Aiwu Zhu

Strengthening effect of unshearable particles of finite size: a computer experimental study

Equilibrium configurations of a dislocation interacting with randomly distributed unshearable obstacles of finite size under an applied stress are analyzed. Ashbys critical dipole spacing Q′ argument for the self-stress effect of dislocations is utilized but analysis suggests that the spacing Q′ varies with the local obstacle distribution as well as with the obstacle shape. Computer simulations of a dislocation slip process through circular or linear obstacles, that are extensions of earlier work by Forman et al., were conducted. The dependence of the strengthening stress on the obstacle size was found to be less than that predicted by equations currently in use, particularly for linear obstacles. New, modified Orowan equations are suggested for the strengthening effects due to spherical, rod-like, and plate-like obstacles. The effect of the orientation distribution of the linear obstacles demonstrated in the simulation is in agreement with recent experimental observations.

The role of plastic deformation on the competitive microstructural evolution and mechanical properties of a novel Al–Li–Cu–X alloy

The role of plastic deformation prior to artificial aging on the microstructural evolution and mechanical properties of a novel Al–Li–Cu–X alloy designated AF/C 458 was investigated. Induced plastic deformation ranged from a non-stretched or 0% stretch condition to an 8% stretch, with intermediate stretches of 2%, 4% and 6%. Tensile properties, fractography and quantitative precipitate analysis were acquired from specimens that were water quenched from a solution heat treatment, immediately stretched and artificially aged at 150°C. Fractography was investigated through scanning electron microscopy (SEM). Quantitative transmission electron microscopy (TEM) determined the variation in precipitate type, number density, size and volume fraction of the major strengthening precipitates Al2CuLi (T1), Al2Cu (θ″/θ′) and Al3Li (δ′). Age hardening curves for each level of mechanical stretch illustrated the enhanced aging kinetics of plastically deformed material. Quantitative TEM indicated that increasing amounts of pre-age stretch were found to greatly affect the competitive precipitation kinetics of T1 and θ″/θ′ in AF/C 458 augmenting the volume fraction of fine matrix T1 plates and dramatically decreasing the volume fraction of θ″/θ′ for isochronal treatments. A quantitative microstructural comparison of specimens exhibiting a given strength demonstrated that the imposed level of cold work dictated the density, size and volume fraction of the competing precipitates. The tensile data indicated a trend of increasing ductility for equivalent yield strengths with the increasing amount of pre-age mechanical stretch and therefore shorter artificial aging times. The quantitative precipitate data were used with a computer simulation for yield strength determination. The theoretical simulation reported calculated yield strengths in good accord with experimental results and can thus be used to predict the optimum microstructural configuration for high strength.

Stress aging of Al–xCu alloys: experiments

Abstract Exposure of age-hardenable aluminum alloys to an elastic loading, either for “age-forming” and other manufacturing processes or during utilization at relatively high temperature, may lead to microstructural changes such as a stress-orienting effect of plate-like coherent or semi-coherent precipitates in the alloys. Preferentially oriented θ″/θ′-precipitate structures were quantitatively examined in single-crystal Al–2.5Cu, Al–4Cu and cube-textured Al–5Cu alloys aged to peak strength under compressive stresses. The dependence of the stress orienting of the θ″/θ′-precipitates on the applied stress, aging temperature and the copper content were determined. The effect is discussed and explained within the frame of classical nucleation and growth theories that incorporate the interaction energy between the external stress and the strain fields due to the lattice misfits between the θ″/θ′-precipitates and the Al matrix.

Precipitation strengthening of stress-aged Al–xCu alloys

Abstract Effects of stress aging on yield strength and yield anisotropy of single crystal and cube-textured polycrystalline Al–xCu alloys were investigated. The resulting microstructures were correlated with the yield stress and analyzed with respect to continuum mechanic models and computer simulations. The yield stresses of the stress-aged specimens, were found to be lower than those of the stress-free-aged specimens whether the test direction was along or perpendicular to the aging-stress direction. This was attributed to the effect of stress-induced preferential orientation of the θ′{100}-plates as well as to their “detrimentally” different volume fraction and/or morphology in the stress-aged specimen subjected to the same thermal treatment as the stress-free-aged specimen. When compared with the continuum models, both of which account for only effects of the orientation and volume fraction of the θ′{100}-plates, the computer simulation of interaction between slip dislocation and unshearable plates, which additionally incorporates the effect of plate morphology, yields the closest results to those obtained experimentally.

Computer experiment on superposition of strengthening effects of different particles

Abstract Particle-hardening materials, particularly high strength aluminum alloys, usually contain two or more types of second-phase particles. While the strengthening effect of mono-dispersed particles has been studied extensively and hence well formulated, a rational and consolidated evaluation of superposed hardening effects of different particle mixtures is still an open problem both experimentally and theoretically. A computer simulation technique is utilized to examine the details of the problem. The technique developed is based on the circle-rolling approach of Morris et al. The strengthening stress τp due to the mixture of different particles is determined by examination of a dislocation-slip process through the particles on one slip plane and along one slip direction under the action of an applied shear stress τ. Two kinds of particle mixtures are investigated. One consists of hard or unshearable point-like particles and soft or shearable point-like ones. The other is a mixture of two types of unshearable plate-like particles. The simulation results indicate that the superposition law can be well described by an equation τα=nα/2AταA+nα/2BταB where nA and nB are the density fractions of A- and B-particles, τA and τB the strengthening stresses due to pure A- and B-particles, and the exponent α varies between 1.0 and 2.0. Application to the spherical precipitates predicts that a bi-modal particle size distribution can give rise to about an 8% increment in strengthening stress with regard to a single size distribution that is normally produced by conventional aging. For a mixture of {θ′ (or θ″)+T1} in Al–Cu–Li alloys the simulation predicts that the larger T1-plates may either reduce or increase the strengthening effect depending on the particular circumstances. A structure comparison factor κAB is introduced to describe the various effects. Calculated values using the simulation method compare favorably with those determined experimentally.

Stress aging of Al–Cu alloys: computer modeling

Abstract Stress aging of Al–Cu alloys was modeled to demonstrate the stress orienting effect of the θ ″/ θ ′-precipitate structures. The precipitation sequence of the Guinier–Preston (GP) zones → θ ″→ θ ′ phases was treated as a continuous evolution—the parameters, such as lattice misfits, of the second-phase particles change depending on their sizes. Numerical simulations were conducted using a concomitant precipitation model that incorporates interaction energy between the external stress and the strain fields due to the lattice misfits of the second phases. The simulations reproduced most of the main features of the effects that were observed in experimental measurements (A. W. Zhu and E. A. Starke Jr, Acta mater ., 2001, 49 (12), 2285–2295).

Application of computational and experimental techniques in intelligent design of age-hardenable aluminum alloys

Age hardenable aluminum alloys are one of the traditional structural materials that are constantly under improvement to meet higher performance requirements. The practical alloys have complex microstructural features, among which the strengthening secondary phases and their dispersions are the controlling factors for major mechanical properties. Usually those phases with their specific crystal structures, elastic constants, strengths and thermal stabilities are formed in complex morphologies with particular shapes, sizes and orientations. Taking examples from Al-Cu-(Li,Mg)-based alloys, this chapter describes the quantitative characterization of the multiple secondary phases and illustrates dislocation slip simulations for evaluation of strengthening effects and for prediction of optimum precipitate structures. A combination of computational and experimental techniques will be described that are useful to study stress-aging treatments.

Atomic Bond Deficiency Defects in Amorphous Metals

Atomic bond deficiency (BD) is considered to be characteristic structural defects in amorphous metals. They are the necessary feature of local atomic configurations that facilitate various atomic transports under different driving forces. Compared with vacancies in crystalline solids, they are “small” in terms of their formation energies, volume costs, and elementary steps involved in atomic transport. This article reviews the authors’ recent efforts made to analyze how various local configurations containing BD are related to amorphous metal’s unique characteristics, such as glass transition, diffusion, shear flow, and structural relaxation.