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 2014 Al-20 vol% Al 2O3 metal matrix composite. The optimum hot working conditions for this material are suggested. © 2004 Elsevier B.V. All rights reserved.

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Journal of Materials Processing Technology

On the hot working characteristics of 2014 Al-20 vol% Al2O 3 metal matrix composite

A simple instability condition is applied to delineate the regions of unstable metal flow in the processing map utilising flow stress data for AFNOR 7020 aluminium alloy, which was fitted in a constitutive equation. Unstable regions identified from the simple instability condition at different strain levels were superimposed in the processing map. This phenomenon takes into account the dependence of strain rate sensitivity and strain hardening effect on plastic instability during hot deformation of the material. The processing map has been developed and validated by comparing microstructural observations of deformed compression specimens.

Materials Science and Technology

Development and validation of a processing map for AFNOR 7020 aluminium alloy

The microstructure characterization by X-ray line profile analysis is possible for determination of dislocation density, micro-strain within grains due to dislocation and average coherent domain size (sub- grain) within the grain. This study presents the X-ray diffraction peaks shape analysis and their broadening with different thermal treatments in Zr-2.5% Nb pressure tube material. The peak shape is analysed using Fourier transformation and information about coherent domain size, micro-strain and dislocation density could be obtained from the Fourier coefficients of the peak. Analysis of broadening of the peaks by integral breadth method also gives the coherent domain size, dislocation density and micro-strain present in the material . The results from the X-ray techniques are comparable to those obtained from direct observation of transmission electron microscopy. The measured yield strength increases with dislocation density. An empirical relationship is obtained for the yield strength from the dislocation density of the material. The measured strength is in agreement with the one calculated from dislocation density.

Fulltext

Bulletin of Materials Science

X-ray diffraction line profile analysis for defect study in Zr-2·5% Nb material

Hydrogen in excess of the terminal solid solubility precipitates out as a brittle hydride phase in zirconium alloys. The hydrides acquire platelet shaped morphology due to their accommodation in the matrix and can cause severe embrittlement, especially when these are oriented normal to the tensile stress axis. The precipitation of hydride platelets normal to the tensile stress when cooled under stress from a solution-annealing temperature is commonly referred to as 'stress-reorientation'. The stress-reorientation is associated with a threshold stress below which no reorientation is observed. In this work, stress-reorientation of hydrides was investigated for unirradiated, cold worked and stress-relieved Zr-2.5 wt% Nb pressure tube material for a reorientation temperature of 423-723 K. The effect of the reoriented hydrides on the tensile properties of the Zr-2.5 wt% Nb pressure tube alloy was evaluated in the temperature range of 298-573 K. © 2003 Elsevier B.V. All rights reserved.

Journal of Nuclear Materials

Stress-reorientation of hydrides and hydride embrittlement of Zr-2.5 wt% Nb pressure tube alloy

This paper presents some of the criteria commonly used to predict ductile fracture in metal-forming operations. To examine the adequacy of these criteria, the test data of spheroidised steel are considered. A simple ductile fracture criterion is proposed in terms of integrals of stress functions. This criterion takes into account the tensile stress ratio and triaxiality. A simple and reliable procedure is also suggested to evaluate the material constants in the criterion. It is found that the proposed ductile fracture model correlates the test data of spheroidised steel. © 2003 Published by Elsevier B.V.

Improved ductile fracture criterion for cold forming of spheroidised steel

Two phase Zr-2.5 wt-%Nb is used as pressure tube material in water cooled and moderated reactors. The in service behaviour of tins hcp zirconium based alloy is dependent on strong textures developed during thermomechanical processing. In the present study, the texture evolution during fabrication of this material, using a modified route, was investigated. Texture, evaluated by the orientation distribution function, was correlated to microstructure during various stages of processing. It was observed that a strong orientation fibre parallel to the working direction develops with hot working (extrusion). With subsequent cold working, the texture formed after extrusion alone becomes strengthened. This observation is attributed to the presence of a soft secondary phase (β) at the α phase grain boundary. In contrast to the above, in the case of single phase Zircaloy 4, new components of texture are generated with cold deformation after hot extrusion, as a result of interaction between grains.

Texture evolution in two phase Zr-2.5 wt-%Nb through modified route

Some inaccuracies are possible during the near-surface residual stress measurement of textured material by X-ray diffraction using the standard multiexposure technique. Diffraction intensities depend on factors like inclination of the sample, the rotation of the beam, and the position of detectors. Low intensity due to texture can result in increased percentage error observed stress values. Anisotropy of elastic constants in textured materials can further increase the errors in measured stress. In the present study, the effect of texture on residual stress has been studied for seamless Ti-3wt%Al-2Vwt% tubes, in pilgered and annealed conditions, by superimposing the residual stress measurements on the pole figure obtained by texture measurement. A general approach has been developed for accurate determination of the stress tensor in textured material. This consists of the selection of inclinations giving rise to high intensity peak, the selection of rotation angles symmetric to the pole figure to get similar intensity from both the detectors (where this is not possible one detector with low intensity is shut off), and the calculation of the bulk X-ray elastic constants using the single crystal elastic data and texture. Cold worked Ti-3wt%Al-2Vwt% tubes had the highest stress along the rolling direction and that is compressive in nature. The principal residual stresses in the pilgered tubes were along the rolling and transverse directions, and annealing leads to reduction and reorientation of the stresses.

Journal of Testing and Evaluation

Influence of crystallographic texture on x-ray residual stress measurement for Ti-3wt% Al-2 Vwt% tube material

A linear relationship Is noticed between the dissipation efficiency parameter (η) of the dynamic materials model (DMM) and the Intrinsic hot workability parameter (ζ) of the polar reciprocity model (PRM); To identify unsafe regions In the processing maps, a simple stability condition is derived and validated Initially by considering the flow stress data of IN 718 with microstructural observations of the deformed compression specimens. Finally, the consistency in predicting the unstable regions in the processing maps is shown by utilizing the flow stress data of titanium and its alloys.

Scandinavian Journal of Metallurgy

On the relationship between the intrinsic hot workability parameters of DMM and PRM

Materials for biology and medicine

The superplastic forming grade sheets of AA8090 Al-Li alloy were observed to contain layers of different microstructure and microtexture across their cross-section along the normal to the rolling direction (RD). The surface layer (SL) material contained coarse equiaxed grains and the dominance of S {1 2 3}[6 3 4] texture whereas the center layer (CL) material contained fine elongated grains and the dominance of Bs {1 1 0}[1 1 2] texture. Tensile specimens, machined to represent the SL of 0.6 mm thickness from the surface towards center (SL), the CL of 0.6 mm thickness, obtained by removing the material of 0.6 mm thickness from each surface towards center (CL), and full thickness (FL) material of 1.8 mm thick, in a sheet of AA8090 Al-Li alloy, were deformed at optimum superplastic condition of strain rate = 1 × 10-3 s-1 and temperature = 803 K to investigate the effect of loading direction. In SL material, the specimen parallel to RD exhibited maximum and the specimen perpendicular to RD exhibited minimum flow stresses. This trend was reserved in CL material. The anisotropy in flow stress could be explained on the basis of texture in the SL material, but the contribution of grain directionality became important in the CL material. The flow behavior of FL material was found to consist of the composite-like contributions of SL and CL materials. © 2002 Elsevier Science B.V. All rights reserved.