Department of Metallurgical & Materials Engineering

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This paper reports the details of a test method that uses elements of elastic-plastic fracture mechanics to assess fracture resistance of zirconium (Zr)-2.5 wt.% niobium (Nb) pressure tubes for a pressurized heavy water reactor. The fracture properties were evaluated on curved specimens, and the effect of certain trace elements on the fracture properties was determined. Significant reduction of trace impurities, produced by using four-stage melting practices rather than the conventional two-stage process, was observed to cause considerable improvement in the fracture resistance of the alloy. Scanning electron microscopy (SEM) of the fracture surfaces of the test specimens confirmed this observation. © ASM International.

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Journal of Failure Analysis and Prevention

Measurement of fracture properties of Zr-2.5 Wt.% Nb pressure tube material and effect of trace impurities

In order to investigate the interrelationship amongst grain size, cell size, cell wall thickness and flow stress in type 316L stainless steel, tensile specimens of grain sizes 3.1-86.7 μm were deformed to a strain of 0.34 at the temperatures 297, 673 and 973 K and at a strain rate 1 × 10-4 s-1. The cell wall thickness is found to be larger than the cell interior size but the two are mutually related and only marginally decrease with the increase in grain size. The difference in the distributions of dislocations and cell structure in the vicinity of grain boundaries and in the grain interior leads to a variation in their relative contributions to strengthening. The smaller cell interior sizes and thicker cell walls result in a greater contribution of grain interior to strengthening of polycrystalline material than the grain boundaries, with the maximum effect observed at 673 K due to dynamic strain aging. © 2002 Acta Materialia Inc. Published by Elsevier Science Ltd Inc. All rights reserved.

Fulltext

Acta Materialia

Towards interrelationship of grain size, cell parameters and flow stress in type 316L stainless steel

For the development of processing maps to zirconium alloys, 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/occurrence of fracture or defects, utilizing the flow stress data of Zr-2.5Nb-0.5Cu. An attempt is made to fit the measured flow stress data in a constitutive equation, useful in the finite element process models. Instability maps at different strain levels were superimposed while delineating the unstable regions in the processing maps. This phenomenon takes into account the dependence of strain rate sensitivity and strain hardening coefficient of the material on the plastic instability during hot deformation. The applicability of the developed processing map has been examined by comparing with the reported microstructural observations of the deformed compression specimens of various zirconium alloys. It is found that the processing map is practically usable in the real fabrication process for the zirconium alloys.

Modelling and Simulation in Materials Science and Engineering

Development and validation of a processing map for zirconium alloys

The unstable flow during hot deformation of α2 titanium aluminide alloy was analysed using α parameter. The optimum hot working conditions for the material were suggested. The test data of Ti-24Al-11Nb was considered because of its high quality and ability to perform numerical analysis. The test data was corrected for the adiabatic temperature rise.

Journal of Materials Science

Processing maps for hot deformation of α2 aluminide alloy Ti-24Al-11Nb

A simple instability condition is derived following Ziegler's continuum principles for identifying the regions of unstable metal flow during hot deformation in the processing maps. This criterion is examined by considering the flow stress data on 2124 Al-SiCp metal matrix composites and comparing with the microstructural observations of the deformed compression specimens in the as-vacuum hot pressed condition and vacuum hot pressed and extruded condition. The optimum hot working conditions are proposed from the identified stable regions (having comparatively high values of workability parameters, namely, strain rate sensitivity parameter and efficiency of power dissipation) in the developed processing maps.

Advanced Composite Materials: The Official Journal of the Japan Society of Composite Materials

On the hot working characteristics of 2124 Al-SiCp metal matrix composites

Hydride blisters were grown over a period of 5-91 days under controlled thermal boundary condition using Zr-2.5wt%Nb pressure tube sections. Rectangular plate type specimens were hydrided to hydrogen concentration in the range of 20-250 ppm by weight and homogenized at 400 °C. These specimens were held in a specially fabricated jig capable of producing the required thermal gradients. The bulk specimen and the cold spot temperatures were maintained in the range of 270-400 °C and 40-100 °C respectively. Depending on the thermal gradients employed, two types of blister morphology were identified. The type I blister was single, round and located at the cold spot region whereas the type II blister consisted of several small blisters along a ring around the cold spot. Microstructural examination of the blister cross-section revealed three regions; a single-phase region consisting of hydrides, a region consisting of matrix containing both radial and circumferential hydrides, and another region consisting of matrix and circumferential hydrides. An attempt was made to rationalize the observed radial-circumferential hydride platelet orientation. Hydride blister growth rates were found to vary strongly with hydrogen concentration and bulk specimen temperature. The observed time for blister growth was found to be in agreement with the Sawatzky's model. © 2002 Published by Elsevier Science B.V.

Journal of Nuclear Materials

Hydride blister formation in Zr-2.5wt%Nb pressure tube alloy

The microstructure and yield behavior of a high aluminum containing titanium alloys was studied. The shear transformations which occured during quenching or deformation were very sensitive to the alloy composition. It was found that the room-temperature deformation of the alloy in the water-quenched condition exhibited low yield stress due to either stress-induced martensitic transformation or movement of glissile martensitic interface.

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Microstructure and yield behavior of a high aluminum containing Ti-Al-Nb alloy

Journal of Materials Processing Technology

Clarification on the physical dimension of K in a constitutive equation for superplastic flow: σ = Kε̇m

Using the test data of extra low interstitial (ELI) grade titanium alloy Ti-6Al-4V (wt.%) with microstructural observations, studies are made to examine the flow localization concepts as well as the simplified metallurgical stability criterion for delineating the regions of flow instabilities in the processing maps. The optimum hot working conditions for the material are suggested.

Zeitschrift fuer Metallkunde/Materials Research and Advanced Techniques

On the hot workability of ELI grade titanium alloy Ti-6Al-4V

The effect of microstructural evolution on superplastic deformation parameters, such as the nature of σ-̇ε plots, strain-rate sensitivity parameter, and activation energy, were studied for unstable and thermally stable microstructures of a Zr-2.5 wt pct Nb pressure-tube alloy. Two types of differential strain-rate tests (increasing temperature (IT) and decreasing temperature (DT), in the temperature range of 610 °C to 810 °C at 20 °C intervals) were conducted within a strain-rate range of 10-5 to 10-3 s-1. Single specimens were used to obtain the σ-̇ε plots for all the test temperatures in the aforementioned temperature range. The effect of orientation (with respect to the axial direction of the tube) on the superplastic deformation parameters was also studied. The microstructural evolution was studied along the three orthogonal planes of the tube by water quenching undeformed samples in the beginning of differential strain-rate tests at each test temperature. The observed apparent activation-energy values associated with deformation were in the two distinct ranges of 287 to 326 and 151 to 211 kJ/mole. In the temperature range of 730 °C to 810 °C, the apparent activation-energy value depended on the direction of approach of the test temperature. The mechanisms of superplastic deformation in this alloy were found to be dislocation climb-controlled creep in region III and grain-boundary sliding accommodated by grain-boundary diffusion or lattice diffusion in the α or β phases in region II. Based on the observed microstructural features, a model to explain the σ-̇ε plots and apparent activation energy has been proposed.