Precipitation Behavior Of Cu-rich Nanoscale Phase And Strengthening Mechanism In High Manganese Austenite

The high manganese austenitic steel is a single phase austenite with Fcc structure.When plastic deformation occurs,the deformation-induced transformation or deformation-induced twinning mechanism brings uniform and continuous work-hardening ability and excellent plasticity and toughness.At present,it has great application potential in automobile body steel(room temperature plasticity and toughness),LNG transport tank steel(ultra-low temperature toughness),high magnetic field structural steel(non-magnetic)and so on.However,the application of high manganese austenitic steel in many fields is limited because of its low yield.Improving yield strength at room temperature without affecting alloy's ductility is still an urgent problem to be solved.In this paper,Cu is added to the high manganese austenitic steel.The Cu element dissolved in the matrix is used to adjust stacking fault energy and control deformation mechanism.Using the same crystal structure and similar lattice constant of copper and matrix,coherent Cu-rich nanoscale phase is precipitated in austenite,resulting in precipitation strengthening.It can improve the yield strength without affecting the steel's ductility.In order to study the precipitation of Cu-rich nanoscale phase in high manganese austenitic steel and the effect of Cu on the deformation mechanism of high manganese austenitic steel,two composition systems are designed: one is the Fe-25Mn-Al-Ni-Cu system with high stacking fault energy(SFE),and the precipitation strengthening mechanism of Cu-rich nanoscale phase is studied;The other is the Fe-18Mn-C-Cu system with low stacking fault energy,which can effectively change the deformation mechanism of matrix by adjusting the concentration of Cu,and study the effect of nano-twins on the precipitation of Cu-rich nanoscale phase.The experimental heat treatment process parameters were designed by using Jmat Pro software combined with literature investigation.The Olson-Cohen thermodynamic model is used to calculate the SFE of high manganese austenitic steel.The microstructure of high manganese austenitic steel was analyzed by OM,SEM,XRD,STEM and other means of microstructure characterization.The mechanical behavior of high manganese austenitic steel was analyzed by means of micro-hardness tester and tensile testing machine.The deformation mechanism of high manganese austenitic steel with high stacking fault energy is mainly dislocation slip,supplemented by deformation twinning,while the deformation mechanism of high manganese austenitic steel with low stacking fault energy in the sensitive area of deformation mechanism is changed from TRIP to TWIP by adding Cu element.It is found that Cu-rich nanoscale phase is a kind of copper cluster in the early stage of growth.It is spherical in nano size and dispersed in the matrix.It has a good relationship with the matrix interface and has a certain thermal stability.However,when Cu-rich nanoscale phase coarsens,its morphology becomes a sheet structure growing along the {110} crystal plane,with thickness of several nanometers,length and width of tens or even hundreds of nanometers.The change of Cu-rich nanoscale phase can change the strengthening mechanism from cutting to bypassing,and the strengthening effect can be greatly weakened.In addition,the shape of Cu-rich nanoscale phase can affect the work hardening behavior.The spherical Cu-rich nanoscale phase can improve the work hardening rate.The sheet Cu-rich nanoscale phase can reduce the work hardening rate and is bad for plasticity.Therefore,through the reasonable control of aging process,the spherical Cu-rich nanoscale phase with nanometer size and dispersion distribution can be obtained,which can improve the yield strength without losing the plasticity.In order to analyze the influence of different matrix on the precipitation behavior of Cu-rich nanoscale phase,high density nano twin was introduced into the high manganese austenitic steel with low stacking fault energy by cold rolling deformation.The thickness of nano twin and matrix layer were mainly distributed within 20 nm,and there were a few nano twin or matrix layer with the thickness of more than 40 nm.The experimental results show that the nucleation of Cu-rich nanoscale phase is affected by defects in the matrix.In the coarse crystal,it is found that the precipitation of Cu-rich nanoscale phase may depend on the vacancy nucleation,and there is "precipitation free zone".In the high manganese austenitic steel with nano twin structure,the nucleation takes priority in the high energy coherent twin boundary,and there is a strip like Cu-rich nanosacle phase.