The grain growth behavior of spray-formed Al-70wt.%Si alloys was studied in the semi-solid state. The specimens were isothermally heat-treated at various temperatures between the solidus and liquidus of Al-Si alloys a...The grain growth behavior of spray-formed Al-70wt.%Si alloys was studied in the semi-solid state. The specimens were isothermally heat-treated at various temperatures between the solidus and liquidus of Al-Si alloys and then quenched in water. The microstructure of reheated specimens was characterized using optical and scanning electron microscopies. The isothermal holding experiment was carried out to investigate grain growth behavior as a function of holding time and temperature in the semi-solid state. The coarsening mechanism and the effect of porosity on microstructure were also studied.展开更多
The growth behavior at the early stage of bainitic transformation was investigated using optical microscopy,X-ray diffraction analysis and transmission electronic microscopy.The bainite was obtained by isothermal tran...The growth behavior at the early stage of bainitic transformation was investigated using optical microscopy,X-ray diffraction analysis and transmission electronic microscopy.The bainite was obtained by isothermal transformation at 200 ℃ only for a short time in a high carbon silicon-containing steel after austenitization at 200 ℃ only for 20 min.Transmission electronic microscopy shows that the bainite appears in the form of plates with a width of about 30 nm,and that the interface of the bainite leading tip is wedge shaped.X-ray diffraction analysis reveals that the bainite plates consist of single ferrite phase,with absence of carbides.The results confirm the occurrence of the moiré which suggests the existence of austenite grain boundaries at the bainite leading tip.Both the lateral growth and longitudinal growth of bainite have weak ability to traverse the lattice-distortion strain fields and austenite grain boundary.The austenite grain boundary impedes the longitudinal growth of the bainite plate,i.e.,the growth of bainite plate stops at the austenite grain boundary.The longitudinal growth of bainite associated with the features of shear mechanism can not completely be in accordance with that of martensitic transformation.展开更多
Ultrafine grained(UFG) materials have attracted considerable attention owing to their unique microstructure and mechanical properties.However,the easy formation of large-scale shear bands and severe grain coarsening d...Ultrafine grained(UFG) materials have attracted considerable attention owing to their unique microstructure and mechanical properties.However,the easy formation of large-scale shear bands and severe grain coarsening during cyclic deformation gives rise to enormous difficulties when investigating the intrinsic fatigue behavior of UFG materials.Herein,we discuss the fabrication of an ideal model material,based on pure Cu,by friction stir processing(FSP),which exhibits equiaxed ultrafine grains,low dislocation density,and a high ratio of high-angle grain boundaries.This model material was used to investigate the intrinsic high cycle fatigue behavior of UFG material.It was found that an enhanced fatigue limit and fatigue ratio can be achieved by FSP Cu due to its uniform and stable UFG structure.Instead of traditional large-scale shear bands,protrusion was found to be the main surface damage morphology for FSP Cu during high cycle fatigue deformation,and no obvious grain coarsening was observed.Dislocation related activity also dominated,but was limited to the ultrafine grains without the formation of regular dislocation structures.展开更多
In current work,Ni-Ti-CeO_(2) nanocomposite coatings were achieved by co-adding Ti microparticles and CeO_(2) nanoparticles.Designed experiments and COMSOL computer simulation were applied to reveal the synergistic ro...In current work,Ni-Ti-CeO_(2) nanocomposite coatings were achieved by co-adding Ti microparticles and CeO_(2) nanoparticles.Designed experiments and COMSOL computer simulation were applied to reveal the synergistic role of Ti microparticles and CeO_(2) nanoparticles in tailoring the spatial microstructures and properties of Ni-Ti-CeO_(2) nanocomposite coating.Unilaterally,the conductive Ti microparticles conducted the growth behavior of Ni grains by current density concentration,distorting electronic feld lines and heterogeneous nucleation.Individual domains consisting of inner nanograins and outer radial columnar grains surrounded Ti microparticles,where Ti microparticles acted as seeds.Ti microparticles tended to be aggregated,leading to spatial heterogeneity of microstructures.Ni deposits buried the Ti microparticles in forms of“covering model”,contributing to the formation of inside voids and rough surface and aggregation of Ti microparticles;on the other hand,the non-conductive CeO_(2)microparticles hardly changed the distribution of current density and electronic feld lines on the cathode surface.Ni deposits buried the CeO_(2)microparticle in forms of“stacking model”,avoiding the inside voids and aggregation of particles.The incorporation of CeO_(2)microparticle brought in microstructure evolutions only on its top side without disturbing the growth behavior of Ni grains on its lateral side or bottom,suggesting the limited effects.This was correlated with the presence of current concentration above the CeO_(2) microparticle at the last stage of burying CeO_(2) microparticle.The co-addition of Ti microparticles and CeO_(2) nanoparticles into Ni deposits exploited the complementary action of the two particles,which gave birth to satisfed spatial microstructures and improved hardness.Ti microparticles took major responsibility for microstructure evolutions,while the CeO_(2) nanoparticles were mainly in charge of the microstructure homogeneity.展开更多
文摘The grain growth behavior of spray-formed Al-70wt.%Si alloys was studied in the semi-solid state. The specimens were isothermally heat-treated at various temperatures between the solidus and liquidus of Al-Si alloys and then quenched in water. The microstructure of reheated specimens was characterized using optical and scanning electron microscopies. The isothermal holding experiment was carried out to investigate grain growth behavior as a function of holding time and temperature in the semi-solid state. The coarsening mechanism and the effect of porosity on microstructure were also studied.
基金Item Sponsored by Tianjin Momentous Technology Supporting Program Foundation of China(11ZCKFGX20500)
文摘The growth behavior at the early stage of bainitic transformation was investigated using optical microscopy,X-ray diffraction analysis and transmission electronic microscopy.The bainite was obtained by isothermal transformation at 200 ℃ only for a short time in a high carbon silicon-containing steel after austenitization at 200 ℃ only for 20 min.Transmission electronic microscopy shows that the bainite appears in the form of plates with a width of about 30 nm,and that the interface of the bainite leading tip is wedge shaped.X-ray diffraction analysis reveals that the bainite plates consist of single ferrite phase,with absence of carbides.The results confirm the occurrence of the moiré which suggests the existence of austenite grain boundaries at the bainite leading tip.Both the lateral growth and longitudinal growth of bainite have weak ability to traverse the lattice-distortion strain fields and austenite grain boundary.The austenite grain boundary impedes the longitudinal growth of the bainite plate,i.e.,the growth of bainite plate stops at the austenite grain boundary.The longitudinal growth of bainite associated with the features of shear mechanism can not completely be in accordance with that of martensitic transformation.
基金supported by the National Natural Science Foundation of China(51301178 and 51331008)
文摘Ultrafine grained(UFG) materials have attracted considerable attention owing to their unique microstructure and mechanical properties.However,the easy formation of large-scale shear bands and severe grain coarsening during cyclic deformation gives rise to enormous difficulties when investigating the intrinsic fatigue behavior of UFG materials.Herein,we discuss the fabrication of an ideal model material,based on pure Cu,by friction stir processing(FSP),which exhibits equiaxed ultrafine grains,low dislocation density,and a high ratio of high-angle grain boundaries.This model material was used to investigate the intrinsic high cycle fatigue behavior of UFG material.It was found that an enhanced fatigue limit and fatigue ratio can be achieved by FSP Cu due to its uniform and stable UFG structure.Instead of traditional large-scale shear bands,protrusion was found to be the main surface damage morphology for FSP Cu during high cycle fatigue deformation,and no obvious grain coarsening was observed.Dislocation related activity also dominated,but was limited to the ultrafine grains without the formation of regular dislocation structures.
文摘In current work,Ni-Ti-CeO_(2) nanocomposite coatings were achieved by co-adding Ti microparticles and CeO_(2) nanoparticles.Designed experiments and COMSOL computer simulation were applied to reveal the synergistic role of Ti microparticles and CeO_(2) nanoparticles in tailoring the spatial microstructures and properties of Ni-Ti-CeO_(2) nanocomposite coating.Unilaterally,the conductive Ti microparticles conducted the growth behavior of Ni grains by current density concentration,distorting electronic feld lines and heterogeneous nucleation.Individual domains consisting of inner nanograins and outer radial columnar grains surrounded Ti microparticles,where Ti microparticles acted as seeds.Ti microparticles tended to be aggregated,leading to spatial heterogeneity of microstructures.Ni deposits buried the Ti microparticles in forms of“covering model”,contributing to the formation of inside voids and rough surface and aggregation of Ti microparticles;on the other hand,the non-conductive CeO_(2)microparticles hardly changed the distribution of current density and electronic feld lines on the cathode surface.Ni deposits buried the CeO_(2)microparticle in forms of“stacking model”,avoiding the inside voids and aggregation of particles.The incorporation of CeO_(2)microparticle brought in microstructure evolutions only on its top side without disturbing the growth behavior of Ni grains on its lateral side or bottom,suggesting the limited effects.This was correlated with the presence of current concentration above the CeO_(2) microparticle at the last stage of burying CeO_(2) microparticle.The co-addition of Ti microparticles and CeO_(2) nanoparticles into Ni deposits exploited the complementary action of the two particles,which gave birth to satisfed spatial microstructures and improved hardness.Ti microparticles took major responsibility for microstructure evolutions,while the CeO_(2) nanoparticles were mainly in charge of the microstructure homogeneity.
