Papers by Keyword: Microstructural Evolution

Abstract: Monotectic alloys are of great industrial importance because of their favorable tribological behavior. Many studies in order to better understand the morphologies obtained by monotectic reaction have been developed. To better understand the morphologies obtained by monotectic Al-1.2wt%Pb alloy reaction, especially in relation to induced convective flow, this paper aims to compare the microstructural evolution of the alloy obteind by directional solidification in transient heat-flow conditions in upward, downwand and horizontal solidification devices. It was observed a morphology transition from particles to fibers on upward and downward cases and a morphology of only particles on the horizontal case. The classical relationship used for eutectic growth, λ²v = C, which was considered applicable to monotectic reactions, didn’t seemed to be valid in the interphase spacing evolution for the downward device, however power functions (λ = C.v^a) were found in all cases.

Abstract: In this work, a 1600 tons forging machine was employed to conduct forging on the Ti-6Al-4V alloy. The microstructural and textural evolution of forged alloy were investigated by optical microscopy (OM), scanning electron microscopy (SEM), electron backscattered diffraction technique (EBSD) and X-ray diffraction (XRD). The results showed that the lamellar α phase in the Ti-6Al-4V alloy changed into a spherical morphology, and the spheroidization rate and the structure uniformity increased with the increasing forging times. The largest pole density in each incomplete pole figure of the α phase decreased gradually and the high-angle grain boundaries (HAGBs) gradually turned to the low-angle grain boundaries (LAGBs), which indicated that the texture was reduced during the forging process.

Abstract: Dynamic recrystallization is responsible for the properties of the final product of TA16 alloy in hot deformation. In this study, a cellular automata model with dynamic recrystallization (DRX-CA) was developed to simulate and predict the microstructural evolution of TA16 alloy during hot deformation with material constants obtained from hot compressive tests. The proposed model has a capability of tracking the deformation history and microstructural evolution. The numerical simulation results obtained by using the developed DRX-CA model were compared to those experimental data obtained for validation and accurately capture the relations among strain, stress, volume fraction recrystallization, recrystallized grain size and deformation temperature.

Abstract: A series of CoFeNi₂W_0.5Ta_x (x = 0-0.6) high entropy alloys (HEAs) were synthesized by arc melting to investigate the alloying effect of Ta element on the microstructure and mechanical properties of the CoFeNi₂W_0.5 alloy system. Phase constitution, microstructure and mechanical properties of the alloys were analyzed by X-ray diffraction (XRD), scanning electron microscopes (SEM), Vickers hardness and compressive test. It was found that when x = 0, the alloy consists of a single-phase face-centered cubic (FCC) solid solution structure and exhibit excellent ductility, the compressive plastic elongation of which can reach 80% without fracture. While with increasing Ta content, the brittle Co₂Ta-type Laves phase appears which leads to a decrease of the plastic strain and an increase of the yield strength, and the Vickers hardness shows an obvious increase from HV 179.5 to HV 753.2.

Abstract: Magnesium and magnesium alloys offer high potential as lightweight materials. Current works are mainly focused on the metal forming technologies and material development for sheet and strips to provide magnesium flat products for industrial applications. However, the technology for the production of magnesium long products for fasteners or other connecting elements is exclusive the extrusion process. A cost-efficient alternative can be the caliber rolling technology for magnesium rods and wire with regard to refined microstructure and specific required properties. But this whole process is rarely applied up to now and all material-specific as well as deformation relevant basics must be developed and additionally validated under industrial conditions. This paper gives the overview for a magnesium-specific wire rolling technology under consideration of chemical composition (AZ31, AZ61, AZ80) and their influence to final mechanical properties in correlation with the microstructure evolution along the whole process line. Therefore, the process-and material-dependent microstructural evolution during rolling process was investigated. The structural constitution is detailed by the grain size and the precipitation conditions. For the determination of the mechanical properties hardness measurement as well as tensile testing was carried out. To preliminary design and determine the material flow, the temperature distribution, and the logarithmic strain, a commercial numerical simulation tool was applied on base of the implemented material-specific deformation and recrystallization behavior. Hence, it was possible to design a magnesium specific caliber sequence for the production of fine-grained magnesium wires with Ø 8 mm and excellent mechanical properties.

Abstract: High energy ball mill tests under the condition of the ball material mass ratio 13:1 and the rotate speed 400 r/min have been employed to investigate the process of mechanical alloying (MA) of Ta and Cr powder mixed in the mole ratio of 1:2.The microstructure evolution process and phase composition were explained useing scanning electron microscope (SEM) and X-ray diffraction (XRD). The results show that, the milled 20h powder existed in Ta (Cr) supersaturated solid solution and amorphous after 40h. Although the hours were spent on ball milling reached to 50h, Laves phase TaCr₂ had not been made during the process.

Abstract: Experiments of vertical unsteady-state directional solidification were carried out in order to permit the influence of copper alloying to Al-Si alloys on the scale of secondary dendritic arm (λ₂) to be investigated. The microstructures of Al-nSi-3wt%Cu alloys, with “n” equal to 5.5wt%Si and 9.0wt%Si, were characterized and correlated with solidification thermal parameters: the growth rate (V_L), the tip cooling rate (Ṫ) and the local solidification time (t_SL). A comparative analysis between the present results and those from the literature related to the secondary dendrite growth during directional solidification of Al-nSi alloys is also conducted. It is shown that the addition of Cu to both Al-nSi alloys decreases λ_2, and experimental growth laws relating λ₂ to V_L and Ṫ_L are proposed for the ternary alloys examined. The experimental scatter of λ₂ is also compared with the only theoretical dendritic growth model from the literature for multicomponent alloys, and it is shown that the theoretical predictions overestimate the present experimental results.

Abstract: In this paper, investigations of the softening behaviour of a supersaturated Al-Mn-Fe-Si alloy during annealing after cold rolling have been carried out. Two different homogenization conditions were considered, of which one gives a condition of a large amount of small pre-existing dispersoids, i.e. providing a significant static Zener drag, while the other gives a condition where both concurrent precipitation and dispersoid drag effects are limited. The homogenized samples with different microchemistry states were then cold-rolled to different strains before subsequent annealing at 300°C. The softening and concurrent precipitation behaviours have been monitored by hardness and electrical conductivity measurements respectively, and the microstructural evolution has been characterized by EBSD. It is clearly demonstrated that the actual microchemistry state, i.e. amount of solutes and second-phase particle structures as determined by the homogenization procedure strongly influence the softening behaviour where a fine dispersion of pre-existing dispersoids together with concurrent precipitation slow down the recrystallization kinetics considerably and give a very coarse and elongated grain structure.

Abstract: Nanometer Al₂O₃ particles reinforced Al-Zn-Mg-Cu composites were processed by cryogenic treatment with different cycle index of 1,2 and 3. Transmission Electronic Microscopy (TEM) was utilized to analyze the microstructural evolution of the treated samples. The results show that with the increase of cycle index the precipitate amount and dislocation density have been increased. The precipitates orientation exhibits some preferred orientation. The high amount of precipitates and dislocation density and preferred orientation of precipitates will benefit to increase the mechanical properties of composites.

Abstract: Material models that couple the evolution of flow stress to the evolution of the microstructure are important for the simulation of hot working processes in which the microstructure undergoes large changes. Among the microstructural evolution mechanisms in hot working, dynamic recrystallization (DRX) plays a central role as it occurs during deformation. When the workpiece deforms, the element shape may deteriorate, which makes re-meshing necessary. At the same time, certain regions of the finite element mesh undergo DRX and a sharp interface between recrystallized and non-recrystallized portions of the workpiece develops. Elements of the old mesh that are cut by the interface contain nodes with a non-zero recrystallized volume fraction and nodes where the recrystallized volume fraction is zero. During re-meshing, when the microstructural data is transferred to the new mesh, nodes or integration points that are actually in region of the workpiece that is not yet recrystallized may be assigned a non-zero recrystallized volume fraction. As a consequence, the interface moves, which is unwanted and may produce large errors when re-meshing is frequently done. In this paper, the problem of the propagation of the DRX interface during re-meshing is treated. It is shown that the propagation occurs with standard data mapping algorithms and produces a large error at the interface. A re-meshing scheme is proposed that uses a smooth mesh-free interpolating function based on radial basis functions to interpolate the recrystallized volume fraction. The interface is the zero level set of this interpolant. Performing the mapping as a least squares fit of the interpolant allows for a substantial reduction of the mapping error and suppresses the propagation of the DRX front.