Solid-solution strengthening and latent hardening of monocrystalline aluminum alloys

Solid-solution strengthening of aluminum single crystals by dilute additions of Cu and Mg solute atoms were investigated in this study. Single crystals of pure aluminum, one ternary Al-Cu-Mg alloy, three binary Al-Cu alloys, and three binary Al-Mg alloys, all monocrystalline with various solute concentrations were grown by the Bridgman method. Compression tests were carried out at four different temperatures: 77 K, 198 K, 298 K and 373 K on specimens oriented for single glide. The critical resolved shear stress (CRSS) of pure aluminum and all the binary alloys was measured and the concentration and temperature-dependences of the CRSS were investigated. We also measured the CRSS of the ternary alloy to determine the superposition rule accounting for the individual contributions of Cu and Mg solutes to the strength of the ternary alloy. The experimental data from single glide tests were analyzed by comparing them with several existing solid-solution-strengthening theories. Constitutive equations that govern the CRSS as a function of temperature and concentration for the binary alloys were also derived. We investigated the latent hardening behavior of one dilute Al-Mg alloy. We thoroughly analyzed all the possible dislocation interactions on different slip systems with those on the primary slip system. The so-called latent hardening tests were performed on each of the five system groups and data were obtained on flow stress-flow strain at room temperature. The latent hardening ratios (LHR) which characterize the latent hardening potential, as well as the hardening rates from the measured stress-strain curves, were calculated. A simplified model was established from which equations relating the LHR to the dislocation densities at various prestrain values were derived. A prediction of the shape of the secondary deformation curve in various latent systems was also made. It is found that the concentration dependence of the CRSS of both Al-Cu and Al-Mg alloys can be equally well described by a c{dollar}sp{lcub}1/2{rcub}{dollar} or c{dollar}sp{lcub}2/3{rcub}{dollar} relationship, although a c{dollar}sp{lcub}1/2{rcub}{dollar} relationship for Al-Cu alloys and a c{dollar}sp{lcub}2/3{rcub}{dollar} relationship for Al-Mg alloys are slightly better. Solute atoms interacting with edge dislocations, instead of screw dislocations, control the CRSS. Both size and modulus effects have been found to contribute to the strengthening effects when Cu or Mg atoms are dissolved in the aluminum matrix. The Pythagorean addition rule and Labusch's superposition law of mixtures equally well describe the superposition of strengthening by Cu and Mg atoms. The latent hardening ratio of systems that form attractive junctions is highest in this investigation. The LHR of systems that form Lomer-Cottrell sessile locks, Hirth locks and cross-slip systems as a group are in the middle range. The coplanar system has the lowest LHR, which is in agreement with the theoretical prediction.