PhD Dissertation Final Oral Defense (September 2024)

Title of dissertation: Microstructures-Based Constitutive Modelling for Dynamic Strain Aging in Metal Alloys
Name of Candidate: Arhum Hassan
Name of supervisor: Farid Abed
Program : PhD in Materials Science and Engineering

Abstract:
Dynamic Strain Aging (DSA) is encountered in some metals at certain combinations of temperatures and strain rates. DSA phenomenon arises due to interactions between mobile dislocations and solute atoms that diffuse around them. This dissertation aims to deepen the understanding of dynamic strain aging in different metals and alloys through a comprehensive approach that incorporates microstructure-based constitutive modeling and finite element (FE) analysis.

The constitutive model developed in this study incorporates the effect of increased waiting time that activates DSA. This is achieved by introducing diffusion parameters into the model to formulate an expression that predicts the temperature ranges for the activation of DSA at different strain rates. The framework is based on material-specific activation energy for diffusion and the diffusion constants of impurity/solute atoms, with the increase in strength associated with the concentration of these atoms. The model is validated through comparison with experimental results over a broad temperature range for pure metals and alloys such Niobium, Vanadium and Titanium, C45 steel alloy and MMFX high strength steel at different strain rates (0.0015/s – 0.15/s) and over a wide range of temperatures (298K – 923K).

The constitutive model was implemented in the commercially available FE software ABAQUS using a user-defined material subroutine coded as VUMAT. The simulation results were validated using available experimental results, both in the presence and absence of DSA, over a broad range of temperatures (200K – 900K) and strain rates (0.001/s – 2200/s) for all the metals and alloys discussed. Moreover, the FE simulations presented in this dissertation indicate that necking initiates earlier in the presence of DSA. The underlying reason is the increased viscosity due to the increased waiting time, which reduces the material’s ability to deform plastically, leading to early failure. Overall, the developed models contribute to a comprehensive understanding of the metallic response under thermomechanical loading scenarios, providing valuable insights into the behavior of metals and alloys subject to DSA.

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