Shahrzad Esmaeili

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.

The sequence of precipitation in the Al-Mg-Si-Cu alloy AA6111

The preciptation behavior of the aluminum alloy AA6111 in the advanced stages of aging has been studied using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM). The results have demonstrated the formation of various precipitate types and morphologies of quaternaryQ phase andβ (Mg2Si) families at the peak-aged condition and during overaging at different temperatures. The work has clarified the sequence ofQ phase formation and showed evidence for a new morphology of this phase. The sequence of precipitation in AA6111 is elaborated in light of the new findings.

A study on the early-stage decomposition in the Al–Mg–Si–Cu alloy AA6111 by electrical resistivity and three-dimensional atom probe

Electrical resistivity measurements and three-dimensional atom probe (3DAP) analysis were employed to investigate early-stage decomposition of the Al alloy AA6111 in the temperature range 60–180°C where electrical resistivity initially increased with ageing time. 3DAP measurements provided information on the shape, number density and solute content of the precipitates, as well as the solute concentration of the matrix, for the ageing conditions corresponding to the resistivity maxima. Using the 3DAP results, the precipitate size distributions for these ageing conditions were determined in terms of the measured number of solute atoms per precipitate. The number density and the Cu content of the precipitates decreased with increasing temperature, whereas the Mg/Si ratio increased. The size distribution of precipitates at the higher ageing temperatures showed the addition of larger size precipitates to the precipitate population. A modification to Matthiessens law was employed to describe the anomalous resistivity increase by considering the effect of solutes and precipitates on the resistivity evolution. Using the 3DAP results in analysing the resistivity anomaly, it was found that the decrease in the resistivity maxima with increasing temperature was associated with the decrease in the number density of precipitates and not the scattering power of precipitates. The 3DAP results were further used to provide information on the mechanisms of early-stage decomposition and the temperature dependence of the nucleation rate. From this, the nucleation rate appeared to be controlled by the migration of solute atoms, which was assisted by quenched-in vacancies.

Characterizations of additive manufactured porous titanium implants

This article describes physical, chemical, and mechanical characterizations of porous titanium implants made by an additive manufacturing method to gain insight into the correlation of process parameters and final physical properties of implants used in orthopedics. For the manufacturing chain, the powder metallurgy technology was combined with the additive manufacturing to fabricate the porous structure from the pure tanium powder. A 3D printing machine was employed in this study to produce porous bar samples. A number of physical parameters such as titanium powder size, polyvinyl alcohol (PVA) amount, sintering temperature and time were investigated to control the mechanical properties and porosity of the structures. The produced samples were characterized through porosity and shrinkage measurements, mechanical compression test and scanning electron microscopy (SEM). The results showed a level of porosity in the samples in the range of 31-43%, which is within the range of the porosity of the cancelluous bone and approaches the range of the porosity of the cortical bone. The results of the mechanical test showed that the compressive strength is in the wide range of 56-509 MPa implying the effect of the process parameters on the mechanical strengths. This technique of manufacturing of Ti porous structures demonstrated a low level of shrinkage with the shrinkage percentage ranging from 1.5 to 5%.

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.

Multicomponent phase-field crystal model for structural transformations in metal alloys

We present a new phase field crystal model for structural transformations in multi-component alloys. The formalism builds upon the two-point correlation kernel developed in Greenwood et al. for describing structural transformations in pure materials [Phys. Rev. Lett. 105, 045702 (2010)]. We introduce an effective twopoint correlation function for multi-component alloys that uses the local species concentrations to interpolate between different crystal structures. A simplified version of the model is derived for the particular case of threecomponent (ternary) alloys, and its equilibrium properties are demonstrated. Dynamical equations of motion for the density and multiple species concentration fields are derived, and the robustness of the model is illustrated with examples of complex microstructure evolution in dendritic solidification and solid-state precipitation.

Effects of Heat Treatment on Grain-Boundary β-Mg17Al12 and Fracture Properties of Resistance Spot-Welded AZ80 Mg Alloy

The distribution and morphology of β-Mg17Al12 intermetallic phase in resistance spot-welded AZ80 Mg alloy were investigated by means of optical microscopy, scanning electron microscopy, and X-ray diffraction. The influence of intermetallic phase on mechanical strength was studied by tensile shear testing and fractography. The results showed that continuous networks of β-Mg17Al12 formed along grain boundaries in both the nugget and heat-affected zone of the spot-welded AZ80 Mg alloy. Those continuous grain-boundary β-Mg17Al12 networks acted as effective crack propagation paths, which had negative effects on the weld strength. Post-weld solution heat treatment effectively reduced the amount of β-Mg17Al12 and broke the grain-boundary intermetallic networks in both the nugget and heat-affected zone. This significantly increased the weld strength of AZ80 Mg alloy and changed the fracture mode from nugget pull-out in the as-welded condition to through-thickness after heat treatment.

Modeling of Solutionizing and Solute Redistribution in a Co-Cast Bi-Layer Al Alloy System

The effect of high-temperature treatment on solutionizing and solute diffusion in a co-cast X609-AA3003 alloy system is examined via a coupled dissolution and diffusion model using finite-element analysis. The model describes the kinetics of the dissolution of intermetallic particles of Mg2Si and Si along with the diffusion of alloying elements of Mg, Si, and Cu across the interface between the two alloy layers. The results are verified using electron probe microanalysis (EPMA) measurements.