D.J. Lloyd

Aspects of fracture in particulate reinforced metal matrix composites

Abstract The tensile deformation and fracture behaviour of the aluminium alloy 6061 reinforced with SiC has been investigated. In the T4 temper plastic deformation occurs throughout the gauge length and the extent of SiC particle cracking increases with increasing strain. In the T6 temper strain becomes localised and particle cracking is more concentrated close to the fracture. The elastic modulus decreases with increasing particle damage and this allows a damage parameter to be identified. The fraction of SiC particles which fracture is less than 5%, and over most of the strain range the damage controlling the tensile ductility can be recovered, indicating that other factors, in addition to particle cracking are important in influencing tensile ductility. It is suggested that macroscopic fracture is initiated by the SiC particle clusters that are present in these composites as a result of the processing. The matrix within the clusters is subjected to high levels of triaxial stress due to elastic misfit and the constraints exerted on the matrix by the surrounding particles. Final fracture is then produced by crack propagation through the matrix between the clusters.

Microstructure and strength of commercial purity aluminium (AA 1200) cold-rolled to large strains

Abstract Commercial purity aluminium has been cold-rolled to reductions from 40 to 99% or true thickness strains ( e t ) from 0.5 to 5.0. Applying transmission electron microscopy techniques the microstructural evolution has been followed and structural parameters such as spacing between dislocation boundaries and misorientation angles across these boundaries have been measured and analyzed. This analysis shows that the microstructural evolution does not saturate at large strains. In parallel, the flow stress and strain hardening behavior has been determined by uniaxial tensile testing of rolled speciments. These stress–strain curves do not saturate in the strain range examined, even when changes in the Taylor M-factor are considered. This agrees with the evolution in the boundary spacing and misorientation angle, which are considered to be the strength determining parameters based on the operation of two mechanisms, dislocation strengthening and grain boundary strengthening. Following this description, the individual strength contributions are calculated and their addition leads to flow stress values and strain hardening behaviour in good agreement with those determined experimentally.

Precipitation hardening in Al–Mg–Si alloys with and without excess Si

Abstract The aluminum alloys of the 6xxx series contain an excess of Si above that required to form stoichiometric Mg 2 Si, which is added to improve the age hardening due mostly to precipitation of metastable β″ precipitates. The excess Si is not believed to alter the precipitation sequence, structure and lattice parameters of the different metastable precursors, but rather promotes formation of additional particles/phases which do not contribute to hardening significantly. The presence of excess Si changes the composition and density of metastable β″ particles, although a systematic study of the Mg/Si ratio in particles from alloys of different composition is lacking. In this paper, it is shown that the precipitation sequence in the balanced alloy is independent of the composition and the strength increases with Mg 2 Si level. This is due primarily to both a higher volume fraction and a refined distribution of the β″ particles. Excess Si increases the effective amount of the hardening phases above ∼0.9 wt.% Si. It modifies the Mg/Si ratio in the clusters/zones and β″ precipitates and improves strength by altering their size, number density and distribution. In addition, the extent and rate of strengthening increases until the overall Mg to Si ratio in the alloy is close to approximately 0.4. The hardening precipitates with reduced Mg to Si ratio become less stable with aging and cause a decrease in strength during over aging.

Microstructural aspects of aluminium-silicon carbide particulate composites produced by a casting method☆

Abstract When metal matrix composites are produced by molten metal methods there are some unique factors which have to be considered. In this paper, the microstructure of SiC-reinforced aluminium alloys produced by this method are considered. It is shown that the stability of SiC in the melt is dependent on the matrix alloy involved and that only alloys with high silicon contents have a low reactivity with this reinforcement. With other alloy matrices, SiC reacts to form Al 4 C 3 , and the nature of this reaction and its kinetics are considered in this paper. Initially, the reaction rates are very rapid but almost saturate after about 1 h. It is also shown that the distribution of the reinforcing particles is dependent on the solidification rate because particles are rejected and pushed ahead of the meniscus. At low solidification rates, and hence for large cell sizes, the reinforcing particles are clustered and form a network which delineates the cell walls. Because the SiC particles are in the interdendritic regions they will be associated with any coarse intermetallic particles present and this can influence the fracture behaviour.

Strengthening of a particulate metal matrix composite by quenching

Abstract The strengthening of a particulate metal matrix composite due to quenching was studied both experimentally and theoretically. The strengthening was attributed to two mechanisms: punched-out dislocations due to CTE mismatch strain and back stress. Both mechanisms were analyzed by theoretical modeling leading to a good agreement between the experimentally observed strengthening and the prediction by the models. A parametric study revealed that the volume fraction of particulate, quenching temperature and its temperature differential and particulate size are the major variables influencing the increase in composite flow stress.

Microstructural control of aluminum sheet used in automotive applications

The microstructure of both 5000 and 6000 series alloy sheet can be controlled to provide the properties required for particular automotive applications. The work-hardening 5000 series alloys, with between 3 and 6% Mg as the major alloying addition, are supplied to the automotive companies in the annealed temper and are characterized by a recrystallized grain structure which is influenced by the insoluble Fe-based intermetallics, dispersoids, and the work-hardening rate. The annealed strength and formability is strongly dependent upon grain size, Mg content, and, to a lesser extent, on crystallographic texture. The 6000 series alloys containing Cu, Mg, and Si are somewhat more complicated to control, because of precipitation of the age-hardening phases during fabrication. It is necessary to control the processes of dispersoid and precipitate formation so as to obtain the desired strength, grain size, and crystallographic texture in the final sheet. These alloys also offer a low solution-treated strength for high formability, combined with rapid age hardening to a relatively high strength in the formed component during the paint bake cycle.

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.

A yield strength model for the Al-Mg-Si-Cu alloy AA6111

Abstract A yield strength model is developed for the Al-Mg-Si-Cu alloy AA6111. The evolution of the strength of precipitates, as obstacles to dislocation motion, during various stages of aging is modeled according to the theories for strengthening mechanisms, as well as the microstructural and mechanical behavior of the alloy. The precipitation hardening component of yield strength is modeled for conditions where precipitates act as either strong or weak obstacles. The dislocation breaking angles for various stages of aging at 180 °C is estimated and the applicability of both strong and weak obstacle models examined. It is found that although the weak obstacle model could be a better choice for describing the very early aging stages and/or the low temperature processes, the entire aging period in the temperature range of 160–220 °C is well described by applying the strong obstacle model. The modeling results are related to the microstructural evolution in this alloy system.

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.

Precipitation hardening processes in an Al-0.4%Mg-1.3%Si-0.25%Fe aluminum alloy

Abstract Aluminum alloys containing Mg and Si as the major solutes are strengthened by precipitation of the metastable precursors of the equilibrium β (Mg2Si) phase in one or more sequences. There are several metastable particles that can form during ageing, but the strengthening potential of these particles relative to one another is not extensively studied. In this paper, the precipitation and hardening potential of different metastable precursors in an Al–0.4%Mg–1.3%Si–0.25%Fe (wt.%) alloy is studied with the help of differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and hardness measurements. It has been found that the hardness increase in a freshly solutionized alloy during natural ageing occurs primarily due to precipitation of clusters and zones, while peak hardness is achieved due to precipitation of β” particles. The particles formed following β” phase are not effective hardeners.