Kinetics of coherent order-disorder transition in Al3ZrAl_3 Zr

Within a phase field approach which takes the strain-induced elasticity into account, the kinetics of the coherent order-disorder transition is investigated for the specific case of $Al_3 Zr$ alloy. It is shown that a microstructure with cubic $L1_2$ precipitates appears as a transient state during the decomposition of a homogeneous disordered solid solution into a microstructure with tetragonal $DO_{23}$ precipitates embedded into a disordered matrix. At low enough temperature, favored by a weak internal stress, only $L1_2$ precipitates grow in the transient microstructure preceding nucleation of the $DO_{23}$ precipitates that occurs exclusively at the interface of the solid solution with the $L1_2$ precipitates. Analysis of microstructures at nanoscopic scale shows a characteristic rod shape for the $DO_{23}$ precipitates due to the combination of their tetragonal symmetry and their large internal stress.Comment: 2 postscript figures and 1 JPG pag

Evaluation of hot formability of an AL-4.6ZN-0.8MG alloy by increasing-strain-rate torsion tests

Evaluation of hot formability of an AL-4.6ZN-0.8MG alloy by increasing-strain-rate torsion testsM. El Mehtedi, N. Ryum, S. Spigarelli, E. Evangelista and B. RonningThe hot formability of an experimental Al-4.6%Zn-0.8%Mg alloy was studied by torsion testing.Conventional constant strain-rate tests were carried out to investigate material response in selected condition of temperature, T, and strain rate, ©.In order to determine the relationship relating flow stress with strain rate and temperature, the conventional procedure, based on straining at constant strain rate, requires a relatively large number of tests.The scope for reducing the number of tests by increasing linearly , i.e. , where K is a constant, from 0 to a definite value, was explored in this work and a series of increasing strain-rate tests were carried out.Nine tests, with K ranging from 0.05 to 0.2 s-1, were performed at 500 ºC; the resulting curves were used to recalculate the isostrain rate curve; comparison between experimental and calculated curve gave encouraging results, confirming the reliability of both testing procedures. The microstructure of torsioned samples was investigated by light (LM) and scanning electron microscopy (SEM).EBSD patterns were obtained to analyse grain size distribution and the presence of substructures. The results are discussed in the light of the more recent theories of high-temperature deformation of Al alloys

Nucleation of Al3Zr and Al3Sc in aluminum alloys: from kinetic Monte Carlo simulations to classical theory

Zr and Sc precipitate in aluminum alloys to form the compounds Al3Zr and Al3Sc which for low supersaturations of the solid solution have the L12 structure. The aim of the present study is to model at an atomic scale this kinetics of precipitation and to build a mesoscopic model based on classical nucleation theory so as to extend the field of supersaturations and annealing times that can be simulated. We use some ab-initio calculations and experimental data to fit an Ising model describing thermodynamics of the Al-Zr and Al-Sc systems. Kinetic behavior is described by means of an atom-vacancy exchange mechanism. This allows us to simulate with a kinetic Monte Carlo algorithm kinetics of precipitation of Al3Zr and Al3Sc. These kinetics are then used to test the classical nucleation theory. In this purpose, we deduce from our atomic model an isotropic interface free energy which is consistent with the one deduced from experimental kinetics and a nucleation free energy. We test di erent mean-field approximations (Bragg-Williams approximation as well as Cluster Variation Method) for these parameters. The classical nucleation theory is coherent with the kinetic Monte Carlo simulations only when CVM is used: it manages to reproduce the cluster size distribution in the metastable solid solution and its evolution as well as the steady-state nucleation rate. We also find that the capillary approximation used in the classical nucleation theory works surprisingly well when compared to a direct calculation of the free energy of formation for small L12 clusters.Comment: submitted to Physical Review B (2004