Austenite Dendrites Formation
Austenite dendrites form when a material undergoes phase transformation from solid austenite to liquid phase. The phenomenon is similar to crystal growth and dendritic growth in metals and alloys, in which successive crystal growth results in dendritic-shaped features. Due to the higher thermoelastic requirements and lower superheat for austenite transformation, a special nucleation process known as austenite dendrites may occur. Austenite dendrite formation takes place at the boundary between the martensite and austenite phases during heat treatment, and is an important factor influencing the mechanical properties of a material.
Austenite dendrite formation is characterized by the growth of successive titanium or iron-carbon austenite dendrites. The austenite dendrites form when the solute atoms at the interface between the austenite and martensite phases are flocculated by the diffusion of atoms or diffusion by stress-induced movement. This process reduces the strain energy at the interface, resulting in nucleation and growth of austenite dendrites. Furthermore, during austenite transformation, the martensite phase is separated from the austenite phase due to the corrosion of its interfaces, and these martensite fragments are incorporated into the austenite grains, producing a dendritic structure.
The austenite dendrites form in the presence of a predetermined amount of superheat. This determines the size, shape and number of austenite dendrites formed. As the amount of superheat increases, the number of dendritic arms increase and their size become bigger. The shape of the austenite dendrite is also determined by the grain size, crystal orientation and the type of microstructures present during the solidification process.
Austenite dendrite formation affects the mechanical properties of a material. Materials with a higher rate of austenite transformation possess enhanced strength, toughness and abrasive wear resistance compared to those with a lower rate. Furthermore, austenite dendrites can lead to heterogeneous grain structure and inhomogeneous material distribution, making it difficult to cast the material into desired shapes and achieve the desired properties.
In summary, austenite dendrite formation occurs when a material undergoes a solid-state phase transformation from austenite to liquid phase. The process is marked by the flocculation of atoms during martensite-austenite interface diffusion and the subsequent corrosion of martensite. The amount of superheat determines the size, shape and number of austenite dendrites formed. The mechanism has an important role in determining the mechanical properties of a material and can make it difficult to achieve homogeneous properties and casting shapes.
