Indiana University-Purdue University Indianapolis (IUPUI) / High-temperature alloys have been extensively used in many applications, such as furnace muffles, fuel nozzles, heat-treating fixtures, and fuel nozzles. Due to such conditions, these materials should have resistance to cyclic loading, oxidation, and high heat. Although there are numerous prior experimental and theoretical studies, there is insufficient understanding of application of the unified viscoplasticity theory to finite element software for fatigue life prediction. Therefore, the goal of this research is to develop a procedure to implement unified viscoplasticity theory in finite element (FE) model to model the complex material deformation pertaining to thermomechanical load and implement an incremental damage lifetime rule to predict the thermomechanical fatigue life of high-temperature alloys. The objectives of the thesis are: 1. Develop a simplified integrated approach to model the fatigue creep deformation under the framework of ‘unified viscoplasticity theory’ 2. Implement a physics - based crack growth damage model into the framework 3. Predict the deformation using the unified viscoplastic material model for ferritic cast iron (Fe-3.2C-4.0Si-0.6Mo) SiMo4.06 4. Predict the isothermal low cycle fatigue (LCF) and LCF-Creep life using the damage model In this work, a unified viscoplastic material model is applied in a FE model with a combination of Chaboche non-linear kinematic hardening, Perzyna rate model, and static recovery model to model rate-dependent plasticity, stress relaxation, and creep-fatigue interaction. Also, an incremental damage rule has been successfully implemented in a FE model. The calibrated viscoplastic model is able to correlate deformations pertaining to isothermal LCF, LCF-Creep, and thermal-mechanical fatigue (TMF) experimental deformations. The life predictions from the FE model have been fairly good at room temperature (20°C), 400°C, and 550°C under Isothermal LCF (0.00001/s and 0.003/s) and LCF-Creep tests. The material calibration techniques proposed for calibrating the model parameters resulted in a fairly good correlation of FE model derived hysteresis loops with experimental hysteresis, pertaining to Isothermal LCF (ranging from 0.00001/s to 0.003/s), Isothermal LCF-Creep tests (withhold time) and TMF responses. In summary, the method and models developed in this work are capable of simulating material deformation dependency on temperature, strain rates, hold time, therefore, they are capable of modeling creep-stress relaxation and fatigue interaction in high-temperature alloy design.