Advances in Microstructural Understanding of Wrought Aluminum Alloys

From Springer Nature via Jisc Publications RouterHistory: received 2020-03-12, registration 2020-06-24, online 2020-07-08, pub-electronic 2020-07-08, pub-print 2020-09Publication status: PublishedAbstract: Wrought aluminum alloys are an attractive option in the quest for lightweight, recyclable, structural materials. Modern wrought aluminum alloys depend on control of complex microstructures to obtain their properties. This requires an understanding of the coupling between alloy composition, processing, and microstructure. This paper summarizes recent work to understand microstructural evolution in such alloys, utilizing the advanced characterization techniques now available such as atom probe tomography, high-resolution electron microscopy, and synchrotron X-ray diffraction and scattering. New insights into precipitation processes, deformation behavior, and texture evolution are discussed. Recent progress in predicting microstructural evolution using computer modeling is also summarized

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

Creep Behavior of an Al-2.0 Wt Pct Li Alloy in the Temperature Range 300 °C to 500 °C

The elevated temperature deformation behavior of an Al-2. 0 wt pct Li alloy in the temperature range 300 °C to 500 °C was studied using constant extension-rate tension testing and constant true-stress creep testing under both isothermal and temperature cycling conditions. Optical microscopy and transmission electron microscopy (TEM) were employed to assess the effect of deformation on microstructure. The data showed that the stress exponent,n, has a value of about 5. 0 at temperatures above theα +δAlLi solvus (approximately 380 °C) and that subgrains form during plastic deformation. Models for dislocation-climb and dislocation-glide control of creep were analyzed for alloys deformed in the temperature range of stability of the terminal AlLi solid solution. A climb model was shown to describe closely the behavior of this material. Anomalous temperature dependence of the activation energy was observed in this same temperature range. This anomalous behavior was ascribed to unusual temperature dependence of either the Young’s modulus or the stacking fault energy, which may be associated, in turn, with a disorder-order transformation on cooling of the alloy.Naval Postgraduate SchoolNaval Surface Weapons Cente

Rheological behavior of Al-Mg-Si-Cu alloys in the mushy state obtained by partial remelting and partial solidification at high cooling rate

This work investigates the mechanical behavior of two aluminum alloys in the mushy state, the alloy AA6056 and an alloy based on mixing AA6056 and AA4047. These alloys have been studied to give insight into the susceptibility to hot tearing, which occurs during laser welding of AA6056 with 4047 filler wire. Two types of isothermal tensile tests have been conducted: (1) tests during partial remelting and (2) tests after partial solidification at a high cooling rate. Results show that the maximum tensile stress increases with increasing solid volume fraction. Both materials exhibit visco-plastic behavior for solid fractions in the range 0.9 to 0.99, except for a critical solid fraction of 0.97, where the semisolid material also shows minimum ductility. The stress levels observed for the remelting experiments are larger than those found for partial solidification experiments at the same solid fraction due to the influence of the microstructure. The influence of temperature and strain rate on the maximum stress is described by using a constitutive law that takes into account the fraction of grain boundaries wetted by the liquid