Abstract:The kinetics of delta-ferrite to austenite phase transformation was investigated using a quenching dilatometer in a Fe-Al-C alloy. The results showed that the austenite phase nucleated along the delta grain boundaries. The transformed austenite morphology changed from cellular to Widmansta¨tten pattern when the holding temperature decreased from 1398 K to 1123 K (1125°C to 850°C). Full partitioning of the substitutional alloying elements was observed and the spacing of the austenite plates was controlled by th… Show more
“…In fact, the long‐range atoms diffusion is strongly relative to the phase change process microscopically . Zhou et al found that growth of the austenite front was controlled by the long‐range diffusion of carbon from the center of the delta grains to the growing front in the delta‐ferrite to austenite phase transformation of a Fe–Al–C alloy. Tanimura et al also pointed out that a long‐range diffusion field has a regular atomic and vacancy configuration, diffusion blocking acts as a nonlinear perturbation in the diffusion kinetics and its appearance should substantially influence the temporal evolution of the macroscopic phase state in a diffusion‐controlled phenomenon of Ni–3(Al,V) alloys.…”
The effect of high-energy electropulsing treatment (EPT) on the phase transition and mechanical properties of the two-phase Ti-6Al-4V alloy strips (in the solid solution state) was studied with the help of uniaxial tensile machine, X-ray diffraction, optical stereo-microscope, scanning electron microscope, and electrical resistance meters. Results show that the ductility of the titanium alloy strips under EPT could be enhanced remarkably at most by 225% while keeping the tensile strength nearly unchanged. EPT facilitates b-Ti phase precipitation noticeably with increasing percentage of the b phase and the average size of the b phase. In addition, precipitated b phase gathers coarsening, forms into continuous strips and migrates from the interior grains to the inter-granular regions transforming the wormlike microstructure into the equiaxed microstructure. The mechanism for rapid phase change during EPT is put forward with increasing the nucleation rate of the a ! b phase transformation and accelerating the diffusion flux of vanadium atoms under the coupling of the thermal and athermal effects. Therefore, a highly efficient aging treatment method for titanium alloys is provided to prepare advanced engineering materials with outstanding mechanical properties, which can be widely applied in the aerospace and biomedical fields.
“…In fact, the long‐range atoms diffusion is strongly relative to the phase change process microscopically . Zhou et al found that growth of the austenite front was controlled by the long‐range diffusion of carbon from the center of the delta grains to the growing front in the delta‐ferrite to austenite phase transformation of a Fe–Al–C alloy. Tanimura et al also pointed out that a long‐range diffusion field has a regular atomic and vacancy configuration, diffusion blocking acts as a nonlinear perturbation in the diffusion kinetics and its appearance should substantially influence the temporal evolution of the macroscopic phase state in a diffusion‐controlled phenomenon of Ni–3(Al,V) alloys.…”
The effect of high-energy electropulsing treatment (EPT) on the phase transition and mechanical properties of the two-phase Ti-6Al-4V alloy strips (in the solid solution state) was studied with the help of uniaxial tensile machine, X-ray diffraction, optical stereo-microscope, scanning electron microscope, and electrical resistance meters. Results show that the ductility of the titanium alloy strips under EPT could be enhanced remarkably at most by 225% while keeping the tensile strength nearly unchanged. EPT facilitates b-Ti phase precipitation noticeably with increasing percentage of the b phase and the average size of the b phase. In addition, precipitated b phase gathers coarsening, forms into continuous strips and migrates from the interior grains to the inter-granular regions transforming the wormlike microstructure into the equiaxed microstructure. The mechanism for rapid phase change during EPT is put forward with increasing the nucleation rate of the a ! b phase transformation and accelerating the diffusion flux of vanadium atoms under the coupling of the thermal and athermal effects. Therefore, a highly efficient aging treatment method for titanium alloys is provided to prepare advanced engineering materials with outstanding mechanical properties, which can be widely applied in the aerospace and biomedical fields.
“…15 In fact, the long-range atom diffusion is strongly relative to the phase change process microscopically. Zhou et al 32 found that growth of the austenite front was controlled by the longrange diffusion of carbon from the center of the delta grains to the growing front in the delta-ferrite to austenite phase transformation of a Fe-Al-C alloy. Tanimura and Koyama 33 also pointed out that a long-range diffusion field has a regular atomic and vacancy configuration, diffusion blocking acts as a nonlinear perturbation in the diffusion kinetics and its appearance should substantially influence the temporal evolution of the macroscopic phase state in a diffusion-controlled phenomenon of Ni-3(Al,V) alloys.…”
The effect of high-energy electropulsing treatment (EPT) on the microstructure evolution, mechanical properties, and fracture behavior of as-treated Ti-6Al-4V alloy strips was investigated. EPT was found to accelerate phase transition and microstructure evolution of quasi-single-phase titanium alloy strips at a relatively low temperature, and obtain characteristic duplex microstructure and Widmanstatten microstructure. The EPT-induced microstructural changes increased elongation-to-failure remarkably with a slight decrease in tensile strength. Fracture surface observation and three-dimensional analysis showed that transition from small-shallow dimple colony to big-deep colony fracture took place with an increase in frequency of EPT. The rapid phase change of the Ti-6Al-4V alloy strip under EPT was attributed to the enhancement of nucleation rate and atomic diffusion resulting from the coupling of the thermal and athermal effects. It is supposed that EPT can provide a highly efficient method for the intermediate-softening annealing of titanium alloy sheet/strips.
“…Although the difference is relatively small, it can be clearly seen after amplification. In addition, some pits and bulges were found around the γ phase (position ); Many previous studies proposed that substructures can also provide additional γ phase nucleation sites, enhancing the phase transition kinetics[14,16,17]. Figure 1 Evolution of the γ phase with decreasing temperature.…”
The δ-ferrite to γ-austenite phase transformation in duplex stainless steels was observed using ultra high-temperature confocal laser scanning microscope, and the orientation relationship between the γ phase, and precipitate is discussed. Owing to mutual promotion action, the γ phase was observed at δ/δ grain boundary at the beginning of δ-ferrite→γ-austenite transformation, followed by two-dimensional γ-phase growth at the same speed (0.625 µm s−1) along the grain boundary and into the δ grain matrix. The γ-phase growth rate decreases to 0.244 µm s−1 when precipitate stops growing. In the process of grain growth at high temperature, the precipitated pinning grain boundary will slow the movement speed of the grain boundary. The mutual promotion action leads to preferential nucleation of the γ phase, and the nucleation and growth of the austenite also promoting the growth of MnS the growth. GRAPHICAL ABSTRACT
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