Channel die compression and initial textures are used to activate different deformation mechanisms in a fine-grained magnesium alloy AZ31. The σ-ε curves, microstructures and, particularly, textures are analyzed to ...Channel die compression and initial textures are used to activate different deformation mechanisms in a fine-grained magnesium alloy AZ31. The σ-ε curves, microstructures and, particularly, textures are analyzed to reveal different deformation mechanisms and to compare with those of coarse grained samples. Dominant double-prismatic slip, {1012} twinning and basal slip are detected in three types of samples, respectively, which is similar to those of coarse grained samples. The detrimental effect of shear band formation or {1011} twinning is limited in fine grained microstructure. In addition to the higher flow stress at low temperature an early decrease in flow stress at higher temperature is also found in fine-grained samples in comparison with their coarse-grained counterparts. This softening is ascribed to the early dynamic recrystallization or grain boundary glide.展开更多
Novel headstand pyrocarbon cones （HPCs） with hollow structure were developed on the surfaces of pyrocarbon layers of the carbon/carbon （C/C） composites at 650-750 °C by the electromagnetic-field-assisted chem...Novel headstand pyrocarbon cones （HPCs） with hollow structure were developed on the surfaces of pyrocarbon layers of the carbon/carbon （C/C） composites at 650-750 °C by the electromagnetic-field-assisted chemical vapor deposition in the absence of catalysts. The fine microstructures of the HPCs were characterized by high-resolution transmission electron microscopy. The results show that the textural features of the HPCs directly transfer from turbostratic structure in roots to a well-ordered high texture in stems. And the degree of high texture ordering decreases gradually from the stem to the tail of the HPCs. The formation mechanism of the HPCs was inferred as the comprehensive effect of polarization induction on electromagnetic fields and particle-filler property under disruptive discharge.展开更多
Phase precipitation and mechanical properties of TC21 titanium alloy with two different initial microstructures during heat treatment were determined. Result indicated that compared with coarse microstructure alloy, f...Phase precipitation and mechanical properties of TC21 titanium alloy with two different initial microstructures during heat treatment were determined. Result indicated that compared with coarse microstructure alloy, fine microstructure alloy developed finer microstructure, more unstable <em>ω</em> and <em>α</em><sub>2</sub> precipitates with much smaller size and lower volume fraction, and obtained better mechanical properties during heat treatment.展开更多
Friction stir processing （FSP）, a variation of FSW （friction stir welding） is an emerging surface engineering technology that can locally eliminate casting defects and refine microstructures, thereby improving the...Friction stir processing （FSP）, a variation of FSW （friction stir welding） is an emerging surface engineering technology that can locally eliminate casting defects and refine microstructures, thereby improving the mechanical properties of material. FSP can also produce fine grained microstructures through the thickness to impart superplasticity. The technology involves plunging a rapidly rotating, non consumable tool, comprising a profiled pin and larger diameter shoulder, into the surface and then traversing the tool across the surface. The pin and the shoulder friction heat the surface which alters the grain structure in the processed area thereby improving the mechanical properties. This paper presents the effects of FSP on microstructure and mechanical properties of extruded cast 2285 aluminum alloy at three different feed rates viz. 10, 12 and 15 mm/min. With the increase in the feed speed the material was observed to have increased impact strength. FSP also increased the tensile and yield strengths with increases in hardness and ductility values also. The observation has been listed in detail and pictorially represented.展开更多
Results of a large set of tensile and compressive creep tests on pure Al were reanalyzed for the influence of low-and high-angle grain boundaries on the deformation resistance at the temperature T = 473 K = 0.51 Tmwhe...Results of a large set of tensile and compressive creep tests on pure Al were reanalyzed for the influence of low-and high-angle grain boundaries on the deformation resistance at the temperature T = 473 K = 0.51 Tmwhere Tm is the melting point.Thermomechanical treatment by equal channel angular pressing followed by heating to T led to strong increase of areal fraction of high-angle boundaries in a structure of subgrains of ≈10^-6m in size,accompanied by significant reduction of subgrain strengthening and of the stress sensitivity of the deformation rate.（Sub）grain strengthening by low-angle boundaries is most effective;the strengthening effect virtually disappears during creep as the boundary spacings coarsen toward their stress-dependent,quasi-stationary size wqs.The same type of coarsening is found for（sub）grain structures with large fraction of high-angle boundaries;in the quasi-stationary state they lead to softening at low and strengthening at high stresses,and a significant increase in tensile fracture strain to values up to 0.8.The results are analogous to former results for Cu and are explained in the same way by the influence of boundaries on storage and recovery of crystal defects and the homogenization of glide.展开更多
Directional solidification experiments were carried out with Al-Pb alloys under the effect of a direct current （DC）. The experimental results show that the DC causes a migration of the minority phase droplets （MPDs...Directional solidification experiments were carried out with Al-Pb alloys under the effect of a direct current （DC）. The experimental results show that the DC causes a migration of the minority phase droplets （MPDs） from the middle part to the surface region of the sample, Samples with either a finely dispersed microstructure or a shell/ core structure were obtained by solidifying the alloy under the effect of the properly selected DC densities, A model was developed to describe the microstructure evolution in an immiscible alloy directionaliy solidified under the effect of the DC. The microstructure formation in the AI-Pb alloys was calculated. The numerical results are in favorable agreement with the experimental ones. They demonstrate that the DC affects the microstructure formation mainly through changing the spatial motions of the MPDs and the temperature field of the melt in front of the solid/liquid interface. The formation mechanisms of the finely dispersed microstructure as well as the shell/core structure were sufficiently clarified.展开更多
基金This work was supported by the National Natural Sci-ence Foundation of China under grant No.50171009the National 863 Program of China(No.2002A A305501)Part work was performed at IMM RWTH A achen,Germany.
文摘Channel die compression and initial textures are used to activate different deformation mechanisms in a fine-grained magnesium alloy AZ31. The σ-ε curves, microstructures and, particularly, textures are analyzed to reveal different deformation mechanisms and to compare with those of coarse grained samples. Dominant double-prismatic slip, {1012} twinning and basal slip are detected in three types of samples, respectively, which is similar to those of coarse grained samples. The detrimental effect of shear band formation or {1011} twinning is limited in fine grained microstructure. In addition to the higher flow stress at low temperature an early decrease in flow stress at higher temperature is also found in fine-grained samples in comparison with their coarse-grained counterparts. This softening is ascribed to the early dynamic recrystallization or grain boundary glide.
基金Project (2011CB605801) supported by the National Basic Research Program of ChinaProject (2011M500127) supported by the China Postdoctoral Science Foundation+1 种基金Projects (50802115, 51102089) supported by the National Natural Science Foundation of ChinaProject supported by the Postdoctoral Fund of the Central South University, China
文摘Novel headstand pyrocarbon cones （HPCs） with hollow structure were developed on the surfaces of pyrocarbon layers of the carbon/carbon （C/C） composites at 650-750 °C by the electromagnetic-field-assisted chemical vapor deposition in the absence of catalysts. The fine microstructures of the HPCs were characterized by high-resolution transmission electron microscopy. The results show that the textural features of the HPCs directly transfer from turbostratic structure in roots to a well-ordered high texture in stems. And the degree of high texture ordering decreases gradually from the stem to the tail of the HPCs. The formation mechanism of the HPCs was inferred as the comprehensive effect of polarization induction on electromagnetic fields and particle-filler property under disruptive discharge.
文摘Phase precipitation and mechanical properties of TC21 titanium alloy with two different initial microstructures during heat treatment were determined. Result indicated that compared with coarse microstructure alloy, fine microstructure alloy developed finer microstructure, more unstable <em>ω</em> and <em>α</em><sub>2</sub> precipitates with much smaller size and lower volume fraction, and obtained better mechanical properties during heat treatment.
文摘Friction stir processing （FSP）, a variation of FSW （friction stir welding） is an emerging surface engineering technology that can locally eliminate casting defects and refine microstructures, thereby improving the mechanical properties of material. FSP can also produce fine grained microstructures through the thickness to impart superplasticity. The technology involves plunging a rapidly rotating, non consumable tool, comprising a profiled pin and larger diameter shoulder, into the surface and then traversing the tool across the surface. The pin and the shoulder friction heat the surface which alters the grain structure in the processed area thereby improving the mechanical properties. This paper presents the effects of FSP on microstructure and mechanical properties of extruded cast 2285 aluminum alloy at three different feed rates viz. 10, 12 and 15 mm/min. With the increase in the feed speed the material was observed to have increased impact strength. FSP also increased the tensile and yield strengths with increases in hardness and ductility values also. The observation has been listed in detail and pictorially represented.
基金support by the Central European Institute of Technology CEITEC(Project CZ.1.05/1.1.00/02.0068 and the European Regional Development Fund)
文摘Results of a large set of tensile and compressive creep tests on pure Al were reanalyzed for the influence of low-and high-angle grain boundaries on the deformation resistance at the temperature T = 473 K = 0.51 Tmwhere Tm is the melting point.Thermomechanical treatment by equal channel angular pressing followed by heating to T led to strong increase of areal fraction of high-angle boundaries in a structure of subgrains of ≈10^-6m in size,accompanied by significant reduction of subgrain strengthening and of the stress sensitivity of the deformation rate.（Sub）grain strengthening by low-angle boundaries is most effective;the strengthening effect virtually disappears during creep as the boundary spacings coarsen toward their stress-dependent,quasi-stationary size wqs.The same type of coarsening is found for（sub）grain structures with large fraction of high-angle boundaries;in the quasi-stationary state they lead to softening at low and strengthening at high stresses,and a significant increase in tensile fracture strain to values up to 0.8.The results are analogous to former results for Cu and are explained in the same way by the influence of boundaries on storage and recovery of crystal defects and the homogenization of glide.
基金the financial support from the National Natural Science Foundation of China(Nos. 51071159,51031003,51271185 and u0837601)
文摘Directional solidification experiments were carried out with Al-Pb alloys under the effect of a direct current （DC）. The experimental results show that the DC causes a migration of the minority phase droplets （MPDs） from the middle part to the surface region of the sample, Samples with either a finely dispersed microstructure or a shell/ core structure were obtained by solidifying the alloy under the effect of the properly selected DC densities, A model was developed to describe the microstructure evolution in an immiscible alloy directionaliy solidified under the effect of the DC. The microstructure formation in the AI-Pb alloys was calculated. The numerical results are in favorable agreement with the experimental ones. They demonstrate that the DC affects the microstructure formation mainly through changing the spatial motions of the MPDs and the temperature field of the melt in front of the solid/liquid interface. The formation mechanisms of the finely dispersed microstructure as well as the shell/core structure were sufficiently clarified.
