Papers by Author: Marco J. Starink

Abstract: The strengthening of AlCuMg(Li) alloys subjected to high pressure torsion (HPT) deformation with strain reversals was studied by microhardness (Hv) tests and differential scanning calorimetry (DSC). It was found that the strengthening is lower for both cyclic HPT (c-HPT) and single reversal HPT (sr-HPT) as compared to monotonic HPT (m-HPT). The DSC results demonstrate that |HPT influences S phase precipitation. With increasing strain, the maximum heat flow (height of the S peak) and the heat content of S formation peaks increases. There is a larger S heat content reaction in the periphery of HPT processed disks compared with those in the centre. Strain reversal also has a significant influence on the S precipitation. The strengthening during HPT deformation is discussed in terms of the density of statistically stored and geometrically necessary dislocations.

Abstract: The evolution of texture and deformation in the grains during one pass of equal-channel angular pressing (ECAP) was examined for fine grained high strength and low strength Al alloys and a coarse grained low strength Al alloy. The materials were analysed using electron back-scatter diffraction (EBSD). The results are consistent with the materials responding to the intense macroscopic shear stress by deformation of individual grains through movement of dislocations on one or more of the slip crystallographic slip planes {hkl} that are favourably oriented, combined with the rotation of grains to directions that bring main crystallographic slip planes parallel to the macroscopic shear direction and crystallographic slip directions parallel to two main shear directions. Contrary to reports claiming up to 4 slip systems are activated, it was observed that only the {111}<110> and {001}<110> shear systems are activated. Macroscopic shear deformation occurs on two shear planes: the main shear plane (MSP), equivalent to the simple shear plane, and a secondary shear plane which is perpendicular to the MSP.

Abstract: To predict strength evolution of precipitation hardening alloys, a wide range of modelling approaches have been proposed. The most accurate published models are physics-based approaches which use both nanoscale processes with their related constants and parameters, as well as parameters calibrated to one or more macroscale measurements of yield strength of one or more samples. Recent developments in submodels including analytical expressions for volume fraction and size evolution including impingement and coarsening are reviewed. It is also shown that Kampmann-Wagner and JMAK models are generally not consistent with data on the progress of precipitations in the main precipitation hardening Al alloys systems, and improved model formulations are described.

Abstract: Analysis of toughness in 6156 Al-Mg-Si-Cu sheet has been performed using enhanced Kahn tear tests on samples quenched at different rates, whilst microstructures of the samples have been assessed using differential scanning calorimetry, scanning electron microscopy and transmission electron microscopy. Crack initiation energies were unaffected by changing water quench temperature from 20°C to 60°C, however a significant reduction was evident on air cooling. Crack propagation resistance was reduced for both 60°C water quenched and air cooled materials. The failure morphology of the air cooled material appears consistent with classical intergranular ductile failure. Coarse voiding and shear decohesion was prevalent in the 20°C water quenched material, whilst the 60°C water quenched material showed a mixture of transgranular and intergranular fracture modes. Changes in microstructure and precipitation behaviour resulting from reduced quenching rate were identified and related to the observed fracture behaviour, particularly in terms of precipitate free zone formation and the simultaneous presence of coarse particles at grain boundaries.

Abstract: A multi-mechanistic model for microstructure development and strengthening during nonisothermal treatment of precipitation strengthened Al-Cu-Mg based alloys is derived. The formation kinetics of the precipitates is modelled using the Kampmann and Wagner numerical model that accounts for complete transformation from the nucleation to the coarsening stages. The increase in critical resolved shear strength of the grains due to the precipitates is based on two mechanisms i.e. the modulus strengthening mechanism for the shearable Cu:Mg co-clusters and the Orowan strengthening mechanism for the non-shearable S phase precipitates. The contributions due to solute and dislocation strengthening are also included in the strength calculations. The model is verified by comparing the predicted results with differential scanning calorimetery and hardness data on 2024 aluminium alloys. The microstructural development and strength/hardness predictions of the model are in reasonable agreement with the experimental data and the differences are discussed in terms of requirements for further model development.

Abstract: In Al-Cu-Mg with compositions in the α+S phase field, precipitation hardening is a twostage process. Experimental evidence shows that the main precipitation sequence in alloys with Cu contents in excess of 1wt% is involves Cu-Mg co-clusters, GPBII/S'' and S. The first stage of the age hardening is due to the formation of Cu-Mg co-clusters, and the hardening can be modelled well by a modulus hardening mechanism. The appearance of the orthorhombic GPBII/S'' does not influence the hardness. The second stage of the hardening is due to the precipitation of S phase, which strengthens the alloy predominantly through the Orowan looping mechanism. These findings are incorporated into a multi-phase, multi mechanism model for yield strength of Al-Cu-Mg based alloys. The model is applied to a range of alloys with Cu:Mg ratios between 0.1 and 1 and to heat treatments ranging from room temperature ageing and artificial isothermal ageing to rapid heating to the solution treatment temperature. The predictive capabilities of this model are reviewed and its constitutive components are compared and contrasted with a range of other methods, such as the Kampmann-Wagner and JMAK models for precipitation as well as the LSW model for coarsening.

Abstract: The texture and grain boundary evolution during equal channel angular pressing (ECAP) of a spray-cast Al-7034 (Al-Zn-Mg-Cu) alloy containing intermetallic particles with a range of sizes was studied through electron backscatter diffraction (EBSD). Up to 8 passes of ECAP using route Bc were employed. The initial ECAP pass leads to the development of low angle grain boundaries and subsequent passes lead to a relatively rapid increase in the fraction of high angle grain boundaries. Before ECAP, the material possessed a strong <111> and <100> fibre texture. On ECAP, the <111> fibre texture component is mostly retained but the <100> fibre develops to a Cube texture after the first ECAP pass. Goss textures form from about 4 passes of ECAP.

Abstract: The precipitation and evolution of microstructure in a spray-cast Al-7034 alloy and a commercial wrought Al-2024 alloy were studied after equal-channel angular pressing (ECAP) using transmission electron microscopy and differential scanning calorimetry (DSC). Microstructural examination showed the grain sizes of both alloys were reduced to the range of ~0.3–0.5 μm through ECAP. The DSC analysis identified the occurrence of thermal effects involving the formation, coarsening, dissolution and melting of the precipitate phases and concurrent recrystallization. The heating and ageing response of the alloys processed by ECAP was identified by micro-hardness testing of the samples after interrupted heating and ageing treatments.