Børge Forbord

The Effect of Sc on the Extrudability and Recrystallisation Resistance of Al-Mn-Zr-Alloys

As cast and precipitation annealed variants of Al-Mn-Zr-alloys with and without Sc have been extruded in order to study the effect of Sc on the extrudability and the recrystallisation resistance after extrusion and subsequent annealing. Both Zr and Sc form dispersoids, which retard recrystallisation very effectively in many aluminium alloys. However, while Al3Zr often is heterogeneously distributed, a dense and homogeneous distribution of Al3(Sc,Zr)-dispersoids is obtained when Sc is added. This was also the case in these alloys, and the Sc-containing variants consequently displayed a far higher recrystallisation resistance than the Sc-free variants during extrusion and subsequent annealing. Another advantage by adding Sc is that precipitation annealing no longer seems to be necessary in order to obtain a high recrystallisation resistance, as the Sccontaining variants displayed an identical structural stability. The Sc-free alloy, on the other hand, had to be precipitation annealed in order to be able to resist recrystallisation during extrusion. However, an addition of Sc leads to a lower extrudability, as the Sc-containing variants displayed significantly higher extrusion pressures than the Sc-free alloys.

Recrystallisation Resistance of Extruded and Cold Rolled Aluminium Alloys with Additions of Hf, Sc and Zr

A high recrystallisation resistance is required in aluminium alloys intended for processing or use at temperatures between 450°C-600°C. Additions of Hf, Sc and Zr significantly improve the resistance to recrystallisation through the formation of Al3X-dispersoids (X=Hf,Sc,Zr), and in this work different concentrations and combinations of these elements were added to five aluminium alloys. The alloys were extruded, subjected to various degrees of cold rolling (0%-80%) and finally annealed at high temperatures in order to study the structural stability. All variants displayed a high resistance towards recrystallisation, but the best results were obtained in the alloy containing only Sc and Zr. In this alloy no signs of recrystallisation were observed even after 1 hour annealing of extruded and 80% cold rolled profiles at 600°C.

X-Ray Diffraction Studies of Grain Growth in an Ultra-fine Grained 6060 Aluminium Alloy

In-situ synchrotron X-ray diffraction has been applied in order to study grain growth in an ultra-fine grained (D~400 nm) 6060 aluminium alloy at 270°C. The submicron grain structure was produced by Equal Channel Angular Pressing (ECAP) to an effective strain of ~6 without rotation of the billet. As the material was textured after ECAP, the initial stages of grain growth were seldom detected, but in the grain size interval available for studies a grain growth exponent of 3.6±0.3 was obtained. By interpolation of the grain growth curves to D=D0 (determined by EBSD) the effect of growth on the softening of the alloy was estimated. The interpolated average curve indicates that the initial stages of softening are not due to uniform grain growth, but rather reconfiguration and annihilation of dislocations as well as overaging of hardening precipitates.

Development of Aluminium Alloys with Ultimate Recrystallisation Resistance

In the present work the precipitation behaviour and recrystallisation resistance of Alalloys containing Hf, Sc and Zr in different concentrations and combinations have been investigated. Special focus has been put on the Hf-containing alloys, as one of the objectives of this work was to find out if Hf can be used as a replacement for Sc. Additions of Sc, either alone or in combination with Zr, leads to the formation of coherent and homogeneously distributed dispersoids, which very efficiently inhibit recrystallisation. Despite these attractive properties, the high price of Sc has limited its use as an alloying element in aluminium. The present investigation has revealed that Hf cannot fully replace Sc, as only heterogeneous dispersoid distributions are obtained in the absence of Sc, i.e. in regions where the number density is low the alloys would still be prone to recrystallisation. However, as an extra addition to the already remarkably stable Sc+Zr-containing alloys, Hf can lead to further improvements and consequently open for the use of aluminium alloys at very high temperatures. Al3(Sc,Zr,Hf)-dispersoids were present at the largest f/r-ratios and also displayed lower coarsening rates than Al3(Sc,Zr)-dispersoids. Very promising results were obtained for an Al-Hf-Sc-Zr alloy, which maintained mainly an unrecrystallised structure after extrusion and large degrees of cold rolling.

The Effect of Boundary Structure on the Mechanical Properties of Aluminium Alloys

The microstructure in heavily deformed metals can be characterized as a complex “mixture” of low and high angle boundaries. By careful annealing of such cold deformed conditions, ultra-fine grained materials can be obtained. This phenomenon has been known for long and utilised in the production of special aluminium sheet qualities, and has received new interest with the emergence of the equal channel angular pressing (ECAP) technique. This work reviews the mechanical properties resulting from plastic deformation and annealing of aluminium, looking at alloys which prior to annealing was subjected to both severe plastic deformation (ECAP) and more conventional deformation by cold rolling. The effect of the resulting microstructures on the subsequent work hardening properties are model, applying the new microstructural metal plasticity model (MMP-model) developed in Trondheim over the last decade.