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Phase transformations
Published in Gregory N. Haidemenopoulos, Physical Metallurgy, 2018
In the previous section it was stated that nucleation proceeds, in most cases, heterogeneously. Consider the following examples. The first example is concerned with the precipitation of Mo2C carbides during tempering of a Fe − 1C − 4Mo steel. A TEM picture of the carbide dispersion is shown in Figure 6.6a. This fine precipitation is due to the heterogeneous nucleation of the carbides on dislocations. An atom probe field ion microscope (AP/FIM) image of the carbides is also shown in Figure 6.6b. The second example is concerned with the precipitation of phase β during artificial aging of aluminum alloy 6061 (Al − Mg − Si). The phase β is the intermetallic compound Mg2Si, which nucleates heterogeneously on dislocations. A dispersion of β rod-shaped particles is shown in Figure 6.7a. A higher magnification shows a Mg2Si particle surrounded by dislocations, depicting the role of dislocations in the nucleation of this phase, Finally, the third example is concerned with heterogeneous nucleation at grain boundaries. Nucleation of ferrite at the prior austenite grain boundaries during cooling of austenite in a Fe − 0.4C proeutectoid steel is depicted in Figure 6.8.
Redistribution of substitutional alloying elements between α-Fe matrix and cementite after long-term tempering in a low alloying steel
Published in Philosophical Magazine, 2022
Zhiquan Zhang, Bangxin Zhou, Jun-an Wang, Wenqing Liu
The earliest evidence of paraequilibrium precipitation of cementite during tempering was obtained by EDS analysis of carbides composition on extraction replicas [10,20,22]. This technique is only capable to measure the average cementite concentration. Later on, atom probe field ion microscopy (APFIM) was employed to characterise the composition of cementite in the tempered martensite [6,15,23–25], which could measure the specific concentration across α/θ. Recently, atom probe tomography (APT) has been utilised increasingly in the characterisation of precipitation as it has a huge advantage over conventional techniques in spatial and mass resolution [26–30]. All of the results acquired by the above three techniques have unanimously confirmed that the substitutional elements were distributed uniformly between the α-Fe matrix and θ phase in the early stage of tempering and that non-carbide forming element would gradually diffuse out of θ phase while carbide forming element would enrich into θ phase with increasing tempering intensity. Silicon was reported to be the first element to be redistributed, followed by the partitioning of carbide-forming elements as Si has higher diffusivity than that of carbide forming elements. Chang and Smith [6] observed a silicon-enriched layer around cementite in a 0.75 wt%C–1.4 wt%Si alloy steel tempered at 380°C for 1 h by atom probe field ion microscope in 1984, and they concluded that the silicon-enriched layer was the kinetic barrier to the further growth of the θ phase, and the diffusion of silicon away from the interface controlled the growth and coarsening processes. Since then, the formation of a non-carbide forming element enriched layer around the cementite was widely accepted as the mechanism by which non-carbide forming elements could retard the growth and coarsening of cementite. As for carbide forming elements, it was the enrichment of them in cementite that was thought to reduce the coarsening of cementite.