LAUSR

Abstract ‘Abnormally’ large grains, whose sizes are greater than twice the critical radius ( 2 R c ), are known to alter the isotropic grain growth phenomena. In the present work, phase-field simulations of 2-dimensional microstructures are extensively analysed to elucidate the deviations from the normal grain growth introduced by the presence of abnormal grains. Polycrystalline microstructures that are ’artificially’ made to resemble physical structures, by governing the distribution and the sizes of the abnormal grains, is employed to analyse the grain growth in the presence of large grains. This study unravels that the abnormal grains induce a period of transition during which its grain size distribution is shifted and confined within 2 R c , indicating a complete disappearance of the abnormality in the microstructure. Furthermore, it is identified that this transition period establishes a bimodal distribution, which subsequently evolves into a unimodal time-invariant distribution. This behaviour noticeably reveals the misconception that the disappearance of the abnormality signifies the onset of normal grain growth. Moreover, despite the apparent disappearance of the abnormal grains, a continued increase in the volume-fraction of these ‘pre-existing’ abnormal grains is recognized, and in the steady-state condition, it is observed that the microstructure predominantly consists of these pre-existing abnormal grains. Influence of the factors like initial volume-fraction of the abnormal grains F o and degree of abnormality U ‾ max on the duration of the transient period is quantified by investigating close to hundred microstructures with unique F o and U ‾ max .