Study On The Nucleation And Growth Mechanism Of Martensite

Martensitic transformation is one of the important phase transformations, through which the properties of materials can be improved, such as the strengthening of steel and the research and development of shape memory alloys. In this paper , we observed martensite and surface relief by direct quenching, austempering and vacuum heat treatment with low carbon steel, medium carbon steel and high carbon steel and Cu-11.42Al-0.35Be-0.18B alloy. Investigated by Axiovert-25CA optical microscope, QUANTA-400 environmental scanning electron microscope, Nano frist-1000 scanning tunneling microscope and JEM-2100 transmission electron microscopy and so on.The nucleation site and morphology of marteensite phase transformation was observed, the results show that the martensite nucleates can not only at austenite intracrystal, but also at austenite crystal grain boundary and phase interface. It is considered that martensitic transformation is not shear transformation, but a crystal lattice reconstruction process from austenite to martensite at the defects which depending on structure fluctuation and energy fluctuation to nucleate.The martensitic morphology configuration in Cu-11.42Al-0.35Be-0.18B shape memory alloy are very complex, the main configuration present spear-like shape, parallel shape and bamboo shape, and the substructure are mainly stacking fault, dislocation and twin.The morphology of martensite in low carbon steel, medium carbon steel and high carbon steel have been observed in this paper, The results show that the two-dimensional of martensite is of lath-, butterfly-, needle or convex lens-type, while the corresponding three-dimensional is of wide-slice-, long-slice- or flat-needle-type. The volume strain energy is incentive for the evolution of martensite morphology, its value depends on different specific volumes between new and old phases, mismatch, elastic modulus and f ( a/b). The nucleation and growth of martensite in accordance with energy-saving principle(that is the least energy consumption). When both value of (a/b) and strain energy are lower, martensitic morphology show lath-type, otherwise, martensite will evolve into long-slice-type or flat-needle-type. sub-structure of low carbon lath-shape martensite is high density dislocation and stacking fault. sub-structure of medium and high carbon martensite is high density dislocation and a large amount of twin crystal. The surface relief of 2Cr13 martensitic plate's is the type of tent(Λ).Its'height between 43nm and 96nm. surface relief of martensite is not caused by shear transformation, but caused by volume expansion.The law of energy change during the formation of martensite crystal nucleus was studied, The calculating equation of the critical dimension and nucleation energy of martensite crystal nucleus was derived, and the critical dimension and nucleation energy of martensite crystal nucleus was calculated. The results show that the critical nucleus size of martensite is about 7²0nm and the nucleation energy is about 200⁶00J·mol^-1. The calculation of critical nucleus size and nucleation energy are reasonable in theory and are in consistent with the general rule of nucleation of phase transformation.Finally, put forward a new mechanism of martensitic transformation. That is the nucleation and growth of martensite is the thermal activation transition displacement of atomic collection, collaboration and non-diffusion.