ICAMS / Interdisciplinary Centre for Advanced Materials Simulation

Evolution of strengthening phases under in-service stresses and temperatures: phase-field and experimental study

Coherent/semi-coherent precipitates obtained per aging are important strengthener of light and tough materials such as Al-alloys, involving phase transformation and change of internal stress and local composition states. While the response of these materials to the external load at service temperatures crucially depends on the fraction, composition and local distribution of the precipitates, in return, the external/internal stresses strongly influence stability, arrangement and growth of the strengthening phases.

Recently we have reported on a coupling phenomenon between elastic constants and concentration around a spherical precipitate which change the concentration profile and stabilize it [1]. This has been applied for precipitation of Ni4Ti3 in shape memory alloys [2]. This effect can be generalized for many precipitates in real materials. We have also investigated mechanically driven motion of vacancies which make nanograin growth possible [3].


Left) Stress-stabilized concentration gradient around a precipitate (prefactor is set to 1). Right) A schematic phase diagram showing concentration shift.

In this project we continue our experience with regards to the coupling and address formation, growth and ripening of the strengthening precipitates and micro-voids during standard aging, aging under load and continued loading (creep) with a specific consideration on the mutual cooperation between external load and internal self-stress to the local chemical composition and vacancy concentration. To express this coupling, a thermodynamically consistent phase-field model will be constructed and will be combined with experimental validations. Al - 4%Cu - 1%Li - 0.25%Mn alloy (wt. %) is chosen as an applied model-alloy with two important classes of precipitates to mimic real multi-component Al-alloys. One important aspect to be addressed is the dynamics of non-equilibrium vacancies on the kinetics of precipitation and stress-driven void formation as it happens under creep conditions. The model development and simulation results will be validated by high-temperature creep experiments and subsequent microstructural characterizations involving SEM, HRTEM, and GPA.

References:
[1] R. Darvishi Kamachali, E. Borukhovich, O. Shchyglo and I. Steinbach; Solutal gradients in strained equilibrium; Phil Mag Lett 93 (2013) 680.
[2] R. Darvishi Kamachali, E. Borukhovich, N. Hatcher and I. Steinbach; DFT-supported phase-field study on the effect of mechanically driven fluxes in Ni4Ti3 precipitation; J Model Sim in Mater Sci Eng (MSMSE) 22 (2013) 034003.
[3] R. Darvishi Kamachali, J. Hua, A. Hartmaier and I. Steinbach; Multiscale simulations on the grain growth process in nanostructured materials; Int J Mater Res 11 (2010) 1332.

Project Files:

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The schematic algorithm of chemo-mechanical coupling within solid state materials to be considered in mesoscale phase-field model.
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Left) Stress-stabilized concentration gradient around a precipitate (prefactor is set to 1). Right) A schematic phase diagram showing concentration shift.
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Mechanical relaxation via vacancies makes grain growth possible on the nanoscale.

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