically deforming under strain or stress control. Experimental measurements on an AlMgSi alloy show that the attendant oscillatory response has characteristic features which may be used to detect and characterize dynamic strain ageing. A constitutive model which accounts for dynamic strain ageing is used to provide a theoretical basis for the analysis. A closed-form solution of the governing equations is obtained under the assumption of small oscillatory perturbations. A full numerical solution is given for the AlMgSi alloy which was tested using the technique proposed. Good agreement of the predictions of the model with the experimental results is obtained.     

Constitutive modeling and characterization of the flow behavior of semi-solid metal alloys slurries—I. The flow response

We present an internal variable constitutive model for semi-solid metal alloy slurries in this two-part paper. The first part discusses the constant internal structure flow equation and the second part derives the structure evolution equation. The model is valid for low (0.1) to moderate solid fractions (0.5-0.6). We also present a set of experiments on two alloy systems, Sn15% Pb and Al7% Si0.6% Mg, performed on a computer controlled, high temperature Couette rheometer to determine the model parameters. We independently validate the internal variable model through separate, primarily hysteresis, experiments.     

Strain rate sensitivity and transient behaviour in an AlMgSi alloy

The effect of strain rate changes on the flow stress of an AlMgSi alloy at strains prior to the onset of the Portevin-Le Chatelier effect has been studied. Measurements of the instantaneous and steady state strain rate sensitivity of the flow stress and the flow stress transient period were carried out as a function of strain and strain rate. Negative values of the steady state strain rate sensitivity were reached prior to the onset of localised yielding, which occurred by the growth of local strain rate oscillations. The results are interpreted in terms of a constitutive flow model which takes into account time dependent changes in the local solute composition at dislocations accompanying a strain rate change.     

On static strain ageing

The phenomenon of static strain ageing (SSA) is reconsidered in order to clear out contradictory experimental results and controversies with their interpretation. The constitutive model used includes the evolutionary behaviour of the dislocation densities and the transient character of ageing kinetics. The numerical solutions for the ageing peak recover all the different types of the strain and ageing time dependence reported in the literature. The SSA properties of b.c.c. alloys, f.c.c. alloys, in particular interstitial NiC, and the h.c.p. Ti alloys containing oxygen can be rationalized. It is concluded that there is no need to assume a vacancy mechanism to explain the observed SSA behaviour. It is rather the evolution of the mobile and forest dislocation densities which determines the strain dependence of static as well as of dynamic strain ageing.     

A process model for friction welding of AlMgSi alloys and AlSiC metal matrix composites—I. Haz temperature and strain rate distribution

The present investigation is concerned with the development of an overall process model for the microstructure and strength evolution during continuous drive friction welding of AlMgSi alloys and AlSiC metal matrix composites. In Part I the different components of the model are outlined and analytical solutions presented which provide quantitative information about the HAZ temperature distribution for a wide range of operational conditions. Moreover, a general procedure for modelling the HAZ strain rate distribution has been developed by introducing a series of kinematically admissible velocity equations which describe the material flow fields in the radial, the rotational, and the axial direction, respectively. Calculations performed for both types of materials show that the effective strain rate may exceed 1000 s⁻¹ in positions close to the contact section due to the high rotational velocities involved. Application of the model for evaluation of the response of AlMgSi alloys and AlSiC metal matrix composites to the imposed heating and plastic deformation is described in an accompanying paper (Part II).     

Equilibrium segregation and interfacial energy in multicomponent systems

A model is developed in the framework of gradient thermodynamics to study the equilibrium composition and energy of interphase boundaries in multicomponent systems. The parameters of the model are related to bulk properties for a nearest-neighbor, ternary regular solution. The model is applied to study the segregation of Au at interphase boundaries in the CuAgAu system, as a function of temperature, bulk composition, and interface orientation. Interfacial segregation is predicted and found to reduce interfacial energy in a manner with the Gibbs adsorption equation.     

The thermodynamics of PdAgH ternary solid solutions

The solubility of hydrogen in PdAgH ternary solutions in equilibrium with H₂ gas at atmospheric pressure has been measured in the temperature range 625–1111 K and in PdAg “binary solvents” containing up to 93 at.% of Ag. Concomitant elastic measurements have provided data which enable the partial thermodynamic functions of the H-atoms deduced from the solubility measurements to be converted so as to refer to a hypothetical PdAg lattice of constant specific volume. The resulting “volume corrected” functions have been discussed in terms of the cell model for ternary solutions and have been shown to vary with temperature and Ag-concentration in a manner in accord with this model.     

Heat flow model of glass formation and crystallization on rapid quenching

The theoretical model of glass formation and partial crystallization during rapid solidification of a metallic melt describes the homogeneous nucleation within the undercooled melt as well as the heat transfer into the metallic chill substrate. The calculated maximum thicknesses of amorphous foils at quenching onto a copper substrate increase in the order of alloys FeC, FeB, NiSiB and PdSi. Reducing the foil-substrate heat transfer coefficient, increasing the casting temperature and utilizing a steel substrate cause the attainable amorphous foil thickness to decrease. In foils with a large volume fraction crystallized the cooling process is not monotonous. The minimum density of quenched-in nuclei is situated at the substrate-side surface in amorphous foils and at the surface away from the substrate in crystalline foils.     

A process model for friction welding of AlMgSi alloys and AlSiC metal matrix composites—II. Haz microstructure and strength evolution

The present investigation is concerned with the development of an overall process model for the microstructure and strength evolution during continuous drive friction welding of AlMgSi alloys and AlSiC metal matrix composites. In Part II the heat and material flow models presented in the first paper (Part I) are utilized for prediction of the HAZ subgrain structure and strength evolution following welding and subsequent natural ageing. The modelling is done on the basis of well established principles from thermodynamics, kinetic theory and simple dislocation mechanics. The models are validated by comparison with experimental data, and are illustrated by means of novel mechanism maps. These show the competition between the different process variables that contribute to microstructural changes and strength losses during friction welding of AlMgSi alloys and AlSiC metal matrix composites.     

The effect of aging on hydrogen trapping in ß-titanium alloys

The ingress of hydrogen in three ß-titanium alloys (Beta-C, Ti10V2Fe3Al, and Ti13V11Cr3Al) and an α-ß titanium alloy (Ti6Al4V) was investigated with a view to characterizing their interaction with hydrogen. A technique referred to as hydrogen ingress analysis by potentiostatic pulsing (HIAPP) was used to obtain anodic current transients for the unaged and aged ß-Ti alloys and as-received Ti-6-4 in an acetate buffer (1 mol L⁻¹ HAc/l mol L⁻¹ NaAc, where Ac = acetate). The transients were analyzed using a diffusion/trapping model under interface control conditions to evaluate the trapping constants and hydrogen entry flux in each case. A marked increase in irreversible trapping was observed for the ß-titanium alloys with aging and was attributed to precipitation of secondary α phase. Aging also induced changes in the passive film and hence the hydrogen entry flux. Ti-13-11-3 and Ti-10-2-3 are predicted to become less resistant to hydrogen embrittlement with aging as a result of increases in both the trapping constant (at least for Ti-13-11-3) and the flux. In contrast, the change in resistance of Beta-C Ti with aging is subject to the opposing effects of a reduced flux and an enhanced trapping capability, though the latter appears to have the primary effect, rendering aged Beta-C Ti less resistant to hydrogen embrittlement than the unaged alloy.     

Nitrogen strengthening of a stable austenitic stainless steel

The low-temperature flow stress of mechanically stable austenitic stainless steels increases with increasing concentration of nitrogen in solution and with decreasing temperature. This phenomenon has been studied in a series of FeNiCrMo alloys with nitrogen contents between 0.04 and 0.36 wt% by measuring the flow stress and the thermal activation parameters for plastic flow as a function of stress, plastic strain and nitrogen concentration in stress relaxation and strain-rate change experiments. Care is taken, when analyzing the data, to distinguish between athermal and thermal effects. The significant increase of the athermal flow stress with increasing nitrogen concentration is attributed to short-range ordering of chromium and nitrogen atoms. The thermally activated component of the flow stress is also dependent on the nitrogen concentration and is thought to be due to localized, predominantly modulus interactions between lattice disturbances in the immediate vicinity of nitrogen atoms and slip dislocations. The thermally activated component is suitably described by Friedel's model of solid solution strengthening.     

A discrete lattice plane analysis of the composition profile and surface energy of binary H.C.P. alloys with applications to γ AlAg

The discrete lattice plane (DLP) model has been used to make a nearest neighbor (n.n.), broken bond, regular solution calculation for the composition profile and surface energy of h.c.p. alloys as a function of surface orientation and temperature. This appears to be the first such calculation for h.c.p. alloys. The surface energy and the anisotropy of surface energy thus calculated were compared to the experimental values of these quantities concurrently determined on a γ AlAg alloy by Hyland et al. using the zero-creep and thermal grooving method. An oscillating, diffuse composition profile was obtained, with the former characteristic arising from the negative regular solution constant. A recent FIM/AP study of γ AlAg precipitates by Osamura et al. is in qualitative agreement with this prediction. Calculations of the surface energy of the γ AlAg phase show rough agreement with the experimentally determined value at 873 K but poor agreement at 773 K. The DLP model predicts a much smaller temperature dependence of surface energy than is experimentally observed when the regular solution model is employed. It appears from these results and those of related studies that the frequently used nearest neighbor model is unable to provide an adequate accounting for the surface energy of Al-base alloys.     

Computer simulation of transformation induced plasticity using finite element method

Some important aspects associated with the transformation induced plasticity in Al₂O₃ZrO₂ are analyzed using a computer simulation based on Finite Element Method (FEM). These aspects include (i) an estimation of the residual stress in the second phase, arising during post fabrication cooling, which affects the critical stress for transformation, (ii) the constitutive behavior of the material during the dilatational transformation of ZrO₂ and (iii) the crack deflection due to the transformation. This simulation study was conducted for angular as well as spherical shapes of second phase particles and also for varying volume fractions of the second phase, using a master finite element mesh. Apart from this numerical experiment, analytical expressions were derived for the residual stress and the constitutive behavior, assuming spherical shape for the second phase particles. The analysis of the constitutive behavior mainly consists of an estimation of the composite modulus and of the irreversible strain due to the transformation. The comparison of the analytical solutions with the results obtained through the simulation shows a very good agreement. Using the simulation of crack deflection, the increase in crack surface area due to the transformation was computed, to approximately estimate the improvement in fracture toughness due to the crack deflection mechanism.     

Kinetics of leaching of a low-grade FeNiCuCo matte in ferric sulphate solution

A study of the dissolution of a low-grade FeNiCuCo matte in acid ferric sulphate solution is reported. This matte contained a number of different phases, and a detailed quantitative mineralogical analysis of the matte was used as a basis for mathematical modelling of the leaching process. The mathematical model assumed that the matte particles were leached by a shrinking-particle mechanism, and that the surface reaction was rate-limiting and electrochemical in nature. The experimental results verified these assumptions, and showed that the redox potential of the solution best approximated the potential of a matte particle during leaching.     

Anomalous X-ray scattering study of early-stage precipitation in AlZnAg

Initial stages of phase separation in an Al14%Zn4%Ag alloy was studied by anomalous small-angle X-ray scattering. Although the three partial structure functions could not be determined individually, it was observed that composition tie-lines rotated in the course of aging time. The rotation was interpreted in terms of a change of the Ag/Zn concentration ratio within the evolving precipitates. An early stage stability analysis was also performed based on an approximate sub-regular solution model for the coherent AlZnAg ternary miscibility gap.     

Phase transformations in the NbAu system by ball milling and the study of the metastable phases

A detailed study on the NbAu binary alloy system upon ball milling is reported. An analysis of the energy levels of the NbAu alloy reveals that the solid solution has a lower enthalpy than the amorphous state for all compositions. The ball milling experiments were performed on five compositions of the NbAu alloys. All five alloys transformed to solid solutions after 40h of grinding. The structures of these solid solutions are different depending on the composition of the starting material. In further experiments, the properties of the metastable phases obtained by ball milling were studied. The lattice parameter of the solid solutions turns out to obey Vegard's law. The exothermic heat of the phase restoration appears to be approximately proportional to the enthalpy as predicted by Miedema's semi-empirical model. The magnitude of the peak temperatures and the activation energies follows the same trend as the melting temperaures. Examination of particle size and shape for two composition gives more insight in the milling process. Finally, the results for prolonged milling are presented.     

The martensitic phases and their stability in CuZn and CuZnAl alloys—III. The transformation between the high temperature phase and the 2H martensite

The stability between the high temperature β phase and the 2H martensite is analyzed in CuZnAl single crystals of a high electron concentration around 1.52. The results are compared with those deduced from previous reports, and the equilibrium temperatures and entropy differences are determined. A phenomenological model for the stress induced transformation explains well the observed crystallography.     

Anomalous strain rate dependence of the serrated flow in NiH and NiCH alloys

Serrated flow in NiH, NiCH, and NiC alloys was studied over a wide range of temperature and strain rates. The results for C related serrated flow in NiC or NiCH alloys were in excellent agreement with previous results and were consistent with dislocation pinning by C solutes at the dislocation cores. Hydrogen related serrated yielding was observed in NiH and NiHC alloys. Solute C had only a small effect on the temperature range of this H related serrated flow. The results could be interpreted on the basis of hydride formation at the dislocation cores and diffusion of H in these hydrides.     

Calculation of the equilibrium phase diagrams and the spinodally decomposed structures of the FeCuNi system

Thermodynamic stability of ternary systems with miscibility gaps is discussed and applied to the f.c.c. phase of FeCuNi alloys. The stability of the system with respect to infinitesimal composition fluctuation is described. The direction most likely for initial composition fluctuation is defined as the one with the largest negative curvature on the Gibbs energy surface. Along this direction, zero-time wavelength at initial stage of spinodally decomposed alloy structure may be calculated. The thermodynamic values of the f.c.c. FeCuNi alloys are obtained from limited amount of thermochemical and phase equilibrium data. It is found that the ternary interaction parameters for both the f.c.c. and liquid phases of FeCuNi may be approximated from the binary interaction parameters of the constituent binaries.     

Unique metallic glass formability and ultra-high tensile strength in AlNiFeGd alloys

The metallic glass formability of aluminum-rich AlNiFeGd alloys has been systematically investigated. The critical cooling rate required to form an amorphous state in this system is generally low, and comparable to that of some of the best metallic glass formers, such as PdCuSi. Amorphous ribbons up to 0.25 mm thick can easily be produced by the single-roller melt-spinning technique. Tensile strengths as high as 1280 MPa and Young's modulus of 75 GPa have been obtained. Bulk amorphous alloys with good mechanical properties are optimized in Al₈₅Ni₆Fe₃Gd₆. DSC and DTA studies reveal that the glass formability is unique for Al-based alloys because the reduced glass temperature T_rg for AlNiFeGd can be as low as 0.44. This is much lower than conventional theory would suggest for easy glass forming systems. A mechanism for the unusual glass formability is suggested.