Molecular Dynamics Investigations On The Mechanism Of Stress Induced FCC-BCC Phase Transformation Of Face-centered Cubic Metal Crystals

Stress-induced phase transformation is a widely existing deformation mechanism in metal structural materials,which has an important influence on the mechanical and physical properties of metal materials.Based on molecular dynamics simulation and theoretical analysis,the FCC-BCC phase transformation mechanism of face-centered cubic metals and L1₂intermetallic compounds was studied.This paper explores the factors influencing the FCC-BCC phase transformation and explains the causes theoretically.The main research contents include the following three parts:(1)The deformation mechanism of Ni,Cu,Au and Ag crystals under uniaxial tension along[100]direction under different strain rates and temperatures was studied.Under high strain rate and low temperature,and beyond the elastic limit,the bifurcation of the FCC phase occurred with sudden contraction along one lateral direction and expansion along the other lateral direction.When the expansion lateral lattice constant gradually converges to the lattice constant in the stretching direction,the FCC phase transformed into an unstressed BCC phase.At low strain rate,the plastic deformation mechanism of crystal becomes dislocation nucleation and slip,while at high strain rate and high temperature,the amorphous structure is formed.We analyze the elastic stability by simulating the change of elastic constants C₂₂and C₂₃in the process of stretching and theoretically analyze the occurrence of phase transformation.(2)The deformation mechanism of Cu nanoplate under uniaxial tension along[100]direction at different strain rates and temperatures was studied.In the state of high strain rate,the negative lateral Poisson's ratio deformation of Cu nanoplate is the same as above when the elastic limit is exceeded,which eventually leads to the appearance of FCC-BCC phase transformation.The difference is that the lateral contraction an expansion process of the nanoplate is gentle rather than sudden,which caused by the free surface of the nanoplate generates surface stress and induces internal compressive stress.The direction of the lateral expansion of the crystal during the FCC-BCC phase transformation can be artificially controlled by using the influence of the surface stress.The free surface also improves the elastic stability criterion of the nanoplate.Since the independent components of the elastic stiffness matrix are changed from 6 to 9,the stability criterion D₃=0 is more accurate for the nanoplate structure.(3)The deformation mechanism of Ni₃Al nanowire under uniaxial stretching along[100]crystal was studied.L1₂-D0₃-D0₁₉(FCC-BCC-HCP)phase transformation occurs when the Ni₃Al nanowires with diameters of 2nm and 4nm are stretched at room temperature at a strain rate of1.0×1 0⁸s^-1.The phase transformation occurs locally and slips with the phase boundary of{100}until the complete phase transformation.With the increase of the model diameter,the whole nanowire underwent a phase transformation process at the same time,while the nanowire with a diameter greater than 8nm became a dislocation mechanism when stretched under the same conditions.The L1₂-D0₃-D0₁₉phase transformation continuity occurs when the model with a diameter of 2nm is stretched at low temperature.The phase transformation occurs in stages and the fracture strain decreases after the temperature increases.Unloading the nanowires with D0₃structure can make them return to the original L1₂structure,indicating that the L1₂-D0₃phase transformation exist pseudo-elasticity.While the nanowire with D0₁₉structure cannot return to the original state after unloading.The reason is that the energy of the structure decreases after the D0₃-D0₁₉phase transformation,resulting in internal defects and plastic deformation.