Remarkable Thermal Stability of Nano-Crystalline Al-Mg-Li Alloys through the Addition of 1%Sr
Authors: Ahmad Sara Iyad , Youssef Khaled 
Published: July 2019

Nanocrystalline metals -with grain sizes less than 100 nm- have proven to attain exceptional mechanical properties such as strengths that exceed those of coarse-grained and alloyed metals with grain size greater than 1 μm. As a result, such materials are to be acknowledged as the new class of high-performance engineering materials and to be implemented in various structural applications. The main reason behind the ultrahigh strength of this class of materials is mainly grain refinement, as in this mechanism, reducing grain size means introducing more grain boundaries. Grain boundaries act as barriers to the intra-grain dislocation motion, which is the main cause for the ductility of materials. Hence in its absence, the material is said to be strong and hard to deform. However, the strength associated with the new reduced grain size is associated with a penalty that leads to microstructural instabilities. This is because the atoms that lie in the grain boundary region are not as ordered and stabile as inside the grains. Hence those atoms are occupying unfavorable interfacial positions energetic wise, which means that they have high energy. As a result of the high volume fraction of grain boundaries, the system tends to pursue stabilization by seeking a configuration that shall allow for the lowest energy possible. The atomic system tends to eliminate the root of the problem which is the large number of grain boundaries and solve the problem by grain growth. Thus the removal of grain boundaries becomes the driving force to decreasing the system's energy. Hence an obstacle is yet to be overcome in order for those materials to be fully utilized to the maximum, as those nano-materials are prone to grain growth at lower temperatures than their conventional counterparts, which limits their service temperatures and expected lifetime. Grain growth can be slowed or even eliminated either thermodynamically, for example by adding solute atoms that segregate to the high energy sites in the grain boundaries, occupying it and lowering the free energy of the grain boundaries, or kinetically by the presence of second phase particles which results in grain boundary pinning, reducing the mobility of the grain boundaries and hence grain growth. This research aimed to studying the effect of adding 1% strontium (Sr) on stabilizing the grain boundaries of an aluminum-based alloy (Al-Mg-Li) which has a very low density yet a specific strength higher than that of steel. In order to achieve the goals of this research, a comparison must be made between the two samples of Al-5Mg-4Li and Al-5Mg-4Li-1Sr. Samples were prepared in a SPEX 8000 shaker mill and annealed at various temperatures up to 600 °C. To study the effect of Sr under various thermal conditions, both the as milled and annealed samples were analyzed using various experimental characterization methods such as X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) to perform a structural analysis and calculate the grain size of each sample. Using the Williamson-Hall model to calculate the average grain size for both samples based on the obtained XRD patterns, the results showed that for the as milled samples of Al-5Mg-4Li and Al-5Mg-4Li-1Sr, the average grain size was calculated to be 36.59 nm and 25.86 nm, respectively. The previous results were further proven when the TEM average grain size calculations gave similar results of 33 nm and 21 nm for the as-milled Al-5Mg-4Li and Al-5Mg-4Li-1Sr samples respectively. The thermal stability of the samples was proven when the grain size was measured after annealing at different temperatures for both Al-5Mg-4Li and Al-5Mg-4Li-1Sr samples. The average grain size was measured for Al-5Mg-4Li (annealed at 400 °C) and Al-5Mg-4Li-1Sr (annealed at 600 °C) to be 172.05 nm and 38.90 nm respectively. This shows that even at a higher temperature, the grain size for the sample that has Sr is much smaller and is still the nano-range. To verify the previous results of thermal stability, Vickers-hardness was measured for each sample after annealing as the mechanical properties of the thermally stabilized sample is expected to exceed those of the conventional sized sample. The plot of hardness variation of the nanocrystalline samples as a function of annealing temperatures showed that at room temperature, the hardness values for Al-5Mg-4Li and Al-5Mg-4Li-1Sr samples were 2.85 GPa and 3.24 GPa, respectively. With increasing annealing temperature, the hardness of Al-5Mg-4Li decreases gradually and reaches a low value of 0.8 GPa after annealing at 600 °C. In contrast, Al-5Mg-4Li-1Sr showed excellent thermal stabilization with increasing annealing temperature. Increasing the annealing temperature to 600 °C decreased the hardness value to 2.73 GPa. This hardness value is almost as high as the hardness of the as milled Al-5Mg-4Li (2.85 GPa) at room temperature. Since the XRD patterns did not show any traces of second phase particles, we suggest that the stabilization of the grain size and hence other mechanical properties such as hardness at high temperatures can be attributed to solute drag or the thermodynamic mechanism. Grain Growth does not only limit the nano-crystalline materials service temperatures, but also its unique technological applications as a consequence. Hence we anticipate that the results of this research will have implications in the development of thermally stabilized ultra-tough nanostructured materials for technological applications.