Investigation On Titanium Oxide Clusters And Nucleation Mechanism In Iron-based Alloys

Dispersed titanium oxides play an important role in the application of"Oxide Metallurgy"technology.The formed composite inclusions from titanium oxides can become the core of precipitation of intragranular ferrite during solidification of steels,is benefit for the grain refinement.The reasonable control of the size,quantity and type of titanium oxides can improve the structure of the steels.In order to further propose a reasonably controlled solution and guide industrial production in practice,it thus requires an accurate and profound understanding of the formation process of titanium oxide.In this dissertation,quenching technology was used to freeze the titanium oxide clusters in the molten Fe-Ti alloy,and the three-dimensional atom probe?3DAP?technique and first-principles simulation were then used to analyze the occurrence of titanium oxide in the alloy.By extracting the space coordinates of solute atoms,the structure of titanium oxide clusters was constructed.The number density and distribution of titanium oxide clusters were calculated,and the structure evolution of the clusters was uncovered.The unit-cluster-assisted nucleation mechanism of titanium oxide was proposed accordingly.Based on the experimental results,we have built the physical model of titanium oxide nucleated in liquid steel,investigated the collision growth path of the elementary clusters,and calculated the parameters such as the number density and nucleation rate of the nucleus,which further reveals the nucleation process of titanium oxide evolves from units to clusters.The distribution,the nearest neighbour relationship of solute atoms and the main forms of Ti and O elements in the quenched Fe-Ti alloys were studied by 3DAP.The existence of TiO units was verified for the first time in Fe-Ti alloys.It was also found that O atoms aggregated in the same region with TiO clusters,implying that it has an interattraction relationship between free O atoms and TiO.The distribution analysis of the nearest neighbor between the solute atoms confirms the tendency of aggregation between TiO and TiO,O and O in Fe-Ti alloy.It was found that the structure of small TiO clusters in the non-TiO aggregated region has a certain heritability such as long chain and three-dimensional.The first-principles calculation also confirmed the existence of the TiO unit.The calculated cluster structure is in good agreement with that obtained from experimental results.It is deduced that the structure tends to maintain Ti/O=1 when the small cluster grows.It was confirmed that the number density of TiO clusters in the TiO accumulation region is mainly related to the O content.Based on the spatial distribution results of TiO clusters,we elucidate that the nucleation process of titanium oxides follows the way of cluster aggregation.The average gyration radius of TiO clusters was calculated,and the clusters containing 2-10 units were determined to be small clusters.It is also confirmed that large clusters have structural fluctuations.Based on the experimental results,a cluster-assisted nucleation mechanism is proposed,i.e.,TiO units are gradually aggregated to form TiO clusters,and O atoms are subsequently aggregated into TiO clusters and continuously moved in,and Ti atoms are continuously aggregated and discharged and,ultimately,the nucleation process of titanium oxides is completed.The physiccal model of cluster collision growth was established and the collision growth path from TiO units to clusters was derived.The evolution of nucleus number density,nucleation rate,transfer mode of nucleation units and the count of clusters with time was studied.The nucleation process can be divided into three stages.Stage I?0<t<1 ns?is the incubation period,during which a large amount of TiO units is gradually transferred from the free units to the small clusters by the aggregation collision and the nucleation rate is close to 0.Stage II?1 ns<t<100 ns?is the productive period,during which the large clusters are rapidly formed and the formation rate first rises and then falls.Stage III?100 ns<t?is the stable period,during which the nucleation number density and nucleation rate no longer change,and the larger the large cluster,the lower the number density.With different oxygen contents,the overall trend of nucleation number density and nucleation rate with time in each system is basically the same.The higher the oxygen content,the earlier the nucleation number density and nucleation rate starts to grow rapidly,the faster the growing rate,the higher the peak value of the nucleation rate,and the higher the nucleation number density during the stable period.When the oxygen content exceeds 5×10^-3 wt.%,the benefit on increasing the nucleation rate by increasing the oxygen content is no longer significant.It is recommended that the oxygenation interval does not exceed 5×10^-3 wt.%,and the nucleation rate can be achieved the peak value of 6.07×10²⁹·s^-1·m^-3.The above experiments and theoretical studies have confirmed the evolution law of the number density,size,and distribution of microstructures in the nucleation stage of titanium oxide,deduced the evolution path of titanium oxide clusters in the pre-nucleation stage,and calculated the effect of oxygen content on the nucleation rate of titanium oxides.The proposed two-step nucleation mechanism with units and clusters provides theoretical guidance for the rational control of the formation and quantity of oxides in practice.