The actuator performance of high temperature shape memory alloys (HTSMAs) can be significantly influenced by viscoplastic mechanisms that appear in metals under high temperature conditions. In this work we investigate experimentally and theoretically the coexistence of phase transformation and viscoplastic behavior in HTSMAs. The experimental study shows that decrease in the thermal cycling rate causes decrease in the actuation strain, as additional irrecoverable strain is generated due to creep mechanisms. The interaction between viscoplasticity and transformation is studied further using TEM. In order to simulate the material response, we propose a constitutive model based on an existing shape memory alloy model, which aims to capture the simultaneous phase transformation and viscoplastic behavior of a HTSMA. After calibration, the proposed model is used in order to identify the impact of the loading rate in the materials response.