The Self-Feedback Mechanism Between Transformation Twinning Elastic Field And Martensitic Configuration

Martenstic transformation has been studied for nearly one hundred years.It has been widely applied in engineering fields.The excellent performance of CuAlNi shape memory alloys are based on the thermalelastic martensitic twin microstructures.However,the twinning mechanism is not fully understood.Its further understanding will promote the application of martensitic transformation.This work is based on the idea that the change of atomic lattice in the martenstic transformation induces eigenstrain between austenite and martensite,leading to selfaccommodated elastic field.With the background of martensitic microstructure and selfadaptive phenomenon,finite element method is applied to calculate the elastic stress and strain fields for different configurations.It is aimed to unveil the relation between the elastic field caused by intrinsic strain and the twinning martensite configuration.Two configurations,the structures of austenite-twinning martensite and wedge twinning martensite are constructed,based on the data from literature work of Hane and Shield.Finite element analysis are then performed with Freefem++ using relevant weak forms of elasticity differential equations.For the austenite-twinning martensite configuration,the elastic field of the fixed boundary,free stress boundary and elastic boundary configurations are analyzed.Results show that the strain energy decreases with the formation of twin structures.And the distribution of eigenstrain has a great influence on the resultant elastic filed.A gradient distribution of eigenstrain through several layers will effectively reduce the system elastic energy.For the wedge twinning martensite,effect of volume fraction of wedge-twinning martensite configurations to the shear stress field is investigated,proved that martensite phase transformation expension along the way of habit plane.It is shown the maximum shear stress along the habit plane will first increase and then decrease with the increasing twin volume fraction.This proves that the growth of the martensite will be finally depressed due to its own elastic field.Also,variation of shear stress along habit plane shows its growth occurs along the plane itself.