Accommodation Mechanism For Grain Boundary Research Articles

Texture Change during Superplastic Deformation in Fine-Grained Magnesium Alloys

Texture change during superplastic deformation was examined and compared in two magnesium alloys with different chemical composition. These alloys were extruded to refine the microstructure. The pre-existing basal texture of both alloys became slightly more random within the bulk probably owing to grain boundary sliding and the accompanying grain rotation. However, the texture changes differed between tensile and compressive deformation along the extrusion (longitudinal) direction. This fact suggests that dislocation slip is important in superplastic deformation. It was concluded that dislocation slip acts primarily as an accommodation mechanism for grain boundary sliding.

Inelastic deformation of nanocrystalline Au thin films as a function of temperature and strain rate

The mechanical behavior of nanocrystalline Au thin films with average grain size of 64nm was investigated at strain rates 10−5–10s−1, and temperatures between 298 and 383K. The yield strength was highly sensitive to both temperature and strain rate: at room temperature it increased by ∼100% within the range of applied strain rates, while it decreased by as much as 50% in the given temperature range at each strain rate. The ductility and activation volume trends pointed to two distinct regimes of plastic deformation: namely, creep-driven and dislocation-mediated plasticity, with the transition occurring at increasing strain rate for increasing temperature. The activation volume for creep-influenced deformation increased monotonically from 6.4b3 to 29.5b3 between 298 and 383K, signifying grain boundary (GB) diffusion processes and dislocation-mediated creep, respectively. Dislocation climb, as an accommodation mechanism for GB sliding, provided an explanation for the increased activation volume at higher temperatures. The activation volumes calculated at high strain rates decreased from 19.7b3 to 11.4b3 between 298 and 383K. A model for thermally activated dislocation depinning was applied to explain this abnormal decreasing trend in the activation volume, resulting in activation energy of 1.2eV.

An Internal Variable Approach to Inelastic Deformation Including Structural Superplasticity

An internal variable theory has been proposed to account for the essential microstructures during inelastic deformation. The framework of the theory is built on the basis of well known dislocation dynamics to provide the concept of an internal strain tensor as the most fundamental deformation state variable. The plastic and inelastic strain rate tensors are then naturally defined and also a kinematics relation among them can further be derived from the time rate of change of this internal strain tensor, which in fact accounts for the evolution of microstructures during inelastic deformation. To complete the theory, the constitutive relations between the various stress variables and their conjugate deformation rate variables are then derived based on the dislocation kinetics. The theory is then further extended to describe the structural superplasticity, taking this slip zone model with dislocation pile-ups as the major accommodation mechanism for grain boundary sliding. The experimental results obtained from the various crystalline materials are then presented and compared with each other in relation to the internal variable theory for inelastic deformation.

Strain Rate Sensitivity of Superplastic Inconel 718

The strain rate sensitivity, m, of a superplastic-quality (SPF) INCONEL 718 alloy has been measured in the temperature and strain rate ranges of 925–1000°C and 10−4–10−2 s−1 using both the classical method of instantaneous strain rate changes and a new method based on the continuous response to a sinusoidal strain rate input. The novel method furnishes continuous information from the whole stress–strain curve on the dependence of m on strain rate transients.In agreement with other recent studies, our results show that the maximum m values of the 718 alloy, although interesting enough for industrial superplastic forming, are lower than those shown by other superplastic materials for typical aeronautical application. A texture analysis indicates that crystallographic slip does not merely act here as an accommodation mechanism for grain boundary sliding. It significantly contributes to the total plastic strain, being thus responsible for the relatively low values observed for the strain rate sensitivity m.

Superplasticity and deformation mechanisms in NiAl(- Mo) alloy

The mechanical behaviour and microstructural evolution in tension of NiAl - 9Mo eutectic alloy at 1100 ° C and strain rates from 10-5 to 10-3 s-1 have been investigated. High values of strain rate sensitivity index, but relatively small elongations between 150 and 200%, have been observed. Tensile specimens with various strains were quenched in water to preserve the dislocation structures for TEM examination. The TEM results show that the dislocation configuration and density change significantly with an increase in strain rate, and therefore correspond to different deformation mechanisms. At a low strain rate (5.5 × 10-5 s-1), the dislocation density is relatively low and dislocation activity is regarded as an accommodation mechanism for grain boundary sliding. In contrast, the high density of dislocations as well as clear subboundaries observed in grains at a high strain rate (5.5 × 10-4 s-1) suggest that the dislocations are active directly in response to the applied stress as well as participating in the relaxation process of grain boundary sliding by subboundary formation. Thus, grain boundary sliding is mainly responsible for superplastic deformation at a low strain rate, while superlastic deformation at a high strain rate is controlled by the combined operation of both grain boundary sliding and dynamic recovery.

Microstructural Evolution of Quasi-Single Phase Alloy during Superplastic Deformation

Flow shifts and changes in microstructure were investigated for superplastic deformation of 7475 Al alloy. The recently proposed internal variable theory of structural superplasticity has been applied to the results of mechanical test. Deformed microstructure was observed using a TEM to check the validity of the constitutive relations. Accommodation mechanism for grain boundary sliding was also examined through the microchemical analysis of deformed microstructures.

Microstructural aspects of superplasticity

This short review summarizes the contribution of microstructural studies to the present understanding of the deformation mechanisms operating during superplastic deformation. A background summary is included that describes the major features of superplasticity, the relevant deformation theories, and the requirements for successful microstructural studies. However, most of the paper is concentrated on a discussion of the microstructural evidence in the literature on two types of material. The first is a representative group of modern commercial nickel, aluminum, and magnesium alloys. The second is the Zn-Al eutectoid, a member of the well characterized “classical” group of superplastic materials based upon eutectic or eutectoid compositions. The importance of dislocation motion at maximumm is emphasized, both as an accommodation mechanism for grain boundary sliding and directly in response to the applied stress. However, it is shown that, in IN100 and Supral 150, other processes occur simultaneously, including recrystallization and diffusion.