Grain size made to measure
How AMAG fulfills the highest customer requirements
At the microscopic scale, wrought aluminum alloys are polycrystals, made of a large number of crystalline grains. The strength of an alloy is higher when the grains are small. Also, the surface appearance of an alloy is smoother when the grains are small. Large grains, however, are beneficial for fatigue resistance. It is not only size that matters. The preferred orientation of crystallites affects the product properties as well. For example, the bending angle of automotive sheets increases when many grains have their lattice aligned with the rolling direction, exhibiting a preferred orientation of the so called cube type. The rolling and annealing steps in the AMAG production process are aligned to optimize the grain size and the grain orientations according to the desired product specifications only with detailed knowledge on the microscopic mechanisms which are activated during annealing it is possible to meet our customers’ specifications in a wide range of applications. The rest of this contribution takes a literal look at recrystallization (REX), i.e. the nucleation and growth of new grains from the cold rolled microstructure and presents the most modern experimental equipment and modelling methods used in AMAG’s research.
Recrystallization and recovery
After rolling, aluminum alloys exhibit a microstructure that consist of elongated grains. The grains are divided into subgrains. While grains are separated by large angle grain boundaries with a misorientation of more than 15°, the misorientation between subgrains is much lower, typically only a few degrees. During rolling, a part of the work required for the plastic deformation is stored in the subgrain boundaries. The smaller the subgrain size the higher is the stored energy. During subsequent annealing at elevated temperatures, this stored energy is reduced through two thermally activated mechanisms: recovery and discontinuous recrystallization, also known as primary or static recrystallization. Recovery involves logarithmic softening due to dislocation annihilation and subgrain growth, and it begins immediately upon annealing. Discontinuous recrystallization is characterized by the nucleation and growth of grains that are separated by large-angle grain boundaries from their surroundings (see Figure 1). Unlike recovery, recrystallization leads to a sigmoidal decline in the softening curve and is preceded by an incubation period necessary for the formation of recrystallization nuclei in the deformed microstructure. In a joint effort with research partners at the Centre of Electron Microscopy in Graz and the Institute for Materials Science and Technology at the TU Wien AMAG researchers developed a model to predict the onset of recrystallization in rolled aluminum alloys. Recrystallization is promoted by the amount of stored energy and the amount of prior deformation. But stored energy alone is not enough. The grain boundary must be able to move freely and surpass the restistance from small dispersoid particles on the one hand and also the drag exerted by solute Mg or Fe atoms. A simple equation could be derived, which relates the stored energy, the grain boundary mobility and the rate of revovery to predict the onset of recrystallization. The model and the associated understanding of the microscopic mechanisms behind recrystallization support the further optimization of AMAG’s production process to meet the most challenging specifications by getting the recrystallization right.
Customer benefit:
Our customers benefit from AMAG's technological expertise in optimizing grain size and orientation through precise heat treatments, ensuring that the final aluminum alloy products meet specific application requirements. This detailed knowledge allows AMAG to deliver high-quality products with tailored properties for various industries, enhancing performance and reliability.
Experimental techniques
Recrystallization is a challenging subject to study due to the rapid transformation in which most of the microstructural changes take place. In order to characterize it, several techniques exist ranging from microscopic investigations to X-ray techniques and mechanical tests. In a research project between AMAG rolling GmbH and FELMI-ZFE several characterization techniques have been applied and evaluated.
One wide spread method is to measure the hardness. The change in hardness values provides a softening curve which is attributed to recovery and recrystallization. The obvious disadvantage of this technique is that only an average value is measured over a large sample volume and no information about the microstructure is obtained. Therefore the method cannot distinguish between recovery and recrystallization. In the case of age-hardenable alloys, simultaneous precipitation can also affect the mechanical properties and thus interfere with the softening.Alternatively, optical microscopy series of ex situ annealed samples provides a convenient overview of the progress of recrystallization. The limitations of this method lie in the limited resolution of an optical microscope, which makes it impossible to obtain information on the onset of recrystallization and nucleation. In addition, grain size and shape cannot be easily assessed because only images without orientational information are obtained.
Using a scanning electron microscope (SEM) equipped with an electron backscatter diffraction (EBSD) detector, the microstructure can be measured directly, including information on grains, orientation, and texture. This method is ideal for capturing the microstructure at multiple stages of recrystallization. Compared to light microscopy, not only is more information available after the measurement, but much better resolution can be achieved. It can also be used to study nucleation and the start of recrystallization. The limits lie in the sample preparation, because EBSD requires good surface quality, which makes the entire measurement more time-consuming.
A further development of the previous method is the use of a heating stage in the SEM. The annealing process can then be performed in the SEM and EBSD is used to follow the change in microstructure in situ with fast scans. The real advantage of this method is that the entire process on a sample can be studied in a single area. This means that the nucleation and growth of individual grains can be studied. Due to the nature of recrystallization, only a limited area can be studied to keep the scan time short. The heating itself also affects the measurement due to thermal drift and thermal radiation.
In conclusion, a wide array of methods exists for studying recrystallization, each with its strengths and limitations. The optimal choice of technique depends heavily on the specific research question being investigated.
Quellen:
[1] G. Falkinger et al.: Modelling Simul. Mater. Sci. Eng. 33 (2025) 025016.
