The characteristics of nanosized precipitates in steels depend on the heat-treatment parameters. The effects of characteristics of vanadium precipitates formed during isothermal heat treatment on the hardness of the f...The characteristics of nanosized precipitates in steels depend on the heat-treatment parameters. The effects of characteristics of vanadium precipitates formed during isothermal heat treatment on the hardness of the ferrite matrix in low-carbon vanadium-alloyed steel were investigated through analysis of transmission electron microscopy images and microhardness measurements. The results show that, during isothermal holding in the temperature range from 675 to 750℃, only interphase precipitation occurs, whereas only random precipitation occurs in the ferrite matrix during holding at 600℃. Furthermore, during isothermal heat treatment between 600 and 675℃, both random and interphase precipitates occurred in the ferrite. Nanoscale vanadium carbides with different atomic ratios of vanadium(V) and carbon(C) were the dominant precipitates in the random and interphase precipitates. The sizes of random precipitation carbides were smaller than those of interphase ones. Also, the sample isothermally heat treated at 650℃ for 900s exhibited a higher hardness with a narrower hardness distribution.展开更多
Grain refinement is one of the effective methods to develop new generation low carbon microalloyed steels possessing excellent combination of mechanical properties. The microstructural evolution and ferrite grain refi...Grain refinement is one of the effective methods to develop new generation low carbon microalloyed steels possessing excellent combination of mechanical properties. The microstructural evolution and ferrite grain refinement at the deformation temperature of 865℃, above Ar3, with different strain rates were investigated using single pass isothermal hot compression experiments for a low carbon Nb-Ti microalloyed steel. The physical processes that occurred during deformation were discussed by observing the optical microstructure and analyzing the true stress-true strain responses. At strain rates of 0.001 and 0.01s^-1, there is no evidence of work hardening behavior during hot deformation and strain-induced transformation （SIT） leads to dynamic flow softening in flow curves. Optical microscopy observation shows that ultrafine and equiaxed ferrite with grain sizes of 2μm can be obtained by applying deformation with strain rate of 0.1 s^-1 due to SIT just after deformation. Furthermore, increasing the strain rate from 0.001 to 0.1 s^-1 reduces both the grain size of the equiaxed ferrite and the amount of deformed ferrite.展开更多
In the present research,aluminum metal matrix composites were processed by the stir casting technique.The effects of TiB2 reinforcement particles,severe plastic deformation through accumulative roll bonding(ARB),and a...In the present research,aluminum metal matrix composites were processed by the stir casting technique.The effects of TiB2 reinforcement particles,severe plastic deformation through accumulative roll bonding(ARB),and aging treatment on the microstructural characteristics and mechanical properties were also evaluated.Uniaxial tensile tests and microhardness measurements were conducted,and the microstructural characteristics were investigated.Notably,the important problems associated with cast samples,including nonuniformity of the reinforcement particles and high porosity content,were solved through the ARB process.At the initial stage,particle-free zones,as well as particle clusters,were observed on the microstructure of the composite.However,after the ARB process,fracturing phenomena occurred in brittle ceramic particles,followed by breaking down of the fragments into fine particles as the number of rolling cycles increased.Subsequently,composites with a uniform distribution of particles were produced.Moreover,the tensile strength and microhardness of the ARB-processed composites increased with the increase in the reinforcement mass fraction.However,their ductility exhibited a different trend.With post-deformation aging treatment(T6),the mechanical properties of composites were improved because of the formation of fine Mg2Si precipitates.展开更多
The tri-metal Ti-Al-Nb composites were processed through three procedures:hot pressing,rolling,and hot pressing,followed by subsequent rolling.The fabricated composites were then subjected to annealing at 600,625,and ...The tri-metal Ti-Al-Nb composites were processed through three procedures:hot pressing,rolling,and hot pressing,followed by subsequent rolling.The fabricated composites were then subjected to annealing at 600,625,and 650℃ temperatures at different times.Microstructure observation at the interfaces reveals that the increase in plastic deformation strain significantly affects TiAl_(3) intermetallic layers’evolution and accelerates the layers’growth.On the contrary,the amount of applied strain does not significantly affect the evolution of the NbAl_(3) intermetallic layer thickness.It was also found that Al and Ti atoms’diffusion has occurred throughout the TiAl_(3) layer,but only Al atoms diffuse through the NbAl_(3) layer.The slow growth rate of the NbAl_(3) intermetallic layer is due to the lack of diffusion of Nb atoms and the high activation energy of Al atoms’reaction with Nb atoms.展开更多
A low carbon Nb-Ti microalloyed steel was subjected to hot torsion testing over the temperature range 850-1100℃ and strain rates 0.01-1 s-1 to study the influence of deformation conditions on the dynamic recrystalliz...A low carbon Nb-Ti microalloyed steel was subjected to hot torsion testing over the temperature range 850-1100℃ and strain rates 0.01-1 s-1 to study the influence of deformation conditions on the dynamic recrystallization characteristics of austenite. The results show that dynamic recrystallization occurs more easily with the decrease of strain rate and the increase of deformation temperature. The complete dynamically recrystallized grain size as a function of Zener-Hollomon parameter was established. It was found that dynamically recrystallized grain sizes decrease with increasing strain rate and decreasing deformation temperature. The effect of microalloying elements on peak strain was investigated and the solute drag corrected peak strain was determined. Also, the dynamic recrystallization map of austenite was obtained by using recrystallization critical parameters.展开更多
文摘The characteristics of nanosized precipitates in steels depend on the heat-treatment parameters. The effects of characteristics of vanadium precipitates formed during isothermal heat treatment on the hardness of the ferrite matrix in low-carbon vanadium-alloyed steel were investigated through analysis of transmission electron microscopy images and microhardness measurements. The results show that, during isothermal holding in the temperature range from 675 to 750℃, only interphase precipitation occurs, whereas only random precipitation occurs in the ferrite matrix during holding at 600℃. Furthermore, during isothermal heat treatment between 600 and 675℃, both random and interphase precipitates occurred in the ferrite. Nanoscale vanadium carbides with different atomic ratios of vanadium(V) and carbon(C) were the dominant precipitates in the random and interphase precipitates. The sizes of random precipitation carbides were smaller than those of interphase ones. Also, the sample isothermally heat treated at 650℃ for 900s exhibited a higher hardness with a narrower hardness distribution.
文摘Grain refinement is one of the effective methods to develop new generation low carbon microalloyed steels possessing excellent combination of mechanical properties. The microstructural evolution and ferrite grain refinement at the deformation temperature of 865℃, above Ar3, with different strain rates were investigated using single pass isothermal hot compression experiments for a low carbon Nb-Ti microalloyed steel. The physical processes that occurred during deformation were discussed by observing the optical microstructure and analyzing the true stress-true strain responses. At strain rates of 0.001 and 0.01s^-1, there is no evidence of work hardening behavior during hot deformation and strain-induced transformation （SIT） leads to dynamic flow softening in flow curves. Optical microscopy observation shows that ultrafine and equiaxed ferrite with grain sizes of 2μm can be obtained by applying deformation with strain rate of 0.1 s^-1 due to SIT just after deformation. Furthermore, increasing the strain rate from 0.001 to 0.1 s^-1 reduces both the grain size of the equiaxed ferrite and the amount of deformed ferrite.
文摘In the present research,aluminum metal matrix composites were processed by the stir casting technique.The effects of TiB2 reinforcement particles,severe plastic deformation through accumulative roll bonding(ARB),and aging treatment on the microstructural characteristics and mechanical properties were also evaluated.Uniaxial tensile tests and microhardness measurements were conducted,and the microstructural characteristics were investigated.Notably,the important problems associated with cast samples,including nonuniformity of the reinforcement particles and high porosity content,were solved through the ARB process.At the initial stage,particle-free zones,as well as particle clusters,were observed on the microstructure of the composite.However,after the ARB process,fracturing phenomena occurred in brittle ceramic particles,followed by breaking down of the fragments into fine particles as the number of rolling cycles increased.Subsequently,composites with a uniform distribution of particles were produced.Moreover,the tensile strength and microhardness of the ARB-processed composites increased with the increase in the reinforcement mass fraction.However,their ductility exhibited a different trend.With post-deformation aging treatment(T6),the mechanical properties of composites were improved because of the formation of fine Mg2Si precipitates.
文摘The tri-metal Ti-Al-Nb composites were processed through three procedures:hot pressing,rolling,and hot pressing,followed by subsequent rolling.The fabricated composites were then subjected to annealing at 600,625,and 650℃ temperatures at different times.Microstructure observation at the interfaces reveals that the increase in plastic deformation strain significantly affects TiAl_(3) intermetallic layers’evolution and accelerates the layers’growth.On the contrary,the amount of applied strain does not significantly affect the evolution of the NbAl_(3) intermetallic layer thickness.It was also found that Al and Ti atoms’diffusion has occurred throughout the TiAl_(3) layer,but only Al atoms diffuse through the NbAl_(3) layer.The slow growth rate of the NbAl_(3) intermetallic layer is due to the lack of diffusion of Nb atoms and the high activation energy of Al atoms’reaction with Nb atoms.
文摘A low carbon Nb-Ti microalloyed steel was subjected to hot torsion testing over the temperature range 850-1100℃ and strain rates 0.01-1 s-1 to study the influence of deformation conditions on the dynamic recrystallization characteristics of austenite. The results show that dynamic recrystallization occurs more easily with the decrease of strain rate and the increase of deformation temperature. The complete dynamically recrystallized grain size as a function of Zener-Hollomon parameter was established. It was found that dynamically recrystallized grain sizes decrease with increasing strain rate and decreasing deformation temperature. The effect of microalloying elements on peak strain was investigated and the solute drag corrected peak strain was determined. Also, the dynamic recrystallization map of austenite was obtained by using recrystallization critical parameters.
