Department of Metallurgical & Materials Engineering

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Flow behaviour and concurrent microstructural evolution during superplastic deformation of the Al-Cu eutectic alloy were investigated in the temperature range 400-540°C. When deformed at strain rates up to 10-4 s-1, strain hardening occurs, which is accompanied by grain growth and cavitation at all temperatures. The observed strengthening can be explained only partly by grain growth. The presence of relatively large cavities surprisingly contributes to strengthening, following the Hall-Petch type relationship between stress and mean free path between cavities. This anomaly is attributed to plastic deformation of the intercavity regions at enhanced strain rates, in analogous to microtensile specimens. An experimental constitutive relationship for cavitation and a modification for flow behaviour are proposed.

Materials Science and Technology

Correlation between flow strengthening and cavitation in superplastic Al-Cu eutectic alloy

A methodology for estimating the stress field in the matrix surrounding a semi-constrained inclusion is proposed. The stress free misfit volume associated with the inclusion was partitioned into semi-constrained and constrained components. The objective was achieved by carrying out a series of sectioning, cutting and imposition of constrained misfit displacement on an imaginary notch of geometry identical to that of the inclusion-matrix contour. The stress field was computed using elastic finite element analysis for plane stress condition. The misfit displacement model thus developed was applied to compute the stress field around a hydride blister grown under controlled thermal boundary condition in Zr-2.5 Nb pressure tube (PT) alloy and was validated by interpreting the hydride platelet orientation in the matrix surrounding the blister. Finally, an assessment of the structural integrity of the Zr-2.5Nb PT material containing hydride blister was made in view of this analysis. © 2003 Elsevier Science B.V. All rights reserved.

World University root node

Materials Science and Engineering A

Stress field estimation around a semi-constrained inclusion and its validation by interpreting hydride platelet orientation around a blister in a Zr-2.5Nb alloy

Preferred orientation or crystallographic texture of Zr-2.5%Nb pressure tubes for a pressurized heavy water reactor (PHWR) affects all the critical in-reactor properties. The preferred orientation has to be measured to standardize the process route for getting the desired in-reactor properties. The quantitative determination involves determination of the complete pole figure and calculation of Kearns parameters from the pole figure data. Independently, neither the X-ray reflection nor the X-ray transmission technique can provide the full pole figure data. The literature reports only partial pole figures. Quantification of texture based on partial pole figures may lead to errors in estimation. For a complete pole figure determination, it is required to merge the data from both the reflection and the transmission techniques. This requires a special test setup that can combine the pole figure data obtained from the two techniques. In this paper, a methodology for determination of the complete pole figure with quantitative texture parameters for Zr-2.5%Nb pressure tubes is reported.

Journal of Testing and Evaluation

Texture measurement in Zr-2.5%Nb pressure tubes for pressurized heavy water reactors

For the development of processing maps, a simple instability condition for assessing the extent of plastic deformation in a workpiece prior to the formation of defects, is derived based on the Ziegler's continuum principles. To demonstrate the potential of the instability condition, the published flow stress data of 6061 Al-10 vol.% Al2O3 particulate reinforced metal matrix composite is considered. Instability maps at different strain levels were superimposed while delineating the unstable regions in the processing maps. This takes into account the dependence of strain rate sensitivity and strain hardening coefficient of the material on the plastic instability during hot deformation. The stable and unstable regions in the map are verified with the microstructural observations of the deformed compression specimens as well as the industrial forging trials. To examine its validity, a comparative study is made with the flow localization concept for titanium alloys and titanium aluminides. The standard flow localization concept shows inconsistency in predicting the unstable regions in the processing maps, whereas, the present instability criterion is found to be consistent. Further studies were made on the hot deformation of 6061 Al with varying volume fractions and sizes of SiC particulate reinforcements. The 'stable' and 'unstable' regions in the processing maps identified from the present instability condition using the measured flow stress data, have been compared with the reported microstructural observations of the deformed compression specimens. It is noted from the processing maps that the domain of instability increases with the increasing volume fraction of the SiC particles. The optimum hot working conditions for these composites are suggested. © 2002 Elsevier Science Ltd. All rights reserved.

Composites Science and Technology

On the hot working characteristics of 6061Al-SiC and 6061-Al2O3 particulate reinforced metal matrix composites

This paper examines some of the criteria commonly used to predict the initiation of fracture during cold forming utilizing the existing test data on low carbon steels. A limiting condition based on integrals of stress functions, which takes into account the tensile stress ratio and triaxiality is proposed, and its applicability demonstrated with test data of low carbon steel of different heats is developed. Material fracture can be estimated during cold forming by adding the proposed criterion to the finite element computer programs. The method of evaluation of the material constants in the proposed criterion is developed and demonstrated. By this way, the traditional procedures of trial and error, which involve cost and time, can be minimized.

Transactions of the Indian Institute of Metals

A limiting condition at the initiation of fracture during cold forming of low carbon steels

The SPF grade sheets of AA8090 Al-Li alloy were observed to possess a composite-like layered microstructure with different microstructures and microtextures near the surface region and in the mid-thickness region in the longitudinal and transverse directions. Tensile specimens representing the microstructures near the surface in the mid-thickness and full thickness of a sheet were deformed under the optimum superplastic condition of strain rate (∈) = 1 × 10-3 s-1 and T = 803 K. The microstructural and microtextural gradient along the sheet thickness were found to result in the highest flow stress in the mid-thickness material which was dominated (60 v%) by brass (110) [12] texture and elongated grains. Grain growth, transformation of elongated grains into equiaxed grains, texture weakening and cavitation occurred during superplastic deformation. Such microstructural evolution in different zones across the thickness direction was found to influence the nature of stress-strain curves to a varying extent in the near surface, mid-thickness and full thickness samples. The mechanical properties and deformation behaviour could be explained by treating the full thickness sheet as a composite of surface and mid-thickness materials.

Canadian Metallurgical Quarterly

Effect of composite-like microstructure on superplastic flow and concurrent microstructural evolution in AA8090 Al-Li alloy

High temperature flow behaviour and microstructural evolution were investigated in an annealed AA 8090 Al-Li alloy over the temperature range 623-803 K and strain rate range ∼6 × 106 -3 × 10 2 s-1. Stress-strain rate data, obtained using a differential strain rate test technique and plotted in log-log scale, exhibited three regions I, II, and III, with increasing strain rate. In these regions, the values of strain rate sensitivity index m and the activation energy for deformation were determined to be 0.17, 0.43, and 0.17; and 758.8, 93.3, and 184.3 kJ mol-1, respectively. The stress-strain curves obtained from constant strain rate tests exhibited flow hardening at lower strain rates and higher temperatures whereas flow softening occurred at higher strain rates and lower temperatures. The microstructural evolution revealed the dominance of grain growth under the former conditions and dynamic recrystallisation under the latter conditions. Ductility and m were found to increase with temperature, with the maximum values of 402% and 0.55, respectively, at a temperature 803 K and strain rate 2 × 10-4 s1. © 2003 IoM Communications Ltd. Published by Maney for the Institute of Materials, Minerals and Mining.

Superplastic behaviour of annealed AA 8090 Al-Li alloy

Superplastic deformation of Al-Li 8090 alloy was carried out to study the effect of local strain developed on the microstructural variation along the gauge length of tensile specimens. For this, separate specimens were deformed to failure at a constant temperature of 530 °C and at the strain rates within the superplastic regime. The strain distribution was found to be non-uniform with more deformation towards fracture tip and less towards the shoulder section of specimens. The grain size was found to decrease with increase in local strain whereas cavity size and cavity volume fraction were found to increase. The cavity growth in longitudinal direction is suggested to be controlled by power law but the same in transverse direction is controlled by diffusional process. A model is proposed for cavity nucleation on the basis of inhomogeneity in microstructure and its implication in deformation mechanism. © 2002 Elsevier Science B.V. All rights reserved.

Fulltext

Effect of local strain distribution on concurrent microstructural evolution during superplastic deformation of Al-Li 8090 alloy

Thin walled calandria tubes for pressurised heavy water reactors are manufactured either by seam welding of Zircaloy-4 sheets or by seamless route. In the present study, the effect of processing on the critical properties such as texture, microstructure, hydriding behaviour and residual stress for both the routes as well as the mechanical anisotropy developed due to seam welding are investigated. The properties of the seam welded tube in the fusion and adjoining region are markedly different from the base material and from the seamless tube. Residual stress measurements indicate that heat affected zone (HAZ) of seam welded tubes have longitudinal tensile residual stress and the seamless tubes have uniform compressive stress along the circumference. The phase transition in the presence of residual stresses due to thermal gradient is found to modify the texture in the HAZ. The hydride orientation and mechanical anisotropy in these regions are found to be dependent on the texture of the material. © 2003 Elsevier Science B.V. All rights reserved.

Journal of Nuclear Materials

Effect of processing on properties of thin walled calandria tubes for pressurised heavy water reactor

Delayed hydride cracking (DHC) is one of the localized forms of hydride embrittlement caused by hydrogen migration up the tensile stress gradient. In this work, DHC velocity was measured along the axial direction of the double melted, cold worked and stress-relieved Zirconium-2.5Niobium pressure tube material in the temperature range of 162-283 °C. The DHC crack growth was monitored using the direct current potential drop (DCPD) technique. The calibration curves between the normalized DCPD output and the normalized crack length at different test temperatures were also used to determine the DHC velocity. A simple model capable of explaining the observed features of DHC is proposed. The model explains the basis for the occurrence of incubation period associated with DHC crack initiation. Activation energy associated with the DHC in this alloy was found to be 56 kJ/mole. © 2002 Elsevier Science B.V. All rights reserved.