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PROJECTS/PROGRAMS

Advanced Materials Design of Structural Materials

Summary

An integrated computational materials engineering (ICME) approach is employed to link experimental and computational models across a variety of time and length scales to design new structural materials and optimize existing materials. Experimental and computational thermodynamics and kinetics are used to identify and model key processing-structure-property relations for specific material systems. The models and data developed for the key processing-structure-property relations form the basis for the alloy design methodologies and will enable industry to tailor materials to specific application property objectives. The current project focus is on the development of high temperature Co-based superalloys, the development of new materials specifically designed AM processing, and the optimization of currently used AM materials.

Description

Designing New High Temperature Co Superalloys

In collaboration with the NIST CHiMaD center, an ICME approach in being used to develop new Co superalloys that are strengthened using an ordered FCC (L1₂) phase (similar to the related Ni-based superalloys). The design goals for these alloys include:

Increased homologous operating temperature (> 50 degrees higher that current Ni-based superalloys), which will increase the turbine engine efficiency and thus decrease fuel consumption and emissions.
Increased wear resistance, which will increase the service life of the engine and lower operational costs.

The current NIST effort is focused on developing the required CALPHAD-based thermodynamic, diffusion, and molar volume databases to enable this design. The database development is focused on the Co-Al-W-Ni-Cr-Ti-Ta-V-Re.

CALPHAD re-assessment of the Co-Al-W Isothermal Section at 900 C

Optimization of Additive Manufacturing Processes and Alloys

AM Ti alloy heat affected zone — CALPHAD predicted melt pool and heat affected zone with the β→α phase transformation at laser speeds: (a) 400 mm/s, (b) 700 mm/s, (c) 1000 mm/s

Metal additive manufacturing has opened new design spaces as the potential now exists to build parts with location-specific properties. However, the rapid solidfication followed by multiple heating and cooling cycles during the build process can produced unexpected phases and composition variations. Post-build thermal processing can be optimized to ensure that the desired properties are achieved. For example, precipitation simulations can be used to determine how to avoid the precipitation of detrimental phases during stress-relieving processing. New alloys are being designed specifically for the additive manufacturing that have smaller freezing range to reduce microsegregation and residual stresses. Current alloy systems of interest include martensitic stainless steels (17-4 PH), Ni-base superalloys (IN625 and IN718) and Ti alloys.

Modeling Creep in Superalloys

A crystal plasticity finite element framework is utilized along with morphology and composition-dependent constitutive models to predict the mechanical responses in superalloys, including stress-strain curves and creep behavior. To include morphology, a multi-scale scheme is employed to acquire a morphology dependent constitutive model. The lack of adequate energy associated with dislocations to cut through the precipitates necessitates the further dislocation movements through climbing the precipitates and gliding across the channel. Hence, a glide and climb constitutive model is applied to capture the creep properties of superalloys.