Papers by Keyword: Continuous Dynamic Recrystallization

Abstract: A new class of superplastic titanium alloy, Ti–4.5Al–2.5Cr–1.2Fe–0.1C–0.3Cu–0.3Ni, was deformed at 1073 K with strain rates of 1×10⁻⁴–1×10⁻¹ s⁻¹, and microstructures in the condition between superplastic regions II and III (= 1×10⁻² s⁻¹) were observed using scanning electron microscope and electron back-scattered diffraction. Continuous dynamic recrystallization was observed, resulting in grain refinement both in α and β phases. The grain size decreased significantly in α phase at the early stage of the deformation and in β phase at the later stage. In the recrystallized microstructure, the major sub-boundaries formed perpendicularly to slip directions <11−20> in α phase and parallel to slip planes {110} in β phase, which might be caused by the difference in the symmetry of the crystal structures.

Abstract: The microstructure evolution and strength properties of a Cu-0.096%Cr-0.057%Zr alloy subjected to equal channel angular pressing (ECAP) at a temperature of 673 K via route B_C to total strains of 1 to 4 were examined. The planar low-angle boundaries with moderate misorientations form within initial grains during the first ECAP passes. Upon further processing the misorientations of these boundaries progressively increase and the formation of new ultrafine grains occurs as a result of continuous dynamic recrystallization. Partially recrystallized ultrafine grained structure evolves at strains above 4. After straining to 4 the (sub) grain size attains 0.65 μm. The large plastic straining provides significant strengthening. The ultimate tensile strength increases from 190 MPa in the initial state to 420 MPa after 4 ECAP passes. A modified Hall-Petch analysis is applied to investigate the contribution of grain refinement and dislocation density to the overall strengthening.

Abstract: The microstructure evolution of martensitic Ti-6Al-4V alloy was investigated through uniaxial hot compression at 700°C and a strain rate of 10^-3 s^-1. A combination of scanning electron microscopy observation in conjunction with high resolution electron back scattered diffraction (EBSD) was used to characterize the microstructure in detail. The development of the microstructure displayed continuous fragmentation of martensitic laths with increasing strain (i.e. continuous dynamic recrystallization), concurrently with decomposition of supersaturated martensite resulting in the formation of equiaxed grains. At a strain of 0.8, an ultrafine equiaxed grained structure with mostly high angle grain boundaries was successfully obtained. The current work proposes a novel approach to produce equiaxed ultrafine grains in a Ti-6Al-4V alloy through thermomechanical processing of a martensitic starting microstructure.

Abstract: Grain refinement taking place in a commercial 7055 aluminum alloy under equal channel angular pressing (ECAP) was examined in the temperature interval 250375°C. It was shown that the formation of recrystallized grains occurs through continuous dynamic recrystallization (CDRX). At 250°C, a low rate of dynamic recovery and high volume fraction of second phase particles provide the rapid formation of stable three-dimensional arrays of low-angle boundaries and their gradual transformation into high-angle boundaries. Increasing temperature leads an increase in the average crystallite size produced by ECAP from 0.7 μm at 250°C to 1.3 μm at 375°C. The effect of temperature on CDRX kinetic is discussed.

Abstract: Dynamic recrystallization mechanisms have been studied after 5182 aluminum Friction Stir Spot Welding (FSSW) and dissimilar friction stir spot welding of 6016 aluminum alloy to IF-steel using EBSD measurements. Moreover, welds have been ice quenched after welding to state on the post-dynamic microstructure evolution after the tool removal. For the Al/Al welds, fine recrystallized grains of the stir zone result from a continuous dynamically recrystallization mechanism followed by a post-dynamic recovery that reduces the low angle boundary fraction in the periphery of the pin. As far as the dissimilar Al/Fe welds are concerned, steel grains of the base metal were fragmented into sub-grains in the thermomechanically affected zone. Nevertheless, recrystallized grains of the stirred zone were about three times larger than these sub-grains. In this case, the continuously recrystallized grains undergo a post-dynamic grain-growth during friction stir welding cooling. In the upper aluminum sheet, the recrystallization mechanisms are the same as in the Al/Al welds.

Abstract: The evolution process of ultrafine grains during hot severe plastic deformation (SPD) was studied in several aluminum alloys. The structural changes can be characterized by the evolution of deformation bands such as microshear bands (MSBs) at moderate strains. The process of strain-induced grain formation can be categorized into the three stages irrespective of deformation mode and temperature: i.e. i) an incubation period for new grain evolution in low strain; ii) a grain fragmentation by frequent development of MSBs and subsequently new grains in medium strain, and iii) a full development of fine grains in large strain. Temperature effect on the new grain formation in aluminum alloys is also analysed in detail and the mechanism operating is discussed.

Abstract: A novel Al-Cu-Mg-Ag alloy with small additions of zirconium and scandium was subjected to equal channel angular pressing (ECAP) by using route B_C at 300°C to strains ranging from ~1 to ~12. Initially, the alloy was subjected to solution treatment followed by water quenching; subsequent overageing was carried out at 380°C for 3 h. It was shown that continuous dynamic recrystallization (CDRX) occurs during ECAP resulting in partially recrystallized structure; at a total strain of ~12, the portion of high-angle boundaries (HAB) attains 50 pct., average misorientation is ~25°. Crystallites having elongated shape and an average size of ~1 μm are evolved after a total strain of ~12.

Abstract: The aim of this research is to investigate the effect of Cr and Al (strong ferrite formers) on the strain-induced γ-to-pearlite transformation in eutectoid steels. The microstructure evolution during the hot deformation of three eutectoid steel grades was investigated using hot torsion testing. More specifically, the steels were deformed to strain levels varying from ε = 0,5 to ε = 1,5 at their specific Ar₁ temperature. Hot deformation of the undercooled austenite leads to strain-induced γ-to-pearlite transformation and to the almost instantaneous spheroidization of the formed carbides. The corresponding microstructures consist of submicronic cementite particles and ferritic grains that are 1-5 μm in size. It is shown that 1,5% Cr addition and 0,5% Al addition increase the equilibrium transformation temperature but slower significantly the kinetics of the strain-induced transformation and consequently reduce the kinetics of cementite spheroidization and of ferrite recrystallization.

Abstract: The influence of temperatures and passes on hot extruded 7075 Al bar were investigated using hydraulic press. The microstructures were observed using optical microscopy. The results showed that the as received AL bar consists of lamellar grain structures, and that equiaxed mixed grain size were observed with different passes at 320°C. Continuous dynamic recrystallization (CDRX) is responsible for the formation of small grain. The large grain tends to increases with more passes due to longer holding time. CDRX were observed for one pass only at 480°C. Lamellar grain structures were formed with different passes at 480°C.The lamellar grain structures are aligned at 50° to 70° to compression direction from one pass to four passes.

Abstract: Microstructural changes during equal channel angular pressing (ECAP) at the temperatures of 250 and 300°C to the strains ~4, ~8 and ~12 were studied in a coarse-grained Al-5.4%Mg-0.5%Mn-0.1%Zr alloy. At a strain of ~4, the microstructural evolution is mainly characterized by the development of well-defined subgrains within interiors of initial grains and the formation of fine grains along original boundaries. Further straining leads to increase in the average misorientation angle, the fraction of high-angle grain boundaries and the fraction of new grains. However, only at 300°C, the plastic deformation to a strain of ~12 leads to the formation of almost uniform submicrocrystalline (SMC) grained structure with an average crystallites size of ~ 0.5 m. At 250°C, the microstructure remains non-uniform and consists of subgrains and new recrystallized grains. The mechanism of new SMC structure formation after ECAP is discussed.