High temperature internal friction in a Tie46Ale1Moe0.2Si intermetallic, comparison with creep behaviour

Advanced g-TiAl based intermetallics Mo-bearing have been developed to obtain the fine-grained microstructure required for superplastic deformation to be used during further processing. In the present work we have studied an alloy of Tie46.8Ale1Moe0.2Si (at%) with two different microstructures, as-cast material with a coarse grain size above 300 mm, and the hot extruded material exhibiting a grain size smaller than 20 mm. We have used a mechanical spectrometer especially developed for high temperature internal friction measurements to study the defect mobility processes taking place at high temperature. The internal friction spectra at different frequencies has been studied and analyzed up to 1360 K in order to characterize the relaxation processes appearing in this temperature range. A relaxation peak, with a maximum in between 900 K and 1080 K, depending on the oscillating frequency, has been attributed to Ti-atoms diffusion by the stress-induced reorientation of AleVTieAl elastic dipoles. The high temperature background in both microstructural states, as-cast and extruded, has been analyzed, measuring the apparent activation parameters, in particular the apparent energies of Ecast(IF) ¼ 4.4 ± 0.05 eV and Eext(IF) ¼ 4.75 ± 0.05 eV respectively. These results have been compared to those obtained on the same materials by creep deformation. We may conclude that the activation parameters obtained by internal friction analysis, are consistent with the ones measured by creep. Furthermore, the analysis of the high temperature background allows establish the difference on creep resistance for both microstructural states.Peer reviewe

On constitutive equations for various diffusion-controlled creep mechanisms

Plastic flow of polycrystalline solids at elevated temperatures occurs by one of three independent deformation mechanisms: slip by dislocation movement, sliding of adjacent grains along grain boundaries, and directional diffusional flow. All three mechenisms are considered to be thermally activated and controlled by the diffusion of atoms. Constitutive equations have been developed which accurately describe each of the three independent mechanisms. These equations center on a power law dependence of the creep rate. Thus the stress exponent, n, in εωασn, is shown to have discrete values depending on the plastic flow mechanism. For deformation by slip, n can take on values ranging from n= 1 to n=8 depending on the specific dislocation mechanism. For grain boundary sliding, n can be either 2 or 4, and for diffusional creep, n is unity. The microstructure is an important factor In establishing the magnitude of the creep rate for each of the three independent deformation mechanisms. Grain size is the principal microstructural feeture in determining the creep rate for deformation by grain boundary sliding and by diffusional flow. On the other hand, the subgrain size and the dislocation density plays an important role in determining the creep rate for deformation by dislocation motion; examples are shown for ODS alloys. Competition between these various mechanisms can be described quantitatively through the use of constitutive equations and deformation mechanism maps. It is shown that diffusional creep is not as dominating a process as has been considered in the literature, and that grain boundary sliding or Harper-Dorn creep are the more likely deformation mechanisms occurring at low stresses and high temperatures for fine grain size materials

Grain shape and microstructural evolution during equal channel angular pressing

The congruence between theoretical and experimental shape changes in flow plane microstructures after repetitive equal channel angular pressing (ECAP) by route A is demonstrated for initially spheroidal grains. The model has been extended to predict shape changes of initially ellipsoidal grains of various aspect ratios during repetitive ECAP by route A. The potential role of initial grain morphology on the grain refinement is suggested

Severe plastic deformation of an as-cast hypoeutectic Al-Si alloy

The article of record as published may be found at http://dx.doi.org/10.1007/s10853-008-2625-yDifferent equal channel angular pressing (ECAP) processing routes have been employed to investigate the flow plane microstructures in a hypoeutectic Al–7wt%Si. In the as-cast condition, this alloy exhibits equiaxed primary aluminum dendrite cells embedded in an Al–Si eutectic constituent. The observed microstructures have been compared to the predicted distortion of a volume element expected during idealized ECAP. The effect of different processing routes on the microstructure refinement, degree of homogenization of second phase particles, and associated mechanical properties are discussed.CICYTU.S. Air Force Office of Scientific ResearchU.S. National Research CouncilProgram MAT2003/01172 (CICYT)No. FIATA06058G001 (AFOSR

Texture analysis of the transition from slip to grain boundary sliding in a continously recrystallized superplastic aluminum alloy

An investigation was conducted into the phenomenon of continuous recrystallization in a superplastic Al-/5%Ca-/5%Zn alloy. The as-processed microstructure includes adjacent regions that have lattice orientations corresponding to the symmetric variants of the most prominent texture component, {2 2 5}<5 5 4). This orientation is near the copper, or C, component, {1 1 2}<1 1 1). A cellular dislocation structure with highly disoriented cell walls was present within these regions. Continuous recrystallization during static annealing resulted in the development of distinct boundaries accompanied by retention and sharpening of the texture and the development of a bimodal grain boundary disorientation distribution. The high-angle boundaries (50 -/62.88) are the interfaces between grains having lattice orientations as symmetric variants of the texture, while the low-angle boundaries (2 -/158) correspond to a cellular structure within the variants. Such a structure persists during superplastic deformation over a wide range of temperature and strain rate conditions. Both dislocation creep and grain boundary sliding operate simultaneously in response to the applied stress under all testing conditions investigated. The relative contribution of each of these mechanisms varies depending on the testing conditions. This is a consequence of the presence of the variants in the microstructure and their persistence during deformation. Fiber texture formation was not observed during either longitudinal or transverse deformation.Spanish Commission of Science and Technology (CICYT)Spanish Ministry of Science and TechnologyGrant MAT 97/070

Influence of microstructural stability on the creep mechanism of Al-7wt% Si alloy processed by equal channel angular pressing

The article of record as published may be located at http://dx.doi.org/10.1016/j.msea.2014.06.017A Na-modified, as-cast Al-7 wt% Si alloy was processed by equal channel angular pressing (ECAP) up to 8 passes by route A at ambient temperature using a 90 degree square section die, obtaining improved strength ductility and work fracture. From the first pass, porosity is removed, the eutectic constituent is refined and the eutectic silicon particles are partially redistributed..

Superplastic behavior of two ultrahigh boron steels

The high-temperature deformation behavior of two ultrahigh boron steels containing 2.2 pct and 4.9 pct B was investigated. Both alloys were processedvia powder metallurgy involving gas atomization and hot isostatic pressing (hipping) at various temperatures. After hipping at 700 °C, the Fe-2.2 pct B alloy showed a fine microstructure consisting of l-µm grains and small elongated borides (less than 1µm) . At 1100 °C, a coarser microstructure with rounded borides was formed. This alloy was superplastic at 850 °C with stress exponents of about two and tensile elongations as high as 435 pct. The microstructure of the Fe-4.9 pct B alloy was similar to that of the Fe-2.2 pct B alloy showing, in addition, coarse borides. This alloy also showed low stress exponent values but lacked high tensile elongation (less than 65 pct), which was attributed to the presence of stress accumulation at the interface between the matrix and the large borides. A change in the activation energy value at theα-γ transformation temperature was seen in the Fe-2.2 pct B alloy. The plastic flow data were in agreement with grain boundary sliding and slip creep models.Peer reviewe

Particle and grain growth in an Al-Si alloy during high-pressure torsion

An Al–7%Si alloy was processed by high-pressure torsion (HPT) for five turns at room temperature under a pressure of 6 GPa. Microstructural examination after HPT revealed submicrometer grains and a homogeneous distribution of silicon particles. Near the peripheries of the HPT disks, there was precipitation of small Si particles, growth of larger Si particles and grain growth in particle-free zones around these larger particles. The effect is attributed to an estimated temperature rise of about 120–140 K during straining.Peer reviewe