Automotive Aluminium?s Strength Unravelled at Atomic Level

Aluminium is essential to modern civilizations, largely because of its excellent combination of lightweight, strength and workability. It is widely used in vehicles for fuel-efficient transportation, e.g., it comprises about 80% of a commercial aircraft's unloaded weight. Aluminium is also an important material for building construction.

Pure aluminium is soft and has little strength or resistance to plastic deformation. This is because in an aluminium grain (atomic-matrix) under stress, a sort of collective atom displacements (called dislocations) can easily form and then rapidly move across the grain, leading to the continuous slipping of portions of the grain with respect to other portions. As such the material is macroscopically deformed.

However, alloyed with small amounts of other elements, aluminium can provide the strength of steel, but with only half the weight. This is because with thermal treatments the added alloying elements form nanometer-sized particles (nanoparticles), which act as obstacles to the dislocation movement in the Al grains, so that aluminium is strengthened. This phenomenon is known as precipitation hardening.

Although they can be visualized with a modern transmission electron microscope, such hardening nanoparticles are too small for structure determination by means of X-ray, neutron and electron diffraction. The atomic structural details inside the particles are generally unclear. Since they are formed under a condition far from the equilibrium, during annealing the hardening nanoparticles may undergo a continuous evolution process, involving changes in composition, structure and morphology. Hence the strength of the alloys varies with the annealing time in the thermal processes. Since the structures of such nanoparticles are poorly understood, the alloys’ property changes cannot be well explained.

AlMgSi occupies a high share of the total aluminum production in the world, because of its excellent combination of lightweight, strength and workability. One important property of AlMgSi alloys is that with appropriate pre-aging treatments (given in the aluminium factories) they can be pressed easily into a given form and then strengthened rapidly by annealing for a very short duration (< 30 minutes) at around 180°C (given in the paint bake circles in the car factories). This ‘super’ property of AlMgSi alloys is called quick bake hardening response (QBHR), which leads to their increasing application in automobile industry, e.g., as excellent outer panel materials for an optimal strength/weight ratio in terms of fuel efficiency and environment protection. However, the mysterious evolution occurring among the hardening nanoparticles in AlMgSi alloys has puzzled scientists for more than half a century. The problem was that the nanoparticles (~2nm) that are responsible for the QBHR are extremely difficult for conventional transmission electron microscopy and diffraction methods to characterize.

Now with advanced modern high-resolution transmission electron microscopy, we have characterized the structures of these nanoparticles and therefore revealed the microscopic mystery of these aluminium alloys’ strength, and particularly the cause for the QBHR of the alloys.

One difficult point previously hampering scientists to understand these nanoparticles is that physicists and metallurgists are used to consider these particles as conventional bulk-phases. In this view, these hardening nanoparticles appeared so complex and difficult to thoroughly understand: different researchers claim different compositions, morphology appearances and atomic structures for these particles---in principle (from this view) there are infinite number of different nanoparticles.
However, we show that these nanoparticles are dynamic objects in the annealing process, somewhat like living objects (such as bamboo or threes): initiating with seeds, then aging in association with growing. A hardening nanoparticle undergoes various physical (compositional, structural and morphological) changes with age (annealing time), but genetically it remains the same object because of its stable identity--the Si2- pillar skeleton, which has been observed by us using atomic-resolution imaging (as shown in Figure 1). In this way, we provided not only a refined scenario but also a complete explanation for the hardening nanoparticles and their evolution.

Figure 1: Left Image: Atomic imaging with quantitative image simulation analysis on an early-stage hardening nanoprecipitate in ALMgSi alloys. The insets are simulated images for the Al matrix and for the particle. The state-of-the-art image reveals that pillar-like silicon-double-columns exist in the nanoprecipitate. Upon annealing, the Si2-pillars provide the skeleton for the nanoparticle to evolve in composition, structure and morphology. Right image: The 3D atomic model for the nanoparticles at Mg2Si3Al7, which are the most crucial particles responsible for the quick bake hardening response of the alloy.

Now we understand that the nanoparticles at their very young age (i.e., the nuclei) contain a lot of Al atoms and are well adapted in the Al-matrix. They have a little pinning force on the moving dislocations. Hence the alloy appears soft and has a good formability for automotive parts. In the next stage, the nanoparticles prefer to grow rapidly in length rather than to become much matured, leading to the rapid increase of total volume fraction of the more effective obstacles for the dislocation movement. Hence the alloy’s strength increases rapidly. The fast maturation occurs on when a new type of atomic orderings or structures appear in the lengthy needle-like nanoparticles. Our study shows that the initial particles at Mg2Si2Al7 and the next particles at Mg2+xSi2+yAl7-x-y are the key particles directly responsible for the QBHR of AlMgSi alloys. Appropriate pre-aging should nucleate a large number of Mg2Si2Al7 particles, but prevent their further evolution (or maturation). Two mechanisms coexist for quickly strengthening the alloys upon a second heating: (i) the quick evolution of the nuclei from Mg2Si2Al7 to Mg2+xSi2+yAl7-x-y and (ii) the one-dimensional growth of the particles.
The new understanding of the hardening nanoprecipitates will lead to better alloys in terms of industry thermal processes and properties.

Our study also indicates that the nanoparticle evolution will be understood in a more quantitative way in the future, if faster computers become available.