Mohammad Reza Ahmadi

Assessment of parameters for precipitation simulation of heat treatable aluminum alloys using differential scanning calorimetry

Abstract Differential scanning calorimetry (DSC) has been used extensively to study different solid state reactions. The signals measured in DSC are associated with the growth and dissolution of different precipitates during a specific heat cycle. The time-temperature dependence of heat cycles and the corresponding heat flow evolution measured in the sample by DSC provide valuable experimental information about the phase evolution and the precipitation kinetics in the material. The thermo-kinetic computer simulation was used to predict the DSC signals of samples taken from 6xxx and 2xxx alloys. In the model, the evolution of different metastable and stable phases and the role and influence of excess quenched-in vacancies in the early stage of precipitation were taken into account. Transmission electron microscopy (TEM) and high-resolution TEM were used to verify the existence of precipitates, their size and number density at specific points of the DSC curves.

Thermo-Kinetic Simulation of the Yield Strength Evolution of AA7075 during Natural Aging

The yield strength evolution in aluminum alloy 7075 is investigated during natural aging. The thermo-kinetic simulation, capable of predicting nucleation, growth, coarsening and dissolution of metastable and stable hardening precipitates in Al-Zn-Mg-Cu during natural aging, is outlined briefly. A recent strengthening model for shearing and bypassing of precipitates by dislocations is utilized to calculate the evolution of the macroscopic yield strength at room temperature. The simulation accounts for vacancy-solute binding energies calculated with the help of first principles simulations that influence the diffusivity of the system due to the presence of excess quenched-in vacancies. These results provide predictions about the amount of excess vacancies trapped by solid solution alloying elements and how the lifetime of vacancies changes due to attractive or repelling binding forces between vacancies and different solid atoms in the aluminum matrix. In our approach, we calculate the strength evolution after quenching due to interaction between dislocations and changes in the microstructure by precipitation of different kinds of secondary phases. The predicted evolution of yield strength is finally verified on experimental measurements.

Simulation of Yield Strength in Allvac® 718PlusTM

In the present study, we describe a comprehensive and consistent physical model for the yield strength change in Allvac® 718PlusTM caused by precipitation strengthening. The model incorporates the effect of different shearing and non-shearing mechanisms with respect to atomic continuity between the lattices of precipitates and matrix. We demonstrate that coherency and anti-phase boundary effects are the major strengthening mechanisms in this alloy. The final yield strength of Allvac® 718PlusTM during aging is investigated using the thermo-kinetic software MatCalc. The calculated final yield strength evolution is consistent with experimental results.