Research On The Grain Ultra-refinement In Austenite By Dynamic Recrystallization And Its Martensitic Transformation

Grain refinement is one of the important means to improve the strength and toughness of the engineering materials, especially the iron and steel materials. The research and development objective of the new generation iron and steel materials is to obtain ultra-fine grained microstructure. Recently, many researchers have fabricated the ultra-fine grained materials in sub-micrometer or nanometer range by severe plastic deformation. The mechanical properties of iron and steel materials can be effectively improved if the microstructure could be further refined through phase transformation. Studies on the austenite grain refinement by severe plastic deformation and its martensitic transformation behavior will not only fulfill the theory of plastic deformation and martensitic transformation under uttermost conditions, but also have great instructional sense on the research and development of new generation iron and steel materials.In the present dissertation, the Fe-32%Ni alloy was firstly severe plastic deformed by multi-axial forging; then the severe plastic deformed Fe-32%Ni alloy austenite was quenched liquid nitrogen to make martensitic transformation take place. The austenite grain refinement mechanism and its martensitic transformation behavior were investigated by a series of micro-analysis techniques such as optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM),electron backscattered diffraction (EBSD) and X-ray diffraction (XRD) etc. The main conclusions of the present dissertation are as followings:1. The stress-strain curves analysis of Fe-32%Ni alloy during single compression shows that there are two categories of stress-strain curves in Fe-32%Ni alloy during deformation. One category is named dynamic recrystallization: the flow stress curve exhibits a single peak stress followed by work softening and then a steady state. The other category is called dynamic recovery: the flow stress increases to a single peak with work hardening and then showed a steady-state-like one during further deformation.2. The Fe-32%Ni alloy was severe plastic deformed by multi-axial forging at the temperature of 500℃and strain rate of 10-2s-1. The results indicated that the austenite grains were refined from 200μm to about 1μm when the cumulative strain amounted to 6.0 and decreased to about 700nm when the cumulative strain reached 10.5. The grain refinement phenomenon was obvious at the initial stage of deformation, and the grain size did not decrease when the strain accumulated to some certain amount. Large amounts of low angle boundaries were formed at the initial deformation, and the proportion of low angle boundaries decreased with strain while the high one increased. The volume fraction of high angle boundaries reached a steady proportion eventually. It is concluded that the grain refinement process in Fe-32%Ni alloy multi-axially forged below the temperature of 0.5Tm is as the following: the deformation bands will be formed when the original austenite grains are compressed in some certain direction, and these deformation bands divide the original grains into subgrians with different orientations; then the deformation bands in another direction will be formed when the sample is deformed by next pass compression with strain path changed, in which way the grains will be further divided into small subgrians. The deformation bands would become complicated and intersected with each other after repeated multi-axial forging, and the original grains would be divided into large amounts of subgrians. The misorientations of subgrain boundaries increase by usual intrinsic slip and the absorption of accomodated dislocations at subgrains boundaries and subgrians rotation, in which way the new ultra-fine grains are formed homogeneously. The grain refinement mechanism is classified as continuous dynamic recrystallization (CDRX).3. The Fe-32%Ni alloy was severe plastic deformed by multi-axial forging at the temperature of 800℃and strain rate of 10-2s-1. The results indicated that the original austenite grians were enlongated with grains boundaries curved at initial deformation, and the nucleation of dynamic recrystallization could be found near the original grain boundaries when the strain was 0.5. The dynamic recrystallized grains increased and gradually replaced the original deformed grains with deformation until the microstructure became stable when the strain accumulated to about 1.5. The grain refinement process is that the dislocations slip and accumulate near the original grain boundaries and make them curved; then the new grains that are almost defect-free are formed by nucleating at the bulged boundaries, and they grow and consume the deformed microstructure until the material is completely recrystallized. The process is classified as discontinuous dynamic recrystallization (DDRX).4. The studies on the microstructure evolution in Fe-32%Ni alloy during multi-axial forging under different deformation conditions indicated that the temperature obviously affected the grain refinement. The grains obtained by low-temperature deformation were much smaller than those obtained through high-temperature deformation because that the continuous dynamic recrystallization (CDRX) tooke place when the Fe-32%Ni alloy was multi-axially forged below the temperature of 0.5Tm, while the discontinuous dynamic recrystallization (DDRX) happened when the Fe-32%Ni alloy austenite was multi-axially forged over the temperature of 0.5Tm. The studies about strain rates effect on the grain refinement indicated that the grain refinement difference was obvious when the cumulative strain was small while the effect was not obvious when the cumulative strain was large when the Fe-32%Ni alloy deformed at low temperatures. The grains were relatively small when the Fe-32%Ni alloy was multi-axially forged at high temperature with high strain rate. The ultra-fine grained Fe-32%Ni alloy only could be obtained by multi-axial forging at low temperatures and high strain rates.5. The studies on the martensite starting temperature (Ms) and the translated martensite volume fraction of severe plastic deormed Fe-32%Ni alloy indicated that the Ms temperature and the volume fraction of martensite under liquid nitrogen temperature decreased with increasing cumulative strain and reached a stble value eventually. The reason is that the parent austenite phase was work-hardened by the grain refinement and the introduction of deformation dislocations. The grain refinement effect decreased with deformation resulted in the stable Ms temperature and the martentie volume fraction.6. The martensite microstructure observation of the severe plastic deformed Fe-32%Ni alloy austenite by multi-axial forging at low temperature indicated that the martensite plates were not integral and its rims were crooked, especially the mid-ribs of some martensite plates were broken. The martensite substructure observation indicated that the main sub-structure of martensite plates were not twins but it just distributed in the moderate of the plates and there were complexed dislocation nets near the plate border. It was also found that the substructure of some martensite plates were high density dislocations. The martensitic transformation took place on the distorted parent phase, therefore the twins was necessary to be crooked; the high density defects and the curved grain boundaries in the severe plastic deformed austenite resulted in the complicated martensite plates morphology. The twins were firstly formed to overcome the strain of martensitic transformation, then large amounts of dislocations emerged when the twins grew after formation for the high strength of the parent phase based on the refinement of the original microstructure. Therefore the martensite plates with twins in the moderate and high density dislocations near the plates'border were formed. Secondly, the introduction of high density dislocation in the parent phase resulted in the severe lattice distortion in austenite, especially in the area circled by the martensite plates formed during initial phase transformation. The twins were difficult to be formed under which kind of condition, and the martensite plates must be formed by dislocation sliding, then the high density dislocations became the substructure of the martensite plates.7. The martensite microstructure observation of high-temperature deformed Fe-32%Ni alloy indicated that the martensite morphology was characterized by crooked midribs and serrated rims, and the main sub-structure of martensite plates was twins but just it distributed in the moderate of the plates and there were complexed dislocation nets near the plates border. The special martensite microstructure was related to the characteristic discontinuous dynamic recrystallization austenite microstructure: the fine grains with low density dislocation and the work-hardened grains with high dislocation density will hinder the martensitic nucleation for the excessive work hardening of the parent-martix, while for the grains with gradient dislocation density, the low dislocation density near the grain boundaries contributed to the martensitic nucleation but the high density dislocation inside the grains will hinder the martensite continuous growth, therefore, the crooked midrib and serrated rim of the martensite plates emerged.