The role of quench rate on the plastic flow and fracture of three aluminium alloys with different grain structure and texture
Bjørn Håkon Frodal, Emil Christiansen, Ole Runar Myhr, Odd Sture Hopperstad
Abstract
The yielding, plastic flow and fracture of age hardenable aluminium alloys depend on the quench rate to room temperature after the solution heat-treatment at elevated temperature and before the artificial ageing. We investigate three AlMgSi alloys with different grain structure and crystallographic texture experimentally to determine the effects of quench rate (either water-quenching or air-cooling) on the precipitate microstructure and the mechanical properties, i.e., yield stress, work hardening and ductility. Tensile tests on smooth and V-notch specimens and Kahn tear tests are performed to study the influence of stress state on plastic flow and fracture. In addition, finite element simulations of the mechanical tests are performed for one of the alloys to investigate the validity of an extension of the Gurson model to high-exponent anisotropic plasticity. Transmission electron microscopy investigations show that the alloys and their precipitation microstructure are differently affected by the quench rate. Common for the three alloys is that the precipitate free zones around dispersoids and grain boundaries become larger, and the yield strength of the alloys becomes lower, after air-cooling than after water-quenching. The nanostructure model NaMo was modified to account for precipitate free zones, and was able to predict both the precipitation parameters and the tensile yield strength of all tempers and materials with a reasonable degree of accuracy, except in one case. In this case, the inhomogeneous precipitation in the material is too complex to be captured by the inherent precipitation model in NaMo. Due to the lower yield strength and higher work-hardening rate after air-cooling, the failure strain is increased for the smooth and V-notch tensile tests. The crack propagation energy, calculated from the Kahn tear tests, is markedly affected by the quench rate and the effect is different depending on the grain structure and plastic anisotropy, caused by the crystallographic texture. The anisotropic porous plasticity model used in the finite element simulations is able to precisely capture the fracture initiation in all the specimen geometries of the considered alloy, whereas the crack propagation energies of the Kahn tear tests are slightly overestimated.