Coarsening Of Lamellar Structure Research Articles

Effect of thermal exposure on microstructure evolution and mechanical properties of cast beta-solidifying TiAl-based alloy doped with Gd

Evolution of an initial nearly-lamellar (γ+α2+β0) structure of a new cast β-solidifying TiAl-based alloy Ti-44.5Al-2V-1Nb-2Cr-0.1Gd (at.%) after isothermal exposures at 800 °C for up to 10,000 h has been systematically investigated using SEM, TEM, EDS, and XRD analyses. During the exposures the alloy microstructure degraded by the α2 laths and β0 phase dissolution, resulting in coarsening of lamellar structure and rising of the γ phase volume fraction. A 2.5- and 3-fold increase in the number of nano-sized gadolinium oxide (Gd2O3) particles have been observed after 10,000-h exposure in lamellar colonies and in grain boundaries, respectively. The gadolinium oxides had monoclinic (C2/m) and cubic (Ia-3) structures and precipitated predominantly along γ/α2 and γ/β0 interfaces and inside the γ lamellae. Furthermore, the applied creep loading was found to contribute significantly to the precipitation process of Gd2O3 nano-particles, since dislocation networks being the sites for their preferred formation. Nanohardness of the γ phase increased from 4.4 ± 0.3 to 5.0 ± 0.4 GPa after the exposures, but no changes were observed for the β0 phase. As a result of structure evolution the tensile strengths reduced by 10–12 and 11–14 % measured at 20 and 750 °C testing temperatures, respectively. Meanwhile, the plastic elongation values remained essentially unchanged. We interpret this as insignificant effect of the precipitation of Gd-rich oxide nano-particles on the alloy ductility.

Evolution of microstructure and texture during recovery and recrystallization in heavily rolled aluminum

The annealing behavior of nanostructured aluminum AA1050 prepared by cold rolling to an ultrahigh strain (εvM = 6.4) has been investigated using both transmission electron microscopy and electron backscatter diffraction techniques, paying particular attention to changes in microstructure and texture during recovery and their influence on subsequent recrystallization. It is found that coarsening of lamellar structures during recovery can occur via triple junction motion, and that this process can modify the proportion of different boundary types and texture components compared to those in the cold rolled material. Additionally, the heavily deformed material is characterized by different textures and different spatial arrangements of rolling texture components in the center and subsurface. It is found that changes in the misorientation distribution and texture during coarsening are greatly affected by the initial spatial distribution of crystallographic orientations. In particular, the reduction in the fraction of high angle boundaries observed during recovery is much more pronounced in the subsurface layers than in the center layer. The initial through-thickness heterogeneity is thus greatly enhanced during recovery, which leads to significant differences in recrystallized microstructure and texture in the different layers.

Influence of the γ-phase volume fraction on the growth kinetics of discontinuous coarsening of the lamellar structure in Ti-(40–46) at.% Al intermetallic compounds

Abstract The growth kinetics of the discontinuous coarsening of lamellar structure in Ti—(40–46) at.% Al intermetallic compounds was studied using optical microscopy, scanning electron microscopy and transmission electron microscopy. The growth rate of discontinuous coarsening shows a maximum near 44 at.% Al. The interlamellar spacing of both primary and secondary lamellar cells also shows a minimum at about 43 at.% Al. The growth rate calculated using solely the elastic strain energy as the driving force shows a linear/relation with the measured value on a normalized scale. This kinetics analysis suggests that the driving force for the migration of grain boundaries is the elastic strain energy rather than the chemical or interfacial energies. The last two energies, being chemical in nature, are believed to be dissipated by the establishment of local equilibrium due to a comparatively rapid volume diffusion in front of the migrating grain boundary. The elastic strain energy depends on the elastic properti...

Gamma titanium aluminides

The first international symposium on gamma titanium aluminides (ISGTA) was held in conjunction with the TMS `95 Annual Meeting, 13--16 February 1995, Las Vegas, under the sponsorship of the TMS Structural Materials Division--Titanium Committee. The selected papers cover almost the entire aspects of the current gamma technology such as fundamentals, alloy development and design, process development, microstructure and property evaluation, and applications. The current status as well as the future of gamma titanium aluminide alloys are discussed, as an emerging engineering materials technology, including potential/real application areas of cast gamma alloys in turbine engines. The topics on environmental effects include oxidation resistance, coating, hydrogen effects and the effects of oxide scale on mechanical properties. Phase relations and transformations are discussed for alpha-phase decomposition, coarsening of lamellar structures, high temperature phase stability, and solidification. Deformation mechanisms are evaluated to explain the flow stress anomaly behavior. Deformation character, fracture behavior, fatigue and creep are evaluated for the lamellar structure of PST crystals. Advanced methodologies are evaluated for calculating the TiAl phase diagram as well as site preferences of various elements in the gamma phase. Studies on strengthening by solid solution as well as precipitation is reported. Measurements are reported of the solubilitiesmore » of oxygen and carbon as well as of the diffusivity and activation energies for various alloying elements in the gamma phase. Melting processes as well as methods for near-net-shaping are discussed. In summary, these proceedings confirm that gamma alloys are emerging as a new generation of structural materials. One hundred and two papers have been processed separately for inclusion on the data base.« less

Coarsening of Lamellar Structures at Elevated Temperatures

The coarsening of lamellar eutectic alloys is controlled by faults in the as-grown structure. A model, based on the use of In cosh cylinder co-ordinates, is proposed to calculate the rate of shrinkage of lamellar terminations. It is found that for high volume fractions and diffusion through the continuous phase around the fault, the fault shape remains approximately constant and a value can be obtained for the rate of shrinkage of an isolated fault. For the case of interface diffusion, the shape is not preserved and only the initial shape changes can be calculated. The problem of coarsening of an array of faulted lamellae is considered. The important variables in the coarsening rate are the lamellar spacing and fault density. These results are compared to the experimental observations of Graham and Kraft on unidirectionally solidified Al–CuAl2 alloys.