bhadeshia123 | Steels: mechanism of the bainite transformation. Lecture 3 of 12 @bhadeshia123 | Uploaded September 2020 | Updated October 2024, 5 hours ago.
My focus in this lecture is purely on some choreography of atoms during the formation of bainite in steels, so that a well-founded understanding of mechanisms can lead to ever greater advances in the discovery of new steels. The composite structure that is known colloquially as bainite is arguably the most interesting of all of the essential microstructures that occur in steels, where the manner in which atoms move is seminal to the design of steels.
The accumulated evidence proves that bainite is nothing but martensite that may be tempered immediately after transformation. There are differences because the driving force available for bainite is smaller than for martensite. So although the nucleation mechanism for bainite, like martensite, involves the dissociation of dislocations, unlike martensite, it is required that carbon partitions during those early stages of genesis. The nucleus then evolves into diffusionless growth. Secondly, the combination of the low driving force and weakness of the austenite at the temperatures where bainite typically forms, induces mechanical stabilisation that leads to a dramatic refinement of the structure. As a result, the platelets of bainite can be much finer than those of martensite.
Associated teaching materials can be found on:
phase-trans.msm.cam.ac.uk/teaching.html
H. K. D. H. Bhadeshia
My focus in this lecture is purely on some choreography of atoms during the formation of bainite in steels, so that a well-founded understanding of mechanisms can lead to ever greater advances in the discovery of new steels. The composite structure that is known colloquially as bainite is arguably the most interesting of all of the essential microstructures that occur in steels, where the manner in which atoms move is seminal to the design of steels.
The accumulated evidence proves that bainite is nothing but martensite that may be tempered immediately after transformation. There are differences because the driving force available for bainite is smaller than for martensite. So although the nucleation mechanism for bainite, like martensite, involves the dissociation of dislocations, unlike martensite, it is required that carbon partitions during those early stages of genesis. The nucleus then evolves into diffusionless growth. Secondly, the combination of the low driving force and weakness of the austenite at the temperatures where bainite typically forms, induces mechanical stabilisation that leads to a dramatic refinement of the structure. As a result, the platelets of bainite can be much finer than those of martensite.
Associated teaching materials can be found on:
phase-trans.msm.cam.ac.uk/teaching.html
H. K. D. H. Bhadeshia
