Investigation On Stress And Strain Partitioning And Micro-Deformation Mechanism In Duplex Stainless Steels

A good matching between strength and plasticity is one of the most important goals for structural material study.Duplex stainless steel composed of austenite and ferrite has a good matching between strength and plasticity.Reasonable stress and strain partitioning between two phases during deformation is one of the main reasons why duplex stainless steel has good mechanical properties.Although a great deal of research has been done in this field,the understanding of stress and strain partitioning between the two phases in duplex stainless steel is still insufficient.In this study,by means of synchrotron-based high-energy X-ray diffraction,micro X-ray diffraction,scanning electron microscopy,and microscopic digital image correlation method,the stress and strain partitioning behavior of the constituent phases in duplex stainless steel is in situ observed at multiple points and scales.On this basis,the deformation mechanism of duplex stainless steel was further systematically analyzed by combining electron backscattering diffraction and transmission electron microscopy.The main research contents and conclusions of the thesis are summarized as follows:1.A multi-scale research method combining synchrotron-based high energy X-ray radiation,microscopic digital image correlation method,electron backscattering diffraction,and transmission electron microscopy is established.Using this method,the non-uniform deformation behavior between austenite and ferrite in 2205 duplex stainless steel was analyzed at multiple scales.With the sample continuous straining,it is found that austenite yields first,while ferrite still keeps elastic.At this stage stress and strain partitioning occurs between these two phases.When both austenite and ferrite yield,due to the small hardness difference between them,the strain in austenite can be transferred into the adjacent ferrite grains through the interface.Additionally,the deformation in austenite grains is more uniform than that in ferrite grains due to the lower stacking fault energy in austenite.Moreover,the pile-up of geometrically necessary dislocation in austenite grains leads to the back stress in austenite grains and forward stress in ferrite grains,respectively,which intensifies the interaction between the constituent phases.2.The dynamic stress and strain partitioning between austenite and ferrite in 2205 duplex stainless steel and their effects on the mechanical properties of the material were studied by in-situ micro X-ray diffraction,dynamic ultra-microhardness tests,and scanning electron microscopy.It is found that austenite is the initial soft phase and bears larger strain at the initial stage of deformation;Meanwhile,comparing with ferrite,austenite has lower stacking fault energy and therefore higher work hardening ability during deformation,which leads to the more obvious work hardening behavior.As a result,the microhardness of austenite will surpass ferrite and become the harder phase,which will increase the strain borne by ferrite in the subsequent deformation process.In this study,the change of stress and strain partitioning caused by the two-phase microhardness conversion is called dynamic stress and strain partitioning.Dynamic stress and strain partitioning can lead ferrite to experience more strain in the later stage of deformation,and therefore to improve the work hardening rate of ferrite and the whole material,and finally improve the matching between strength and plasticity of the material.3.The interaction between dislocation and interface in 2205 duplex stainless steel was studied by scanning electron microscopy,electron backscattering diffraction,and transmission electron microscopy.It is found that during deformation,the interface between austenite and ferrite can prevent the slip band in the austenite grain from entering into the ferrite grain through the phase boundary.However,there are also cases where slip bands in some austenite grains can cross the interface and enter into the adjacent ferrite grains.The transmission scalar m' at the interface is the key factor to determine whether the slip in austenite can pass through the interface and enter into ferrite.When m' is large,the slip band in austenite can pass through the interface to enter into the ferrite.At this time,the slip mode in ferrite will be changed,and dislocation concentration is not easy to form.When m' is small,the slip band in austenite is hard to enter ferrite through the interface.in this case,ferrite shows the wavy slip traces,and dislocation pile-up is easy to occur at the interface.The interaction between dislocations and different interfaces in the microstructure was further observed by transmission electron microscope.It is found that dislocations rotate after entering the interface and emit dislocations into the adjacent grains.4.The phenomenon of deformation-induced phase transformation in Fe-20Cr-0.5Ni duplex stainless steel and its effects on the stress and strain partitioning were studied by synchrotron-based high energy X-ray diffraction and electron backscattering diffraction.It is found that austenite in Fe-20Cr-0.5Ni duplex stainless steel can undergo deformation-induced transformation to form the hard martensite phase during deformation,which improves the work hardening rate of the material.However,when austenite cannot continue to undergo deformation-induced transformation,the hard martensite cannot accommodate the subsequent deformation well.This leads the work hardening rate of material to drop rapidly and cause necking.At the micro elastic deformation stage,some austenite began to undergo deformation-induced phase transformation;When austenite yields,the rate of deformation-induced phase transformation increases;When ferrite yields as the strain continues,the rate of deformation-induced phase transformation reaches the maximum.In the whole process of deformation,the yield of austenite will lead to the stress transfer to harder ferrite and martensite;After ferrite yields,stress will be transferred to austenite and martensite.Martensite always keeps elastic during the deformation.Additionally,by adjusting the content of C and N in the alloy,it is found that the influence of C on the mechanical stability of austenite is greater than that of N.