| The fine structure of the quenched martensite in Fe-C(0.05-2.0 wt.%)binary alloy was investigated by using transmission electron microscopy at the nanometer scale.It was found that the quenched twinned martensite was actually composed of ultra-fine a-Fe grains.The characteristics of twin structure in martensite and the behavior of detwinning in tempering process were studied by using selective electron diffraction and in-situ heating transmission electron microscopy.The results show that:1.The twins in the twin martensite structure belong to the typical body-centered cubic{112}<111>type twins,both the matrix and the twin are composed of a-Fe fine particles,and the size of the ultra-fine particles is approximately 1-2 nm.Due to the solid-solid phase transition,these fine a-Fe grains are formed by phase transformation from the same parent phase(austenite)grains,so they have almost the same crystallographic orientation distribution relative to the parent phase,and thus single crystal diffraction characteristics have appeared in the electron difiraction pattern.A new metastable ?-Fe(C)phase particle with the same fine particles at the twin grain boundary,the phase has a hexagonal structure with a lattice parameter of a?-Fe=?2a-Fe,c?-Fe=?3/2aa-Fe.The phase has a special orientation relationship with its inseparable a-Fe,and its crystal structure is very similar to that of other body-centered cubic metals and alloys.This kind of martensitic structure is ubiquitous in the quenched carbon steel,regardless of the carbon content.2.In the quenched low-carbon Fe-0.05C samples,there are also a large number of body-centered cubic {112}<111>-type twin martensite,and their sub-structure characteristics are not different from those in high-carbon.The difference in overall organization is due to the higher martensitic transformation start temperature(Ms)in the low carbon steel and the lower Ms in the high carbon steel.Thus,the twinned martensite structure which is first formed at the Ms inevitably undergoes the detwinning process caused by auto-tempering during its subsequent cooling to room temperature.In the ultra-high carbon alloy,the Ms point is low,the auto-tempering effect is not serious,and the detwinning behavior is not obvious,and the initial martensite structure characteristics are preserved,that is,the twin structure.Therefore,the product of the initial martensitic transformation should be the body-centered cubic{112}<111>type twin with the ?-Fe(C)phase particles at the twin grain boundary.3.The formation of the microstructure in various carbon steel is inseparable from the detwinning behavior of twinned martensite.Direct in-situ heating TEM observation shows that there is no significant change in the twinned martensite from room temperature to 200?.However,between 200 and 250?,the ?-Fe(C)phase particles at the twin grain boundaries are instantaneously transformed into cementite,and the grain size is significantly increased.At the same time,the matrix of the twin structure and the fine ?-Fe particles in the twin crystal are instantaneously roughened,similar to the recrystallization behavior.This process is accompanied by the disappearance of the twin structure,that is,the detwinning process.The various carbon steel configurations are inseparable from the temperature and time at which the detwinning process takes place.The above findings,especially the twinned martensite is composed of fine a-Fe particles,which will give us a deeper understanding of an important basic theory in steel materials,namely the martensitic transformation process.Thus,a more comprehensive understanding and control of the microstructure evolution can effectively change the production process and control the microstructure to achieve the best performance.Due to the ubiquity of the ? phase in body-centered cubic metal system,these findings can be extended to other metals and alloys,not just carbon steel. |
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