Papers by Keyword: Dynamic Recrystallization (DRX)

Abstract: Orientation-controlled Mg single crystals were tensile tested at temperatures between 473 K and 673 K at a strain rate of 4.2 x 10-4 s-1 in vacuum. Though all the single crystals showed high ductility compared with that of polycrystals, the ductility of the single crystals strongly depended on the crystal orientation. The [27 -1 -26 1] single crystal showed 0.57 fracture strain, while the [3 8 -11 -1] single crystal showed superplastic behavior of ductility over 1.8 strain. The observed strong orientation dependence of ductility seemed to be caused by orientation dependence of ease occurrence of dynamic recrystallization (DRX) in the single crystals during high-temperature deformation. The orientation dependence of ductility of Mg single crystals will be discussed in detail concerning crystallographical orientations of the single crystals, occurrence of DRX and fracture.

Abstract: Superplastic forming provides a good way for Ti alloys which are usually difficult to be deformed. Ti75 alloy with a nominal composition of Ti-3Al-2Mo-2Zr is a newly developed corrosion resistant alloy, with a middle strength and high toughness. In the present paper, superplastic behavior of the alloy was investigated, the microstructural evolution in superplastic deformation was observed and the superplastic deformation mechanisms were analyzed. The results showed that the strain rate sensitivity, m, of the Ti75 alloy was larger than 0.3 and the strain was over 2.0 without surface cracking at 800°C and 5×10-4s-1 in compressive testing. During the first stage of superplastic deformation, a phase grains became equiaxed, fine and homogeneous due to the recrystallization in a phase and diffusion in b phase. Newly formed equiaxed a grains then could slide and rotate, exhibiting superplastic features. The stress concentration caused by grain sliding of a grains could be released by slip and diffusion in b phase between the a phase grains, which acted as accommodation mechanisms.

Abstract: The commercial hot-rolled bars of LD10 alloy (grain size 25~75µm) are pretreated via proper recrystallization annealing (temperature 380°C, 420°C and 460°C) and solution annealing (temperature 500°C). The average elongation is increased from 158% to 270%, 223%, 219% and 233% respectively and the maximum elongation is 321% (500°C, 3.3×10-4s-1). The test results show that the alloy can present certain superplasticity owing to structural effect of dynamic recrystallization refining during superplastic deformation of coarse-grained LD10 alloy. Large and deep cavities form in grain boundaries and result in intergranular fracture after larger superplastic tensile deformation. And a large amount of metal filaments appear on grain surfaces. The formation of filaments is attributed to viscous flow of solid-liquid mixture in grain boundaries caused by grain-boundary sliding. The appearance of solid-liquid mixture makes grain-boundary sliding easier.

Abstract: High temperature deformation behavior of AZ31 Mg alloy was investigated in this study on the basis of a processing map (e » 0.6). To construct a processing map, compression tests were carried out at various temperatures and strain rates. Two regions of high deformation efficiency (h) were identified as: (1) a dynamic recrystalization (DRX) domain at 250°C and 1/s and (2) a superplasticity domain at 450°C and 10-4/s. The average grain size observed in the DRX region was considerably smaller (2.9µm) than in any other region. In the superplastic condition, tensile elongation to failure approached to 1040%. At the high Z regions, flow softening occurred resulting from the dynamic recrystallization but below 1010 of Z value, flow hardening occurred due to the grain growth. Possible deformation mechanisms operating at high temperature were discussed in relation to the activation energy. A two-stage deformation method was found to be effective in enhancing the superplasticity of AZ31 Mg alloy.

Abstract: In this study, isothermal torsion tests were carried out on magnesium AZ31B alloy under constant strain rate conditions, in the range of 250 to 400oC at 0.01, 0.1, and 1.0 s-1. Alloy flow stress dependence on strain rate and temperature can be described by a power law with activation energy of 130 kJ/mol. Microstructural examination of hot deformed samples shows very fine recrystallized grains decorating grain boundaries of larger gains in the form of a necklace. These fine grains are produced by dynamic recrystallization at the grain boundaries of original grains. Microstructure evolution, based on samples quenched at different strain levels, indicates that increasing deformation strain has little effect on recrystallized grain size but widens the recrystallized region, with full recrystallization achieved at a certain high strain level. Recrystallized grain size increases with increasing deformation temperature and strain rate. The latter suggests recrystallization in AZ31 to be essentially a time dependant phenomenon.

Abstract: The initial coarse grain size of 400㎛ of a as-cast AZ31 alloy was refined to 130㎛ by carbon addition method. The microstructural evolution, recrystallization, edge cracking phenomena of the coarse grained AZ31 alloy during hot rolling were investigated and compared with those of grain refined AZ31 alloy.

Abstract: A constitutive relation is proposed in this study by introducing a function prescrbing the volume fraction of dynamically recrystallized grains into the Voce’s equation to predict the stress-strain relations for three microalloyed medium carbon steels. We performed hot torsion test in temperature range of 900-1100 and strain rate range of 0.05-5.0s-1 to obtain the flow stress-strain curves. The calculated flow stresses are in good agreement with the measured ones. We have then applied the equations in FE analysis to predict the distribution of dynamic recrystallization volume fraction and flow stress for hot forging.

Abstract: The hot deformation behavior of a low carbon Nb-microalloyed steel is investigated by hot compression test in the ferrite phase region compared with a low carbon steel with similar compositions, and the effect of Nb on dynamic recrystallization of ferrite is analyzed. Results indicate that during hot deformation in the ferrite phase region, the effect of Nb solely depends on the size of NbC precipitates. Tiny particles which average size is about 7.5nm have a retarding effect on dynamic recrystallization process of ferrite, on the contrary, coarser particles which average size is about 30.6nm have a promoting effect and are of benefit to the refinement of recrystallized grains.

Abstract: The design of base chemistry and optimization of rolling schedule are the two important factors that influence large strain accumulation in multi-pass rolling in order to obtain ultra-fine grain size by dynamic recrystallization. A base chemistry of 0.03C-0.003N-0.08Nb-0.015Ti-1.8Mn (all in weight %) of HTP steel design was chosen in order to control the time evolution of strain induced precipitation of NbC and the strain accumulation through precipitate interaction with recovery and recrystallization at short inter-pass times characteristic of strip rolling. Experimental data on the critical strain for static and dynamic recrystallisation for HTP steel are used in a quantitative model to predict strain accumulation pass by pass and to achieve grain refinement by dynamic recrystallisation through large strain accumulation. The model is used to optimize the time-temperature-deformation schedule to prevent static recrystallization during the inter-pass times and to target ultra-fine grain size through dynamic recrystallization by large strain accumulation. The model predictions are validated by simulation of strip rolling of HTP steel on the thermo-mechanical simulator (WUMSI) to obtain a uniform ultra-fine ferrite grain size of about 1.5 micrometer diameter in final ferrite microstructure.

Abstract: It is generally agreed that the driving force (plastic strain energy) is much too small to allow "classical" nucleation during static and dynamic recrystallisation, and that rotation/growth of subgrains is an alternative. The latter explanation predicts that new grains should begin at low angles to old grains. We have used electron backscatter diffraction on an experimentally deformed quartz polycrystal that has deformed by dislocation creep and partially recrystallised. In a region shortened by about 30% new grains are at high angles (much greater than 15º) to adjacent parent grains. A histogram of misorientation versus number of boundaries shows a gap at 15-20º. In its simple form we expect the subgrain rotation model to predict a spectrum of misorientations but with most of them being low angle. Instead, the histogram suggests that new boundaries began life as high-angle structures, so current models for deformation-induced nucleation require refinement.