Atomistic insights into sluggish crystal growth in an undercooled CoNiCrFe multi-principal element alloy

Crystal growth in undercooled CoNi, CoNiCr and CoNiCrFe equi-atomic alloys was investigated by molecular-dynamics simulation to show atomistic insights into sluggish crystal growth in the undercooled CoNiCrFe multi-principal element alloy (MPEA). The simulation results showed that the growth velocity decreases gradually and the crystallization kinetics became sluggish as the increasing number of principal elements, being qualitatively consistent with the experiments. Independent of the number of principal elements, the activation controlled crystal growth mechanism always played a dominative role. The sluggish crystal growth phenomenon in the undercooled CoNiCrFe MPEA was found to be due to the increased growth activation energy and the decreased ultimate growth velocity. By converting the atomic displacement arose at the process of crystallization into timescales, the contribution of each element to the growth activation energy was quantified for the first time. Consequently, the important role of the Cr element in increasing the growth activation energy was revealed, being qualitatively consistent with experimental findings that the adding amount of Cr element played a dominate role in sluggish crystal growth. An implementation of the time dependent Ginzburg-Landau theory for solidification kinetics indicated that the weakened density relaxation capacity of the melt led to a significant decrease of the ultimate growth velocity as the increasing number of principal elements.