Department of Metallurgical & Materials Engineering

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A simple instability condition based on the Ziegler's continuum principles as applied to large plastic flow, is extended for delineating the regions of unstable metal flow during hot deformation of Nimonic AP-1 superalloy. The optimum hot working conditions for this material are suggested.

World University root node

Powder Metallurgy

Hot working characteristics of powder metallurgy nimonic AP-1 superalloy

Tensile specimens of superplastic forming grade AA 8090 Al-Li alloy were deformed at constant strain rates in the range 1 × 10-5 - 1 × 10-2 s-1 and at constant temperatures in the range 298-843 K, to investigate their effects on the nature of stress-strain (σ-ε) curves and on the concurrent microstructures, substructures, and microtextures. The σ-ε curves exhibited flow hardening in the early part of deformation, the rate of which was found to increase with an increase in strain rate and decrease in temperature. Grain growth, cavitation, dislocation interactions, and texture weakening were observed to occur during superplastic deformation. The early part of deformation involved a substantial dislocation slip contribution to flow hardening and to the mechanism for superplastic flow. However, the microstructural evolution facilitated the conventional mechanism of grain boundary sliding and its accommodation by the diffusional process, as suggested by the strain rate sensitivity index and activation energy determined.

Materials Science and Technology

Effects of strain rate and test temperature on flow behaviour and microstructural evolution in AA 8090 Al-Li alloy

The elements that are needed to describe a deformation structure-grains, grains boundaries, macroscale banding within crystals, cell blocks, geometrically necessary boundaries and incidental dislocation boundaries (cell boundaries)-are presented and described for fcc metals and alloys of medium to high stacking fault energy. Pertinent to this quantitative description are the local orientations, structural morphology, different boundary misorientations and spacings as a function of strain and deformation conditions. Three specific examples are given in which particular aspects of the deformation structure are shown to direct the recrystallization behavior.

Fulltext

Surface and Interface Analysis

Deformation microstructures and selected examples of their recrystallization

Superplastic forming grade sheets of AA 8090 Al-Li alloy were observed to contain layers of different microstructure and microtexture across the thickness cross-section. Superplastic behaviour and its relationship to the concurrent microstructural and microtextural evolution of this sheet were studied at 803 K by tensile testing of specimens taken from the full thickness and the near surface and midthickness layers. Initially, the surface layers contained nearly equiaxed and relatively coarse grains with a strong S {123}&lt;634&gt; type texture, whereas the midthickness section had elongated fine grains and a dominant Bs {011}&lt;211&gt; texture. The stress-strain rate (σ-ε̇) curves exhibited minimum flow stress for the full thickness material. Varying levels of grain growth and texture weakening occurred in the above two layers, the extent of which depended on whether the layers were in separated form or as coexistents in the full thickness material. The maximum values of strain rate sensitivity index for the full thickness, surface, and centre materials were 0.82, 0.64, and 0.56, respectively. The corresponding ductility values were 475, 420, and 286% at ε̇ = 1 × 10-3 s-1.

Effect of layered microstructure and its evolution on superplastic behaviour of AA 8090 Al-Li alloy

A brief review of orientation imaging microscopy and an overview of the results of investigations related to textural evolution during superplastic deformation of a recent series of superplastic grade aluminium alloys, and specifically the results of the authors' study on microtextural evolution in AA 8090 Al-Li alloy, are presented. It was observed that the surface layer of the AA 8090 alloy sheet was dominated by S {011}(634) type texture, whereas the midthickness layer was dominated by brass {011}(211) type texture. In the course of superplastic deformation, textural weakening occurred, the rate and extent of which were different in the surface and centre regions, depending upon whether the two layers were deformed separately or as a composite in the full thickness tensile specimen. Textural weakening was generally more extensive at higher test temperatures and lower strain rates. Based on textural evolution and the presence of a large proportion of low angle boundaries, and their near elimination at larger strains, dislocation slip could be considered to be the dominant mechanism during the initial stages of deformation, whereas the conventional mechanism for superplastic deformation is thought to be operating during the later stages.

Microtextural evolution during superplastic deformation of AA 8090 Al-Li alloy

A simple instability condition based on the Ziegler's continuum principles as applied to large plastic flow, is extended for delineating the regions of unstable metal flow during hot deformation of powder metallurgy materials. This criterion has been applied to the existing flow stress data of powder metallurgy superalloy IN 100 and compared with the reported microstructural observations. The optimum hot working conditions for the PM superalloy IN 100 are suggested.

Identification of flow instabilities during hot working of powder metallurgy superalloy IN 100

An aluminum-lithium alloy AA8090 was deformed under superplastic condition to investigate the effect of strain, strain rate and test temperature on deformation anisotropy and microstructural evolution. The plastic anisotropy ratio was found to increase towards 1 with the increase in strain and strain rate. The initially elongated microstructure with rolling texture underwent dynamic recrystallization and tended to become equiaxed with cubic or random texture during superplastic deformation. The grain size increased with increasing strain and test temperature but decreased with the increase in strain rate. Such microstructural changes and the mechanisms involved in superplastic deformation, including the dislocation-based accommodation process, are suggested to develop isotropy. © 2000 Elsevier Science S.A.

Materials Science and Engineering A

Stain anisotropy in AA8090 Al-Li alloy during high temperature deformation

The methods for determining the extent of deformation a metal can withstand before cracking or fracture occurs are reviewed. Flow stress data are essential in the development of constitutive equations and processing maps. One of the requirements for process modelling is a knowledge of the material flow behavior for defining the deformation maps that delineate `safe' and `non-safe' hot working conditions. These maps show in the processing space, the processing conditions for stable and unstable deformation.

International Materials Reviews

Instability criteria for hot deformation of materials

A simple instability condition based on the Ziegler's instability criterion is found to be more appropriate in delineating the regimes of flow instabilities in the processing maps of 2124 Al-20 Vol% SiCp metal matrix composite. Applicability of the criterion is demonstrated using the reported microstructural observations in open literature.

Advanced Composites Letters

Processing map for hot working of powder metallurgy 2124 Al - 20 Vol% SiCp metal matrix composite

The strain rate sensitivity index (m) and activation energy (Q) parameters of the constitutive relationship for superplastic deformation of the IN718 superalloy were evaluated after prestraining. The flow behavior of IN718 superalloy over the strain rate range of approximately 1×10-5×10-4 s-1 and the temperature range of 1173-1248 K were examined. The log(σ)-log(ε) plot delineated the predominantly superplastic region over the entire temperature range, with m increasing from 0.37 at 1173 K to 0.58 at 1248 J. The Arrhenius plot revealed two activation energy values for superplastic deformation at a transition temperature of 1218 K.