Molecular Dynamics Simulation Of Phase Transition And Mechanical Behavior Of NiTi Alloy Containing Nanocrystalline And Amorphous Phase

Nitinol(NiTi)shape memory alloys are widely used in engineering fields such as biomedicine and aerospace due to their excellent shape memory effect and superelasticity.It is generally desirable to improve their mechanical properties without destroying their overall shape memory effect.It is easy to achieve amorphization on the surface of NiTi shape memory alloy by surface severe plastic deformation technology,and subsequent heat treatment process can realize nanocrystallization of amorphous phase on the surface of NiTi shape memory alloy,which contributes to improving its comprehensive mechanical properties.At present,the microscopic mechanisms of the phase transition and mechanical behavior of this multilayer NiTi shape memory alloy with complex structures under external loading are not clear sufficiently.However,molecular dynamics simulations can reveal the evolution of the microstructure of NiTi shape memory alloys under external loading from a smaller scale.Therefore,it is of great scientific significance to study the phase transition and mechanical behavior of the above complex NiTi alloys by molecular dynamics simulation.In this thesis,a variety of NiTi shape memory alloys with complex structures are constructed with the help of molecular dynamics modeling software,and the phase transition and mechanical behavior of NiTi shape memory alloys during the stress-induced martensitic transformation under tensile loading are mainly studied.The main research results are as follows:The behaviors of temperature-induced and stress-induced phase transition of single crystal NiTi shape memory alloy are investigated based on molecular dynamics simulation.It is found that the temperature change rate has little effect on the transformation temperature,where the starting temperature of martensitic transformation is about 200 K,while the finishing temperature of austenitic transformation does not exceed 450 K,which provides a theoretical basis for determining the simulation temperature of phase transition and mechanical behavior NiTi alloy in the following chapters.If the cooling rate is too high,a multi-domain structure shall be formed.The stress-induced martensitic transformation of single crystal NiTi shape memory alloy does not show obvious strain rate dependence.The single crystal NiTi shape memory alloys under different strain rates all show local martensitic transformation instability.Molecular dynamics models of NiTi shape memory alloy with polymorphic structures including single crystal,amorphous phase and nanocrystalline grain are constructed,and the orientation dependence of martensitic transformation is mainly studied by molecular dynamics simulation.The results indicate that in the case of tensile loading,the[100]-oriented single crystal is in a non-preferential orientation and thus it is difficult to undergo stress-induced martensitic transformation,whereas the [110]-oriented single crystal is in a preferential orientation and martensitic transformation can occur under lower tensile stress.Furthermore,due to the synergistic effect of martensitic transformation between adjacent grains,the preferentially oriented grains can promote the martensitic transformation of their adjacent non-preferentially oriented grains.However,the amorphous phase inhibits the martensitic transformation and mechanically stabilizes the B19’ martensite phase.Molecular dynamics models of NiTi alloy subjected to surface amorphization are constructed.The stress-induced martensitic transformation and mechanical behavior of the surface-amorphized NiTi shape memory alloy are revealed based on molecular dynamics simulation.It is found that the presence of the amorphous phase shall inhibit the stress-induced martensitic transformation within some grains with larger size,which ultimately leads to a reduction in the number of martensitic variants.In addition,the presence of the amorphous phase can increase the tensile stress of NiTi shape memory alloys,but the plastic deformation of the amorphous phase leads to the degradation of its superelasticity.Molecular dynamics models of gradient-structured NiTi shape memory alloy with nanocrystalline grains on the surface are constructed and the stress-induced martensitic transformation and mechanical behavior of the gradient-structured NiTi shape memory alloy with nanocrystalline grains on the surface are revealed based on molecular dynamics simulation.The simulation results show that the existence of surface nanocrystals helps to enhance the tensile stress of NiTi shape memory alloy and the tensile stress increases with the decrease of nanocrystal size.It is found that the existence of grain boundary,phase boundary and antiphase domain boundary can cause superelastic degradation of NiTi alloy.