Papers by Keyword: Grain Growth

Abstract: In this study, the effect of second step sintering holding time on LTD of 3 mol% yttria tetragonal zirconia polycrystals (3Y-TZP) ceramic was studied. The Zirconia powder was die pressed followed by cold isostatically pressed (CIP) at 200 MPa, and the samples were initially heated to a first step temperature of 1400°C with constant heating rate 10°C/min and the samples were stand still for 1 minute. Then cooled down to the second step temperature of 1200°C and maintained at that temperature for 2 and 10 hours before cooled down to the room temperature. The other sample was sintered by using Single Step Sintering (SSS) at 1400°C for two hours. The phase content in the zirconia samples was measured by using X-ray diffractometer. Average grain size was calculated by using Field Emission Scanning Electron Microscope (FESEM). The low temperature degradation study was conducted in an autoclave containing superheated steam at 180°C and 10 bar pressures for periods up to 60 hours. Results revealed that the two-step sintering (TSS) effectively controlled the grain growth than SSS and subsequently ageing. Increase in second step sintering holding time from two to ten hours also increased the densification and hydrothermal ageing resistance of Y-TZP ceramics.

Abstract: The understanding of the softening behaviour during the hot rolling process is required to optimize the hot rolling schedule. Therefore, the microstructural evolution in the hot rolling of austenitic stainless steel was simulated. In this work, kinetics of grain growth was investigated by means of compression tests using the Gleeble HDS V40 and described by appropriate kinetic equations based on the obtained experimental results. Moreover, numerical simulation was performed using the Simufact.forming software. The results of the numerical simulation were further validated by experimental data, which were obtained from the labour continuous hot rolling of the austenitic stainless steel.

Abstract: It is widely accepted that the dominant deformation mechanism of fine-grained superplasticity is through grain boundary sliding (GBS) that occurs in fine-grained materials. However, it has been reported that in “Class I” solid solution alloys, superplastic-like behavior controlled by trans-granular deformation occurs by solute drag creep. In this study, we have investigated superplastic behavior in a fine-grained aluminum solid solution alloy with a thermally unstable microstructure. To obtain fine-grained microstructure, friction stir processing (FSP) was applied to a commercial 5083 aluminum (Al−Mg) alloy. An equiaxial fine-grained microstructure with a grain size of 7.4 μm was obtained after FSP; however, this microstructure was unstable at high temperatures. Generally, for fine-grained superplasticity or GBS to occur or continue, the fine-grained microstructure must be smaller than 10 μm during high-temperature deformation. However, a large elongation of over 200% was observed at high temperatures despite the occurrence of grain growth. From microstructural observations, it was determined that a fine-grained microstructure is maintained in the early stage of deformation, but at strain levels greater than 100%, trans-granular deformation occurs. The microstructural feature of this trans-granular deformation is similar to the deformation microstructure of solute drag creep observed in “Class I” solid solution alloys. This indicates that a change in the deformation mechanism from GBS to solute drag creep takes place during high-temperature deformation. Here, based on our observations on our model system, which is a thermally unstable aluminum solid solution alloy, we discuss the possibility of a superplastic elongation occurring by means of a transition of the deformation mechanism.

Abstract: The crystallographic aspects of nucleation of cube grains during annealing have been analyzed in (234)[20-28 11] - oriented aluminum single crystal. The samples were plane strain compressed in a channel-die up to logarithmic strains of 0.5 (40%) and then annealed to develop initial and final stages of primary recrystallization. The deformed and annealed samples were analyzed using scanning electron microscopy equipped with EBSD facility. Local orientation measurements reveled that significant part of the sample deforms homogeneously with only small deviation from the initial crystal orientation. The heterogeneities were thin bands of localized strain in which the crystal lattice rotate towards another variant of S orientation. After annealing the orientations identified inside deformed/recovered areas were similar to that observed in the sample just after deformation. The crystal lattice of recrystallized grains exhibit a well-defined clockwise and anticlockwise rotations around the axes grouped near all normals of the {111} planes of the deformed/recovered state. The cube grains were observed in both homogeneously and heterogeneously deformed areas despite the cube-oriented nuclei surrounded by high angle boundary were not present in the as-deformed structure.

Abstract: The mechanical properties of steels are strictly connected to chemical composition as well as to microstructural features obtained after thermo-mechanical processing. As a consequence, recrystallization and grain growth are relevant to the mechanical properties of steels, thus suggesting the necessity of mathematical models able to predict the microstructural evolution after thermo-mechanical cycles. In particular, in stainless steel grades, mechanical characteristics, and a proper microstructure with an adequate grain size distribution, are very important in order to achieve the required formability and deep drawing properties for many applications. This paper deals with the study of microstructural changes, such as grain size variations and recrystallized volume fraction in stainless steels during isothermal treatments through the application of a mathematical model, able in general to describe the primary recrystallization and grain growth in metals. The developed model takes into account the recrystallization phenomenon and Zener drag effect. A general continuity equation is proposed describing in continuous way recrystallization and grain growth phenomena without taking into account textures effect. The influence of input parameters is analyzed.

Abstract: The microstructures, mechanical properties, deformation mechanism, and recrystallization behavior of Cu-Zn-Bi alloys for cartridge case application have been investigated in this research. Cu-28Zn-1.1Bi wt. % alloys were produced by gravity casting and subjected to a homogenization – cold rolling – annealing sequences with variations on reduction level and annealing temperature. Samples characterizations were done through optical emission spectroscopy, optical microscopy, SEM-EDS imaging, and X-ray mapping modes, while hardness measurements were performed using micro Vickers method. The presence of Bi was found to increase cartridge brass hardness through a dispersoid strengthening mechanism in which dislocation movements are rendered by Bi particles. Higher deformation levels resulted in higher microhardness of the alloy. Recrystallization took place at grain boundaries and areas surrounding Bi dispersoid at 400 °C, while further heating resulted in grain growth phenomenon. Bismuth addition accelerated the recrystallization process in cartridge brass by a particles stimulated nucleation (PSN) mechanism.

Abstract: Grain growth and grain boundary character distribution relationships in Fe-Ga rolled sheet is investigated to study the influence of H₂S gas content in argon on the development of selective grain growth through secondary recrystallization. Abnormal growth of (011) grains was predominant at the low content of H₂S gas, while (113) grain growth was well developed at contents higher than 1.33% H₂S. On the other hand, the development of (001) grains was challenging to produce because it is very sensitive to the anneal environment and has a relatively low fraction of high energy grain boundaries associated with misorientation angles that determine the mobility of boundaries.

Abstract: The results of the studies presented here are devoted to understanding of microstructure effect on the processes and properties driven by diffusion. The role of various interfaces (intergranular, phase, free surface), as the high-energy defects, is underlined and investigated with special attention. The methodology relevant to analyses of the microstructural processes is first briefly presented. The capability and limitations of classical molecular dynamics, mesoscale Monte Carlo and cellular automaton techniques are described. Two examples of the diffusion driven processes analyzed at various length and time scale are shown: namely, grain growth in nanometallic materials and melting of thin embedded films. The modeling results are also accompanied with experimental studies. Thanks to application of numerical methods, models of relevant processes were proposed, which enabled to provide quantitative relationships between microstructure and the process kinetics. Such relationships can be later used for design of optimized materials for wide range of applications.

Abstract: The effect of solution treatment temperature on the grain growth behavior of fine grained nickel-base superalloy FGH96 has been investigated. The results showed that with the increase of solution treatment temperature, the grain growth of fine grained FGH96 alloy (<5μm) experienced 3 stages, the slow growth stage below 1120°C, the steady grain size stage from 1120°Cto 1250°C, which revealed the fine grained FGH96 alloy with wide solution treatment window compared to coarse grain, and the exponential growth stage. At the same time, the primary γ′ and MC carbides on impeding grain boundary migration to limit grain growth was demonstrated. The grain growth model has been established, and the activation energy of grain growth was calculated to be 288 KJ/mol through linear fitting.

Abstract: This paper deals with the analysis of microstructure in the conditions after austenitising at high temperatures (1150 °C, 1180 °C a 1210 °C/45 min) for 9CrNB steel, and with the analysis of microstructure in the conditions after subsequent tempering at temperatures 740 °C, 770 °C, 800 °C/1 hr. After austenitising the microstructure was formed of lath martensite with secondary carbides. The average size of initial austenitic grains ranged from 144 μm (1150 °C) to 163 μm (1210 °C). Increasing the austenitising temperature led to insignificant improvement in strength properties, while the plasticity of the steel decreased. Microstructure in tempered conditions was formed from tempered martensite. This martensite had lath morphology, with inhomogeneus distribution of precipitates. Higher austenitising temperature and increased tempering temperature led to spheroidal and growth of secondary particles. The tempering effect on strength and plastic properties of steel are also described.