Papers by Author: Jostein Røyset

Abstract: In the structures of all metastable precipitates in Al-Mg-Cu and Al-Mg-Si alloys, we find that column surrounding of an element column in the needle/lath direction order according to simple principles. Advanced transmission electron microscopy and DFT calculations support the principles originate with a line defect, which is a segment of a <100>Al column shifted to interstitial positions. We propose the defect aids solute decomposition by partitioning the FCC matrix locally into columns of fewer and higher number of nearest neighbours, which suit smaller and larger size solute atoms, respectively. The defect explains how <100> directionality of the precipitates can arise in a cluster. Ordering of a few defects leads naturally to GPB zones in Al-Mg-Cu and to β'' in Al-Mg-Si.

Abstract: Two 6xxx alloys with different Mn-content have been homogenised in a furnace at 575 ^oC for 2 hours and 15 minutes. Three different heating rates to the homogenisation holding temperature were chosen, as this was expected to affect the precipitation behaviour of the dispersoids. The study focused on developing a reliable procedure for the characterization of the density and spatial distribution of dispersoids in aluminium alloys; both in terms of sample preparation, microscopic techniques and quantitative analyses of results. Scanning electron microscopy (SEM) has been used to evaluate the dispersoid characteristics for the different alloys and heating rates. The results indicate an increase in dispersoid number density and a more uniform distribution of dispersoids for the lowest heating rate, as compared to the more rapid heating rates, for the alloy with 0.05 wt% Mn. For the alloy with 0.15 wt% Mn the number density increased with the heating rate. This is suggested to be due to particle coarsening as an effect of the low heating rate where the samples spend longer time in the furnace.

Abstract: The effect on hardness and precipitate microstructure of elastically straining a 6060 Al-Mg-Si alloy during natural ageing or artificial ageing has been investigated. The elastic strain is here defined as 50 % of the material yield strength. All heat treatments where elastic straining was applied led to an increased hardness compared to the unstrained reference material. Quantitative investigations of the precipitate microstructure using transmission electron microscopy (TEM) did not indicate any significant difference in precipitate parameters as compared to the unstrained reference material. Therefore the increased strength in the elastically strained material is being linked to strain induced dislocations based on faster ageing kinetics compared to unstrained reference samples.

Abstract: The effect of cooling rate after solution heat treatment and its combination with 1% pre-deformation on precipitation hardening in two Al-Mg-Si alloys is investigated by transmission electron microscopy (TEM), and related to material hardness. Two alloys have been used, one Cu-free and the other with low Cu additions (~0.1 wt%), both having the same amounts of other solutes. A double peak hardness evolution during an isothermal heat treatment was observed with slow cooling after solution heat treatment. This effect was less pronounced in the Cu-added alloy. The 1% pre-deformation also made this effect less pronounced, but it led to faster initial hardness evolution and delayed over-aging. Maximum hardness was not influenced by cooling rate and the pre-deformation. Hardness was directly related to precipitate number densities.

Abstract: Quench sensitivity in two Al-Mg-Si alloys, one Cu-free and the other with low Cu additions (~0.1 wt%), both having the same amounts of other solutes, has been investigated using transmission electron microscopy (TEM) and corresponding material hardness. A two stage hardness evolution during an isothermal heat treatment was observed with slow cooling after solution heat treatment. This effect was less pronounced in the Cu-added alloy. However maximum hardness was not influenced by cooling rate, which could be related to higher precipitate number densities and volume fractions. Both alloys were over-aged faster in the slow cooling conditions.

Abstract: Twelve different aluminium alloys with constant iron (Fe) and scandium (Sc) contents of 0.5 wt.% and 0.2 wt.%, respectively, were cast and subsequently homogenised. The distribution of Sc in the microstructure was examined by means of Energy Dispersive X-ray Spectroscopy (EDX) in Scanning Electron Microscope (SEM). Emphasis was put on measuring the solubility of Sc in the Fe-bearing phases of the investigated alloys. It was found that the amount of Sc tied up in Febearing phases is so low that it can be regarded as negligible, with the possible exception of α- AlFeMnSi (Al15(Fe,Mn)3Si2) and the π-phase (Al8FeMg3Si6). A quaternary AlFeSiSc phase with a composition close to Al10Fe3Si5Sc2 may have been discovered.

Abstract: Specimens of three Al-Mg-Si alloys, 6060, 6005 and 6082, were solution heat treated, stored at different temperatures for different time, and artificially aged. Properties were measured before and after artificial ageing. The natural ageing response of the alloys is dependent on the storage temperature. Decreasing storage temperature leads to a delayed onset of natural ageing, but also to a higher strength after prolonged ageing, particularly for lean alloys such as 6060. The temperature and time of intermediate storage between solution heat treatment and artificial ageing has a significant effect on the strength of the artificially aged material. For the 6005 and 6082 alloys the processes that take place during natural ageing lead to a reduced strength after artificial ageing.