ICME Overview of predicting the Specific Strength of Aluminum-Lithium Alloys

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Background

Lowering the costs for both maintenance and operation in the next generation of aircrafts has become a significant deciding factor with many airline companies currently in the process of replacing their aging fleets. The challenge then falls to the aircraft manufactures to meet the demands of their airline customers. Cost reduction can be accomplished by decreasing the fuel consumption and damage tolerant maintenance required, while increasing the service life, load capacity, and range of the aircrafts. Reducing the overall weight of the aircraft by using less dense structural materials, without sacrificing strength, results in lower fuel consumption, a higher load capacity, and a larger potential range. Optimizing the material composition and applying beneficial processing techniques can generate higher damage tolerance, fatigue crack propagation resistance, and corrosion resistance, which results in a longer service life with less maintenance.

Multi-scale modeling diagram of determining an Al-Li Alloy's specific strength based on material composition and a downscaling parameter of plasticity and damage.

Aluminum-Lithium alloys are a lightweight, high performance aluminum alloy developed to compete with conventional aluminum alloys and polymer composites commonly used in aerospace applications today. Lithium is one of the few elements with a high solubility in aluminum. The addition of lithium to aluminum is special for a variety of reasons. For each 1% added, the density of the aluminum alloy is reduced by 3% and the elastic modulus is increased by 6%.[1][2] Not to mention that aluminum alloys containing lithium respond to age conditioning. Also, by adding lithium to aluminum the fatigue crack growth resistance increases and allows the formation of potent hardening precipitates.[1][2]

The purpose of the proposed project is to develop a model using multiscale analysis that can be used to accurately predict the specific strength of a given Al-Li alloy based on the composition. After the hierarchical multiscale methodology has been applied to specific strength, in subsequent projects, the same methodology can then be applied to other material properties significant for aerospace applications, such as fatigue life, fracture toughness, corrosion resistance, and damage tolerance. The goal is to eventually optimize the process for developing new and better aluminum-lithium alloys for aerospace applications.

The creation of a lightweight alloy including Aluminum and Lithium with the focus on specific strength has been investigated in the past.[3][4] In addition, there is existing research focusing on the structure-material property relationships of Al-Li alloys using both experimental and analytical methods[5][6][7], however most of these studies incorporate only one or two length scales in their analysis.

Plasticity-Damage Mechanisms

Stress-Strain curve showing ultimate strength in the context of plastic deformation and damage.

Specific Strength is defined as the ultimate tensile strength of a material normalized by its density. To get the most accurate ultimate tensile strength both damage and plasticity mechanisms must be taken into consideration and used as the downscaling parameters.

Plastic deformation is what happens after atomic bonds have broken. Sometimes those atomic bonds stay broken and sometimes they reform and rearrange themselves. When the atomic bonds stay broken, a new void/dislocation forms in the lattice structure of the material. When bonds reform, existing voids and dislocations shift while planes shear based on their crystallographic orientation. The plastic deformation region is when strain hardening occurs.

The ultimate strength, also known as the Ultimate Tensile Strength or just Tensile Strength, is the point when the energy absorption potential of a material peaks because the summation of the localized energy throughout the material cannot be dissipated by the moving of existing dislocations or the nucleation of new ones. Void growth mechanisms must also be considered when trying to determine the ultimate strength because some portions of the material become saturated with dislocations sooner than others, which will result in void coalescence and growth at those locations. From a localized perspective this is possible before the overall maximum work hardening point of the material and remote necking begins to occur.

Mesoscale

As described above, to completely model the crystal plasticity and damage of the aluminum lithium alloy the macroscale continuum needs the void growth, nucleation, and void-void interactions calculated using the crystal plasticity finite element model, ABAQUS FEM, in the mesoscale. A polycrystalline stress-strain plot will ultimately be generated using crystal plasticity.

Microscale

Macroscale needs dislocation motion and material density from this scale so, we need a microscale dislocation dynamics model, Micro-3D, to input the dislocation motion defined from lower scales to obtain the hardening rule of the Al-Li Alloy. The mesoscale needs the dislocation interactions and the hardening rules to define the overall crystal plasticity.

Nanoscale

Using the material composition, this scale must give the deformation kinematics of aluminum-lithium alloys so, we need to use crystal plasticity models to account for the slip along the FCC crystal structure close-packed planes, as well as the stack fault energies. Atom mobility, interfacial energy, and elastic moduli are the inputs of this scale.

Atomistics Scale

Atomistic calculations (MD) using the Modified Embedded Atom Method (MEAM) can give us mobility for the microscale, besides atomistic calculations can give high rate mechanism to component scale. It is crucial that the interaction potentials used in the MD and MEAM simulations be assessed and calibrated to accurately describe this system.

Electronic scale

At this scale by using DFT we can calculate cohesive energy, elastic moduli, and general stack fault energy curves for aluminum-lithium alloys. The elastic moduli will be passed directly up to the macroscale continuum and the cohesive energies and stack fault energies will be used to further define the mobility in the Atomistics Scale.

References

  1. 2.0 2.1 T. Dursun, C. Soutis. Recent developments in advanced aircraft aluminum alloys. Materials and Design 2014;56:862-871
  2. R. Rioja. Fabrication methods to manufacture isotropic Al-Li alloys and products for space and aerospace applications. Materials Science and Engineering 1998;A257:100-107
  3. G. Park, J. Kim, H. Park, Y. Kim, H. Jeong, N. Lee, Y. Seo, J. Suh, H. Son, W. Wang, J. Park, K. Kim. Development of lightweight Mg-Li-Al alloys with high specific strength. Journal of Alloys and Compounds 2016;680:116-120
  4. N. Behnood, J. Evans. Plastic deformation and the flow of stress of aluminum-lithium alloys. Acta Metallurgica 1989;37:687-695
  5. P. Khanikar, Y. Liu, M. Zikry. Experimental and computational investigation of the dynamic behavior of Al-Cu-Li alloys. Materials Science & Engineering A 2014;604:67-77
  6. J. Fragomeni. An iterative approach to determine composition and heat treatment from the mechanical yield strength of an aluminum-lithium alloy. Conference: Intelligent Processing and Manufacturing of Materials 1999;1:577-583
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