Academic literature on the topic 'Secondary recrystallisation; Zener pinning'

In recent decades, microstructure and texture engineering has become an indispensable factor in meeting the rising demands in mechanical properties and forming behavior of aluminum alloys. Alloying elements, such as Fe and Mn in AlMg(Mn) alloys, affect the number density, size and morphology of both the primary and secondary phases, thus altering the grain size and orientation of the final annealed sheet by Zener pinning and particle stimulated nucleation (PSN). The present study investigates the grain size and texture of four laboratory processed AlMg(Mn) alloys with various Fe and Mn levels (see Part I). Common models for deriving the Zener-limit grain size are discussed in the light of the experimental data. The results underline the significant grain refinement by dispersoids in high Mn alloys and show a good correlation with the Smith–Zener equation, when weighting the volume fraction of the dispersoids with an exponent of 0.33. Moreover, for high Fe alloys a certain reduction in the average grain size is obtained due to pinning effects and PSN of coarse primary phases. The texture analysis focuses on characteristic texture transformations occurring with pinning effects and PSN. However, the discussion of the texture and typical PSN components is only possible in terms of trends, as all alloys exhibit an almost random distribution of orientations.

Heat treatment has a significant impact on the microstructure and the mechanical properties of Al-Mg-Si alloys. The present study presents a first Phase-Field modelling approach on the recrystallisation and grain growth mechanism during annealing. It focuses on the precipitate fraction, radius, and Mg-Si concentration in the matrix phase, which are used as input data for the calculation of the yield strength and hardness at the end of different ageing treatments. Annealing and artificial ageing simulations have been conducted on the MultiPhase-Field based MICRESS@ software, while the ThermoCalc@ software has been used to construct the pseudo-binary Al-Mg phase-diagrams and the atomic-mobility databases of MgxSiy precipitates. Recrystallisation simulation estimates the recrystallisation kinetics, the grain growth, and the interface mobility with the presence/absence of secondary particles, selecting as annealing temperature 400 °C and a microstructure previously subjected to cold rolling. The pinning force of secondary particles decelerates the overall recrystallisation time, causing a slight decrease in the final grain radius due to the reduction of interface mobility. The ageing simulation examines different ageing temperatures (180 and 200 °C) for two distinct ternary systems (Al-0.9Mg-0.6Si/Al-1.0Mg-1.1Si wt.%) considering the interface energy and the chemical free energy as the driving force for precipitation. The combination of Phase-Field and the Deschamps–Brechet model predicted the under-ageing condition for the 180 °C ageing treatment and the peak-ageing condition for the 200 °C ageing treatment.

This work describes the microstructure and properties of a range of Al-(4-6)Mg- (1.2-1.6)Li-(0.3-0.4)Zr-(0-0.2)Sc alloys produced at Oxford University by spraycasting. Follow- ing hot isostatic pressing of the as-spraycast billets to close any porosity and to precipitate a ¯ne, coherent dispersoid population, forging to a true strain of 1 at 250 and 400±C led to a substantial re¯nement of the microstructure with grain sizes in the range 0.8 to 5¹m. A large intra-granular orientation gradient with distance measured using EBSD showed that at 250±C, partial dynamic recrystallisation by progressive lattice rotation led to a `necklace' structure of very ¯ne grains surrounding larger deformed grains. At 400±C, dynamic recrystallisation oc- curred by nucleation of new grains at prior grain boundaries and triple points. The strength of as-forged alloys was 200-350MPa and the high ductilities of up to 30% rendered the alloys amenable to post-forging cold work. A proof strength of 460MPa with 9.5% elongation was achieved in a non-heat-treatable spraycast Al-6Mg-1.3Li-0.4Zr alloy, matching the best prop- erties of similar mechanically alloyed AA5091, and exceeding the properties of AA7010-T74. The as-forged alloys showed excellent thermal stability up to » 0.9Tm, with no abnormal grain growth and grain size stagnation due to Zener pinning. Finally, strain rate sensitivity testing revealed the potential for superplasticity at 400 and 500±C and strain rates of 0.001-0.05s¡1.

The increasing demands for Al sheets with superior mechanical properties and excellent formability require a profound knowledge of the microstructure and texture evolution in the course of their production. The present study gives a comprehensive overview on the primary- and secondary phase formation in AlMg(Mn) alloys with varying Fe and Mn additions, including variations in processing parameters such as solidification conditions, homogenization temperature, and degree of cold rolling. Higher Fe alloying levels increase the primary phase fraction and favor the needle-shaped morphology of the constituent phases. Increasing Mn additions alter both the shape and composition of the primary phase particles, but also promote the formation of dispersoids as secondary phases. The size, morphology, and composition of primary and secondary phases is further affected by the processing parameters. The average dispersoid size increases significantly with higher homogenization temperature and large primary particles tend to fragment during cold rolling. The microstructures of the final soft annealed states reflect the important effects of the primary and secondary phase particles on their evolution. The results presented in this paper regarding the relevant secondary phases provide the basis for an in-depth discussion of the mechanisms underlying the microstructure formation, such as Zener pinning, particle stimulated nucleation, and texture evolution, which is presented in Part II of this study.

The effect of individual and combined zirconium and manganese additions have been compared for an AA2198 6 mm thick sheet in T351 temper regarding their influence primarily on recrystallisation resistance and secondly on fracture toughness and overageing resistance. A complete characterisation of the dispersoid distributions was carried out for a deeper understanding of the effects of the Al3Zr and Al20Cu2Mn3 particles, involving studying their formation from the as-cast and homogenised stage.The most important finding in this work was the lower recrystallisation resistance in the alloy containing 0.1 wt%Zr + 0.3 wt%Mn compared to that containing only 0.1 wt%Zr. This result was rather unexpected, if one considers the opposite microsegregation patterns of Zr and Mn during casting, which leads to dispersoids occupying the majority of the grains’ volume and minimising dispersoid-free zones that could be potential sites for nucleation of recrystallisation. The other two alloys with dispersoid additions 0.05 wt%Zr + 0.3 wt%Mn and 0.4 wt%Mn, were partially and fully recrystallised respectively in the rolled T351 condition.Equally important in this work, was the observation that the opposite microsegregation trend of Zr and Mn sufficed to restrict grain growth in unrecrystallised areas. The 0.1Zr-0.3Mn alloy exhibited the lowest grain size of all alloys, both in the T351 temper and after annealing at 535oC for up to 144 hours. The reason for this was the combined action of Al20Cu2Mn3 dispersoids and Mn solute in the regions where the Zr concentration was low (i.e. near the grain boundaries), which offered additional pinning pressure to those areas compared to the 0.1Zr alloy.The lower recrystallisation resistance of the 0.1Zr-0.3Mn alloy was explained on the grounds of two main factors. The first was the lower subgrain size and hence stored energy within bands of Al20Cu2Mn3 dispersoids, which increased the driving force for recrystallisation in these regions. The second was the interaction between Zr and Mn that led to a decrease in the Al3Zr number density and pinning pressure. Since Zr was the dominant dispersoid family in terms of inhibiting recrystallisation, inevitably this alloy became more prone to recrystallisation. The Al3Zr pinning pressure was found to be much lower especially within bands of Al20Cu2Mn3 dispersoids. The detrimental effect of the Mn addition on the Al3Zr distribution was proven not to result from the dissolution of Zr within Mn-containing phases, and several other phases, at the grain interior and also in grain boundaries. The observed effect could not be precisely explained at this stage.Concerning mechanical properties, the 0.1Zr alloy exhibited the best combination of properties in the Kahn tear tests for fracture toughness. Further, it had a higher overageing resistance compared to the 0.1Zr-0.3Mn alloy.As an overall conclusion from this work, considering all the studied properties here that are essential for damage tolerant applications, the addition of 0.1 wt%Zr to the AA2198 6 mm thick sheet was found to be superior to that of the combined addition of 0.1 wt%Zr + 0.3 wt%Mn.