Effect of the Zener-Hollomon parameter on the microstructure evolution of dual phase TWIP steel subjected to friction stir processing

The present work is devoted to investigate the correlation between the Zener-Hollomon parameter and the grain structure of the thermo-mechanically processed dual phase twinning induced plasticity (TWIP) steel utilizing the friction stir processing (FSP) technique. To this end 3 mm thick workpieces were subjected to FSP under rotational speeds of 800-2500 rpm and constant traveling speed of 50 mm/min. Additionally, isothermal hot compression tests were conducted at temperatures in the range of 800-1100 °C under the strain rates of 0.001-0.1 s−1. Employing the flow stress data acquired from compression tests, the precise value of deformation activation energy (Q) was determined through Arrhenius-type constitutive model. The results indicate that increasing the rotational speed from 800 to 2500 rpm has led to Z-value variation between 1.04×1020 and 0.03×1020 s−1. However, the scanning electron microscopy (SEM) characterization shows that the grain size reaches a certain minimum value at 1600 rpm. Three different models have been established to interpret the correlation between Z-value and the size of FSP-induced grains in the case of the experimental TWIP steel.