Warren J. Poole

Modeling of precipitation hardening for the naturally aged Al-Mg-Si-Cu alloy AA6111

The effect of natural aging on the artificial aging behavior of the Al-Mg-Si-Cu alloy AA6111 is examined by isothermal calorimetry and the results are analyzed in a new kinetic model. The model describes the kinetics of concurrent precipitate formation and cluster dissolution during artificial aging of the alloy with variable levels of natural aging. The kinetic model is then combined with a recently developed yield strength model for AA6111 to predict the precipitation hardening behavior of the naturally aged alloy. The validity of both models is verified by agreement between the predictions of the models and independent experimental results.

On the precipitation-hardening behavior of the Al-Mg-Si-Cu alloy AA6111

The precipitation-hardening behavior of aluminum alloy AA6111 during artificial aging and the influence of prior natural aging on the aging behavior were investigated. The evolution of microstructure was studied using quantitative transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The evolution of the relative volume fraction of precipitates for the solution-treated alloy was determined using isothermal calorimetry and a new analysis based on the DSC technique. Quantitative TEM was also used to obtain the rate of precipitation of microscopically resolvable phases during aging at 180 °C. Three types of precipitates, i.e., unresolved Guinier-Preston (GP) zones, β″, and Q′, were found to form during aging at 180 °C. The evolution of yield strength was related to the evolution of microstructure. It was found that the high hardening rate during artificial aging for the solution-treated alloy is due to the rapid precipitation of the β″ phase. Natural aging prior to artificial aging was found to decrease the rate of precipitation of β″. The slow hardening rate for the naturally aged alloy was attributed to the slower nucleation and growth of β″ phase.

The shearable–non-shearable transition in Al–Mg–Si–Cu precipitation hardening alloys: implications on the distribution of slip, work hardening and fracture

A systematic study has been conducted to evaluate the nature of the dislocation–precipitate interaction and its relationship to the mechanical properties for a commercial Al–Mg–Si–Cu alloy. A variety of experimental techniques employed including transmission electron microscopy, slip line observations and macroscopic work hardening behaviour. The results from this work indicate that a clear transition in macroscopic behaviour of the alloy can be observed when the precipitates become large enough so that they are not sheared by dislocations. Direct observations using a transmission electron microscope (TEM) indicate that the precursor to the Q phase becomes impenetrable to dislocations when its equivalent diameter is above 2.5–3.0 nm. The transition from shearable to non-shearable precipitates manifests itself in a number of ways including: (i) a change in the local distribution of slip from a banded to a more homogeneous structure and (ii) a characteristic change in macroscopic work hardening behaviour. In addition, observations on intergranular fracture suggest that the distribution of slip and the intrinsic fracture properties of the grain boundary are critical in controlling this process. Finally, an integrated view of the relationship between the basic dislocation–precipitate interaction and the global response of the alloy is rationalized.

On the impact of grain size distribution on the plastic behaviour of polycrystalline metals

Recent experimental studies have reported improved combinations of strength and uniform elongation in ultrafine-grained polycrystals with bi-modal grain size distributions. Despite these results, the extent to which the grain size distribution affects macroscopic tensile response, particularly at large strains, is unclear. This issue is examined here for polycrystals with varying grain sizes and grain size distributions using a grain size dependent constitutive model within the viscoplastic self-consistent formalism. The evolution of the macroscopic and grain-level stresses and strains has been monitored as a function of the width and mean of the grain size distribution. As an example of highly heterogeneous structures, the stress–strain response of a number of bi-modal microstructures have been examined and compared with their uni-modal counterparts.

The role of internal stresses on the plastic deformation of the Al–Mg–Si–Cu alloy AA6111

In this work, we have investigated the internal stress contribution to the flow stress for a commercial 6xxx aluminium alloy (AA6111). In contrast to stresses from forest and precipitation hardening, the internal stress cannot be assessed properly with a uniaxial tensile test. Instead, tension–compression tests have been used to measure the Bauschinger stress and produce a comprehensive study which examines its evolution with (i) the precipitation structure, and (ii) a wide range of applied strain. A large set of ageing conditions was investigated to explore the effect of the precipitation state on the development of internal stress within the material. It is shown that the Bauschinger stress generally increases with the applied strain and critically depends on the average radius of the precipitate and is thus linked to the shearable/non-shearable transition. Further work in the case of non-shearable particles shows that higher strain eventually leads to particle fracture and the Bauschinger stress then rapidly decreases. Following the seminal work of Brown et al. a physically based approach including plastic relaxation and particle fracture is developed to predict the evolution of the internal stress as a function of the applied strain. Knowing the main characteristics of the precipitation structure–such as the average precipitate radius, length and volume fraction–allows one to estimate accurately the internal stress contribution to the flow stress with this model.

The deformation behaviour of AA6111 as a function of temperature and precipitation state

Abstract Tensile tests were conducted on the aluminum alloy, AA6111, over a range of test temperatures from 4.2 to 293 K. The state of precipitation was also varied with tests on material in the supersaturated solid solution, after 1, 6 h and 6 months at 180°C. The work hardening behaviour was quantified by numerically differentiating the data from tensile tests. In addition, strain rate change tests were conducted during tensile tests at 77 K. It was observed that the work hardening behaviour was a strong function of test temperature and precipitation condition. In particular, it was found that the supersaturated solid solution showed particularly high work hardening characteristics. For the samples aged for 1 and 6 h, a high initial work hardening rate was observed followed by a rapid drop in the hardening rate. Finally, for the overaged material, it was observed that the hardening rate decreased at a higher rate than high purity aluminum. Strain rate change data showed that for the obstacles formed in the early stages of ageing, thermal activation was important, while for overaged conditions, thermal activation became less significant. This was attributed to the change in the precipitate characteristics during ageing.

The effect of natural ageing on the evolution of yield strength during artificial ageing for Al-Mg-Si-Cu alloys

Abstract Tensile tests have been conducted at 77 and 293 K in order to examine the effect of natural ageing on the subsequent artificial ageing behaviour of the industrial alloy AA6111. The evolution of the yield stress as a function of ageing time at 180 °C was measured for samples with no natural ageing time and a prolonged period of natural ageing (i.e. T4 material). The nature of the obstacles was characterized in terms of the yield stress temperature dependence by measuring the ratio of the yield stress at 293 K to the yield stress at 77 K. From these measurements, it was possible to characterize the variation in the obstacle properties as a function of artificial ageing time.

Characterization of reinforcement distribution inhomogeneity in metal matrix composites

Abstract The non-uniformity of reinforcement distribution in metal matrix composites (MMCs) can have significant effects on the plastic deformation and failure characteristics of these materials. Characterization of the reinforcement arrangement, in terms of a ‘random’ or ‘clustered’ distribution, is thus important. In the present work, microstructures involving ‘hard-core’, ‘single cluster’ and ‘triple cluster’ reinforcement arrangements were generated on a computer, and were characterized using a new approach to develop local reinforcement area-fraction contour maps. The construction of the contour maps followed a Voronoi tessellation-based algorithm. The entire microstructure was discretized into a number of elements, and the elemental area-fractions were determined from the ratio of the reinforcement area to the area of the enclosing Voronoi cell. The contour maps were able to distinguish between the different distributions, and capture the position, size, shape and the reinforcement area-fraction in the clusters. The cluster characteristics obtained from the area-fraction contour maps may be useful in modeling the MMC failure mechanism, which is often dominated by damage evolution in the reinforcement clusters.

The characterization of dislocation-nanocluster interactions in Al-Mg-Si(-Cu/Ag) alloys

The quantification of the interaction between nanoclusters and dislocation motion has received relatively little experimental or theoretical research. In this work, the relationship between nanoclusters and dislocations was investigated by conducting tensile tests at different temperatures for a variety of nanoclusters in Al–Mg–Si alloys. Further, the nanoclusters were characterized by 3D atom probe. The normalized energy required for a dislocation to shear through a nanocluster, go , was estimated by using the results from the tensile tests and thermal activation theory. It was possible to characterize differences in nanoclusters for different ageing times as well as changes due to the addition of Cu or Ag. Specifically, it was found that the nanoclusters that formed at 293 K could be differentiated from those formed at 393 K, even after correcting for the nanocluster size. Finally, it was found that the addition of small amounts of Cu or Ag fundamentally altered the dislocation–nanocluster interaction.

In situ measurement of reinforcement stress in an aluminum–alumina metal matrix composite under compressive loading

Abstract The phenomena of stress partitioning between the matrix and the reinforcements in a loaded metal matrix composite dominate the mechanical behavior of these materials. Numerical models for estimating the stress in the matrix and the reinforcement under load are well developed. However, direct experimental measurements (e.g. measurement of reinforcement stress) are more difficult and have not been widely undertaken at present. The objective of the present work was to measure in situ the hydrostatic stress in the ceramic reinforcements in a continuously reinforced metal matrix composite loaded under transverse compression (i.e. loading perpendicular to the fiber axis). A single crystal sapphire reinforced AA6061 matrix model composite (reinforcement volume fraction ∼10%) was used for the measurements, which were undertaken at applied strains of 5, 10 and 20%. The stress measurements utilized the piezo-spectroscopic property of the Cr 3+ ions which were present as impurities in the sapphire reinforcements. The compressive deformation of the composite was simulated using an isotropic, plane strain finite element model. The reinforcement hydrostatic stress estimates from the isotropic FEM model were suitably modified to incorporate the effects of anisotropy in properties of the sapphire single crystal. The mean values of the experimental measurements of reinforcement hydrostatic stress matched well with the numerical estimates.