115 research outputs found
Kinetics of coherent order-disorder transition in
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 alloy. It is shown that a microstructure
with cubic precipitates appears as a transient state during the
decomposition of a homogeneous disordered solid solution into a microstructure
with tetragonal precipitates embedded into a disordered matrix. At
low enough temperature, favored by a weak internal stress, only
precipitates grow in the transient microstructure preceding nucleation of the
precipitates that occurs exclusively at the interface of the solid
solution with the precipitates. Analysis of microstructures at
nanoscopic scale shows a characteristic rod shape for the
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
Pinning Effect of Oxide Particles on Grain Boundaries of a Low Aluminum Non‐oriented Electrical Steel
On the formation of fan-shaped precipitates during the decomposition of a highly supersaturated AlZr solid solution
- …